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Versions: (draft-shelby-core-coap) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 7252

CoRE                                                           Z. Shelby
Internet-Draft                                                 Sensinode
Intended status: Standards Track                                B. Frank
Expires: April 29, 2011                                       SkyFoundry
                                                               D. Sturek
                                                  Pacific Gas & Electric
                                                        October 26, 2010


                Constrained Application Protocol (CoAP)
                        draft-ietf-core-coap-03

Abstract

   This document specifies the Constrained Application Protocol (CoAP),
   a specialized web 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 a 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 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 April 29, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  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  . . . . . . . . . . . . . . . . 12
   3.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 12
     3.1.  CoAP header  . . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  Header options . . . . . . . . . . . . . . . . . . . . . . 14
       3.2.1.  Content-type Option  . . . . . . . . . . . . . . . . . 16
       3.2.2.  Uri-Authority Option . . . . . . . . . . . . . . . . . 16
       3.2.3.  Uri-Path Option  . . . . . . . . . . . . . . . . . . . 16
       3.2.4.  Uri-Query Option . . . . . . . . . . . . . . . . . . . 17
       3.2.5.  Location Option  . . . . . . . . . . . . . . . . . . . 17
       3.2.6.  Max-age Option . . . . . . . . . . . . . . . . . . . . 17
       3.2.7.  Etag Option  . . . . . . . . . . . . . . . . . . . . . 17
       3.2.8.  Token Option . . . . . . . . . . . . . . . . . . . . . 17
   4.  UDP Binding  . . . . . . . . . . . . . . . . . . . . . . . . . 18



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     4.1.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.2.  Retransmission . . . . . . . . . . . . . . . . . . . . . . 19
     4.3.  Congestion Control . . . . . . . . . . . . . . . . . . . . 19
     4.4.  Default Port . . . . . . . . . . . . . . . . . . . . . . . 19
   5.  Caching  . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     5.1.  Cache control  . . . . . . . . . . . . . . . . . . . . . . 20
     5.2.  Cache refresh  . . . . . . . . . . . . . . . . . . . . . . 21
     5.3.  Proxying . . . . . . . . . . . . . . . . . . . . . . . . . 21
   6.  Resource Discovery . . . . . . . . . . . . . . . . . . . . . . 22
   7.  HTTP Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 22
   8.  Protocol Constants . . . . . . . . . . . . . . . . . . . . . . 23
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 26
     10.1. Securing CoAP with IPsec . . . . . . . . . . . . . . . . . 27
     10.2. Securing CoAP with DTLS  . . . . . . . . . . . . . . . . . 28
       10.2.1. SharedKey & MultiKey Modes . . . . . . . . . . . . . . 28
       10.2.2. Certificate Mode . . . . . . . . . . . . . . . . . . . 28
     10.3. Threat analysis and protocol limitations . . . . . . . . . 29
       10.3.1. Processing URIs  . . . . . . . . . . . . . . . . . . . 29
       10.3.2. Proxying and Caching . . . . . . . . . . . . . . . . . 29
       10.3.3. Risk of amplification  . . . . . . . . . . . . . . . . 30
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 31
     11.1. Codes  . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     11.2. Content Types  . . . . . . . . . . . . . . . . . . . . . . 32
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 33
   13. Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     14.2. Informative References . . . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38





















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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 web 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 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 web 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 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 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 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 requests and responses.  These
   transactions are transparent to the request/response 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 request/response interactions using
   Method and Response codes.


                            +---------------------+
                            |     Application     |
                            +---------------------+
                            +---------------------+
                            |     CoAP Req/Res    |
                            |---------------------|
                            |  CoAP Transactions  |
                            +---------------------+
                            +---------------------+
                            |        UDP          |
                            +---------------------+


                    Figure 1: Abstract layering of CoAP



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2.1.1.  Synchronous response

   The most basic interaction between the Req/Res and Transaction layers
   works by sending a request in a confirmable CoAP message and waiting
   for an acknowledgment message that also carries the 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
   Req/Res layer.  In effect, the 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 Token
   Option supplied by the client.  We say that a response carried in an
   ACK _pertains_ to the 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 response is
   decoupled from the transaction layer acknowledgment.  Actually, the
   latter does not carry any message at all.

