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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 Working Group                                             Z. Shelby
Internet-Draft                                                 Sensinode
Intended status: Standards Track                               K. Hartke
Expires: April 4, 2013                                        C. Bormann
                                                 Universitaet Bremen TZI
                                                                B. Frank
                                                              SkyFoundry
                                                         October 1, 2012


                Constrained Application Protocol (CoAP)
                        draft-ietf-core-coap-12

Abstract

   The Constrained Application Protocol (CoAP) is a specialized web
   transfer protocol for use with constrained nodes and constrained
   (e.g., low-power, lossy) networks.  The nodes often have 8-bit
   microcontrollers with small amounts of ROM and RAM, while constrained
   networks such as 6LoWPAN often have high packet error rates and a
   typical throughput of 10s of kbit/s.  The protocol is designed for
   machine-to-machine (M2M) applications such as smart energy and
   building automation.

   CoAP provides a request/response interaction model between
   application endpoints, supports built-in discovery of services and
   resources, and includes key concepts of the Web such as URIs and
   Internet media types.  CoAP easily interfaces with HTTP for
   integration with the Web while meeting specialized requirements such
   as multicast support, very low overhead and simplicity for
   constrained environments.

Status of this Memo

   This 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 4, 2013.



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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.1.  Features  . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
   2.  Constrained Application Protocol  . . . . . . . . . . . . . .  10
     2.1.  Messaging Model . . . . . . . . . . . . . . . . . . . . .  11
     2.2.  Request/Response Model  . . . . . . . . . . . . . . . . .  12
     2.3.  Intermediaries and Caching  . . . . . . . . . . . . . . .  15
     2.4.  Resource Discovery  . . . . . . . . . . . . . . . . . . .  15
   3.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .  15
     3.1.  Header Format . . . . . . . . . . . . . . . . . . . . . .  16
     3.2.  Option Format . . . . . . . . . . . . . . . . . . . . . .  17
     3.3.  Option Jump . . . . . . . . . . . . . . . . . . . . . . .  19
     3.4.  Option Value Formats  . . . . . . . . . . . . . . . . . .  20
       3.4.1.   uint . . . . . . . . . . . . . . . . . . . . . . . .  21
       3.4.2.   string . . . . . . . . . . . . . . . . . . . . . . .  21
       3.4.3.   opaque . . . . . . . . . . . . . . . . . . . . . . .  21
       3.4.4.   empty  . . . . . . . . . . . . . . . . . . . . . . .  22
   4.  Message Transmission  . . . . . . . . . . . . . . . . . . . .  22
     4.1.  Messages and Endpoints  . . . . . . . . . . . . . . . . .  22
     4.2.  Messages Transmitted Reliably . . . . . . . . . . . . . .  23
     4.3.  Messages Transmitted Without Reliability  . . . . . . . .  24
     4.4.  Message Correlation . . . . . . . . . . . . . . . . . . .  24
     4.5.  Message Deduplication . . . . . . . . . . . . . . . . . .  25
     4.6.  Message Size  . . . . . . . . . . . . . . . . . . . . . .  26
     4.7.  Congestion Control  . . . . . . . . . . . . . . . . . . .  27
     4.8.  Transmission Parameters . . . . . . . . . . . . . . . . .  27
       4.8.1.   Changing The Parameters  . . . . . . . . . . . . . .  28
       4.8.2.   Time Values derived from Transmission Parameters . .  29
   5.  Request/Response Semantics  . . . . . . . . . . . . . . . . .  31
     5.1.  Requests  . . . . . . . . . . . . . . . . . . . . . . . .  31



