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CoRE                                                           Z. Shelby
Internet-Draft                                                 Sensinode
Intended status: Standards Track                                B. Frank
Expires: January 9, 2011                                      SkyFoundry
                                                               D. Sturek
                                                  Pacific Gas & Electric
                                                            July 8, 2010


                Constrained Application Protocol (CoAP)
                        draft-ietf-core-coap-01

Abstract

   This document specifies the Constrained Application Protocol (CoAP),
   a specialized RESTful transfer protocol for use with constrained
   networks and nodes for machine-to-machine applications such as smart
   energy and building automation.  These constrained nodes often have
   8-bit microcontrollers with small amounts of ROM and RAM, while
   networks such as 6LoWPAN often have high packet error rates and a
   typical throughput of 10s of kbit/s.  CoAP provides the REST Method/
   Response interaction model between application end-points, supports
   built-in resource discovery, and includes key web concepts such as
   URIs and content-types.  CoAP easily translates to 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 Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.



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   This Internet-Draft will expire on January 9, 2011.

Copyright Notice

   Copyright (c) 2010 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 BSD License.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Constrained Application Protocol . . . . . . . . . . . . . . .  5
     2.1.  Interaction Model  . . . . . . . . . . . . . . . . . . . .  5
       2.1.1.  Synchronous response . . . . . . . . . . . . . . . . .  6
       2.1.2.  Asynchronous response  . . . . . . . . . . . . . . . .  6
     2.2.  Transaction messages . . . . . . . . . . . . . . . . . . .  8
       2.2.1.  Confirmable (CON)  . . . . . . . . . . . . . . . . . .  8
       2.2.2.  Non-Confirmable (NON)  . . . . . . . . . . . . . . . .  8
       2.2.3.  Acknowledgment (ACK) . . . . . . . . . . . . . . . . .  8
       2.2.4.  Reset (RST)  . . . . . . . . . . . . . . . . . . . . .  8
       2.2.5.  Transaction IDs  . . . . . . . . . . . . . . . . . . .  9
     2.3.  Methods  . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.3.1.  GET  . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.3.2.  POST . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.3.3.  PUT  . . . . . . . . . . . . . . . . . . . . . . . . . 10
       2.3.4.  DELETE . . . . . . . . . . . . . . . . . . . . . . . . 10
     2.4.  Response Codes . . . . . . . . . . . . . . . . . . . . . . 10
     2.5.  Options  . . . . . . . . . . . . . . . . . . . . . . . . . 10
       2.5.1.  Option Processing  . . . . . . . . . . . . . . . . . . 10
       2.5.2.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . 11
       2.5.3.  Content-type encoding  . . . . . . . . . . . . . . . . 11
   3.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 12
     3.1.  CoAP header  . . . . . . . . . . . . . . . . . . . . . . . 12
     3.2.  Header options . . . . . . . . . . . . . . . . . . . . . . 13
       3.2.1.  Content-type Option  . . . . . . . . . . . . . . . . . 15
       3.2.2.  Uri-Scheme Option  . . . . . . . . . . . . . . . . . . 15
       3.2.3.  Uri-Authority Option . . . . . . . . . . . . . . . . . 16
       3.2.4.  Uri-Path Option  . . . . . . . . . . . . . . . . . . . 16
       3.2.5.  Location Option  . . . . . . . . . . . . . . . . . . . 16
       3.2.6.  Max-age Option . . . . . . . . . . . . . . . . . . . . 16
       3.2.7.  Etag Option  . . . . . . . . . . . . . . . . . . . . . 16
   4.  UDP Binding  . . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.1.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 17
     4.2.  Retransmission . . . . . . . . . . . . . . . . . . . . . . 17
     4.3.  Congestion Control . . . . . . . . . . . . . . . . . . . . 18
     4.4.  Default Port . . . . . . . . . . . . . . . . . . . . . . . 18
   5.  Caching  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     5.1.  Cache control  . . . . . . . . . . . . . . . . . . . . . . 19
     5.2.  Cache refresh  . . . . . . . . . . . . . . . . . . . . . . 19
     5.3.  Proxying . . . . . . . . . . . . . . . . . . . . . . . . . 20
   6.  Resource Discovery . . . . . . . . . . . . . . . . . . . . . . 21
     6.1.  Link Format  . . . . . . . . . . . . . . . . . . . . . . . 22
     6.2.  Naming . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     6.3.  Query Filtering  . . . . . . . . . . . . . . . . . . . . . 24
     6.4.  HTTP Resource Discovery  . . . . . . . . . . . . . . . . . 25
   7.  HTTP Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 25



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   8.  Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 26
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
     11.1. Codes  . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     11.2. Content Types  . . . . . . . . . . . . . . . . . . . . . . 30
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 31
   13. Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 32
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 34
     14.2. Informative References . . . . . . . . . . . . . . . . . . 34
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 35







































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1.  Introduction

   The use of web services on the Internet has become ubiquitous in most
   applications, and depends on the fundamental Representational State
   Transfer (REST) architecture of the web.

   The Constrained RESTful Environments (CoRE) working group 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).  Constrained networks like 6LoWPAN
   support the expensive fragmentation of IPv6 packets into small link-
   layer frames.  One design goal of CoRE has been to keep message
   overhead small, thus limiting the use of fragmentation.

   One of the main goals of CoRE is to design a generic RESTful protocol
   for the special requirements of this constrained environment,
   especially considering energy, building automation and other M2M
   applications.  The goal of CoAP is not to blindly compress HTTP, but
   rather to realize a subset of REST common with HTTP but optimized for
   M2M applications.  Although CoRE could be used for compressing simple
   HTTP interfaces, it more importantly also offers features for M2M
   such as built-in discovery, multicast support and asynchronous
   transactions.

   This document specifies the RESTful 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 [I-D.shelby-core-coap-req].  CoAP
   has the following main features:

   o  Constrained RESTful protocol fulfilling M2M requirements.

   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  UDP binding with reliable unicast and best-effort multicast
      support.

   o  Asynchronous transaction support.

   o  Low header overhead and parsing complexity.

   o  URI and Content-type support.

   o  Built-in resource discovery.




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   o  Simple proxy and caching capabilities.


2.  Constrained Application Protocol

   This section specifies the basic functionality and processing rules
   of the protocol.

2.1.  Interaction Model

   The REST 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 (called an end-point).  A CoAP REST exchange 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.  A
   response is then sent with a Response Code and resource
   representation if appropriate.

   Unlike HTTP, CoAP deals with these REST interchanges asynchronously
   over a UDP transport with support for both unicast and multicast
   interactions.  This is achieved using transaction messages
   (Confirmable, Non-Confirmable, Acknowledgment, Reset) supporting
   optional reliability (with exponential back-off) and transaction IDs
   between end-points to carry REST requests and responses.  These
   transactions are transparent to the REST interchanges.  The only
   difference being that responses may arrive asynchronously.