   As the client cannot know that this will be the case, it sends
   exactly the same confirmable message with the same request.  The



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   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 request will be
   acted upon, see Figure 3.  Since no Token Option was included in the
   initial request, an "Token Option required by server (CoAP 240)"
   error will be returned in the ACK.  The client would then repeat the
   request, now including a Token Option.  For a request where an
   asynchronous response is expeced, the Token Option can be included in
   the first request.

   Client             Server
      |                 |
      |   CON tid=48    |
      |   TOKEN = 3a    |
      |  GET http://n.. |
      +---------------->|
      |                 |
      |   ACK tid=48    |
      |<----------------+
      |                 |
      ... Time Passes ...
      |                 |
      |   CON tid=783   |
      |   TOKEN = 3a    |
      |   200 "<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
   Token Option is echoed back to the client in order to associate the
   response to the original 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   |
      |   TOKEN = 3a    |
      |  200 "<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
   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 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.  Unlike PUT,
   POST is not idempotent because the URI in the request indicates the
   resource that will handle the enclosed body.  This resource indicated
   by the POST may be used for data processing, a gateway to other
   protocols and it may create a new resource as a result of the POST.

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
   subordinate resource under the requested parent URI.  If a resource
   has been created on the server, the response SHOULD be 201 (Created)



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   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.

   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 or created 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 along with some
   CoAP specific 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



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   class "elective" MUST be silently skipped.  Unknown options of class
   "critical" in a Confirmable MUST cause the return of a response code
   "Critical Option not supported (CoAP 242)" including a copy of the
   critical option number in the payload of the response.

2.5.2.  URIs

   The Universal Resource Identifier (URI) [RFC3986] is an important
   feature of the web architecture.  CoAP supports URIs similarly to
   HTTP, e.g. coap://[2001:DB8::101]/s/temp, where the authority and
   path identify the resource to be manipulated.

   The CoAP header splits the URI up into four parts with the default
   coap:// scheme plus Uri-Authority, Uri-Path and Uri-Query CoAP header
   options.  The full URI is reconstructed as follows:


      ( "coap:" )
      ( "//" Uri-Authority ) only if Uri-Authority is present
      ( "/" Uri-Path )
      ( "?"  Uri-Query ) only if Uri-Query is present


   Where reference to an option is replaced by the value of that option
   if the option is in the header, and the default value for the option
   if it is not present.  The default value for Uri-Authority and Uri-
   Path is the empty string.  Uri-Query has no default value.

   CoAP does not support "." or ".." in URIs, IRIs, nor fragment "#"
   processing.  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".

   The authority part of a URI is important in determining the correct
   representation to return on end-points maintaining virtual servers
   and for intermediate components such as proxies.  For this reason it
   is important that the full URI can be reconstructed when needed.
   However, at the same time, it is often advantageous for CoAP to elide
   the Uri-Authority when it is unknown or identical to the IPv6
   destination address for efficiency.  The following rules apply to
   processing a CoAP request:

   1.  If the Uri-Authority option is absent and the remainder of the
       URI uniquely identifies a resource the server MAY proceed to
       execute the request.





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   2.  If an origin server is able to determine the IP destination
       address of the request, it MAY assume this as the authority of
       the URI.

   3.  If no authority can be determined and the server requires the
       authority to identify the resource it MUST reject the request
       with "Uri-Authority Option required by server (CoAP 241)".

   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.  The CoRE Link Format includes an attribute to
   indicate if a short alternative URI of a resource is available
   [I-D.ietf-core-link-format].