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     5.2.  Responses . . . . . . . . . . . . . . . . . . . . . . . .  31
       5.2.1.   Piggy-backed . . . . . . . . . . . . . . . . . . . .  32
       5.2.2.   Separate . . . . . . . . . . . . . . . . . . . . . .  33
       5.2.3.   Non-Confirmable  . . . . . . . . . . . . . . . . . .  34
     5.3.  Request/Response Matching . . . . . . . . . . . . . . . .  34
     5.4.  Options . . . . . . . . . . . . . . . . . . . . . . . . .  35
       5.4.1.   Critical/Elective  . . . . . . . . . . . . . . . . .  37
       5.4.2.   Proxy Unsafe/Safe and Cache-Key  . . . . . . . . . .  37
       5.4.3.   Length . . . . . . . . . . . . . . . . . . . . . . .  38
       5.4.4.   Default Values . . . . . . . . . . . . . . . . . . .  38
       5.4.5.   Repeatable Options . . . . . . . . . . . . . . . . .  38
       5.4.6.   Option Numbers . . . . . . . . . . . . . . . . . . .  38
     5.5.  Payload . . . . . . . . . . . . . . . . . . . . . . . . .  39
       5.5.1.   Representation . . . . . . . . . . . . . . . . . . .  39
       5.5.2.   Diagnostic Message . . . . . . . . . . . . . . . . .  39
     5.6.  Caching . . . . . . . . . . . . . . . . . . . . . . . . .  39
       5.6.1.   Freshness Model  . . . . . . . . . . . . . . . . . .  40
       5.6.2.   Validation Model . . . . . . . . . . . . . . . . . .  40
     5.7.  Proxying  . . . . . . . . . . . . . . . . . . . . . . . .  41
       5.7.1.   Proxy Operation  . . . . . . . . . . . . . . . . . .  42
       5.7.2.   Forward-Proxies  . . . . . . . . . . . . . . . . . .  43
       5.7.3.   Reverse-Proxies  . . . . . . . . . . . . . . . . . .  43
     5.8.  Method Definitions  . . . . . . . . . . . . . . . . . . .  44
       5.8.1.   GET  . . . . . . . . . . . . . . . . . . . . . . . .  44
       5.8.2.   POST . . . . . . . . . . . . . . . . . . . . . . . .  44
       5.8.3.   PUT  . . . . . . . . . . . . . . . . . . . . . . . .  45
       5.8.4.   DELETE . . . . . . . . . . . . . . . . . . . . . . .  45
     5.9.  Response Code Definitions . . . . . . . . . . . . . . . .  45
       5.9.1.   Success 2.xx . . . . . . . . . . . . . . . . . . . .  45
       5.9.2.   Client Error 4.xx  . . . . . . . . . . . . . . . . .  47
       5.9.3.   Server Error 5.xx  . . . . . . . . . . . . . . . . .  48
     5.10. Option Definitions  . . . . . . . . . . . . . . . . . . .  49
       5.10.1.  Token  . . . . . . . . . . . . . . . . . . . . . . .  50
       5.10.2.  Uri-Host, Uri-Port, Uri-Path and Uri-Query . . . . .  50
       5.10.3.  Proxy-Uri  . . . . . . . . . . . . . . . . . . . . .  51
       5.10.4.  Content-Format . . . . . . . . . . . . . . . . . . .  51
       5.10.5.  Accept . . . . . . . . . . . . . . . . . . . . . . .  52
       5.10.6.  Max-Age  . . . . . . . . . . . . . . . . . . . . . .  52
       5.10.7.  ETag . . . . . . . . . . . . . . . . . . . . . . . .  52
       5.10.8.  Location-Path and Location-Query . . . . . . . . . .  53
       5.10.9.  If-Match . . . . . . . . . . . . . . . . . . . . . .  53
       5.10.10. If-None-Match  . . . . . . . . . . . . . . . . . . .  54
   6.  CoAP URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  54
     6.1.  coap URI Scheme . . . . . . . . . . . . . . . . . . . . .  55
     6.2.  coaps URI Scheme  . . . . . . . . . . . . . . . . . . . .  56
     6.3.  Normalization and Comparison Rules  . . . . . . . . . . .  56
     6.4.  Decomposing URIs into Options . . . . . . . . . . . . . .  57
     6.5.  Composing URIs from Options . . . . . . . . . . . . . . .  58