   One could think of CoAP as using a two-layer approach, a
   transactional layer used to deal with UDP and the asynchronous nature
   of the interactions, and the REST interactions using Method and
   Response codes.


                            +---------------------+
                            |     Application     |
                            +---------------------+
                            +---------------------+
                            |      CoAP REST      |
                            |---------------------|
                            |  CoAP Transactions  |
                            +---------------------+
                            +---------------------+
                            |        UDP          |
                            +---------------------+


                    Figure 1: Abstract layering of CoAP



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2.1.1.  Synchronous response

   The most basic interaction between the REST and Transaction layers
   works by sending a REST request in a confirmable CoAP message and
   waiting for an acknowledgment message that also carries the REST
   response.  E.g., two possible interactions for a basic GET are shown
   in Figure 2.

   Client             Server       Client             Server
      |                 |             |                 |
      |    CON tid=47   |             |    CON tid=53   |
      |     GET /foo    |             |     GET /baz    |
      +---------------->|             +---------------->|
      |                 |             |                 |
      |    ACK tid=47   |             |    ACK tid=53   |
      |  200 "<temp...  |             |    404 "Not...  |
      |<----------------+             |<----------------+
      |                 |             |                 |

    Figure 2: Two basic GET transactions, one successful, one not found

   Note that at the transaction layer, the response is returned in an
   ACK message, independent of whether the request was successful at the
   REST layer.  In effect, the REST response is piggy-backed on the ACK
   message, so no separate acknowledgment is required that the GET
   message was received.

   The relationship between the confirmable message (CON) and the
   acknowledgment message (ACK) is indicated by the transaction ID,
   which is echoed back by the server in the ACK.  Transaction IDs are
   short-lived, they only serve to couple CON and ACK messages.

   The tight coupling between CON and ACK also relieves the ACK of the
   need to echo back information from the request, such as the URI or a
   request token supplied by the client.  We say that a REST response
   carried in an ACK _pertains_ to the REST request in the corresponding
   CON.

2.1.2.  Asynchronous response

   Not all interactions are as simple as the basic synchronous exchange
   shown.  For example, a server might need longer to obtain the
   representation of the resource requested than it can wait sending
   back the acknowledgment, without risking the client to repeatedly
   retransmit the request.  To handle this case, the REST response is
   decoupled from the transaction layer acknowledgment.  Actually, the
   latter does not carry any REST message at all.




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   As the client cannot know that this will be the case, it sends
   exactly the same confirmable message with the same REST request.  The
   server maybe attempts to obtain the resource (e.g., by acting as a
   proxy) and times out an ACK timer, or it immediately sends an
   acknowledgment knowing in advance that there will be no quick answer.
   The acknowledgment effectively is a promise that the REST request
   will be acted upon, see Figure 3.

   Client             Server
      |                 |
      |   CON tid=48    |
      |  GET http://n.. |
      +---------------->|
      |                 |
      |   ACK tid=48    |
      |<----------------+
      |                 |
      ... Time Passes ...
      |                 |
      |   CON tid=783   |
      |  200 http://n.. |
      |     "<html..    |
      |<----------------+
      |                 |
      |   ACK tid=783   |
      +---------------->|
      |                 |

                 Figure 3: An asynchronous GET transaction

   When the server finally has obtained the resource representation and
   is ready to send the response, it initiates a transaction to the
   client.  This new transaction has its own transaction ID, so there is
   no automatic coupling of the response to the request.  Instead, the
   URI (and possibly token) is echoed back to the client in order to
   associate the REST response to the original REST request.  To ensure
   that this message is not lost, it is again sent as a confirmable
   message and answered by the client with an ACK, citing the new TID
   chosen by the server.

   As a special failure situation, a client may no longer be aware that
   it sent a request, e.g., if it does not have stable storage and was
   rebooted in the meantime.  This can be indicated by a special "Reset"
   message, as shown in Figure 4.







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   Client             Server
    ... Client reboots ...
      |                 |
      |   CON tid=783   |
      |  200 http://n.. |
      |     "<html..    |
      |<----------------+
      |                 |
      |   RST tid=783   |
      +---------------->|
      |                 |

                     Figure 4: An orphaned transaction

2.2.  Transaction messages

   The CoAP transactions make use of four different message types,
   described in this section.  These messages are transparent to the
   REST request/response carried over them.

2.2.1.  Confirmable (CON)

   Some messages require an acknowledgment, either just to know they did
   arrive or also to deliver the reply to a request.  We call these
   messages "Confirmable".  When no packets are lost, each Confirmable
   message elicits exactly one return message of type Acknowledgment or
   type Reset.

2.2.2.  Non-Confirmable (NON)

   Some other messages do not require an acknowledgment.  This is
   particularly true for messages that are repeated regularly for
   application requirements, such as repeated readings from a sensor
   where eventual arrival is sufficient.

2.2.3.  Acknowledgment (ACK)

   An Acknowledgment message acknowledges that a specific Confirmable
   message (identified by its Transaction ID) arrived.  As with all of
   the message types itself, it may carry a payload and some options to
   provide more details, such as the result of a request that was
   carried in the Confirmable.

2.2.4.  Reset (RST)

   A Reset message indicates that a specific Confirmable message was
   received, but some context is missing to properly process it.  This
   condition is usually caused when the receiving node has rebooted and



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   has forgotten some state that would be required to interpret the
   message.

2.2.5.  Transaction IDs

   The Transaction ID is an unsigned integer kept by a CoAP end-point
   for all of the CoAP Confirmable or Non-Confirmable messages it sends.
   Each CoAP end-point keeps a single Transaction ID variable, which is
   changed each time a new Confirmable or Non-Confirmable message is
   sent regardless of the destination address or port.  The Transaction
   ID is used to match an Acknowledgment with an outstanding request,
   for retransmission and to discard duplicate messages.  The initial
   Transaction ID should be randomized.  The same Transaction ID MUST
   NOT be re-used within the potential retransmission window, calculated
   as RESPONSE_TIMEOUT * (2 ^ MAX_RETRANSMIT - 1).

2.3.  Methods

   CoAP supports the basic RESTful methods of GET, POST, PUT, DELETE,
   which are easily mapped to HTTP.  In this section each method is
   defined along with its behavior.  A unicast request with an unknown
   or unsupported Method Code MUST generate a message with a "405 Method
   Not Allowed" Response Code.

   As CoAP methods manipulate resources, they have the same properties
   of safe (only retrieval) and idempotent (you can invoke it multiple
   times with the same effects) as HTTP Section 9.1 [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.

2.3.1.  GET

   The GET method retrieves the information of the resource identified
   by the request URI.  Upon success a 200 (OK) response SHOULD be sent.

   The response to a GET is cacheable if it meets the requirements in
   Section 5.

2.3.2.  POST

   The POST method is used to request the server to create a new
   resource under the requested URI.  If a resource has been created on
   the server, the response SHOULD be 201 (Created) including the URI of
   the new resource in a Location Option with any possible status in the
   message body.  If the POST succeeds but does not result in a new
   resource being created on the server, a 200 (OK) response code SHOULD
   be returned.



