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-rw-r--r--.gitignore6
-rw-r--r--COPYING674
-rw-r--r--Makefile34
-rw-r--r--README.md19
-rw-r--r--include/coaperl.hrl52
-rw-r--r--include/corerl.hrl8
-rwxr-xr-xrebarbin0 -> 160323 bytes
-rw-r--r--rebar.config3
-rw-r--r--rfc/core-parameters.txt237
-rw-r--r--rfc/draft-ietf-core-block-17.txt1848
-rw-r--r--rfc/draft-ietf-core-observe-16.txt1904
-rw-r--r--rfc/rfc6690.txt1235
-rw-r--r--rfc/rfc7252.txt6275
-rw-r--r--src/coap_parse.erl137
-rw-r--r--src/coap_unparse.erl195
-rw-r--r--src/coaperl.app.src11
-rw-r--r--src/coaperl_sup.erl29
-rw-r--r--src/corerl.erl72
-rw-r--r--test/coap_parse_test.erl19
-rw-r--r--test/coap_unparse_test.erl31
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diff --git a/.gitignore b/.gitignore
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+*~
+.eunit/
+.rebar/
+deps/
+ebin/
+doc/
diff --git a/COPYING b/COPYING
new file mode 100644
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--- /dev/null
+++ b/COPYING
@@ -0,0 +1,674 @@
+ GNU GENERAL PUBLIC LICENSE
+ Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
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+author or copyright holder as a result of your choosing to follow a
+later version.
+
+ 15. Disclaimer of Warranty.
+
+ THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+ 16. Limitation of Liability.
+
+ IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+ 17. Interpretation of Sections 15 and 16.
+
+ If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+ END OF TERMS AND CONDITIONS
+
+ How to Apply These Terms to Your New Programs
+
+ If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+ To do so, attach the following notices to the program. It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+ <one line to give the program's name and a brief idea of what it does.>
+ Copyright (C) <year> <name of author>
+
+ This program is free software: you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation, either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+ If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+ <program> Copyright (C) <year> <name of author>
+ This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+ This is free software, and you are welcome to redistribute it
+ under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License. Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+ You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<http://www.gnu.org/licenses/>.
+
+ The GNU General Public License does not permit incorporating your program
+into proprietary programs. If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library. If this is what you want to do, use the GNU Lesser General
+Public License instead of this License. But first, please read
+<http://www.gnu.org/philosophy/why-not-lgpl.html>.
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..a329f76
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,34 @@
+ERL ?= erl
+APP := coaperl
+
+.PHONY: deps
+
+all: deps
+ @./rebar compile
+
+deps:
+ @./rebar get-deps
+
+clean:
+ @./rebar clean
+
+distclean: clean
+ @./rebar delete-deps
+
+docs:
+ @erl -noshell -run edoc_run application '$(APP)' '"."' '[]'
+
+test: all
+ @(./rebar skip_deps=true eunit)
+
+tags:
+ find . -name "*.[he]rl" -print | etags -
+
+DEPSOLVER_PLT=$(CURDIR)/.depsolver_plt
+$(DEPSOLVER_PLT):
+ dialyzer --output_plt $(DEPSOLVER_PLT) --build_plt \
+ --apps erts kernel stdlib crypto public_key # -r deps
+
+dialyzer: $(DEPSOLVER_PLT)
+ dialyzer --plt $(DEPSOLVER_PLT) -Wrace_conditions --src src
+
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..de5494f
--- /dev/null
+++ b/README.md
@@ -0,0 +1,19 @@
+Some erlang helpers for COAP messages (RFC7252)
+
+Copyright 2015 Guido G√ľnther <agx@sigxcpu.org>
+
+This is by no means a full coap implementation. It merely allows to ease
+parsing of messages a bit by putting things into a record.
+
+Let's assume you receive UDP datagrams in a gen_server. You can then match on
+the incoming coap message like:
+
+ handle_info({udp, Socket, Host, Port, Bin}, State) ->
+ Msg = coap_parse:parse(Bin),
+ case Msg of
+ #coap_request{method=post, path= <<"/switches">>, query=[<<"name=", Name/binary>>], payload=Payload} ->
+ ...
+
+Which would allow you to process a request like:
+
+ echo '{"mode": "on"}' | coap-client -f- -t application/json -m post "coap://[::1]/switches?name=Lever1"
diff --git a/include/coaperl.hrl b/include/coaperl.hrl
new file mode 100644
index 0000000..be3b0f1
--- /dev/null
+++ b/include/coaperl.hrl
@@ -0,0 +1,52 @@
+
+% RFC7252, Section 12.2
+% 0 (Reserved)
+% 1 if-match
+% 3 uri-host
+% 4 eTag
+% 5 if-none-match
+% 7 uri-port
+% 8 location-path
+-define(COAP_OPTION_URI_PATH, 11).
+-define(COAP_OPTION_CONTENT_FORMAT, 12).
+% 14 max-age
+-define(COAP_OPTION_URI_QUERY, 15).
+% 17 accept
+% 20 location-query
+% 35 proxy-uri
+% 39 proxy-scheme
+% 60 size1
+%128 (Reserved)
+%132 (Reserved)
+%136 (Reserved)
+%140 (Reserved)
+
+% draft-ietf-core-block-17
+-define(COAP_OPTION_BLOCK2, 23).
+-define(COAP_OPTION_BLOCK1, 27).
+-define(COAP_OPTION_SIZE2, 28).
+
+-define(COAP_PAYLOAD_MARKER, 16#FF).
+
+-record(coap_request, {
+ method,
+ path,
+ query,
+ mid,
+ ct,
+ payload
+ }).
+
+-record(coap_response, {
+ class, % response class
+ detail, % repsonse detail
+ mid, % the meassage id
+ ct, % the content type
+ block2, % block2 option
+ options, % all coap options
+ payload}).
+% 2.01 Location-Path, Location-Query
+% 2.03 ETag, Max-Age
+% 4.13 Size1
+
+-record(coap_empty_message, {mid}).
diff --git a/include/corerl.hrl b/include/corerl.hrl
new file mode 100644
index 0000000..80b93ba
--- /dev/null
+++ b/include/corerl.hrl
@@ -0,0 +1,8 @@
+
+-record(core_link, {uri,
+ title,
+ sz, % size
+ rt, % resouce type
+ ct, % content type
+ 'if' % interface
+ }).
diff --git a/rebar b/rebar
new file mode 100755
index 0000000..14e5c22
--- /dev/null
+++ b/rebar
Binary files differ
diff --git a/rebar.config b/rebar.config
new file mode 100644
index 0000000..9f18e25
--- /dev/null
+++ b/rebar.config
@@ -0,0 +1,3 @@
+{erl_opts, [
+ debug_info
+]}.
diff --git a/rfc/core-parameters.txt b/rfc/core-parameters.txt
new file mode 100644
index 0000000..659d0e2
--- /dev/null
+++ b/rfc/core-parameters.txt
@@ -0,0 +1,237 @@
+ Constrained RESTful Environments (CoRE) Parameters
+
+ Created
+ 2012-06-08
+
+ Last Updated
+ 2015-07-03
+
+ Available Formats
+ [IMG]
+ XML [IMG]
+ HTML [IMG]
+ Plain text
+
+ Registries included below
+
+ * Resource Type (rt=) Link Target Attribute Values
+ * Interface Description (if=) Link Target Attribute Values
+ * CoAP Codes
+
+ * CoAP Method Codes
+ * CoAP Response Codes
+
+ * CoAP Option Numbers
+ * CoAP Content-Formats
+
+Resource Type (rt=) Link Target Attribute Values
+
+ Expert(s)
+
+ Unassigned
+
+ Reference
+ [RFC6690]
+
+ Available Formats
+ [IMG]
+ CSV
+
+ Range Registration Procedures
+ value starts with "core" IETF Review
+ all other values Specification Required
+
+ Value Description Reference Notes
+ core.gp Group Configuration resource. This resource is used to query/manage the group membership of a CoAP [RFC7390, Section 2.6.2]
+ server.
+
+Interface Description (if=) Link Target Attribute Values
+
+ Expert(s)
+
+ Unassigned
+
+ Reference
+ [RFC6690]
+
+ Range Registration Procedures
+ value starts with "core" IETF Review
+ all other values Specification Required
+
+ Value Description Reference Notes
+ No registrations at this time.
+
+CoAP Codes
+
+ Registration Procedure(s)
+
+ Reserved ranges may be allocated in accordance with section 4.3 of [RFC5226].
+
+ Reference
+ [RFC7252, Section 12.1]
+
+ Available Formats
+ [IMG]
+ CSV
+
+ Code Description Reference
+ 0.00 Indicates an Empty message. [RFC7252, section 4.1]
+ 0.01-0.31 Indicates a request. Values in this range are assigned by the [CoAP Method Codes] sub-registry. [RFC7252, section 12.1.1]
+ 1.00-1.31 Reserved [RFC7252]
+ 2.00-5.31 Indicates a response. Values in this range are assigned by the [CoAP Response Codes] sub-registry. [RFC7252, section 12.1.2]
+ 6.00-7.31 Reserved [RFC7252]
+
+ CoAP Method Codes
+
+ Registration Procedure(s)
+
+ IETF Review or IESG Approval
+
+ Reference
+ [RFC7252, Section 12.1.1]
+
+ Available Formats
+ [IMG]
+ CSV
+
+ Code Name Reference
+ 0.01 GET [RFC7252]
+ 0.02 POST [RFC7252]
+ 0.03 PUT [RFC7252]
+ 0.04 DELETE [RFC7252]
+ 0.05-0.31 Unassigned
+
+ CoAP Response Codes
+
+ Registration Procedure(s)
+
+ IETF Review or IESG Approval
+
+ Reference
+ [RFC7252, Section 12.1.2]
+
+ Available Formats
+ [IMG]
+ CSV
+
+ Code Description Reference
+ 2.00 Unassigned
+ 2.01 Created [RFC7252]
+ 2.02 Deleted [RFC7252]
+ 2.03 Valid [RFC7252]
+ 2.04 Changed [RFC7252]
+ 2.05 Content [RFC7252]
+ 2.06-2.31 Unassigned
+ 3.00-3.31 Reserved [RFC7252]
+ 4.00 Bad Request [RFC7252]
+ 4.01 Unauthorized [RFC7252]
+ 4.02 Bad Option [RFC7252]
+ 4.03 Forbidden [RFC7252]
+ 4.04 Not Found [RFC7252]
+ 4.05 Method Not Allowed [RFC7252]
+ 4.06 Not Acceptable [RFC7252]
+ 4.07-4.11 Unassigned
+ 4.12 Precondition Failed [RFC7252]
+ 4.13 Request Entity Too Large [RFC7252]
+ 4.14 Unassigned
+ 4.15 Unsupported Content-Format [RFC7252]
+ 4.16-4.31 Unassigned
+ 5.00 Internal Server Error [RFC7252]
+ 5.01 Not Implemented [RFC7252]
+ 5.02 Bad Gateway [RFC7252]
+ 5.03 Service Unavailable [RFC7252]
+ 5.04 Gateway Timeout [RFC7252]
+ 5.05 Proxying Not Supported [RFC7252]
+ 5.06-5.31 Unassigned
+
+CoAP Option Numbers
+
+ Expert(s)
+
+ Zach Shelby
+
+ Reference
+ [RFC7252, Section 12.2]
+
+ Available Formats
+ [IMG]
+ CSV
+
+ Range Registration Procedures
+ 0-255 IETF Review or IESG Approval
+ 256-2047 Specification Required
+ 2048-64999 Expert Review
+ 65000-65535 Experimental use (no operational use)
+
+ Number Name Reference
+ 0 Reserved [RFC7252]
+ 1 If-Match [RFC7252]
+ 2 Unassigned
+ 3 Uri-Host [RFC7252]
+ 4 ETag [RFC7252]
+ 5 If-None-Match [RFC7252]
+ 6 Observe [RFC-ietf-core-observe-16]
+ 7 Uri-Port [RFC7252]
+ 8 Location-Path [RFC7252]
+ 9-10 Unassigned
+ 11 Uri-Path [RFC7252]
+ 12 Content-Format [RFC7252]
+ 13 Unassigned
+ 14 Max-Age [RFC7252]
+ 15 Uri-Query [RFC7252]
+ 16 Unassigned
+ 17 Accept [RFC7252]
+ 18-19 Unassigned
+ 20 Location-Query [RFC7252]
+ 21-34 Unssigned
+ 35 Proxy-Uri [RFC7252]
+ 36-38 Unassigned
+ 39 Proxy-Scheme [RFC7252]
+ 40-59 Unassigned
+ 60 Size1 [RFC7252]
+ 61-127 Unassigned
+ 128 Reserved [RFC7252]
+ 129-131 Unassigned
+ 132 Reserved [RFC7252]
+ 133-135 Unassigned
+ 136 Reserved [RFC7252]
+ 137-139 Unassigned
+ 140 Reserved [RFC7252]
+ 141-283 Unassigned
+ 284 No-Response [draft-tcs-coap-no-response-option]
+ 285-64999 Unassigned
+ 65000-65535 Reserved for Experimental Use [RFC7252]
+
+CoAP Content-Formats
+
+ Expert(s)
+
+ Zach Shelby
+
+ Reference
+ [RFC7252, Section 12.3]
+
+ Available Formats
+ [IMG]
+ CSV
+
+ Range Registration Procedures
+ 0-255 Expert Review
+ 256-9999 IETF Review or IESG Approval
+ 10000-64999 First Come First Served
+ 65000-65535 Experimental use (no operational use)
+
+ Media Type Encoding ID Reference
+ text/plain; charset=utf-8 0 [RFC2046][RFC3676][RFC5147]
+ Unassigned 1-39
+ application/link-format 40 [RFC6690]
+ application/xml 41 [RFC3023]
+ application/octet-stream 42 [RFC2045][RFC2046]
+ Unassigned 43-46
+ application/exi 47 ["Efficient XML Interchange (EXI) Format 1.0 (Second Edition)", February 2014]
+ Unassigned 48-49
+ application/json 50 [RFC4627]
+ Unassigned 51-59
+ application/cbor 60 [RFC7049]
+ Unassigned 61-64999
+ Reserved for Experimental Use 65000-65535 [RFC7252]
diff --git a/rfc/draft-ietf-core-block-17.txt b/rfc/draft-ietf-core-block-17.txt
new file mode 100644
index 0000000..60bed7a
--- /dev/null
+++ b/rfc/draft-ietf-core-block-17.txt
@@ -0,0 +1,1848 @@
+
+
+
+
+CoRE Working Group C. Bormann
+Internet-Draft Universitaet Bremen TZI
+Intended status: Standards Track Z. Shelby, Ed.
+Expires: September 10, 2015 ARM
+ March 09, 2015
+
+
+ Block-wise transfers in CoAP
+ draft-ietf-core-block-17
+
+Abstract
+
+ CoAP is a RESTful transfer protocol for constrained nodes and
+ networks. Basic CoAP messages work well for the small payloads we
+ expect from temperature sensors, light switches, and similar
+ building-automation devices. Occasionally, however, applications
+ will need to transfer larger payloads -- for instance, for firmware
+ updates. With HTTP, TCP does the grunt work of slicing large
+ payloads up into multiple packets and ensuring that they all arrive
+ and are handled in the right order.
+
+ CoAP is based on datagram transports such as UDP or DTLS, which
+ limits the maximum size of resource representations that can be
+ transferred without too much fragmentation. Although UDP supports
+ larger payloads through IP fragmentation, it is limited to 64 KiB
+ and, more importantly, doesn't really work well for constrained
+ applications and networks.
+
+ Instead of relying on IP fragmentation, this specification extends
+ basic CoAP with a pair of "Block" options, for transferring multiple
+ blocks of information from a resource representation in multiple
+ request-response pairs. In many important cases, the Block options
+ enable a server to be truly stateless: the server can handle each
+ block transfer separately, with no need for a connection setup or
+ other server-side memory of previous block transfers.
+
+ In summary, the Block options provide a minimal way to transfer
+ larger representations in a block-wise fashion.
+
+Status of This Memo
+
+ This Internet-Draft is submitted in full conformance with the
+ provisions of BCP 78 and BCP 79.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF). Note that other groups may also distribute
+ working documents as Internet-Drafts. The list of current Internet-
+ Drafts is at http://datatracker.ietf.org/drafts/current/.
+
+
+
+Bormann & Shelby Expires September 10, 2015 [Page 1]
+
+Internet-Draft Block-wise transfers in CoAP March 2015
+
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than as "work in progress."
+
+ This Internet-Draft will expire on September 10, 2015.
+
+Copyright Notice
+
+ Copyright (c) 2015 IETF Trust and the persons identified as the
+ document authors. All rights reserved.
+
+ This document is subject to BCP 78 and the IETF Trust's Legal
+ Provisions Relating to IETF Documents
+ (http://trustee.ietf.org/license-info) in effect on the date of
+ publication of this document. Please review these documents
+ carefully, as they describe your rights and restrictions with respect
+ to this document. Code Components extracted from this document must
+ include Simplified BSD License text as described in Section 4.e of
+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 2. Block-wise transfers . . . . . . . . . . . . . . . . . . . . 5
+ 2.1. The Block2 and Block1 Options . . . . . . . . . . . . . . 5
+ 2.2. Structure of a Block Option . . . . . . . . . . . . . . . 6
+ 2.3. Block Options in Requests and Responses . . . . . . . . . 8
+ 2.4. Using the Block2 Option . . . . . . . . . . . . . . . . . 10
+ 2.5. Using the Block1 Option . . . . . . . . . . . . . . . . . 12
+ 2.6. Combining Block-wise Transfers with the Observe Option . 13
+ 2.7. Combining Block1 and Block2 . . . . . . . . . . . . . . . 14
+ 2.8. Combining Block2 with Multicast . . . . . . . . . . . . . 14
+ 2.9. Response Codes . . . . . . . . . . . . . . . . . . . . . 14
+ 2.9.1. 2.31 Continue . . . . . . . . . . . . . . . . . . . . 15
+ 2.9.2. 4.08 Request Entity Incomplete . . . . . . . . . . . 15
+ 2.9.3. 4.13 Request Entity Too Large . . . . . . . . . . . . 15
+ 2.10. Caching Considerations . . . . . . . . . . . . . . . . . 15
+ 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
+ 3.1. Block2 Examples . . . . . . . . . . . . . . . . . . . . . 16
+ 3.2. Block1 Examples . . . . . . . . . . . . . . . . . . . . . 20
+ 3.3. Combining Block1 and Block2 . . . . . . . . . . . . . . . 21
+ 3.4. Combining Observe and Block2 . . . . . . . . . . . . . . 23
+ 4. The Size2 and Size1 Options . . . . . . . . . . . . . . . . . 26
+ 5. HTTP Mapping Considerations . . . . . . . . . . . . . . . . . 27
+ 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
+ 7. Security Considerations . . . . . . . . . . . . . . . . . . . 29
+
+
+
+Bormann & Shelby Expires September 10, 2015 [Page 2]
+
+Internet-Draft Block-wise transfers in CoAP March 2015
+
+
+ 7.1. Mitigating Resource Exhaustion Attacks . . . . . . . . . 30
+ 7.2. Mitigating Amplification Attacks . . . . . . . . . . . . 31
+ 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
+ 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 31
+ 9.1. Normative References . . . . . . . . . . . . . . . . . . 31
+ 9.2. Informative References . . . . . . . . . . . . . . . . . 32
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
+
+1. Introduction
+
+ The work on Constrained RESTful Environments (CoRE) aims at realizing
+ the REST architecture in a suitable form for the most constrained
+ nodes (such as microcontrollers with limited RAM and ROM [RFC7228])
+ and networks (such as 6LoWPAN, [RFC4944]) [RFC7252]. The CoAP
+ protocol is intended to provide RESTful [REST] services not unlike
+ HTTP [RFC7230], while reducing the complexity of implementation as
+ well as the size of packets exchanged in order to make these services
+ useful in a highly constrained network of themselves highly
+ constrained nodes.
+
+ This objective requires restraint in a number of sometimes
+ conflicting ways:
+
+ o reducing implementation complexity in order to minimize code size,
+
+ o reducing message sizes in order to minimize the number of
+ fragments needed for each message (in turn to maximize the
+ probability of delivery of the message), the amount of
+ transmission power needed and the loading of the limited-bandwidth
+ channel,
+
+ o reducing requirements on the environment such as stable storage,
+ good sources of randomness or user interaction capabilities.
+
+ CoAP is based on datagram transports such as UDP, which limit the
+ maximum size of resource representations that can be transferred
+ without creating unreasonable levels of IP fragmentation. In
+ addition, not all resource representations will fit into a single
+ link layer packet of a constrained network, which may cause
+ adaptation layer fragmentation even if IP layer fragmentation is not
+ required. Using fragmentation (either at the adaptation layer or at
+ the IP layer) for the transport of larger representations would be
+ possible up to the maximum size of the underlying datagram protocol
+ (such as UDP), but the fragmentation/reassembly process burdens the
+ lower layers with conversation state that is better managed in the
+ application layer.
+
+
+
+
+
+Bormann & Shelby Expires September 10, 2015 [Page 3]
+
+Internet-Draft Block-wise transfers in CoAP March 2015
+
+
+ The present specification defines a pair of CoAP options to enable
+ _block-wise_ access to resource representations. The Block options
+ provide a minimal way to transfer larger resource representations in
+ a block-wise fashion. The overriding objective is to avoid the need
+ for creating conversation state at the server for block-wise GET
+ requests. (It is impossible to fully avoid creating conversation
+ state for POST/PUT, if the creation/replacement of resources is to be
+ atomic; where that property is not needed, there is no need to create
+ server conversation state in this case, either.)
+
+ In summary, this specification adds a pair of Block options to CoAP
+ that can be used for block-wise transfers. Benefits of using these
+ options include:
+
+ o Transfers larger than what can be accommodated in constrained-
+ network link-layer packets can be performed in smaller blocks.
+
+ o No hard-to-manage conversation state is created at the adaptation
+ layer or IP layer for fragmentation.
+
+ o The transfer of each block is acknowledged, enabling individual
+ retransmission if required.
+
+ o Both sides have a say in the block size that actually will be
+ used.
+
+ o The resulting exchanges are easy to understand using packet
+ analyzer tools and thus quite accessible to debugging.
+
+ o If needed, the Block options can also be used (without changes) to
+ provide random access to power-of-two sized blocks within a
+ resource representation.
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
+ "OPTIONAL" in this document are to be interpreted as described in RFC
+ 2119, BCP 14 [RFC2119] and indicate requirement levels for compliant
+ CoAP implementations.
+
+ In this document, the term "byte" is used in its now customary sense
+ as a synonym for "octet".
+
+ Where bit arithmetic is explained, this document uses the notation
+ familiar from the programming language C, except that the operator
+ "**" stands for exponentiation.
+
+
+
+
+
+
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+
+2. Block-wise transfers
+
+ As discussed in the introduction, there are good reasons to limit the
+ size of datagrams in constrained networks:
+
+ o by the maximum datagram size (~ 64 KiB for UDP)
+
+ o by the desire to avoid IP fragmentation (MTU of 1280 for IPv6)
+
+ o by the desire to avoid adaptation layer fragmentation (60-80 bytes
+ for 6LoWPAN [RFC4919])
+
+ When a resource representation is larger than can be comfortably
+ transferred in the payload of a single CoAP datagram, a Block option
+ can be used to indicate a block-wise transfer. As payloads can be
+ sent both with requests and with responses, this specification
+ provides two separate options for each direction of payload transfer.
+ In identifying these options, we use the number 1 to refer to the
+ transfer of the resource representation that pertains to the request,
+ and the number 2 to refer to the transfer of the resource
+ representation for the response.
+
+ In the following, the term "payload" will be used for the actual
+ content of a single CoAP message, i.e. a single block being
+ transferred, while the term "body" will be used for the entire
+ resource representation that is being transferred in a block-wise
+ fashion. The Content-Format option applies to the body, not to the
+ payload, in particular the boundaries between the blocks may be in
+ places that are not separating whole units in terms of the structure,
+ encoding, or content-coding used by the Content-Format.
+
+ In most cases, all blocks being transferred for a body (except for
+ the last one) will be of the same size. The block size is not fixed
+ by the protocol. To keep the implementation as simple as possible,
+ the Block options support only a small range of power-of-two block
+ sizes, from 2**4 (16) to 2**10 (1024) bytes. As bodies often will
+ not evenly divide into the power-of-two block size chosen, the size
+ need not be reached in the final block (but even for the final block,
+ the chosen power-of-two size will still be indicated in the block
+ size field of the Block option).
+
+2.1. The Block2 and Block1 Options
+
+
+
+
+
+
+
+
+
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+ +-----+---+---+---+---+--------+--------+--------+---------+
+ | No. | C | U | N | R | Name | Format | Length | Default |
+ +-----+---+---+---+---+--------+--------+--------+---------+
+ | 23 | C | U | - | - | Block2 | uint | 0-3 | (none) |
+ | | | | | | | | | |
+ | 27 | C | U | - | - | Block1 | uint | 0-3 | (none) |
+ +-----+---+---+---+---+--------+--------+--------+---------+
+
+ Table 1: Block Option Numbers
+
+ Both Block1 and Block2 options can be present both in request and
+ response messages. In either case, the Block1 Option pertains to the
+ request payload, and the Block2 Option pertains to the response
+ payload.
+
+ Hence, for the methods defined in [RFC7252], Block1 is useful with
+ the payload-bearing POST and PUT requests and their responses.
+ Block2 is useful with GET, POST, and PUT requests and their payload-
+ bearing responses (2.01, 2.02, 2.04, 2.05 -- see section "Payload" of
+ [RFC7252]).
+
+ Where Block1 is present in a request or Block2 in a response (i.e.,
+ in that message to the payload of which it pertains) it indicates a
+ block-wise transfer and describes how this specific block-wise
+ payload forms part of the entire body being transferred ("descriptive
+ usage"). Where it is present in the opposite direction, it provides
+ additional control on how that payload will be formed or was
+ processed ("control usage").
+
+ Implementation of either Block option is intended to be optional.
+ However, when it is present in a CoAP message, it MUST be processed
+ (or the message rejected); therefore it is identified as a critical
+ option. It MUST NOT occur more than once.
+
+2.2. Structure of a Block Option
+
+ Three items of information may need to be transferred in a Block
+ (Block1 or Block2) option:
+
+ o The size of the block (SZX);
+
+ o whether more blocks are following (M);
+
+ o the relative number of the block (NUM) within a sequence of blocks
+ with the given size.
+
+ The value of the Block Option is a variable-size (0 to 3 byte)
+ unsigned integer (uint, see Section 3.2 of [RFC7252]). This integer
+
+
+
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+
+ value encodes these three fields, see Figure 1. (Due to the CoAP
+ uint encoding rules, when all of NUM, M, and SZX happen to be zero, a
+ zero-byte integer will be sent.)
+
+ 0
+ 0 1 2 3 4 5 6 7
+ +-+-+-+-+-+-+-+-+
+ | NUM |M| SZX |
+ +-+-+-+-+-+-+-+-+
+
+ 0 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | NUM |M| SZX |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ 0 1 2
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | NUM |M| SZX |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Figure 1: Block option value
+
+ The block size is encoded using a three-bit unsigned integer (0 for
+ 2**4 to 6 for 2**10 bytes), which we call the "SZX" ("size
+ exponent"); the actual block size is then "2**(SZX + 4)". SZX is
+ transferred in the three least significant bits of the option value
+ (i.e., "val & 7" where "val" is the value of the option).
+
+ The fourth least significant bit, the M or "more" bit ("val & 8"),
+ indicates whether more blocks are following or the current block-wise
+ transfer is the last block being transferred.
+
+ The option value divided by sixteen (the NUM field) is the sequence
+ number of the block currently being transferred, starting from zero.
+ The current transfer is therefore about the "size" bytes starting at
+ byte "NUM << (SZX + 4)".
+
+ Implementation note: As an implementation convenience, "(val & ~0xF)
+ << (val & 7)", i.e., the option value with the last 4 bits masked
+ out, shifted to the left by the value of SZX, gives the byte
+ position of the first byte of the block being transferred.
+
+ More specifically, within the option value of a Block1 or Block2
+ Option, the meaning of the option fields is defined as follows:
+
+
+
+
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+
+ NUM: Block Number, indicating the block number being requested or
+ provided. Block number 0 indicates the first block of a body
+ (i.e., starting with the first byte of the body).
+
+ M: More Flag ("not last block"). For descriptive usage, this flag,
+ if unset, indicates that the payload in this message is the last
+ block in the body; when set it indicates that there are one or
+ more additional blocks available. When a Block2 Option is used in
+ a request to retrieve a specific block number ("control usage"),
+ the M bit MUST be sent as zero and ignored on reception. (In a
+ Block1 Option in a response, the M flag is used to indicate
+ atomicity, see below.)
+
+ SZX: Block Size. The block size is represented as three-bit
+ unsigned integer indicating the size of a block to the power of
+ two. Thus block size = 2**(SZX + 4). The allowed values of SZX
+ are 0 to 6, i.e., the minimum block size is 2**(0+4) = 16 and the
+ maximum is 2**(6+4) = 1024. The value 7 for SZX (which would
+ indicate a block size of 2048) is reserved, i.e. MUST NOT be sent
+ and MUST lead to a 4.00 Bad Request response code upon reception
+ in a request.
+
+ There is no default value for the Block1 and Block2 Options. Absence
+ of one of these options is equivalent to an option value of 0 with
+ respect to the value of NUM and M that could be given in the option,
+ i.e. it indicates that the current block is the first and only block
+ of the transfer (block number 0, M bit not set). However, in
+ contrast to the explicit value 0, which would indicate an SZX of 0
+ and thus a size value of 16 bytes, there is no specific explicit size
+ implied by the absence of the option -- the size is left unspecified.
+ (As for any uint, the explicit value 0 is efficiently indicated by a
+ zero-length option; this, therefore, is different in semantics from
+ the absence of the option.)
+
+2.3. Block Options in Requests and Responses
+
+ The Block options are used in one of three roles:
+
+ o In descriptive usage, i.e., a Block2 Option in a response (such as
+ a 2.05 response for GET), or a Block1 Option in a request (a PUT
+ or POST):
+
+ * The NUM field in the option value describes what block number
+ is contained in the payload of this message.
+
+ * The M bit indicates whether further blocks need to be
+ transferred to complete the transfer of that body.
+
+
+
+
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+
+ * The block size implied by SZX MUST match the size of the
+ payload in bytes, if the M bit is set. (SZX does not govern
+ the payload size if M is unset). For Block2, if the request
+ suggested a larger value of SZX, the next request MUST move SZX
+ down to the size given in the response. (The effect is that,
+ if the server uses the smaller of (1) its preferred block size
+ and (2) the block size requested, all blocks for a body use the
+ same block size.)
+
+ o A Block2 Option in control usage in a request (e.g., GET):
+
+ * The NUM field in the Block2 Option gives the block number of
+ the payload that is being requested to be returned in the
+ response.
+
+ * In this case, the M bit has no function and MUST be set to
+ zero.
+
+ * The block size given (SZX) suggests a block size (in the case
+ of block number 0) or repeats the block size of previous blocks
+ received (in the case of a non-zero block number).
+
+ o A Block1 Option in control usage in a response (e.g., a 2.xx
+ response for a PUT or POST request):
+
+ * The NUM field of the Block1 Option indicates what block number
+ is being acknowledged.
+
+ * If the M bit was set in the request, the server can choose
+ whether to act on each block separately, with no memory, or
+ whether to handle the request for the entire body atomically,
+ or any mix of the two.
+
+ + If the M bit is also set in the response, it indicates that
+ this response does not carry the final response code to the
+ request, i.e. the server collects further blocks from the
+ same endpoint and plans to implement the request atomically
+ (e.g., acts only upon reception of the last block of
+ payload). In this case, the response MUST NOT carry a
+ Block2 option.
+
+ + Conversely, if the M bit is unset even though it was set in
+ the request, it indicates the block-wise request was enacted
+ now specifically for this block, and the response carries
+ the final response to this request (and to any previous ones
+ with the M bit set in the response's Block1 Option in this
+ sequence of block-wise transfers); the client is still
+
+
+
+
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+
+ expected to continue sending further blocks, the request
+ method for which may or may not also be enacted per-block.
+
+ * Finally, the SZX block size given in a control Block1 Option
+ indicates the largest block size preferred by the server for
+ transfers toward the resource that is the same or smaller than
+ the one used in the initial exchange; the client SHOULD use
+ this block size or a smaller one in all further requests in the
+ transfer sequence, even if that means changing the block size
+ (and possibly scaling the block number accordingly) from now
+ on.
+
+ Using one or both Block options, a single REST operation can be split
+ into multiple CoAP message exchanges. As specified in [RFC7252],
+ each of these message exchanges uses their own CoAP Message ID.
+
+ The Content-Format Option sent with the requests or responses MUST
+ reflect the content-format of the entire body. If blocks of a
+ response body arrive with different content-format options, it is up
+ to the client how to handle this error (it will typically abort any
+ ongoing block-wise transfer). If blocks of a request arrive at a
+ server with mismatching content-format options, the server MUST NOT
+ assemble them into a single request; this usually leads to a 4.08
+ (Request Entity Incomplete, Section 2.9.2) error response on the
+ mismatching block.
+
+2.4. Using the Block2 Option
+
+ When a request is answered with a response carrying a Block2 Option
+ with the M bit set, the requester may retrieve additional blocks of
+ the resource representation by sending further requests with the same
+ options as the initial request and a Block2 Option giving the block
+ number and block size desired. In a request, the client MUST set the
+ M bit of a Block2 Option to zero and the server MUST ignore it on
+ reception.
+
+ To influence the block size used in a response, the requester MAY
+ also use the Block2 Option on the initial request, giving the desired
+ size, a block number of zero and an M bit of zero. A server MUST use
+ the block size indicated or a smaller size. Any further block-wise
+ requests for blocks beyond the first one MUST indicate the same block
+ size that was used by the server in the response for the first
+ request that gave a desired size using a Block2 Option.
+
+ Once the Block2 Option is used by the requester and a first response
+ has been received with a possibly adjusted block size, all further
+ requests in a single block-wise transfer SHOULD ultimately use the
+ same size, except that there may not be enough content to fill the
+
+
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+ last block (the one returned with the M bit not set). (Note that the
+ client may start using the Block2 Option in a second request after a
+ first request without a Block2 Option resulted in a Block2 option in
+ the response.) The server SHOULD use the block size indicated in the
+ request option or a smaller size, but the requester MUST take note of
+ the actual block size used in the response it receives to its initial
+ request and proceed to use it in subsequent requests. The server
+ behavior MUST ensure that this client behavior results in the same
+ block size for all responses in a sequence (except for the last one
+ with the M bit not set, and possibly the first one if the initial
+ request did not contain a Block2 Option).
+
+ Block-wise transfers can be used to GET resources the representations
+ of which are entirely static (not changing over time at all, such as
+ in a schema describing a device), or for dynamically changing
+ resources. In the latter case, the Block2 Option SHOULD be used in
+ conjunction with the ETag Option, to ensure that the blocks being
+ reassembled are from the same version of the representation: The
+ server SHOULD include an ETag option in each response. If an ETag
+ option is available, the client's reassembler, when reassembling the
+ representation from the blocks being exchanged, MUST compare ETag
+ Options. If the ETag Options do not match in a GET transfer, the
+ requester has the option of attempting to retrieve fresh values for
+ the blocks it retrieved first. To minimize the resulting
+ inefficiency, the server MAY cache the current value of a
+ representation for an ongoing sequence of requests. (The server may
+ identify the sequence by the combination of the requesting end-point
+ and the URI being the same in each block-wise request.) Note well
+ that this specification makes no requirement for the server to
+ establish any state; however, servers that offer quickly changing
+ resources may thereby make it impossible for a client to ever
+ retrieve a consistent set of blocks. Clients that want to retrieve
+ all blocks of a resource SHOULD strive to do so without undue delay.
+ Servers can fully expect to be free to discard any cached state after
+ a period of EXCHANGE_LIFETIME ([RFC7252], Section 4.8.2) after the
+ last access to the state, however, there is no requirement to always
+ keep the state for as long.
+
+ The Block2 option provides no way for a single endpoint to perform
+ multiple concurrently proceeding block-wise response payload transfer
+ (e.g., GET) operations to the same resource. This is rarely a
+ requirement, but as a workaround, a client may vary the cache key
+ (e.g., by using one of several URIs accessing resources with the same
+ semantics, or by varying a proxy-safe elective option).
+
+
+
+
+
+
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+2.5. Using the Block1 Option
+
+ In a request with a request payload (e.g., PUT or POST), the Block1
+ Option refers to the payload in the request (descriptive usage).
+
+ In response to a request with a payload (e.g., a PUT or POST
+ transfer), the block size given in the Block1 Option indicates the
+ block size preference of the server for this resource (control
+ usage). Obviously, at this point the first block has already been
+ transferred by the client without benefit of this knowledge. Still,
+ the client SHOULD heed the preference indicated and, for all further
+ blocks, use the block size preferred by the server or a smaller one.
+ Note that any reduction in the block size may mean that the second
+ request starts with a block number larger than one, as the first
+ request already transferred multiple blocks as counted in the smaller
+ size.
+
+ To counter the effects of adaptation layer fragmentation on packet
+ delivery probability, a client may want to give up retransmitting a
+ request with a relatively large payload even before MAX_RETRANSMIT
+ has been reached, and try restating the request as a block-wise
+ transfer with a smaller payload. Note that this new attempt is then
+ a new message-layer transaction and requires a new Message ID.
+ (Because of the uncertainty whether the request or the
+ acknowledgement was lost, this strategy is useful mostly for
+ idempotent requests.)
+
+ In a block-wise transfer of a request payload (e.g., a PUT or POST)
+ that is intended to be implemented in an atomic fashion at the
+ server, the actual creation/replacement takes place at the time the
+ final block, i.e. a block with the M bit unset in the Block1 Option,
+ is received. In this case, all success responses to non-final blocks
+ carry the response code 2.31 (Continue, Section 2.9.1). If not all
+ previous blocks are available at the server at the time of processing
+ the final block, the transfer fails and error code 4.08 (Request
+ Entity Incomplete, Section 2.9.2) MUST be returned. A server MAY
+ also return a 4.08 error code for any (final or non-final) Block1
+ transfer that is not in sequence; clients that do not have specific
+ mechanisms to handle this case therefore SHOULD always start with
+ block zero and send the following blocks in order.
+
+ One reason that a client might encounter a 4.08 error code is that
+ the server has already timed out and discarded the partial request
+ body being assembled. Clients SHOULD strive to send all blocks of a
+ request without undue delay. Servers can fully expect to be free to
+ discard any partial request body when a period of EXCHANGE_LIFETIME
+ ([RFC7252], Section 4.8.2) has elapsed after the most recent block
+
+
+
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+ was transferred; however, there is no requirement on a server to
+ always keep the partial request body for as long.
+
+ The error code 4.13 (Request Entity Too Large) can be returned at any
+ time by a server that does not currently have the resources to store
+ blocks for a block-wise request payload transfer that it would intend
+ to implement in an atomic fashion. (Note that a 4.13 response to a
+ request that does not employ Block1 is a hint for the client to try
+ sending Block1, and a 4.13 response with a smaller SZX in its Block1
+ option than requested is a hint to try a smaller SZX.)
+
+ The Block1 option provides no way for a single endpoint to perform
+ multiple concurrently proceeding block-wise request payload transfer
+ (e.g., PUT or POST) operations to the same resource. Starting a new
+ block-wise sequence of requests to the same resource (before an old
+ sequence from the same endpoint was finished) simply overwrites the
+ context the server may still be keeping. (This is probably exactly
+ what one wants in this case - the client may simply have restarted
+ and lost its knowledge of the previous sequence.)
+
+2.6. Combining Block-wise Transfers with the Observe Option
+
+ The Observe Option provides a way for a client to be notified about
+ changes over time of a resource [I-D.ietf-core-observe]. Resources
+ observed by clients may be larger than can be comfortably processed
+ or transferred in one CoAP message. The following rules apply to the
+ combination of block-wise transfers with notifications.
+
+ Observation relationships always apply to an entire resource; the
+ Block2 option does not provide a way to observe a single block of a
+ resource.
+
+ As with basic GET transfers, the client can indicate its desired
+ block size in a Block2 Option in the GET request establishing or
+ renewing the observation relationship. If the server supports block-
+ wise transfers, it SHOULD take note of the block size and apply it as
+ a maximum size to all notifications/responses resulting from the GET
+ request (until the client is removed from the list of observers or
+ the entry in that list is updated by the server receiving a new GET
+ request for the resource from the client).
+
+ When sending a 2.05 (Content) notification, the server only sends the
+ first block of the representation. The client retrieves the rest of
+ the representation as if it had caused this first response by a GET
+ request, i.e., by using additional GET requests with Block2 options
+ containing NUM values greater than zero. (This results in the
+ transfer of the entire representation, even if only some of the
+ blocks have changed with respect to a previous notification.)
+
+
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+ As with other dynamically changing resources, to ensure that the
+ blocks being reassembled are from the same version of the
+ representation, the server SHOULD include an ETag option in each
+ response, and the reassembling client MUST compare the ETag options
+ (Section 2.4). Even more so than for the general case of Block2,
+ clients that want to retrieve all blocks of a resource they have been
+ notified about with a first block SHOULD strive to do so without
+ undue delay.
+
+ See Section 3.4 for examples.
+
+2.7. Combining Block1 and Block2
+
+ In PUT and particularly in POST exchanges, both the request body and
+ the response body may be large enough to require the use of block-
+ wise transfers. First, the Block1 transfer of the request body
+ proceeds as usual. In the exchange of the last slice of this block-
+ wise transfer, the response carries the first slice of the Block2
+ transfer (NUM is zero). To continue this Block2 transfer, the client
+ continues to send requests similar to the requests in the Block1
+ phase, but leaves out the Block1 options and includes a Block2
+ request option with non-zero NUM.
+
+ Block2 transfers that retrieve the response body for a request that
+ used Block1 MUST be performed in sequential order.
+
+2.8. Combining Block2 with Multicast
+
+ A client can use the Block2 option in a multicast GET request with
+ NUM = 0 to aid in limiting the size of the response.
+
+ Similarly, a response to a multicast GET request can use a Block2
+ option with NUM = 0 if the representation is large, or to further
+ limit the size of the response.
+
+ In both cases, the client retrieves any further blocks using unicast
+ exchanges; in the unicast requests, the client SHOULD heed any block
+ size preferences indicated by the server in the response to the
+ multicast request.
+
+ Other uses of the Block options in conjunction with multicast
+ messages are for further study.
+
+2.9. Response Codes
+
+ Two response codes are defined by this specification beyond those
+ already defined in [RFC7252], and another response code is extended
+ in its meaning.
+
+
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+2.9.1. 2.31 Continue
+
+ This new success status code indicates that the transfer of this
+ block of the request body was successful and that the server
+ encourages sending further blocks, but that a final outcome of the
+ whole block-wise request cannot yet be determined. No payload is
+ returned with this response code.
+
+2.9.2. 4.08 Request Entity Incomplete
+
+ This new client error status code indicates that the server has not
+ received the blocks of the request body that it needs to proceed.
+ The client has not sent all blocks, not sent them in the order
+ required by the server, or has sent them long enough ago that the
+ server has already discarded them.
+
+2.9.3. 4.13 Request Entity Too Large
+
+ In [RFC7252], section 5.9.2.9, the response code 4.13 (Request Entity
+ Too Large) is defined to be like HTTP 413 "Request Entity Too Large".
+ [RFC7252] also recommends that this response SHOULD include a Size1
+ Option (Section 4) to indicate the maximum size of request entity the
+ server is able and willing to handle, unless the server is not in a
+ position to make this information available.
+
+ The present specification allows the server to return this response
+ code at any time during a Block1 transfer to indicate that it does
+ not currently have the resources to store blocks for a transfer that
+ it would intend to implement in an atomic fashion. It also allows
+ the server to return a 4.13 response to a request that does not
+ employ Block1 as a hint for the client to try sending Block1.
+ Finally, a 4.13 response to a request with a Block1 option (control
+ usage, see Section 2.3) where the response carries a smaller SZX in
+ its Block1 option is a hint to try that smaller SZX.
+
+2.10. Caching Considerations
+
+ This specification attempts to leave a variety of implementation
+ strategies open for caches, in particular those in caching proxies.
+ E.g., a cache is free to cache blocks individually, but also could
+ wait to obtain the complete representation before it serves parts of
+ it. Partial caching may be more efficient in a cross-proxy
+ (equivalent to a streaming HTTP proxy). A cached block (partial
+ cached response) can be used in place of a complete response to
+ satisfy a block-wise request that is presented to a cache. Note that
+ different blocks can have different Max-Age values, as they are
+ transferred at different times. A response with a block updates the
+ freshness of the complete representation. Individual blocks can be
+
+
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+ validated, and validating a single block validates the complete
+ representation. A response with a Block1 Option in control usage
+ with the M bit set invalidates cached responses for the target URI.
+
+ A cache or proxy that combines responses (e.g., to split blocks in a
+ request or increase the block size in a response, or a cross-proxy)
+ may need to combine 2.31 and 2.01/2.04 responses; a stateless server
+ may be responding with 2.01 only on the first Block1 block
+ transferred, which dominates any 2.04 responses for later blocks.
+
+ If-None-Match only works correctly on Block1 requests with (NUM=0)
+ and MUST NOT be used on Block1 requests with NUM != 0.
+
+3. Examples
+
+ This section gives a number of short examples with message flows for
+ a block-wise GET, and for a PUT or POST. These examples demonstrate
+ the basic operation, the operation in the presence of
+ retransmissions, and examples for the operation of the block size
+ negotiation.
+
+ In all these examples, a Block option is shown in a decomposed way
+ indicating the kind of Block option (1 or 2) followed by a colon, and
+ then the block number (NUM), more bit (M), and block size exponent
+ (2**(SZX+4)) separated by slashes. E.g., a Block2 Option value of 33
+ would be shown as 2:2/0/32), or a Block1 Option value of 59 would be
+ shown as 1:3/1/128.
+
+3.1. Block2 Examples
+
+ The first example (Figure 2) shows a GET request that is split into
+ three blocks. The server proposes a block size of 128, and the
+ client agrees. The first two ACKs contain 128 bytes of payload each,
+ and third ACK contains between 1 and 128 bytes.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], GET, /status ------> |
+ | |
+ | <------ ACK [MID=1234], 2.05 Content, 2:0/1/128 |
+ | |
+ | CON [MID=1235], GET, /status, 2:1/0/128 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.05 Content, 2:1/1/128 |
+ | |
+ | CON [MID=1236], GET, /status, 2:2/0/128 ------> |
+ | |
+ | <------ ACK [MID=1236], 2.05 Content, 2:2/0/128 |
+
+ Figure 2: Simple block-wise GET
+
+ In the second example (Figure 3), the client anticipates the block-
+ wise transfer (e.g., because of a size indication in the link-format
+ description [RFC6690]) and sends a block size proposal. All ACK
+ messages except for the last carry 64 bytes of payload; the last one
+ carries between 1 and 64 bytes.
+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], GET, /status, 2:0/0/64 ------> |
+ | |
+ | <------ ACK [MID=1234], 2.05 Content, 2:0/1/64 |
+ | |
+ | CON [MID=1235], GET, /status, 2:1/0/64 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.05 Content, 2:1/1/64 |
+ : :
+ : ... :
+ : :
+ | CON [MID=1238], GET, /status, 2:4/0/64 ------> |
+ | |
+ | <------ ACK [MID=1238], 2.05 Content, 2:4/1/64 |
+ | |
+ | CON [MID=1239], GET, /status, 2:5/0/64 ------> |
+ | |
+ | <------ ACK [MID=1239], 2.05 Content, 2:5/0/64 |
+
+ Figure 3: Block-wise GET with early negotiation
+
+ In the third example (Figure 4), the client is surprised by the need
+ for a block-wise transfer, and unhappy with the size chosen
+ unilaterally by the server. As it did not send a size proposal
+ initially, the negotiation only influences the size from the second
+
+
+
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+
+ message exchange onward. Since the client already obtained both the
+ first and second 64-byte block in the first 128-byte exchange, it
+ goes on requesting the third 64-byte block ("2/0/64"). None of this
+ is (or needs to be) understood by the server, which simply responds
+ to the requests as it best can.
+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], GET, /status ------> |
+ | |
+ | <------ ACK [MID=1234], 2.05 Content, 2:0/1/128 |
+ | |
+ | CON [MID=1235], GET, /status, 2:2/0/64 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.05 Content, 2:2/1/64 |
+ | |
+ | CON [MID=1236], GET, /status, 2:3/0/64 ------> |
+ | |
+ | <------ ACK [MID=1236], 2.05 Content, 2:3/1/64 |
+ | |
+ | CON [MID=1237], GET, /status, 2:4/0/64 ------> |
+ | |
+ | <------ ACK [MID=1237], 2.05 Content, 2:4/1/64 |
+ | |
+ | CON [MID=1238], GET, /status, 2:5/0/64 ------> |
+ | |
+ | <------ ACK [MID=1238], 2.05 Content, 2:5/0/64 |
+
+ Figure 4: Block-wise GET with late negotiation
+
+ In all these (and the following) cases, retransmissions are handled
+ by the CoAP message exchange layer, so they don't influence the block
+ operations (Figure 5, Figure 6).
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+
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+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], GET, /status ------> |
+ | |
+ | <------ ACK [MID=1234], 2.05 Content, 2:0/1/128 |
+ | |
+ | CON [MID=1235], GE///////////////////////// |
+ | |
+ | (timeout) |
+ | |
+ | CON [MID=1235], GET, /status, 2:2/0/64 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.05 Content, 2:2/1/64 |
+ : :
+ : ... :
+ : :
+ | CON [MID=1238], GET, /status, 2:5/0/64 ------> |
+ | |
+ | <------ ACK [MID=1238], 2.05 Content, 2:5/0/64 |
+
+ Figure 5: Block-wise GET with late negotiation and lost CON
+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], GET, /status ------> |
+ | |
+ | <------ ACK [MID=1234], 2.05 Content, 2:0/1/128 |
+ | |
+ | CON [MID=1235], GET, /status, 2:2/0/64 ------> |
+ | |
+ | //////////////////////////////////tent, 2:2/1/64 |
+ | |
+ | (timeout) |
+ | |
+ | CON [MID=1235], GET, /status, 2:2/0/64 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.05 Content, 2:2/1/64 |
+ : :
+ : ... :
+ : :
+ | CON [MID=1238], GET, /status, 2:5/0/64 ------> |
+ | |
+ | <------ ACK [MID=1238], 2.05 Content, 2:5/0/64 |
+
+ Figure 6: Block-wise GET with late negotiation and lost ACK
+
+
+
+
+
+
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+
+3.2. Block1 Examples
+
+ The following examples demonstrate a PUT exchange; a POST exchange
+ looks the same, with different requirements on atomicity/idempotence.