   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
   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



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   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 request/response 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.

      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.



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      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).

   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






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   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).

   +-----+----+----------------------+----------+--------+-------------+
   | Typ | C/ | Name                 | Data     | Length | Default     |
   | e   | E  |                      | type     |        |             |
   +-----+----+----------------------+----------+--------+-------------+
   | 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  | -                    | Reserved | -      | -           |
   | 4   | E  | Etag                 | Sequence | 1-4 B  | -           |
   |     |    |                      | of bytes |        |             |
   | 5   | C  | Uri-Authority        | String   | 1-270  | ""          |
   |     |    |                      |          | B      |             |
   | 6   | E  | Location             | String   | 1-270  | -           |
   |     |    |                      |          | B      |             |
   | 7   | -  | -                    | -        | -      | -           |
   | 9   | C  | Uri-Path             | String   | 1-270  | ""          |
   |     |    |                      |          | B      |             |
   | 10  | E  | Subscription         | Variable | 0-4 B  | 0           |
   |     |    | lifetime             | length   |        |             |
   |     |    | [I-D.ietf-core-obser | unsigned |        |             |
   |     |    | ve]                  | integer  |        |             |



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   | 11  | C  | Token                | Sequence | 1-2 B  | -           |
   |     |    |                      | of bytes |        |             |
   | 13  | C  | Block                | Unsigned | 1-3 B  | 0           |
   |     |    | [I-D.ietf-core-block | Integer  |        |             |
   |     |    | ]                    |          |        |             |
   | 15  | C  | Uri-Query            | 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.
   This option MUST NOT occur more than once in a header.

3.2.2.  Uri-Authority Option

   The Uri-Authority Option indicates the authority (host + port) part
   of a URI, and conforms to "host [ : port ]" (section 3.2 of
   [RFC3986], excluding use of userinfo) Examples of this option include
   "[2001:DB8::101]", "198.51.100.0:8000" and "sensor.example.com".
   This option is used by servers to determine which resource to return
   and by intermediate components, e.g. when accessing a resource via a
   proxy.  Section 2.5.2 specifies the rules for URIs in CoAP.  This
   option SHOULD be included in a request when the authority of the URI
   is known.  This option MUST be supported by an end-point implementing
   proxy functionality.  This option MUST NOT occur more than once in a
   header.

3.2.3.  Uri-Path Option

   The Uri-Path Option indicates the absolute path part of a URI, and
   confirms to the path-noscheme (path-rootless?) rule in section 3.3 of
   [RFC3986].  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.  This option MUST NOT occur more than
   once in a header.






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3.2.4.  Uri-Query Option

   The Uri-Query Option indicates the query part of a URI (if any), and
   conforms to the query rule in section 3.4 of [RFC3986].  In the
   absence of this option, there is assumed to be no query string.
   Section 2.5.2 specifies the rules for URIs in CoAP.  This option MUST
   be supported by all end-points.  This option MUST NOT occur more than
   once in a header.

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.
   This option MUST NOT occur more than once in a header.

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.  This
   option MUST NOT occur more than once in a header.

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

3.2.8.  Token Option

   The Token Option is an opaque sequence of 1-2 bytes which is used to
   match a request with a response and is meant for use with
   asynchronous responses by this specification.  The Token is generated
   by a client and included in a way that Token values currently in use



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   are unique.  Tokens have the following rules:

      If a Token Option is included in a request, the response (and any
      subsequent delayed responses) MUST include the same value in a
      Token Option.

      If a Token Option is included in a request, any resulting delayed
      response SHOULD NOT include the URI option (for sake of
      efficiency) as the Token is sufficient for matching it with the
      request.

      If a request does not include a Token option, the server MUST
      provide its ReST response within the transaction response.  If it
      cannot do so (i.e., can only satisfy the request through an
      asynchronous response), it MUST respond with error "Token Option
      required by server (CoAP 240)".