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   Responses to this method are not cacheable.

2.3.3.  PUT

   The PUT method requests that the resource identified by the request
   URI be updated with the enclosed message body.  If a resource exists
   at that URI the message body SHOULD be considered a modified version
   of that resource, and a 200 (OK) response SHOULD be returned.  If no
   resource exists then the server MAY create a new resource with that
   URI, resulting in a 201 (Created) response.  If the resource could
   not be created or modified, then an appropriate error response code
   SHOULD be sent.

   Responses to this method are not cacheable.

2.3.4.  DELETE

   The DELETE method requests that the resource identified by the
   request URI be deleted.  The response 200 (OK) SHOULD be sent on
   success.

   Responses to this method are not cacheable.

2.4.  Response Codes

   CoAP makes use of a subset of HTTP response codes as defined in
   Section 11.1.

2.5.  Options

   CoAP makes use of compact, extensible Type-Length-Value (TLV) style
   options.  This section explains the processing of CoAP options along
   with a summary of the main features implemented in options such as
   URIs and Content-types.

2.5.1.  Option Processing

   If no options are to be included, the Option Count field is set to 0
   below and the Payload (if any) immediately follows the Transaction
   ID.  If options are to be included, the following rules apply.  The
   number of options is placed in the Option Count field.  Each option
   is then placed in order of Type, immediately following the
   Transaction ID with no padding.  Upon reception, unknown options of
   class "elective" MUST be silently skipped.  Unknown options of class
   "critical" in a Confirmable SHOULD cause the return of a response
   code "400 Bad Request" (TBD) including a copy of the critical option
   number.




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2.5.2.  URIs

   The Universal Resource Identifier (URI) [RFC3986] is an important
   feature of the REST architecture, where the relative part of the URI
   indicates the resource being manipulated.  CoAP supports URIs
   similarly to HTTP, e.g. coap://[2001:DB8::101]/s/temp.  As this URI
   is used purely as a locator, CoAP only supports Universal Resource
   Locator features of [RFC3986] although throughout the document we
   refer to URI.

   CoAP splits the URI up into its three parts with the Uri-Scheme, Uri-
   Authority and Uri-Path Options.  The full URI can be created by
   concatenating those options (or their defaults if not present).  Uri-
   Scheme and Uri-Authority options are used for proxying, whereas Uri-
   Path is used for all requests.  CoAP does not support "." or ".." in
   URIs nor does it support IRIs.  A CoAP implementation SHOULD support
   query "?" processing (see Section 6 on the use of the query string in
   discovery), however fragment "#" processing is not supported.  All
   URI strings in CoAP MUST use the US-ASCII encoding defined in
   [RFC3986].  When using the Uri-Path Option the leading slash MUST be
   omitted.  Thus the above example "/s/temp" is included in the Uri-
   Path Option as "s/temp".

   Application designers are encouraged to make use of short, but
   descriptive URIs.  For example URIs 14 or less bytes in length fit in
   a more compact option header.  In addition, very short URIs such as
   "/1" can be assigned as an alternative short URI for a resource by
   the application.  CoAP resource descriptions includes an attribute to
   indicate if a short alternative URI of a resource is available (see
   Section 6).

   The CoAP protocol scheme is identified in URIs with "coap://"
   [IANA_TBD_SCHEME].

2.5.3.  Content-type encoding

   In order to support heterogeneous uses, CoAP is transparent to the
   use of different application payloads.  In order for the application
   process receiving a packet to properly parse a payload, its content-
   type should be explicitly known from the header (as e.g. with HTTP).
   The use of typical binary encodings for XML is discussed in
   [I-D.shelby-6lowapp-encoding].

   String names of Internet media types (MIME types) [RFC2046] are not
   optimal for use in the CoAP header.  Instead, CoAP simply assigns
   identifiers to a subset of common media and content transfer encoding
   types.  The content-type identifier is optionally included in the
   Content-type Option Header of messages to indicate the type of the



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   message body.  CoAP Content-type identifiers are defined in
   Section 11.2.  In the absence of the Content-type Option the MIME
   type "text/plain" MUST BE assumed.


3.  Message Formats

   CoAP makes use of asynchronous transactions using a simple binary
   header format.  This base header may be followed by options in Type-
   Length-Value (TLV) format.  CoAP is bound to UDP as described in
   Section 4.

   Any bytes after the headers in the packet are considered the message
   payload, if any.  The length of the message payload is implied by the
   datagram length.  See Section 4 for further message length
   requirements.

3.1.  CoAP header

   This section defines the CoAP header, which is shared for all CoAP
   messages.  CoAP makes use of an asynchronous transaction model.
   These transactions are used to carry RESTful exchanges, either using
   a Method Code (GET/PUT/POST/DELETE) to invoke interaction with a
   resource, or a Response Code carried in an immediate or asynchronous
   response.


     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 |  OC   |      Code     |        Transaction ID         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Payload (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                       Figure 5: CoAP header format

   Header Fields:

      Ver:  Version. 2-bit unsigned integer.  Indicates the version of
         CoAP.  Implementations of this specification MUST set this
         field to 1.  Other values are reserved for future versions.





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      T: 2-bit unsigned integer Transaction Type field.  Indicates if
         this message is Confirmable (0), Non-Confirmable (1),
         Acknowledgment (2) or Reset (3).

      OC:  4-bit unsigned integer Option Count field.  Indicates if
         there are Option Headers following the base header.  If set to
         0 the payload (if any) immediately follows the base header.  If
         greater than zero the field indicates the number of options to
         immediately follow the header.

      Code:  8-bit unsigned integer.  This field indicates the Method or
         Response Code of a message.  The value 0 indicates no code.
         The values 1-10 are used for Method Codes as defined in
         Table 1.  The values 11-39 are reserved for future use.  The
         values 40-255 are used for Response Codes as defined in
         Section 11.1.

      Transaction ID:  16-bit unsigned integer.  A unique Transaction ID
         assigned by the source and used to match responses.  The
         Transaction ID MUST be changed for each new request (regardless
         of the end-point) and MUST NOT be changed when retransmitting a
         request (see Section 2.2.5).

                             +--------+------+
                             | Method | Code |
                             +--------+------+
                             | GET    | 1    |
                             | POST   | 2    |
                             | PUT    | 3    |
                             | DELETE | 4    |
                             +--------+------+

                           Table 1: Method Codes

3.2.  Header options

   CoAP messages may also include one or more header options in TLV
   format.  Options MUST appear in order of option type (see Table 2).
   A delta encoding is used between each option header, with the Type
   identifier for each Option calculated as the sum of its Option Delta
   field and the Type identifier of the preceding Option in the message,
   if any, or zero otherwise.

   Each option header includes a Length field which can be extended by
   an octet for options with values longer than 14 octets.  CoAP options
   include the concept of Critical (odd value) and Elective (even value)
   options (see Section 2.5.1).