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   7.  Discovery . . . . . . . . . . . . . . . . . . . . . . . . . .  59
     7.1.  Service Discovery . . . . . . . . . . . . . . . . . . . .  59
     7.2.  Resource Discovery  . . . . . . . . . . . . . . . . . . .  60
       7.2.1.   'ct' Attribute . . . . . . . . . . . . . . . . . . .  60
   8.  Multicast CoAP  . . . . . . . . . . . . . . . . . . . . . . .  60
     8.1.  Messaging Layer . . . . . . . . . . . . . . . . . . . . .  61
     8.2.  Request/Response Layer  . . . . . . . . . . . . . . . . .  61
       8.2.1.   Caching  . . . . . . . . . . . . . . . . . . . . . .  62
       8.2.2.   Proxying . . . . . . . . . . . . . . . . . . . . . .  62
   9.  Securing CoAP . . . . . . . . . . . . . . . . . . . . . . . .  63
     9.1.  DTLS-secured CoAP . . . . . . . . . . . . . . . . . . . .  64
       9.1.1.   Messaging Layer  . . . . . . . . . . . . . . . . . .  65
       9.1.2.   Request/Response Layer . . . . . . . . . . . . . . .  65
       9.1.3.   Endpoint Identity  . . . . . . . . . . . . . . . . .  66
     9.2.  Using CoAP with IPsec . . . . . . . . . . . . . . . . . .  68
   10. Cross-Protocol Proxying between CoAP and HTTP . . . . . . . .  68
     10.1. CoAP-HTTP Proxying  . . . . . . . . . . . . . . . . . . .  69
       10.1.1.  GET  . . . . . . . . . . . . . . . . . . . . . . . .  70
       10.1.2.  PUT  . . . . . . . . . . . . . . . . . . . . . . . .  70
       10.1.3.  DELETE . . . . . . . . . . . . . . . . . . . . . . .  70
       10.1.4.  POST . . . . . . . . . . . . . . . . . . . . . . . .  71
     10.2. HTTP-CoAP Proxying  . . . . . . . . . . . . . . . . . . .  71
       10.2.1.  OPTIONS and TRACE  . . . . . . . . . . . . . . . . .  71
       10.2.2.  GET  . . . . . . . . . . . . . . . . . . . . . . . .  71
       10.2.3.  HEAD . . . . . . . . . . . . . . . . . . . . . . . .  72
       10.2.4.  POST . . . . . . . . . . . . . . . . . . . . . . . .  72
       10.2.5.  PUT  . . . . . . . . . . . . . . . . . . . . . . . .  73
       10.2.6.  DELETE . . . . . . . . . . . . . . . . . . . . . . .  73
       10.2.7.  CONNECT  . . . . . . . . . . . . . . . . . . . . . .  73
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  73
     11.1. Protocol Parsing, Processing URIs . . . . . . . . . . . .  74
     11.2. Proxying and Caching  . . . . . . . . . . . . . . . . . .  74
     11.3. Risk of amplification . . . . . . . . . . . . . . . . . .  75
     11.4. IP Address Spoofing Attacks . . . . . . . . . . . . . . .  76
     11.5. Cross-Protocol Attacks  . . . . . . . . . . . . . . . . .  76
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  78
     12.1. CoAP Code Registry  . . . . . . . . . . . . . . . . . . .  78
       12.1.1.  Method Codes . . . . . . . . . . . . . . . . . . . .  79
       12.1.2.  Response Codes . . . . . . . . . . . . . . . . . . .  80
     12.2. Option Number Registry  . . . . . . . . . . . . . . . . .  81
     12.3. Content-Format Registry . . . . . . . . . . . . . . . . .  83
     12.4. URI Scheme Registration . . . . . . . . . . . . . . . . .  84
     12.5. Secure URI Scheme Registration  . . . . . . . . . . . . .  85
     12.6. Service Name and Port Number Registration . . . . . . . .  86
     12.7. Secure Service Name and Port Number Registration  . . . .  87
     12.8. Multicast Address Registration  . . . . . . . . . . . . .  88
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  88
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  89