   This option MUST NOT occur ore than once in a header.


4.  UDP Binding

   The CoAP protocol operates by default over UDP.  CoAP may also be
   used with Datagram Transport Layer Security (DTLS) as described in
   Section 10.  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



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   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
   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.



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   When a CoAP server is hosted by a 6LoWPAN node, it SHOULD also
   support a port in the 61616-61631 compressed UDP port space defined
   in [RFC4944].  The specific port number in use will be communicated
   using e.g.  CoRE discovery [I-D.ietf-core-link-format].


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

   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



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   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
   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.  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).



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   When a request is made to a proxy, then the following steps are
   taken:

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

      2.  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.

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

      4.  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.  A CoAP
   end-point SHOULD support the CoRE Link Format of discoverable
   resources as described in [I-D.ietf-core-link-format].


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 web
   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



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   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
   request/response exchanges 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/core
   [I-D.ietf-core-link-format], 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/core [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 a multicast request.  Here a client
   sends a request for /.well-known/core with a query for ?n=Light



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   (Resource name = Light) to all-nodes link-scope multicast.  There are
   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/core?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

   This section describes mechanisms that can be used to secure CoAP and
   analyzes the possible threats to the protocol and its limitations.
   Security bootstrapping (authenticating nodes and setting up keys) in
   constrained environments is considered in
   [I-D.oflynn-core-bootstrapping].

   During the bootstrap and enrollment phases, a CoAP device is provided
   with the key security information that it needs.  How this is done is
   out of scope for this specification but a couple ways of doing this
   are described in [I-D.oflynn-core-bootstrapping].  At the end of the
   enrollment and bootstrap, the device will be in one of four security
   modes with the following information for the given mode:

   NoSec:  There is no protocol level security.

   SharedKey:  There is one shared key between all the nodes that this
      CoAP nodes needs to communicate with.







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   MultiKey:  There is a list of shared keys and each key includes a
      list of which nodes it can be used to communicate with.  At the
      extreme there may be one key for each node this CoAP node needs to
      communicate with.

   Certificate:  The device has an asymmetric key pair with a X.509
      [RFC5280] certificate that binds it to its Authority Name and is
      signed by a some common trust root.  The device also has a list or
      root trust anchors that can be used for validating a certificate.
      There may be an optional shared key that all the nodes that
      communicate have access too.

   The Authority Name in the certificate is the name that would be used
   in the Authority part of a CoAP URI.  It is worth noting that this
   would typically not be either an IP address or DNS name but would
   instead be a long term unique identifier for the device such as the
   EUI-64.  The discovery process used in the system would build up the
   mapping between IP addresses of the given devices and the Authority
   Name for each device.  Some devices could have more than one
   Authority and would need more than a single certificate.

   In the "NoSec" mode, the system simply sends the packets over normal
   UDP over IP.  The system is secured only by physically keeping
   attackers from being able to send or receive packets from the network
   with the CoAP nodes.  The other three security modes can be achieved
   with IPsec or DTLS.

10.1.  Securing CoAP with IPsec

   One mechanism to secure CoAP in constrained environments is the IPsec
   Encapsulating Security Payload (ESP) [RFC4303].  Using IPsec ESP with
   the appropriate configuration, it is possible for many constrained
   devices to support encryption with built-in link-layer encryption
   hardware.  For example, many IEEE 802.15.4 radio chips are compatible
   with AES-CBC (with 128-bit keys) [RFC3602] as defined for use with
   IPsec in [RFC4835].  Alternatively, and to avoid the need for
   padding, nodes could directly use the more widely supported AES-CCM
   as defined for use with IPsec in [RFC4309], if the security
   considerations in section 9 of that specification can be fulfilled.

   When using IPsec to secure CoAP, both authentication and
   confidentiality SHOULD be applied as recommended in [RFC4303].  The
   use of IPsec between CoAP end-points is transparent to the
   application layer and does not require special consideration for a
   CoAP implementation.