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   Each option has the following format:


         0   1   2   3   4   5   6   7
       +---+---+---+---+---+---+---+---+
       | option delta  |    length     | for 0..14
       +---+---+---+---+---+---+---+---+
                                                  for 15..270:
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
       | option delta  | 1   1   1   1 |          length - 15          |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


                      Figure 6: Header option format

   Option delta:  4-bit unsigned integer.  This field defines the
      difference between the option Type of this option and the previous
      option (or zero for the first option).  In other words, the Type
      identifier is calculated by simply summing the Option delta fields
      of this and previous options before it.  The Option Values 14, 28,
      ... are reserved for no-op options with no value (they are
      ignored) and are used for deltas larger than 14.  Thus these can
      be used as "fenceposts" if deltas larger than 15 would otherwise
      be required.

   Length:  Length Field.  Normally Length is a 4-bit unsigned integer
      allowing values of 0-14 octets.  When the length is 15 or more,
      another byte is added as an 8-bit unsigned integer plus 15
      allowing values of 15-270 octets.

   Option Value  The value in the format defined for that option in
      Table 2 of Length octets.  Options MAY use variable length values.

   The following options are defined in this document.  The Default
   column indicates the value to be assumed in the absence of this
   option (if any).















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   +------+-----+---------------+--------------+--------+--------------+
   | Type | C/E | Name          | Data type    | Length | Default      |
   +------+-----+---------------+--------------+--------+--------------+
   | 0    | -   | Reserved      | -            | -      | -            |
   | 1    | C   | Content-type  | 8-bit        | 1 B    | 0            |
   |      |     |               | unsigned     |        | (text/plain) |
   |      |     |               | integer      |        |              |
   | 2    | E   | Max-age       | Variable     | 1-4 B  | 60 seconds   |
   |      |     |               | length       |        |              |
   |      |     |               | unsigned     |        |              |
   |      |     |               | integer      |        |              |
   | 3    | C   | Uri-Scheme    | String       | 1-270  | "coap"       |
   |      |     |               |              | B      |              |
   | 4    | E   | Etag          | Sequence of  | 1-4 B  | -            |
   |      |     |               | bytes        |        |              |
   | 5    | C   | Uri-Authority | String       | 1-270  | ""           |
   |      |     |               |              | B      |              |
   | 6    | E   | Location      | String       | 1-270  | -            |
   |      |     |               |              | B      |              |
   | 7    | -   | Reserved      | -            | -      | -            |
   | 9    | C   | Uri-Path      | String       | 1-270  | ""           |
   |      |     |               |              | B      |              |
   +------+-----+---------------+--------------+--------+--------------+

                          Table 2: Option headers

3.2.1.  Content-type Option

   The Content-type Identifier Option indicates the Internet media type
   identifier of the message-body, see Section 11.2 for the encoding and
   identifier tables.  A Content-type Identifier Option SHOULD be
   included if there is a payload included with a CoAP message.  In the
   absence of the Content-type Option the MIME type "text/plain" (0)
   MUST be assumed.  This option MUST be supported by all end-points.

3.2.2.  Uri-Scheme Option

   The Uri-Scheme Option indicates the schema part of the URI without
   any ":" or "://" glue.  This option is most often used to access a
   resource via a proxy.  For example, to access an HTTP resource via a
   proxy the option value would be "http".  In the absence of this
   option, the URI scheme is assumed to be "coap".  Section 2.5.2
   specifies the rules for URIs in CoAP.  This option MUST be supported
   by an end-point implementing proxy functionality.







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3.2.3.  Uri-Authority Option

   The Uri-Authority Option indicates the authority (host + port) part
   of a URI.  Examples of this option include "[2001:DB8::101]",
   "198.51.100.0:8000" and "sensor.example.com".  This option is used
   when accessing a resource via a proxy.  In the absence of this
   option, the authority is assumed to be "".  Section 2.5.2 specifies
   the rules for URIs in CoAP.  This option MUST be supported by an end-
   point implementing proxy functionality.

3.2.4.  Uri-Path Option

   The Uri-Path Option indicates the absolute path part of a URI.  One
   example of an absolute path in his option is "s/light".  In the
   absence of this option, the path is assumed to be "/".  Section 2.5.2
   specifies the rules for URIs in CoAP.  The leading slash is assumed
   and MUST be omitted.  This option MUST be supported by all end-
   points.

3.2.5.  Location Option

   The Location Option indicates the location of a resource as an
   absolute path URI and is similar to the Uri-Path Option.  The
   Location Option MAY be included in a response to indicate the
   Location of a new resource created with POST or together with a 30x
   response code.  The leading slash is assumed and MUST be omitted.

3.2.6.  Max-age Option

   The Max-age Option indicates the maximum age of the resource for use
   in cache control in seconds.  The option value is represented as a
   variable length unsigned integer between 8 and 32 bits.  A default
   value of 60 seconds is assumed in the absence of this option.

   When included in a request, Max-age indicates the maximum age of a
   cached representation of that resource the client will accept.  When
   included in a response, Max-age indicates the maximum time the
   representation may be cached before it MUST be discarded.

3.2.7.  Etag Option

   The Etag Option is an opaque sequence of bytes which specifies the
   version of a resource representation.  An Etag may be generated for a
   resource in any number of ways including a version, checksum, hash or
   time.  An end-point receiving an Etag MUST treat it as opaque and
   make no assumptions about its format.  The Etag MAY be included in a
   response to indicate to a client if a resource has changed.  The Etag
   SHOULD be included in a request used for a cache refresh to indicate



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   the client's current version of the resource (see Section 5.2).


4.  UDP Binding

   The CoAP protocol operates by default over UDP.  CoAP could also be
   used over other transports such as TCP or SCTP, the specification of
   which is out of this document's scope.

   The goal of binding CoAP to UDP is to provide the bare minimum
   features for the protocol to operate over UDP, without trying to re-
   create the full feature set of a transport like TCP.  CoAP over UDP
   has the following features:

   o  Simple stop-and-wait retransmission reliability with exponential
      back-off as described in Section 4.2 for Confirmable messages.

   o  Transaction ID for response matching as described in
      Section 2.2.5.

   o  Multicast support as described in Section 4.1.

   The length of the Payload in a CoAP message is calculated from the
   datagram length.  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.  A CoAP message SHOULD fit within a single IP packet and MUST
   fit within a single IP datagram.  If the Path MTU is not known for a
   destination, an MTU of 1280 octets SHOULD be assumed.

4.1.  Multicast

   CoAP supports the use of multicast destination addresses.  Multicast
   messages SHOULD be Non-Confirmable.  If a Confirmable multicast
   message is sent then retransmission MUST NOT be performed.
   Furthermore, a destination end-point to a multicast Confirmable
   message MUST only send an Acknowledgment if the response code
   included indicates success (Code = 2XX) in order to eliminate error
   code response floods.  Other mechanisms for avoiding congestion from
   multicast requests are being considered in
   [I-D.eggert-core-congestion-control].