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     14.1. Normative References  . . . . . . . . . . . . . . . . . .  89
     14.2. Informative References  . . . . . . . . . . . . . . . . .  91
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  93
   Appendix B.  URI Examples . . . . . . . . . . . . . . . . . . . .  98
   Appendix C.  Changelog  . . . . . . . . . . . . . . . . . . . . .  99
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 107













































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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) work aims at realizing
   the REST architecture in a suitable form for the most constrained
   nodes (e.g. 8-bit microcontrollers with limited RAM and ROM) and
   networks (e.g. 6LoWPAN, [RFC4944]).  Constrained networks like
   6LoWPAN support the expensive fragmentation of IPv6 packets into
   small link-layer frames.  One design goal of CoAP has been to keep
   message overhead small, thus limiting the use of fragmentation.

   One of the main goals of CoAP is to design a generic web protocol for
   the special requirements of this constrained environment, especially
   considering energy, building automation and other machine-to-machine
   (M2M) applications.  The goal of CoAP is not to blindly compress HTTP
   [RFC2616], but rather to realize a subset of REST common with HTTP
   but optimized for M2M applications.  Although CoAP could be used for
   compressing simple HTTP interfaces, it more importantly also offers
   features for M2M such as built-in discovery, multicast support and
   asynchronous message exchanges.

   This document specifies the Constrained Application Protocol (CoAP),
   which easily translates to HTTP for integration with the existing web
   while meeting specialized requirements such as multicast support,
   very low overhead and simplicity for constrained environments and M2M
   applications.

1.1.  Features

   CoAP has the following main features:

   o  Constrained web protocol fulfilling M2M requirements.

   o  UDP binding with optional reliability supporting unicast and
      multicast requests.

   o  Asynchronous message exchanges.

   o  Low header overhead and parsing complexity.

   o  URI and Content-type support.

   o  Simple proxy and caching capabilities.





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   o  A stateless HTTP mapping, allowing proxies to be built providing
      access to CoAP resources via HTTP in a uniform way or for HTTP
      simple interfaces to be realized alternatively over CoAP.

   o  Security binding to Datagram Transport Layer Security (DTLS).

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119] when they appear in ALL CAPS.  These words may also appear
   in this document in lower case as plain English words, absent their
   normative meanings.

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in [RFC2616].  In addition, this
   specification defines the following terminology:

   Endpoint
      An entity participating in the CoAP protocol.  Colloquially, an
      endpoint lives on a "Node", although "Host" would be more
      consistent with Internet standards usage, and is further
      identified by transport layer multiplexing information that can
      include a UDP port number and a security association
      (Section 4.1).

   Sender
      The originating endpoint of a message.  When the aspect of
      identification of the specific sender is in focus, also "source
      endpoint".

   Recipient
      The destination endpoint of a message.  When the aspect of
      identification of the specific recipient is in focus, also
      "destination endpoint".

   Client
      The originating endpoint of a request; the destination endpoint of
      a response.

   Server
      The destination endpoint of a request; the originating endpoint of
      a response.







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   Origin Server
      The server on which a given resource resides or is to be created.

   Intermediary
      A CoAP endpoint that acts both as a server and as a client towards
      (possibly via further intermediaries) an origin server.  A common
      form of an intermediary is a proxy; several classes of such
      proxies are discussed in this specification.

   Proxy
      An intermediary that mainly is concerned with forwarding requests
      and relaying back responses, possibly performing caching,
      namespace translation, or protocol translation in the process.  As
      opposed to intermediaries in the general sense, proxies generally
      do not implement specific application semantics.  Based on the
      position in the overall structure of the request forwarding, there
      are two common forms of proxy: forward-proxy and reverse-proxy.
      In some cases, a single endpoint might act as an origin server,
      forward-proxy, or reverse-proxy, switching behavior based on the
      nature of each request.