+ Note that, similar to GET, the responses to the requests that have a
+ more bit in the request Block1 Option are provisional and carry the
+ response code 2.31 (Continue); only the final response tells the
+ client that the PUT did succeed.
+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], PUT, /options, 1:0/1/128 ------> |
+ | |
+ | <------ ACK [MID=1234], 2.31 Continue, 1:0/1/128 |
+ | |
+ | CON [MID=1235], PUT, /options, 1:1/1/128 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.31 Continue, 1:1/1/128 |
+ | |
+ | CON [MID=1236], PUT, /options, 1:2/0/128 ------> |
+ | |
+ | <------ ACK [MID=1236], 2.04 Changed, 1:2/0/128 |
+
+ Figure 7: Simple atomic block-wise PUT
+
+ A stateless server that simply builds/updates the resource in place
+ (statelessly) may indicate this by not setting the more bit in the
+ response (Figure 8); in this case, the response codes are valid
+ separately for each block being updated. This is of course only an
+ acceptable behavior of the server if the potential inconsistency
+ present during the run of the message exchange sequence does not lead
+ to problems, e.g. because the resource being created or changed is
+ not yet or not currently in use.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], PUT, /options, 1:0/1/128 ------> |
+ | |
+ | <------ ACK [MID=1234], 2.04 Changed, 1:0/0/128 |
+ | |
+ | CON [MID=1235], PUT, /options, 1:1/1/128 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.04 Changed, 1:1/0/128 |
+ | |
+ | CON [MID=1236], PUT, /options, 1:2/0/128 ------> |
+ | |
+ | <------ ACK [MID=1236], 2.04 Changed, 1:2/0/128 |
+
+ Figure 8: Simple stateless block-wise PUT
+
+ Finally, a server receiving a block-wise PUT or POST may want to
+ indicate a smaller block size preference (Figure 9). In this case,
+ the client SHOULD continue with a smaller block size; if it does, it
+ MUST adjust the block number to properly count in that smaller size.
+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], PUT, /options, 1:0/1/128 ------> |
+ | |
+ | <------ ACK [MID=1234], 2.31 Continue, 1:0/1/32 |
+ | |
+ | CON [MID=1235], PUT, /options, 1:4/1/32 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.31 Continue, 1:4/1/32 |
+ | |
+ | CON [MID=1236], PUT, /options, 1:5/1/32 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.31 Continue, 1:5/1/32 |
+ | |
+ | CON [MID=1237], PUT, /options, 1:6/0/32 ------> |
+ | |
+ | <------ ACK [MID=1236], 2.04 Changed, 1:6/0/32 |
+
+ Figure 9: Simple atomic block-wise PUT with negotiation
+
+3.3. Combining Block1 and Block2
+
+ Block options may be used in both directions of a single exchange.
+ The following example demonstrates a block-wise POST request,
+ resulting in a separate block-wise response.
+
+
+
+
+
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+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], POST, /soap, 1:0/1/128 ------> |
+ | |
+ | <------ ACK [MID=1234], 2.31 Continue, 1:0/1/128 |
+ | |
+ | CON [MID=1235], POST, /soap, 1:1/1/128 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.31 Continue, 1:1/1/128 |
+ | |
+ | CON [MID=1236], POST, /soap, 1:2/0/128 ------> |
+ | |
+ | <------ ACK [MID=1236], 2.04 Changed, 2:0/1/128, 1:2/0/128 |
+ | |
+ | CON [MID=1237], POST, /soap, 2:1/0/128 ------> |
+ | (no payload for requests with Block2 with NUM != 0) |
+ | (could also do late negotiation by requesting e.g. 2:2/0/64) |
+ | |
+ | <------ ACK [MID=1237], 2.04 Changed, 2:1/1/128 |
+ | |
+ | CON [MID=1238], POST, /soap, 2:2/0/128 ------> |
+ | |
+ | <------ ACK [MID=1238], 2.04 Changed, 2:2/1/128 |
+ | |
+ | CON [MID=1239], POST, /soap, 2:3/0/128 ------> |
+ | |
+ | <------ ACK [MID=1239], 2.04 Changed, 2:3/0/128 |
+
+ Figure 10: Atomic block-wise POST with block-wise response
+
+ This model does provide for early negotiation input to the Block2
+ block-wise transfer, as shown below.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+ CLIENT SERVER
+ | |
+ | CON [MID=1234], POST, /soap, 1:0/1/128 ------> |
+ | |
+ | <------ ACK [MID=1234], 2.31 Continue, 1:0/1/128 |
+ | |
+ | CON [MID=1235], POST, /soap, 1:1/1/128 ------> |
+ | |
+ | <------ ACK [MID=1235], 2.31 Continue, 1:1/1/128 |
+ | |
+ | CON [MID=1236], POST, /soap, 1:2/0/128, 2:0/0/64 ------> |
+ | |
+ | <------ ACK [MID=1236], 2.04 Changed, 1:2/0/128, 2:0/1/64 |
+ | |
+ | CON [MID=1237], POST, /soap, 2:1/0/64 ------> |
+ | (no payload for requests with Block2 with NUM != 0) |
+ | |
+ | <------ ACK [MID=1237], 2.04 Changed, 2:1/1/64 |
+ | |
+ | CON [MID=1238], POST, /soap, 2:2/0/64 ------> |
+ | |
+ | <------ ACK [MID=1238], 2.04 Changed, 2:2/1/64 |
+ | |
+ | CON [MID=1239], POST, /soap, 2:3/0/64 ------> |
+ | |
+ | <------ ACK [MID=1239], 2.04 Changed, 2:3/0/64 |
+
+ Figure 11: Atomic block-wise POST with block-wise response, early
+ negotiation
+
+3.4. Combining Observe and Block2
+
+ In the following example, the server first sends a direct response
+ (Observe sequence number 62350) to the initial GET request (the
+ resulting block-wise transfer is as in Figure 4 and has therefore
+ been left out). The second transfer is started by a 2.05
+ notification that contains just the first block (Observe sequence
+ number 62354); the client then goes on to obtain the rest of the
+ blocks.
+
+ CLIENT SERVER
+ | |
+ +----->| Header: GET 0x41011636
+ | GET | Token: 0xfb
+ | | Uri-Path: status-icon
+ | | Observe: (empty)
+ | |
+ |<-----+ Header: 2.05 0x61451636
+
+
+
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+
+ | 2.05 | Token: 0xfb
+ | | Block2: 0/1/128
+ | | Observe: 62350
+ | | ETag: 6f00f38e
+ | | Payload: [128 bytes]
+ | |
+ | | (Usual GET transfer left out)
+ ...
+ | | (Notification of first block:)
+ | |
+ |<-----+ Header: 2.05 0x4145af9c
+ | 2.05 | Token: 0xfb
+ | | Block2: 0/1/128
+ | | Observe: 62354
+ | | ETag: 6f00f392
+ | | Payload: [128 bytes]
+ | |
+ +- - ->| Header: 0x6000af9c
+ | |
+ | | (Retrieval of remaining blocks)
+ | |
+ +----->| Header: GET 0x41011637
+ | GET | Token: 0xfc
+ | | Uri-Path: status-icon
+ | | Block2: 1/0/128
+ | |
+ |<-----+ Header: 2.05 0x61451637
+ | 2.05 | Token: 0xfc
+ | | Block2: 1/1/128
+ | | ETag: 6f00f392
+ | | Payload: [128 bytes]
+ | |
+ +----->| Header: GET 0x41011638
+ | GET | Token: 0xfc
+ | | Uri-Path: status-icon
+ | | Block2: 2/0/128
+ | |
+ |<-----+ Header: 2.05 0x61451638
+ | 2.05 | Token: 0xfc
+ | | Block2: 2/0/128
+ | | ETag: 6f00f392
+ | | Payload: [53 bytes]
+
+
+ Figure 12: Observe sequence with block-wise response
+
+ (Note that the choice of token 0xfc in this examples is arbitrary;
+ tokens are just shown in this example to illustrate that the requests
+
+
+
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+
+ for additional blocks cannot make use of the token of the Observation
+ relationship. As a general comment on tokens, there is no other
+ mention of tokens in this document, as block-wise transfers handle
+ tokens like any other CoAP exchange. As usual the client is free to
+ choose tokens for each exchange as it likes.)
+
+ In the following example, the client also uses early negotiation to
+ limit the block size to 64 bytes.
+
+ CLIENT SERVER
+ | |
+ +----->| Header: GET 0x41011636
+ | GET | Token: 0xfb
+ | | Uri-Path: status-icon
+ | | Observe: (empty)
+ | | Block2: 0/0/64
+ | |
+ |<-----+ Header: 2.05 0x61451636
+ | 2.05 | Token: 0xfb
+ | | Block2: 0/1/64
+ | | Observe: 62350
+ | | ETag: 6f00f38e
+ | | Max-Age: 60
+ | | Payload: [64 bytes]
+ | |
+ | | (Usual GET transfer left out)
+ ...
+ | | (Notification of first block:)
+ | |
+ |<-----+ Header: 2.05 0x4145af9c
+ | 2.05 | Token: 0xfb
+ | | Block2: 0/1/64
+ | | Observe: 62354
+ | | ETag: 6f00f392
+ | | Payload: [64 bytes]
+ | |
+ +- - ->| Header: 0x6000af9c
+ | |
+ | | (Retrieval of remaining blocks)
+ | |
+ +----->| Header: GET 0x41011637
+ | GET | Token: 0xfc
+ | | Uri-Path: status-icon
+ | | Block2: 1/0/64
+ | |
+ |<-----+ Header: 2.05 0x61451637
+ | 2.05 | Token: 0xfc
+ | | Block2: 1/1/64
+
+
+
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+
+ | | ETag: 6f00f392
+ | | Payload: [64 bytes]
+ ....
+ | |
+ +----->| Header: GET 0x41011638
+ | GET | Token: 0xfc
+ | | Uri-Path: status-icon
+ | | Block2: 4/0/64
+ | |
+ |<-----+ Header: 2.05 0x61451638
+ | 2.05 | Token: 0xfc
+ | | Block2: 4/0/64
+ | | ETag: 6f00f392
+ | | Payload: [53 bytes]
+
+ Figure 13: Observe sequence with early negotiation
+
+4. The Size2 and Size1 Options
+
+ In many cases when transferring a large resource representation block
+ by block, it is advantageous to know the total size early in the
+ process. Some indication may be available from the maximum size
+ estimate attribute "sz" provided in a resource description [RFC6690].
+ However, the size may vary dynamically, so a more up-to-date
+ indication may be useful.
+
+ This specification defines two CoAP Options, Size1 for indicating the
+ size of the representation transferred in requests, and Size2 for
+ indicating the size of the representation transferred in responses.
+ (Size1 is already defined in [RFC7252] for the narrow case of
+ indicating in 4.13 responses the maximum size of request payload that
+ the server is able and willing to handle.)
+
+ The Size2 Option may be used for two purposes:
+
+ o in a request, to ask the server to provide a size estimate along
+ with the usual response ("size request"). For this usage, the
+ value MUST be set to 0.
+
+ o in a response carrying a Block2 Option, to indicate the current
+ estimate the server has of the total size of the resource
+ representation, measured in bytes ("size indication").
+
+ Similarly, the Size1 Option may be used for two purposes:
+
+ o in a request carrying a Block1 Option, to indicate the current
+ estimate the client has of the total size of the resource
+ representation, measured in bytes ("size indication").
+
+
+
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+
+ o in a 4.13 response, to indicate the maximum size that would have
+ been acceptable [RFC7252], measured in bytes.
+
+ Apart from conveying/asking for size information, the Size options
+ have no other effect on the processing of the request or response.
+ If the client wants to minimize the size of the payload in the
+ resulting response, it should add a Block2 option to the request with
+ a small block size (e.g., setting SZX=0).
+
+ The Size Options are "elective", i.e., a client MUST be prepared for
+ the server to ignore the size estimate request. The Size Options
+ MUST NOT occur more than once.
+
+ +-----+---+---+---+---+-------+--------+--------+---------+
+ | No. | C | U | N | R | Name | Format | Length | Default |
+ +-----+---+---+---+---+-------+--------+--------+---------+
+ | 60 | | | x | | Size1 | uint | 0-4 | (none) |
+ | | | | | | | | | |
+ | 28 | | | x | | Size2 | uint | 0-4 | (none) |
+ +-----+---+---+---+---+-------+--------+--------+---------+
+
+ Table 2: Size Option Numbers
+
+ Implementation Notes:
+
+ o As a quality of implementation consideration, block-wise transfers
+ for which the total size considerably exceeds the size of one
+ block are expected to include size indications, whenever those can
+ be provided without undue effort (preferably with the first block
+ exchanged). If the size estimate does not change, the indication
+ does not need to be repeated for every block.
+
+ o The end of a block-wise transfer is governed by the M bits in the
+ Block Options, _not_ by exhausting the size estimates exchanged.
+
+ o As usual for an option of type uint, the value 0 is best expressed
+ as an empty option (0 bytes). There is no default value for
+ either Size Option.
+
+ o The Size Options are neither critical nor unsafe, and are marked
+ as No-Cache-Key.
+
+5. HTTP Mapping Considerations
+
+ In this subsection, we give some brief examples for the influence the
+ Block options might have on intermediaries that map between CoAP and
+ HTTP.
+
+
+
+
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+
+ For mapping CoAP requests to HTTP, the intermediary may want to map
+ the sequence of block-wise transfers into a single HTTP transfer.
+ E.g., for a GET request, the intermediary could perform the HTTP
+ request once the first block has been requested and could then
+ fulfill all further block requests out of its cache. A constrained
+ implementation may not be able to cache the entire object and may use
+ a combination of TCP flow control and (in particular if timeouts
+ occur) HTTP range requests to obtain the information necessary for
+ the next block transfer at the right time.
+
+ For PUT or POST requests, historically there was more variation in
+ how HTTP servers might implement ranges; recently, [RFC7233] has
+ defined that Range header fields received with a request method other
+ than GET are not to be interpreted. So, in general, the CoAP-to-HTTP
+ intermediary will have to try sending the payload of all the blocks
+ of a block-wise transfer for these other methods within one HTTP
+ request. If enough buffering is available, this request can be
+ started when the last CoAP block is received. A constrained
+ implementation may want to relieve its buffering by already starting
+ to send the HTTP request at the time the first CoAP block is
+ received; any HTTP 408 status code that indicates that the HTTP
+ server became impatient with the resulting transfer can then be
+ mapped into a CoAP 4.08 response code (similarly, 413 maps to 4.13).
+
+ For mapping HTTP to CoAP, the intermediary may want to map a single
+ HTTP transfer into a sequence of block-wise transfers. If the HTTP
+ client is too slow delivering a request body on a PUT or POST, the
+ CoAP server might time out and return a 4.08 response code, which in
+ turn maps well to an HTTP 408 status code (again, 4.13 maps to 413).
+ HTTP range requests received on the HTTP side may be served out of a
+ cache and/or mapped to GET requests that request a sequence of blocks
+ overlapping the range.
+
+ (Note that, while the semantics of CoAP 4.08 and HTTP 408 differ,
+ this difference is largely due to the different way the two protocols
+ are mapped to transport. HTTP has an underlying TCP connection,
+ which supplies connection state, so a HTTP 408 status code can
+ immediately be used to indicate that a timeout occurred during
+ transmitting a request through that active TCP connection. The CoAP
+ 4.08 response code indicates one or more missing blocks, which may be
+ due to timeouts or resource constraints; as there is no connection
+ state, there is no way to deliver such a response immediately;
+ instead, it is delivered on the next block transfer. Still, HTTP 408
+ is probably the best mapping back to HTTP, as the timeout is the most
+ likely cause for a CoAP 4.08. Note that there is no way to
+ distinguish a timeout from a missing block for a server without
+ creating additional state, the need for which we want to avoid.)
+
+
+
+
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+
+6. IANA Considerations
+
+ This draft adds the following option numbers to the CoAP Option
+ Numbers registry of [RFC7252]:
+
+ +--------+--------+-----------+
+ | Number | Name | Reference |
+ +--------+--------+-----------+
+ | 23 | Block2 | [RFCXXXX] |
+ | | | |
+ | 27 | Block1 | [RFCXXXX] |
+ | | | |
+ | 28 | Size2 | [RFCXXXX] |
+ +--------+--------+-----------+
+
+ Table 3: CoAP Option Numbers
+
+ This draft adds the following response code to the CoAP Response
+ Codes registry of [RFC7252]:
+
+ +------+---------------------------+-----------+
+ | Code | Description | Reference |
+ +------+---------------------------+-----------+
+ | 2.31 | Continue | [RFCXXXX] |
+ | | | |
+ | 4.08 | Request Entity Incomplete | [RFCXXXX] |
+ +------+---------------------------+-----------+
+
+ Table 4: CoAP Response Codes
+
+7. Security Considerations
+
+ Providing access to blocks within a resource may lead to surprising
+ vulnerabilities. Where requests are not implemented atomically, an
+ attacker may be able to exploit a race condition or confuse a server
+ by inducing it to use a partially updated resource representation.
+ Partial transfers may also make certain problematic data invisible to
+ intrusion detection systems; it is RECOMMENDED that an intrusion
+ detection system (IDS) that analyzes resource representations
+ transferred by CoAP implement the Block options to gain access to
+ entire resource representations. Still, approaches such as
+ transferring even-numbered blocks on one path and odd-numbered blocks
+ on another path, or even transferring blocks multiple times with
+ different content and obtaining a different interpretation of
+ temporal order at the IDS than at the server, may prevent an IDS from
+ seeing the whole picture. These kinds of attacks are well understood
+ from IP fragmentation and TCP segmentation; CoAP does not add
+ fundamentally new considerations.
+
+
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+ Where access to a resource is only granted to clients making use of
+ specific security associations, all blocks of that resource MUST be
+ subject to the same security checks; it MUST NOT be possible for
+ unprotected exchanges to influence blocks of an otherwise protected
+ resource. As a related consideration, where object security is
+ employed, PUT/POST should be implemented in the atomic fashion,
+ unless the object security operation is performed on each access and
+ the creation of unusable resources can be tolerated.
+
+ A stateless server might be susceptible to an attack where the
+ adversary sends a Block1 (e.g., PUT) block with a high block number:
+ A naive implementation might exhaust its resources by creating a huge
+ resource representation.
+
+ Misleading size indications may be used by an attacker to induce
+ buffer overflows in poor implementations, for which the usual
+ considerations apply.
+
+7.1. Mitigating Resource Exhaustion Attacks
+
+ Certain block-wise requests may induce the server to create state,
+ e.g. to create a snapshot for the block-wise GET of a fast-changing
+ resource to enable consistent access to the same version of a
+ resource for all blocks, or to create temporary resource
+ representations that are collected until pressed into service by a
+ final PUT or POST with the more bit unset. All mechanisms that
+ induce a server to create state that cannot simply be cleaned up
+ create opportunities for denial-of-service attacks. Servers SHOULD
+ avoid being subject to resource exhaustion based on state created by
+ untrusted sources. But even if this is done, the mitigation may
+ cause a denial-of-service to a legitimate request when it is drowned
+ out by other state-creating requests. Wherever possible, servers
+ should therefore minimize the opportunities to create state for
+ untrusted sources, e.g. by using stateless approaches.
+
+ Performing segmentation at the application layer is almost always
+ better in this respect than at the transport layer or lower (IP
+ fragmentation, adaptation layer fragmentation), for instance because
+ there is application layer semantics that can be used for mitigation
+ or because lower layers provide security associations that can
+ prevent attacks. However, it is less common to apply timeouts and
+ keepalive mechanisms at the application layer than at lower layers.
+ Servers MAY want to clean up accumulated state by timing it out (cf.
+ response code 4.08), and clients SHOULD be prepared to run block-wise
+ transfers in an expedient way to minimize the likelihood of running
+ into such a timeout.
+
+
+
+
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+7.2. Mitigating Amplification Attacks
+
+ [RFC7252] discusses the susceptibility of CoAP end-points for use in
+ amplification attacks.
+
+ A CoAP server can reduce the amount of amplification it provides to
+ an attacker by offering large resource representations only in
+ relatively small blocks. With this, e.g., for a 1000 byte resource,
+ a 10-byte request might result in an 80-byte response (with a 64-byte
+ block) instead of a 1016-byte response, considerably reducing the
+ amplification provided.
+
+8. Acknowledgements
+
+ Much of the content of this draft is the result of discussions with
+ the [RFC7252] authors, and via many CoRE WG discussions.
+
+ Charles Palmer provided extensive editorial comments to a previous
+ version of this draft, some of which the authors hope to have covered
+ in this version. Esko Dijk reviewed a more recent version, leading
+ to a number of further editorial improvements, a solution to the 4.13
+ ambiguity problem, and the section about combining Block and
+ multicast. Markus Becker proposed getting rid of an ill-conceived
+ default value for the Block2 and Block1 options. Peter Bigot
+ insisted on a more systematic coverage of the options and response
+ code.
+
+ Kepeng Li, Linyi Tian, and Barry Leiba wrote up an early version of
+ the Size Option, which has informed this draft. Klaus Hartke wrote
+ some of the text describing the interaction of Block2 with Observe.
+ Matthias Kovatsch provided a number of significant simplifications of
+ the protocol.
+
+9. References
+
+9.1. Normative References
+
+ [I-D.ietf-core-observe]
+ Hartke, K., "Observing Resources in CoAP", draft-ietf-
+ core-observe-16 (work in progress), December 2014.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
+ Application Protocol (CoAP)", RFC 7252, June 2014.
+
+
+
+
+
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+
+9.2. Informative References
+
+ [REST] Fielding, R., "Architectural Styles and the Design of
+ Network-based Software Architectures", Ph.D. Dissertation,
+ University of California, Irvine, 2000,
+ <http://www.ics.uci.edu/~fielding/pubs/dissertation/
+ fielding_dissertation.pdf>.
+
+ [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
+ over Low-Power Wireless Personal Area Networks (6LoWPANs):
+ Overview, Assumptions, Problem Statement, and Goals", RFC
+ 4919, August 2007.
+
+ [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
+ "Transmission of IPv6 Packets over IEEE 802.15.4
+ Networks", RFC 4944, September 2007.
+
+ [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
+ Format", RFC 6690, August 2012.
+
+ [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
+ Constrained-Node Networks", RFC 7228, May 2014.
+
+ [RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
+ (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
+ 2014.
+
+ [RFC7233] Fielding, R., Lafon, Y., and J. Reschke, "Hypertext
+ Transfer Protocol (HTTP/1.1): Range Requests", RFC 7233,
+ June 2014.
+
+Authors' Addresses
+
+ Carsten Bormann
+ Universitaet Bremen TZI
+ Postfach 330440
+ Bremen D-28359
+ Germany
+
+ Phone: +49-421-218-63921
+ Email: cabo@tzi.org
+
+
+
+
+
+
+
+
+
+
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+
+ Zach Shelby (editor)
+ ARM
+ 150 Rose Orchard
+ San Jose, CA 95134
+ USA
+
+ Phone: +1-408-203-9434
+ Email: zach.shelby@arm.com
+
+
+
+
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+
+
+
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+
+
+
+
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+
+
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+
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+
+
+
+CoRE Working Group K. Hartke
+Internet-Draft Universitaet Bremen TZI
+Intended status: Standards Track December 30, 2014
+Expires: July 3, 2015
+
+
+ Observing Resources in CoAP
+ draft-ietf-core-observe-16
+
+Abstract
+
+ The Constrained Application Protocol (CoAP) is a RESTful application
+ protocol for constrained nodes and networks. The state of a resource
+ on a CoAP server can change over time. This document specifies a
+ simple protocol extension for CoAP that enables CoAP clients to
+ "observe" resources, i.e., to retrieve a representation of a resource
+ and keep this representation updated by the server over a period of
+ time. The protocol follows a best-effort approach for sending new
+ representations to clients and provides eventual consistency between
+ the state observed by each client and the actual resource state at
+ the server.
+
+Status of This Memo
+
+ This Internet-Draft is submitted in full conformance with the
+ provisions of BCP 78 and BCP 79.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF). Note that other groups may also distribute
+ working documents as Internet-Drafts. The list of current Internet-
+ Drafts is at http://datatracker.ietf.org/drafts/current/.
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than as "work in progress."
+
+ This Internet-Draft will expire on July 3, 2015.
+
+Copyright Notice
+
+ Copyright (c) 2014 IETF Trust and the persons identified as the
+ document authors. All rights reserved.
+
+ This document is subject to BCP 78 and the IETF Trust's Legal
+ Provisions Relating to IETF Documents
+ (http://trustee.ietf.org/license-info) in effect on the date of
+ publication of this document. Please review these documents
+
+
+
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+
+ carefully, as they describe your rights and restrictions with respect
+ to this document. Code Components extracted from this document must
+ include Simplified BSD License text as described in Section 4.e of
+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 1.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 3
+ 1.3. Consistency Model . . . . . . . . . . . . . . . . . . . . 5
+ 1.4. Observable Resources . . . . . . . . . . . . . . . . . . . 6
+ 1.5. Requirements Notation . . . . . . . . . . . . . . . . . . 7
+ 2. The Observe Option . . . . . . . . . . . . . . . . . . . . . . 7
+ 3. Client-side Requirements . . . . . . . . . . . . . . . . . . . 8
+ 3.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 8
+ 3.2. Notifications . . . . . . . . . . . . . . . . . . . . . . 8
+ 3.3. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 9
+ 3.4. Reordering . . . . . . . . . . . . . . . . . . . . . . . . 10
+ 3.5. Transmission . . . . . . . . . . . . . . . . . . . . . . . 11
+ 3.6. Cancellation . . . . . . . . . . . . . . . . . . . . . . . 11
+ 4. Server-side Requirements . . . . . . . . . . . . . . . . . . . 12
+ 4.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 12
+ 4.2. Notifications . . . . . . . . . . . . . . . . . . . . . . 13
+ 4.3. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 13
+ 4.4. Reordering . . . . . . . . . . . . . . . . . . . . . . . . 14
+ 4.5. Transmission . . . . . . . . . . . . . . . . . . . . . . . 15
+ 5. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 18
+ 6. Web Linking . . . . . . . . . . . . . . . . . . . . . . . . . 19
+ 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
+ 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
+ 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
+ 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
+ 10.1. Normative References . . . . . . . . . . . . . . . . . . . 21
+ 10.2. Informative References . . . . . . . . . . . . . . . . . . 21
+ Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 22
+ A.1. Client/Server Examples . . . . . . . . . . . . . . . . . . 22
+ A.2. Proxy Examples . . . . . . . . . . . . . . . . . . . . . . 26
+ Appendix B. Changelog . . . . . . . . . . . . . . . . . . . . . . 28
+
+
+
+
+
+
+
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+1. Introduction
+
+1.1. Background
+
+ The Constrained Application Protocol (CoAP) [RFC7252] is intended to
+ provide RESTful services [REST] not unlike HTTP [RFC7230] while
+ reducing the complexity of implementation as well as the size of
+ packets exchanged in order to make these services useful in a highly
+ constrained network of themselves highly constrained nodes [RFC7228].
+
+ The model of REST is that of a client exchanging representations of
+ resources with a server, where a representation captures the current
+ or intended state of a resource. The server is the authority for
+ representations of the resources in its namespace. A client
+ interested in the state of a resource initiates a request to the
+ server, the server then returns a response with a representation of
+ the resource that is current at the time of the request.
+
+ This model does not work well when a client is interested in having a
+ current representation of a resource over a period of time. Existing
+ approaches from HTTP, such as repeated polling or HTTP long polling
+ [RFC6202], generate significant complexity and/or overhead and thus
+ are less applicable in a constrained environment.
+
+ The protocol specified in this document extends the CoAP core
+ protocol with a mechanism for a CoAP client to "observe" a resource
+ on a CoAP server: the client retrieves a representation of the
+ resource and requests this representation be updated by the server
+ as long as the client is interested in the resource.
+
+ The protocol keeps the architectural properties of REST. It enables
+ high scalability and efficiency through the support of caches and
+ proxies. There is no intention, though, to solve the full set of
+ problems that the existing HTTP solutions solve, or to replace
+ publish/subscribe networks that solve a much more general problem
+ [RFC5989].
+
+1.2. Protocol Overview
+
+ The protocol is based on the well-known observer design pattern
+ [GOF]. In this design pattern, components called "observers"
+ register at a specific, known provider called the "subject" that they
+ are interested in being notified whenever the subject undergoes a
+ change in state. The subject is responsible for administering its
+ list of registered observers. If multiple subjects are of interest
+ to an observer, the observer must register separately for all of
+ them.
+
+
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+ Observer Subject
+ | |
+ | Registration |
+ +------------------->|
+ | |
+ | Notification |
+ |<-------------------+
+ | |
+ | Notification |
+ |<-------------------+
+ | |
+ | Notification |
+ |<-------------------+
+ | |
+
+ Figure 1: The Observer Design Pattern
+
+ The observer design pattern is realized in CoAP as follows:
+
+ Subject: In the context of CoAP, the subject is a resource in the
+ namespace of a CoAP server. The state of the resource can change
+ over time, ranging from infrequent updates to continuous state
+ transformations.
+
+ Observer: An observer is a CoAP client that is interested in having
+ a current representation of the resource at any given time.
+
+ Registration: A client registers its interest in a resource by
+ initiating an extended GET request to the server. In addition to
+ returning a representation of the target resource, this request
+ causes the server to add the client to the list of observers of
+ the resource.
+
+ Notification: Whenever the state of a resource changes, the server
+ notifies each client in the list of observers of the resource.
+ Each notification is an additional CoAP response sent by the
+ server in reply to the single extended GET request, and includes a
+ complete, updated representation of the new resource state.
+
+ Figure 2 below shows an example of a CoAP client registering its
+ interest in a resource and receiving three notifications: the first
+ with the current state upon registration, and then two upon changes
+ to the resource state. Both the registration request and the
+ notifications are identified as such by the presence of the Observe
+ Option defined in this document. In notifications, the Observe
+ Option additionally provides a sequence number for reordering
+ detection. All notifications carry the token specified by the
+ client, so the client can easily correlate them to the request.
+
+
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+ Client Server
+ | |
+ | GET /temperature |
+ | Token: 0x4a | Registration
+ | Observe: 0 |
+ +------------------->|
+ | |
+ | 2.05 Content |
+ | Token: 0x4a | Notification of
+ | Observe: 12 | the current state
+ | Payload: 22.9 Cel |
+ |<-------------------+
+ | |
+ | 2.05 Content |
+ | Token: 0x4a | Notification upon
+ | Observe: 44 | a state change
+ | Payload: 22.8 Cel |
+ |<-------------------+
+ | |
+ | 2.05 Content |
+ | Token: 0x4a | Notification upon
+ | Observe: 60 | a state change
+ | Payload: 23.1 Cel |
+ |<-------------------+
+ | |
+
+ Figure 2: Observing a Resource in CoAP
+
+ A client remains on the list of observers as long as the server can
+ determine the client's continued interest in the resource. The
+ server may send a notification in a confirmable CoAP message to
+ request an acknowledgement from the client. When the client
+ deregisters, rejects a notification, or the transmission of a
+ notification times out after several transmission attempts, the
+ client is considered no longer interested in the resource and is
+ removed by the server from the list of observers.
+
+1.3. Consistency Model
+
+ While a client is in the list of observers of a resource, the goal of
+ the protocol is to keep the resource state observed by the client as
+ closely in sync with the actual state at the server as possible.
+
+ It cannot be avoided that the client and the server become out of
+ sync at times: First, there is always some latency between the change
+ of the resource state and the receipt of the notification. Second,
+ CoAP messages with notifications can get lost, which will cause the
+ client to assume an old state until it receives a new notification.
+
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+ And third, the server may erroneously come to the conclusion that the
+ client is no longer interested in the resource, which will cause the
+ server to stop sending notifications and the client to assume an old
+ state until it eventually registers its interest again.
+
+ The protocol addresses this issue as follows:
+
+ o It follows a best-effort approach for sending the current
+ representation to the client after a state change: Clients should
+ see the new state after a state change as soon as possible, and
+ they should see as many states as possible. This is limited by
+ congestion control, however, so a client cannot rely on observing
+ every single state that a resource might go through.
+
+ o It labels notifications with a maximum duration up to which it is
+ acceptable for the observed state and the actual state to be out
+ of sync. When the age of the notification received reaches this
+ limit, the client cannot use the enclosed representation until it
+ receives a new notification.
+
+ o It is designed on the principle of eventual consistency: The
+ protocol guarantees that, if the resource does not undergo a new
+ change in state, eventually all registered observers will have a
+ current representation of the latest resource state.
+
+1.4. Observable Resources
+
+ A CoAP server is the authority for determining under what conditions
+ resources change their state and thus when observers are notified of
+ new resource states. The protocol does not offer explicit means for
+ setting up triggers or thresholds; it is up to the server to expose
+ observable resources that change their state in a way that is useful
+ in the application context.
+
+ For example, a CoAP server with an attached temperature sensor could
+ expose one or more of the following resources:
+
+ o <coap://server/temperature>, which changes its state every few
+ seconds to a current reading of the temperature sensor;
+
+ o <coap://server/temperature/felt>, which changes its state to
+ "COLD" whenever the temperature reading drops below a certain pre-
+ configured threshold, and to "WARM" whenever the reading exceeds a
+ second, slightly higher threshold;
+
+ o <coap://server/temperature/critical?above=42>, which changes its
+ state based on the client-specified parameter value: every few
+ seconds to the current temperature reading if the temperature
+
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+ exceeds the threshold, or to "OK" when the reading drops below;
+
+ o <coap://server/?query=select+avg(temperature)+from+Sensor.window:
+ time(30sec)>, which accepts expressions of arbitrary complexity
+ and changes its state accordingly.
+
+ Thus, by designing CoAP resources that change their state on certain
+ conditions, it is possible to update the client only when these
+ conditions occur instead of supplying it continuously with raw sensor
+ data. By parameterizing resources, this is not limited to conditions
+ defined by the server, but can be extended to arbitrarily complex
+ queries specified by the client. The application designer therefore
+ can choose exactly the right level of complexity for the application
+ envisioned and devices involved, and is not constrained to a "one
+ size fits all" mechanism built into the protocol.
+
+1.5. Requirements Notation
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+ document are to be interpreted as described in RFC 2119 [RFC2119].
+
+2. The Observe Option
+
+ The Observe Option has the following properties. Its meaning depends
+ on whether it is included in a GET request or in a response.
+
+ +-----+---+---+---+---+---------+--------+--------+---------+
+ | No. | C | U | N | R | Name | Format | Length | Default |
+ +-----+---+---+---+---+---------+--------+--------+---------+
+ | 6 | | x | - | | Observe | uint | 0-3 B | (none) |
+ +-----+---+---+---+---+---------+--------+--------+---------+
+
+ C=Critical, U=Unsafe, N=No-Cache-Key, R=Repeatable
+
+ Table 1: The Observe Option
+
+ When included in a GET request, the Observe Option extends the GET
+ method so it does not only retrieve a current representation of the
+ target resource, but also requests the server to add or remove an
+ entry in the list of observers of the resource, depending on the
+ option value. The list entry consists of the client endpoint and the
+ token specified by the client in the request. Possible values are:
+
+ 0 (register) adds the entry to the list, if not present;
+
+ 1 (deregister) removes the entry from the list, if present.
+
+
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+ The Observe Option is not critical for processing the request. If
+ the server is unwilling or unable to add a new entry to the list of
+ observers, then the request falls back to a normal GET request, and
+ the response does not include the Observe Option.
+
+ The Observe Option is not part of the cache-key: a cacheable response
+ obtained with an Observe Option in the request can be used to satisfy
+ a request without an Observe Option, and vice versa. When a stored
+ response with an Observe Option is used to satisfy a normal GET
+ request, the option MUST be removed before the response is returned.
+
+ When included in a response, the Observe Option identifies the
+ message as a notification. This implies that a matching entry exists
+ in the list of observers and that the server will notify the client
+ of changes to the resource state. The option value is a sequence
+ number for reordering detection (see Section 3.4 and Section 4.4).
+
+ The value of the Observe Option is encoded as an unsigned integer in
+ network byte order using a variable number of bytes ('uint' option
+ format); see Section 3.2 of RFC 7252 [RFC7252].
+
+3. Client-side Requirements
+
+3.1. Request
+
+ A client registers its interest in a resource by issuing a GET
+ request with an Observe Option set to 0 (register). If the server
+ returns a 2.xx response that includes an Observe Option as well, the
+ server has successfully added an entry with the client endpoint and
+ request token to the list of observers of the target resource and the
+ client will be notified of changes to the resource state.
+
+ Like a fresh response can be used to satisfy a request without
+ contacting the server, the stream of updates resulting from one
+ observation request can be used to satisfy another (observation or
+ normal GET) request if the target resource is the same. A client
+ MUST aggregate such requests and MUST NOT register more than once for
+ the same target resource. The target resource is identified by all
+ options in the request that are part of the cache-key. This
+ includes, for example, the full request URI and the Accept Option.
+
+3.2. Notifications
+
+ Notifications are additional responses sent by the server in reply to
+ the single extended GET request that created the registration. Each
+ notification includes the token specified by the client in the
+ request. The only difference between a notification and a normal
+ response is the presence of the Observe Option.
+
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+ Notifications typically have a 2.05 (Content) response code. They
+ include an Observe Option with a sequence number for reordering
+ detection (see Section 3.4), and a payload in the same Content-Format
+ as the initial response. If the client included one or more ETag
+ Options in the GET request (see Section 3.3), notifications can have
+ a 2.03 (Valid) response code rather than a 2.05 (Content) response
+ code. Such notifications include an Observe Option with a sequence
+ number but no payload.
+
+ In the event that the resource changes in a way that would cause a
+ normal GET request at that time to return a non-2.xx response (for
+ example, when the resource is deleted), the server sends a
+ notification with an appropriate response code (such as 4.04 Not
+ Found) and removes the client's entry from the list of observers of
+ the resource. Non-2.xx responses do not include an Observe Option.
+
+3.3. Caching
+
+ As notifications are just additional responses to a GET request,
+ notifications partake in caching as defined in Section 5.6 of RFC
+ 7252 [RFC7252]. Both the freshness model and the validation model
+ are supported.
+
+3.3.1. Freshness
+
+ A client MAY store a notification like a response in its cache and
+ use a stored notification that is fresh without contacting the
+ server. Like a response, a notification is considered fresh while
+ its age is not greater than the value indicated by the Max-Age Option
+ (and no newer notification/response has been received).
+
+ The server will do its best to keep the resource state observed by
+ the client as closely in sync with the actual state as possible.
+ However, a client cannot rely on observing every single state that a
+ resource might go through. For example, if the network is congested
+ or the state changes more frequently than the network can handle, the
+ server can skip notifications for any number of intermediate states.
+
+ The server uses the Max-Age Option to indicate an age up to which it
+ is acceptable that the observed state and the actual state are
+ inconsistent. If the age of the latest notification becomes greater
+ than its indicated Max-Age, then the client MUST NOT assume that the
+ enclosed representation reflects the actual resource state.
+
+ To make sure it has a current representation and/or to re-register
+ its interest in a resource, a client MAY issue a new GET request with
+ the same token as the original at any time. All options MUST be
+ identical to those in the original request, except for the set of
+
+
+
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+
+ ETag Options. It is RECOMMENDED that the client does not issue the
+ request while it still has a fresh notification/response for the
+ resource in its cache. Additionally, the client SHOULD at least wait
+ for a random amount of time between 5 and 15 seconds after Max-Age
+ expired to reduce collisions with other clients.
+
+3.3.2. Validation
+
+ When a client has one or more notifications stored in its cache for a
+ resource, it can use the ETag Option in the GET request to give the
+ server an opportunity to select a stored notification to be used.
+
+ The client MAY include an ETag Option for each stored response that
+ is applicable in the GET request. Whenever the observed resource
+ changes to a representation identified by one of the ETag Options,
+ the server can select a stored response by sending a 2.03 (Valid)
+ notification with an appropriate ETag Option instead of a 2.05
+ (Content) notification.
+
+ A client implementation needs to keep all candidate responses in its
+ cache until it is no longer interested in the target resource or it
+ re-registers with a new set of entity-tags.
+
+3.4. Reordering
+
+ Messages with notifications can arrive in a different order than they
+ were sent. Since the goal is to keep the observed state as closely
+ in sync with the actual state as possible, a client MUST consider the
+ notification that was sent most recently as the freshest, regardless
+ of the order of arrival.
+
+ To provide an order among notifications for the client, the server
+ sets the value of the Observe Option in each notification to the 24
+ least-significant bits of a strictly increasing sequence number. An
+ incoming notification was sent more recently than the freshest
+ notification so far when one of the following conditions is met:
+
+ (V1 < V2 and V2 - V1 < 2^23) or
+ (V1 > V2 and V1 - V2 > 2^23) or
+ (T2 > T1 + 128 seconds)
+
+ where V1 is the value of the Observe Option in the freshest
+ notification so far, V2 the value of the Observe Option in the
+ incoming notification, T1 a client-local timestamp for the freshest
+ notification so far, and T2 a client-local timestamp for the incoming
+ notification.
+
+
+
+
+
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+
+ Design Note: The first two conditions verify that V1 is less than V2
+ in 24-bit serial number arithmetic [RFC1982]. The third condition
+ ensures that, if the server is generating serial numbers based on
+ a local clock, the time elapsed between the two incoming messages
+ is not so large that the difference between V1 and V2 has become
+ larger than the largest integer that it is meaningful to add to a
+ 24-bit serial number; in other words, after 128 seconds have
+ elapsed without any notification, a client does not need to check
+ the sequence numbers to assume that an incoming notification was
+ sent more recently than the freshest notification it has received
+ so far.
+
+ The duration of 128 seconds was chosen as a nice round number
+ greater than MAX_LATENCY (Section 4.8.2 of RFC 7252 [RFC7252]).
+
+3.5. Transmission
+
+ A notification can be confirmable or non-confirmable, i.e., it can be
+ sent in a confirmable or a non-confirmable message. The message type
+ used for a notification is independent of the type used for the
+ request and of any previous notification.
+
+ If a client does not recognize the token in a confirmable
+ notification, it MUST NOT acknowledge the message and SHOULD reject
+ it with a Reset message; otherwise, the client MUST acknowledge the
+ message as usual. In the case of a non-confirmable notification,
+ rejecting the message with a Reset message is OPTIONAL.
+
+ An acknowledgement message signals to the server that the client is
+ alive and interested in receiving further notifications; if the
+ server does not receive an acknowledgement in reply to a confirmable
+ notification, it will assume that the client is no longer interested
+ and will eventually remove the associated entry from the list of
+ observers.
+
+3.6. Cancellation
+
+ A client that is no longer interested in receiving notifications for
+ a resource can simply "forget" the observation. When the server then
+ sends the next notification, the client will not recognize the token
+ in the message and thus will return a Reset message. This causes the
+ server to remove the associated entry from the list of observers.
+ The entries in lists of observers are effectively "garbage collected"
+ by the server.
+
+
+
+
+
+
+
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+
+ Implementation Note: Due to potential message loss, the Reset
+ message may not reach the server. The client may therefore have
+ to reject multiple notifications, each with one Reset message,
+ until the server finally removes the associated entry from the
+ list of observers and stops sending notifications.
+
+ In some circumstances, it may be desirable to cancel an observation
+ and release the resources allocated by the server to it more eagerly.
+ In this case, a client MAY explicitly deregister by issuing a GET
+ request which has the Token field set to the token of the observation
+ to be cancelled and includes an Observe Option with the value set to
+ 1 (deregister). All other options MUST be identical to those in the
+ registration request, except for the set of ETag Options. When the
+ server receives such a request, it will remove any matching entry
+ from the list of observers and process the GET request as usual.
+
+4. Server-side Requirements
+
+4.1. Request
+
+ A GET request with an Observe Option set to 0 (register) requests the
+ server not only to return a current representation of the target
+ resource, but also to add the client to the list of observers of that
+ resource. Upon success, the server returns a current representation
+ of the resource and MUST keep this representation updated (as
+ described in Section 1.3) as long as the client is on the list of
+ observers.
+
+ The entry in the list of observers is keyed by the client endpoint
+ and the token specified by the client in the request. If an entry
+ with a matching endpoint/token pair is already present in the list
+ (which, for example, happens when the client wishes to reinforce its
+ interest in a resource), the server MUST NOT add a new entry but MUST
+ replace or update the existing one.
+
+ A server that is unable or unwilling to add a new entry to the list
+ of observers of a resource MAY silently ignore the registration
+ request and process the GET request as usual. The resulting response
+ MUST NOT include an Observe Option, the absence of which signals to
+ the client that it will not be notified of changes to the resource
+ and, e.g., needs to poll the resource for its state instead.
+
+ If the Observe Option in a GET request is set to 1 (deregister), then
+ the server MUST remove any existing entry with a matching endpoint/
+ token pair from the list of observers and process the GET request as
+ usual. The resulting response MUST NOT include an Observe Option.
+
+
+
+
+
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+
+4.2. Notifications
+
+ A client is notified of changes to the resource state by additional
+ responses sent by the server in reply to the GET request. Each such
+ notification response (including the initial response) MUST echo the
+ token specified by the client in the GET request. If there are
+ multiple entries in the list of observers, the order in which the
+ clients are notified is not defined; the server is free to use any
+ method to determine the order.
+
+ A notification SHOULD have a 2.05 (Content) or 2.03 (Valid) response
+ code. However, in the event that the state of a resource changes in
+ a way that would cause a normal GET request at that time to return a
+ non-2.xx response (for example, when the resource is deleted), the
+ server SHOULD notify the client by sending a notification with an
+ appropriate response code (such as 4.04 Not Found) and subsequently
+ MUST remove the associated entry from the list of observers of the
+ resource.
+
+ The Content-Format specified in a 2.xx notification MUST be the same
+ as the one used in the initial response to the GET request. If the
+ server is unable to continue sending notifications in this format, it
+ SHOULD send a notification with a 4.06 (Not Acceptable) response code
+ and subsequently MUST remove the associated entry from the list of
+ observers of the resource.
+
+ A 2.xx notification MUST include an Observe Option with a sequence
+ number as specified in Section 4.4 below; a non-2.xx notification
+ MUST NOT include an Observe Option.
+
+4.3. Caching
+
+ As notifications are just additional responses sent by the server in
+ reply to a GET request, they are subject to caching as defined in
+ Section 5.6 of RFC 7252 [RFC7252].
+
+4.3.1. Freshness
+
+ After returning the initial response, the server MUST keep the
+ resource state that is observed by the client as closely in sync with
+ the actual resource state as possible.
+
+ Since becoming out of sync at times cannot be avoided, the server
+ MUST indicate for each representation an age up to which it is
+ acceptable that the observed state and the actual state are
+ inconsistent. This age is application-dependent and MUST be
+ specified in notifications using the Max-Age Option.
+
+
+
+
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+
+ When the resource does not change and the client has a current
+ representation, the server does not need to send a notification.
+ However, if the client does not receive a notification, the client
+ cannot tell if the observed state and the actual state are still in
+ sync. Thus, when the age of the latest notification becomes greater
+ than its indicated Max-Age, the client no longer has a usable
+ representation of the resource state. The server MAY wish to prevent
+ that by sending a new notification with the unchanged representation
+ and a new Max-Age just before the Max-Age indicated earlier expires.
+
+4.3.2. Validation
+
+ A client can include a set of entity-tags in its request using the
+ ETag Option. When a observed resource changes its state and the
+ origin server is about to send a 2.05 (Content) notification, then,
+ whenever that notification has an entity-tag in the set of entity-
+ tags specified by the client, the server MAY send a 2.03 (Valid)
+ response with an appropriate ETag Option instead.
+
+4.4. Reordering
+
+ Because messages can get reordered, the client needs a way to
+ determine if a notification arrived later than a newer notification.
+ For this purpose, the server MUST set the value of the Observe Option
+ of each notification it sends to the 24 least-significant bits of a
+ strictly increasing sequence number. The sequence number MAY start
+ at any value and MUST NOT increase so fast that it increases by more
+ than 2^23 within less than 256 seconds.
+
+ The sequence number selected for a notification MUST be greater than
+ that of any preceding notification sent to the same client with the
+ same token for the same resource. The value of the Observe Option
+ MUST be current at the time of transmission; if a notification is
+ retransmitted, the server MUST update the value of the option to the
+ sequence number that is current at that time before retransmission.
+
+ Implementation Note: A simple implementation that satisfies the
+ requirements is to obtain a timestamp from a local clock. The
+ sequence number then is the timestamp in ticks, where 1 tick =
+ (256 seconds)/(2^23) = 30.52 microseconds. It is not necessary
+ that the clock reflects the current time/date.
+
+ Another valid implementation is to store a 24-bit unsigned integer
+ variable per resource and increment this variable each time the
+ resource undergoes a change of state (provided that the resource
+ changes its state less than 2^23 times in the first 256 seconds
+ after every state change). This removes the need to update the
+ value of the Observe Option on retransmission when the resource
+
+
+
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+
+ state did not change.
+
+ Design Note: The choice of a 24-bit option value and a time span of
+ 256 seconds theoretically allows for a notification rate of up to
+ 65536 notifications per second. Constrained nodes often have
+ rather imprecise clocks, though, and inaccuracies of the client
+ and server side may cancel out or add in effect. Therefore, the
+ maximum notification rate is reduced to 32768 notifications per
+ second. This is still well beyond the highest known design
+ objective of around 1 kHz (most CoAP applications will be several
+ orders of magnitude below that), but allows total clock
+ inaccuracies of up to -50/+100 %.