4.2.  Retransmission

   A CoAP end-point keeps track of open Confirmable messages it sent
   that are waiting for a response.  Each entry includes at least the
   destination IP address and port of the original message, the message



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   itself, a retransmission counter and a timeout.  When a Confirmable
   is sent, an entry is made for that message with a default initial
   timeout of RESPONSE_TIMEOUT and the retransmission counter set to 0.
   When a matching Acknowledgment is received for an entry, the entry is
   invalidated.  When a timeout is triggered for an entry and the
   retransmission counter is less than MAX_RETRANSMIT, the original
   message is retransmitted to the destination without modification, the
   retransmission counter is incremented, and the timeout is doubled.
   If the retransmission counter reaches MAX_RETRANSMIT on a timeout,
   then the entry is removed and the application process informed of
   delivery failure.

   For CoAP messages sent to IP multicast addresses, retransmission MUST
   NOT be performed.  Therefore MAX_RETRANSMIT is always set to 0 when
   the destination address is multicast.

4.3.  Congestion Control

   In addition to the exponential back-off mechanism in Section 4.2,
   further congestion control optimizations are being considered and
   tested for CoAP.  These congestion control mechanism under
   consideration are described in [I-D.eggert-core-congestion-control].

4.4.  Default Port

   The CoAP default port number [IANA_TBD_PORT] MUST be supported by a
   server for resource discovery (see Section 6) and SHOULD be supported
   for providing access to other resources.  In addition other end-
   points may be hosted in the dynamic port space.

   When a CoAP server is hosted by a 6LoWPAN node, it SHOULD support a
   port in the 61616-61631 compressed UDP port space defined in
   [RFC4944].  The specific port number in use will be communicated in a
   URI and/or by some other discovery mechanism.


5.  Caching

   CoAP end-points are by definition constrained by bandwidth and
   processing power.  To optimize the performance of data transfer under
   these constraints, we use caching features consistent with HTTP.
   Caching includes the following concepts:

   o  Cache life of a resource is controlled via the Max-Age header
      option

   o  Cache refresh and versioning of a resource is controlled via the
      Etag header option



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   o  Proxies between a client and end-point may participate in the
      caching process on behalf of sleeping end-points and to avoid
      unnecessary traffic on the constrained network

5.1.  Cache control

   When an end-point responds to a GET request by sending a
   representation of the resource, it SHOULD specify the Max-Age header
   option.  The Max-Age specifies the cache life of the resource in
   seconds.  Resources which change rapidly will have a short cache
   life, and resources which change infrequently should specify a long
   cache life.  If Max-Age is unspecified in a GET response, then it is
   assumed to be 60 seconds.  If an end-point wishes to disable caching,
   it must explicitly specify a Max-Age of zero seconds.

   When a client reads the response from a GET request, it should cache
   the resource representation for the cache lifetime as specified by
   the Max-Age header.  During the cache lifetime, the client SHOULD use
   its cached version and avoid performing additional GETs for the
   resource.

   In general, the origin server end-point is responsible for
   determining cache age.  However, in some cases a client may wish to
   determine its own tolerance for cache staleness.  In this case, a
   client may specify the Max-Age header during a GET request.  If the
   client's Max-Age is of a shorter duration than the age of a cached
   resource, then the proxy or end-point SHOULD perform a cache refresh.
   If the client specifies a Max-Age of zero seconds, then the response
   MUST discard the cached representation and return a fresh
   representation.

5.2.  Cache refresh

   After the expiration of the cache lifetime, clients and proxies can
   refresh their cached representation of a resource.  Cache refresh is
   accomplished using a GET request which will return a representation
   of the resource's current state.

   If the end-point has the capability to version the resource, then the
   end-point should include the Etag header option in the response to a
   GET request.  The Etag is a variable length sequence of bytes which
   captures a version identifier of the resource.  The Etag is an opaque
   identifier; clients MUST NOT infer any semantics from the Etag value.

   If an end-point specifies the Etag header option with a response,
   then the client SHOULD specify a matching Etag header option in their
   GET request during cache refresh.  If the end-point's version of the
   resource is unmodified, then the server SHOULD return a 304 response



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   with no payload to avoid retransmitting the resource representation.

5.3.  Proxying

   A proxy is defined as a CoAP end-point which services cached requests
   on behalf of other CoAP end-points.  Any node in a CoAP network may
   act as a proxy, although in general the node between the constrained
   network and the Internet at large SHOULD always support proxy
   functionality.

   Proxies should be used under the following scenarios:

   o  Clients external to the constrained network SHOULD always make
      requests through a proxy to limit traffic on the constrained
      network

   o  Clients internal to the constrained network MAY use a proxy based
      on network topology when performance warrants

   o  Clients of sleeping devices MUST use a proxy to access resources
      while the device is sleeping

   Proxy requests are made as normal CON requests to the proxy end-
   point.  All proxy requests MUST use the Uri-Authority header to
   indicate the origin server's IP address using the URI format defined
   by RFC 3986:


     full uri  = "coap://" + authority + path
     authority = host [ ":" port ]
     host      = IP-literal / IPv4address / reg-name
                 (as defined by RFC 3986)
     port      = *DIGIT


   The host part is case insensitive and may be an IPv4 literal, IPv6
   literal in square brackets, or a registered name.  The port number is
   optional, if omitted or zero-length it is assumed to be the default
   CoAP port (see Section 4.4).

   If a request is made using the Uri-Authority header, then the
   following steps are taken:

      1.  If the authority (host and port) is the same as the proxy end-
      point, then the request MUST be treated as a local request and the
      path part is used as Uri-Path





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      2.  If the end-point does not support proxy functionality for the
      given URI or Method code, then the 400 Bad Request (TBD) error
      code MUST be returned

      3.  If the proxy does not contain a fresh cached representation of
      the resource, then the proxy MUST attempt to refresh its cache
      according to section 5.2.  The origin server's IP address and port
      is determined by the authority part of the full URI.  The Uri-Path
      option for the refresh request is determined by the path part of
      the full URI.

      4.  If the proxy fails to obtain a fresh cached representation,
      then a 502 Bad Gateway error code MUST be returned

      5.  The proxy returns the cached representation on behalf of the
      origin server

   All CoAP options are considered end-to-end and MUST be stored as part
   of the cache entry and MUST be transmitted in the proxy's response.
   The Max-Age option should 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
   20sec ago and had an original Max-Age of 60sec, then that resource's
   proxied Max-Age is now 40sec.


6.  Resource Discovery

   The discovery of resources offered by a CoAP end-point is extremely
   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 Web Server is typically
   called Web Discovery.  In this document we refer to the discovery of
   resources offered by a CoAP end-point as resource discovery.  CoAP
   resource discovery enables both unicast and multicast discovery of
   resources including filtering on attributes of resource descriptions.
   CoAP resource discovery can also be used to discovery HTTP resources.