   Forward-Proxy
      A "forward-proxy" is an endpoint selected by a client, usually via
      local configuration rules, to perform requests on behalf of the
      client, doing any necessary translations.  Some translations are
      minimal, such as for proxy requests for "coap" URIs, whereas other
      requests might require translation to and from entirely different
      application-layer protocols.

   Reverse-Proxy
      A "reverse-proxy" is an endpoint that stands in for one or more
      other server(s) and satisfies requests on behalf of these, doing
      any necessary translations.  Unlike a forward-proxy, the client
      may not be aware that it is communicating with a reverse-proxy; a
      reverse-proxy receives requests as if it was the origin server for
      the target resource.

   Cross-Proxy
      A cross-protocol proxy, or "cross-proxy" for short, is a proxy
      that translates between different protocols, such as a CoAP-to-
      HTTP proxy or an HTTP-to-CoAP proxy.  While this specification
      makes very specific demands of CoAP-to-CoAP proxies, there is more
      variation possible in cross-proxies.

   Confirmable Message
      Some messages require an acknowledgement.  These messages are
      called "Confirmable".  When no packets are lost, each confirmable
      message elicits exactly one return message of type Acknowledgement



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      or type Reset.

   Non-Confirmable Message
      Some other messages do not require an acknowledgement.  This is
      particularly true for messages that are repeated regularly for
      application requirements, such as repeated readings from a sensor
      where eventual success is sufficient.

   Acknowledgement Message
      An Acknowledgement message acknowledges that a specific
      Confirmable Message arrived.  It does not indicate success or
      failure of any encapsulated request.

   Reset Message
      A Reset message indicates that a specific message (confirmable or
      non-confirmable) was received, but some context is missing to
      properly process it.  This condition is usually caused when the
      receiving node has rebooted and has forgotten some state that
      would be required to interpret the message.

   Piggy-backed Response
      A Piggy-backed Response is included right in a CoAP
      Acknowledgement (ACK) message that is sent to acknowledge receipt
      of the Request for this Response (Section 5.2.1).

   Separate Response
      When a Confirmable message carrying a Request is acknowledged with
      an empty message (e.g., because the server doesn't have the answer
      right away), a Separate Response is sent in a separate message
      exchange (Section 5.2.2).

   Critical Option
      An option that would need to be understood by the endpoint
      receiving the message in order to properly process the message
      (Section 5.4.1).  Note that the implementation of critical options
      is, as the name "Option" implies, generally optional: unsupported
      critical options lead to an error response or summary rejection of
      the message.

   Elective Option
      An option that is intended to be ignored by an endpoint that does
      not understand it.  Processing the message even without
      understanding the option is acceptable (Section 5.4.1).

   Unsafe Option
      An option that would need to be understood by a proxy receiving
      the message in order to safely forward the message
      (Section 5.4.2).



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   Safe Option
      An option that is intended to be safe for forwarding by a proxy
      that does not understand it.  Forwarding the message even without
      understanding the option is acceptable (Section 5.4.2).

   Resource Discovery
      The process where a CoAP client queries a server for its list of
      hosted resources (i.e., links, Section 7).

   Content-Format
      The combination of an Internet media type, potentially with
      specific parameters given, and a content-coding (which is often
      the identity content-coding), identified by a numeric identifier
      defined by the CoAP Content-Format registry.

   In this specification, the term "byte" is used in its now customary
   sense as a synonym for "octet".

   All multi-byte integers in this protocol are interpreted in network
   byte order.

   Where arithmetic is used, this specification uses the notation
   familiar from the programming language C, except that the operator
   "**" stands for exponentiation.


2.  Constrained Application Protocol

   The interaction model of CoAP is similar to the client/server model
   of HTTP.  However, machine-to-machine interactions typically result
   in a CoAP implementation acting in both client and server roles.  A
   CoAP request is equivalent to that of HTTP, and is sent by a client
   to request an action (using a method code) on a resource (identified
   by a URI) on a server.  The server then sends a response with a
   response code; this response may include a resource representation.