+
+4.5. Transmission
+
+ A notification can be sent in a confirmable or a non-confirmable
+ message. The message type used is typically application-dependent
+ and may be determined by the server for each notification
+ individually.
+
+ For example, for resources that change in a somewhat predictable or
+ regular fashion, notifications can be sent in non-confirmable
+ messages; for resources that change infrequently, notifications can
+ be sent in confirmable messages. The server can combine these two
+ approaches depending on the frequency of state changes and the
+ importance of individual notifications.
+
+ A server MAY choose to skip sending a notification if it knows that
+ it will send another notification soon, for example, when the state
+ of a resource is changing frequently. It also MAY choose to send
+ more than one notification for the same resource state. However,
+ above all, the server MUST ensure that a client in the list of
+ observers of a resource eventually observes the latest state if the
+ resource does not undergo a new change in state.
+
+ For example, when state changes occur in bursts, the server can skip
+ some notifications, send the notifications in non-confirmable
+ messages, and make sure that the client observes the latest state
+ change by repeating the last notification in a confirmable message
+ when the burst is over.
+
+ The client's acknowledgement of a confirmable notification signals
+ that the client is interested in receiving further notifications. If
+ a client rejects a confirmable or non-confirmable notification with a
+ Reset message, or if the last attempt to retransmit a confirmable
+ notification times out, then the client is considered no longer
+ interested and the server MUST remove the associated entry from the
+ list of observers.
+
+
+
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+
+ Implementation Note: To properly process a Reset message that
+ rejects a non-confirmable notification, a server needs to remember
+ the message IDs of the non-confirmable notifications it sends.
+ This may be challenging for a server with constrained resources.
+ However, since Reset messages are transmitted unreliably, the
+ client must be prepared that its Reset messages aren't received by
+ the server. A server thus can always pretend that a Reset message
+ rejecting a non-confirmable notification was lost. If a server
+ does this, it could accelerate cancellation by sending the
+ following notifications to that client in confirmable messages.
+
+ A server that transmits notifications mostly in non-confirmable
+ messages MUST send a notification in a confirmable message instead of
+ a non-confirmable message at least every 24 hours. This prevents a
+ client that went away or is no longer interested from remaining in
+ the list of observers indefinitely.
+
+4.5.1. Congestion Control
+
+ Basic congestion control for CoAP is provided by the exponential
+ back-off mechanism in Section 4.2 of RFC 7252 [RFC7252] and the
+ limitations in Section 4.7 of RFC 7252 [RFC7252]. However, CoAP
+ places the responsibility of congestion control for simple request/
+ response interactions only on the clients: rate limiting request
+ transmission implicitly controls the transmission of the responses.
+ When a single request yields a potentially infinite number of
+ notifications, additional responsibility needs to be placed on the
+ server.
+
+ In order not to cause congestion, servers MUST strictly limit the
+ number of simultaneous outstanding notifications/responses that they
+ transmit to a given client to NSTART (1 by default; see Section 4.7
+ of RFC 7252 [RFC7252]). An outstanding notification/response is
+ either a confirmable message for which an acknowledgement has not yet
+ been received and whose last retransmission attempt has not yet timed
+ out, or a non-confirmable message for which the waiting time that
+ results from the following rate limiting rules has not yet elapsed.
+
+ The server SHOULD NOT send more than one non-confirmable notification
+ per round-trip time (RTT) to a client on average. If the server
+ cannot maintain an RTT estimate for a client, it SHOULD NOT send more
+ than one non-confirmable notification every 3 seconds, and SHOULD use
+ an even less aggressive rate when possible (see also Section 3.1.2 of
+ RFC 5405 [RFC5405]).
+
+ Further congestion control optimizations and considerations are
+ expected in the future with advanced CoAP congestion control
+ mechanisms.
+
+
+
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+
+4.5.2. Advanced Transmission
+
+ The state of an observed resource may change while the number of the
+ number of simultaneous outstanding notifications/responses to a
+ client on the list of observers is greater than or equal to NSTART.
+ In this case, the server cannot notify the client of the new resource
+ state immediately but has to wait for an outstanding notification/
+ response to complete first.
+
+ If there exists an outstanding notification/response that the server
+ transmits to the client and that pertains to the changed resource,
+ then it is desirable for the server to stop working towards getting
+ the representation of the old resource state to the client, and to
+ start transmitting the current representation to the client instead,
+ so the resource state observed by the client stays closer in sync
+ with the actual state at the server.
+
+ For this purpose, the server MAY optimize the transmission process by
+ aborting the transmission of the old notification (but not before the
+ current transmission attempt completed) and starting a new
+ transmission for the new notification (but with the retransmission
+ timer and counter of the aborted transmission retained).
+
+ In more detail, a server MAY supersede an outstanding transmission
+ that pertains to an observation as follows:
+
+ 1. Wait for the current (re-)transmission attempt to be
+ acknowledged, rejected or to time out (confirmable transmission);
+ or wait for the waiting time to elapse or the transmission to be
+ rejected (non-confirmable transmission).
+
+ 2. If the transmission is rejected or it was the last attempt to
+ retransmit a notification, remove the associated entry from the
+ list of observers of the observed resource.
+
+ 3. If the entry is still in the list of observers, start to transmit
+ a new notification with a representation of the current resource
+ state. Should the resource have changed its state more than once
+ in the meantime, the notifications for the intermediate states
+ are silently skipped.
+
+ 4. The new notification is transmitted with a new Message ID and the
+ following transmission parameters: If the previous
+ (re-)transmission attempt timed out, retain its transmission
+ parameters, increment the retransmission counter and double the
+ timeout; otherwise, initialize the transmission parameters as
+ usual (see Section 4.2 of RFC 7252 [RFC7252]).
+
+
+
+
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+
+ It is possible that the server later receives an acknowledgement for
+ a confirmable notification that it superseded this way. Even though
+ this does not signal consistency, it is valuable in that it signals
+ the client's further interest in the resource. The server therefore
+ should avoid inadvertently removing the associated entry from the
+ list of observers.
+
+5. Intermediaries
+
+ A client may be interested in a resource in the namespace of a server
+ that is reached through a chain of one or more CoAP intermediaries.
+ In this case, the client registers its interest with the first
+ intermediary towards the server, acting as if it was communicating
+ with the server itself, as specified in Section 3. It is the task of
+ this intermediary to provide the client with a current representation
+ of the target resource and to keep the representation updated upon
+ changes to the resource state, as specified in Section 4.
+
+ To perform this task, the intermediary SHOULD make use of the
+ protocol specified in this document, taking the role of the client
+ and registering its own interest in the target resource with the next
+ hop towards the server. If the response returned by the next hop
+ doesn't include an Observe Option, the intermediary MAY resort to
+ polling the next hop or MAY itself return a response without an
+ Observe Option.
+
+ The communication between each pair of hops is independent; each hop
+ in the server role MUST determine individually how many notifications
+ to send, of which message type, and so on. Each hop MUST generate
+ its own values for the Observe Option in notifications, and MUST set
+ the value of the Max-Age Option according to the age of the local
+ current representation.
+
+ If two or more clients have registered their interest in a resource
+ with an intermediary, the intermediary MUST register itself only once
+ with the next hop and fan out the notifications it receives to all
+ registered clients. This relieves the next hop from sending the same
+ notifications multiple times and thus enables scalability.
+
+ An intermediary is not required to act on behalf of a client to
+ observe a resource; an intermediary MAY observe a resource, for
+ example, just to keep its own cache up to date.
+
+ See Appendix A.2 for examples.
+
+
+
+
+
+
+
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+
+6. Web Linking
+
+ A web link [RFC5988] to a resource accessible over CoAP (for example,
+ in a link-format document [RFC6690]) MAY include the target attribute
+ "obs".
+
+ The "obs" attribute, when present, is a hint indicating that the
+ destination of a link is useful for observation and thus, for
+ example, should have a suitable graphical representation in a user
+ interface. Note that this is only a hint; it is not a promise that
+ the Observe Option can actually be used to perform the observation.
+ A client may need to resort to polling the resource if the Observe
+ Option is not returned in the response to the GET request.
+
+ A value MUST NOT be given for the "obs" attribute; any present value
+ MUST be ignored by parsers. The "obs" attribute MUST NOT appear more
+ than once in a given link-value; occurrences after the first MUST be
+ ignored by parsers.
+
+7. Security Considerations
+
+ The security considerations in Section 11 of the CoAP specification
+ [RFC7252] apply.
+
+ Observing resources can dramatically increase the negative effects of
+ amplification attacks. That is, not only can notifications messages
+ be much larger than the request message, but the nature of the
+ protocol can cause a significant number of notifications to be
+ generated. Without client authentication, a server therefore MUST
+ strictly limit the number of notifications that it sends between
+ receiving acknowledgements that confirm the actual interest of the
+ client in the data; i.e., any notifications sent in non-confirmable
+ messages MUST be interspersed with confirmable messages. (An
+ attacker may still spoof the acknowledgements if the confirmable
+ messages are sufficiently predictable.)
+
+ The protocol follows a best-effort approach for keeping the state
+ observed by a client and the actual resource state at a server in
+ sync. This may have the client and the server become out of sync at
+ times. Depending on the sensitivity of the observed resource,
+ operating on an old state might be a security threat. The client
+ therefore must be careful not to use a representation after its Max-
+ Age expires, and the server must set the Max-Age Option to a sensible
+ value.
+
+ As with any protocol that creates state, attackers may attempt to
+ exhaust the resources that the server has available for maintaining
+ the list of observers for each resource. Servers may want to apply
+
+
+
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+
+
+ access controls to this creation of state. As degraded behavior, the
+ server can always fall back to processing the request as a normal GET
+ request (without an Observe Option) if it is unwilling or unable to
+ add a client to the list of observers of a resource, including if
+ system resources are exhausted or nearing exhaustion.
+
+ Intermediaries must be careful to ensure that notifications cannot be
+ employed to create a loop. A simple way to break any loops is to
+ employ caches for forwarding notifications in intermediaries.
+
+ Resources can be observed over DTLS-secured CoAP using any of the
+ security modes described in Section 9 of RFC 7252. The use of DTLS
+ is indicated by the "coaps" URI scheme. All notifications resulting
+ from a GET request with an Observe Option MUST be returned within the
+ same epoch of the same connection as the request.
+
+8. IANA Considerations
+
+ The following entry is added to the CoAP Option Numbers registry:
+
+ +--------+---------+-----------+
+ | Number | Name | Reference |
+ +--------+---------+-----------+
+ | 6 | Observe | [RFCXXXX] |
+ +--------+---------+-----------+
+
+ [Note to RFC Editor: Please replace XXXX with the RFC number of this
+ specification.]
+
+9. Acknowledgements
+
+ Carsten Bormann was an original author of this draft and is
+ acknowledged for significant contribution to this document.
+
+ Thanks to Daniele Alessandrelli, Jari Arkko, Peter A. Bigot, Angelo
+ P. Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank,
+ Bert Greevenbosch, Jeroen Hoebeke, Cullen Jennings, Matthias
+ Kovatsch, Barry Leiba, Salvatore Loreto, Charles Palmer, Akbar
+ Rahman, Zach Shelby, and Floris Van den Abeele for helpful comments
+ and discussions that have shaped the document.
+
+ This work was supported in part by Klaus Tschira Foundation, Intel,
+ Cisco, and Nokia.
+
+10. References
+
+
+
+
+
+
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+
+
+10.1. Normative References
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
+
+ [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
+ Application Protocol (CoAP)", RFC 7252, June 2014.
+
+10.2. Informative References
+
+ [GOF] Gamma, E., Helm, R., Johnson, R., and J. Vlissides,
+ "Design Patterns: Elements of Reusable Object-Oriented
+ Software", Addison-Wesley, Reading, MA, USA,
+ November 1994.
+
+ [REST] Fielding, R., "Architectural Styles and the Design of
+ Network-based Software Architectures", Ph.D. Dissertation,
+ University of California, Irvine, 2000, <http://
+ www.ics.uci.edu/~fielding/pubs/dissertation/
+ fielding_dissertation.pdf>.
+
+ [RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
+ August 1996.
+
+ [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
+ for Application Designers", BCP 145, RFC 5405,
+ November 2008.
+
+ [RFC5989] Roach, A., "A SIP Event Package for Subscribing to Changes
+ to an HTTP Resource", RFC 5989, October 2010.
+
+ [RFC6202] Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
+ "Known Issues and Best Practices for the Use of Long
+ Polling and Streaming in Bidirectional HTTP", RFC 6202,
+ April 2011.
+
+ [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
+ Format", RFC 6690, August 2012.
+
+ [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
+ Constrained-Node Networks", RFC 7228, May 2014.
+
+ [RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
+ (HTTP/1.1): Message Syntax and Routing", RFC 7230,
+ June 2014.
+
+
+
+
+Hartke Expires July 3, 2015 [Page 21]
+
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+
+
+Appendix A. Examples
+
+A.1. Client/Server Examples
+
+ Observed CLIENT SERVER Actual
+ t State | | State
+ ____________ | | ____________
+ 1 | |
+ 2 unknown | | 18.5 Cel
+ 3 +----->| Header: GET 0x41011633
+ 4 | GET | Token: 0x4a
+ 5 | | Uri-Path: temperature
+ 6 | | Observe: 0 (register)
+ 7 | |
+ 8 | |
+ 9 ____________ |<-----+ Header: 2.05 0x61451633
+ 10 | 2.05 | Token: 0x4a
+ 11 18.5 Cel | | Observe: 9
+ 12 | | Max-Age: 15
+ 13 | | Payload: "18.5 Cel"
+ 14 | |
+ 15 | | ____________
+ 16 ____________ |<-----+ Header: 2.05 0x51457b50
+ 17 | 2.05 | 19.2 Cel Token: 0x4a
+ 18 19.2 Cel | | Observe: 16
+ 29 | | Max-Age: 15
+ 20 | | Payload: "19.2 Cel"
+ 21 | |
+
+ Figure 3: A client registers and receives one notification of the
+ current state and one of a new state upon a state change
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+ Observed CLIENT SERVER Actual
+ t State | | State
+ ____________ | | ____________
+ 22 | |
+ 23 19.2 Cel | | 19.2 Cel
+ 24 | | ____________
+ 25 | X----+ Header: 2.05 0x51457b51
+ 26 | 2.05 | 19.7 Cel Token: 0x4a
+ 27 | | Observe: 25
+ 28 | | Max-Age: 15
+ 29 | | Payload: "19.7 Cel"
+ 30 | |
+ 31 ____________ | |
+ 32 | |
+ 33 19.2 Cel | |
+ 34 (stale) | |
+ 35 | |
+ 36 | |
+ 37 | |
+ 38 +----->| Header: GET 0x41011634
+ 39 | GET | Token: 0xb2
+ 40 | | Uri-Path: temperature
+ 41 | | Observe: 0 (register)
+ 42 | |
+ 43 | |
+ 44 ____________ |<-----+ Header: 2.05 0x61451634
+ 45 | 2.05 | Token: 0xb2
+ 46 19.7 Cel | | Observe: 44
+ 47 | | Max-Age: 15
+ 48 | | ETag: 0x78797a7a79
+ 49 | | Payload: "19.7 Cel"
+ 50 | |
+
+ Figure 4: The client re-registers after Max-Age ends
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hartke Expires July 3, 2015 [Page 23]
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+
+
+ Observed CLIENT SERVER Actual
+ t State | | State
+ ____________ | | ____________
+ 51 | |
+ 52 19.7 Cel | | 19.7 Cel
+ 53 | |
+ 54 | | ____________
+ 55 | crash
+ 56 |
+ 57 |
+ 58 |
+ 59 ____________ |
+ 60 |
+ 61 19.7 Cel |
+ 62 (stale) |
+ 63 | reboot____________
+ 64 | |
+ 65 | | 20.0 Cel
+ 66 | |
+ 67 +----->| Header: GET 0x41011635
+ 68 | GET | Token: 0xf9
+ 69 | | Uri-Path: temperature
+ 70 | | Observe: 0 (register)
+ 71 | | ETag: 0x78797a7a79
+ 72 | |
+ 73 | |
+ 74 ____________ |<-----+ Header: 2.05 0x61451635
+ 75 | 2.05 | Token: 0xf9
+ 76 20.0 Cel | | Observe: 74
+ 77 | | Max-Age: 15
+ 78 | | Payload: "20.0 Cel"
+ 79 | |
+ 80 | | ____________
+ 81 ____________ |<-----+ Header: 2.03 0x5143aa0c
+ 82 | 2.03 | 19.7 Cel Token: 0xf9
+ 83 19.7 Cel | | Observe: 81
+ 84 | | ETag: 0x78797a7a79
+ 85 | | Max-Age: 15
+ 86 | |
+
+ Figure 5: The client re-registers and gives the server the
+ opportunity to select a stored response
+
+
+
+
+
+
+
+
+
+Hartke Expires July 3, 2015 [Page 24]
+
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+
+
+ Observed CLIENT SERVER Actual
+ t State | | State
+ ____________ | | ____________
+ 87 | |
+ 88 19.7 Cel | | 19.7 Cel
+ 89 | |
+ 90 | | ____________
+ 91 ____________ |<-----+ Header: 2.05 0x4145aa0f
+ 92 | 2.05 | 19.3 Cel Token: 0xf9
+ 93 19.3 Cel | | Observe: 91
+ 94 | | Max-Age: 15
+ 95 | | Payload: "19.3 Cel"
+ 96 | |
+ 97 | |
+ 98 +- - ->| Header: 0x7000aa0f
+ 99 | |
+ 100 | |
+ 101 | |
+ 102 | | ____________
+ 103 | |
+ 104 | | 19.0 Cel
+ 105 | |
+ 106 ____________ | |
+ 107 | |
+ 108 19.3 Cel | |
+ 109 (stale) | |
+ 110 | |
+
+ Figure 6: The client rejects a notification and thereby cancels the
+ observation
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+
+
+A.2. Proxy Examples
+
+ CLIENT PROXY SERVER
+ | | |
+ | +----->| Header: GET 0x41015fb8
+ | | GET | Token: 0x1a
+ | | | Uri-Host: sensor.example
+ | | | Uri-Path: status
+ | | | Observe: 0 (register)
+ | | |
+ | |<-----+ Header: 2.05 0x61455fb8
+ | | 2.05 | Token: 0x1a
+ | | | Observe: 42
+ | | | Max-Age: 60
+ | | | Payload: "ready"
+ | | |
+ +----->| | Header: GET 0x41011633
+ | GET | | Token: 0x9a
+ | | | Proxy-Uri: coap://sensor.example/status
+ | | |
+ |<-----+ | Header: 2.05 0x61451633
+ | 2.05 | | Token: 0x9a
+ | | | Max-Age: 53
+ | | | Payload: "ready"
+ | | |
+ | |<-----+ Header: 2.05 0x514505fc0
+ | | 2.05 | Token: 0x1a
+ | | | Observe: 135
+ | | | Max-Age: 60
+ | | | Payload: "busy"
+ | | |
+ +----->| | Header: GET 0x41011634
+ | GET | | Token: 0x9b
+ | | | Proxy-Uri: coap://sensor.example/status
+ | | |
+ |<-----+ | Header: 2.05 0x61451634
+ | 2.05 | | Token: 0x9b
+ | | | Max-Age: 49
+ | | | Payload: "busy"
+ | | |
+
+ Figure 7: A proxy observes a resource to keep its cache up to date
+
+
+
+
+
+
+
+
+
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+
+
+ CLIENT PROXY SERVER
+ | | |
+ +----->| | Header: GET 0x41011635
+ | GET | | Token: 0x6a
+ | | | Proxy-Uri: coap://sensor.example/status
+ | | | Observe: 0 (register)
+ | | |
+ |<- - -+ | Header: 0x60001635
+ | | |
+ | +----->| Header: GET 0x4101af90
+ | | GET | Token: 0xaa
+ | | | Uri-Host: sensor.example
+ | | | Uri-Path: status
+ | | | Observe: 0 (register)
+ | | |
+ | |<-----+ Header: 2.05 0x6145af90
+ | | 2.05 | Token: 0xaa
+ | | | Observe: 67
+ | | | Max-Age: 60
+ | | | Payload: "ready"
+ | | |
+ |<-----+ | Header: 2.05 0x4145af94
+ | 2.05 | | Token: 0x6a
+ | | | Observe: 17346
+ | | | Max-Age: 60
+ | | | Payload: "ready"
+ | | |
+ +- - ->| | Header: 0x6000af94
+ | | |
+ | |<-----+ Header: 2.05 0x51455a20
+ | | 2.05 | Token: 0xaa
+ | | | Observe: 157
+ | | | Max-Age: 60
+ | | | Payload: "busy"
+ | | |
+ |<-----+ | Header: 2.05 0x5145af9b
+ | 2.05 | | Token: 0x6a
+ | | | Observe: 17436
+ | | | Max-Age: 60
+ | | | Payload: "busy"
+ | | |
+
+ Figure 8: A client observes a resource through a proxy
+
+
+
+
+
+
+
+
+Hartke Expires July 3, 2015 [Page 27]
+
+Internet-Draft Observing Resources in CoAP December 2014
+
+
+Appendix B. Changelog
+
+ [Note to RFC Editor: Please remove this section before publication.]
+
+ Changes from ietf-15 to ietf-16:
+
+ o Clarified several points based on AD, GenART, IESG, and Secdir
+ reviews.
+
+ Changes from ietf-14 to ietf-15:
+
+ o Clarified several points based on AD, GenART, IESG, and Secdir
+ reviews.
+
+ Changes from ietf-13 to ietf-14:
+
+ o Updated references.
+
+ Changes from ietf-12 to ietf-13:
+
+ o Extended the Observe Option in requests to not only add but also
+ remove an entry in the list of observers, depending on the option
+ value.
+
+ Note: The value of the Observe Option in a registration request
+ may now be any sequence of bytes that encodes the unsigned
+ integer 0, i.e., 0x'', 0x'00', 0x'00 00' or 0x'00 00 00'.
+
+ o Removed the 7.31 Code for cancellation.
+
+ Changes from ietf-11 to ietf-12:
+
+ o Introduced the 7.31 Code to request the cancellation of a pending
+ request.
+
+ o Made the algorithm for superseding an outstanding transmission
+ OPTIONAL.
+
+ o Clarified that the entry in the list of observers is removed if
+ the client fails to acknowledge a confirmable notification before
+ the last retransmission attempt times out (#350).
+
+ o Simplified the text on cancellation (#352) and the handling of
+ Reset messages (#353).
+
+ Changes from ietf-10 to ietf-11:
+
+
+
+
+
+Hartke Expires July 3, 2015 [Page 28]
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+
+ o Pointed out that client and server clocks may differ in their
+ realization of the SI second, and added robustness to the existing
+ reordering scheme by reducing the maximum notification rate to
+ 32768 notifications per second (#341).
+
+ Changes from ietf-09 to ietf-10:
+
+ o Required consistent sequence numbers across requests (#333).
+
+ o Clarified that a server needs to update the entry in the list of
+ observers instead of adding a new entry if the endpoint/token pair
+ is already present.
+
+ o Allowed that a client uses a token that is currently in use to
+ ensure that it's still in the list of observers. This is possible
+ because sequence numbers are now consistent across requests and
+ servers won't add a new entry for the same token.
+
+ o Improved text on the transmission of non-confirmable notifications
+ to match Section 3.1.2 of RFC 5405 more closely.
+
+ o Updated examples to use UCUM units.
+
+ o Moved Appendix B into the introduction.
+
+ Changes from ietf-08 to ietf-09:
+
+ o Removed the side effects of requests on existing observations.
+ This includes removing that
+
+ * the client can use a GET request to cancel an observation;
+
+ * the server updates the entry in the list of observers instead
+ of adding a new entry if the client is already present (#258,
+ #281).
+
+ o Clarified that a resource (and hence an observation relationship)
+ is identified by the request options that are part of the Cache-
+ Key (#258).
+
+ o Clarified that a non-2.xx notification MUST NOT include an Observe
+ Option.
+
+ o Moved block-wise transfer of notifications to [I-D.ietf-core-
+ block].
+
+ Changes from ietf-07 to ietf-08:
+
+
+
+
+Hartke Expires July 3, 2015 [Page 29]
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+
+ o Expanded text on transmitting a notification while a previous
+ transmission is pending (#242).
+
+ o Changed reordering detection to use a fixed time span of 128
+ seconds instead of EXCHANGE_LIFETIME (#276).
+
+ o Removed the use of the freshness model to determine if the client
+ is still on the list of observers. This includes removing that
+
+ * the client assumes that it has been removed from the list of
+ observers when Max-Age ends;
+
+ * the server sets the Max-Age Option of a notification to a value
+ that indicates when the server will send the next notification;
+
+ * the server uses a number of retransmit attempts such that
+ removing a client from the list of observers before Max-Age
+ ends is avoided (#235);
+
+ * the server may remove the client from all lists of observers
+ when the transmission of a confirmable notification ultimately
+ times out.
+
+ o Changed that an unrecognized critical option in a request must
+ actually have no effect on the state of any observation
+ relationship to any resource, as the option could lead to a
+ different target resource.
+
+ o Clarified that client implementations must be prepared to receive
+ each notification equally as a confirmable or a non-confirmable
+ message, regardless of the message type of the request and of any
+ previous notification.
+
+ o Added a requirement for sending a confirmable notification at
+ least every 24 hours before continuing with non-confirmable
+ notifications (#221).
+
+ o Added congestion control considerations from [I-D.bormann-core-
+ congestion-control-02].
+
+ o Recommended that the client waits for a randomized time after the
+ freshness of the latest notification expired before re-
+ registering. This prevents that multiple clients observing a
+ resource perform a GET request at the same time when the need to
+ re-register arises.
+
+ o Changed reordering detection from 'MAY' to 'SHOULD', as the goal
+ of the protocol (to keep the observed state as closely in sync
+
+
+
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+
+
+ with the actual state as possible) is not optional.
+
+ o Fixed the length of the Observe Option (3 bytes) in the table in
+ Section 2.
+
+ o Replaced the 'x' in the No-Cache-Key column in the table in
+ Section 2 with a '-', as the Observe Option doesn't have the No-
+ Cache-Key flag set, even though it is not part of the cache key.
+
+ o Updated examples.
+
+ Changes from ietf-06 to ietf-07:
+
+ o Moved to 24-bit sequence numbers to allow for up to 15000
+ notifications per second per client and resource (#217).
+
+ o Re-numbered option number to use Unsafe/Safe and Cache-Key
+ compliant numbers (#241).
+
+ o Clarified how to react to a Reset message that is sent in reply to
+ a non-confirmable notification (#225).
+
+ o Clarified the semantics of the "obs" link target attribute (#236).
+
+ Changes from ietf-05 to ietf-06:
+
+ o Improved abstract and introduction to say that the protocol is
+ about best effort and eventual consistency (#219).
+
+ o Clarified that the value of the Observe Option in a request must
+ have zero length.
+
+ o Added requirement that the sequence number must be updated each
+ time a server retransmits a notification.
+
+ o Clarified that a server must remove a client from the list of
+ observers when it receives a GET request with an unrecognized
+ critical option.
+
+ o Updated the text to use the endpoint concept from [I-D.ietf-core-
+ coap] (#224).
+
+ o Improved the reordering text (#223).
+
+ Changes from ietf-04 to ietf-05:
+
+ o Recommended that a client does not re-register while a new
+ notification from the server is still likely to arrive. This is
+
+
+
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+
+
+ to avoid that the request of the client and the last notification
+ after max-age cross over each other (#174).
+
+ o Relaxed requirements when sending a Reset message in reply to non-
+ confirmable notifications.
+
+ o Added an implementation note about careless GET requests (#184).
+
+ o Updated examples.
+
+ Changes from ietf-03 to ietf-04:
+
+ o Removed the "Max-OFE" Option.
+
+ o Allowed a Reset message in reply to non-confirmable notifications.
+
+ o Added a section on cancellation.
+
+ o Updated examples.
+
+ Changes from ietf-02 to ietf-03:
+
+ o Separated client-side and server-side requirements.
+
+ o Fixed uncertainty if client is still on the list of observers by
+ introducing a liveliness model based on Max-Age and a new option
+ called "Max-OFE" (#174).
+
+ o Simplified the text on message reordering (#129).
+
+ o Clarified requirements for intermediaries.
+
+ o Clarified the combination of blockwise transfers with
+ notifications (#172).
+
+ o Updated examples to show how the state observed by the client
+ becomes eventually consistent with the actual state on the server.
+
+ o Added examples for parameterization of observable resource.
+
+ Changes from ietf-01 to ietf-02:
+
+ o Removed the requirement of periodic refreshing (#126).
+
+ o The new "Observe" Option replaces the "Lifetime" Option.
+
+ o Introduced a new mechanism to detect message reordering.
+
+
+
+
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+
+ o Changed 2.00 (OK) notifications to 2.05 (Content) notifications.
+
+ Changes from ietf-00 to ietf-01:
+
+ o Changed terminology from "subscriptions" to "observation
+ relationships" (#33).
+
+ o Changed the name of the option to "Lifetime".
+
+ o Clarified establishment of observation relationships.
+
+ o Clarified that an observation is only identified by the URI of the
+ observed resource and the identity of the client (#66).
+
+ o Clarified rules for establishing observation relationships (#68).
+
+ o Clarified conditions under which an observation relationship is
+ terminated.
+
+ o Added explanation on how clients can terminate an observation
+ relationship before the lifetime ends (#34).
+
+ o Clarified that the overriding objective for notifications is
+ eventual consistency of the actual and the observed state (#67).
+
+ o Specified how a server needs to deal with clients not
+ acknowledging confirmable messages carrying notifications (#69).
+
+ o Added a mechanism to detect message reordering (#35).
+
+ o Added an explanation of how notifications can be cached,
+ supporting both the freshness and the validation model (#39, #64).
+
+ o Clarified that non-GET requests do not affect observation
+ relationships, and that GET requests without "Lifetime" Option
+ affecting relationships is by design (#65).
+
+ o Described interaction with blockwise transfers (#36).
+
+ o Added Resource Discovery section (#99).
+
+ o Added IANA Considerations.
+
+ o Added Security Considerations (#40).
+
+ o Added examples (#38).
+
+
+
+
+
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+
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+
+
+Author's Address
+
+ Klaus Hartke
+ Universitaet Bremen TZI
+ Postfach 330440
+ Bremen D-28359
+ Germany
+
+ Phone: +49-421-218-63905
+ EMail: hartke@tzi.org
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
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+
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diff --git a/rfc/rfc6690.txt b/rfc/rfc6690.txt
new file mode 100644
index 0000000..37209df
--- /dev/null
+++ b/rfc/rfc6690.txt
@@ -0,0 +1,1235 @@
+
+
+
+
+
+
+Internet Engineering Task Force (IETF) Z. Shelby
+Request for Comments: 6690 Sensinode
+Category: Standards Track August 2012
+ISSN: 2070-1721
+
+
+ Constrained RESTful Environments (CoRE) Link Format
+
+Abstract
+
+ This specification defines Web Linking using a link format for use by
+ constrained web servers to describe hosted resources, their
+ attributes, and other relationships between links. Based on the HTTP
+ Link Header field defined in RFC 5988, the Constrained RESTful
+ Environments (CoRE) Link Format is carried as a payload and is
+ assigned an Internet media type. "RESTful" refers to the
+ Representational State Transfer (REST) architecture. A well-known
+ URI is defined as a default entry point for requesting the links
+ hosted by a server.
+
+Status of This Memo
+
+ This is an Internet Standards Track document.
+
+ This document is a product of the Internet Engineering Task Force
+ (IETF). It represents the consensus of the IETF community. It has
+ received public review and has been approved for publication by the
+ Internet Engineering Steering Group (IESG). Further information on
+ Internet Standards is available in Section 2 of RFC 5741.
+
+ Information about the current status of this document, any errata,
+ and how to provide feedback on it may be obtained at
+ http://www.rfc-editor.org/info/rfc6690.
+
+Copyright Notice
+
+ Copyright (c) 2012 IETF Trust and the persons identified as the
+ document authors. All rights reserved.
+
+ This document is subject to BCP 78 and the IETF Trust's Legal
+ Provisions Relating to IETF Documents
+ (http://trustee.ietf.org/license-info) in effect on the date of
+ publication of this document. Please review these documents
+ carefully, as they describe your rights and restrictions with respect
+ to this document. Code Components extracted from this document must
+ include Simplified BSD License text as described in Section 4.e of
+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+
+
+Shelby Standards Track [Page 1]
+
+RFC 6690 CoRE Link Format August 2012
+
+
+Table of Contents
+
+ 1. Introduction ....................................................3
+ 1.1. Web Linking in CoRE ........................................3
+ 1.2. Use Cases ..................................................4
+ 1.2.1. Discovery ...........................................4
+ 1.2.2. Resource Collections ................................5
+ 1.2.3. Resource Directory ..................................5
+ 1.3. Terminology ................................................6
+ 2. Link Format .....................................................6
+ 2.1. Target and Context URIs ....................................8
+ 2.2. Link Relations .............................................8
+ 2.3. Use of Anchors .............................................9
+ 3. CoRE Link Attributes ............................................9
+ 3.1. Resource Type 'rt' Attribute ...............................9
+ 3.2. Interface Description 'if' Attribute ......................10
+ 3.3. Maximum Size Estimate 'sz' Attribute ......................10
+ 4. Well-Known Interface ...........................................10
+ 4.1. Query Filtering ...........................................12
+ 5. Examples .......................................................13
+ 6. Security Considerations ........................................15
+ 7. IANA Considerations ............................................16
+ 7.1. Well-Known 'core' URI .....................................16
+ 7.2. New 'hosts' Relation Type .................................16
+ 7.3. New 'link-format' Internet Media Type .....................17
+ 7.4. Constrained RESTful Environments (CoRE) Parameters
+ Registry ..................................................18
+ 8. Acknowledgments ................................................19
+ 9. References .....................................................20
+ 9.1. Normative References ......................................20
+ 9.2. Informative References ....................................20
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby Standards Track [Page 2]
+
+RFC 6690 CoRE Link Format August 2012
+
+
+1. Introduction
+
+ The Constrained RESTful Environments (CoRE) realizes the
+ Representational State Transfer (REST) architecture [REST] in a
+ suitable form for the most constrained nodes (e.g., 8-bit
+ microcontrollers with limited memory) and networks (e.g., IPv6 over
+ Low-Power Wireless Personal Area Networks (6LoWPANs) [RFC4919]).
+ CoRE is aimed at Machine-to-Machine (M2M) applications such as smart
+ energy and building automation.
+
+ The discovery of resources hosted by a constrained server is very
+ important in machine-to-machine applications where there are no
+ humans in the loop and static interfaces result in fragility. The
+ discovery of resources provided by an HTTP [RFC2616] web server is
+ typically called "Web Discovery" and the description of relations
+ between resources is called "Web Linking" [RFC5988]. In the present
+ specification, we refer to the discovery of resources hosted by a
+ constrained web server, their attributes, and other resource
+ relations as CoRE Resource Discovery.
+
+ The main function of such a discovery mechanism is to provide
+ Universal Resource Identifiers (URIs, called links) for the resources
+ hosted by the server, complemented by attributes about those
+ resources and possible further link relations. In CoRE, this
+ collection of links is carried as a resource of its own (as opposed
+ to HTTP headers delivered with a specific resource). This document
+ specifies a link format for use in CoRE Resource Discovery by
+ extending the HTTP Link Header format [RFC5988] to describe these
+ link descriptions. The CoRE Link Format is carried as a payload and
+ is assigned an Internet media type. A well-known relative URI
+ "/.well-known/core" is defined as a default entry point for
+ requesting the list of links about resources hosted by a server and
+ thus performing CoRE Resource Discovery. This specification is
+ applicable for use with Constrained Application Protocol (CoAP)
+ [COAP], HTTP, or any other suitable web transfer protocol. The link
+ format can also be saved in file format.
+
+1.1. Web Linking in CoRE
+
+ Technically, the CoRE Link Format is a serialization of a typed link
+ as specified in [RFC5988], used to describe relationships between
+ resources, so-called "Web Linking". In this specification, Web
+ Linking is extended with specific constrained M2M attributes; links
+ are carried as a message payload rather than in an HTTP Link Header
+ field, and a default interface is defined to discover resources
+ hosted by a server. This specification also defines a new relation
+
+
+
+
+
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+
+
+ type "hosts" (from the verb "to host"), which indicates that the
+ resource is hosted by the server from which the link document was
+ requested.
+
+ In HTTP, the Link Header can be used to carry link information about
+ a resource along with an HTTP response. This works well for the
+ typical use case for a web server and browser, where further
+ information about a particular resource is useful after accessing it.
+ In CoRE, the main use case for Web Linking is the discovery of which
+ resources a server hosts in the first place. Although some resources
+ may have further links associated with them, this is expected to be
+ an exception. For that reason, the CoRE Link Format serialization is
+ carried as a resource representation of a well-known URI. The CoRE
+ Link Format does reuse the format of the HTTP Link Header
+ serialization defined in [RFC5988].
+
+1.2. Use Cases
+
+ Typical use cases for Web Linking on today's web include, e.g.,
+ describing the author of a web page or describing relations between
+ web pages (next chapter, previous chapter, etc.). Web Linking can
+ also be applied to M2M applications, where typed links are used to
+ assist a machine client in finding and understanding how to use
+ resources on a server. In this section a few use cases are described
+ for how the CoRE Link Format could be used in M2M applications. For
+ further technical examples, see Section 5. As there is a large range
+ of M2M applications, these use cases are purposely generic. This
+ specification assumes that different deployments or application
+ domains will define the appropriate REST Interface Descriptions along
+ with Resource Types to make discovery meaningful.
+
+1.2.1. Discovery
+
+ In M2M applications, for example, home or building automation, there
+ is a need for local clients and servers to find and interact with
+ each other without human intervention. The CoRE Link Format can be
+ used by servers in such environments to enable Resource Discovery of
+ the resources hosted by the server.
+
+ Resource Discovery can be performed either unicast or multicast.
+ When a server's IP address is already known, either a priori or
+ resolved via the Domain Name System (DNS) [RFC1034][RFC1035], unicast
+ discovery is performed in order to locate the entry point to the
+ resource of interest. In this specification, this is performed using
+ a GET to "/.well-known/core" on the server, which returns a payload
+ in the CoRE Link Format. A client would then match the appropriate
+ Resource Type, Interface Description, and possible media type
+
+
+
+
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+
+
+ [RFC2045] for its application. These attributes may also be included
+ in the query string in order to filter the number of links returned
+ in a response.
+
+ Multicast Resource Discovery is useful when a client needs to locate
+ a resource within a limited scope, and that scope supports IP
+ multicast. A GET request to the appropriate multicast address is
+ made for "/.well-known/core". In order to limit the number and size
+ of responses, a query string is recommended with the known
+ attributes. Typically, a resource would be discovered based on its
+ Resource Type and/or Interface Description, along with possible
+ application-specific attributes.
+
+1.2.2. Resource Collections
+
+ RESTful designs of M2M interfaces often make use of collections of
+ resources. For example, an index of temperature sensors on a data
+ collection node or a list of alarms on a home security controller.
+ The CoRE Link Format can be used to make it possible to find the
+ entry point to a collection and traverse its members. The entry
+ point of a collection would always be included in "/.well-known/core"
+ to enable its discovery. The members of the collection can be
+ defined either through the Interface Description of the resource
+ along with a parameter resource for the size of the collection or by
+ using the link format to describe each resource in the collection.
+ These links could be located under "/.well-known/core" or hosted, for
+ example, in the root resource of the collection.
+
+1.2.3. Resource Directory
+
+ In many deployment scenarios, for example, constrained networks with
+ sleeping servers or large M2M deployments with bandwidth limited
+ access networks, it makes sense to deploy resource directory entities
+ that store links to resources stored on other servers. Think of this
+ as a limited search engine for constrained M2M resources.
+
+ The CoRE Link Format can be used by a server to register resources
+ with a resource directory or to allow a resource directory to poll
+ for resources. Resource registration can be achieved by having each
+ server POST their resources to "/.well-known/core" on the resource
+ directory. This, in turn, adds links to the resource directory under
+ an appropriate resource. These links can then be discovered by any
+ client by making a request to a resource directory lookup interface.
+
+
+
+
+
+
+
+
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+
+
+1.3. Terminology
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+ specification are to be interpreted as described in [RFC2119].
+
+ This specification makes use of the Augmented Backus-Naur Form (ABNF)
+ [RFC5234] notation, including the core rules defined in Appendix B of
+ that document.
+
+ This specification requires readers to be familiar with all the terms
+ and concepts that are discussed in [RFC5988] and [RFC6454]. In
+ addition, this specification makes use of the following terminology:
+
+ Web Linking
+ A framework for indicating the relationships between web
+ resources.
+
+ Link
+ Also called "typed links" in [RFC5988]. A link is a typed
+ connection between two resources identified by URI and is made up
+ of a context URI, a link relation type, a target URI, and optional
+ target attributes.
+
+ Link Format
+ A particular serialization of typed links.
+
+ CoRE Link Format
+ A particular serialization of typed links based on the HTTP Link
+ Header field serialization defined in Section 5 of [RFC5988] but
+ carried as a resource representation with a media type.
+
+ Attribute
+ Properly called "Target Attribute" in [RFC5988]. A key/value pair
+ that describes the link or its target.
+
+ CoRE Resource Discovery
+ When a client discovers the list of resources hosted by a server,
+ their attributes, and other link relations by accessing
+ "/.well-known/core".
+
+2. Link Format
+
+ The CoRE Link Format extends the HTTP Link Header field specified in
+ [RFC5988]. The format does not require special XML or binary
+ parsing, is fairly compact, and is extensible -- all important
+ characteristics for CoRE. It should be noted that this link format
+ is just one serialization of typed links defined in [RFC5988]; others
+
+
+
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+
+
+ include HTML links, Atom feed links [RFC4287], or HTTP Link Header
+ fields. It is expected that resources discovered in the CoRE Link
+ Format may also be made available in alternative formats on the
+ greater Internet. The CoRE Link Format is only expected to be
+ supported in constrained networks and M2M systems.
+
+ Section 5 of [RFC5988] did not require an Internet media type for the
+ defined link format, as it was defined to be carried in an HTTP
+ header. This specification thus defines the Internet media type
+ 'application/link-format' for the CoRE Link Format (see Section 7.3).
+ Whereas the HTTP Link Header field depends on [RFC2616] for its
+ encoding, the CoRE Link Format is encoded as UTF-8 [RFC3629]. A
+ decoder of the format is not expected to validate UTF-8 encoding (but
+ is not prohibited from doing so) and doesn't need to perform any
+ UTF-8 normalization. UTF-8 data can be compared bitwise, which
+ allows values to contain UTF-8 data without any added complexity for
+ constrained nodes.
+
+ The CoRE Link Format is equivalent to the [RFC5988] link format;
+ however, the ABNF in the present specification is repeated with
+ improvements to be compliant with [RFC5234] and includes new link
+ parameters. The link parameter "href" is reserved for use as a query
+ parameter for filtering in this specification (see Section 4.1) and
+ MUST NOT be defined as a link parameter. As in [RFC5988], multiple
+ link descriptions are separated by commas. Note that commas can also
+ occur in quoted strings and URIs but do not end a description. In
+ order to convert an HTTP Link Header field to this link format, first
+ the "Link:" HTTP header is removed, any linear whitespace (LWS) is
+ removed, the header value is converted to UTF-8, and any percent-
+ encodings are decoded.
+
+ Link = link-value-list
+ link-value-list = [ link-value *[ "," link-value ]]
+ link-value = "<" URI-Reference ">" *( ";" link-param )
+ link-param = ( ( "rel" "=" relation-types )
+ / ( "anchor" "=" DQUOTE URI-Reference DQUOTE )
+ / ( "rev" "=" relation-types )
+ / ( "hreflang" "=" Language-Tag )
+ / ( "media" "=" ( MediaDesc
+ / ( DQUOTE MediaDesc DQUOTE ) ) )
+ / ( "title" "=" quoted-string )
+ / ( "title*" "=" ext-value )
+ / ( "type" "=" ( media-type / quoted-mt ) )
+ / ( "rt" "=" relation-types )
+ / ( "if" "=" relation-types )
+ / ( "sz" "=" cardinal )
+ / ( link-extension ) )
+ link-extension = ( parmname [ "=" ( ptoken / quoted-string ) ] )
+
+
+
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+
+
+ / ( ext-name-star "=" ext-value )
+ ext-name-star = parmname "*" ; reserved for RFC-2231-profiled
+ ; extensions. Whitespace NOT
+ ; allowed in between.
+ ptoken = 1*ptokenchar
+ ptokenchar = "!" / "#" / "$" / "%" / "&" / "'" / "("
+ / ")" / "*" / "+" / "-" / "." / "/" / DIGIT
+ / ":" / "<" / "=" / ">" / "?" / "@" / ALPHA
+ / "[" / "]" / "^" / "_" / "`" / "{" / "|"
+ / "}" / "~"
+ media-type = type-name "/" subtype-name
+ quoted-mt = DQUOTE media-type DQUOTE
+ relation-types = relation-type
+ / DQUOTE relation-type *( 1*SP relation-type ) DQUOTE
+ relation-type = reg-rel-type / ext-rel-type
+ reg-rel-type = LOALPHA *( LOALPHA / DIGIT / "." / "-" )
+ ext-rel-type = URI
+ cardinal = "0" / ( %x31-39 *DIGIT )
+ LOALPHA = %x61-7A ; a-z
+ quoted-string = <defined in [RFC2616]>
+ URI = <defined in [RFC3986]>
+ URI-Reference = <defined in [RFC3986]>
+ type-name = <defined in [RFC4288]>
+ subtype-name = <defined in [RFC4288]>
+ MediaDesc = <defined in [W3C.HTML.4.01]>
+ Language-Tag = <defined in [RFC5646]>
+ ext-value = <defined in [RFC5987]>
+ parmname = <defined in [RFC5987]>
+
+2.1. Target and Context URIs
+
+ Each link conveys one target URI as a URI-reference inside angle
+ brackets ("<>"). The context URI of a link (also called the base URI
+ in [RFC3986]) is determined by the following rules in this
+ specification:
+
+ (a) The context URI is set to the anchor parameter, when specified.
+
+ (b) Origin of the target URI, when specified.
+
+ (c) Origin of the link format resource's base URI.
+
+2.2. Link Relations
+
+ Since links in the CoRE Link Format are typically used to describe
+ resources hosted by a server, the new relation type "hosts" is
+ assumed in the absence of the relation parameter (see Section 7.2).
+ The "hosts" relation type (from the verb "to host") indicates that
+
+
+
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+
+
+ the target URI is a resource hosted by the server (i.e., server hosts
+ resource) indicated by the context URI. The target URI MUST be a
+ relative URI of the context URI for this relation type.
+
+ To express other relations, links can make use of any registered
+ relation by including the relation parameter. The context of a
+ relation can be defined using the anchor parameter. In this way,
+ relations between resources hosted on a server or between hosted
+ resources and external resources can be expressed.
+
+2.3. Use of Anchors
+
+ As per Section 5.2 of [RFC5988], a link description MAY include an
+ "anchor" parameter, in which case the context is the URI included in
+ that attribute. This is used to describe a relationship between two
+ resources. A consuming implementation can, however, choose to ignore
+ such links. It is not expected that all implementations will be able
+ to derive useful information from explicitly anchored links.
+
+3. CoRE Link Attributes
+
+ The following CoRE-specific target attributes are defined in addition
+ to those already defined in [RFC5988]. These attributes describe
+ information useful in accessing the target link of the relation and,
+ in some cases, can use the syntactical form of a URI. Such a URI MAY
+ be dereferenced (for instance, to obtain a description of the link
+ relation), but that is not part of the protocol and MUST NOT be done
+ automatically on link evaluation. When the values of attributes are
+ compared, they MUST be compared as strings.
+
+3.1. Resource Type 'rt' Attribute
+
+ The Resource Type 'rt' attribute is an opaque string used to assign
+ an application-specific semantic type to a resource. One can think
+ of this as a noun describing the resource. In the case of a
+ temperature resource, this could be, e.g., an application-specific
+ semantic type like "outdoor-temperature" or a URI referencing a
+ specific concept in an ontology like
+ "http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature". Multiple
+ Resource Types MAY be included in the value of this parameter, each
+ separated by a space, similar to the relation attribute. The
+ registry for Resource Type values is defined in Section 7.4.
+
+ The Resource Type attribute is not meant to be used to assign a
+ human-readable name to a resource. The "title" attribute defined in
+ [RFC5988] is meant for that purpose. The Resource Type attribute
+ MUST NOT appear more than once in a link.
+
+
+
+
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+
+
+3.2. Interface Description 'if' Attribute
+
+ The Interface Description 'if' attribute is an opaque string used to
+ provide a name or URI indicating a specific interface definition used
+ to interact with the target resource. One can think of this as
+ describing verbs usable on a resource. The Interface Description
+ attribute is meant to describe the generic REST interface to interact
+ with a resource or a set of resources. It is expected that an
+ Interface Description will be reused by different Resource Types.
+ For example, the Resource Types "outdoor-temperature", "dew-point",
+ and "rel-humidity" could all be accessible using the Interface
+ Description "http://www.example.org/myapp.wadl#sensor". Multiple
+ Interface Descriptions MAY be included in the value of this
+ parameter, each separated by a space, similar to the relation
+ attribute. The registry for Interface Description values is defined
+ in Section 7.4.
+
+ The Interface Description could be, for example, the URI of a Web
+ Application Description Language (WADL) [WADL] definition of the
+ target resource "http://www.example.org/myapp.wadl#sensor", a URN
+ indicating the type of interface to the resource "urn:myapp:sensor",
+ or an application-specific name "sensor". The Interface Description
+ attribute MUST NOT appear more than once in a link.
+
+3.3. Maximum Size Estimate 'sz' Attribute
+
+ The maximum size estimate attribute 'sz' gives an indication of the
+ maximum size of the resource representation returned by performing a
+ GET on the target URI. For links to CoAP resources, this attribute
+ is not expected to be included for small resources that can
+ comfortably be carried in a single Maximum Transmission Unit (MTU)
+ but SHOULD be included for resources larger than that. The maximum
+ size estimate attribute MUST NOT appear more than once in a link.