   CoAP makes the assumption that all CoAP servers host an end-point on
   the default CoAP port (see Section 4.4) hosting a well-known
   discovery resource, or otherwise have been configured or discovered
   using some general service discovery mechanism such as
   [I-D.cheshire-dnsext-dns-sd].  This section assumes that such a
   configuration or service discovery has already been performed a
   priori, if needed.

   Resource discovery in CoAP is accomplished through the use of well-
   known resources which describe the links (resource descriptions)



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   offered by that CoAP server.  Well-known resources use the URI form
   "/.well-known/" as specified in [RFC5785].  CoAP discovery defines a
   new well-known URI for discovery "/.well-known/r"
   [IANA_TBD_WELLKNOWN].  A CoAP end-point MUST support this URI for the
   purpose of resource discovery.  It is however up to the application
   which resources descriptions are included and how they are organized.
   The resource representation of this resource is described in
   Section 6.1.

   CoAP Resource Discovery supports the following interactions:

   o  [GET /.well-known/r] returns a list of links available from a CoAP
      server (if any).  This request may be unicast or multicast.

   o  Filtering may be performed on any of the resource description
      attributes using a query string as specified in Section 6.3.  For
      example [GET /.well-known/r?n=Temperature] would return resources
      with the name Temperature.

   o  More capable end-points such as proxies could support a resource
      directory by requesting the resource descriptions of other end-
      points or accepting [POST /.well-known/r] messages from other CoAP
      end-points.  This adds the resources of other end-points to a
      resource directory in which absolute URIs are included for the
      links.  The details of such an entity are however out of scope for
      this document.

   o  CoAP may also be used to discovery HTTP resources or an HTTP
      server may use the well-known resource and link-format as
      discussed in Section 6.4.

   End-points with a large number of resources SHOULD include resource
   descriptions only for important collections and organize their
   resource descriptions into a hierarchy of link resources.  This is
   done by including links in the /.well-known/r list which point to
   other resource lists, e.g. "</.well-known/r/s>;n=Sensors;ct=40".
   Such a hierarchy SHOULD be under the /.well-known/r path.

6.1.  Link Format

   CoAP resource discovery makes use of the HTTP Link Header format
   specified in [I-D.nottingham-http-link-header] which is in Augmented
   Backus-Naur Form (ABNF) notation [RFC5234].  This specification
   allows for the use of this simple link format by other protocols,
   thus not limiting it to the actual HTTP Link Header.  The format does
   not require special XML or binary parsing, and is extensible.  The
   format is also compatible with the naming and descriptions used in
   [I-D.cheshire-dnsext-dns-sd].



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   [I-D.nottingham-http-link-header] did not request a MIME type for
   this link format, as it assumes that would usually be carried in an
   HTTP header.  This specification thus requests a MIME type for this
   header as in Section 11.2 [IANA_TBD_LINK].

   The CoAP link-format defines default attributes for CoAP resource
   discovery.  All link parameters are optional and custom link-
   extensions MAY be defined.  Multiple descriptions are separated by
   commas as in [I-D.nottingham-http-link-header].  The following ABNF
   specification defines the format:


      link-value        = "<" URI-Reference ">" *( ";" link-param )
      link-param        = ( ( "d" "=" URI )
                        | ( "sh" "=" URI )
                        | ( "n" "=" string )
                        | ( "ct" "=" media-code *( "," media-code ) )
                        | ( "id" "=" integer )
                        | ( link-extension ) )
      link-extension    = ( parmname [ "=" ( ptoken | string ) ] )
      ptoken            = 1*ptokenchar
      ptokenchar        = "!" | "#" | "$" | "%" | "&" | "'" | "("
                        | ")" | "*" | "+" | "-" | "." | "/" | DIGIT
                        | ":" | "<" | "=" | ">" | "?" | "@" | ALPHA
                        | "[" | "]" | "^" | "_" | "`" | "{" | "|"
                        | "}" | "~"
      media-code        = Identifier for media type in decimal ASCII,
                          see Section 11.2
      URI-reference     = Imported from [RFC3986]
      URI               = Imported from [RFC3986]


   link-value:  The link-value is the relative URI of the resource on
      that end-point or an absolute full URI in the case of e.g. a
      directory agent or HTTP resource.  The key "uri=" is used to match
      this field in a query string.  Protocol identifier, host and port
      parts of the URI MAY be included if they differ from the default
      CoAP values.

   d: Description.  A URI that points to the definition of that resource
      interface, for example in WADL.

   sh:  Short URI.  Alternative short URI which can also be used to
      access this resource.







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   n: Instance Name.  See Section 6.2.

   ct:  Content-type.  The Internet media type this resource returns in
      the CoAP identifier code format as a decimal ASCII integer as per
      Section 11.2.  For example application/xml would be indicated as
      "ct=41".  If a resource supports multiple content-types they can
      be separated by a comma in this attribute.  If no Content-type
      attribute is present then text/plain is assumed.

   id:  Unique Identifier.  The id field is a unique identifier (e.g.
      UUID) for this resource for use in e.g. search directories.

   An example of three typical CoAP link descriptions in this format
   follows.  Multiple resource descriptions in a representation are
   separated by commas.  Linefeeds are included in the example just for
   readability.

   </>;n=Info,
   </sensor/temp>;sh=/t;ct=0,41;n=Temperature,
   </sensor/light>;sh=/l;ct=41;n=Light,

6.2.  Naming

   CoAP resource naming is designed to be compatible with DNS-based
   Service Discovery naming.  The n= attribute carries what can be
   considered the "Instance" part of a DNS-SD name.  The "Service" and
   "Domain" parts of DNS naming are implied as Service is assumed to be
   "_coap._udp" and Domain is assumed to be .local.  The Service part
   MAY be included as _http._tcp in the case of an HTTP resource.  The
   Domain MAY be included if different than .local.

   Whereas service discovery is used to find the IP address, port and
   protocol of an unknown service, resource discovery is a fine-grained
   discovery of resource URIs within a web service.  CoAP resource
   descriptions can be imported into DNS-SD for exposure to service
   discovery by using the n= attribute as the "Instance", "_coap_udp" as
   the Service and a default of ".local" as the Domain.  The DNS TXT
   record can be generated by importing the resource description
   attributes as they share the same format as Section 6 of
   [I-D.cheshire-dnsext-dns-sd].

6.3.  Query Filtering

   A CoAP end-point SHOULD support the query string /.well-known/r? with
   uri= or any of the resource description attributes for the purpose of
   filtering a discovery.  Wildcard * endings SHOULD be supported.  An
   exact match is performed on the query string, and a 200 OK response
   is returned with a link-style structure that contains the matching



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   entries (if any).  If resource descriptions are organized
   hierarchically, a query on the root resource /.well-known/r SHOULD
   return all matching resource descriptions from the entire hierarchy
   if possible.

   For a unicast query with no matching entries a 404 Not Found code is
   returned, however no response is sent in case of a multicast query as
   described in Section 4.1 to prevent a flood of responses.