   IPsec may not be appropriate for all environments.  For example,
   IPsec support is not available for many embedded IP stacks and even



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   in full PC operating systems or on backend web servers, application
   developers may not have sufficient access to configure or enable
   IPsec or to add a security gateway to the infrastructure.  Problems
   with firewalls and NATs may furthermore limit the use of IPsec.

10.2.  Securing CoAP with DTLS

   Just as HTTP may be secured using Transport Layer Security (TLS) over
   TCP, CoAP may be secured using Datagram TLS (DTLS) [RFC4347] over
   UDP.  This section describes how to secure CoAP with DTLS , along
   with the minimal configurations appropriate for constrained
   environments.  DTLS is in practice TLS with added features to deal
   with the unreliable nature of the UDP transport.

   In some constrained nodes (limited flash and/or RAM) and networks
   (limited bandwidth or high scalability requirements) DTLS may not be
   applicable.  The protocol is an order of magnitude more complex than
   CoAP and has appreciable handshake overhead needed to maintain
   security sessions.  DTLS makes sense for applications where the
   session maintenance makes it compatible with application flows and
   sufficient resources are available on the constrained nodes and for
   the added network overhead.

   Devices SHOULD support the Server Name Indication (SNI) to indicate
   their Authority Name in the SNI HostName field as defined in Section
   3 of draft-ietf-tls-rfc4366-bis.  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.

   DTLS connections with certificates are set up using mutual
   authentication so they can remain up and be reused for future
   transaction 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 transaction is very inefficient.

10.2.1.  SharedKey & MultiKey Modes

   When forming a connection to a new node, the system selects an
   appropriate key based on which nodes it is trying to reach then forms
   a DTLS session using a PSK (Pre-Shared Key) mode of DTLS.
   Implementations SHOULD support the mandatory to implement cipher
   suite TLS_PSK_WITH_AES_128_CBC_SHA as specified in [RFC5246].

10.2.2.  Certificate Mode

   As with IPsec, DTLS should be configured with a cypher suite
   compatible with any possible hardware engine on the node, for example



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   AES-CBC in the case of IEEE 802.15.4.  Implementations SHOULD support
   the mandatory to implement cipher suite TLS_RSA_WITH_AES_128_CBC_SHA
   as specified in [RFC5246].

   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 the validity dates are of
   the certificate are within range.  The certificate MUST also be
   signed by an appropriate chain of trust.  If the certificate contains
   a SubjectAltName, then the Authority Name MUST match at least one of
   the authority names of any CoAP URI found in a URI type fields in the
   SubjectAltName set.  If there is no SubjectAltName in the
   certificate, then the Authoritative Name must match the CN found in
   the certificate using the matching rules defined in [RFC2818] with
   the exception that certificates with wildcards are not allowed.

   If the system has a shared key in addition to the certificate, then a
   cipher suite that includes the shared key such as
   TLS_RSA_PSK_WITH_AES_128_CBC_SHA SHOULD be used.

10.3.  Threat analysis and protocol limitations

   This section 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
   and CoRE.

10.3.1.  Processing URIs

   Complex parsers are known as a likely source of vulnerabilities, such
   as the ability to remotely crash a node, or even remotely execute
   arbitrary code on it.  The URI processing code in CoAP
   implementations should be subjected to stringent tests with various
   forms of malformed parameters.  (See also section 15.2 of [RFC2616].)

10.3.2.  Proxying and Caching

   As mentioned in 15.2 of [RFC2616], which see, proxies are by their
   very nature men-in-the-middle, breaking any IPsec or DTLS protection
   that a direct CoAP transaction might have.  They are therefore
   interesting targets for breaking confidentiality or integrity of CoAP
   transactions.  As noted in [RFC2616], they are also interesting
   targets for breaking availability.