+
+ Note that there is no defined upper limit to the value of the 'sz'
+ attributes. Implementations MUST be prepared to accept large values.
+ One implementation strategy is to convert any value larger than a
+ reasonable size limit for this implementation to a special value
+ "Big", which in further processing would indicate that a size value
+ was given that was so big that it cannot be processed by this
+ implementation.
+
+4. Well-Known Interface
+
+ Resource discovery in CoRE is accomplished through the use of a well-
+ known resource URI that returns a list of links about resources
+ hosted by that server and other link relations. Well-known resources
+
+
+
+
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+
+
+ have a path component that begins with "/.well-known/" as specified
+ in [RFC5785]. This specification defines a new well-known resource
+ for CoRE Resource Discovery: "/.well-known/core".
+
+ A server implementing this specification MUST support this resource
+ on the default port appropriate for the protocol for the purpose of
+ resource discovery. It is, however, up to the application which
+ links are included and how they are organized. The resource
+ "/.well-known/core" is meant to be used to return links to the entry
+ points of resource interfaces on a server. More sophisticated link
+ organization can be achieved by including links to CoRE Link Format
+ resources located elsewhere on the server, for example, to achieve an
+ index. In the absence of any links, a zero-length payload is
+ returned. The resource representation of this resource MUST be the
+ CoRE Link Format described in Section 2.
+
+ The CoRE resource discovery interface supports the following
+ interactions:
+
+ o Performing a GET on "/.well-known/core" to the default port
+ returns a set of links available from the server (if any) in the
+ CoRE Link Format. These links might describe resources hosted on
+ that server or on other servers or express other kinds of link
+ relations as described in Section 2.
+
+ o Filtering may be performed on any of the link format attributes
+ using a query string as specified in Section 4.1. For example,
+ [GET /.well-known/core?rt=temperature-c] would request resources
+ with the Resource Type temperature-c. A server is not, however,
+ required to support filtering.
+
+ o More capable servers such as proxies could support a resource
+ directory by requesting the resource descriptions of other end-
+ points or allowing servers to POST requests to "/.well-known/
+ core". The details of such resource directory functionality is,
+ however, out of the scope of this specification and is expected to
+ be specified separately.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
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+
+
+4.1. Query Filtering
+
+ A server implementing this specification MAY recognize the query part
+ of a resource discovery URI as a filter on the resources to be
+ returned. The path and query components together should conform to
+ the following level-4 URI Template [RFC6570]:
+
+ /.well-known/core{?search*}
+
+ where the variable "search" is a 1-element list that has a single
+ name/value pair, where
+
+ o name is either "href", a link-param name defined in this
+ specification, or any other link-extension name, and
+
+ o value is either a Complete Value String that does not end in an
+ "*" (%2A), or a Prefix Value String followed by an "*" (%2A).
+
+ The search name "href" refers to the URI-reference between the "<"
+ and ">" characters of a link. Both Value Strings match a target
+ attribute only if it exists. Value Strings are percent-decoded
+ ([RFC3986], Section 2.1) before matching; similarly, any target
+ attributes notated as quoted-string are interpreted as defined in
+ Section 2.2 of [RFC2616]. After these steps, a Complete Value String
+ matches a target attribute if it is bitwise identical. A Prefix
+ Value String matches a target attribute if it is a bitwise prefix of
+ the target attribute (where any string is a prefix of itself). Empty
+ Prefix Value Strings are allowed; by the definition above, they match
+ any target attribute that does exist. Note that relation-type target
+ attributes can contain multiple values, and each value MUST be
+ treated as a separate target attribute when matching.
+
+ It is not expected that very constrained nodes support filtering.
+ Implementations not supporting filtering MUST simply ignore the query
+ string and return the whole resource for unicast requests.
+
+ When using a transfer protocol like the Constrained Application
+ Protocol (CoAP) that supports multicast requests, special care needs
+ to be taken. A multicast request with a query string SHOULD NOT be
+ responded to if filtering is not supported or if the filter does not
+ match (to avoid a needless response storm). The exception is in
+ cases where the IP stack interface is not able to indicate that the
+ destination address was multicast.
+
+
+
+
+
+
+
+
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+
+
+ The following are examples of valid query URIs:
+
+ o ?href=/foo matches a link-value that is anchored at /foo
+
+ o ?href=/foo* matches a link-value that is anchored at a URI that
+ starts with /foo
+
+ o ?foo=bar matches a link-value that has a target attribute named
+ foo with the exact value bar
+
+ o ?foo=bar* matches a link-value that has a target attribute named
+ foo, the value of which starts with bar, e.g., bar or barley
+
+ o ?foo=* matches a link-value that has a target attribute named foo
+
+5. Examples
+
+ A few examples of typical link descriptions in this format follows.
+ Multiple resource descriptions in a representation are separated by
+ commas. Linefeeds are also included in these examples for
+ readability. Although the following examples use CoAP response
+ codes, the examples are applicable to HTTP as well (the corresponding
+ response code would be 200 OK).
+
+ This example includes links to two different sensors sharing the same
+ Interface Description. Note that the default relation type for this
+ link format is "hosts" in links with no rel= target attribute. Thus,
+ the links in this example tell that the Origin server from which
+ /.well-known/core was requested (the context) hosts the resources
+ /sensors/temp and /sensors/light (each a target).
+
+ REQ: GET /.well-known/core
+
+ RES: 2.05 Content
+ </sensors/temp>;if="sensor",
+ </sensors/light>;if="sensor"
+
+ Without the linefeeds inserted here for readability, the format
+ actually looks as follows.
+
+ </sensors/temp>;if="sensor",</sensors/light>;if="sensor"
+
+ This example arranges link descriptions hierarchically, with the
+ entry point including a link to a sub-resource containing links about
+ the sensors.
+
+
+
+
+
+
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+
+
+ REQ: GET /.well-known/core
+
+ RES: 2.05 Content
+ </sensors>;ct=40
+
+ REQ: GET /sensors
+
+ RES: 2.05 Content
+ </sensors/temp>;rt="temperature-c";if="sensor",
+ </sensors/light>;rt="light-lux";if="sensor"
+
+ An example query filter may look like:
+
+ REQ: GET /.well-known/core?rt=light-lux
+
+ RES: 2.05 Content
+ </sensors/light>;rt="light-lux";if="sensor"
+
+ Note that relation-type attributes like 'rt', 'if', and 'rel' can
+ have multiple values separated by spaces. A query filter parameter
+ can match any one of those values, as in this example:
+
+ REQ: GET /.well-known/core?rt=light-lux
+
+ RES: 2.05 Content
+ </sensors/light>;rt="light-lux core.sen-light";if="sensor"
+
+ This example shows the use of an "anchor" attribute to relate the
+ temperature sensor resource to an external description and to an
+ alternative URI.
+
+ REQ: GET /.well-known/core
+
+ RES: 2.05 Content
+ </sensors>;ct=40;title="Sensor Index",
+ </sensors/temp>;rt="temperature-c";if="sensor",
+ </sensors/light>;rt="light-lux";if="sensor",
+ <http://www.example.com/sensors/t123>;anchor="/sensors/temp"
+ ;rel="describedby",
+ </t>;anchor="/sensors/temp";rel="alternate"
+
+
+
+
+
+
+
+
+
+
+
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+
+
+ If a client is interested in finding relations about a particular
+ resource, it can perform a query on the anchor parameter:
+
+ REQ: GET /.well-known/core?anchor=/sensors/temp
+
+ RES: 2.05 Content
+ <http://www.example.com/sensors/temp123>;anchor="/sensors/temp"
+ ;rel="describedby",
+ </t>;anchor="/sensors/temp";rel="alternate"
+
+ The following example shows a large firmware resource with a size
+ attribute. The consumer of this link would use the 'sz' attribute to
+ determine if the resource representation is too large and if block
+ transfer would be required to request it. In this case, a client
+ with only a 64 KiB flash might only support a 16-bit integer for
+ storing the 'sz' attribute. Thus, a special flag or value should be
+ used to indicate "Big" (larger than 64 KiB).
+
+ REQ: GET /.well-known/core?rt=firmware
+
+ RES: 2.05 Content
+ </firmware/v2.1>;rt="firmware";sz=262144
+
+6. Security Considerations
+
+ This specification has the same security considerations as described
+ in Section 7 of [RFC5988]. The "/.well-known/core" resource MAY be
+ protected, e.g., using Datagram Transport Layer Security (DTLS) when
+ hosted on a CoAP server as per [COAP], Section 9.1.
+
+ Some servers might provide resource discovery services to a mix of
+ clients that are trusted to different levels. For example, a
+ lighting control system might allow any client to read state
+ variables, but only certain clients to write state (turn lights on or
+ off). Servers that have authentication and authorization features
+ SHOULD support authentication features of the underlying transport
+ protocols (HTTP or DTLS/TLS) and allow servers to return different
+ lists of links based on a client's identity and authorization. While
+ such servers might not return all links to all requesters, not
+ providing the link does not, by itself, control access to the
+ relevant resource -- a bad actor could know or guess the right URIs.
+ Servers can also lie about the resources available. If it is
+ important for a client to only get information from a known source,
+ then that source needs to be authenticated.
+
+
+
+
+
+
+
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+
+
+ Multicast requests using CoAP for the well-known link-format
+ resources could be used to perform denial of service on a constrained
+ network. A multicast request SHOULD only be accepted if the request
+ is sufficiently authenticated and secured using, e.g., IPsec or an
+ appropriate object security mechanism.
+
+ CoRE Link Format parsers should be aware that a link description may
+ be cyclical, i.e., contain a link to itself. These cyclical links
+ could be direct or indirect (i.e., through referenced link
+ resources). Care should be taken when parsing link descriptions and
+ accessing cyclical links.
+
+7. IANA Considerations
+
+7.1. Well-Known 'core' URI
+
+ This memo registers the 'core' well-known URI in the Well-Known URIs
+ registry as defined by [RFC5785].
+
+ URI suffix: core
+
+ Change controller: IETF
+
+ Specification document(s): RFC 6690
+
+ Related information: None
+
+7.2. New 'hosts' Relation Type
+
+ This memo registers the new "hosts" Web Linking relation type as per
+ [RFC5988].
+
+ Relation Name: hosts
+
+ Description: Refers to a resource hosted by the server indicated by
+ the link context.
+
+ Reference: RFC 6690
+
+ Notes: This relation is used in CoRE where links are retrieved as a
+ "/.well-known/core" resource representation and is the default
+ relation type in the CoRE Link Format.
+
+ Application Data: None
+
+
+
+
+
+
+
+Shelby Standards Track [Page 16]
+
+RFC 6690 CoRE Link Format August 2012
+
+
+7.3. New 'link-format' Internet Media Type
+
+ This memo registers the a new Internet media type for the CoRE Link
+ Format, 'application/link-format'.
+
+ Type name: application
+
+ Subtype name: link-format
+
+ Required parameters: None
+
+ Optional parameters: None
+
+ Encoding considerations: Binary data (UTF-8)
+
+ Security considerations:
+
+ Multicast requests using CoAP for the well-known link-format
+ resources could be used to perform denial of service on a constrained
+ network. A multicast request SHOULD only be accepted if the request
+ is sufficiently authenticated and secured using, e.g., IPsec or an
+ appropriate object security mechanism.
+
+ CoRE Link Format parsers should be aware that a link description may
+ be cyclical, i.e., contain a link to itself. These cyclical links
+ could be direct or indirect (i.e., through referenced link
+ resources). Care should be taken when parsing link descriptions and
+ accessing cyclical links.
+
+ Interoperability considerations: None
+
+ Published specification: RFC 6690
+
+ Applications that use this media type: CoAP server and client
+ implementations for resource discovery and HTTP applications that use
+ the link-format as a payload.
+
+ Additional information:
+
+ Magic number(s):
+
+ File extension(s): *.wlnk
+
+ Macintosh file type code(s):
+
+ Intended usage: COMMON
+
+ Restrictions on usage: None
+
+
+
+Shelby Standards Track [Page 17]
+
+RFC 6690 CoRE Link Format August 2012
+
+
+ Author: CoRE WG
+
+ Change controller: IETF
+
+7.4. Constrained RESTful Environments (CoRE) Parameters Registry
+
+ This specification establishes a new Constrained RESTful Environments
+ (CoRE) Parameters registry, which contains two new sub-registries of
+ Link Target Attribute values (defined in [RFC5988]), one for Resource
+ Type (rt=) Link Target Attribute values and the other for Interface
+ Description (if=) Link Target Attribute values. No initial entries
+ are defined by this specification for either sub-registry.
+
+ For both sub-registries, values starting with the characters "core"
+ are registered using the IETF Review registration policy [RFC5226].
+ All other values are registered using the Specification Required
+ policy, which requires review by a designated expert appointed by the
+ IESG or their delegate.
+
+ The designated expert will enforce the following requirements:
+
+ o Registration values MUST be related to the intended purpose of
+ these attributes as described in Section 3.
+
+ o Registered values MUST conform to the ABNF reg-rel-type definition
+ of Section 2, meaning that the value starts with a lowercase
+ alphabetic character, followed by a sequence of lowercase
+ alphabetic, numeric, ".", or "-" characters, and contains no white
+ space.
+
+ o It is recommended that the period "." character be used for
+ dividing name segments and that the dash "-" character be used for
+ making a segment more readable. Example Interface Description
+ values might be "core.batch" and "core.link-batch".
+
+ o URIs are reserved for free use as extension values for these
+ attributes and MUST NOT be registered.
+
+ Registration requests consist of the completed registration template
+ below, with the reference pointing to the required specification. To
+ allow for the allocation of values prior to publication, the
+ designated expert may approve registration once they are satisfied
+ that a specification will be published.
+
+ Note that Link Target Attribute Values can be registered by third
+ parties if the Designated Expert determines that an unregistered Link
+ Target Attribute Value is widely deployed and not likely to be
+ registered in a timely manner.
+
+
+
+Shelby Standards Track [Page 18]
+
+RFC 6690 CoRE Link Format August 2012
+
+
+ The registration template for both sub-registries is:
+
+ o Attribute Value:
+
+ o Description:
+
+ o Reference:
+
+ o Notes: [optional]
+
+ Registration requests should be sent to the core-parameters@ietf.org
+ mailing list, marked clearly in the subject line (e.g., "NEW RESOURCE
+ TYPE - example" to register an "example" relation type or "NEW
+ INTERFACE DESCRIPTION - example" to register an "example" Interface
+ Description).
+
+ Within at most 14 days of the request, the Designated Expert(s) will
+ either approve or deny the registration request, communicating this
+ decision to the review list and IANA. Denials should include an
+ explanation and, if applicable, suggestions as to how to make the
+ request successful.
+
+ Decisions (or lack thereof) made by the Designated Expert can be
+ first appealed to Application Area Directors (contactable using the
+ app-ads@tools.ietf.org email address or directly by looking up their
+ email addresses on http://www.iesg.org/ website) and, if the
+ appellant is not satisfied with the response, to the full IESG (using
+ the iesg@ietf.org mailing list).
+
+8. Acknowledgments
+
+ Special thanks to Peter Bigot, who has made a considerable number of
+ reviews and text contributions that greatly improved the document.
+ In particular, Peter is responsible for early improvements to the
+ ABNF descriptions and the idea for a new 'hosts' relation type.
+
+ Thanks to Mark Nottingham and Eran Hammer-Lahav for the discussions
+ and ideas that led to this document, and to Carsten Bormann, Martin
+ Thomson, Alexey Melnikov, Julian Reschke, Joel Halpern, Richard
+ Barnes, Barry Leiba, and Peter Saint-Andre for extensive comments and
+ contributions that improved the text.
+
+ Thanks to Michael Stuber, Richard Kelsey, Cullen Jennings, Guido
+ Moritz, Peter Van Der Stok, Adriano Pezzuto, Lisa Dussealt, Alexey
+ Melnikov, Gilbert Clark, Salvatore Loreto, Petri Mutka, Szymon Sasin,
+ Robert Quattlebaum, Robert Cragie, Angelo Castellani, Tom Herbst, Ed
+ Beroset, Gilman Tolle, Robby Simpson, Colin O'Flynn, and David Ryan
+ for helpful comments and discussions that have shaped the document.
+
+
+
+Shelby Standards Track [Page 19]
+
+RFC 6690 CoRE Link Format August 2012
+
+
+9. References
+
+9.1. Normative References
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
+ Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
+ Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
+
+ [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
+ 10646", STD 63, RFC 3629, November 2003.
+
+ [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
+ Resource Identifier (URI): Generic Syntax", STD 66,
+ RFC 3986, January 2005.
+
+ [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and
+ Registration Procedures", BCP 13, RFC 4288, December 2005.
+
+ [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
+ IANA Considerations Section in RFCs", BCP 26, RFC 5226,
+ May 2008.
+
+ [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
+ Specifications: ABNF", STD 68, RFC 5234, January 2008.
+
+ [RFC5646] Phillips, A. and M. Davis, "Tags for Identifying
+ Languages", BCP 47, RFC 5646, September 2009.
+
+ [RFC5987] Reschke, J., "Character Set and Language Encoding for
+ Hypertext Transfer Protocol (HTTP) Header Field
+ Parameters", RFC 5987, August 2010.
+
+ [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
+
+ [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
+ and D. Orchard, "URI Template", RFC 6570, March 2012.
+
+9.2. Informative References
+
+ [COAP] Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
+ "Constrained Application Protocol (CoAP)", Work in
+ Progress, July 2012.
+
+
+
+
+
+
+Shelby Standards Track [Page 20]
+
+RFC 6690 CoRE Link Format August 2012
+
+
+ [REST] Fielding, R., "Architectural Styles and the Design of
+ Network-based Software Architectures", 2000,
+ <http://www.ics.uci.edu/~fielding/pubs/dissertation/
+ top.htm>.
+
+ [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
+ STD 13, RFC 1034, November 1987.
+
+ [RFC1035] Mockapetris, P., "Domain names - implementation and
+ specification", STD 13, RFC 1035, November 1987.
+
+ [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
+ Extensions (MIME) Part One: Format of Internet Message
+ Bodies", RFC 2045, November 1996.
+
+ [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
+ Word Extensions: Character Sets, Languages, and
+ Continuations", RFC 2231, November 1997.
+
+ [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
+ Syndication Format", RFC 4287, December 2005.
+
+ [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
+ over Low-Power Wireless Personal Area Networks (6LoWPANs):
+ Overview, Assumptions, Problem Statement, and Goals",
+ RFC 4919, August 2007.
+
+ [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
+ Uniform Resource Identifiers (URIs)", RFC 5785,
+ April 2010.
+
+ [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
+ December 2011.
+
+ [W3C.HTML.4.01]
+ Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
+ Specification", World Wide Web Consortium
+ Recommendation REC-html401-19991224, December 1999,
+ <http://www.w3.org/TR/1999/REC-html401-19991224>.
+
+ [WADL] Hadley, M., "Web Application Description Language (WADL)",
+ 2009, <http://java.net/projects/wadl/sources/svn/content/
+ trunk/www/wadl20090202.pdf>.
+
+
+
+
+
+
+
+
+Shelby Standards Track [Page 21]
+
+RFC 6690 CoRE Link Format August 2012
+
+
+Author's Address
+
+ Zach Shelby
+ Sensinode
+ Kidekuja 2
+ Vuokatti 88600
+ Finland
+
+ Phone: +358407796297
+ EMail: zach@sensinode.com
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby Standards Track [Page 22]
+
diff --git a/rfc/rfc7252.txt b/rfc/rfc7252.txt
new file mode 100644
index 0000000..7ba5d03
--- /dev/null
+++ b/rfc/rfc7252.txt
@@ -0,0 +1,6275 @@
+
+
+
+
+
+
+Internet Engineering Task Force (IETF) Z. Shelby
+Request for Comments: 7252 ARM
+Category: Standards Track K. Hartke
+ISSN: 2070-1721 C. Bormann
+ Universitaet Bremen TZI
+ June 2014
+
+
+ The Constrained Application Protocol (CoAP)
+
+Abstract
+
+ The Constrained Application Protocol (CoAP) is a specialized web
+ transfer protocol for use with constrained nodes and constrained
+ (e.g., low-power, lossy) networks. The nodes often have 8-bit
+ microcontrollers with small amounts of ROM and RAM, while constrained
+ networks such as IPv6 over Low-Power Wireless Personal Area Networks
+ (6LoWPANs) often have high packet error rates and a typical
+ throughput of 10s of kbit/s. The protocol is designed for machine-
+ to-machine (M2M) applications such as smart energy and building
+ automation.
+
+ CoAP provides a request/response interaction model between
+ application endpoints, supports built-in discovery of services and
+ resources, and includes key concepts of the Web such as URIs and
+ Internet media types. CoAP is designed to easily interface with HTTP
+ for integration with the Web while meeting specialized requirements
+ such as multicast support, very low overhead, and simplicity for
+ constrained environments.
+
+Status of This Memo
+
+ This is an Internet Standards Track document.
+
+ This document is a product of the Internet Engineering Task Force
+ (IETF). It represents the consensus of the IETF community. It has
+ received public review and has been approved for publication by the
+ Internet Engineering Steering Group (IESG). Further information on
+ Internet Standards is available in Section 2 of RFC 5741.
+
+ Information about the current status of this document, any errata,
+ and how to provide feedback on it may be obtained at
+ http://www.rfc-editor.org/info/rfc7252.
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 1]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+Copyright Notice
+
+ Copyright (c) 2014 IETF Trust and the persons identified as the
+ document authors. All rights reserved.
+
+ This document is subject to BCP 78 and the IETF Trust's Legal
+ Provisions Relating to IETF Documents
+ (http://trustee.ietf.org/license-info) in effect on the date of
+ publication of this document. Please review these documents
+ carefully, as they describe your rights and restrictions with respect
+ to this document. Code Components extracted from this document must
+ include Simplified BSD License text as described in Section 4.e of
+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
+ 1.1. Features . . . . . . . . . . . . . . . . . . . . . . . . 5
+ 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
+ 2. Constrained Application Protocol . . . . . . . . . . . . . . 10
+ 2.1. Messaging Model . . . . . . . . . . . . . . . . . . . . . 11
+ 2.2. Request/Response Model . . . . . . . . . . . . . . . . . 12
+ 2.3. Intermediaries and Caching . . . . . . . . . . . . . . . 15
+ 2.4. Resource Discovery . . . . . . . . . . . . . . . . . . . 15
+ 3. Message Format . . . . . . . . . . . . . . . . . . . . . . . 15
+ 3.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 17
+ 3.2. Option Value Formats . . . . . . . . . . . . . . . . . . 19
+ 4. Message Transmission . . . . . . . . . . . . . . . . . . . . 20
+ 4.1. Messages and Endpoints . . . . . . . . . . . . . . . . . 20
+ 4.2. Messages Transmitted Reliably . . . . . . . . . . . . . . 21
+ 4.3. Messages Transmitted without Reliability . . . . . . . . 23
+ 4.4. Message Correlation . . . . . . . . . . . . . . . . . . . 24
+ 4.5. Message Deduplication . . . . . . . . . . . . . . . . . . 24
+ 4.6. Message Size . . . . . . . . . . . . . . . . . . . . . . 25
+ 4.7. Congestion Control . . . . . . . . . . . . . . . . . . . 26
+ 4.8. Transmission Parameters . . . . . . . . . . . . . . . . . 27
+ 4.8.1. Changing the Parameters . . . . . . . . . . . . . . . 27
+ 4.8.2. Time Values Derived from Transmission Parameters . . 28
+ 5. Request/Response Semantics . . . . . . . . . . . . . . . . . 31
+ 5.1. Requests . . . . . . . . . . . . . . . . . . . . . . . . 31
+ 5.2. Responses . . . . . . . . . . . . . . . . . . . . . . . . 31
+ 5.2.1. Piggybacked . . . . . . . . . . . . . . . . . . . . . 33
+ 5.2.2. Separate . . . . . . . . . . . . . . . . . . . . . . 33
+ 5.2.3. Non-confirmable . . . . . . . . . . . . . . . . . . . 34
+ 5.3. Request/Response Matching . . . . . . . . . . . . . . . . 34
+ 5.3.1. Token . . . . . . . . . . . . . . . . . . . . . . . . 34
+ 5.3.2. Request/Response Matching Rules . . . . . . . . . . . 35
+
+
+
+Shelby, et al. Standards Track [Page 2]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ 5.4. Options . . . . . . . . . . . . . . . . . . . . . . . . . 36
+ 5.4.1. Critical/Elective . . . . . . . . . . . . . . . . . . 37
+ 5.4.2. Proxy Unsafe or Safe-to-Forward and NoCacheKey . . . 38
+ 5.4.3. Length . . . . . . . . . . . . . . . . . . . . . . . 38
+ 5.4.4. Default Values . . . . . . . . . . . . . . . . . . . 38
+ 5.4.5. Repeatable Options . . . . . . . . . . . . . . . . . 39
+ 5.4.6. Option Numbers . . . . . . . . . . . . . . . . . . . 39
+ 5.5. Payloads and Representations . . . . . . . . . . . . . . 40
+ 5.5.1. Representation . . . . . . . . . . . . . . . . . . . 40
+ 5.5.2. Diagnostic Payload . . . . . . . . . . . . . . . . . 41
+ 5.5.3. Selected Representation . . . . . . . . . . . . . . . 41
+ 5.5.4. Content Negotiation . . . . . . . . . . . . . . . . . 41
+ 5.6. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 42
+ 5.6.1. Freshness Model . . . . . . . . . . . . . . . . . . . 43
+ 5.6.2. Validation Model . . . . . . . . . . . . . . . . . . 43
+ 5.7. Proxying . . . . . . . . . . . . . . . . . . . . . . . . 44
+ 5.7.1. Proxy Operation . . . . . . . . . . . . . . . . . . . 44
+ 5.7.2. Forward-Proxies . . . . . . . . . . . . . . . . . . . 46
+ 5.7.3. Reverse-Proxies . . . . . . . . . . . . . . . . . . . 46
+ 5.8. Method Definitions . . . . . . . . . . . . . . . . . . . 47
+ 5.8.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 47
+ 5.8.2. POST . . . . . . . . . . . . . . . . . . . . . . . . 47
+ 5.8.3. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 48
+ 5.8.4. DELETE . . . . . . . . . . . . . . . . . . . . . . . 48
+ 5.9. Response Code Definitions . . . . . . . . . . . . . . . . 48
+ 5.9.1. Success 2.xx . . . . . . . . . . . . . . . . . . . . 48
+ 5.9.2. Client Error 4.xx . . . . . . . . . . . . . . . . . . 50
+ 5.9.3. Server Error 5.xx . . . . . . . . . . . . . . . . . . 51
+ 5.10. Option Definitions . . . . . . . . . . . . . . . . . . . 52
+ 5.10.1. Uri-Host, Uri-Port, Uri-Path, and Uri-Query . . . . 53
+ 5.10.2. Proxy-Uri and Proxy-Scheme . . . . . . . . . . . . . 54
+ 5.10.3. Content-Format . . . . . . . . . . . . . . . . . . . 55
+ 5.10.4. Accept . . . . . . . . . . . . . . . . . . . . . . . 55
+ 5.10.5. Max-Age . . . . . . . . . . . . . . . . . . . . . . 55
+ 5.10.6. ETag . . . . . . . . . . . . . . . . . . . . . . . . 56
+ 5.10.7. Location-Path and Location-Query . . . . . . . . . . 57
+ 5.10.8. Conditional Request Options . . . . . . . . . . . . 57
+ 5.10.9. Size1 Option . . . . . . . . . . . . . . . . . . . . 59
+ 6. CoAP URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 59
+ 6.1. coap URI Scheme . . . . . . . . . . . . . . . . . . . . . 59
+ 6.2. coaps URI Scheme . . . . . . . . . . . . . . . . . . . . 60
+ 6.3. Normalization and Comparison Rules . . . . . . . . . . . 61
+ 6.4. Decomposing URIs into Options . . . . . . . . . . . . . . 61
+ 6.5. Composing URIs from Options . . . . . . . . . . . . . . . 62
+ 7. Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 64
+ 7.1. Service Discovery . . . . . . . . . . . . . . . . . . . . 64
+ 7.2. Resource Discovery . . . . . . . . . . . . . . . . . . . 64
+ 7.2.1. 'ct' Attribute . . . . . . . . . . . . . . . . . . . 64
+
+
+
+Shelby, et al. Standards Track [Page 3]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ 8. Multicast CoAP . . . . . . . . . . . . . . . . . . . . . . . 65
+ 8.1. Messaging Layer . . . . . . . . . . . . . . . . . . . . . 65
+ 8.2. Request/Response Layer . . . . . . . . . . . . . . . . . 66
+ 8.2.1. Caching . . . . . . . . . . . . . . . . . . . . . . . 67
+ 8.2.2. Proxying . . . . . . . . . . . . . . . . . . . . . . 67
+ 9. Securing CoAP . . . . . . . . . . . . . . . . . . . . . . . . 68
+ 9.1. DTLS-Secured CoAP . . . . . . . . . . . . . . . . . . . . 69
+ 9.1.1. Messaging Layer . . . . . . . . . . . . . . . . . . . 70
+ 9.1.2. Request/Response Layer . . . . . . . . . . . . . . . 71
+ 9.1.3. Endpoint Identity . . . . . . . . . . . . . . . . . . 71
+ 10. Cross-Protocol Proxying between CoAP and HTTP . . . . . . . . 74
+ 10.1. CoAP-HTTP Proxying . . . . . . . . . . . . . . . . . . . 75
+ 10.1.1. GET . . . . . . . . . . . . . . . . . . . . . . . . 76
+ 10.1.2. PUT . . . . . . . . . . . . . . . . . . . . . . . . 77
+ 10.1.3. DELETE . . . . . . . . . . . . . . . . . . . . . . . 77
+ 10.1.4. POST . . . . . . . . . . . . . . . . . . . . . . . . 77
+ 10.2. HTTP-CoAP Proxying . . . . . . . . . . . . . . . . . . . 77
+ 10.2.1. OPTIONS and TRACE . . . . . . . . . . . . . . . . . 78
+ 10.2.2. GET . . . . . . . . . . . . . . . . . . . . . . . . 78
+ 10.2.3. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 79
+ 10.2.4. POST . . . . . . . . . . . . . . . . . . . . . . . . 79
+ 10.2.5. PUT . . . . . . . . . . . . . . . . . . . . . . . . 79
+ 10.2.6. DELETE . . . . . . . . . . . . . . . . . . . . . . . 80
+ 10.2.7. CONNECT . . . . . . . . . . . . . . . . . . . . . . 80
+ 11. Security Considerations . . . . . . . . . . . . . . . . . . . 80
+ 11.1. Parsing the Protocol and Processing URIs . . . . . . . . 80
+ 11.2. Proxying and Caching . . . . . . . . . . . . . . . . . . 81
+ 11.3. Risk of Amplification . . . . . . . . . . . . . . . . . 81
+ 11.4. IP Address Spoofing Attacks . . . . . . . . . . . . . . 83
+ 11.5. Cross-Protocol Attacks . . . . . . . . . . . . . . . . . 84
+ 11.6. Constrained-Node Considerations . . . . . . . . . . . . 86
+ 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 86
+ 12.1. CoAP Code Registries . . . . . . . . . . . . . . . . . . 86
+ 12.1.1. Method Codes . . . . . . . . . . . . . . . . . . . . 87
+ 12.1.2. Response Codes . . . . . . . . . . . . . . . . . . . 88
+ 12.2. CoAP Option Numbers Registry . . . . . . . . . . . . . . 89
+ 12.3. CoAP Content-Formats Registry . . . . . . . . . . . . . 91
+ 12.4. URI Scheme Registration . . . . . . . . . . . . . . . . 93
+ 12.5. Secure URI Scheme Registration . . . . . . . . . . . . . 94
+ 12.6. Service Name and Port Number Registration . . . . . . . 95
+ 12.7. Secure Service Name and Port Number Registration . . . . 96
+ 12.8. Multicast Address Registration . . . . . . . . . . . . . 97
+ 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 97
+ 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 98
+ 14.1. Normative References . . . . . . . . . . . . . . . . . . 98
+ 14.2. Informative References . . . . . . . . . . . . . . . . . 100
+ Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 104
+ Appendix B. URI Examples . . . . . . . . . . . . . . . . . . . . 110
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+1. Introduction
+
+ The use of web services (web APIs) on the Internet has become
+ ubiquitous in most applications and depends on the fundamental
+ Representational State Transfer [REST] architecture of the Web.
+
+ The work on Constrained RESTful Environments (CoRE) aims at realizing
+ the REST architecture in a suitable form for the most constrained
+ nodes (e.g., 8-bit microcontrollers with limited RAM and ROM) and
+ networks (e.g., 6LoWPAN, [RFC4944]). Constrained networks such as
+ 6LoWPAN support the fragmentation of IPv6 packets into small link-
+ layer frames; however, this causes significant reduction in packet
+ delivery probability. One design goal of CoAP has been to keep
+ message overhead small, thus limiting the need for fragmentation.
+
+ One of the main goals of CoAP is to design a generic web protocol for
+ the special requirements of this constrained environment, especially
+ considering energy, building automation, and other machine-to-machine
+ (M2M) applications. The goal of CoAP is not to blindly compress HTTP
+ [RFC2616], but rather to realize a subset of REST common with HTTP
+ but optimized for M2M applications. Although CoAP could be used for
+ refashioning simple HTTP interfaces into a more compact protocol,
+ more importantly it also offers features for M2M such as built-in
+ discovery, multicast support, and asynchronous message exchanges.
+
+ This document specifies the Constrained Application Protocol (CoAP),
+ which easily translates to HTTP for integration with the existing Web
+ while meeting specialized requirements such as multicast support,
+ very low overhead, and simplicity for constrained environments and
+ M2M applications.
+
+1.1. Features
+
+ CoAP has the following main features:
+
+ o Web protocol fulfilling M2M requirements in constrained
+ environments
+
+ o UDP [RFC0768] binding with optional reliability supporting unicast
+ and multicast requests.
+
+ o Asynchronous message exchanges.
+
+ o Low header overhead and parsing complexity.
+
+ o URI and Content-type support.
+
+ o Simple proxy and caching capabilities.
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ o A stateless HTTP mapping, allowing proxies to be built providing
+ access to CoAP resources via HTTP in a uniform way or for HTTP
+ simple interfaces to be realized alternatively over CoAP.
+
+ o Security binding to Datagram Transport Layer Security (DTLS)
+ [RFC6347].
+
+1.2. Terminology
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
+ "OPTIONAL" in this document are to be interpreted as described in
+ [RFC2119] when they appear in ALL CAPS. These words may also appear
+ in this document in lowercase, absent their normative meanings.
+
+ This specification requires readers to be familiar with all the terms
+ and concepts that are discussed in [RFC2616], including "resource",
+ "representation", "cache", and "fresh". (Having been completed
+ before the updated set of HTTP RFCs, RFC 7230 to RFC 7235, became
+ available, this specification specifically references the predecessor
+ version -- RFC 2616.) In addition, this specification defines the
+ following terminology:
+
+ Endpoint
+ An entity participating in the CoAP protocol. Colloquially, an
+ endpoint lives on a "Node", although "Host" would be more
+ consistent with Internet standards usage, and is further
+ identified by transport-layer multiplexing information that can
+ include a UDP port number and a security association
+ (Section 4.1).
+
+ Sender
+ The originating endpoint of a message. When the aspect of
+ identification of the specific sender is in focus, also "source
+ endpoint".
+
+ Recipient
+ The destination endpoint of a message. When the aspect of
+ identification of the specific recipient is in focus, also
+ "destination endpoint".
+
+ Client
+ The originating endpoint of a request; the destination endpoint of
+ a response.
+
+ Server
+ The destination endpoint of a request; the originating endpoint of
+ a response.
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Origin Server
+ The server on which a given resource resides or is to be created.
+
+ Intermediary
+ A CoAP endpoint that acts both as a server and as a client towards
+ an origin server (possibly via further intermediaries). A common
+ form of an intermediary is a proxy; several classes of such
+ proxies are discussed in this specification.
+
+ Proxy
+ An intermediary that mainly is concerned with forwarding requests
+ and relaying back responses, possibly performing caching,
+ namespace translation, or protocol translation in the process. As
+ opposed to intermediaries in the general sense, proxies generally
+ do not implement specific application semantics. Based on the
+ position in the overall structure of the request forwarding, there
+ are two common forms of proxy: forward-proxy and reverse-proxy.
+ In some cases, a single endpoint might act as an origin server,
+ forward-proxy, or reverse-proxy, switching behavior based on the
+ nature of each request.
+
+ Forward-Proxy
+ An endpoint selected by a client, usually via local configuration
+ rules, to perform requests on behalf of the client, doing any
+ necessary translations. Some translations are minimal, such as
+ for proxy requests for "coap" URIs, whereas other requests might
+ require translation to and from entirely different application-
+ layer protocols.
+
+ Reverse-Proxy
+ An endpoint that stands in for one or more other server(s) and
+ satisfies requests on behalf of these, doing any necessary
+ translations. Unlike a forward-proxy, the client may not be aware
+ that it is communicating with a reverse-proxy; a reverse-proxy
+ receives requests as if it were the origin server for the target
+ resource.
+
+ CoAP-to-CoAP Proxy
+ A proxy that maps from a CoAP request to a CoAP request, i.e.,
+ uses the CoAP protocol both on the server and the client side.
+ Contrast to cross-proxy.
+
+ Cross-Proxy
+ A cross-protocol proxy, or "cross-proxy" for short, is a proxy
+ that translates between different protocols, such as a CoAP-to-
+ HTTP proxy or an HTTP-to-CoAP proxy. While this specification
+ makes very specific demands of CoAP-to-CoAP proxies, there is more
+ variation possible in cross-proxies.
+
+
+
+Shelby, et al. Standards Track [Page 7]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Confirmable Message
+ Some messages require an acknowledgement. These messages are
+ called "Confirmable". When no packets are lost, each Confirmable
+ message elicits exactly one return message of type Acknowledgement
+ or type Reset.
+
+ Non-confirmable Message
+ Some other messages do not require an acknowledgement. This is
+ particularly true for messages that are repeated regularly for
+ application requirements, such as repeated readings from a sensor.
+
+ Acknowledgement Message
+ An Acknowledgement message acknowledges that a specific
+ Confirmable message arrived. By itself, an Acknowledgement
+ message does not indicate success or failure of any request
+ encapsulated in the Confirmable message, but the Acknowledgement
+ message may also carry a Piggybacked Response (see below).
+
+ Reset Message
+ A Reset message indicates that a specific message (Confirmable or
+ Non-confirmable) was received, but some context is missing to
+ properly process it. This condition is usually caused when the
+ receiving node has rebooted and has forgotten some state that
+ would be required to interpret the message. Provoking a Reset
+ message (e.g., by sending an Empty Confirmable message) is also
+ useful as an inexpensive check of the liveness of an endpoint
+ ("CoAP ping").
+
+ Piggybacked Response
+ A piggybacked Response is included right in a CoAP Acknowledgement
+ (ACK) message that is sent to acknowledge receipt of the Request
+ for this Response (Section 5.2.1).
+
+ Separate Response
+ When a Confirmable message carrying a request is acknowledged with
+ an Empty message (e.g., because the server doesn't have the answer
+ right away), a Separate Response is sent in a separate message
+ exchange (Section 5.2.2).
+
+ Empty Message
+ A message with a Code of 0.00; neither a request nor a response.
+ An Empty message only contains the 4-byte header.
+
+
+
+
+
+
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Critical Option
+ An option that would need to be understood by the endpoint
+ ultimately receiving the message in order to properly process the
+ message (Section 5.4.1). Note that the implementation of critical
+ options is, as the name "Option" implies, generally optional:
+ unsupported critical options lead to an error response or summary
+ rejection of the message.
+
+ Elective Option
+ An option that is intended to be ignored by an endpoint that does
+ not understand it. Processing the message even without
+ understanding the option is acceptable (Section 5.4.1).
+
+ Unsafe Option
+ An option that would need to be understood by a proxy receiving
+ the message in order to safely forward the message
+ (Section 5.4.2). Not every critical option is an unsafe option.
+
+ Safe-to-Forward Option
+ An option that is intended to be safe for forwarding by a proxy
+ that does not understand it. Forwarding the message even without
+ understanding the option is acceptable (Section 5.4.2).
+
+ Resource Discovery
+ The process where a CoAP client queries a server for its list of
+ hosted resources (i.e., links as defined in Section 7).
+
+ Content-Format
+ The combination of an Internet media type, potentially with
+ specific parameters given, and a content-coding (which is often
+ the identity content-coding), identified by a numeric identifier
+ defined by the "CoAP Content-Formats" registry. When the focus is
+ less on the numeric identifier than on the combination of these
+ characteristics of a resource representation, this is also called
+ "representation format".
+
+ Additional terminology for constrained nodes and constrained-node
+ networks can be found in [RFC7228].
+
+ In this specification, the term "byte" is used in its now customary
+ sense as a synonym for "octet".
+
+ All multi-byte integers in this protocol are interpreted in network
+ byte order.
+
+ Where arithmetic is used, this specification uses the notation
+ familiar from the programming language C, except that the operator
+ "**" stands for exponentiation.
+
+
+
+Shelby, et al. Standards Track [Page 9]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+2. Constrained Application Protocol
+
+ The interaction model of CoAP is similar to the client/server model
+ of HTTP. However, machine-to-machine interactions typically result
+ in a CoAP implementation acting in both client and server roles. A
+ CoAP request is equivalent to that of HTTP and is sent by a client to
+ request an action (using a Method Code) on a resource (identified by
+ a URI) on a server. The server then sends a response with a Response
+ Code; this response may include a resource representation.
+
+ Unlike HTTP, CoAP deals with these interchanges asynchronously over a
+ datagram-oriented transport such as UDP. This is done logically
+ using a layer of messages that supports optional reliability (with
+ exponential back-off). CoAP defines four types of messages:
+ Confirmable, Non-confirmable, Acknowledgement, Reset. Method Codes
+ and Response Codes included in some of these messages make them carry
+ requests or responses. The basic exchanges of the four types of
+ messages are somewhat orthogonal to the request/response
+ interactions; requests can be carried in Confirmable and Non-
+ confirmable messages, and responses can be carried in these as well
+ as piggybacked in Acknowledgement messages.
+
+ One could think of CoAP logically as using a two-layer approach, a
+ CoAP messaging layer used to deal with UDP and the asynchronous
+ nature of the interactions, and the request/response interactions
+ using Method and Response Codes (see Figure 1). CoAP is however a
+ single protocol, with messaging and request/response as just features
+ of the CoAP header.
+
+ +----------------------+
+ | Application |
+ +----------------------+
+ +----------------------+ \
+ | Requests/Responses | |
+ |----------------------| | CoAP
+ | Messages | |
+ +----------------------+ /
+ +----------------------+
+ | UDP |
+ +----------------------+
+
+ Figure 1: Abstract Layering of CoAP
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 10]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+2.1. Messaging Model
+
+ The CoAP messaging model is based on the exchange of messages over
+ UDP between endpoints.
+
+ CoAP uses a short fixed-length binary header (4 bytes) that may be
+ followed by compact binary options and a payload. This message
+ format is shared by requests and responses. The CoAP message format
+ is specified in Section 3. Each message contains a Message ID used
+ to detect duplicates and for optional reliability. (The Message ID
+ is compact; its 16-bit size enables up to about 250 messages per
+ second from one endpoint to another with default protocol
+ parameters.)
+
+ Reliability is provided by marking a message as Confirmable (CON). A
+ Confirmable message is retransmitted using a default timeout and
+ exponential back-off between retransmissions, until the recipient
+ sends an Acknowledgement message (ACK) with the same Message ID (in
+ this example, 0x7d34) from the corresponding endpoint; see Figure 2.
+ When a recipient is not at all able to process a Confirmable message
+ (i.e., not even able to provide a suitable error response), it
+ replies with a Reset message (RST) instead of an Acknowledgement
+ (ACK).
+
+ Client Server
+ | |
+ | CON [0x7d34] |
+ +----------------->|
+ | |
+ | ACK [0x7d34] |
+ |<-----------------+
+ | |
+
+ Figure 2: Reliable Message Transmission
+
+ A message that does not require reliable transmission (for example,
+ each single measurement out of a stream of sensor data) can be sent
+ as a Non-confirmable message (NON). These are not acknowledged, but
+ still have a Message ID for duplicate detection (in this example,
+ 0x01a0); see Figure 3. When a recipient is not able to process a
+ Non-confirmable message, it may reply with a Reset message (RST).
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 11]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Client Server
+ | |
+ | NON [0x01a0] |
+ +----------------->|
+ | |
+
+ Figure 3: Unreliable Message Transmission
+
+ See Section 4 for details of CoAP messages.
+
+ As CoAP runs over UDP, it also supports the use of multicast IP
+ destination addresses, enabling multicast CoAP requests. Section 8
+ discusses the proper use of CoAP messages with multicast addresses
+ and precautions for avoiding response congestion.
+
+ Several security modes are defined for CoAP in Section 9 ranging from
+ no security to certificate-based security. This document specifies a
+ binding to DTLS for securing the protocol; the use of IPsec with CoAP
+ is discussed in [IPsec-CoAP].
+
+2.2. Request/Response Model
+
+ CoAP request and response semantics are carried in CoAP messages,
+ which include either a Method Code or Response Code, respectively.
+ Optional (or default) request and response information, such as the
+ URI and payload media type are carried as CoAP options. A Token is
+ used to match responses to requests independently from the underlying
+ messages (Section 5.3). (Note that the Token is a concept separate
+ from the Message ID.)
+
+ A request is carried in a Confirmable (CON) or Non-confirmable (NON)
+ message, and, if immediately available, the response to a request
+ carried in a Confirmable message is carried in the resulting
+ Acknowledgement (ACK) message. This is called a piggybacked
+ response, detailed in Section 5.2.1. (There is no need for
+ separately acknowledging a piggybacked response, as the client will
+ retransmit the request if the Acknowledgement message carrying the
+ piggybacked response is lost.) Two examples for a basic GET request
+ with piggybacked response are shown in Figure 4, one successful, one
+ resulting in a 4.04 (Not Found) response.
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 12]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Client Server Client Server
+ | | | |
+ | CON [0xbc90] | | CON [0xbc91] |
+ | GET /temperature | | GET /temperature |
+ | (Token 0x71) | | (Token 0x72) |
+ +----------------->| +----------------->|
+ | | | |
+ | ACK [0xbc90] | | ACK [0xbc91] |
+ | 2.05 Content | | 4.04 Not Found |
+ | (Token 0x71) | | (Token 0x72) |
+ | "22.5 C" | | "Not found" |
+ |<-----------------+ |<-----------------+
+ | | | |
+
+ Figure 4: Two GET Requests with Piggybacked Responses
+
+ If the server is not able to respond immediately to a request carried
+ in a Confirmable message, it simply responds with an Empty
+ Acknowledgement message so that the client can stop retransmitting
+ the request. When the response is ready, the server sends it in a
+ new Confirmable message (which then in turn needs to be acknowledged
+ by the client). This is called a "separate response", as illustrated
+ in Figure 5 and described in more detail in Section 5.2.2.
+
+ Client Server
+ | |
+ | CON [0x7a10] |
+ | GET /temperature |
+ | (Token 0x73) |
+ +----------------->|
+ | |
+ | ACK [0x7a10] |
+ |<-----------------+
+ | |
+ ... Time Passes ...
+ | |
+ | CON [0x23bb] |
+ | 2.05 Content |
+ | (Token 0x73) |
+ | "22.5 C" |
+ |<-----------------+
+ | |
+ | ACK [0x23bb] |
+ +----------------->|
+ | |
+
+ Figure 5: A GET Request with a Separate Response
+
+
+
+
+Shelby, et al. Standards Track [Page 13]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ If a request is sent in a Non-confirmable message, then the response
+ is sent using a new Non-confirmable message, although the server may
+ instead send a Confirmable message. This type of exchange is
+ illustrated in Figure 6.
+
+ Client Server
+ | |
+ | NON [0x7a11] |
+ | GET /temperature |
+ | (Token 0x74) |
+ +----------------->|
+ | |
+ | NON [0x23bc] |
+ | 2.05 Content |
+ | (Token 0x74) |
+ | "22.5 C" |
+ |<-----------------+
+ | |
+
+ Figure 6: A Request and a Response Carried in Non-confirmable
+ Messages
+
+ CoAP makes use of GET, PUT, POST, and DELETE methods in a similar
+ manner to HTTP, with the semantics specified in Section 5.8. (Note
+ that the detailed semantics of CoAP methods are "almost, but not
+ entirely unlike" [HHGTTG] those of HTTP methods: intuition taken from
+ HTTP experience generally does apply well, but there are enough
+ differences that make it worthwhile to actually read the present
+ specification.)
+
+ Methods beyond the basic four can be added to CoAP in separate
+ specifications. New methods do not necessarily have to use requests
+ and responses in pairs. Even for existing methods, a single request
+ may yield multiple responses, e.g., for a multicast request
+ (Section 8) or with the Observe option [OBSERVE].
+
+ URI support in a server is simplified as the client already parses
+ the URI and splits it into host, port, path, and query components,
+ making use of default values for efficiency. Response Codes relate
+ to a small subset of HTTP status codes with a few CoAP-specific codes
+ added, as defined in Section 5.9.
+
+
+
+
+
+
+
+
+
+
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+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+2.3. Intermediaries and Caching
+
+ The protocol supports the caching of responses in order to
+ efficiently fulfill requests. Simple caching is enabled using
+ freshness and validity information carried with CoAP responses. A
+ cache could be located in an endpoint or an intermediary. Caching
+ functionality is specified in Section 5.6.
+
+ Proxying is useful in constrained networks for several reasons,
+ including to limit network traffic, to improve performance, to access
+ resources of sleeping devices, and for security reasons. The
+ proxying of requests on behalf of another CoAP endpoint is supported
+ in the protocol. When using a proxy, the URI of the resource to
+ request is included in the request, while the destination IP address
+ is set to the address of the proxy. See Section 5.7 for more
+ information on proxy functionality.