6.4.  HTTP Resource Discovery

   This mechanism may also be used to discover HTTP resources as HTTP
   lacks a built-in discovery mechanism and multicast support.  When
   HTTP resources are listed in the link-format HTTP they SHOULD be
   indicated by including http:// in the link-value field or the
   _http._tcp Service in the name attribute.

   An HTTP server may make use of the /.well-known/r URI scheme and
   link-format representation defined in this section for the purpose of
   discovery using HTTP.  This may especially be useful for a CoAP-HTTP
   proxy to expose the resources available in a constrained domain to
   HTTP clients.


7.  HTTP Mapping

   CoAP supports a limited subset of HTTP functionality, and thus a
   mapping to HTTP is straightforward.  There might be several reasons
   for mapping between CoAP and HTTP, for example when designing a REST
   interface for use over either protocol or when realizing a CoAP-HTTP
   proxy.  Likewise, CoAP could equally be mapped to other protocols
   such as XMPP or SIP, the definition of which is out of scope.

   The mapping of CoAP to HTTP is a straightforward conversion of the
   CoAP method or response code, content-type and options to the
   corresponding HTTP feature.  The payload is carried in an equivalent
   way by both protocols.  The mapping of HTTP to CoAP requires checking
   for methods, response codes, options and content-types that are not
   supported by CoAP.  A mapping SHOULD attempt to map options, response
   codes and content-types to a suitable alternative if possible.
   Otherwise the unsupported feature SHOULD be silently dropped if
   possible, or an appropriate error code generated otherwise.

   The caching and proxying of CoAP is specified in Section 5.  In a
   similar manner, caching and proxying MAY be performed between CoAP
   and HTTP by an intermediate node.  A proxy SHOULD respond with a 502
   (Bad Gateway) error to HTTP requests which can not be successfully
   mapped to CoAP.  CoAP transaction messages are transparent to REST



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   and MUST have no affect on a proxy function.


8.  Protocol Constants

   This section defines the relevant protocol constants defined in this
   document:

   RESPONSE_TIMEOUT                        1 second

   MAX_RETRANSMIT                          5


9.  Examples

   Figure 7 shows a basic request sequence.  A client makes a
   Confirmable GET request for the resource /temperature to the server
   with a Transaction ID of 1234.  The request includes one Uri-Path
   Option (delta 0 + 9 = 9) "temperature" of Len = 11.  This request is
   a total of 16 octets long.  The corresponding Acknowledgment is of
   Code 200 OK and includes a Payload of "22.3 C".  The Transaction ID
   is 1234, thus the transaction is successfully completed.  The
   response is 10 octets long and a Content-type of 0 (text/plain) is
   assumed as there is no Content-type Option.



























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   CLIENT                                                     SERVER
     |                                                          |
     |    ----- CON + GET /temperature [TID=1234]  ------>      |
     |                                                          |


    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    |           TID=1234            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   9   |   11  |       "temperature" (11 Octets) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   CLIENT                                                     SERVER
     |                                                          |
     |       <-------- ACK + 200 OK [TID=1234] ---------        |
     |                                                          |


    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   |    Code=80    |           TID=1234            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    "22.3 C" (6 Octets) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                     Figure 7: Basic request/response

   Figure 8 shows an example of a retransmission using the previous
   request.  The first ACK from the server is lost, and after
   RESPONSE_TIMEOUT seconds the client retransmits the request.














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   CLIENT                                                     SERVER
     |                                                          |
     |    ----- CON + GET /temperature [TID=1234]  ------>      |
     |                                                          |
     |                           X------------------------      |
     |                                                          |
                         RESPONSE_TIMEOUT
     |                                                          |
     |    ----- CON + GET /temperature [TID=1234]  ------>      |
     |                                                          |
     |                                                          |
     |       <-------- ACK + 200 OK [TID=1234] ---------        |
   Payload:
   22.3 C



                     Figure 8: Basic request/response

   Figure 9 shows an example of resource discovery.  Here a unicast GET
   request is made to the server for /.well-known/r, which returns a
   list of two resource descriptions.  The client then decides to make a
   request for the short URI of /sensor/light (/l).  Requesting
   /sensors/light would result in the same representation.


   CLIENT                                                     SERVER
     |                                                          |
     |   ----- CON + GET /.well-known/r [TID=5068]  ------>     |
     |                                                          |
     |      <----- ACK + 200 OK [TID=5068, CT=40] ------        |
   Payload:
   </sensor/temp>;sh=/t;ct=0,41;n=Temperature,
   </sensor/light>;sh=/l;ct=41;n=Light

     |                                                          |
     |          ----- CON + GET /l [TID=5069]  ------>          |
     |                                                          |
     |        <---- ACK + 200 OK [TID=5069, CT=41] -----        |
   Payload:
   <?xml?><Light unit="Lux">45</Light>


                     Figure 9: Basic request/response

   Figure 10 shows an example of multicast resource discovery.  Here a
   client sends a request for /.well-known/r with a query for ?n=Light
   (Resource name = Light) to all-nodes link-scope multicast.  There are



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   3 servers on the link: A, B and C. Servers A and B have a matching
   resource, therefore they send back a successful 200 OK response with
   the matching resource in the payload.  C does not attempt to send a
   response.


   CLIENT                                                    FF02::1
     |                                                          |
     |  --- CON + GET /.well-known/r?n=Light [TID=7000]  --->   |
     |                                                          |
     |      <----- ACK + 200 OK [TID=7000, CT=40] ------     SERVER A
   Payload:
   </sensor/light>;sh=/l;ct=41;n=Light

     |                                                          |
     |      <----- ACK + 200 OK [TID=7000, CT=40] ------     SERVER B
   Payload:
   </light>;ct=41;n=Light



                     Figure 10: Basic request/response


10.  Security Considerations

   TODO: Expand this section to a full security analysis, including how
   to use CoAP with various security options.

   Some of the features considered in this document will need further
   security considerations during a protocol design.  For example the
   use of string URLs may have entail security risks due to complex
   processing on limited microcontroller implementations.

   The CoAP protocol will be designed for use with e.g.  (D)TLS, IPsec
   or object security.  A protocol design should consider how
   integration with these security methods will be done, how to secure
   the CoAP header and other implications.


11.  IANA Considerations

   [IANA_TBD_SCHEME] This document suggests the scheme coap:// to
   identify this protocol in a URI.  The string "coap" should similarly
   be used in well-known port and service discovery registrations.

   [IANA_TBD_PORT] Apply for a well-known port number in the 0-1023
   space as CoAP end-points are usually executed by an operating system



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   or root process. http://www.iana.org/assignments/port-numbers

   [IANA_TBD_MIME] A new registry is required for the Internet MIME type
   identifier space for CoAP as described in Section 11.2.

   [IANA_TBD_LINK] MIME type for link format, application/link-format to
   be requested.