   The threat to confidentiality and integrity of transaction data is
   amplified where proxies also cache.  Note that CoAP does not define



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   any of the cache-suppressing Cache-Control options that HTTP/1.1
   provides to better protect sensitive data.

   Finally, a proxy that fans out asynchronous responses to multiple
   original requesters may provide additional amplification (see below).

10.3.3.  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.

   This is particularly a problem in nodes that enable NoSec access and
   that 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 network 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.

   A CoAP server can reduce the amount of amplification it provides to
   an attacker by using slicing/blocking modes of CoAP
   [I-D.ietf-core-block] and offering large resource representations
   only in relatively small slices.  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.

   CoAP supports also the use of multicast IP addresses in requests, an
   important requirement for M2M. Multicast CoAP requests may be the
   source of accidental or deliberate denial of service 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



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   be authenticated.  If possible a CoAP server SHOULD limit the support
   for multicast requests to specific resources where the feature is
   required.

   On some general purpose operating systems providing a Posix-style
   API, 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.


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
   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.

11.1.  Codes

   CoAP makes use of (a subset of) the HTTP status codes defined in
   [RFC2616] plus some CoAP-specific status codes.  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 (except for CoAP-specific codes).

















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            +------+-----------------------------------------+
            | 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                         |
            | 203  | 503 Service Unavailable                 |
            | 204  | 504 Gateway Timeout                     |
            | 240  | Token Option required by server         |
            | 241  | Uri-Authority Option required by server |
            | 242  | Critical Option not supported           |
            +------+-----------------------------------------+

                            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.
   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.












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   +-----------------------------------------------------+------------+
   | 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 [I-D.ietf-core-link-format] | 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, Klaus Hartke, Peter Bigot and
   Cullen Jennings 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, Section 10), along with countless detailed reviews and
   discussions.

   Thanks to Michael Stuber, Richard Kelsey, 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.







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13.  Changelog

   Changes from ietf-02 to ietf-03:

      o Token Option and related use in asynchronous requests added
      (#25).

      o CoAP specific error codes added (#26).

      o Erroring out on unknown critical options changed to a MUST
      (#27).

      o Uri-Query option added.

      o Terminology and definitions of URIs improved.

      o Security section completed (#22).

   Changes from ietf-01 to ietf-02:

      o Sending an error on a critical option clarified (#18).

      o Clarification on behavior of PUT and idempotent operations
      (#19).

      o Use of Uri-Authority clarified along with server processing
      rules.  Uri-Scheme option removed. (#20, #23)

      o Resource discovery section removed to a separate CoRE Link
      Format draft (#21)

      o Initial security section outline added.

   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)





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      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.

      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:





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      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

14.1.  Normative References

   [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.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602,
              September 2003.

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



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   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [RFC4309]  Housley, R., "Using Advanced Encryption Standard (AES) CCM
              Mode with IPsec Encapsulating Security Payload (ESP)",
              RFC 4309, December 2005.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [RFC4835]  Manral, V., "Cryptographic Algorithm Implementation
              Requirements for Encapsulating Security Payload (ESP) and
              Authentication Header (AH)", RFC 4835, April 2007.

   [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.

14.2.  Informative References

   [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.ietf-core-block]
              Shelby, Z. and C. Bormann, "Blockwise transfers in CoAP",
              draft-ietf-core-block-00 (work in progress), October 2010.

   [I-D.ietf-core-link-format]
              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-01 (work in progress),
              October 2010.

   [I-D.ietf-core-observe]
              Hartke, K. and Z. Shelby, "Observing Resources in CoAP",
              draft-ietf-core-observe-00 (work in progress),
              October 2010.

   [I-D.oflynn-core-bootstrapping]
              Sarikaya, B. and R. Cragie, "Initial Configuration of
              Resource-Constrained Devices",
              draft-oflynn-core-bootstrapping-01 (work in progress),



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              July 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),
              April 2010.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.

   [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










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   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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