+
+ As CoAP was designed according to the REST architecture [REST], and
+ thus exhibits functionality similar to that of the HTTP protocol, it
+ is quite straightforward to map from CoAP to HTTP and from HTTP to
+ CoAP. Such a mapping may be used to realize an HTTP REST interface
+ using CoAP or to convert between HTTP and CoAP. This conversion can
+ be carried out by a cross-protocol proxy ("cross-proxy"), which
+ converts the Method or Response Code, media type, and options to the
+ corresponding HTTP feature. Section 10 provides more detail about
+ HTTP mapping.
+
+2.4. Resource Discovery
+
+ Resource discovery is important for machine-to-machine interactions
+ and is supported using the CoRE Link Format [RFC6690] as discussed in
+ Section 7.
+
+3. Message Format
+
+ CoAP is based on the exchange of compact messages that, by default,
+ are transported over UDP (i.e., each CoAP message occupies the data
+ section of one UDP datagram). CoAP may also be used over Datagram
+ Transport Layer Security (DTLS) (see Section 9.1). It could also be
+ used over other transports such as SMS, TCP, or SCTP, the
+ specification of which is out of this document's scope. (UDP-lite
+ [RFC3828] and UDP zero checksum [RFC6936] are not supported by CoAP.)
+
+ CoAP messages are encoded in a simple binary format. The message
+ format starts with a fixed-size 4-byte header. This is followed by a
+ variable-length Token value, which can be between 0 and 8 bytes long.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 15]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Following the Token value comes a sequence of zero or more CoAP
+ Options in Type-Length-Value (TLV) format, optionally followed by a
+ payload that takes up the rest of the datagram.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |Ver| T | TKL | Code | Message ID |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Token (if any, TKL bytes) ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Options (if any) ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |1 1 1 1 1 1 1 1| Payload (if any) ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Figure 7: Message Format
+
+ The fields in the header are defined as follows:
+
+ Version (Ver): 2-bit unsigned integer. Indicates the CoAP version
+ number. Implementations of this specification MUST set this field
+ to 1 (01 binary). Other values are reserved for future versions.
+ Messages with unknown version numbers MUST be silently ignored.
+
+ Type (T): 2-bit unsigned integer. Indicates if this message is of
+ type Confirmable (0), Non-confirmable (1), Acknowledgement (2), or
+ Reset (3). The semantics of these message types are defined in
+ Section 4.
+
+ Token Length (TKL): 4-bit unsigned integer. Indicates the length of
+ the variable-length Token field (0-8 bytes). Lengths 9-15 are
+ reserved, MUST NOT be sent, and MUST be processed as a message
+ format error.
+
+ Code: 8-bit unsigned integer, split into a 3-bit class (most
+ significant bits) and a 5-bit detail (least significant bits),
+ documented as "c.dd" where "c" is a digit from 0 to 7 for the
+ 3-bit subfield and "dd" are two digits from 00 to 31 for the 5-bit
+ subfield. The class can indicate a request (0), a success
+ response (2), a client error response (4), or a server error
+ response (5). (All other class values are reserved.) As a
+ special case, Code 0.00 indicates an Empty message. In case of a
+ request, the Code field indicates the Request Method; in case of a
+ response, a Response Code. Possible values are maintained in the
+ CoAP Code Registries (Section 12.1). The semantics of requests
+ and responses are defined in Section 5.
+
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Message ID: 16-bit unsigned integer in network byte order. Used to
+ detect message duplication and to match messages of type
+ Acknowledgement/Reset to messages of type Confirmable/Non-
+ confirmable. The rules for generating a Message ID and matching
+ messages are defined in Section 4.
+
+ The header is followed by the Token value, which may be 0 to 8 bytes,
+ as given by the Token Length field. The Token value is used to
+ correlate requests and responses. The rules for generating a Token
+ and correlating requests and responses are defined in Section 5.3.1.
+
+ Header and Token are followed by zero or more Options (Section 3.1).
+ An Option can be followed by the end of the message, by another
+ Option, or by the Payload Marker and the payload.
+
+ Following the header, token, and options, if any, comes the optional
+ payload. If present and of non-zero length, it is prefixed by a
+ fixed, one-byte Payload Marker (0xFF), which indicates the end of
+ options and the start of the payload. The payload data extends from
+ after the marker to the end of the UDP datagram, i.e., the Payload
+ Length is calculated from the datagram size. The absence of the
+ Payload Marker denotes a zero-length payload. The presence of a
+ marker followed by a zero-length payload MUST be processed as a
+ message format error.
+
+ Implementation Note: The byte value 0xFF may also occur within an
+ option length or value, so simple byte-wise scanning for 0xFF is
+ not a viable technique for finding the payload marker. The byte
+ 0xFF has the meaning of a payload marker only where the beginning
+ of another option could occur.
+
+3.1. Option Format
+
+ CoAP defines a number of options that can be included in a message.
+ Each option instance in a message specifies the Option Number of the
+ defined CoAP option, the length of the Option Value, and the Option
+ Value itself.
+
+ Instead of specifying the Option Number directly, the instances MUST
+ appear in order of their Option Numbers and a delta encoding is used
+ between them: the Option Number for each instance is calculated as
+ the sum of its delta and the Option Number of the preceding instance
+ in the message. For the first instance in a message, a preceding
+ option instance with Option Number zero is assumed. Multiple
+ instances of the same option can be included by using a delta of
+ zero.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 17]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Option Numbers are maintained in the "CoAP Option Numbers" registry
+ (Section 12.2). See Section 5.4 for the semantics of the options
+ defined in this document.
+
+ 0 1 2 3 4 5 6 7
+ +---------------+---------------+
+ | | |
+ | Option Delta | Option Length | 1 byte
+ | | |
+ +---------------+---------------+
+ \ \
+ / Option Delta / 0-2 bytes
+ \ (extended) \
+ +-------------------------------+
+ \ \
+ / Option Length / 0-2 bytes
+ \ (extended) \
+ +-------------------------------+
+ \ \
+ / /
+ \ \
+ / Option Value / 0 or more bytes
+ \ \
+ / /
+ \ \
+ +-------------------------------+
+
+ Figure 8: Option Format
+
+ The fields in an option are defined as follows:
+
+ Option Delta: 4-bit unsigned integer. A value between 0 and 12
+ indicates the Option Delta. Three values are reserved for special
+ constructs:
+
+ 13: An 8-bit unsigned integer follows the initial byte and
+ indicates the Option Delta minus 13.
+
+ 14: A 16-bit unsigned integer in network byte order follows the
+ initial byte and indicates the Option Delta minus 269.
+
+ 15: Reserved for the Payload Marker. If the field is set to this
+ value but the entire byte is not the payload marker, this MUST
+ be processed as a message format error.
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 18]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ The resulting Option Delta is used as the difference between the
+ Option Number of this option and that of the previous option (or
+ zero for the first option). In other words, the Option Number is
+ calculated by simply summing the Option Delta values of this and
+ all previous options before it.
+
+ Option Length: 4-bit unsigned integer. A value between 0 and 12
+ indicates the length of the Option Value, in bytes. Three values
+ are reserved for special constructs:
+
+ 13: An 8-bit unsigned integer precedes the Option Value and
+ indicates the Option Length minus 13.
+
+ 14: A 16-bit unsigned integer in network byte order precedes the
+ Option Value and indicates the Option Length minus 269.
+
+ 15: Reserved for future use. If the field is set to this value,
+ it MUST be processed as a message format error.
+
+ Value: A sequence of exactly Option Length bytes. The length and
+ format of the Option Value depend on the respective option, which
+ MAY define variable-length values. See Section 3.2 for the
+ formats used in this document; options defined in other documents
+ MAY make use of other option value formats.
+
+3.2. Option Value Formats
+
+ The options defined in this document make use of the following option
+ value formats.
+
+ empty: A zero-length sequence of bytes.
+
+ opaque: An opaque sequence of bytes.
+
+ uint: A non-negative integer that is represented in network byte
+ order using the number of bytes given by the Option Length
+ field.
+
+ An option definition may specify a range of permissible
+ numbers of bytes; if it has a choice, a sender SHOULD
+ represent the integer with as few bytes as possible, i.e.,
+ without leading zero bytes. For example, the number 0 is
+ represented with an empty option value (a zero-length
+ sequence of bytes) and the number 1 by a single byte with
+ the numerical value of 1 (bit combination 00000001 in most
+ significant bit first notation). A recipient MUST be
+ prepared to process values with leading zero bytes.
+
+
+
+
+Shelby, et al. Standards Track [Page 19]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Implementation Note: The exceptional behavior permitted
+ for the sender is intended for highly constrained,
+ templated implementations (e.g., hardware
+ implementations) that use fixed-size options in the
+ templates.
+
+ string: A Unicode string that is encoded using UTF-8 [RFC3629] in
+ Net-Unicode form [RFC5198].
+
+ Note that here, and in all other places where UTF-8
+ encoding is used in the CoAP protocol, the intention is
+ that the encoded strings can be directly used and compared
+ as opaque byte strings by CoAP protocol implementations.
+ There is no expectation and no need to perform
+ normalization within a CoAP implementation (except where
+ Unicode strings that are not known to be normalized are
+ imported from sources outside the CoAP protocol). Note
+ also that ASCII strings (that do not make use of special
+ control characters) are always valid UTF-8 Net-Unicode
+ strings.
+
+4. Message Transmission
+
+ CoAP messages are exchanged asynchronously between CoAP endpoints.
+ They are used to transport CoAP requests and responses, the semantics
+ of which are defined in Section 5.
+
+ As CoAP is bound to unreliable transports such as UDP, CoAP messages
+ may arrive out of order, appear duplicated, or go missing without
+ notice. For this reason, CoAP implements a lightweight reliability
+ mechanism, without trying to re-create the full feature set of a
+ transport like TCP. It has the following features:
+
+ o Simple stop-and-wait retransmission reliability with exponential
+ back-off for Confirmable messages.
+
+ o Duplicate detection for both Confirmable and Non-confirmable
+ messages.
+
+4.1. Messages and Endpoints
+
+ A CoAP endpoint is the source or destination of a CoAP message. The
+ specific definition of an endpoint depends on the transport being
+ used for CoAP. For the transports defined in this specification, the
+ endpoint is identified depending on the security mode used (see
+ Section 9): With no security, the endpoint is solely identified by an
+ IP address and a UDP port number. With other security modes, the
+ endpoint is identified as defined by the security mode.
+
+
+
+Shelby, et al. Standards Track [Page 20]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ There are different types of messages. The type of a message is
+ specified by the Type field of the CoAP Header.
+
+ Separate from the message type, a message may carry a request, a
+ response, or be Empty. This is signaled by the Request/Response Code
+ field in the CoAP Header and is relevant to the request/response
+ model. Possible values for the field are maintained in the CoAP Code
+ Registries (Section 12.1).
+
+ An Empty message has the Code field set to 0.00. The Token Length
+ field MUST be set to 0 and bytes of data MUST NOT be present after
+ the Message ID field. If there are any bytes, they MUST be processed
+ as a message format error.
+
+4.2. Messages Transmitted Reliably
+
+ The reliable transmission of a message is initiated by marking the
+ message as Confirmable in the CoAP header. A Confirmable message
+ always carries either a request or response, unless it is used only
+ to elicit a Reset message, in which case it is Empty. A recipient
+ MUST either (a) acknowledge a Confirmable message with an
+ Acknowledgement message or (b) reject the message if the recipient
+ lacks context to process the message properly, including situations
+ where the message is Empty, uses a code with a reserved class (1, 6,
+ or 7), or has a message format error. Rejecting a Confirmable
+ message is effected by sending a matching Reset message and otherwise
+ ignoring it. The Acknowledgement message MUST echo the Message ID of
+ the Confirmable message and MUST carry a response or be Empty (see
+ Sections 5.2.1 and 5.2.2). The Reset message MUST echo the Message
+ ID of the Confirmable message and MUST be Empty. Rejecting an
+ Acknowledgement or Reset message (including the case where the
+ Acknowledgement carries a request or a code with a reserved class, or
+ the Reset message is not Empty) is effected by silently ignoring it.
+ More generally, recipients of Acknowledgement and Reset messages MUST
+ NOT respond with either Acknowledgement or Reset messages.
+
+ The sender retransmits the Confirmable message at exponentially
+ increasing intervals, until it receives an acknowledgement (or Reset
+ message) or runs out of attempts.
+
+ Retransmission is controlled by two things that a CoAP endpoint MUST
+ keep track of for each Confirmable message it sends while waiting for
+ an acknowledgement (or reset): a timeout and a retransmission
+ counter. For a new Confirmable message, the initial timeout is set
+ to a random duration (often not an integral number of seconds)
+ between ACK_TIMEOUT and (ACK_TIMEOUT * ACK_RANDOM_FACTOR) (see
+ Section 4.8), and the retransmission counter is set to 0. When the
+ timeout is triggered and the retransmission counter is less than
+
+
+
+Shelby, et al. Standards Track [Page 21]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ MAX_RETRANSMIT, the message is retransmitted, the retransmission
+ counter is incremented, and the timeout is doubled. If the
+ retransmission counter reaches MAX_RETRANSMIT on a timeout, or if the
+ endpoint receives a Reset message, then the attempt to transmit the
+ message is canceled and the application process informed of failure.
+ On the other hand, if the endpoint receives an acknowledgement in
+ time, transmission is considered successful.
+
+ This specification makes no strong requirements on the accuracy of
+ the clocks used to implement the above binary exponential back-off
+ algorithm. In particular, an endpoint may be late for a specific
+ retransmission due to its sleep schedule and may catch up on the next
+ one. However, the minimum spacing before another retransmission is
+ ACK_TIMEOUT, and the entire sequence of (re-)transmissions MUST stay
+ in the envelope of MAX_TRANSMIT_SPAN (see Section 4.8.2), even if
+ that means a sender may miss an opportunity to transmit.
+
+ A CoAP endpoint that sent a Confirmable message MAY give up in
+ attempting to obtain an ACK even before the MAX_RETRANSMIT counter
+ value is reached. For example, the application has canceled the
+ request as it no longer needs a response, or there is some other
+ indication that the CON message did arrive. In particular, a CoAP
+ request message may have elicited a separate response, in which case
+ it is clear to the requester that only the ACK was lost and a
+ retransmission of the request would serve no purpose. However, a
+ responder MUST NOT in turn rely on this cross-layer behavior from a
+ requester, i.e., it MUST retain the state to create the ACK for the
+ request, if needed, even if a Confirmable response was already
+ acknowledged by the requester.
+
+ Another reason for giving up retransmission MAY be the receipt of
+ ICMP errors. If it is desired to take account of ICMP errors, to
+ mitigate potential spoofing attacks, implementations SHOULD take care
+ to check the information about the original datagram in the ICMP
+ message, including port numbers and CoAP header information such as
+ message type and code, Message ID, and Token; if this is not possible
+ due to limitations of the UDP service API, ICMP errors SHOULD be
+ ignored. Packet Too Big errors [RFC4443] ("fragmentation needed and
+ DF set" for IPv4 [RFC0792]) cannot properly occur and SHOULD be
+ ignored if the implementation note in Section 4.6 is followed;
+ otherwise, they SHOULD feed into a path MTU discovery algorithm
+ [RFC4821]. Source Quench and Time Exceeded ICMP messages SHOULD be
+ ignored. Host, network, port, or protocol unreachable errors or
+ parameter problem errors MAY, after appropriate vetting, be used to
+ inform the application of a failure in sending.
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 22]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+4.3. Messages Transmitted without Reliability
+
+ Some messages do not require an acknowledgement. This is
+ particularly true for messages that are repeated regularly for
+ application requirements, such as repeated readings from a sensor
+ where eventual success is sufficient.
+
+ As a more lightweight alternative, a message can be transmitted less
+ reliably by marking the message as Non-confirmable. A Non-
+ confirmable message always carries either a request or response and
+ MUST NOT be Empty. A Non-confirmable message MUST NOT be
+ acknowledged by the recipient. A recipient MUST reject the message
+ if it lacks context to process the message properly, including the
+ case where the message is Empty, uses a code with a reserved class
+ (1, 6, or 7), or has a message format error. Rejecting a Non-
+ confirmable message MAY involve sending a matching Reset message, and
+ apart from the Reset message the rejected message MUST be silently
+ ignored.
+
+ At the CoAP level, there is no way for the sender to detect if a Non-
+ confirmable message was received or not. A sender MAY choose to
+ transmit multiple copies of a Non-confirmable message within
+ MAX_TRANSMIT_SPAN (limited by the provisions of Section 4.7, in
+ particular, by PROBING_RATE if no response is received), or the
+ network may duplicate the message in transit. To enable the receiver
+ to act only once on the message, Non-confirmable messages specify a
+ Message ID as well. (This Message ID is drawn from the same number
+ space as the Message IDs for Confirmable messages.)
+
+ Summarizing Sections 4.2 and 4.3, the four message types can be used
+ as in Table 1. "*" means that the combination is not used in normal
+ operation but only to elicit a Reset message ("CoAP ping").
+
+ +----------+-----+-----+-----+-----+
+ | | CON | NON | ACK | RST |
+ +----------+-----+-----+-----+-----+
+ | Request | X | X | - | - |
+ | Response | X | X | X | - |
+ | Empty | * | - | X | X |
+ +----------+-----+-----+-----+-----+
+
+ Table 1: Usage of Message Types
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 23]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+4.4. Message Correlation
+
+ An Acknowledgement or Reset message is related to a Confirmable
+ message or Non-confirmable message by means of a Message ID along
+ with additional address information of the corresponding endpoint.
+ The Message ID is a 16-bit unsigned integer that is generated by the
+ sender of a Confirmable or Non-confirmable message and included in
+ the CoAP header. The Message ID MUST be echoed in the
+ Acknowledgement or Reset message by the recipient.
+
+ The same Message ID MUST NOT be reused (in communicating with the
+ same endpoint) within the EXCHANGE_LIFETIME (Section 4.8.2).
+
+ Implementation Note: Several implementation strategies can be
+ employed for generating Message IDs. In the simplest case, a CoAP
+ endpoint generates Message IDs by keeping a single Message ID
+ variable, which is changed each time a new Confirmable or Non-
+ confirmable message is sent, regardless of the destination address
+ or port. Endpoints dealing with large numbers of transactions
+ could keep multiple Message ID variables, for example, per prefix
+ or destination address. (Note that some receiving endpoints may
+ not be able to distinguish unicast and multicast packets addressed
+ to it, so endpoints generating Message IDs need to make sure these
+ do not overlap.) It is strongly recommended that the initial
+ value of the variable (e.g., on startup) be randomized, in order
+ to make successful off-path attacks on the protocol less likely.
+
+ For an Acknowledgement or Reset message to match a Confirmable or
+ Non-confirmable message, the Message ID and source endpoint of the
+ Acknowledgement or Reset message MUST match the Message ID and
+ destination endpoint of the Confirmable or Non-confirmable message.
+
+4.5. Message Deduplication
+
+ A recipient might receive the same Confirmable message (as indicated
+ by the Message ID and source endpoint) multiple times within the
+ EXCHANGE_LIFETIME (Section 4.8.2), for example, when its
+ Acknowledgement went missing or didn't reach the original sender
+ before the first timeout. The recipient SHOULD acknowledge each
+ duplicate copy of a Confirmable message using the same
+ Acknowledgement or Reset message but SHOULD process any request or
+ response in the message only once. This rule MAY be relaxed in case
+ the Confirmable message transports a request that is idempotent (see
+ Section 5.1) or can be handled in an idempotent fashion. Examples
+ for relaxed message deduplication:
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 24]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ o A server might relax the requirement to answer all retransmissions
+ of an idempotent request with the same response (Section 4.2), so
+ that it does not have to maintain state for Message IDs. For
+ example, an implementation might want to process duplicate
+ transmissions of a GET, PUT, or DELETE request as separate
+ requests if the effort incurred by duplicate processing is less
+ expensive than keeping track of previous responses would be.
+
+ o A constrained server might even want to relax this requirement for
+ certain non-idempotent requests if the application semantics make
+ this trade-off favorable. For example, if the result of a POST
+ request is just the creation of some short-lived state at the
+ server, it may be less expensive to incur this effort multiple
+ times for a request than keeping track of whether a previous
+ transmission of the same request already was processed.
+
+ A recipient might receive the same Non-confirmable message (as
+ indicated by the Message ID and source endpoint) multiple times
+ within NON_LIFETIME (Section 4.8.2). As a general rule that MAY be
+ relaxed based on the specific semantics of a message, the recipient
+ SHOULD silently ignore any duplicated Non-confirmable message and
+ SHOULD process any request or response in the message only once.
+
+4.6. Message Size
+
+ While specific link layers make it beneficial to keep CoAP messages
+ small enough to fit into their link-layer packets (see Section 1),
+ this is a matter of implementation quality. The CoAP specification
+ itself provides only an upper bound to the message size. Messages
+ larger than an IP packet result in undesirable packet fragmentation.
+ A CoAP message, appropriately encapsulated, SHOULD fit within a
+ single IP packet (i.e., avoid IP fragmentation) and (by fitting into
+ one UDP payload) obviously needs to fit within a single IP datagram.
+ If the Path MTU is not known for a destination, an IP MTU of 1280
+ bytes SHOULD be assumed; if nothing is known about the size of the
+ headers, good upper bounds are 1152 bytes for the message size and
+ 1024 bytes for the payload size.
+
+ Implementation Note: CoAP's choice of message size parameters works
+ well with IPv6 and with most of today's IPv4 paths. (However,
+ with IPv4, it is harder to absolutely ensure that there is no IP
+ fragmentation. If IPv4 support on unusual networks is a
+ consideration, implementations may want to limit themselves to
+ more conservative IPv4 datagram sizes such as 576 bytes; per
+ [RFC0791], the absolute minimum value of the IP MTU for IPv4 is as
+ low as 68 bytes, which would leave only 40 bytes minus security
+ overhead for a UDP payload. Implementations extremely focused on
+ this problem set might also set the IPv4 DF bit and perform some
+
+
+
+Shelby, et al. Standards Track [Page 25]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ form of path MTU discovery [RFC4821]; this should generally be
+ unnecessary in realistic use cases for CoAP, however.) A more
+ important kind of fragmentation in many constrained networks is
+ that on the adaptation layer (e.g., 6LoWPAN L2 packets are limited
+ to 127 bytes including various overheads); this may motivate
+ implementations to be frugal in their packet sizes and to move to
+ block-wise transfers [BLOCK] when approaching three-digit message
+ sizes.
+
+ Message sizes are also of considerable importance to
+ implementations on constrained nodes. Many implementations will
+ need to allocate a buffer for incoming messages. If an
+ implementation is too constrained to allow for allocating the
+ above-mentioned upper bound, it could apply the following
+ implementation strategy for messages not using DTLS security:
+ Implementations receiving a datagram into a buffer that is too
+ small are usually able to determine if the trailing portion of a
+ datagram was discarded and to retrieve the initial portion. So,
+ at least the CoAP header and options, if not all of the payload,
+ are likely to fit within the buffer. A server can thus fully
+ interpret a request and return a 4.13 (Request Entity Too Large;
+ see Section 5.9.2.9) Response Code if the payload was truncated.
+ A client sending an idempotent request and receiving a response
+ larger than would fit in the buffer can repeat the request with a
+ suitable value for the Block Option [BLOCK].
+
+4.7. Congestion Control
+
+ Basic congestion control for CoAP is provided by the exponential
+ back-off mechanism in Section 4.2.
+
+ In order not to cause congestion, clients (including proxies) MUST
+ strictly limit the number of simultaneous outstanding interactions
+ that they maintain to a given server (including proxies) to NSTART.
+ An outstanding interaction is either a CON for which an ACK has not
+ yet been received but is still expected (message layer) or a request
+ for which neither a response nor an Acknowledgment message has yet
+ been received but is still expected (which may both occur at the same
+ time, counting as one outstanding interaction). The default value of
+ NSTART for this specification is 1.
+
+ Further congestion control optimizations and considerations are
+ expected in the future, may for example provide automatic
+ initialization of the CoAP transmission parameters defined in
+ Section 4.8, and thus may allow a value for NSTART greater than one.
+
+ After EXCHANGE_LIFETIME, a client stops expecting a response to a
+ Confirmable request for which no acknowledgment message was received.
+
+
+
+Shelby, et al. Standards Track [Page 26]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ The specific algorithm by which a client stops to "expect" a response
+ to a Confirmable request that was acknowledged, or to a Non-
+ confirmable request, is not defined. Unless this is modified by
+ additional congestion control optimizations, it MUST be chosen in
+ such a way that an endpoint does not exceed an average data rate of
+ PROBING_RATE in sending to another endpoint that does not respond.
+
+ Note: CoAP places the onus of congestion control mostly on the
+ clients. However, clients may malfunction or actually be
+ attackers, e.g., to perform amplification attacks (Section 11.3).
+ To limit the damage (to the network and to its own energy
+ resources), a server SHOULD implement some rate limiting for its
+ response transmission based on reasonable assumptions about
+ application requirements. This is most helpful if the rate limit
+ can be made effective for the misbehaving endpoints, only.
+
+4.8. Transmission Parameters
+
+ Message transmission is controlled by the following parameters:
+
+ +-------------------+---------------+
+ | name | default value |
+ +-------------------+---------------+
+ | ACK_TIMEOUT | 2 seconds |
+ | ACK_RANDOM_FACTOR | 1.5 |
+ | MAX_RETRANSMIT | 4 |
+ | NSTART | 1 |
+ | DEFAULT_LEISURE | 5 seconds |
+ | PROBING_RATE | 1 byte/second |
+ +-------------------+---------------+
+
+ Table 2: CoAP Protocol Parameters
+
+4.8.1. Changing the Parameters
+
+ The values for ACK_TIMEOUT, ACK_RANDOM_FACTOR, MAX_RETRANSMIT,
+ NSTART, DEFAULT_LEISURE (Section 8.2), and PROBING_RATE may be
+ configured to values specific to the application environment
+ (including dynamically adjusted values); however, the configuration
+ method is out of scope of this document. It is RECOMMENDED that an
+ application environment use consistent values for these parameters;
+ the specific effects of operating with inconsistent values in an
+ application environment are outside the scope of the present
+ specification.
+
+ The transmission parameters have been chosen to achieve a behavior in
+ the presence of congestion that is safe in the Internet. If a
+ configuration desires to use different values, the onus is on the
+
+
+
+Shelby, et al. Standards Track [Page 27]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ configuration to ensure these congestion control properties are not
+ violated. In particular, a decrease of ACK_TIMEOUT below 1 second
+ would violate the guidelines of [RFC5405]. ([RTO-CONSIDER] provides
+ some additional background.) CoAP was designed to enable
+ implementations that do not maintain round-trip-time (RTT)
+ measurements. However, where it is desired to decrease the
+ ACK_TIMEOUT significantly or increase NSTART, this can only be done
+ safely when maintaining such measurements. Configurations MUST NOT
+ decrease ACK_TIMEOUT or increase NSTART without using mechanisms that
+ ensure congestion control safety, either defined in the configuration
+ or in future standards documents.
+
+ ACK_RANDOM_FACTOR MUST NOT be decreased below 1.0, and it SHOULD have
+ a value that is sufficiently different from 1.0 to provide some
+ protection from synchronization effects.
+
+ MAX_RETRANSMIT can be freely adjusted, but a value that is too small
+ will reduce the probability that a Confirmable message is actually
+ received, while a larger value than given here will require further
+ adjustments in the time values (see Section 4.8.2).
+
+ If the choice of transmission parameters leads to an increase of
+ derived time values (see Section 4.8.2), the configuration mechanism
+ MUST ensure the adjusted value is also available to all the endpoints
+ with which these adjusted values are to be used to communicate.
+
+4.8.2. Time Values Derived from Transmission Parameters
+
+ The combination of ACK_TIMEOUT, ACK_RANDOM_FACTOR, and MAX_RETRANSMIT
+ influences the timing of retransmissions, which in turn influences
+ how long certain information items need to be kept by an
+ implementation. To be able to unambiguously reference these derived
+ time values, we give them names as follows:
+
+ o MAX_TRANSMIT_SPAN is the maximum time from the first transmission
+ of a Confirmable message to its last retransmission. For the
+ default transmission parameters, the value is (2+4+8+16)*1.5 = 45
+ seconds, or more generally:
+
+ ACK_TIMEOUT * ((2 ** MAX_RETRANSMIT) - 1) * ACK_RANDOM_FACTOR
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 28]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ o MAX_TRANSMIT_WAIT is the maximum time from the first transmission
+ of a Confirmable message to the time when the sender gives up on
+ receiving an acknowledgement or reset. For the default
+ transmission parameters, the value is (2+4+8+16+32)*1.5 = 93
+ seconds, or more generally:
+
+ ACK_TIMEOUT * ((2 ** (MAX_RETRANSMIT + 1)) - 1) *
+ ACK_RANDOM_FACTOR
+
+ In addition, some assumptions need to be made on the characteristics
+ of the network and the nodes.
+
+ o MAX_LATENCY is the maximum time a datagram is expected to take
+ from the start of its transmission to the completion of its
+ reception. This constant is related to the MSL (Maximum Segment
+ Lifetime) of [RFC0793], which is "arbitrarily defined to be 2
+ minutes" ([RFC0793] glossary, page 81). Note that this is not
+ necessarily smaller than MAX_TRANSMIT_WAIT, as MAX_LATENCY is not
+ intended to describe a situation when the protocol works well, but
+ the worst-case situation against which the protocol has to guard.
+ We, also arbitrarily, define MAX_LATENCY to be 100 seconds. Apart
+ from being reasonably realistic for the bulk of configurations as
+ well as close to the historic choice for TCP, this value also
+ allows Message ID lifetime timers to be represented in 8 bits
+ (when measured in seconds). In these calculations, there is no
+ assumption that the direction of the transmission is irrelevant
+ (i.e., that the network is symmetric); there is just the
+ assumption that the same value can reasonably be used as a maximum
+ value for both directions. If that is not the case, the following
+ calculations become only slightly more complex.
+
+ o PROCESSING_DELAY is the time a node takes to turn around a
+ Confirmable message into an acknowledgement. We assume the node
+ will attempt to send an ACK before having the sender time out, so
+ as a conservative assumption we set it equal to ACK_TIMEOUT.
+
+ o MAX_RTT is the maximum round-trip time, or:
+
+ (2 * MAX_LATENCY) + PROCESSING_DELAY
+
+ From these values, we can derive the following values relevant to the
+ protocol operation:
+
+ o EXCHANGE_LIFETIME is the time from starting to send a Confirmable
+ message to the time when an acknowledgement is no longer expected,
+ i.e., message-layer information about the message exchange can be
+ purged. EXCHANGE_LIFETIME includes a MAX_TRANSMIT_SPAN, a
+ MAX_LATENCY forward, PROCESSING_DELAY, and a MAX_LATENCY for the
+
+
+
+Shelby, et al. Standards Track [Page 29]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ way back. Note that there is no need to consider
+ MAX_TRANSMIT_WAIT if the configuration is chosen such that the
+ last waiting period (ACK_TIMEOUT * (2 ** MAX_RETRANSMIT) or the
+ difference between MAX_TRANSMIT_SPAN and MAX_TRANSMIT_WAIT) is
+ less than MAX_LATENCY -- which is a likely choice, as MAX_LATENCY
+ is a worst-case value unlikely to be met in the real world. In
+ this case, EXCHANGE_LIFETIME simplifies to:
+
+ MAX_TRANSMIT_SPAN + (2 * MAX_LATENCY) + PROCESSING_DELAY
+
+ or 247 seconds with the default transmission parameters.
+
+ o NON_LIFETIME is the time from sending a Non-confirmable message to
+ the time its Message ID can be safely reused. If multiple
+ transmission of a NON message is not used, its value is
+ MAX_LATENCY, or 100 seconds. However, a CoAP sender might send a
+ NON message multiple times, in particular for multicast
+ applications. While the period of reuse is not bounded by the
+ specification, an expectation of reliable detection of duplication
+ at the receiver is on the timescales of MAX_TRANSMIT_SPAN.
+ Therefore, for this purpose, it is safer to use the value:
+
+ MAX_TRANSMIT_SPAN + MAX_LATENCY
+
+ or 145 seconds with the default transmission parameters; however,
+ an implementation that just wants to use a single timeout value
+ for retiring Message IDs can safely use the larger value for
+ EXCHANGE_LIFETIME.
+
+ Table 3 lists the derived parameters introduced in this subsection
+ with their default values.
+
+ +-------------------+---------------+
+ | name | default value |
+ +-------------------+---------------+
+ | MAX_TRANSMIT_SPAN | 45 s |
+ | MAX_TRANSMIT_WAIT | 93 s |
+ | MAX_LATENCY | 100 s |
+ | PROCESSING_DELAY | 2 s |
+ | MAX_RTT | 202 s |
+ | EXCHANGE_LIFETIME | 247 s |
+ | NON_LIFETIME | 145 s |
+ +-------------------+---------------+
+
+ Table 3: Derived Protocol Parameters
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 30]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5. Request/Response Semantics
+
+ CoAP operates under a similar request/response model as HTTP: a CoAP
+ endpoint in the role of a "client" sends one or more CoAP requests to
+ a "server", which services the requests by sending CoAP responses.
+ Unlike HTTP, requests and responses are not sent over a previously
+ established connection but are exchanged asynchronously over CoAP
+ messages.
+
+5.1. Requests
+
+ A CoAP request consists of the method to be applied to the resource,
+ the identifier of the resource, a payload and Internet media type (if
+ any), and optional metadata about the request.
+
+ CoAP supports the basic methods of GET, POST, PUT, and DELETE, which
+ are easily mapped to HTTP. They have the same properties of safe
+ (only retrieval) and idempotent (you can invoke it multiple times
+ with the same effects) as HTTP (see Section 9.1 of [RFC2616]). The
+ GET method is safe; therefore, it MUST NOT take any other action on a
+ resource other than retrieval. The GET, PUT, and DELETE methods MUST
+ be performed in such a way that they are idempotent. POST is not
+ idempotent, because its effect is determined by the origin server and
+ dependent on the target resource; it usually results in a new
+ resource being created or the target resource being updated.
+
+ A request is initiated by setting the Code field in the CoAP header
+ of a Confirmable or a Non-confirmable message to a Method Code and
+ including request information.
+
+ The methods used in requests are described in detail in Section 5.8.
+
+5.2. Responses
+
+ After receiving and interpreting a request, a server responds with a
+ CoAP response that is matched to the request by means of a client-
+ generated token (Section 5.3); note that this is different from the
+ Message ID that matches a Confirmable message to its Acknowledgement.
+
+ A response is identified by the Code field in the CoAP header being
+ set to a Response Code. Similar to the HTTP Status Code, the CoAP
+ Response Code indicates the result of the attempt to understand and
+ satisfy the request. These codes are fully defined in Section 5.9.
+ The Response Code numbers to be set in the Code field of the CoAP
+ header are maintained in the CoAP Response Code Registry
+ (Section 12.1.2).
+
+
+
+
+
+Shelby, et al. Standards Track [Page 31]
+
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+
+
+ 0
+ 0 1 2 3 4 5 6 7
+ +-+-+-+-+-+-+-+-+
+ |class| detail |
+ +-+-+-+-+-+-+-+-+
+
+ Figure 9: Structure of a Response Code
+
+ The upper three bits of the 8-bit Response Code number define the
+ class of response. The lower five bits do not have any
+ categorization role; they give additional detail to the overall class
+ (Figure 9).
+
+ As a human-readable notation for specifications and protocol
+ diagnostics, CoAP code numbers including the Response Code are
+ documented in the format "c.dd", where "c" is the class in decimal,
+ and "dd" is the detail as a two-digit decimal. For example,
+ "Forbidden" is written as 4.03 -- indicating an 8-bit code value of
+ hexadecimal 0x83 (4*0x20+3) or decimal 131 (4*32+3).
+
+ There are 3 classes of Response Codes:
+
+ 2 - Success: The request was successfully received, understood, and
+ accepted.
+
+ 4 - Client Error: The request contains bad syntax or cannot be
+ fulfilled.
+
+ 5 - Server Error: The server failed to fulfill an apparently valid
+ request.
+
+ The Response Codes are designed to be extensible: Response Codes in
+ the Client Error or Server Error class that are unrecognized by an
+ endpoint are treated as being equivalent to the generic Response Code
+ of that class (4.00 and 5.00, respectively). However, there is no
+ generic Response Code indicating success, so a Response Code in the
+ Success class that is unrecognized by an endpoint can only be used to
+ determine that the request was successful without any further
+ details.
+
+ The possible Response Codes are described in detail in Section 5.9.
+
+ Responses can be sent in multiple ways, which are defined in the
+ following subsections.
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 32]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.2.1. Piggybacked
+
+ In the most basic case, the response is carried directly in the
+ Acknowledgement message that acknowledges the request (which requires
+ that the request was carried in a Confirmable message). This is
+ called a "Piggybacked Response".
+
+ The response is returned in the Acknowledgement message, independent
+ of whether the response indicates success or failure. In effect, the
+ response is piggybacked on the Acknowledgement message, and no
+ separate message is required to return the response.
+
+ Implementation Note: The protocol leaves the decision whether to
+ piggyback a response or not (i.e., send a separate response) to
+ the server. The client MUST be prepared to receive either. On
+ the quality-of-implementation level, there is a strong expectation
+ that servers will implement code to piggyback whenever possible --
+ saving resources in the network and both at the client and at the
+ server.
+
+5.2.2. Separate
+
+ It may not be possible to return a piggybacked response in all cases.
+ For example, a server might need longer to obtain the representation
+ of the resource requested than it can wait to send back the
+ Acknowledgement message, without risking the client repeatedly
+ retransmitting the request message (see also the discussion of
+ PROCESSING_DELAY in Section 4.8.2). The response to a request
+ carried in a Non-confirmable message is always sent separately (as
+ there is no Acknowledgement message).
+
+ One way to implement this in a server is to initiate the attempt to
+ obtain the resource representation and, while that is in progress,
+ time out an acknowledgement timer. A server may also immediately
+ send an acknowledgement if it knows in advance that there will be no
+ piggybacked response. In both cases, the acknowledgement effectively
+ is a promise that the request will be acted upon later.
+
+ When the server finally has obtained the resource representation, it
+ sends the response. When it is desired that this message is not
+ lost, it is sent as a Confirmable message from the server to the
+ client and answered by the client with an Acknowledgement, echoing
+ the new Message ID chosen by the server. (It may also be sent as a
+ Non-confirmable message; see Section 5.2.3.)
+
+ When the server chooses to use a separate response, it sends the
+ Acknowledgement to the Confirmable request as an Empty message. Once
+ the server sends back an Empty Acknowledgement, it MUST NOT send back
+
+
+
+Shelby, et al. Standards Track [Page 33]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ the response in another Acknowledgement, even if the client
+ retransmits another identical request. If a retransmitted request is
+ received (perhaps because the original Acknowledgement was delayed),
+ another Empty Acknowledgement is sent, and any response MUST be sent
+ as a separate response.
+
+ If the server then sends a Confirmable response, the client's
+ Acknowledgement to that response MUST also be an Empty message (one
+ that carries neither a request nor a response). The server MUST stop
+ retransmitting its response on any matching Acknowledgement (silently
+ ignoring any Response Code or payload) or Reset message.
+
+ Implementation Notes: Note that, as the underlying datagram
+ transport may not be sequence-preserving, the Confirmable message
+ carrying the response may actually arrive before or after the
+ Acknowledgement message for the request; for the purposes of
+ terminating the retransmission sequence, this also serves as an
+ acknowledgement. Note also that, while the CoAP protocol itself
+ does not make any specific demands here, there is an expectation
+ that the response will come within a time frame that is reasonable
+ from an application point of view. As there is no underlying
+ transport protocol that could be instructed to run a keep-alive
+ mechanism, the requester may want to set up a timeout that is
+ unrelated to CoAP's retransmission timers in case the server is
+ destroyed or otherwise unable to send the response.
+
+5.2.3. Non-confirmable
+
+ If the request message is Non-confirmable, then the response SHOULD
+ be returned in a Non-confirmable message as well. However, an
+ endpoint MUST be prepared to receive a Non-confirmable response
+ (preceded or followed by an Empty Acknowledgement message) in reply
+ to a Confirmable request, or a Confirmable response in reply to a
+ Non-confirmable request.
+
+5.3. Request/Response Matching
+
+ Regardless of how a response is sent, it is matched to the request by
+ means of a token that is included by the client in the request, along
+ with additional address information of the corresponding endpoint.
+
+5.3.1. Token
+
+ The Token is used to match a response with a request. The token
+ value is a sequence of 0 to 8 bytes. (Note that every message
+ carries a token, even if it is of zero length.) Every request
+ carries a client-generated token that the server MUST echo (without
+ modification) in any resulting response.
+
+
+
+Shelby, et al. Standards Track [Page 34]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ A token is intended for use as a client-local identifier for
+ differentiating between concurrent requests (see Section 5.3); it
+ could have been called a "request ID".
+
+ The client SHOULD generate tokens in such a way that tokens currently
+ in use for a given source/destination endpoint pair are unique.
+ (Note that a client implementation can use the same token for any
+ request if it uses a different endpoint each time, e.g., a different
+ source port number.) An empty token value is appropriate e.g., when
+ no other tokens are in use to a destination, or when requests are
+ made serially per destination and receive piggybacked responses.
+ There are, however, multiple possible implementation strategies to
+ fulfill this.
+
+ A client sending a request without using Transport Layer Security
+ (Section 9) SHOULD use a nontrivial, randomized token to guard
+ against spoofing of responses (Section 11.4). This protective use of
+ tokens is the reason they are allowed to be up to 8 bytes in size.
+ The actual size of the random component to be used for the Token
+ depends on the security requirements of the client and the level of
+ threat posed by spoofing of responses. A client that is connected to
+ the general Internet SHOULD use at least 32 bits of randomness,
+ keeping in mind that not being directly connected to the Internet is
+ not necessarily sufficient protection against spoofing. (Note that
+ the Message ID adds little in protection as it is usually
+ sequentially assigned, i.e., guessable, and can be circumvented by
+ spoofing a separate response.) Clients that want to optimize the
+ Token length may further want to detect the level of ongoing attacks
+ (e.g., by tallying recent Token mismatches in incoming messages) and
+ adjust the Token length upwards appropriately. [RFC4086] discusses
+ randomness requirements for security.
+
+ An endpoint receiving a token it did not generate MUST treat the
+ token as opaque and make no assumptions about its content or
+ structure.
+
+5.3.2. Request/Response Matching Rules
+
+ The exact rules for matching a response to a request are as follows:
+
+ 1. The source endpoint of the response MUST be the same as the
+ destination endpoint of the original request.
+
+ 2. In a piggybacked response, the Message ID of the Confirmable
+ request and the Acknowledgement MUST match, and the tokens of the
+ response and original request MUST match. In a separate
+ response, just the tokens of the response and original request
+ MUST match.
+
+
+
+Shelby, et al. Standards Track [Page 35]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ In case a message carrying a response is unexpected (the client is
+ not waiting for a response from the identified endpoint, at the
+ endpoint addressed, and/or with the given token), the response is
+ rejected (Sections 4.2 and 4.3).
+
+ Implementation Note: A client that receives a response in a CON
+ message may want to clean up the message state right after sending
+ the ACK. If that ACK is lost and the server retransmits the CON,
+ the client may no longer have any state to which to correlate this
+ response, making the retransmission an unexpected message; the
+ client will likely send a Reset message so it does not receive any
+ more retransmissions. This behavior is normal and not an
+ indication of an error. (Clients that are not aggressively
+ optimized in their state memory usage will still have message
+ state that will identify the second CON as a retransmission.
+ Clients that actually expect more messages from the server
+ [OBSERVE] will have to keep state in any case.)
+
+5.4. Options
+
+ Both requests and responses may include a list of one or more
+ options. For example, the URI in a request is transported in several
+ options, and metadata that would be carried in an HTTP header in HTTP
+ is supplied as options as well.
+
+ CoAP defines a single set of options that are used in both requests
+ and responses:
+
+ o Content-Format
+
+ o ETag
+
+ o Location-Path
+
+ o Location-Query
+
+ o Max-Age
+
+ o Proxy-Uri
+
+ o Proxy-Scheme
+
+ o Uri-Host
+
+ o Uri-Path
+
+ o Uri-Port
+
+
+
+
+Shelby, et al. Standards Track [Page 36]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ o Uri-Query
+
+ o Accept
+
+ o If-Match
+
+ o If-None-Match
+
+ o Size1
+
+ The semantics of these options along with their properties are
+ defined in detail in Section 5.10.
+
+ Not all options are defined for use with all methods and Response
+ Codes. The possible options for methods and Response Codes are
+ defined in Sections 5.8 and 5.9, respectively. In case an option is
+ not defined for a Method or Response Code, it MUST NOT be included by
+ a sender and MUST be treated like an unrecognized option by a
+ recipient.
+
+5.4.1. Critical/Elective
+
+ Options fall into one of two classes: "critical" or "elective". The
+ difference between these is how an option unrecognized by an endpoint
+ is handled:
+
+ o Upon reception, unrecognized options of class "elective" MUST be
+ silently ignored.
+
+ o Unrecognized options of class "critical" that occur in a
+ Confirmable request MUST cause the return of a 4.02 (Bad Option)
+ response. This response SHOULD include a diagnostic payload
+ describing the unrecognized option(s) (see Section 5.5.2).
+
+ o Unrecognized options of class "critical" that occur in a
+ Confirmable response, or piggybacked in an Acknowledgement, MUST
+ cause the response to be rejected (Section 4.2).
+
+ o Unrecognized options of class "critical" that occur in a Non-
+ confirmable message MUST cause the message to be rejected
+ (Section 4.3).
+
+ Note that, whether critical or elective, an option is never
+ "mandatory" (it is always optional): these rules are defined in order
+ to enable implementations to stop processing options they do not
+ understand or implement.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 37]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Critical/elective rules apply to non-proxying endpoints. A proxy
+ processes options based on Unsafe/Safe-to-Forward classes as defined
+ in Section 5.7.
+
+5.4.2. Proxy Unsafe or Safe-to-Forward and NoCacheKey
+
+ In addition to an option being marked as critical or elective,
+ options are also classified based on how a proxy is to deal with the
+ option if it does not recognize it. For this purpose, an option can
+ either be considered Unsafe to forward (UnSafe is set) or Safe-to-
+ Forward (UnSafe is clear).
+
+ In addition, for an option that is marked Safe-to-Forward, the option
+ number indicates whether or not it is intended to be part of the
+ Cache-Key (Section 5.6) in a request. If some of the NoCacheKey bits
+ are 0, it is; if all NoCacheKey bits are 1, it is not (see
+ Section 5.4.6).
+
+ Note: The Cache-Key indication is relevant only for proxies that do
+ not implement the given option as a request option and instead
+ rely on the Unsafe/Safe-to-Forward indication only. For example,
+ for ETag, actually using the request option as a part of the
+ Cache-Key is grossly inefficient, but it is the best thing one can
+ do if ETag is not implemented by a proxy, as the response is going
+ to differ based on the presence of the request option. A more
+ useful proxy that does implement the ETag request option is not
+ using ETag as a part of the Cache-Key.
+
+ NoCacheKey is indicated in three bits so that only one out of
+ eight codepoints is qualified as NoCacheKey, leaving seven out of
+ eight codepoints for what appears to be the more likely case.
+
+ Proxy behavior with regard to these classes is defined in
+ Section 5.7.
+
+5.4.3. Length
+
+ Option values are defined to have a specific length, often in the
+ form of an upper and lower bound. If the length of an option value
+ in a request is outside the defined range, that option MUST be
+ treated like an unrecognized option (see Section 5.4.1).
+
+5.4.4. Default Values
+
+ Options may be defined to have a default value. If the value of an
+ option is intended to be this default value, the option SHOULD NOT be
+ included in the message. If the option is not present, the default
+ value MUST be assumed.
+
+
+
+Shelby, et al. Standards Track [Page 38]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Where a critical option has a default value, this is chosen in such a
+ way that the absence of the option in a message can be processed
+ properly both by implementations unaware of the critical option and
+ by implementations that interpret this absence as the presence of the
+ default value for the option.
+
+5.4.5. Repeatable Options
+
+ The definition of some options specifies that those options are
+ repeatable. An option that is repeatable MAY be included one or more
+ times in a message. An option that is not repeatable MUST NOT be
+ included more than once in a message.
+
+ If a message includes an option with more occurrences than the option
+ is defined for, each supernumerary option occurrence that appears
+ subsequently in the message MUST be treated like an unrecognized
+ option (see Section 5.4.1).
+
+5.4.6. Option Numbers
+
+ An Option is identified by an option number, which also provides some
+ additional semantics information, e.g., odd numbers indicate a
+ critical option, while even numbers indicate an elective option.
+ Note that this is not just a convention, it is a feature of the
+ protocol: Whether an option is elective or critical is entirely
+ determined by whether its option number is even or odd.
+
+ More generally speaking, an Option number is constructed with a bit
+ mask to indicate if an option is Critical or Elective, Unsafe or
+ Safe-to-Forward, and, in the case of Safe-to-Forward, to provide a
+ Cache-Key indication as shown by the following figure. In the
+ following text, the bit mask is expressed as a single byte that is
+ applied to the least significant byte of the option number in
+ unsigned integer representation. When bit 7 (the least significant
+ bit) is 1, an option is Critical (and likewise Elective when 0).
+ When bit 6 is 1, an option is Unsafe (and likewise Safe-to-Forward
+ when 0). When bit 6 is 0, i.e., the option is not Unsafe, it is not
+ a Cache-Key (NoCacheKey) if and only if bits 3-5 are all set to 1;
+ all other bit combinations mean that it indeed is a Cache-Key. These
+ classes of options are explained in the next sections.
+
+ 0 1 2 3 4 5 6 7
+ +---+---+---+---+---+---+---+---+
+ | | NoCacheKey| U | C |
+ +---+---+---+---+---+---+---+---+
+
+ Figure 10: Option Number Mask (Least Significant Byte)
+
+
+
+
+Shelby, et al. Standards Track [Page 39]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ An endpoint may use an equivalent of the C code in Figure 11 to
+ derive the characteristics of an option number "onum".
+
+ Critical = (onum & 1);
+ UnSafe = (onum & 2);
+ NoCacheKey = ((onum & 0x1e) == 0x1c);
+
+ Figure 11: Determining Characteristics from an Option Number
+
+ The option numbers for the options defined in this document are
+ listed in the "CoAP Option Numbers" registry (Section 12.2).