   [IANA_TBD_WELLKNOWN] Apply for a well-known URI for the purpose of
   resource discovery at /.well-known/r as per [RFC5785].

11.1.  Codes

   CoAP makes use of (a subset of) the HTTP status codes defined in
   [RFC2616].  The HTTP status code is encoded into an 8-bit unsigned
   integer code with the mapping defined in Table 3.  The use of these
   codes is defined throughout this document using the HTTP Name.

                   +------+----------------------------+
                   | Code | HTTP Name                  |
                   +------+----------------------------+
                   | 40   | 100 Continue               |
                   | 80   | 200 OK                     |
                   | 81   | 201 Created                |
                   | 124  | 304 Not Modified           |
                   | 160  | 400 Bad Request            |
                   | 164  | 404 Not Found              |
                   | 165  | 405 Method Not Allowed     |
                   | 175  | 415 Unsupported Media Type |
                   | 200  | 500 Internal Server Error  |
                   | 202  | 502 Bad Gateway            |
                   | 204  | 504 Gateway Timeout        |
                   +------+----------------------------+

                            Table 3: CoAP Codes

11.2.  Content Types

   Internet media types are identified by a string in HTTP, such as
   "application/xml".  This string is made up of a top-level type
   "application" and a sub-type "xml" [RFC2046].  In order to minimize
   the overhead of using these media types to indicate the type of
   message payload, CoAP defines an identifier encoding scheme for a
   subset of Internet media types.  It is expected that this table of
   identifiers will be extensible and maintained by IANA for values of
   0-200 [IANA_TBD_MIME].

   The Content-type Option is formatted as an 8-bit unsigned integer.



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   Initial mappings from Internet media types to a suitable identifier
   is shown in Table 4.  Composite high-level types (multipart and
   message) are not supported.  Identifier values from 201-255 are
   reserved for vendor specific, application specific or experimental
   use and are not maintained by IANA.

         +-----------------------------------------+------------+
         | Internet media type                     | Identifier |
         +-----------------------------------------+------------+
         | text/plain (UTF-8)                      | 0          |
         | text/xml (UTF-8)                        | 1          |
         | text/csv (UTF-8)                        | 2          |
         | text/html (UTF-8)                       | 3          |
         | image/gif                               | 21         |
         | image/jpeg                              | 22         |
         | image/png                               | 23         |
         | image/tiff                              | 24         |
         | audio/raw                               | 25         |
         | video/raw                               | 26         |
         | application/link-format [IANA_TBD_LINK] | 40         |
         | application/xml                         | 41         |
         | application/octet-stream                | 42         |
         | application/rdf+xml                     | 43         |
         | application/soap+xml                    | 44         |
         | application/atom+xml                    | 45         |
         | application/xmpp+xml                    | 46         |
         | application/exi                         | 47         |
         | application/x-bxml                      | 48         |
         | application/fastinfoset                 | 49         |
         | application/soap+fastinfoset            | 50         |
         | application/json                        | 51         |
         +-----------------------------------------+------------+

                      Table 4: Media type identifiers


12.  Acknowledgments

   Special thanks to Carsten Bormann and Klaus Hartke for substantial
   contributions to the ideas and text in the document (Section 2.1.1,
   Section 2.1.2, Section 2.2, Section 3.2), along with countless
   detailed reviews and discussions.

   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, Peter Bigot, Colin O'Flynn and



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   David Ryan for helpful comments and discussions that have shaped the
   document.


13.  Changelog

   Changes from ietf-00 to ietf-01:

      o New cleaner transaction message model and header (#5)

      o Removed subscription while being designed (#1)

      o Section 2 re-written (#3)

      o Text added about use of short URIs (#4)

      o Improved header option scheme (#5, #14)

      o Date option removed whiled being designed (#6)

      o New text for CoAP default port (#7)

      o Completed proxying section (#8)

      o Completed resource discovery section (#9)

      o Completed HTTP mapping section (#10)

      o Several new examples added (#11)

      o URI split into 3 options (#12)

      o MIME type defined for link-format (#13, #16)

      o New text on maximum message size (#15)

      o Location Option added

   Changes from shelby-01 to ietf-00:

      o Removed the TCP binding section, left open for the future.

      o Fixed a bug in the example.

      o Marked current Sub/Notify as (Experimental) while under WG
      discussion.





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      o Fixed maximum datagram size to 1280 for both IPv4 and IPv6 (for
      CoAP-CoAP proxying to work).

      o Temporarily removed the Magic Byte header as TCP is no longer
      included as a binding.

      o Removed the Uri-code Option as different URI encoding schemes
      are being discussed.

      o Changed the rel= field to desc= for resource discovery.

      o Changed the maximum message size to 1024 bytes to allow for IP/
      UDP headers.

      o Made the URI slash optimization and method impotence MUSTs

      o Minor editing and bug fixing.

   Changes from shelby-00 to shelby-01:

      o Unified the message header and added a notify message type.

      o Renamed methods with HTTP names and removed the NOTIFY method.

      o Added a number of options field to the header.

      o Combines the Option Type and Length into an 8-bit field.

      o Added the magic byte header.

      o Added new Etag option.

      o Added new Date option.

      o Added new Subscription option.

      o Completed the HTTP Code - CoAP Code mapping table appendix.

      o Completed the Content-type Identifier appendix and tables.

      o Added more simplifications for URI support.

      o Initial subscription and discovery sections.

      o A Flag requirements simplified.


14.  References



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14.1.  Normative References

   [I-D.nottingham-http-link-header]
              Nottingham, M., "Web Linking",
              draft-nottingham-http-link-header-09 (work in progress),
              April 2010.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

   [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.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

14.2.  Informative References

   [I-D.cheshire-dnsext-dns-sd]
              Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", draft-cheshire-dnsext-dns-sd-06 (work in
              progress), March 2010.

   [I-D.eggert-core-congestion-control]
              Eggert, L., "Congestion Control for the Constrained
              Application Protocol (CoAP)",
              draft-eggert-core-congestion-control-00 (work in
              progress), June 2010.

   [I-D.shelby-6lowapp-encoding]
              Shelby, Z., Luimula, M., and D. Peintner, "Efficient XML
              Encoding and 6LowApp", draft-shelby-6lowapp-encoding-00
              (work in progress), October 2009.

   [I-D.shelby-core-coap-req]
              Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
              Kelsey, "CoAP Requirements and Features",
              draft-shelby-core-coap-req-01 (work in progress),



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              April 2010.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.


Authors' Addresses

   Zach Shelby
   Sensinode
   Kidekuja 2
   Vuokatti  88600
   FINLAND

   Phone: +358407796297
   Email: zach@sensinode.com


   Brian Frank
   SkyFoundry
   Richmond, VA
   USA

   Phone:
   Email: brian@skyfoundry.com


   Don Sturek
   Pacific Gas & Electric
   77 Beale Street
   San Francisco, CA
   USA

   Phone: +1-619-504-3615
   Email: d.sturek@att.net















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