+
+5.5. Payloads and Representations
+
+ Both requests and responses may include a payload, depending on the
+ Method or Response Code, respectively. If a Method or Response Code
+ is not defined to have a payload, then a sender MUST NOT include one,
+ and a recipient MUST ignore it.
+
+5.5.1. Representation
+
+ The payload of requests or of responses indicating success is
+ typically a representation of a resource ("resource representation")
+ or the result of the requested action ("action result"). Its format
+ is specified by the Internet media type and content coding given by
+ the Content-Format Option. In the absence of this option, no default
+ value is assumed, and the format will need to be inferred by the
+ application (e.g., from the application context). Payload "sniffing"
+ SHOULD only be attempted if no content type is given.
+
+ Implementation Note: On a quality-of-implementation level, there is
+ a strong expectation that a Content-Format indication will be
+ provided with resource representations whenever possible. This is
+ not a "SHOULD" level requirement solely because it is not a
+ protocol requirement, and it also would be difficult to outline
+ exactly in what cases this expectation can be violated.
+
+ For responses indicating a client or server error, the payload is
+ considered a representation of the result of the requested action
+ only if a Content-Format Option is given. In the absence of this
+ option, the payload is a Diagnostic Payload (Section 5.5.2).
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 40]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.5.2. Diagnostic Payload
+
+ If no Content-Format option is given, the payload of responses
+ indicating a client or server error is a brief human-readable
+ diagnostic message, explaining the error situation. This diagnostic
+ message MUST be encoded using UTF-8 [RFC3629], more specifically
+ using Net-Unicode form [RFC5198].
+
+ The message is similar to the Reason-Phrase on an HTTP status line.
+ It is not intended for end users but for software engineers that
+ during debugging need to interpret it in the context of the present,
+ English-language specification; therefore, no mechanism for language
+ tagging is needed or provided. In contrast to what is usual in HTTP,
+ the payload SHOULD be empty if there is no additional information
+ beyond the Response Code.
+
+5.5.3. Selected Representation
+
+ Not all responses carry a payload that provides a representation of
+ the resource addressed by the request. It is, however, sometimes
+ useful to be able to refer to such a representation in relation to a
+ response, independent of whether it actually was enclosed.
+
+ We use the term "selected representation" to refer to the current
+ representation of a target resource that would have been selected in
+ a successful response if the corresponding request had used the
+ method GET and excluded any conditional request options
+ (Section 5.10.8).
+
+ Certain response options provide metadata about the selected
+ representation, which might differ from the representation included
+ in the message for responses to some state-changing methods. Of the
+ response options defined in this specification, only the ETag
+ response option (Section 5.10.6) is defined as metadata about the
+ selected representation.
+
+5.5.4. Content Negotiation
+
+ A server may be able to supply a representation for a resource in one
+ of multiple representation formats. Without further information from
+ the client, it will provide the representation in the format it
+ prefers.
+
+ By using the Accept Option (Section 5.10.4) in a request, the client
+ can indicate which content-format it prefers to receive.
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 41]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.6. Caching
+
+ CoAP endpoints MAY cache responses in order to reduce the response
+ time and network bandwidth consumption on future, equivalent
+ requests.
+
+ The goal of caching in CoAP is to reuse a prior response message to
+ satisfy a current request. In some cases, a stored response can be
+ reused without the need for a network request, reducing latency and
+ network round-trips; a "freshness" mechanism is used for this purpose
+ (see Section 5.6.1). Even when a new request is required, it is
+ often possible to reuse the payload of a prior response to satisfy
+ the request, thereby reducing network bandwidth usage; a "validation"
+ mechanism is used for this purpose (see Section 5.6.2).
+
+ Unlike HTTP, the cacheability of CoAP responses does not depend on
+ the request method, but it depends on the Response Code. The
+ cacheability of each Response Code is defined along the Response Code
+ definitions in Section 5.9. Response Codes that indicate success and
+ are unrecognized by an endpoint MUST NOT be cached.
+
+ For a presented request, a CoAP endpoint MUST NOT use a stored
+ response, unless:
+
+ o the presented request method and that used to obtain the stored
+ response match,
+
+ o all options match between those in the presented request and those
+ of the request used to obtain the stored response (which includes
+ the request URI), except that there is no need for a match of any
+ request options marked as NoCacheKey (Section 5.4) or recognized
+ by the Cache and fully interpreted with respect to its specified
+ cache behavior (such as the ETag request option described in
+ Section 5.10.6; see also Section 5.4.2), and
+
+ o the stored response is either fresh or successfully validated as
+ defined below.
+
+ The set of request options that is used for matching the cache entry
+ is also collectively referred to as the "Cache-Key". For URI schemes
+ other than coap and coaps, matching of those options that constitute
+ the request URI may be performed under rules specific to the URI
+ scheme.
+
+
+
+
+
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.6.1. Freshness Model
+
+ When a response is "fresh" in the cache, it can be used to satisfy
+ subsequent requests without contacting the origin server, thereby
+ improving efficiency.
+
+ The mechanism for determining freshness is for an origin server to
+ provide an explicit expiration time in the future, using the Max-Age
+ Option (see Section 5.10.5). The Max-Age Option indicates that the
+ response is to be considered not fresh after its age is greater than
+ the specified number of seconds.
+
+ The Max-Age Option defaults to a value of 60. Thus, if it is not
+ present in a cacheable response, then the response is considered not
+ fresh after its age is greater than 60 seconds. If an origin server
+ wishes to prevent caching, it MUST explicitly include a Max-Age
+ Option with a value of zero seconds.
+
+ If a client has a fresh stored response and makes a new request
+ matching the request for that stored response, the new response
+ invalidates the old response.
+
+5.6.2. Validation Model
+
+ When an endpoint has one or more stored responses for a GET request,
+ but cannot use any of them (e.g., because they are not fresh), it can
+ use the ETag Option (Section 5.10.6) in the GET request to give the
+ origin server an opportunity both to select a stored response to be
+ used, and to update its freshness. This process is known as
+ "validating" or "revalidating" the stored response.
+
+ When sending such a request, the endpoint SHOULD add an ETag Option
+ specifying the entity-tag of each stored response that is applicable.
+
+ A 2.03 (Valid) response indicates the stored response identified by
+ the entity-tag given in the response's ETag Option can be reused
+ after updating it as described in Section 5.9.1.3.
+
+ Any other Response Code indicates that none of the stored responses
+ nominated in the request is suitable. Instead, the response SHOULD
+ be used to satisfy the request and MAY replace the stored response.
+
+
+
+
+
+
+
+
+
+
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+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.7. Proxying
+
+ A proxy is a CoAP endpoint that can be tasked by CoAP clients to
+ perform requests on their behalf. This may be useful, for example,
+ when the request could otherwise not be made, or to service the
+ response from a cache in order to reduce response time and network
+ bandwidth or energy consumption.
+
+ In an overall architecture for a Constrained RESTful Environment,
+ proxies can serve quite different purposes. Proxies can be
+ explicitly selected by clients, a role that we term "forward-proxy".
+ Proxies can also be inserted to stand in for origin servers, a role
+ that we term "reverse-proxy". Orthogonal to this distinction, a
+ proxy can map from a CoAP request to a CoAP request (CoAP-to-CoAP
+ proxy) or translate from or to a different protocol ("cross-proxy").
+ Full definitions of these terms are provided in Section 1.2.
+
+ Notes: The terminology in this specification has been selected to be
+ culturally compatible with the terminology used in the wider web
+ application environments, without necessarily matching it in every
+ detail (which may not even be relevant to Constrained RESTful
+ Environments). Not too much semantics should be ascribed to the
+ components of the terms (such as "forward", "reverse", or
+ "cross").
+
+ HTTP proxies, besides acting as HTTP proxies, often offer a
+ transport-protocol proxying function ("CONNECT") to enable end-to-
+ end transport layer security through the proxy. No such function
+ is defined for CoAP-to-CoAP proxies in this specification, as
+ forwarding of UDP packets is unlikely to be of much value in
+ Constrained RESTful Environments. See also Section 10.2.7 for the
+ cross-proxy case.
+
+ When a client uses a proxy to make a request that will use a secure
+ URI scheme (e.g., "coaps" or "https"), the request towards the proxy
+ SHOULD be sent using DTLS except where equivalent lower-layer
+ security is used for the leg between the client and the proxy.
+
+5.7.1. Proxy Operation
+
+ A proxy generally needs a way to determine potential request
+ parameters for a request it places to a destination, based on the
+ request it received from its client. This way is fully specified for
+ a forward-proxy but may depend on the specific configuration for a
+ reverse-proxy. In particular, the client of a reverse-proxy
+ generally does not indicate a locator for the destination,
+
+
+
+
+
+Shelby, et al. Standards Track [Page 44]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ necessitating some form of namespace translation in the reverse-
+ proxy. However, some aspects of the operation of proxies are common
+ to all its forms.
+
+ If a proxy does not employ a cache, then it simply forwards the
+ translated request to the determined destination. Otherwise, if it
+ does employ a cache but does not have a stored response that matches
+ the translated request and is considered fresh, then it needs to
+ refresh its cache according to Section 5.6. For options in the
+ request that the proxy recognizes, it knows whether the option is
+ intended to act as part of the key used in looking up the cached
+ value or not. For example, since requests for different Uri-Path
+ values address different resources, Uri-Path values are always part
+ of the Cache-Key, while, e.g., Token values are never part of the
+ Cache-Key. For options that the proxy does not recognize but that
+ are marked Safe-to-Forward in the option number, the option also
+ indicates whether it is to be included in the Cache-Key (NoCacheKey
+ is not all set) or not (NoCacheKey is all set). (Options that are
+ unrecognized and marked Unsafe lead to 4.02 Bad Option.)
+
+ If the request to the destination times out, then a 5.04 (Gateway
+ Timeout) response MUST be returned. If the request to the
+ destination returns a response that cannot be processed by the proxy
+ (e.g, due to unrecognized critical options or message format errors),
+ then a 5.02 (Bad Gateway) response MUST be returned. Otherwise, the
+ proxy returns the response to the client.
+
+ If a response is generated out of a cache, the generated (or implied)
+ Max-Age Option MUST NOT extend the max-age originally set by the
+ server, considering the time the resource representation spent in the
+ cache. For example, the Max-Age Option could be adjusted by the
+ proxy for each response using the formula:
+
+ proxy-max-age = original-max-age - cache-age
+
+ For example, if a request is made to a proxied resource that was
+ refreshed 20 seconds ago and had an original Max-Age of 60 seconds,
+ then that resource's proxied max-age is now 40 seconds. Considering
+ potential network delays on the way from the origin server, a proxy
+ should be conservative in the max-age values offered.
+
+ All options present in a proxy request MUST be processed at the
+ proxy. Unsafe options in a request that are not recognized by the
+ proxy MUST lead to a 4.02 (Bad Option) response being returned by the
+ proxy. A CoAP-to-CoAP proxy MUST forward to the origin server all
+ Safe-to-Forward options that it does not recognize. Similarly,
+
+
+
+
+
+Shelby, et al. Standards Track [Page 45]
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+
+
+ Unsafe options in a response that are not recognized by the CoAP-to-
+ CoAP proxy server MUST lead to a 5.02 (Bad Gateway) response. Again,
+ Safe-to-Forward options that are not recognized MUST be forwarded.
+
+ Additional considerations for cross-protocol proxying between CoAP
+ and HTTP are discussed in Section 10.
+
+5.7.2. Forward-Proxies
+
+ CoAP distinguishes between requests made (as if) to an origin server
+ and requests made through a forward-proxy. CoAP requests to a
+ forward-proxy are made as normal Confirmable or Non-confirmable
+ requests to the forward-proxy endpoint, but they specify the request
+ URI in a different way: The request URI in a proxy request is
+ specified as a string in the Proxy-Uri Option (see Section 5.10.2),
+ while the request URI in a request to an origin server is split into
+ the Uri-Host, Uri-Port, Uri-Path, and Uri-Query Options (see
+ Section 5.10.1). Alternatively, the URI in a proxy request can be
+ assembled from a Proxy-Scheme option and the split options mentioned.
+
+ When a proxy request is made to an endpoint and the endpoint is
+ unwilling or unable to act as proxy for the request URI, it MUST
+ return a 5.05 (Proxying Not Supported) response. If the authority
+ (host and port) is recognized as identifying the proxy endpoint
+ itself (see Section 5.10.2), then the request MUST be treated as a
+ local (non-proxied) request.
+
+ Unless a proxy is configured to forward the proxy request to another
+ proxy, it MUST translate the request as follows: the scheme of the
+ request URI defines the outgoing protocol and its details (e.g., CoAP
+ is used over UDP for the "coap" scheme and over DTLS for the "coaps"
+ scheme.) For a CoAP-to-CoAP proxy, the origin server's IP address
+ and port are determined by the authority component of the request
+ URI, and the request URI is decoded and split into the Uri-Host, Uri-
+ Port, Uri-Path and Uri-Query Options. This consumes the Proxy-Uri or
+ Proxy-Scheme option, which is therefore not forwarded to the origin
+ server.
+
+5.7.3. Reverse-Proxies
+
+ Reverse-proxies do not make use of the Proxy-Uri or Proxy-Scheme
+ options but need to determine the destination (next hop) of a request
+ from information in the request and information in their
+ configuration. For example, a reverse-proxy might offer various
+ resources as if they were its own resources, after having learned of
+ their existence through resource discovery. The reverse-proxy is
+ free to build a namespace for the URIs that identify these resources.
+ A reverse-proxy may also build a namespace that gives the client more
+
+
+
+Shelby, et al. Standards Track [Page 46]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ control over where the request goes, e.g., by embedding host
+ identifiers and port numbers into the URI path of the resources
+ offered.
+
+ In processing the response, a reverse-proxy has to be careful that
+ ETag option values from different sources are not mixed up on one
+ resource offered to its clients. In many cases, the ETag can be
+ forwarded unchanged. If the mapping from a resource offered by the
+ reverse-proxy to resources offered by its various origin servers is
+ not unique, the reverse-proxy may need to generate a new ETag, making
+ sure the semantics of this option are properly preserved.
+
+5.8. Method Definitions
+
+ In this section, each method is defined along with its behavior. A
+ request with an unrecognized or unsupported Method Code MUST generate
+ a 4.05 (Method Not Allowed) piggybacked response.
+
+5.8.1. GET
+
+ The GET method retrieves a representation for the information that
+ currently corresponds to the resource identified by the request URI.
+ If the request includes an Accept Option, that indicates the
+ preferred content-format of a response. If the request includes an
+ ETag Option, the GET method requests that ETag be validated and that
+ the representation be transferred only if validation failed. Upon
+ success, a 2.05 (Content) or 2.03 (Valid) Response Code SHOULD be
+ present in the response.
+
+ The GET method is safe and idempotent.
+
+5.8.2. POST
+
+ The POST method requests that the representation enclosed in the
+ request be processed. The actual function performed by the POST
+ method is determined by the origin server and dependent on the target
+ resource. It usually results in a new resource being created or the
+ target resource being updated.
+
+ If a resource has been created on the server, the response returned
+ by the server SHOULD have a 2.01 (Created) Response Code and SHOULD
+ include the URI of the new resource in a sequence of one or more
+ Location-Path and/or Location-Query Options (Section 5.10.7). If the
+ POST succeeds but does not result in a new resource being created on
+ the server, the response SHOULD have a 2.04 (Changed) Response Code.
+ If the POST succeeds and results in the target resource being
+ deleted, the response SHOULD have a 2.02 (Deleted) Response Code.
+ POST is neither safe nor idempotent.
+
+
+
+Shelby, et al. Standards Track [Page 47]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.8.3. PUT
+
+ The PUT method requests that the resource identified by the request
+ URI be updated or created with the enclosed representation. The
+ representation format is specified by the media type and content
+ coding given in the Content-Format Option, if provided.
+
+ If a resource exists at the request URI, the enclosed representation
+ SHOULD be considered a modified version of that resource, and a 2.04
+ (Changed) Response Code SHOULD be returned. If no resource exists,
+ then the server MAY create a new resource with that URI, resulting in
+ a 2.01 (Created) Response Code. If the resource could not be created
+ or modified, then an appropriate error Response Code SHOULD be sent.
+
+ Further restrictions to a PUT can be made by including the If-Match
+ (see Section 5.10.8.1) or If-None-Match (see Section 5.10.8.2)
+ options in the request.
+
+ PUT is not safe but is idempotent.
+
+5.8.4. DELETE
+
+ The DELETE method requests that the resource identified by the
+ request URI be deleted. A 2.02 (Deleted) Response Code SHOULD be
+ used on success or in case the resource did not exist before the
+ request.
+
+ DELETE is not safe but is idempotent.
+
+5.9. Response Code Definitions
+
+ Each Response Code is described below, including any options required
+ in the response. Where appropriate, some of the codes will be
+ specified in regards to related Response Codes in HTTP [RFC2616];
+ this does not mean that any such relationship modifies the HTTP
+ mapping specified in Section 10.
+
+5.9.1. Success 2.xx
+
+ This class of Response Code indicates that the clients request was
+ successfully received, understood, and accepted.
+
+5.9.1.1. 2.01 Created
+
+ Like HTTP 201 "Created", but only used in response to POST and PUT
+ requests. The payload returned with the response, if any, is a
+ representation of the action result.
+
+
+
+
+Shelby, et al. Standards Track [Page 48]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ If the response includes one or more Location-Path and/or Location-
+ Query Options, the values of these options specify the location at
+ which the resource was created. Otherwise, the resource was created
+ at the request URI. A cache receiving this response MUST mark any
+ stored response for the created resource as not fresh.
+
+ This response is not cacheable.
+
+5.9.1.2. 2.02 Deleted
+
+ This Response Code is like HTTP 204 "No Content" but only used in
+ response to requests that cause the resource to cease being
+ available, such as DELETE and, in certain circumstances, POST. The
+ payload returned with the response, if any, is a representation of
+ the action result.
+
+ This response is not cacheable. However, a cache MUST mark any
+ stored response for the deleted resource as not fresh.
+
+5.9.1.3. 2.03 Valid
+
+ This Response Code is related to HTTP 304 "Not Modified" but only
+ used to indicate that the response identified by the entity-tag
+ identified by the included ETag Option is valid. Accordingly, the
+ response MUST include an ETag Option and MUST NOT include a payload.
+
+ When a cache that recognizes and processes the ETag response option
+ receives a 2.03 (Valid) response, it MUST update the stored response
+ with the value of the Max-Age Option included in the response
+ (explicitly, or implicitly as a default value; see also
+ Section 5.6.2). For each type of Safe-to-Forward option present in
+ the response, the (possibly empty) set of options of this type that
+ are present in the stored response MUST be replaced with the set of
+ options of this type in the response received. (Unsafe options may
+ trigger similar option-specific processing as defined by the option.)
+
+5.9.1.4. 2.04 Changed
+
+ This Response Code is like HTTP 204 "No Content" but only used in
+ response to POST and PUT requests. The payload returned with the
+ response, if any, is a representation of the action result.
+
+ This response is not cacheable. However, a cache MUST mark any
+ stored response for the changed resource as not fresh.
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 49]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.9.1.5. 2.05 Content
+
+ This Response Code is like HTTP 200 "OK" but only used in response to
+ GET requests.
+
+ The payload returned with the response is a representation of the
+ target resource.
+
+ This response is cacheable: Caches can use the Max-Age Option to
+ determine freshness (see Section 5.6.1) and (if present) the ETag
+ Option for validation (see Section 5.6.2).
+
+5.9.2. Client Error 4.xx
+
+ This class of Response Code is intended for cases in which the client
+ seems to have erred. These Response Codes are applicable to any
+ request method.
+
+ The server SHOULD include a diagnostic payload under the conditions
+ detailed in Section 5.5.2.
+
+ Responses of this class are cacheable: Caches can use the Max-Age
+ Option to determine freshness (see Section 5.6.1). They cannot be
+ validated.
+
+5.9.2.1. 4.00 Bad Request
+
+ This Response Code is Like HTTP 400 "Bad Request".
+
+5.9.2.2. 4.01 Unauthorized
+
+ The client is not authorized to perform the requested action. The
+ client SHOULD NOT repeat the request without first improving its
+ authentication status to the server. Which specific mechanism can be
+ used for this is outside this document's scope; see also Section 9.
+
+5.9.2.3. 4.02 Bad Option
+
+ The request could not be understood by the server due to one or more
+ unrecognized or malformed options. The client SHOULD NOT repeat the
+ request without modification.
+
+5.9.2.4. 4.03 Forbidden
+
+ This Response Code is like HTTP 403 "Forbidden".
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 50]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.9.2.5. 4.04 Not Found
+
+ This Response Code is like HTTP 404 "Not Found".
+
+5.9.2.6. 4.05 Method Not Allowed
+
+ This Response Code is like HTTP 405 "Method Not Allowed" but with no
+ parallel to the "Allow" header field.
+
+5.9.2.7. 4.06 Not Acceptable
+
+ This Response Code is like HTTP 406 "Not Acceptable", but with no
+ response entity.
+
+5.9.2.8. 4.12 Precondition Failed
+
+ This Response Code is like HTTP 412 "Precondition Failed".
+
+5.9.2.9. 4.13 Request Entity Too Large
+
+ This Response Code is like HTTP 413 "Request Entity Too Large".
+
+ The response SHOULD include a Size1 Option (Section 5.10.9) to
+ indicate the maximum size of request entity the server is able and
+ willing to handle, unless the server is not in a position to make
+ this information available.
+
+5.9.2.10. 4.15 Unsupported Content-Format
+
+ This Response Code is like HTTP 415 "Unsupported Media Type".
+
+5.9.3. Server Error 5.xx
+
+ This class of Response Code indicates cases in which the server is
+ aware that it has erred or is incapable of performing the request.
+ These Response Codes are applicable to any request method.
+
+ The server SHOULD include a diagnostic payload under the conditions
+ detailed in Section 5.5.2.
+
+ Responses of this class are cacheable: Caches can use the Max-Age
+ Option to determine freshness (see Section 5.6.1). They cannot be
+ validated.
+
+5.9.3.1. 5.00 Internal Server Error
+
+ This Response Code is like HTTP 500 "Internal Server Error".
+
+
+
+
+Shelby, et al. Standards Track [Page 51]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.9.3.2. 5.01 Not Implemented
+
+ This Response Code is like HTTP 501 "Not Implemented".
+
+5.9.3.3. 5.02 Bad Gateway
+
+ This Response Code is like HTTP 502 "Bad Gateway".
+
+5.9.3.4. 5.03 Service Unavailable
+
+ This Response Code is like HTTP 503 "Service Unavailable" but uses
+ the Max-Age Option in place of the "Retry-After" header field to
+ indicate the number of seconds after which to retry.
+
+5.9.3.5. 5.04 Gateway Timeout
+
+ This Response Code is like HTTP 504 "Gateway Timeout".
+
+5.9.3.6. 5.05 Proxying Not Supported
+
+ The server is unable or unwilling to act as a forward-proxy for the
+ URI specified in the Proxy-Uri Option or using Proxy-Scheme (see
+ Section 5.10.2).
+
+5.10. Option Definitions
+
+ The individual CoAP options are summarized in Table 4 and explained
+ in the subsections of this section.
+
+ In this table, the C, U, and N columns indicate the properties
+ Critical, UnSafe, and NoCacheKey, respectively. Since NoCacheKey
+ only has a meaning for options that are Safe-to-Forward (not marked
+ Unsafe), the column is filled with a dash for UnSafe options.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 52]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ +-----+---+---+---+---+----------------+--------+--------+----------+
+ | No. | C | U | N | R | Name | Format | Length | Default |
+ +-----+---+---+---+---+----------------+--------+--------+----------+
+ | 1 | x | | | x | If-Match | opaque | 0-8 | (none) |
+ | 3 | x | x | - | | Uri-Host | string | 1-255 | (see |
+ | | | | | | | | | below) |
+ | 4 | | | | x | ETag | opaque | 1-8 | (none) |
+ | 5 | x | | | | If-None-Match | empty | 0 | (none) |
+ | 7 | x | x | - | | Uri-Port | uint | 0-2 | (see |
+ | | | | | | | | | below) |
+ | 8 | | | | x | Location-Path | string | 0-255 | (none) |
+ | 11 | x | x | - | x | Uri-Path | string | 0-255 | (none) |
+ | 12 | | | | | Content-Format | uint | 0-2 | (none) |
+ | 14 | | x | - | | Max-Age | uint | 0-4 | 60 |
+ | 15 | x | x | - | x | Uri-Query | string | 0-255 | (none) |
+ | 17 | x | | | | Accept | uint | 0-2 | (none) |
+ | 20 | | | | x | Location-Query | string | 0-255 | (none) |
+ | 35 | x | x | - | | Proxy-Uri | string | 1-1034 | (none) |
+ | 39 | x | x | - | | Proxy-Scheme | string | 1-255 | (none) |
+ | 60 | | | x | | Size1 | uint | 0-4 | (none) |
+ +-----+---+---+---+---+----------------+--------+--------+----------+
+
+ C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
+
+ Table 4: Options
+
+5.10.1. Uri-Host, Uri-Port, Uri-Path, and Uri-Query
+
+ The Uri-Host, Uri-Port, Uri-Path, and Uri-Query Options are used to
+ specify the target resource of a request to a CoAP origin server.
+ The options encode the different components of the request URI in a
+ way that no percent-encoding is visible in the option values and that
+ the full URI can be reconstructed at any involved endpoint. The
+ syntax of CoAP URIs is defined in Section 6.
+
+ The steps for parsing URIs into options is defined in Section 6.4.
+ These steps result in zero or more Uri-Host, Uri-Port, Uri-Path, and
+ Uri-Query Options being included in a request, where each option
+ holds the following values:
+
+ o the Uri-Host Option specifies the Internet host of the resource
+ being requested,
+
+ o the Uri-Port Option specifies the transport-layer port number of
+ the resource,
+
+ o each Uri-Path Option specifies one segment of the absolute path to
+ the resource, and
+
+
+
+Shelby, et al. Standards Track [Page 53]
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+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ o each Uri-Query Option specifies one argument parameterizing the
+ resource.
+
+ Note: Fragments ([RFC3986], Section 3.5) are not part of the request
+ URI and thus will not be transmitted in a CoAP request.
+
+ The default value of the Uri-Host Option is the IP literal
+ representing the destination IP address of the request message.
+ Likewise, the default value of the Uri-Port Option is the destination
+ UDP port. The default values for the Uri-Host and Uri-Port Options
+ are sufficient for requests to most servers. Explicit Uri-Host and
+ Uri-Port Options are typically used when an endpoint hosts multiple
+ virtual servers.
+
+ The Uri-Path and Uri-Query Option can contain any character sequence.
+ No percent-encoding is performed. The value of a Uri-Path Option
+ MUST NOT be "." or ".." (as the request URI must be resolved before
+ parsing it into options).
+
+ The steps for constructing the request URI from the options are
+ defined in Section 6.5. Note that an implementation does not
+ necessarily have to construct the URI; it can simply look up the
+ target resource by examining the individual options.
+
+ Examples can be found in Appendix B.
+
+5.10.2. Proxy-Uri and Proxy-Scheme
+
+ The Proxy-Uri Option is used to make a request to a forward-proxy
+ (see Section 5.7). The forward-proxy is requested to forward the
+ request or service it from a valid cache and return the response.
+
+ The option value is an absolute-URI ([RFC3986], Section 4.3).
+
+ Note that the forward-proxy MAY forward the request on to another
+ proxy or directly to the server specified by the absolute-URI. In
+ order to avoid request loops, a proxy MUST be able to recognize all
+ of its server names, including any aliases, local variations, and the
+ numeric IP addresses.
+
+ An endpoint receiving a request with a Proxy-Uri Option that is
+ unable or unwilling to act as a forward-proxy for the request MUST
+ cause the return of a 5.05 (Proxying Not Supported) response.
+
+ The Proxy-Uri Option MUST take precedence over any of the Uri-Host,
+ Uri-Port, Uri-Path or Uri-Query options (each of which MUST NOT be
+ included in a request containing the Proxy-Uri Option).
+
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ As a special case to simplify many proxy clients, the absolute-URI
+ can be constructed from the Uri-* options. When a Proxy-Scheme
+ Option is present, the absolute-URI is constructed as follows: a CoAP
+ URI is constructed from the Uri-* options as defined in Section 6.5.
+ In the resulting URI, the initial scheme up to, but not including,
+ the following colon is then replaced by the content of the Proxy-
+ Scheme Option. Note that this case is only applicable if the
+ components of the desired URI other than the scheme component
+ actually can be expressed using Uri-* options; for example, to
+ represent a URI with a userinfo component in the authority, only
+ Proxy-Uri can be used.
+
+5.10.3. Content-Format
+
+ The Content-Format Option indicates the representation format of the
+ message payload. The representation format is given as a numeric
+ Content-Format identifier that is defined in the "CoAP Content-
+ Formats" registry (Section 12.3). In the absence of the option, no
+ default value is assumed, i.e., the representation format of any
+ representation message payload is indeterminate (Section 5.5).
+
+5.10.4. Accept
+
+ The CoAP Accept option can be used to indicate which Content-Format
+ is acceptable to the client. The representation format is given as a
+ numeric Content-Format identifier that is defined in the "CoAP
+ Content-Formats" registry (Section 12.3). If no Accept option is
+ given, the client does not express a preference (thus no default
+ value is assumed). The client prefers the representation returned by
+ the server to be in the Content-Format indicated. The server returns
+ the preferred Content-Format if available. If the preferred Content-
+ Format cannot be returned, then a 4.06 "Not Acceptable" MUST be sent
+ as a response, unless another error code takes precedence for this
+ response.
+
+5.10.5. Max-Age
+
+ The Max-Age Option indicates the maximum time a response may be
+ cached before it is considered not fresh (see Section 5.6.1).
+
+ The option value is an integer number of seconds between 0 and
+ 2**32-1 inclusive (about 136.1 years). A default value of 60 seconds
+ is assumed in the absence of the option in a response.
+
+ The value is intended to be current at the time of transmission.
+ Servers that provide resources with strict tolerances on the value of
+ Max-Age SHOULD update the value before each retransmission. (See
+ also Section 5.7.1.)
+
+
+
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+
+
+5.10.6. ETag
+
+ An entity-tag is intended for use as a resource-local identifier for
+ differentiating between representations of the same resource that
+ vary over time. It is generated by the server providing the
+ resource, which may generate it in any number of ways including a
+ version, checksum, hash, or time. An endpoint receiving an entity-
+ tag MUST treat it as opaque and make no assumptions about its content
+ or structure. (Endpoints that generate an entity-tag are encouraged
+ to use the most compact representation possible, in particular in
+ regards to clients and intermediaries that may want to store multiple
+ ETag values.)
+
+5.10.6.1. ETag as a Response Option
+
+ The ETag Option in a response provides the current value (i.e., after
+ the request was processed) of the entity-tag for the "tagged
+ representation". If no Location-* options are present, the tagged
+ representation is the selected representation (Section 5.5.3) of the
+ target resource. If one or more Location-* options are present and
+ thus a location URI is indicated (Section 5.10.7), the tagged
+ representation is the representation that would be retrieved by a GET
+ request to the location URI.
+
+ An ETag response option can be included with any response for which
+ there is a tagged representation (e.g., it would not be meaningful in
+ a 4.04 or 4.00 response). The ETag Option MUST NOT occur more than
+ once in a response.
+
+ There is no default value for the ETag Option; if it is not present
+ in a response, the server makes no statement about the entity-tag for
+ the tagged representation.
+
+5.10.6.2. ETag as a Request Option
+
+ In a GET request, an endpoint that has one or more representations
+ previously obtained from the resource, and has obtained ETag response
+ options with these, can specify an instance of the ETag Option for
+ one or more of these stored responses.
+
+ A server can issue a 2.03 Valid response (Section 5.9.1.3) in place
+ of a 2.05 Content response if one of the ETags given is the entity-
+ tag for the current representation, i.e., is valid; the 2.03 Valid
+ response then echoes this specific ETag in a response option.
+
+ In effect, a client can determine if any of the stored
+ representations is current (see Section 5.6.2) without needing to
+ transfer them again.
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ The ETag Option MAY occur zero, one, or multiple times in a request.
+
+5.10.7. Location-Path and Location-Query
+
+ The Location-Path and Location-Query Options together indicate a
+ relative URI that consists either of an absolute path, a query
+ string, or both. A combination of these options is included in a
+ 2.01 (Created) response to indicate the location of the resource
+ created as the result of a POST request (see Section 5.8.2). The
+ location is resolved relative to the request URI.
+
+ If a response with one or more Location-Path and/or Location-Query
+ Options passes through a cache that interprets these options and the
+ implied URI identifies one or more currently stored responses, those
+ entries MUST be marked as not fresh.
+
+ Each Location-Path Option specifies one segment of the absolute path
+ to the resource, and each Location-Query Option specifies one
+ argument parameterizing the resource. The Location-Path and
+ Location-Query Option can contain any character sequence. No
+ percent-encoding is performed. The value of a Location-Path Option
+ MUST NOT be "." or "..".
+
+ The steps for constructing the location URI from the options are
+ analogous to Section 6.5, except that the first five steps are
+ skipped and the result is a relative URI-reference, which is then
+ interpreted relative to the request URI. Note that the relative URI-
+ reference constructed this way always includes an absolute path
+ (e.g., leaving out Location-Path but supplying Location-Query means
+ the path component in the URI is "/").
+
+ The options that are used to compute the relative URI-reference are
+ collectively called Location-* options. Beyond Location-Path and
+ Location-Query, more Location-* options may be defined in the future
+ and have been reserved option numbers 128, 132, 136, and 140. If any
+ of these reserved option numbers occurs in addition to Location-Path
+ and/or Location-Query and are not supported, then a 4.02 (Bad Option)
+ error MUST be returned.
+
+5.10.8. Conditional Request Options
+
+ Conditional request options enable a client to ask the server to
+ perform the request only if certain conditions specified by the
+ option are fulfilled.
+
+
+
+
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ For each of these options, if the condition given is not fulfilled,
+ then the server MUST NOT perform the requested method. Instead, the
+ server MUST respond with the 4.12 (Precondition Failed) Response
+ Code.
+
+ If the condition is fulfilled, the server performs the request method
+ as if the conditional request options were not present.
+
+ If the request would, without the conditional request options, result
+ in anything other than a 2.xx or 4.12 Response Code, then any
+ conditional request options MAY be ignored.
+
+5.10.8.1. If-Match
+
+ The If-Match Option MAY be used to make a request conditional on the
+ current existence or value of an ETag for one or more representations
+ of the target resource. If-Match is generally useful for resource
+ update requests, such as PUT requests, as a means for protecting
+ against accidental overwrites when multiple clients are acting in
+ parallel on the same resource (i.e., the "lost update" problem).
+
+ The value of an If-Match option is either an ETag or the empty
+ string. An If-Match option with an ETag matches a representation
+ with that exact ETag. An If-Match option with an empty value matches
+ any existing representation (i.e., it places the precondition on the
+ existence of any current representation for the target resource).
+
+ The If-Match Option can occur multiple times. If any of the options
+ match, then the condition is fulfilled.
+
+ If there is one or more If-Match Options, but none of the options
+ match, then the condition is not fulfilled.
+
+5.10.8.2. If-None-Match
+
+ The If-None-Match Option MAY be used to make a request conditional on
+ the nonexistence of the target resource. If-None-Match is useful for
+ resource creation requests, such as PUT requests, as a means for
+ protecting against accidental overwrites when multiple clients are
+ acting in parallel on the same resource. The If-None-Match Option
+ carries no value.
+
+ If the target resource does exist, then the condition is not
+ fulfilled.
+
+ (It is not very useful to combine If-Match and If-None-Match options
+ in one request, because the condition will then never be fulfilled.)
+
+
+
+
+Shelby, et al. Standards Track [Page 58]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+5.10.9. Size1 Option
+
+ The Size1 option provides size information about the resource
+ representation in a request. The option value is an integer number
+ of bytes. Its main use is with block-wise transfers [BLOCK]. In the
+ present specification, it is used in 4.13 responses (Section 5.9.2.9)
+ to indicate the maximum size of request entity that the server is
+ able and willing to handle.
+
+6. CoAP URIs
+
+ CoAP uses the "coap" and "coaps" URI schemes for identifying CoAP
+ resources and providing a means of locating the resource. Resources
+ are organized hierarchically and governed by a potential CoAP origin
+ server listening for CoAP requests ("coap") or DTLS-secured CoAP
+ requests ("coaps") on a given UDP port. The CoAP server is
+ identified via the generic syntax's authority component, which
+ includes a host component and optional UDP port number. The
+ remainder of the URI is considered to be identifying a resource that
+ can be operated on by the methods defined by the CoAP protocol. The
+ "coap" and "coaps" URI schemes can thus be compared to the "http" and
+ "https" URI schemes, respectively.
+
+ The syntax of the "coap" and "coaps" URI schemes is specified in this
+ section in Augmented Backus-Naur Form (ABNF) [RFC5234]. The
+ definitions of "host", "port", "path-abempty", "query", "segment",
+ "IP-literal", "IPv4address", and "reg-name" are adopted from
+ [RFC3986].
+
+ Implementation Note: Unfortunately, over time, the URI format has
+ acquired significant complexity. Implementers are encouraged to
+ examine [RFC3986] closely. For example, the ABNF for IPv6
+ addresses is more complicated than maybe expected. Also,
+ implementers should take care to perform the processing of
+ percent-decoding or percent-encoding exactly once on the way from
+ a URI to its decoded components or back. Percent-encoding is
+ crucial for data transparency but may lead to unusual results such
+ as a slash character in a path component.
+
+6.1. coap URI Scheme
+
+ coap-URI = "coap:" "//" host [ ":" port ] path-abempty [ "?" query ]
+
+ If the host component is provided as an IP-literal or IPv4address,
+ then the CoAP server can be reached at that IP address. If host is a
+ registered name, then that name is considered an indirect identifier
+ and the endpoint might use a name resolution service, such as DNS, to
+ find the address of that host. The host MUST NOT be empty; if a URI
+
+
+
+Shelby, et al. Standards Track [Page 59]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ is received with a missing authority or an empty host, then it MUST
+ be considered invalid. The port subcomponent indicates the UDP port
+ at which the CoAP server is located. If it is empty or not given,
+ then the default port 5683 is assumed.
+
+ The path identifies a resource within the scope of the host and port.
+ It consists of a sequence of path segments separated by a slash
+ character (U+002F SOLIDUS "/").
+
+ The query serves to further parameterize the resource. It consists
+ of a sequence of arguments separated by an ampersand character
+ (U+0026 AMPERSAND "&"). An argument is often in the form of a
+ "key=value" pair.
+
+ The "coap" URI scheme supports the path prefix "/.well-known/"
+ defined by [RFC5785] for "well-known locations" in the namespace of a
+ host. This enables discovery of policy or other information about a
+ host ("site-wide metadata"), such as hosted resources (see
+ Section 7).
+
+ Application designers are encouraged to make use of short but
+ descriptive URIs. As the environments that CoAP is used in are
+ usually constrained for bandwidth and energy, the trade-off between
+ these two qualities should lean towards the shortness, without
+ ignoring descriptiveness.
+
+6.2. coaps URI Scheme
+
+ coaps-URI = "coaps:" "//" host [ ":" port ] path-abempty
+ [ "?" query ]
+
+ All of the requirements listed above for the "coap" scheme are also
+ requirements for the "coaps" scheme, except that a default UDP port
+ of 5684 is assumed if the port subcomponent is empty or not given,
+ and the UDP datagrams MUST be secured through the use of DTLS as
+ described in Section 9.1.
+
+ Considerations for caching of responses to "coaps" identified
+ requests are discussed in Section 11.2.
+
+ Resources made available via the "coaps" scheme have no shared
+ identity with the "coap" scheme even if their resource identifiers
+ indicate the same authority (the same host listening to the same UDP
+ port). They are distinct namespaces and are considered to be
+ distinct origin servers.
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 60]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+6.3. Normalization and Comparison Rules
+
+ Since the "coap" and "coaps" schemes conform to the URI generic
+ syntax, such URIs are normalized and compared according to the
+ algorithm defined in [RFC3986], Section 6, using the defaults
+ described above for each scheme.
+
+ If the port is equal to the default port for a scheme, the normal
+ form is to elide the port subcomponent. Likewise, an empty path
+ component is equivalent to an absolute path of "/", so the normal
+ form is to provide a path of "/" instead. The scheme and host are
+ case insensitive and normally provided in lowercase; IP-literals are
+ in recommended form [RFC5952]; all other components are compared in a
+ case-sensitive manner. Characters other than those in the "reserved"
+ set are equivalent to their percent-encoded bytes (see [RFC3986],
+ Section 2.1): the normal form is to not encode them.
+
+ For example, the following three URIs are equivalent and cause the
+ same options and option values to appear in the CoAP messages:
+
+ coap://example.com:5683/~sensors/temp.xml
+ coap://EXAMPLE.com/%7Esensors/temp.xml
+ coap://EXAMPLE.com:/%7esensors/temp.xml
+
+6.4. Decomposing URIs into Options
+
+ The steps to parse a request's options from a string |url| are as
+ follows. These steps either result in zero or more of the Uri-Host,
+ Uri-Port, Uri-Path, and Uri-Query Options being included in the
+ request or they fail.
+
+ 1. If the |url| string is not an absolute URI ([RFC3986]), then fail
+ this algorithm.
+
+ 2. Resolve the |url| string using the process of reference
+ resolution defined by [RFC3986]. At this stage, the URL is in
+ ASCII encoding [RFC0020], even though the decoded components will
+ be interpreted in UTF-8 [RFC3629] after steps 5, 8, and 9.
+
+ NOTE: It doesn't matter what it is resolved relative to, since we
+ already know it is an absolute URL at this point.
+
+ 3. If |url| does not have a <scheme> component whose value, when
+ converted to ASCII lowercase, is "coap" or "coaps", then fail
+ this algorithm.
+
+ 4. If |url| has a <fragment> component, then fail this algorithm.
+
+
+
+
+Shelby, et al. Standards Track [Page 61]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ 5. If the <host> component of |url| does not represent the request's
+ destination IP address as an IP-literal or IPv4address, include a
+ Uri-Host Option and let that option's value be the value of the
+ <host> component of |url|, converted to ASCII lowercase, and then
+ convert all percent-encodings ("%" followed by two hexadecimal
+ digits) to the corresponding characters.
+
+ NOTE: In the usual case where the request's destination IP
+ address is derived from the host part, this ensures that a Uri-
+ Host Option is only used for a <host> component of the form reg-
+ name.
+
+ 6. If |url| has a <port> component, then let |port| be that
+ component's value interpreted as a decimal integer; otherwise,
+ let |port| be the default port for the scheme.
+
+ 7. If |port| does not equal the request's destination UDP port,
+ include a Uri-Port Option and let that option's value be |port|.
+
+ 8. If the value of the <path> component of |url| is empty or
+ consists of a single slash character (U+002F SOLIDUS "/"), then
+ move to the next step.
+
+ Otherwise, for each segment in the <path> component, include a
+ Uri-Path Option and let that option's value be the segment (not
+ including the delimiting slash characters) after converting each
+ percent-encoding ("%" followed by two hexadecimal digits) to the
+ corresponding byte.
+
+ 9. If |url| has a <query> component, then, for each argument in the
+ <query> component, include a Uri-Query Option and let that
+ option's value be the argument (not including the question mark
+ and the delimiting ampersand characters) after converting each
+ percent-encoding to the corresponding byte.
+
+ Note that these rules completely resolve any percent-encoding.
+
+6.5. Composing URIs from Options
+
+ The steps to construct a URI from a request's options are as follows.
+ These steps either result in a URI or they fail. In these steps,
+ percent-encoding a character means replacing each of its
+ (UTF-8-encoded) bytes by a "%" character followed by two hexadecimal
+ digits representing the byte, where the digits A-F are in uppercase
+ (as defined in Section 2.1 of [RFC3986]; to reduce variability, the
+ hexadecimal notation for percent-encoding in CoAP URIs MUST use
+ uppercase letters). The definitions of "unreserved" and "sub-delims"
+ are adopted from [RFC3986].
+
+
+
+Shelby, et al. Standards Track [Page 62]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ 1. If the request is secured using DTLS, let |url| be the string
+ "coaps://". Otherwise, let |url| be the string "coap://".
+
+ 2. If the request includes a Uri-Host Option, let |host| be that
+ option's value, where any non-ASCII characters are replaced by
+ their corresponding percent-encoding. If |host| is not a valid
+ reg-name or IP-literal or IPv4address, fail the algorithm. If
+ the request does not include a Uri-Host Option, let |host| be
+ the IP-literal (making use of the conventions of [RFC5952]) or
+ IPv4address representing the request's destination IP address.
+
+ 3. Append |host| to |url|.
+
+ 4. If the request includes a Uri-Port Option, let |port| be that
+ option's value. Otherwise, let |port| be the request's
+ destination UDP port.
+
+ 5. If |port| is not the default port for the scheme, then append a
+ single U+003A COLON character (:) followed by the decimal
+ representation of |port| to |url|.
+
+ 6. Let |resource name| be the empty string. For each Uri-Path
+ Option in the request, append a single character U+002F SOLIDUS
+ (/) followed by the option's value to |resource name|, after
+ converting any character that is not either in the "unreserved"
+ set, in the "sub-delims" set, a U+003A COLON (:) character, or a
+ U+0040 COMMERCIAL AT (@) character to its percent-encoded form.
+
+ 7. If |resource name| is the empty string, set it to a single
+ character U+002F SOLIDUS (/).
+
+ 8. For each Uri-Query Option in the request, append a single
+ character U+003F QUESTION MARK (?) (first option) or U+0026
+ AMPERSAND (&) (subsequent options) followed by the option's
+ value to |resource name|, after converting any character that is
+ not either in the "unreserved" set, in the "sub-delims" set
+ (except U+0026 AMPERSAND (&)), a U+003A COLON (:), a U+0040
+ COMMERCIAL AT (@), a U+002F SOLIDUS (/), or a U+003F QUESTION
+ MARK (?) character to its percent-encoded form.
+
+ 9. Append |resource name| to |url|.
+
+ 10. Return |url|.
+
+ Note that these steps have been designed to lead to a URI in normal
+ form (see Section 6.3).
+
+
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+7. Discovery
+
+7.1. Service Discovery
+
+ As a part of discovering the services offered by a CoAP server, a
+ client has to learn about the endpoint used by a server.
+
+ A server is discovered by a client (knowing or) learning a URI that
+ references a resource in the namespace of the server. Alternatively,
+ clients can use multicast CoAP (see Section 8) and the "All CoAP
+ Nodes" multicast address to find CoAP servers.
+
+ Unless the port subcomponent in a "coap" or "coaps" URI indicates the
+ UDP port at which the CoAP server is located, the server is assumed
+ to be reachable at the default port.
+
+ The CoAP default port number 5683 MUST be supported by a server that
+ offers resources for resource discovery (see Section 7.2 below) and
+ SHOULD be supported for providing access to other resources. The
+ default port number 5684 for DTLS-secured CoAP MAY be supported by a
+ server for resource discovery and for providing access to other
+ resources. In addition, other endpoints may be hosted at other
+ ports, e.g., in the dynamic port space.
+
+ Implementation Note: When a CoAP server is hosted by a 6LoWPAN node,
+ header compression efficiency is improved when it also supports a
+ port number in the 61616-61631 compressed UDP port space defined
+ in [RFC4944] and [RFC6282]. (Note that, as its UDP port differs
+ from the default port, it is a different endpoint from the server
+ at the default port.)
+
+7.2. Resource Discovery
+
+ The discovery of resources offered by a CoAP endpoint is extremely
+ important in machine-to-machine applications where there are no
+ humans in the loop and static interfaces result in fragility. To
+ maximize interoperability in a CoRE environment, a CoAP endpoint
+ SHOULD support the CoRE Link Format of discoverable resources as
+ described in [RFC6690], except where fully manual configuration is
+ desired. It is up to the server which resources are made
+ discoverable (if any).
+
+7.2.1. 'ct' Attribute
+
+ This section defines a new Web Linking [RFC5988] attribute for use
+ with [RFC6690]. The Content-Format code "ct" attribute provides a
+ hint about the Content-Formats this resource returns. Note that this
+ is only a hint, and it does not override the Content-Format Option of
+
+
+
+Shelby, et al. Standards Track [Page 64]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ a CoAP response obtained by actually requesting the representation of
+ the resource. The value is in the CoAP identifier code format as a
+ decimal ASCII integer and MUST be in the range of 0-65535 (16-bit
+ unsigned integer). For example, "application/xml" would be indicated
+ as "ct=41". If no Content-Format code attribute is present, then
+ nothing about the type can be assumed. The Content-Format code
+ attribute MAY include a space-separated sequence of Content-Format
+ codes, indicating that multiple content-formats are available. The
+ syntax of the attribute value is summarized in the production "ct-
+ value" in Figure 12, where "cardinal", "SP", and "DQUOTE" are defined
+ as in [RFC6690].
+
+ ct-value = cardinal
+ / DQUOTE cardinal *( 1*SP cardinal ) DQUOTE
+
+ Figure 12
+
+8. Multicast CoAP
+
+ CoAP supports making requests to an IP multicast group. This is
+ defined by a series of deltas to unicast CoAP. A more general
+ discussion of group communication with CoAP is in [GROUPCOMM].
+
+ CoAP endpoints that offer services that they want other endpoints to
+ be able to find using multicast service discovery join one or more of
+ the appropriate all-CoAP-node multicast addresses (Section 12.8) and
+ listen on the default CoAP port. Note that an endpoint might receive
+ multicast requests on other multicast addresses, including the all-
+ nodes IPv6 address (or via broadcast on IPv4); an endpoint MUST
+ therefore be prepared to receive such messages but MAY ignore them if
+ multicast service discovery is not desired.
+
+8.1. Messaging Layer
+
+ A multicast request is characterized by being transported in a CoAP
+ message that is addressed to an IP multicast address instead of a
+ CoAP endpoint. Such multicast requests MUST be Non-confirmable.
+
+ A server SHOULD be aware that a request arrived via multicast, e.g.,
+ by making use of modern APIs such as IPV6_RECVPKTINFO [RFC3542], if
+ available.
+
+ To avoid an implosion of error responses, when a server is aware that
+ a request arrived via multicast, it MUST NOT return a Reset message
+ in reply to a Non-confirmable message. If it is not aware, it MAY
+ return a Reset message in reply to a Non-confirmable message as
+ usual. Because such a Reset message will look identical to one for a
+
+
+
+
+Shelby, et al. Standards Track [Page 65]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ unicast message from the sender, the sender MUST avoid using a
+ Message ID that is also still active from this endpoint with any
+ unicast endpoint that might receive the multicast message.
+
+ At the time of writing, multicast messages can only be carried in UDP
+ not in DTLS. This means that the security modes defined for CoAP in
+ this document are not applicable to multicast.
+
+8.2. Request/Response Layer
+
+ When a server is aware that a request arrived via multicast, the
+ server MAY always ignore the request, in particular if it doesn't
+ have anything useful to respond (e.g., if it only has an empty
+ payload or an error response). The decision for this may depend on
+ the application. (For example, in query filtering as described in
+ [RFC6690], a server should not respond to a multicast request if the
+ filter does not match. More examples are in [GROUPCOMM].)
+
+ If a server does decide to respond to a multicast request, it should
+ not respond immediately. Instead, it should pick a duration for the
+ period of time during which it intends to respond. For the purposes
+ of this exposition, we call the length of this period the Leisure.
+ The specific value of this Leisure may depend on the application or
+ MAY be derived as described below. The server SHOULD then pick a
+ random point of time within the chosen leisure period to send back
+ the unicast response to the multicast request. If further responses
+ need to be sent based on the same multicast address membership, a new
+ leisure period starts at the earliest after the previous one
+ finishes.
+
+ To compute a value for Leisure, the server should have a group size
+ estimate G, a target data transfer rate R (which both should be
+ chosen conservatively), and an estimated response size S; a rough
+ lower bound for Leisure can then be computed as
+
+ lb_Leisure = S * G / R
+
+ For example, for a multicast request with link-local scope on a 2.4
+ GHz IEEE 802.15.4 (6LoWPAN) network, G could be (relatively
+ conservatively) set to 100, S to 100 bytes, and the target rate to 8
+ kbit/s = 1 kB/s. The resulting lower bound for the Leisure is 10
+ seconds.
+
+ If a CoAP endpoint does not have suitable data to compute a value for
+ Leisure, it MAY resort to DEFAULT_LEISURE.
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 66]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ When matching a response to a multicast request, only the token MUST
+ match; the source endpoint of the response does not need to (and will
+ not) be the same as the destination endpoint of the original request.
+
+ For the purposes of interpreting the Location-* options and any links
+ embedded in the representation, the request URI (i.e., the base URI
+ relative to which the response is interpreted) is formed by replacing
+ the multicast address in the Host component of the original request
+ URI by the literal IP address of the endpoint actually responding.
+
+8.2.1. Caching
+
+ When a client makes a multicast request, it always makes a new
+ request to the multicast group (since there may be new group members
+ that joined meanwhile or ones that did not get the previous request).
+ It MAY update a cache with the received responses. Then, it uses
+ both cached-still-fresh and new responses as the result of the
+ request.
+
+ A response received in reply to a GET request to a multicast group
+ MAY be used to satisfy a subsequent request on the related unicast
+ request URI. The unicast request URI is obtained by replacing the
+ authority part of the request URI with the transport-layer source
+ address of the response message.
+
+ A cache MAY revalidate a response by making a GET request on the
+ related unicast request URI.
+
+ A GET request to a multicast group MUST NOT contain an ETag option.
+ A mechanism to suppress responses the client already has is left for
+ further study.
+
+8.2.2. Proxying
+
+ When a forward-proxy receives a request with a Proxy-Uri or URI
+ constructed from Proxy-Scheme that indicates a multicast address, the
+ proxy obtains a set of responses as described above and sends all
+ responses (both cached-still-fresh and new) back to the original
+ client.
+
+ This specification does not provide a way to indicate the unicast-
+ modified request URI (base URI) in responses thus forwarded.
+ Proxying multicast requests is discussed in more detail in
+ [GROUPCOMM]; one proposal to address the base URI issue can be found
+ in Section 3 of [CoAP-MISC].
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 67]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+9. Securing CoAP
+
+ This section defines the DTLS binding for CoAP.
+
+ During the provisioning phase, a CoAP device is provided with the
+ security information that it needs, including keying materials and
+ access control lists. This specification defines provisioning for
+ the RawPublicKey mode in Section 9.1.3.2.1. At the end of the
+ provisioning phase, the device will be in one of four security modes
+ with the following information for the given mode. The NoSec and
+ RawPublicKey modes are mandatory to implement for this specification.
+
+ NoSec: There is no protocol-level security (DTLS is disabled).
+ Alternative techniques to provide lower-layer security SHOULD be
+ used when appropriate. The use of IPsec is discussed in
+ [IPsec-CoAP]. Certain link layers in use with constrained nodes
+ also provide link-layer security, which may be appropriate with
+ proper key management.
+
+ PreSharedKey: DTLS is enabled, there is a list of pre-shared keys
+ [RFC4279], and each key includes a list of which nodes it can be
+ used to communicate with as described in Section 9.1.3.1. At the
+ extreme, there may be one key for each node this CoAP node needs
+ to communicate with (1:1 node/key ratio). Conversely, if more
+ than two entities share a specific pre-shared key, this key only
+ enables the entities to authenticate as a member of that group and
+ not as a specific peer.
+
+ RawPublicKey: DTLS is enabled and the device has an asymmetric key
+ pair without a certificate (a raw public key) that is validated
+ using an out-of-band mechanism [RFC7250] as described in
+ Section 9.1.3.2. The device also has an identity calculated from
+ the public key and a list of identities of the nodes it can
+ communicate with.
+
+ Certificate: DTLS is enabled and the device has an asymmetric key
+ pair with an X.509 certificate [RFC5280] that binds it to its
+ subject and is signed by some common trust root as described in
+ Section 9.1.3.3. The device also has a list of root trust anchors
+ that can be used for validating a certificate.
+
+ In the "NoSec" mode, the system simply sends the packets over normal
+ UDP over IP and is indicated by the "coap" scheme and the CoAP
+ default port. The system is secured only by keeping attackers from
+ being able to send or receive packets from the network with the CoAP
+ nodes; see Section 11.5 for an additional complication with this
+ approach.
+
+
+
+
+Shelby, et al. Standards Track [Page 68]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ The other three security modes are achieved using DTLS and are
+ indicated by the "coaps" scheme and DTLS-secured CoAP default port.
+ The result is a security association that can be used to authenticate
+ (within the limits of the security model) and, based on this
+ authentication, authorize the communication partner. CoAP itself
+ does not provide protocol primitives for authentication or
+ authorization; where this is required, it can either be provided by
+ communication security (i.e., IPsec or DTLS) or by object security
+ (within the payload). Devices that require authorization for certain
+ operations are expected to require one of these two forms of
+ security. Necessarily, where an intermediary is involved,
+ communication security only works when that intermediary is part of
+ the trust relationships. CoAP does not provide a way to forward
+ different levels of authorization that clients may have with an
+ intermediary to further intermediaries or origin servers -- it
+ therefore may be required to perform all authorization at the first
+ intermediary.
+
+9.1. DTLS-Secured CoAP
+
+ Just as HTTP is secured using Transport Layer Security (TLS) over
+ TCP, CoAP is secured using Datagram TLS (DTLS) [RFC6347] over UDP
+ (see Figure 13). This section defines the CoAP binding to DTLS,
+ along with the minimal mandatory-to-implement configurations
+ appropriate for constrained environments. The binding is defined by
+ a series of deltas to unicast CoAP. In practice, DTLS is TLS with
+ added features to deal with the unreliable nature of the UDP
+ transport.
+
+ +----------------------+
+ | Application |
+ +----------------------+
+ +----------------------+
+ | Requests/Responses |
+ |----------------------| CoAP
+ | Messages |
+ +----------------------+
+ +----------------------+
+ | DTLS |
+ +----------------------+
+ +----------------------+
+ | UDP |
+ +----------------------+
+
+ Figure 13: Abstract Layering of DTLS-Secured CoAP
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 69]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ In some constrained nodes (limited flash and/or RAM) and networks
+ (limited bandwidth or high scalability requirements), and depending
+ on the specific cipher suites in use, all modes of DTLS may not be
+ applicable. Some DTLS cipher suites can add significant
+ implementation complexity as well as some initial handshake overhead
+ needed when setting up the security association. Once the initial
+ handshake is completed, DTLS adds a limited per-datagram overhead of
+ approximately 13 bytes, not including any initialization vectors/
+ nonces (e.g., 8 bytes with TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]),
+ integrity check values (e.g., 8 bytes with TLS_PSK_WITH_AES_128_CCM_8
+ [RFC6655]), and padding required by the cipher suite. Whether the
+ use of a given mode of DTLS is applicable for a CoAP-based
+ application should be carefully weighed considering the specific
+ cipher suites that may be applicable, whether the session maintenance
+ makes it compatible with application flows, and whether sufficient
+ resources are available on the constrained nodes and for the added
+ network overhead. (For some modes of using DTLS, this specification
+ identifies a mandatory-to-implement cipher suite. This is an
+ implementation requirement to maximize interoperability in those
+ cases where these cipher suites are indeed appropriate. The specific
+ security policies of an application may determine the actual set of
+ cipher suites that can be used.) DTLS is not applicable to group
+ keying (multicast communication); however, it may be a component in a
+ future group key management protocol.
+
+9.1.1. Messaging Layer
+
+ The endpoint acting as the CoAP client should also act as the DTLS
+ client. It should initiate a session to the server on the
+ appropriate port. When the DTLS handshake has finished, the client
+ may initiate the first CoAP request. All CoAP messages MUST be sent
+ as DTLS "application data".
+
+ The following rules are added for matching an Acknowledgement message
+ or Reset message to a Confirmable message, or a Reset message to a
+ Non-confirmable message: The DTLS session MUST be the same, and the
+ epoch MUST be the same.
+
+ A message is the same when it is sent within the same DTLS session
+ and same epoch and has the same Message ID.
+
+ Note: When a Confirmable message is retransmitted, a new DTLS
+ sequence_number is used for each attempt, even though the CoAP
+ Message ID stays the same. So a recipient still has to perform
+ deduplication as described in Section 4.5. Retransmissions MUST NOT
+ be performed across epochs.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 70]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ DTLS connections in RawPublicKey and Certificate mode are set up
+ using mutual authentication so they can remain up and be reused for
+ future message exchanges in either direction. Devices can close a
+ DTLS connection when they need to recover resources, but in general
+ they should keep the connection up for as long as possible. Closing
+ the DTLS connection after every CoAP message exchange is very
+ inefficient.
+
+9.1.2. Request/Response Layer
+
+ The following rules are added for matching a response to a request:
+ The DTLS session MUST be the same, and the epoch MUST be the same.
+
+ This means the response to a DTLS secured request MUST always be DTLS
+ secured using the same security session and epoch. Any attempt to
+ supply a NoSec response to a DTLS request simply does not match the
+ request and therefore MUST be rejected (unless it does match an
+ unrelated NoSec request).
+
+9.1.3. Endpoint Identity
+
+ Devices SHOULD support the Server Name Indication (SNI) to indicate
+ their authority in the SNI HostName field as defined in Section 3 of
+ [RFC6066]. This is needed so that when a host that acts as a virtual
+ server for multiple Authorities receives a new DTLS connection, it
+ knows which keys to use for the DTLS session.
+
+9.1.3.1. Pre-Shared Keys
+
+ When forming a connection to a new node, the system selects an
+ appropriate key based on which nodes it is trying to reach and then
+ forms a DTLS session using a PSK (Pre-Shared Key) mode of DTLS.
+ Implementations in these modes MUST support the mandatory-to-
+ implement cipher suite TLS_PSK_WITH_AES_128_CCM_8 as specified in
+ [RFC6655].
+
+ Depending on the commissioning model, applications may need to define
+ an application profile for identity hints (as required and detailed
+ in Section 5.2 of [RFC4279]) to enable the use of PSK identity hints.
+
+ The security considerations of Section 7 of [RFC4279] apply. In
+ particular, applications should carefully weigh whether or not they
+ need Perfect Forward Secrecy (PFS) and select an appropriate cipher
+ suite (Section 7.1 of [RFC4279]). The entropy of the PSK must be
+ sufficient to mitigate against brute-force and (where the PSK is not
+ chosen randomly but by a human) dictionary attacks (Section 7.2 of
+ [RFC4279]). The cleartext communication of client identities may
+ leak data or compromise privacy (Section 7.3 of [RFC4279]).
+
+
+
+Shelby, et al. Standards Track [Page 71]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+9.1.3.2. Raw Public Key Certificates
+
+ In this mode, the device has an asymmetric key pair but without an
+ X.509 certificate (called a raw public key); for example, the
+ asymmetric key pair is generated by the manufacturer and installed on
+ the device (see also Section 11.6). A device MAY be configured with
+ multiple raw public keys. The type and length of the raw public key
+ depends on the cipher suite used. Implementations in RawPublicKey
+ mode MUST support the mandatory-to-implement cipher suite
+ TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as specified in [RFC7251],
+ [RFC5246], and [RFC4492]. The key used MUST be ECDSA capable. The
+ curve secp256r1 MUST be supported [RFC4492]; this curve is equivalent
+ to the NIST P-256 curve. The hash algorithm is SHA-256.
+ Implementations MUST use the Supported Elliptic Curves and Supported
+ Point Formats Extensions [RFC4492]; the uncompressed point format
+ MUST be supported; [RFC6090] can be used as an implementation method.
+ Some guidance relevant to the implementation of this cipher suite can
+ be found in [W3CXMLSEC]. The mechanism for using raw public keys
+ with TLS is specified in [RFC7250].
+
+ Implementation Note: Specifically, this means the extensions listed
+ in Figure 14 with at least the values listed will be present in
+ the DTLS handshake.
+
+ Extension: elliptic_curves
+ Type: elliptic_curves (0x000a)
+ Length: 4
+ Elliptic Curves Length: 2
+ Elliptic curves (1 curve)
+ Elliptic curve: secp256r1 (0x0017)
+
+ Extension: ec_point_formats
+ Type: ec_point_formats (0x000b)
+ Length: 2
+ EC point formats Length: 1
+ Elliptic curves point formats (1)
+ EC point format: uncompressed (0)
+
+ Extension: signature_algorithms
+ Type: signature_algorithms (0x000d)
+ Length: 4
+ Data (4 bytes): 00 02 04 03
+ HashAlgorithm: sha256 (4)
+ SignatureAlgorithm: ecdsa (3)
+
+ Figure 14: DTLS Extensions Present for
+ TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
+
+
+
+
+Shelby, et al. Standards Track [Page 72]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+9.1.3.2.1. Provisioning
+
+ The RawPublicKey mode was designed to be easily provisioned in M2M
+ deployments. It is assumed that each device has an appropriate
+ asymmetric public key pair installed. An identifier is calculated by
+ the endpoint from the public key as described in Section 2 of
+ [RFC6920]. All implementations that support checking RawPublicKey
+ identities MUST support at least the sha-256-120 mode (SHA-256
+ truncated to 120 bits). Implementations SHOULD also support longer
+ length identifiers and MAY support shorter lengths. Note that the
+ shorter lengths provide less security against attacks, and their use
+ is NOT RECOMMENDED.
+
+ Depending on how identifiers are given to the system that verifies
+ them, support for URI, binary, and/or human-speakable format
+ [RFC6920] needs to be implemented. All implementations SHOULD
+ support the binary mode, and implementations that have a user
+ interface SHOULD also support the human-speakable format.
+
+ During provisioning, the identifier of each node is collected, for
+ example, by reading a barcode on the outside of the device or by
+ obtaining a pre-compiled list of the identifiers. These identifiers
+ are then installed in the corresponding endpoint, for example, an M2M
+ data collection server. The identifier is used for two purposes, to
+ associate the endpoint with further device information and to perform
+ access control. During (initial and ongoing) provisioning, an access
+ control list of identifiers with which the device may start DTLS
+ sessions SHOULD also be installed and maintained.
+
+9.1.3.3. X.509 Certificates
+
+ Implementations in Certificate Mode MUST support the mandatory-to-
+ implement cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as
+ specified in [RFC7251], [RFC5246], and [RFC4492]. Namely, the
+ certificate includes a SubjectPublicKeyInfo that indicates an
+ algorithm of id-ecPublicKey with namedCurves secp256r1 [RFC5480]; the
+ public key format is uncompressed [RFC5480]; the hash algorithm is
+ SHA-256; if included, the key usage extension indicates
+ digitalSignature. Certificates MUST be signed with ECDSA using
+ secp256r1, and the signature MUST use SHA-256. The key used MUST be
+ ECDSA capable. The curve secp256r1 MUST be supported [RFC4492]; this
+ curve is equivalent to the NIST P-256 curve. The hash algorithm is
+ SHA-256. Implementations MUST use the Supported Elliptic Curves and
+ Supported Point Formats Extensions [RFC4492]; the uncompressed point
+ format MUST be supported; [RFC6090] can be used as an implementation
+ method.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 73]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ The subject in the certificate would be built out of a long-term
+ unique identifier for the device such as the EUI-64 [EUI64]. The
+ subject could also be based on the Fully Qualified Domain Name (FQDN)
+ that was used as the Host part of the CoAP URI. However, the
+ device's IP address should not typically be used as the subject, as
+ it would change over time. The discovery process used in the system
+ would build up the mapping between IP addresses of the given devices
+ and the subject for each device. Some devices could have more than
+ one subject and would need more than a single certificate.
+
+ When a new connection is formed, the certificate from the remote
+ device needs to be verified. If the CoAP node has a source of
+ absolute time, then the node SHOULD check that the validity dates of
+ the certificate are within range. The certificate MUST be validated
+ as appropriate for the security requirements, using functionality
+ equivalent to the algorithm specified in Section 6 of [RFC5280]. If
+ the certificate contains a SubjectAltName, then the authority of the
+ request URI MUST match at least one of the authorities of any CoAP
+ URI found in a field of URI type in the SubjectAltName set. If there
+ is no SubjectAltName in the certificate, then the authority of the
+ request URI MUST match the Common Name (CN) found in the certificate
+ using the matching rules defined in [RFC3280] with the exception that
+ certificates with wildcards are not allowed.
+
+ CoRE support for certificate status checking requires further study.
+ As a mapping of the Online Certificate Status Protocol (OCSP)
+ [RFC6960] onto CoAP is not currently defined and OCSP may also not be
+ easily applicable in all environments, an alternative approach may be
+ using the TLS Certificate Status Request extension (Section 8 of
+ [RFC6066]; also known as "OCSP stapling") or preferably the Multiple
+ Certificate Status Extension ([RFC6961]), if available.
+
+ If the system has a shared key in addition to the certificate, then a
+ cipher suite that includes the shared key such as
+ TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA [RFC5489] SHOULD be used.
+
+10. Cross-Protocol Proxying between CoAP and HTTP
+
+ CoAP supports a limited subset of HTTP functionality, and thus cross-
+ protocol proxying to HTTP is straightforward. There might be several
+ reasons for proxying between CoAP and HTTP, for example, when
+ designing a web interface for use over either protocol or when
+ realizing a CoAP-HTTP proxy. Likewise, CoAP could equally be proxied
+ to other protocols such as XMPP [RFC6120] or SIP [RFC3264]; the
+ definition of these mechanisms is out of scope for this
+ specification.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 74]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ There are two possible directions to access a resource via a forward-
+ proxy:
+
+ CoAP-HTTP Proxying: Enables CoAP clients to access resources on HTTP
+ servers through an intermediary. This is initiated by including
+ the Proxy-Uri or Proxy-Scheme Option with an "http" or "https" URI
+ in a CoAP request to a CoAP-HTTP proxy.
+
+ HTTP-CoAP Proxying: Enables HTTP clients to access resources on CoAP
+ servers through an intermediary. This is initiated by specifying
+ a "coap" or "coaps" URI in the Request-Line of an HTTP request to
+ an HTTP-CoAP proxy.
+
+ Either way, only the request/response model of CoAP is mapped to
+ HTTP. The underlying model of Confirmable or Non-confirmable
+ messages, etc., is invisible and MUST have no effect on a proxy
+ function. The following sections describe the handling of requests
+ to a forward-proxy. Reverse-proxies are not specified, as the proxy
+ function is transparent to the client with the proxy acting as if it
+ were the origin server. However, similar considerations apply to
+ reverse-proxies as to forward-proxies, and there generally will be an
+ expectation that reverse-proxies operate in a similar way forward-
+ proxies would. As an implementation note, HTTP client libraries may
+ make it hard to operate an HTTP-CoAP forward-proxy by not providing a
+ way to put a CoAP URI on the HTTP Request-Line; reverse-proxying may
+ therefore lead to wider applicability of a proxy. A separate
+ specification may define a convention for URIs operating such an
+ HTTP-CoAP reverse-proxy [MAPPING].
+
+10.1. CoAP-HTTP Proxying
+
+ If a request contains a Proxy-Uri or Proxy-Scheme Option with an
+ 'http' or 'https' URI [RFC2616], then the receiving CoAP endpoint
+ (called "the proxy" henceforth) is requested to perform the operation
+ specified by the request method on the indicated HTTP resource and
+ return the result to the client. (See also Section 5.7 for how the
+ request to the proxy is formulated, including security requirements.)
+
+ This section specifies for any CoAP request the CoAP response that
+ the proxy should return to the client. How the proxy actually
+ satisfies the request is an implementation detail, although the
+ typical case is expected to be that the proxy translates and forwards
+ the request to an HTTP origin server.
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 75]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Since HTTP and CoAP share the basic set of request methods,
+ performing a CoAP request on an HTTP resource is not so different
+ from performing it on a CoAP resource. The meanings of the
+ individual CoAP methods when performed on HTTP resources are
+ explained in the subsections of this section.
+
+ If the proxy is unable or unwilling to service a request with an HTTP
+ URI, a 5.05 (Proxying Not Supported) response is returned to the
+ client. If the proxy services the request by interacting with a
+ third party (such as the HTTP origin server) and is unable to obtain
+ a result within a reasonable time frame, a 5.04 (Gateway Timeout)
+ response is returned; if a result can be obtained but is not
+ understood, a 5.02 (Bad Gateway) response is returned.
+
+10.1.1. GET
+
+ The GET method requests the proxy to return a representation of the
+ HTTP resource identified by the request URI.
+
+ Upon success, a 2.05 (Content) Response Code SHOULD be returned. The
+ payload of the response MUST be a representation of the target HTTP
+ resource, and the Content-Format Option MUST be set accordingly. The
+ response MUST indicate a Max-Age value that is no greater than the
+ remaining time the representation can be considered fresh. If the
+ HTTP entity has an entity-tag, the proxy SHOULD include an ETag
+ Option in the response and process ETag Options in requests as
+ described below.
+
+ A client can influence the processing of a GET request by including
+ the following option:
+
+ Accept: The request MAY include an Accept Option, identifying the
+ preferred response content-format.
+
+ ETag: The request MAY include one or more ETag Options, identifying
+ responses that the client has stored. This requests the proxy to
+ send a 2.03 (Valid) response whenever it would send a 2.05
+ (Content) response with an entity-tag in the requested set
+ otherwise. Note that CoAP ETags are always strong ETags in the
+ HTTP sense; CoAP does not have the equivalent of HTTP weak ETags,
+ and there is no good way to make use of these in a cross-proxy.
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 76]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+10.1.2. PUT
+
+ The PUT method requests the proxy to update or create the HTTP
+ resource identified by the request URI with the enclosed
+ representation.
+
+ If a new resource is created at the request URI, a 2.01 (Created)
+ response MUST be returned to the client. If an existing resource is
+ modified, a 2.04 (Changed) response MUST be returned to indicate
+ successful completion of the request.
+
+10.1.3. DELETE
+
+ The DELETE method requests the proxy to delete the HTTP resource
+ identified by the request URI at the HTTP origin server.
+
+ A 2.02 (Deleted) response MUST be returned to the client upon success
+ or if the resource does not exist at the time of the request.
+
+10.1.4. POST
+
+ The POST method requests the proxy to have the representation
+ enclosed in the request be processed by the HTTP origin server. The
+ actual function performed by the POST method is determined by the
+ origin server and dependent on the resource identified by the request
+ URI.
+
+ If the action performed by the POST method does not result in a
+ resource that can be identified by a URI, a 2.04 (Changed) response
+ MUST be returned to the client. If a resource has been created on
+ the origin server, a 2.01 (Created) response MUST be returned.
+
+10.2. HTTP-CoAP Proxying
+
+ If an HTTP request contains a Request-URI with a "coap" or "coaps"
+ URI, then the receiving HTTP endpoint (called "the proxy" henceforth)
+ is requested to perform the operation specified by the request method
+ on the indicated CoAP resource and return the result to the client.
+
+ This section specifies for any HTTP request the HTTP response that
+ the proxy should return to the client. Unless otherwise specified,
+ all the statements made are RECOMMENDED behavior; some highly
+ constrained implementations may need to resort to shortcuts. How the
+ proxy actually satisfies the request is an implementation detail,
+ although the typical case is expected to be that the proxy translates
+ and forwards the request to a CoAP origin server. The meanings of
+ the individual HTTP methods when performed on CoAP resources are
+ explained in the subsections of this section.
+
+
+
+Shelby, et al. Standards Track [Page 77]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ If the proxy is unable or unwilling to service a request with a CoAP
+ URI, a 501 (Not Implemented) response is returned to the client. If
+ the proxy services the request by interacting with a third party
+ (such as the CoAP origin server) and is unable to obtain a result
+ within a reasonable time frame, a 504 (Gateway Timeout) response is
+ returned; if a result can be obtained but is not understood, a 502
+ (Bad Gateway) response is returned.
+
+10.2.1. OPTIONS and TRACE
+
+ As the OPTIONS and TRACE methods are not supported in CoAP, a 501
+ (Not Implemented) error MUST be returned to the client.
+
+10.2.2. GET
+
+ The GET method requests the proxy to return a representation of the
+ CoAP resource identified by the Request-URI.
+
+ Upon success, a 200 (OK) response is returned. The payload of the
+ response MUST be a representation of the target CoAP resource, and
+ the Content-Type and Content-Encoding header fields MUST be set
+ accordingly. The response MUST indicate a max-age directive that
+ indicates a value no greater than the remaining time the
+ representation can be considered fresh. If the CoAP response has an
+ ETag option, the proxy should include an ETag header field in the
+ response.
+
+ A client can influence the processing of a GET request by including
+ the following options:
+
+ Accept: The most-preferred media type of the HTTP Accept header
+ field in a request is mapped to a CoAP Accept option. HTTP Accept
+ media-type ranges, parameters, and extensions are not supported by
+ the CoAP Accept option. If the proxy cannot send a response that
+ is acceptable according to the combined Accept field value, then
+ the proxy sends a 406 (Not Acceptable) response. The proxy MAY
+ then retry the request with further media types from the HTTP
+ Accept header field.
+
+ Conditional GETs: Conditional HTTP GET requests that include an "If-
+ Match" or "If-None-Match" request-header field can be mapped to a
+ corresponding CoAP request. The "If-Modified-Since" and "If-
+ Unmodified-Since" request-header fields are not directly supported
+ by CoAP but are implemented locally by a caching proxy.
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 78]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+10.2.3. HEAD
+
+ The HEAD method is identical to GET except that the server MUST NOT
+ return a message-body in the response.
+
+ Although there is no direct equivalent of HTTP's HEAD method in CoAP,
+ an HTTP-CoAP proxy responds to HEAD requests for CoAP resources, and
+ the HTTP headers are returned without a message-body.
+
+ Implementation Note: An HTTP-CoAP proxy may want to try using a
+ block-wise transfer option [BLOCK] to minimize the amount of data
+ actually transferred, but it needs to be prepared for the case
+ that the origin server does not support block-wise transfers.
+
+10.2.4. POST
+
+ The POST method requests the proxy to have the representation
+ enclosed in the request be processed by the CoAP origin server. The
+ actual function performed by the POST method is determined by the
+ origin server and dependent on the resource identified by the request
+ URI.
+
+ If the action performed by the POST method does not result in a
+ resource that can be identified by a URI, a 200 (OK) or 204 (No
+ Content) response MUST be returned to the client. If a resource has
+ been created on the origin server, a 201 (Created) response MUST be
+ returned.
+
+ If any of the Location-* Options are present in the CoAP response, a
+ Location header field constructed from the values of these options is
+ returned.
+
+10.2.5. PUT
+
+ The PUT method requests the proxy to update or create the CoAP
+ resource identified by the Request-URI with the enclosed
+ representation.
+
+ If a new resource is created at the Request-URI, a 201 (Created)
+ response is returned to the client. If an existing resource is
+ modified, either the 200 (OK) or 204 (No Content) Response Codes is
+ sent to indicate successful completion of the request.
+
+
+
+
+
+
+
+
+
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+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+10.2.6. DELETE
+
+ The DELETE method requests the proxy to delete the CoAP resource
+ identified by the Request-URI at the CoAP origin server.
+
+ A successful response is 200 (OK) if the response includes an entity
+ describing the status or 204 (No Content) if the action has been
+ enacted but the response does not include an entity.
+
+10.2.7. CONNECT
+
+ This method cannot currently be satisfied by an HTTP-CoAP proxy
+ function, as TLS to DTLS tunneling has not yet been specified. For
+ now, a 501 (Not Implemented) error is returned to the client.
+
+11. Security Considerations
+
+ This section analyzes the possible threats to the protocol. It is
+ meant to inform protocol and application developers about the
+ security limitations of CoAP as described in this document. As CoAP
+ realizes a subset of the features in HTTP/1.1, the security
+ considerations in Section 15 of [RFC2616] are also pertinent to CoAP.
+ This section concentrates on describing limitations specific to CoAP.
+
+11.1. Parsing the Protocol and Processing URIs
+
+ A network-facing application can exhibit vulnerabilities in its
+ processing logic for incoming packets. Complex parsers are well-
+ known as a likely source of such vulnerabilities, such as the ability
+ to remotely crash a node, or even remotely execute arbitrary code on
+ it. CoAP attempts to narrow the opportunities for introducing such
+ vulnerabilities by reducing parser complexity, by giving the entire
+ range of encodable values a meaning where possible, and by
+ aggressively reducing complexity that is often caused by unnecessary
+ choice between multiple representations that mean the same thing.
+ Much of the URI processing has been moved to the clients, further
+ reducing the opportunities for introducing vulnerabilities into the
+ servers. Even so, the URI processing code in CoAP implementations is
+ likely to be a large source of remaining vulnerabilities and should
+ be implemented with special care. CoAP access control
+ implementations need to ensure they don't introduce vulnerabilities
+ through discrepancies between the code deriving access control
+ decisions from a URI and the code finally serving up the resource
+ addressed by the URI. The most complex parser remaining could be the
+ one for the CoRE Link Format, although this also has been designed
+ with a goal of reduced implementation complexity [RFC6690]. (See
+ also Section 15.2 of [RFC2616].)
+
+
+
+
+Shelby, et al. Standards Track [Page 80]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+11.2. Proxying and Caching
+
+ As mentioned in Section 15.7 of [RFC2616], proxies are by their very
+ nature men-in-the-middle, breaking any IPsec or DTLS protection that
+ a direct CoAP message exchange might have. They are therefore
+ interesting targets for breaking confidentiality or integrity of CoAP
+ message exchanges. As noted in [RFC2616], they are also interesting
+ targets for breaking availability.
+
+ The threat to confidentiality and integrity of request/response data
+ is amplified where proxies also cache. Note that CoAP does not
+ define any of the cache-suppressing Cache-Control options that
+ HTTP/1.1 provides to better protect sensitive data.
+
+ For a caching implementation, any access control considerations that
+ would apply to making the request that generated the cache entry also
+ need to be applied to the value in the cache. This is relevant for
+ clients that implement multiple security domains, as well as for
+ proxies that may serve multiple clients. Also, a caching proxy MUST
+ NOT make cached values available to requests that have lesser
+ transport-security properties than those the proxy would require to
+ perform request forwarding in the first place.
+
+ Unlike the "coap" scheme, responses to "coaps" identified requests
+ are never "public" and thus MUST NOT be reused for shared caching,
+ unless the cache is able to make equivalent access control decisions
+ to the ones that led to the cached entry. They can, however, be
+ reused in a private cache if the message is cacheable by default in
+ CoAP.
+
+ Finally, a proxy that fans out Separate Responses (as opposed to
+ piggybacked Responses) to multiple original requesters may provide
+ additional amplification (see Section 11.3).
+
+11.3. Risk of Amplification
+
+ CoAP servers generally reply to a request packet with a response
+ packet. This response packet may be significantly larger than the
+ request packet. An attacker might use CoAP nodes to turn a small
+ attack packet into a larger attack packet, an approach known as
+ amplification. There is therefore a danger that CoAP nodes could
+ become implicated in denial-of-service (DoS) attacks by using the
+ amplifying properties of the protocol: an attacker that is attempting
+ to overload a victim but is limited in the amount of traffic it can
+ generate can use amplification to generate a larger amount of
+ traffic.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 81]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ This is particularly a problem in nodes that enable NoSec access, are
+ accessible from an attacker, and can access potential victims (e.g.,
+ on the general Internet), as the UDP protocol provides no way to
+ verify the source address given in the request packet. An attacker
+ need only place the IP address of the victim in the source address of
+ a suitable request packet to generate a larger packet directed at the
+ victim.
+
+ As a mitigating factor, many constrained networks will only be able
+ to generate a small amount of traffic, which may make CoAP nodes less
+ attractive for this attack. However, the limited capacity of the
+ constrained network makes the network itself a likely victim of an
+ amplification attack.
+
+ Therefore, large amplification factors SHOULD NOT be provided in the
+ response if the request is not authenticated. A CoAP server can
+ reduce the amount of amplification it provides to an attacker by
+ using slicing/blocking modes of CoAP [BLOCK] and offering large
+ resource representations only in relatively small slices. For
+ example, for a 1000-byte resource, a 10-byte request might result in
+ an 80-byte response (with a 64-byte block) instead of a 1016-byte
+ response, considerably reducing the amplification provided.
+
+ CoAP also supports the use of multicast IP addresses in requests, an
+ important requirement for M2M. Multicast CoAP requests may be the
+ source of accidental or deliberate DoS attacks, especially over
+ constrained networks. This specification attempts to reduce the
+ amplification effects of multicast requests by limiting when a
+ response is returned. To limit the possibility of malicious use,
+ CoAP servers SHOULD NOT accept multicast requests that can not be
+ authenticated in some way, cryptographically or by some multicast
+ boundary limiting the potential sources. If possible, a CoAP server
+ SHOULD limit the support for multicast requests to the specific
+ resources where the feature is required.
+
+ On some general-purpose operating systems providing a POSIX-style API
+ [IEEE1003.1], it is not straightforward to find out whether a packet
+ received was addressed to a multicast address. While many
+ implementations will know whether they have joined a multicast group,
+ this creates a problem for packets addressed to multicast addresses
+ of the form FF0x::1, which are received by every IPv6 node.
+ Implementations SHOULD make use of modern APIs such as
+ IPV6_RECVPKTINFO [RFC3542], if available, to make this determination.
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 82]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+11.4. IP Address Spoofing Attacks
+
+ Due to the lack of a handshake in UDP, a rogue endpoint that is free
+ to read and write messages carried by the constrained network (i.e.,
+ NoSec or PreSharedKey deployments with a nodes/key ratio > 1:1), may
+ easily attack a single endpoint, a group of endpoints, as well as the
+ whole network, e.g., by:
+
+ 1. spoofing a Reset message in response to a Confirmable message or
+ Non-confirmable message, thus making an endpoint "deaf"; or
+
+ 2. spoofing an ACK in response to a CON message, thus potentially
+ preventing the sender of the CON message from retransmitting, and
+ drowning out the actual response; or
+
+ 3. spoofing the entire response with forged payload/options (this
+ has different levels of impact: from single-response disruption,
+ to much bolder attacks on the supporting infrastructure, e.g.,
+ poisoning proxy caches, or tricking validation/lookup interfaces
+ in resource directories and, more generally, any component that
+ stores global network state and uses CoAP as the messaging
+ facility to handle setting or updating state is a potential
+ target.); or
+
+ 4. spoofing a multicast request for a target node; this may result
+ in network congestion/collapse, a DoS attack on the victim, or
+ forced wake-up from sleeping; or
+
+ 5. spoofing observe messages, etc.
+
+ Response spoofing by off-path attackers can be detected and mitigated
+ even without transport layer security by choosing a nontrivial,
+ randomized token in the request (Section 5.3.1). [RFC4086] discusses
+ randomness requirements for security.
+
+ In principle, other kinds of spoofing can be detected by CoAP only in
+ case Confirmable message semantics is used, because of unexpected
+ Acknowledgement or Reset messages coming from the deceived endpoint.
+ But this imposes keeping track of the used Message IDs, which is not
+ always possible, and moreover detection becomes available usually
+ after the damage is already done. This kind of attack can be
+ prevented using security modes other than NoSec.
+
+ With or without source address spoofing, a client can attempt to
+ overload a server by sending requests, preferably complex ones, to a
+ server; address spoofing makes tracing back, and blocking, this
+ attack harder. Given that the cost of a CON request is small, this
+ attack can easily be executed. Under this attack, a constrained node
+
+
+
+Shelby, et al. Standards Track [Page 83]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ with limited total energy available may exhaust that energy much more
+ quickly than planned (battery depletion attack). Also, if the client
+ uses a Confirmable message and the server responds with a Confirmable
+ separate response to a (possibly spoofed) address that does not
+ respond, the server will have to allocate buffer and retransmission
+ logic for each response up to the exhaustion of MAX_TRANSMIT_SPAN,
+ making it more likely that it runs out of resources for processing
+ legitimate traffic. The latter problem can be mitigated somewhat by
+ limiting the rate of responses as discussed in Section 4.7. An
+ attacker could also spoof the address of a legitimate client; this
+ might cause the server, if it uses separate responses, to block
+ legitimate responses to that client because of NSTART=1. All these
+ attacks can be prevented using a security mode other than NoSec, thus
+ leaving only attacks on the security protocol.
+
+11.5. Cross-Protocol Attacks
+
+ The ability to incite a CoAP endpoint to send packets to a fake
+ source address can be used not only for amplification, but also for
+ cross-protocol attacks against a victim listening to UDP packets at a
+ given address (IP address and port). This would occur as follows:
+
+ o The attacker sends a message to a CoAP endpoint with the given
+ address as the fake source address.
+
+ o The CoAP endpoint replies with a message to the given source
+ address.
+
+ o The victim at the given address receives a UDP packet that it
+ interprets according to the rules of a different protocol.
+
+ This may be used to circumvent firewall rules that prevent direct
+ communication from the attacker to the victim but happen to allow
+ communication from the CoAP endpoint (which may also host a valid
+ role in the other protocol) to the victim.
+
+ Also, CoAP endpoints may be the victim of a cross-protocol attack
+ generated through an endpoint of another UDP-based protocol such as
+ DNS. In both cases, attacks are possible if the security properties
+ of the endpoints rely on checking IP addresses (and firewalling off
+ direct attacks sent from outside using fake IP addresses). In
+ general, because of their lack of context, UDP-based protocols are
+ relatively easy targets for cross-protocol attacks.
+
+ Finally, CoAP URIs transported by other means could be used to incite
+ clients to send messages to endpoints of other protocols.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 84]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ One mitigation against cross-protocol attacks is strict checking of
+ the syntax of packets received, combined with sufficient difference
+ in syntax. As an example, it might help if it were difficult to
+ incite a DNS server to send a DNS response that would pass the checks
+ of a CoAP endpoint. Unfortunately, the first two bytes of a DNS
+ reply are an ID that can be chosen by the attacker and that map into
+ the interesting part of the CoAP header, and the next two bytes are
+ then interpreted as CoAP's Message ID (i.e., any value is
+ acceptable). The DNS count words may be interpreted as multiple
+ instances of a (nonexistent but elective) CoAP option 0, or possibly
+ as a Token. The echoed query finally may be manufactured by the
+ attacker to achieve a desired effect on the CoAP endpoint; the
+ response added by the server (if any) might then just be interpreted
+ as added payload.
+
+ 1 1 1 1 1 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | ID | T, TKL, code
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ |QR| Opcode |AA|TC|RD|RA| Z | RCODE | Message ID
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | QDCOUNT | (options 0)
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | ANCOUNT | (options 0)
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | NSCOUNT | (options 0)
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+ | ARCOUNT | (options 0)
+ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+
+ Figure 15: DNS Header ([RFC1035], Section 4.1.1) vs. CoAP Message
+
+ In general, for any pair of protocols, one of the protocols can very
+ well have been designed in a way that enables an attacker to cause
+ the generation of replies that look like messages of the other
+ protocol. It is often much harder to ensure or prove the absence of
+ viable attacks than to generate examples that may not yet completely
+ enable an attack but might be further developed by more creative
+ minds. Cross-protocol attacks can therefore only be completely
+ mitigated if endpoints don't authorize actions desired by an attacker
+ just based on trusting the source IP address of a packet.
+ Conversely, a NoSec environment that completely relies on a firewall
+ for CoAP security not only needs to firewall off the CoAP endpoints
+ but also all other endpoints that might be incited to send UDP
+ messages to CoAP endpoints using some other UDP-based protocol.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 85]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ In addition to the considerations above, the security considerations
+ for DTLS with respect to cross-protocol attacks apply. For example,
+ if the same DTLS security association ("connection") is used to carry
+ data of multiple protocols, DTLS no longer provides protection
+ against cross-protocol attacks between these protocols.
+
+11.6. Constrained-Node Considerations
+
+ Implementers on constrained nodes often find themselves without a
+ good source of entropy [RFC4086]. If that is the case, the node MUST
+ NOT be used for processes that require good entropy, such as key
+ generation. Instead, keys should be generated externally and added
+ to the device during manufacturing or commissioning.
+
+ Due to their low processing power, constrained nodes are particularly
+ susceptible to timing attacks. Special care must be taken in
+ implementation of cryptographic primitives.
+
+ Large numbers of constrained nodes will be installed in exposed
+ environments and will have little resistance to tampering, including
+ recovery of keying materials. This needs to be considered when
+ defining the scope of credentials assigned to them. In particular,
+ assigning a shared key to a group of nodes may make any single
+ constrained node a target for subverting the entire group.
+
+12. IANA Considerations
+
+12.1. CoAP Code Registries
+
+ This document defines two sub-registries for the values of the Code
+ field in the CoAP header within the "Constrained RESTful Environments
+ (CoRE) Parameters" registry, hereafter referred to as the "CoRE
+ Parameters" registry.
+
+ Values in the two sub-registries are eight-bit values notated as
+ three decimal digits c.dd separated by a period between the first and
+ the second digit; the first digit c is between 0 and 7 and denotes
+ the code class; the second and third digits dd denote a decimal
+ number between 00 and 31 for the detail.
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 86]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ All Code values are assigned by sub-registries according to the
+ following ranges:
+
+ 0.00 Indicates an Empty message (see Section 4.1).
+
+ 0.01-0.31 Indicates a request. Values in this range are assigned by
+ the "CoAP Method Codes" sub-registry (see Section 12.1.1).
+
+ 1.00-1.31 Reserved
+
+ 2.00-5.31 Indicates a response. Values in this range are assigned by
+ the "CoAP Response Codes" sub-registry (see
+ Section 12.1.2).
+
+ 6.00-7.31 Reserved
+
+12.1.1. Method Codes
+
+ The name of the sub-registry is "CoAP Method Codes".
+
+ Each entry in the sub-registry must include the Method Code in the
+ range 0.01-0.31, the name of the method, and a reference to the
+ method's documentation.
+
+ Initial entries in this sub-registry are as follows:
+
+ +------+--------+-----------+
+ | Code | Name | Reference |
+ +------+--------+-----------+
+ | 0.01 | GET | [RFC7252] |
+ | 0.02 | POST | [RFC7252] |
+ | 0.03 | PUT | [RFC7252] |
+ | 0.04 | DELETE | [RFC7252] |
+ +------+--------+-----------+
+
+ Table 5: CoAP Method Codes
+
+ All other Method Codes are Unassigned.
+
+ The IANA policy for future additions to this sub-registry is "IETF
+ Review or IESG Approval" as described in [RFC5226].
+
+ The documentation of a Method Code should specify the semantics of a
+ request with that code, including the following properties:
+
+ o The Response Codes the method returns in the success case.
+
+ o Whether the method is idempotent, safe, or both.
+
+
+
+Shelby, et al. Standards Track [Page 87]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+12.1.2. Response Codes
+
+ The name of the sub-registry is "CoAP Response Codes".
+
+ Each entry in the sub-registry must include the Response Code in the
+ range 2.00-5.31, a description of the Response Code, and a reference
+ to the Response Code's documentation.
+
+ Initial entries in this sub-registry are as follows:
+
+ +------+------------------------------+-----------+
+ | Code | Description | Reference |
+ +------+------------------------------+-----------+
+ | 2.01 | Created | [RFC7252] |
+ | 2.02 | Deleted | [RFC7252] |
+ | 2.03 | Valid | [RFC7252] |
+ | 2.04 | Changed | [RFC7252] |
+ | 2.05 | Content | [RFC7252] |
+ | 4.00 | Bad Request | [RFC7252] |
+ | 4.01 | Unauthorized | [RFC7252] |
+ | 4.02 | Bad Option | [RFC7252] |
+ | 4.03 | Forbidden | [RFC7252] |
+ | 4.04 | Not Found | [RFC7252] |
+ | 4.05 | Method Not Allowed | [RFC7252] |
+ | 4.06 | Not Acceptable | [RFC7252] |
+ | 4.12 | Precondition Failed | [RFC7252] |
+ | 4.13 | Request Entity Too Large | [RFC7252] |
+ | 4.15 | Unsupported Content-Format | [RFC7252] |
+ | 5.00 | Internal Server Error | [RFC7252] |
+ | 5.01 | Not Implemented | [RFC7252] |
+ | 5.02 | Bad Gateway | [RFC7252] |
+ | 5.03 | Service Unavailable | [RFC7252] |
+ | 5.04 | Gateway Timeout | [RFC7252] |
+ | 5.05 | Proxying Not Supported | [RFC7252] |
+ +------+------------------------------+-----------+
+
+ Table 6: CoAP Response Codes
+
+ The Response Codes 3.00-3.31 are Reserved for future use. All other
+ Response Codes are Unassigned.
+
+ The IANA policy for future additions to this sub-registry is "IETF
+ Review or IESG Approval" as described in [RFC5226].
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 88]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ The documentation of a Response Code should specify the semantics of
+ a response with that code, including the following properties:
+
+ o The methods the Response Code applies to.
+
+ o Whether payload is required, optional, or not allowed.
+
+ o The semantics of the payload. For example, the payload of a 2.05
+ (Content) response is a representation of the target resource; the
+ payload in an error response is a human-readable diagnostic
+ payload.
+
+ o The format of the payload. For example, the format in a 2.05
+ (Content) response is indicated by the Content-Format Option; the
+ format of the payload in an error response is always Net-Unicode
+ text.
+
+ o Whether the response is cacheable according to the freshness
+ model.
+
+ o Whether the response is validatable according to the validation
+ model.
+
+ o Whether the response causes a cache to mark responses stored for
+ the request URI as not fresh.
+
+12.2. CoAP Option Numbers Registry
+
+ This document defines a sub-registry for the Option Numbers used in
+ CoAP options within the "CoRE Parameters" registry. The name of the
+ sub-registry is "CoAP Option Numbers".
+
+ Each entry in the sub-registry must include the Option Number, the
+ name of the option, and a reference to the option's documentation.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 89]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Initial entries in this sub-registry are as follows:
+
+ +--------+------------------+-----------+
+ | Number | Name | Reference |
+ +--------+------------------+-----------+
+ | 0 | (Reserved) | [RFC7252] |
+ | 1 | If-Match | [RFC7252] |
+ | 3 | Uri-Host | [RFC7252] |
+ | 4 | ETag | [RFC7252] |
+ | 5 | If-None-Match | [RFC7252] |
+ | 7 | Uri-Port | [RFC7252] |
+ | 8 | Location-Path | [RFC7252] |
+ | 11 | Uri-Path | [RFC7252] |
+ | 12 | Content-Format | [RFC7252] |
+ | 14 | Max-Age | [RFC7252] |
+ | 15 | Uri-Query | [RFC7252] |
+ | 17 | Accept | [RFC7252] |
+ | 20 | Location-Query | [RFC7252] |
+ | 35 | Proxy-Uri | [RFC7252] |
+ | 39 | Proxy-Scheme | [RFC7252] |
+ | 60 | Size1 | [RFC7252] |
+ | 128 | (Reserved) | [RFC7252] |
+ | 132 | (Reserved) | [RFC7252] |
+ | 136 | (Reserved) | [RFC7252] |
+ | 140 | (Reserved) | [RFC7252] |
+ +--------+------------------+-----------+
+
+ Table 7: CoAP Option Numbers
+
+ The IANA policy for future additions to this sub-registry is split
+ into three tiers as follows. The range of 0..255 is reserved for
+ options defined by the IETF (IETF Review or IESG Approval). The
+ range of 256..2047 is reserved for commonly used options with public
+ specifications (Specification Required). The range of 2048..64999 is
+ for all other options including private or vendor-specific ones,
+ which undergo a Designated Expert review to help ensure that the
+ option semantics are defined correctly. The option numbers between
+ 65000 and 65535 inclusive are reserved for experiments. They are not
+ meant for vendor-specific use of any kind and MUST NOT be used in
+ operational deployments.
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 90]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ +-------------+---------------------------------------+
+ | Range | Registration Procedures |
+ +-------------+---------------------------------------+
+ | 0-255 | IETF Review or IESG Approval |
+ | 256-2047 | Specification Required |
+ | 2048-64999 | Expert Review |
+ | 65000-65535 | Experimental use (no operational use) |
+ +-------------+---------------------------------------+
+
+ Table 8: CoAP Option Numbers: Registration Procedures
+
+ The documentation of an Option Number should specify the semantics of
+ an option with that number, including the following properties:
+
+ o The meaning of the option in a request.
+
+ o The meaning of the option in a response.
+
+ o Whether the option is critical or elective, as determined by the
+ Option Number.
+
+ o Whether the option is Safe-to-Forward, and, if yes, whether it is
+ part of the Cache-Key, as determined by the Option Number (see
+ Section 5.4.2).
+
+ o The format and length of the option's value.
+
+ o Whether the option must occur at most once or whether it can occur
+ multiple times.
+
+ o The default value, if any. For a critical option with a default
+ value, a discussion on how the default value enables processing by
+ implementations that do not support the critical option
+ (Section 5.4.4).
+
+12.3. CoAP Content-Formats Registry
+
+ Internet media types are identified by a string, such as
+ "application/xml" [RFC2046]. In order to minimize the overhead of
+ using these media types to indicate the format of payloads, this
+ document defines a sub-registry for a subset of Internet media types
+ to be used in CoAP and assigns each, in combination with a content-
+ coding, a numeric identifier. The name of the sub-registry is "CoAP
+ Content-Formats", within the "CoRE Parameters" registry.
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 91]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Each entry in the sub-registry must include the media type registered
+ with IANA, the numeric identifier in the range 0-65535 to be used for
+ that media type in CoAP, the content-coding associated with this
+ identifier, and a reference to a document describing what a payload
+ with that media type means semantically.
+
+ CoAP does not include a separate way to convey content-encoding
+ information with a request or response, and for that reason the
+ content-encoding is also specified for each identifier (if any). If
+ multiple content-encodings will be used with a media type, then a
+ separate Content-Format identifier for each is to be registered.
+ Similarly, other parameters related to an Internet media type, such
+ as level, can be defined for a CoAP Content-Format entry.
+
+ Initial entries in this sub-registry are as follows:
+
+ +--------------------------+----------+----+------------------------+
+ | Media type | Encoding | ID | Reference |
+ +--------------------------+----------+----+------------------------+
+ | text/plain; | - | 0 | [RFC2046] [RFC3676] |
+ | charset=utf-8 | | | [RFC5147] |
+ | application/link-format | - | 40 | [RFC6690] |
+ | application/xml | - | 41 | [RFC3023] |
+ | application/octet-stream | - | 42 | [RFC2045] [RFC2046] |
+ | application/exi | - | 47 | [REC-exi-20140211] |
+ | application/json | - | 50 | [RFC7159] |
+ +--------------------------+----------+----+------------------------+
+
+ Table 9: CoAP Content-Formats
+
+ The identifiers between 65000 and 65535 inclusive are reserved for
+ experiments. They are not meant for vendor-specific use of any kind
+ and MUST NOT be used in operational deployments. The identifiers
+ between 256 and 9999 are reserved for future use in IETF
+ specifications (IETF Review or IESG Approval). All other identifiers
+ are Unassigned.
+
+ Because the namespace of single-byte identifiers is so small, the
+ IANA policy for future additions in the range 0-255 inclusive to the
+ sub-registry is "Expert Review" as described in [RFC5226]. The IANA
+ policy for additions in the range 10000-64999 inclusive is "First
+ Come First Served" as described in [RFC5226]. This is summarized in
+ the following table.
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 92]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ +-------------+---------------------------------------+
+ | Range | Registration Procedures |
+ +-------------+---------------------------------------+
+ | 0-255 | Expert Review |
+ | 256-9999 | IETF Review or IESG Approval |
+ | 10000-64999 | First Come First Served |
+ | 65000-65535 | Experimental use (no operational use) |
+ +-------------+---------------------------------------+
+
+ Table 10: CoAP Content-Formats: Registration Procedures
+
+ In machine-to-machine applications, it is not expected that generic
+ Internet media types such as text/plain, application/xml or
+ application/octet-stream are useful for real applications in the long
+ term. It is recommended that M2M applications making use of CoAP
+ request new Internet media types from IANA indicating semantic
+ information about how to create or parse a payload. For example, a
+ Smart Energy application payload carried as XML might request a more
+ specific type like application/se+xml or application/se-exi.
+
+12.4. URI Scheme Registration
+
+ This document contains the request for the registration of the
+ Uniform Resource Identifier (URI) scheme "coap". The registration
+ request complies with [RFC4395].
+
+ URI scheme name.
+ coap
+
+ Status.
+ Permanent.
+
+ URI scheme syntax.
+ Defined in Section 6.1 of [RFC7252].
+
+ URI scheme semantics.
+ The "coap" URI scheme provides a way to identify resources that
+ are potentially accessible over the Constrained Application
+ Protocol (CoAP). The resources can be located by contacting the
+ governing CoAP server and operated on by sending CoAP requests to
+ the server. This scheme can thus be compared to the "http" URI
+ scheme [RFC2616]. See Section 6 of [RFC7252] for the details of
+ operation.
+
+ Encoding considerations.
+ The scheme encoding conforms to the encoding rules established for
+ URIs in [RFC3986], i.e., internationalized and reserved characters
+ are expressed using UTF-8-based percent-encoding.
+
+
+
+Shelby, et al. Standards Track [Page 93]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Applications/protocols that use this URI scheme name.
+ The scheme is used by CoAP endpoints to access CoAP resources.
+
+ Interoperability considerations.
+ None.
+
+ Security considerations.
+ See Section 11.1 of [RFC7252].
+
+ Contact.
+ IETF Chair <chair@ietf.org>
+
+ Author/Change controller.
+ IESG <iesg@ietf.org>
+
+ References.
+ [RFC7252]
+
+12.5. Secure URI Scheme Registration
+
+ This document contains the request for the registration of the
+ Uniform Resource Identifier (URI) scheme "coaps". The registration
+ request complies with [RFC4395].
+
+ URI scheme name.
+ coaps
+
+ Status.
+ Permanent.
+
+ URI scheme syntax.
+ Defined in Section 6.2 of [RFC7252].
+
+ URI scheme semantics.
+ The "coaps" URI scheme provides a way to identify resources that
+ are potentially accessible over the Constrained Application
+ Protocol (CoAP) using Datagram Transport Layer Security (DTLS) for
+ transport security. The resources can be located by contacting
+ the governing CoAP server and operated on by sending CoAP requests
+ to the server. This scheme can thus be compared to the "https"
+ URI scheme [RFC2616]. See Section 6 of [RFC7252] for the details
+ of operation.
+
+ Encoding considerations.
+ The scheme encoding conforms to the encoding rules established for
+ URIs in [RFC3986], i.e., internationalized and reserved characters
+ are expressed using UTF-8-based percent-encoding.
+
+
+
+
+Shelby, et al. Standards Track [Page 94]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Applications/protocols that use this URI scheme name.
+ The scheme is used by CoAP endpoints to access CoAP resources
+ using DTLS.
+
+ Interoperability considerations.
+ None.
+
+ Security considerations.
+ See Section 11.1 of [RFC7252].
+
+ Contact.
+ IETF Chair <chair@ietf.org>
+
+ Author/Change controller.
+ IESG <iesg@ietf.org>
+
+ References.
+ [RFC7252]
+
+12.6. Service Name and Port Number Registration
+
+ One of the functions of CoAP is resource discovery: a CoAP client can
+ ask a CoAP server about the resources offered by it (see Section 7).
+ To enable resource discovery just based on the knowledge of an IP
+ address, the CoAP port for resource discovery needs to be
+ standardized.
+
+ IANA has assigned the port number 5683 and the service name "coap",
+ in accordance with [RFC6335].
+
+ Besides unicast, CoAP can be used with both multicast and anycast.
+
+ Service Name.
+ coap
+
+ Transport Protocol.
+ udp
+
+ Assignee.
+ IESG <iesg@ietf.org>
+
+ Contact.
+ IETF Chair <chair@ietf.org>
+
+ Description.
+ Constrained Application Protocol (CoAP)
+
+
+
+
+
+Shelby, et al. Standards Track [Page 95]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Reference.
+ [RFC7252]
+
+ Port Number.
+ 5683
+
+12.7. Secure Service Name and Port Number Registration
+
+ CoAP resource discovery may also be provided using the DTLS-secured
+ CoAP "coaps" scheme. Thus, the CoAP port for secure resource
+ discovery needs to be standardized.
+
+ IANA has assigned the port number 5684 and the service name "coaps",
+ in accordance with [RFC6335].
+
+ Besides unicast, DTLS-secured CoAP can be used with anycast.
+
+ Service Name.
+ coaps
+
+ Transport Protocol.
+ udp
+
+ Assignee.
+ IESG <iesg@ietf.org>
+
+ Contact.
+ IETF Chair <chair@ietf.org>
+
+ Description.
+ DTLS-secured CoAP
+
+ Reference.
+ [RFC7252]
+
+ Port Number.
+ 5684
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 96]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+12.8. Multicast Address Registration
+
+ Section 8, "Multicast CoAP", defines the use of multicast. IANA has
+ assigned the following multicast addresses for use by CoAP nodes:
+
+ IPv4 -- "All CoAP Nodes" address 224.0.1.187, from the "IPv4
+ Multicast Address Space Registry". As the address is used for
+ discovery that may span beyond a single network, it has come from
+ the Internetwork Control Block (224.0.1.x, RFC 5771).
+
+ IPv6 -- "All CoAP Nodes" address FF0X::FD, from the "IPv6 Multicast
+ Address Space Registry", in the "Variable Scope Multicast
+ Addresses" space (RFC 3307). Note that there is a distinct
+ multicast address for each scope that interested CoAP nodes should
+ listen to; CoAP needs the Link-Local and Site-Local scopes only.
+
+13. Acknowledgements
+
+ Brian Frank was a contributor to and coauthor of early versions of
+ this specification.
+
+ Special thanks to Peter Bigot, Esko Dijk, and Cullen Jennings for
+ substantial contributions to the ideas and text in the document,
+ along with countless detailed reviews and discussions.
+
+ Thanks to Floris Van den Abeele, Anthony Baire, Ed Beroset, Berta
+ Carballido, Angelo P. Castellani, Gilbert Clark, Robert Cragie,
+ Pierre David, Esko Dijk, Lisa Dusseault, Mehmet Ersue, Thomas
+ Fossati, Tobias Gondrom, Bert Greevenbosch, Tom Herbst, Jeroen
+ Hoebeke, Richard Kelsey, Sye Loong Keoh, Ari Keranen, Matthias
+ Kovatsch, Avi Lior, Stephan Lohse, Salvatore Loreto, Kerry Lynn,
+ Andrew McGregor, Alexey Melnikov, Guido Moritz, Petri Mutka, Colin
+ O'Flynn, Charles Palmer, Adriano Pezzuto, Thomas Poetsch, Robert
+ Quattlebaum, Akbar Rahman, Eric Rescorla, Dan Romascanu, David Ryan,
+ Peter Saint-Andre, Szymon Sasin, Michael Scharf, Dale Seed, Robby
+ Simpson, Peter van der Stok, Michael Stuber, Linyi Tian, Gilman
+ Tolle, Matthieu Vial, Maciej Wasilak, Fan Xianyou, and Alper Yegin
+ for helpful comments and discussions that have shaped the document.
+ Special thanks also to the responsible IETF area director at the time
+ of completion, Barry Leiba, and the IESG reviewers, Adrian Farrel,
+ Martin Stiemerling, Pete Resnick, Richard Barnes, Sean Turner,
+ Spencer Dawkins, Stephen Farrell, and Ted Lemon, who contributed in-
+ depth reviews.
+
+ Some of the text has been borrowed from the working documents of the
+ IETF HTTPBIS working group.
+
+
+
+
+
+Shelby, et al. Standards Track [Page 97]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+14. References
+
+14.1. Normative References
+
+ [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
+ August 1980.
+
+ [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
+ Extensions (MIME) Part One: Format of Internet Message
+ Bodies", RFC 2045, November 1996.
+
+ [RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
+ Extensions (MIME) Part Two: Media Types", RFC 2046,
+ November 1996.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
+ Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
+ Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
+
+ [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media
+ Types", RFC 3023, January 2001.
+
+ [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
+ 10646", STD 63, RFC 3629, November 2003.
+
+ [RFC3676] Gellens, R., "The Text/Plain Format and DelSp Parameters",
+ RFC 3676, February 2004.
+
+ [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
+ Resource Identifier (URI): Generic Syntax", STD 66, RFC
+ 3986, January 2005.
+
+ [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
+ for Transport Layer Security (TLS)", RFC 4279, December
+ 2005.
+
+ [RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
+ Registration Procedures for New URI Schemes", BCP 35, RFC
+ 4395, February 2006.
+
+ [RFC5147] Wilde, E. and M. Duerst, "URI Fragment Identifiers for the
+ text/plain Media Type", RFC 5147, April 2008.
+
+ [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
+ Interchange", RFC 5198, March 2008.
+
+
+
+Shelby, et al. Standards Track [Page 98]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
+ IANA Considerations Section in RFCs", BCP 26, RFC 5226,
+ May 2008.
+
+ [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
+ Specifications: ABNF", STD 68, RFC 5234, January 2008.
+
+ [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
+ (TLS) Protocol Version 1.2", RFC 5246, August 2008.
+
+ [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
+ Housley, R., and W. Polk, "Internet X.509 Public Key
+ Infrastructure Certificate and Certificate Revocation List
+ (CRL) Profile", RFC 5280, May 2008.
+
+ [RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
+ "Elliptic Curve Cryptography Subject Public Key
+ Information", RFC 5480, March 2009.
+
+ [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
+ Uniform Resource Identifiers (URIs)", RFC 5785, April
+ 2010.
+
+ [RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
+ Address Text Representation", RFC 5952, August 2010.
+
+ [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
+
+ [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
+ Extension Definitions", RFC 6066, January 2011.
+
+ [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
+ Security Version 1.2", RFC 6347, January 2012.
+
+ [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
+ Format", RFC 6690, August 2012.
+
+ [RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
+ Keranen, A., and P. Hallam-Baker, "Naming Things with
+ Hashes", RFC 6920, April 2013.
+
+ [RFC7250] Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
+ T. Kivinen, "Using Raw Public Keys in Transport Layer
+ Security (TLS) and Datagram Transport Layer Security
+ (DTLS)", RFC 7250, June 2014.
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 99]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ [RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
+ CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
+ Transport Layer Security (TLS)", RFC 7251, June 2014.
+
+14.2. Informative References
+
+ [BLOCK] Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
+ Work in Progress, October 2013.
+
+ [CoAP-MISC]
+ Bormann, C. and K. Hartke, "Miscellaneous additions to
+ CoAP", Work in Progress, December 2013.
+
+ [EUI64] IEEE Standards Association, "Guidelines for 64-bit Global
+ Identifier (EUI-64 (TM))", Registration Authority
+ Tutorials, April 2010, <http://standards.ieee.org/regauth/
+ oui/tutorials/EUI64.html>.
+
+ [GROUPCOMM]
+ Rahman, A. and E. Dijk, "Group Communication for CoAP",
+ Work in Progress, December 2013.
+
+ [HHGTTG] Adams, D., "The Hitchhiker's Guide to the Galaxy", Pan
+ Books ISBN 3320258648, 1979.
+
+ [IEEE1003.1]
+ IEEE and The Open Group, "Portable Operating System
+ Interface (POSIX)", The Open Group Base Specifications
+ Issue 7, IEEE 1003.1, 2013 Edition,
+ <http://pubs.opengroup.org/onlinepubs/9699919799/>.
+
+ [IPsec-CoAP]
+ Bormann, C., "Using CoAP with IPsec", Work in Progress,
+ December 2012.
+
+ [MAPPING] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
+ E. Dijk, "Guidelines for HTTP-CoAP Mapping
+ Implementations", Work in Progress, February 2014.
+
+ [OBSERVE] Hartke, K., "Observing Resources in CoAP", Work in
+ Progress, April 2014.
+
+ [REC-exi-20140211]
+ Schneider, J., Kamiya, T., Peintner, D., and R. Kyusakov,
+ "Efficient XML Interchange (EXI) Format 1.0 (Second
+ Edition)", W3C Recommendation REC-exi-20140211, February
+ 2014, <http://www.w3.org/TR/2014/REC-exi-20140211/>.
+
+
+
+
+Shelby, et al. Standards Track [Page 100]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ [REST] Fielding, R., "Architectural Styles and the Design of
+ Network-based Software Architectures", Ph.D. Dissertation,
+ University of California, Irvine, 2000,
+ <http://www.ics.uci.edu/~fielding/pubs/dissertation/
+ fielding_dissertation.pdf>.
+
+ [RFC0020] Cerf, V., "ASCII format for network interchange", RFC 20,
+ October 1969.
+
+ [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
+ 1981.
+
+ [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
+ RFC 792, September 1981.
+
+ [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
+ 793, September 1981.
+
+ [RFC1035] Mockapetris, P., "Domain names - implementation and
+ specification", STD 13, RFC 1035, November 1987.
+
+ [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
+ with Session Description Protocol (SDP)", RFC 3264, June
+ 2002.
+
+ [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
+ X.509 Public Key Infrastructure Certificate and
+ Certificate Revocation List (CRL) Profile", RFC 3280,
+ April 2002.
+
+ [RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
+ "Advanced Sockets Application Program Interface (API) for
+ IPv6", RFC 3542, May 2003.
+
+ [RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
+ G. Fairhurst, "The Lightweight User Datagram Protocol
+ (UDP-Lite)", RFC 3828, July 2004.
+
+ [RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
+ Requirements for Security", BCP 106, RFC 4086, June 2005.
+
+ [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
+ Message Protocol (ICMPv6) for the Internet Protocol
+ Version 6 (IPv6) Specification", RFC 4443, March 2006.
+
+ [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
+ Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
+ for Transport Layer Security (TLS)", RFC 4492, May 2006.
+
+
+
+Shelby, et al. Standards Track [Page 101]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
+ Discovery", RFC 4821, March 2007.
+
+ [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
+ "Transmission of IPv6 Packets over IEEE 802.15.4
+ Networks", RFC 4944, September 2007.
+
+ [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
+ for Application Designers", BCP 145, RFC 5405, November
+ 2008.
+
+ [RFC5489] Badra, M. and I. Hajjeh, "ECDHE_PSK Cipher Suites for
+ Transport Layer Security (TLS)", RFC 5489, March 2009.
+
+ [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
+ Curve Cryptography Algorithms", RFC 6090, February 2011.
+
+ [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
+ Protocol (XMPP): Core", RFC 6120, March 2011.
+
+ [RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
+ Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
+ September 2011.
+
+ [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
+ Cheshire, "Internet Assigned Numbers Authority (IANA)
+ Procedures for the Management of the Service Name and
+ Transport Protocol Port Number Registry", BCP 165, RFC
+ 6335, August 2011.
+
+ [RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
+ Transport Layer Security (TLS)", RFC 6655, July 2012.
+
+ [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
+ for the Use of IPv6 UDP Datagrams with Zero Checksums",
+ RFC 6936, April 2013.
+
+ [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
+ Galperin, S., and C. Adams, "X.509 Internet Public Key
+ Infrastructure Online Certificate Status Protocol - OCSP",
+ RFC 6960, June 2013.
+
+ [RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
+ Multiple Certificate Status Request Extension", RFC 6961,
+ June 2013.
+
+ [RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
+ Interchange Format", RFC 7159, March 2014.
+
+
+
+Shelby, et al. Standards Track [Page 102]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
+ Constrained-Node Networks", RFC 7228, May 2014.
+
+ [RTO-CONSIDER]
+ Allman, M., "Retransmission Timeout Considerations", Work
+ in Progress, May 2012.
+
+ [W3CXMLSEC]
+ Wenning, R., "Report of the XML Security PAG", W3C XML
+ Security PAG, October 2012,
+ <http://www.w3.org/2011/xmlsec-pag/pagreport.html>.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 103]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+Appendix A. Examples
+
+ This section gives a number of short examples with message flows for
+ GET requests. These examples demonstrate the basic operation, the
+ operation in the presence of retransmissions, and multicast.
+
+ Figure 16 shows a basic GET request causing a piggybacked response:
+ The client sends a Confirmable GET request for the resource
+ coap://server/temperature to the server with a Message ID of 0x7d34.
+ The request includes one Uri-Path Option (Delta 0 + 11 = 11, Length
+ 11, Value "temperature"); the Token is left empty. This request is a
+ total of 16 bytes long. A 2.05 (Content) response is returned in the
+ Acknowledgement message that acknowledges the Confirmable request,
+ echoing both the Message ID 0x7d34 and the empty Token value. The
+ response includes a Payload of "22.3 C" and is 11 bytes long.
+
+ Client Server
+ | |
+ | |
+ +----->| Header: GET (T=CON, Code=0.01, MID=0x7d34)
+ | GET | Uri-Path: "temperature"
+ | |
+ | |
+ |<-----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d34)
+ | 2.05 | Payload: "22.3 C"
+ | |
+
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 1 | 0 | 0 | GET=1 | MID=0x7d34 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 11 | 11 | "temperature" (11 B) ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 1 | 2 | 0 | 2.05=69 | MID=0x7d34 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |1 1 1 1 1 1 1 1| "22.3 C" (6 B) ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Figure 16: Confirmable Request; Piggybacked Response
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 104]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Figure 17 shows a similar example, but with the inclusion of an non-
+ empty Token (Value 0x20) in the request and the response, increasing
+ the sizes to 17 and 12 bytes, respectively.
+
+ Client Server
+ | |
+ | |
+ +----->| Header: GET (T=CON, Code=0.01, MID=0x7d35)
+ | GET | Token: 0x20
+ | | Uri-Path: "temperature"
+ | |
+ | |
+ |<-----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d35)
+ | 2.05 | Token: 0x20
+ | | Payload: "22.3 C"
+ | |
+
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 1 | 0 | 1 | GET=1 | MID=0x7d35 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 0x20 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 11 | 11 | "temperature" (11 B) ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 1 | 2 | 1 | 2.05=69 | MID=0x7d35 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | 0x20 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |1 1 1 1 1 1 1 1| "22.3 C" (6 B) ...
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+ Figure 17: Confirmable Request; Piggybacked Response
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 105]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ In Figure 18, the Confirmable GET request is lost. After ACK_TIMEOUT
+ seconds, the client retransmits the request, resulting in a
+ piggybacked response as in the previous example.
+
+ Client Server
+ | |
+ | |
+ +----X | Header: GET (T=CON, Code=0.01, MID=0x7d36)
+ | GET | Token: 0x31
+ | | Uri-Path: "temperature"
+ TIMEOUT |
+ | |
+ +----->| Header: GET (T=CON, Code=0.01, MID=0x7d36)
+ | GET | Token: 0x31
+ | | Uri-Path: "temperature"
+ | |
+ | |
+ |<-----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d36)
+ | 2.05 | Token: 0x31
+ | | Payload: "22.3 C"
+ | |
+
+ Figure 18: Confirmable Request (Retransmitted); Piggybacked Response
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 106]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ In Figure 19, the first Acknowledgement message from the server to
+ the client is lost. After ACK_TIMEOUT seconds, the client
+ retransmits the request.
+
+ Client Server
+ | |
+ | |
+ +----->| Header: GET (T=CON, Code=0.01, MID=0x7d37)
+ | GET | Token: 0x42
+ | | Uri-Path: "temperature"
+ | |
+ | |
+ | X----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d37)
+ | 2.05 | Token: 0x42
+ | | Payload: "22.3 C"
+ TIMEOUT |
+ | |
+ +----->| Header: GET (T=CON, Code=0.01, MID=0x7d37)
+ | GET | Token: 0x42
+ | | Uri-Path: "temperature"
+ | |
+ | |
+ |<-----+ Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d37)
+ | 2.05 | Token: 0x42
+ | | Payload: "22.3 C"
+ | |
+
+ Figure 19: Confirmable Request; Piggybacked Response (Retransmitted)
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 107]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ In Figure 20, the server acknowledges the Confirmable request and
+ sends a 2.05 (Content) response separately in a Confirmable message.
+ Note that the Acknowledgement message and the Confirmable response do
+ not necessarily arrive in the same order as they were sent. The
+ client acknowledges the Confirmable response.
+
+ Client Server
+ | |
+ | |
+ +----->| Header: GET (T=CON, Code=0.01, MID=0x7d38)
+ | GET | Token: 0x53
+ | | Uri-Path: "temperature"
+ | |
+ | |
+ |<- - -+ Header: (T=ACK, Code=0.00, MID=0x7d38)
+ | |
+ | |
+ |<-----+ Header: 2.05 Content (T=CON, Code=2.05, MID=0xad7b)
+ | 2.05 | Token: 0x53
+ | | Payload: "22.3 C"
+ | |
+ | |
+ +- - ->| Header: (T=ACK, Code=0.00, MID=0xad7b)
+ | |
+
+ Figure 20: Confirmable Request; Separate Response
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 108]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Figure 21 shows an example where the client loses its state (e.g.,
+ crashes and is rebooted) right after sending a Confirmable request,
+ so the separate response arriving some time later comes unexpected.
+ In this case, the client rejects the Confirmable response with a
+ Reset message. Note that the unexpected ACK is silently ignored.
+
+ Client Server
+ | |
+ | |
+ +----->| Header: GET (T=CON, Code=0.01, MID=0x7d39)
+ | GET | Token: 0x64
+ | | Uri-Path: "temperature"
+ CRASH |
+ | |
+ |<- - -+ Header: (T=ACK, Code=0.00, MID=0x7d39)
+ | |
+ | |
+ |<-----+ Header: 2.05 Content (T=CON, Code=2.05, MID=0xad7c)
+ | 2.05 | Token: 0x64
+ | | Payload: "22.3 C"
+ | |
+ | |
+ +- - ->| Header: (T=RST, Code=0.00, MID=0xad7c)
+ | |
+
+ Figure 21: Confirmable Request; Separate Response (Unexpected)
+
+ Figure 22 shows a basic GET request where the request and the
+ response are Non-confirmable, so both may be lost without notice.
+
+ Client Server
+ | |
+ | |
+ +----->| Header: GET (T=NON, Code=0.01, MID=0x7d40)
+ | GET | Token: 0x75
+ | | Uri-Path: "temperature"
+ | |
+ | |
+ |<-----+ Header: 2.05 Content (T=NON, Code=2.05, MID=0xad7d)
+ | 2.05 | Token: 0x75
+ | | Payload: "22.3 C"
+ | |
+
+ Figure 22: Non-confirmable Request; Non-confirmable Response
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 109]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ In Figure 23, the client sends a Non-confirmable GET request to a
+ multicast address: all nodes in link-local scope. There are 3
+ servers on the link: A, B and C. Servers A and B have a matching
+ resource, therefore they send back a Non-confirmable 2.05 (Content)
+ response. The response sent by B is lost. C does not have matching
+ response, therefore it sends a Non-confirmable 4.04 (Not Found)
+ response.
+
+ Client ff02::1 A B C
+ | | | | |
+ | | | | |
+ +------>| | | | Header: GET (T=NON, Code=0.01, MID=0x7d41)
+ | GET | | | | Token: 0x86
+ | | | | Uri-Path: "temperature"
+ | | | |
+ | | | |
+ |<------------+ | | Header: 2.05 (T=NON, Code=2.05, MID=0x60b1)
+ | 2.05 | | | Token: 0x86
+ | | | | Payload: "22.3 C"
+ | | | |
+ | | | |
+ | X------------+ | Header: 2.05 (T=NON, Code=2.05, MID=0x01a0)
+ | 2.05 | | | Token: 0x86
+ | | | | Payload: "20.9 C"
+ | | | |
+ | | | |
+ |<------------------+ Header: 4.04 (T=NON, Code=4.04, MID=0x952a)
+ | 4.04 | | | Token: 0x86
+ | | | |
+
+ Figure 23: Non-confirmable Request (Multicast); Non-confirmable
+ Response
+
+Appendix B. URI Examples
+
+ The following examples demonstrate different sets of Uri options, and
+ the result after constructing an URI from them. In addition to the
+ options, Section 6.5 refers to the destination IP address and port,
+ but not all paths of the algorithm cause the destination IP address
+ and port to be included in the URI.
+
+ o Input:
+
+ Destination IP Address = [2001:db8::2:1]
+ Destination UDP Port = 5683
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 110]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ Output:
+
+ coap://[2001:db8::2:1]/
+
+ o Input:
+
+ Destination IP Address = [2001:db8::2:1]
+ Destination UDP Port = 5683
+ Uri-Host = "example.net"
+
+ Output:
+
+ coap://example.net/
+
+ o Input:
+
+ Destination IP Address = [2001:db8::2:1]
+ Destination UDP Port = 5683
+ Uri-Host = "example.net"
+ Uri-Path = ".well-known"
+ Uri-Path = "core"
+
+ Output:
+
+ coap://example.net/.well-known/core
+
+ o Input:
+
+ Destination IP Address = [2001:db8::2:1]
+ Destination UDP Port = 5683
+ Uri-Host = "xn--18j4d.example"
+ Uri-Path = the string composed of the Unicode characters U+3053
+ U+3093 U+306b U+3061 U+306f, usually represented in UTF-8 as
+ E38193E38293E381ABE381A1E381AF hexadecimal
+
+ Output:
+
+ coap://xn--18j4d.example/
+ %E3%81%93%E3%82%93%E3%81%AB%E3%81%A1%E3%81%AF
+
+ (The line break has been inserted for readability; it is not
+ part of the URI.)
+
+
+
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 111]
+
+RFC 7252 The Constrained Application Protocol (CoAP) June 2014
+
+
+ o Input:
+
+ Destination IP Address = 198.51.100.1
+ Destination UDP Port = 61616
+ Uri-Path = ""
+ Uri-Path = "/"
+ Uri-Path = ""
+ Uri-Path = ""
+ Uri-Query = "//"
+ Uri-Query = "?&"
+
+ Output:
+
+ coap://198.51.100.1:61616//%2F//?%2F%2F&?%26
+
+Authors' Addresses
+
+ Zach Shelby
+ ARM
+ 150 Rose Orchard
+ San Jose, CA 95134
+ USA
+
+ Phone: +1-408-203-9434
+ EMail: zach.shelby@arm.com
+
+
+ Klaus Hartke
+ Universitaet Bremen TZI
+ Postfach 330440
+ Bremen D-28359
+ Germany
+
+ Phone: +49-421-218-63905
+ EMail: hartke@tzi.org
+
+
+ Carsten Bormann
+ Universitaet Bremen TZI
+ Postfach 330440
+ Bremen D-28359
+ Germany
+
+ Phone: +49-421-218-63921
+ EMail: cabo@tzi.org
+
+
+
+
+
+
+Shelby, et al. Standards Track [Page 112]
+
diff --git a/src/coap_parse.erl b/src/coap_parse.erl
new file mode 100644
index 0000000..bb61a28
--- /dev/null
+++ b/src/coap_parse.erl
@@ -0,0 +1,137 @@
+-module(coap_parse).
+-export([parse/1,
+ get_option_values/2]).
+
+-include("coaperl.hrl").
+
+
+-spec parse(<<>>) -> #coap_request{} | #coap_response{} | #coap_empty_message{}.
+% @doc: parse a binary coap message
+parse(Packet) ->
+ {coap, Parsed} = parse_raw(Packet),
+ parse_to_rec(Parsed).
+
+
+% @doc get the named option values from the given response record
+get_option_values(#coap_response{options=Options}, OptNum) ->
+ [ Val || {coap_option, Num, Val} <- Options, Num =:= OptNum ].
+
+
+% @doc: convert raw message to request, response or empty message records
+parse_to_rec({_Type, Code, MID, _Token, Options, Payload}) when Code > 0, Code =< 31 ->
+ #coap_request{method=coap_unparse:method_name(Code),
+ path=parse_path(Options),
+ query=parse_query(Options),
+ mid=MID,
+ ct=parse_content_type(Options),
+ payload=Payload};
+parse_to_rec({_Type, 0, MID , _Token, _Options, <<>>}) ->
+ #coap_empty_message{mid=MID};
+parse_to_rec({_Type, Code, MID, _Token, Options, Payload}) ->
+ <<Class:3, Detail:5>> = <<Code>>,
+ #coap_response{mid=MID,
+ ct=parse_content_type(Options),
+ options=Options,
+ class=Class,
+ detail=Detail,
+ block2=parse_block2(Options),
+ payload=Payload}.
+
+
+% @doc: parse path from raw options
+parse_path(Options) ->
+ erlang:iolist_to_binary(lists:reverse(parse_path(Options, []))).
+
+parse_path([{coap_option, ?COAP_OPTION_URI_PATH, Comp}|T], Acc) ->
+ parse_path(T, [["/"|Comp]|Acc]);
+parse_path([{coap_option, _, _}|T], Acc) ->
+ parse_path(T, Acc);
+parse_path([], Acc) ->
+ Acc.
+
+
+% @doc: parse query from raw options
+parse_query(Options) ->
+ parse_query(Options, []).
+
+parse_query([{coap_option, ?COAP_OPTION_URI_QUERY, Opt}|T], Acc) ->
+ parse_query(T, [Opt|Acc]);
+parse_query([{coap_option, _, _}|T], Acc) ->
+ parse_query(T, Acc);
+parse_query([], Acc) ->
+ Acc.
+
+
+
+% @doc: parse content type from raw options
+parse_content_type(Options) ->
+ OptVals = [Value || {coap_option, Format, Value} <- Options, Format =:= ?COAP_OPTION_CONTENT_FORMAT],
+ case OptVals of
+ [<<Val:8>>] -> corerl:content_format_type(Val);
+ _ -> undefined
+ end.
+
+
+% @doc: parse block2 option from raw options
+parse_block2(Options) ->
+ OptVals = [Value || {coap_option, Format, Value} <- Options, Format =:= ?COAP_OPTION_BLOCK2],
+ case OptVals of
+ [Val] ->
+ NumLen = bit_size(Val)-4,
+ <<Num:NumLen, M:1, SZX:3>> = Val,
+ {Num, M, 1 bsl (SZX+4)};
+ _ -> undefined
+ end.
+
+
+% @doc: parse raw packet
+parse_raw(<<Version:2, Type:2, TKL: 4, Code:8, MID:16, Token:TKL/bytes, Tail/bytes>>) when Version =:= 1 ->
+ {Options, Payload} = parse_raw_options(Tail),
+ {coap, {Type, Code, MID, Token, Options, Payload}};
+parse_raw(_AnythingElse) ->
+ {bad, "unable to parse packet"}.
+
+% @doc: parse options in raw packet, don't interpret any option types yet
+parse_raw_options(OptionBin) ->
+ parse_raw_options(OptionBin, 0, []).
+
+parse_raw_options(<<>>, _LastNum, OptionList) ->
+ {OptionList, <<>>};
+parse_raw_options(<<16#FF, Payload/bytes>>, _LastNum, OptionList) ->
+ {OptionList, Payload};
+parse_raw_options(<<BaseOptNum:4, BaseOptLen:4, Tail/bytes>>, LastNum, OptionList) ->
+ {Tail1, OptNum} = case BaseOptNum of
+ X when X < 13 ->
+ {Tail, BaseOptNum + LastNum};
+ X when X =:= 13 ->
+ <<ExtOptNum, NewTail/bytes>> = Tail,
+ {NewTail, ExtOptNum + 13 + LastNum};
+ X when X =:= 14 ->
+ <<ExtOptNum:2/bytes, NewTail/bytes>> = Tail,
+ {NewTail, ExtOptNum + 269 + LastNum}
+ end,
+ {OptLen, Tail3} = case BaseOptLen of
+ Y when Y < 13 ->
+ {BaseOptLen, Tail1};
+ Y when Y =:= 13 ->
+ <<ExtOptLen, Tail2/bytes>> = Tail1,
+ {ExtOptLen + 13, Tail2};
+ Y when Y =:= 14 ->
+ <<ExtOptLen:2/bytes, Tail2/bytes>> = Tail1,
+ {ExtOptLen + 269, Tail2}
+ end,
+ <<OptVal:OptLen/bytes, NextOpt/bytes>> = Tail3,
+ parse_raw_options(NextOpt, OptNum, [{coap_option, OptNum, OptVal}|OptionList]).
+
+-ifdef(TEST).
+-include_lib("eunit/include/eunit.hrl").
+
+parse_to_rec_test() ->
+ % Check if parse to the right rec
+ ?assertEqual(parse_to_rec({egal, 100, 1, egal, [{coap_option, ?COAP_OPTION_CONTENT_FORMAT, <<50:8>>}], payload}),
+ {coap_response,3,4,1,
+ <<"application/json">>,
+ undefined,
+ [{coap_option,12,<<"2">>}],
+ payload}).
+-endif.
diff --git a/src/coap_unparse.erl b/src/coap_unparse.erl
new file mode 100644
index 0000000..aaca450
--- /dev/null
+++ b/src/coap_unparse.erl
@@ -0,0 +1,195 @@
+-module(coap_unparse).
+-export([build_request/2,
+ build_response/3,
+ build_text_response/4,
+ method_name/1]).
+
+-include("coaperl.hrl").
+
+coap_scheme_defaults() ->
+ [{coap, 5683},
+ {coaps, 5684},
+ {'coap+tcp', 5683},
+ {'coaps+tcp', 443}
+ ].
+
+
+% RFC7252, Section 3
+types() ->
+ [{confirmable, 0},
+ {nonconfirmable, 1},
+ {acknowledgement, 2},
+ {reset, 3}].
+
+type_code(Type) ->
+ proplists:get_value(Type, types()).
+
+
+% RFC7252, Section 3
+%% code() ->
+%% [{request, 0},
+%% {success, 2},
+%% {clienterror, 4},
+%% {servererror, 5}].
+
+
+
+% RFC7252, Section 12.1.1
+methods() ->
+ [{get, 1},
+ {post, 2},
+ {put, 3},
+ {delete, 4}
+ ].
+method_code(Method) ->
+ proplists:get_value(Method, methods()).
+method_name(Number) ->
+ {Method, Number} = lists:keyfind(Number, 2, methods()),
+ Method.
+
+
+-spec uri_to_options(string()) -> [{string(), integer(), list()}].
+% convert an URI to the corresponding COAP Options
+uri_to_options(Uri) ->
+ {ok, {_, _, Host, Port, Path, _}} = http_uri:parse(Uri,
+ [{scheme_defaults, coap_scheme_defaults()},
+ {ipv6_host_with_brackets, true}]),
+
+ % Remove brackets from addr if present
+ Host2 = case Host of
+ [$[|T] -> [X || X <- T, X =/= $]];
+ _ -> Host
+ end,
+ {Host2, Port, path_to_options(Path)}.
+
+
+% convert an URI path to the corresponding COAP Options
+path_to_options([$/|Path]) ->
+ lists:reverse(path_to_options(string:tokens(Path, "/"), [])).
+
+path_to_options([], Options) ->
+ Options;
+path_to_options([H|T], Options) ->
+ path_to_options(T, [{?COAP_OPTION_URI_PATH, H}|Options]).
+
+
+-spec content_format_to_options(string()) -> [{integer(), integer()}].
+% @doc: convert a content format into a number (12.3)
+content_format_to_options(ContentFormat) ->
+ FormatNum = corerl:content_format_number(ContentFormat),
+ [{?COAP_OPTION_CONTENT_FORMAT, FormatNum}].
+
+
+-spec num_bytes(integer()) -> integer().
+num_bytes(Val) when is_integer(Val), Val =< 16#ff ->
+ 1;
+num_bytes(Val) when is_integer(Val), Val =< 16#ffff ->
+ 2;
+num_bytes(Val) when is_integer(Val), Val =< 16#ffffff ->
+ 3.
+
+
+-spec option_length(integer() | binary() | list()) -> integer().
+option_length(Val) when is_integer(Val) ->
+ num_bytes(Val);
+option_length(Val) when is_binary(Val) ->
+ byte_size(Val);
+option_length(Val) when is_list(Val) ->
+ string:len(Val).
+
+
+-spec option_format_int(integer()) -> {integer(), integer(), integer()}.
+option_format_int(Val) ->
+ case Val of
+ X when X < 13 ->
+ {X, 0, 0};
+ X when 13 =< X, X < 269 ->
+ {13, X-13, 8};
+ X when X >= 14 ->
+ {14, X-269, 16}
+ end.
+
+
+-spec options_to_bin(list()) -> iolist().
+options_to_bin(Options) ->
+ % FIXME: sort options by number here
+ options_to_bin(Options, 0, []).
+
+options_to_bin([], _, Acc) ->
+ lists:reverse(Acc);
+options_to_bin([{Num, Val}|T], OldNum, Acc) when is_integer(Num)->
+ Diff = Num - OldNum,
+ if Diff < 0 -> erlang:error(badarith);
+ true -> true
+ end,
+ Len = option_length(Val),
+
+ {Delta, ExtDelta, ExtDeltaBits} = option_format_int(Diff),
+ {Length, ExtLength, ExtLengthBits} = option_format_int(Len),
+
+ NewBin = [[<<Delta:4, Length:4,
+ ExtDelta:ExtDeltaBits,
+ ExtLength:ExtLengthBits>>,
+ Val] | Acc],
+ options_to_bin(T, Num, NewBin).
+
+
+build_request([{url, Url}, {method, Method}], Msgid, Options) ->
+ {Host, Port, UriOptions} = uri_to_options(Url),
+ BinOptions = options_to_bin(UriOptions ++ Options),
+ MethodNumber = method_code(Method),
+ Type = type_code(confirmable),
+ {Host, Port, [<<1:2, Type:2, 0:4, 0:3, MethodNumber:5, Msgid:16>>, BinOptions]}.
+
+build_request(Url, Msgid) ->
+ build_request([{url, Url}, {method, get}], Msgid, []).
+
+
+build_response(Class, Detail, Msgid) ->
+ Type = type_code(confirmable),
+ [<<1:2, Type:2, 0:4, Class:3, Detail:5, Msgid:16>>].
+
+build_text_response(Class, Detail, Msgid, Payload) ->
+ Options = content_format_to_options("text/plain"),
+ BinOptions = options_to_bin(Options),
+ Type = type_code(confirmable),
+ [<<1:2, Type:2, 0:4, Class:3, Detail:5, Msgid:16>>, BinOptions, ?COAP_PAYLOAD_MARKER, Payload].
+
+
+-ifdef(TEST).
+-include_lib("eunit/include/eunit.hrl").
+
+option_delta_test() ->
+ % Check if we encode the option delta correctly
+ ?assertEqual(options_to_bin([{11, "foo"}]),
+ [[<<11:4, 3:4>>, "foo"]]),
+ ?assertEqual(options_to_bin([{15, "foo"}]),
+ [[<<13:4, 3:4, 2:8>>, "foo"]]),
+ ?assertEqual(options_to_bin([{275, "foo"}]),
+ [[<<14:4, 3:4, 6:16>>, "foo"]]).
+
+option_length_test() ->
+ % Check if we encode the option value length correctly
+ ?assertEqual(options_to_bin([{11, "12345678901234"}]),
+ [[<<11:4, 13:4, 1:8>>, "12345678901234"]]),
+ % integer represented in one byte
+ ?assertEqual(options_to_bin([{11, 24}]),
+ [[<<11:4, 1:4>>, 24]]),
+ % integer represented in two bytes
+ ?assertEqual(options_to_bin([{11, 324}]),
+ [[<<11:4, 2:4>>, 324]]),
+ % integer represented in three bytes
+ ?assertEqual(options_to_bin([{11, 70024}]),
+ [[<<11:4, 3:4>>, 70024]]).
+
+path_to_options_test() ->
+ Path = "/.well-known/core",
+ Ret = path_to_options(Path),
+ ?assertEqual(Ret, [{11,".well-known"},{11,"core"}]).
+
+uri_to_options_test() ->
+ Uri = "coap://[::1]/.well-known/core",
+ Ret = uri_to_options(Uri),
+ ?assertEqual(Ret, {"::1", 5683, [{11,".well-known"},{11,"core"}]}).
+
+-endif.
diff --git a/src/coaperl.app.src b/src/coaperl.app.src
new file mode 100644
index 0000000..df8c84c
--- /dev/null
+++ b/src/coaperl.app.src
@@ -0,0 +1,11 @@
+{application, coaperl,
+ [
+ {description, ""},
+ {vsn, "1"},
+ {registered, []},
+ {applications, [
+ kernel,
+ stdlib
+ ]},
+ {env, []}
+ ]}.
diff --git a/src/coaperl_sup.erl b/src/coaperl_sup.erl
new file mode 100644
index 0000000..b57c918
--- /dev/null
+++ b/src/coaperl_sup.erl
@@ -0,0 +1,29 @@
+-module(coaperl_sup).
+
+-behaviour(supervisor).
+
+%% API
+-export([start_link/0]).
+
+%% Supervisor callbacks
+-export([init/1]).
+
+%% Helper macro for declaring children of supervisor
+-define(CHILD(I, Type, Args), {I, {I, start_link, [Args]}, permanent, 5000, Type, [I]}).
+
+%% ===================================================================
+%% API functions
+%% ===================================================================
+
+start_link() ->
+ supervisor:start_link({local, ?MODULE}, ?MODULE, []).
+
+%% ===================================================================
+%% Supervisor callbacks
+%% ===================================================================
+
+init([]) ->
+ {ok, { {one_for_one, 5, 10}
+% [?CHILD(udp, worker, [])]
+ } }.
+
diff --git a/src/corerl.erl b/src/corerl.erl
new file mode 100644
index 0000000..6aa4af5
--- /dev/null
+++ b/src/corerl.erl
@@ -0,0 +1,72 @@
+-module(corerl).
+-export([parse/1,
+ content_format_number/1,
+ content_format_type/1]).
+
+-include("corerl.hrl").
+
+
+content_formats() ->
+ [{"text/plain;charset=utf-8", 0},
+ % shortcut for the above
+ {"text/plain", 0},
+ {"application/link-format", 40},
+ {"application/xml", 41},
+ {"application/octext-stream", 42},
+ {"application/exi", 47},
+ {"application/json", 50}].
+
+content_format_number(ContentFormat) ->
+ proplists:get_value(ContentFormat, content_formats()).
+
+content_format_type(Number) ->
+ list_to_binary(proplists:get_value(Number,
+ [{N,F} || {F,N} <- content_formats()]
+ )).
+
+
+-spec parse(<<>>) -> list(#core_link{}).
+% @doc: parse a core message
+parse(Data) ->
+ parse_link_values(binary:split(Data, <<",">>, [global]), []).
+
+
+parse_link_values([LinkValue|T], Acc) ->
+ Link = parse_link_value(LinkValue),
+ parse_link_values(T, [Link|Acc]);
+parse_link_values([], Acc) ->
+ Acc.
+
+parse_link_value(<<$<, Data/binary>>) ->
+ [Value, Params] = binary:split(Data, <<$>>>),
+ parse_link_params(binary:split(Params, <<$;>>, [global]), #core_link{uri=Value}).
+
+
+parse_link_params([LinkParam|T], Rec) ->
+ NewRec = case binary:split(LinkParam, <<$=>>, [global]) of
+ [<<"title">>, V] ->
+ Rec#core_link{title=V};
+ [<<"ct">>, V] ->
+ Rec#core_link{ct=content_format_type(binary_to_integer(V))};
+ [<<"rt">>, V] ->
+ Rec#core_link{rt=V};
+ [<<"if">>, V] ->
+ Rec#core_link{'if'=V};
+ _ -> Rec
+ end,
+ parse_link_params(T, NewRec);
+parse_link_params([], Rec) ->
+ Rec.
+
+
+-ifdef(TEST).
+-include_lib("eunit/include/eunit.hrl").
+
+parse_link_values_test() ->
+ Data = <<"</>;title=\"General Info\";ct=0,</time>;if=\"clock\";rt=\"Ticks\";title=\"Internal Clock\";ct=0;obs,</async>;ct=50">>,
+ ?assertEqual(parse(Data),
+ [{core_link,<<"/async">>,undefined,undefined,undefined,<<"application/json">>,undefined},
+ {core_link,<<"/time">>,<<"\"Internal Clock\"">>,undefined,<<"\"Ticks\"">>, <<"text/plain;charset=utf-8">>,<<"\"clock\"">>},
+ {core_link,<<"/">>,<<"\"General Info\"">>,undefined,undefined,<<"text/plain;charset=utf-8">>,undefined}
+ ]).
+-endif.
diff --git a/test/coap_parse_test.erl b/test/coap_parse_test.erl
new file mode 100644
index 0000000..59b8510
--- /dev/null
+++ b/test/coap_parse_test.erl
@@ -0,0 +1,19 @@
+-module(coap_parse_test).
+-include_lib("eunit/include/eunit.hrl").
+
+-include("coaperl.hrl").
+
+% Test descriptions
+parse_test_() ->
+ [{"Option extration works",
+ check_get_option_values()}
+ ].
+
+% Tests
+check_get_option_values() ->
+ Ret = coap_parse:get_option_values(#coap_response{options=[{coap_option,11,<<"bar">>},
+ {coap_option,11,<<"baz">>},
+ {coap_option,12,<<"foo">>}]},
+ 11),
+ [?_assertEqual(Ret, [<<"bar">>, <<"baz">>])].
+
diff --git a/test/coap_unparse_test.erl b/test/coap_unparse_test.erl
new file mode 100644
index 0000000..777913e
--- /dev/null
+++ b/test/coap_unparse_test.erl
@@ -0,0 +1,31 @@
+-module(coap_unparse_test).
+-include_lib("eunit/include/eunit.hrl").
+
+% Test descriptions
+unparse_test_() ->
+ [{"Building a get request should result in binary data",
+ check_build_request()},
+ {"Building a response without payload in binary data",
+ check_build_response()},
+ {"Building a response with payload in binary data",
+ check_build_text_response()}
+ ].
+
+% Tests
+check_build_request() ->
+ Uri = "coap://[::1]/.well-known/core",
+ Ret = coap_unparse:build_request(Uri, 1),
+ [?_assertEqual(Ret, {"::1", 5683,
+ [<<64,1,0,1>>,
+ [[<<11:4,11:4>>,".well-known"],
+ [<<0:4,4:4>>,"core"]]]})].
+
+check_build_response() ->
+ Ret = coap_unparse:build_response(4, 4, 1),
+ [?_assertEqual(Ret, [<<64,4:3, 4:5,0,1>>])].
+
+check_build_text_response() ->
+ Ret = coap_unparse:build_text_response(2, 5, 1, "payload"),
+ [?_assertEqual(Ret, [<<64,2:3, 5:5,0,1>>,
+ [[<<12:4,1:4>>,0]],
+ 255, "payload"])].