   Unlike HTTP, CoAP deals with these interchanges asynchronously over a
   datagram-oriented transport such as UDP.  This is done logically
   using a layer of messages that supports optional reliability (with
   exponential back-off).  CoAP defines four types of messages:
   Confirmable, Non-Confirmable, Acknowledgement, Reset; method codes
   and response codes included in some of these messages make them carry
   requests or responses.  The basic exchanges of the four types of
   messages are somewhat orthogonal to the request/response
   interactions; requests can be carried in Confirmable and Non-
   Confirmable messages, and responses can be carried in these as well
   as piggy-backed in Acknowledgement messages.




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   One could think of CoAP logically as using a two-layer approach, a
   CoAP messaging layer used to deal with UDP and the asynchronous
   nature of the interactions, and the request/response interactions
   using Method and Response codes (see Figure 1).  CoAP is however a
   single protocol, with messaging and request/response just features of
   the CoAP header.

                         +----------------------+
                         |      Application     |
                         +----------------------+
                         +----------------------+  \
                         |  Requests/Responses  |  |
                         |----------------------|  | CoAP
                         |       Messages       |  |
                         +----------------------+  /
                         +----------------------+
                         |          UDP         |
                         +----------------------+

                    Figure 1: Abstract layering of CoAP

2.1.  Messaging Model

   The CoAP messaging model is based on the exchange of messages over
   UDP between endpoints.

   CoAP uses a short fixed-length binary header (4 bytes) that may be
   followed by compact binary options and a payload.  This message
   format is shared by requests and responses.  The CoAP message format
   is specified in Section 3.  Each message contains a Message ID used
   to detect duplicates and for optional reliability.

   Reliability is provided by marking a message as Confirmable (CON).  A
   Confirmable message is retransmitted using a default timeout and
   exponential back-off between retransmissions, until the recipient
   sends an Acknowledgement message (ACK) with the same Message ID (for
   example, 0x7d34) from the corresponding endpoint; see Figure 2.  When
   a recipient is not at all able to process a Confirmable message
   (i.e., not even able to provide a suitable error response), it
   replies with a Reset message (RST) instead of an Acknowledgement
   (ACK).










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                         Client              Server
                            |                  |
                            |   CON [0x7d34]   |
                            +----------------->|
                            |                  |
                            |   ACK [0x7d34]   |
                            |<-----------------+
                            |                  |

                  Figure 2: Reliable message transmission

   A message that does not require reliable transmission, for example
   each single measurement out of a stream of sensor data, can be sent
   as a Non-confirmable message (NON).  These are not acknowledged, but
   still have a Message ID for duplicate detection; see Figure 3.  When
   a recipient is not able to process a Non-confirmable message, it may
   reply with a Reset message (RST).

                         Client              Server
                            |                  |
                            |   NON [0x01a0]   |
                            +----------------->|
                            |                  |

                 Figure 3: Unreliable message transmission

   See Section 4 for details of CoAP messages.

   As CoAP is based on UDP, it also supports the use of multicast IP
   destination addresses, enabling multicast CoAP requests.  Section 8
   discusses the proper use of CoAP messages with multicast addresses
   and precautions for avoiding response congestion.

   Several security modes are defined for CoAP in Section 9 ranging from
   no security to certificate-based security.  The use of IPsec along
   with a binding to DTLS are specified for securing the protocol.

2.2.  Request/Response Model

   CoAP request and response semantics are carried in CoAP messages,
   which include either a Method code or Response code, respectively.
   Optional (or default) request and response information, such as the
   URI and payload media type are carried as CoAP options.  A Token
   Option is used to match responses to requests independently from the
   underlying messages (Section 5.3).

   A request is carried in a Confirmable (CON) or Non-confirmable (NON)
   message, and if immediately available, the response to a request



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   carried in a Confirmable message is carried in the resulting
   Acknowledgement (ACK) message.  This is called a piggy-backed
   response, detailed in Section 5.2.1.  Two examples for a basic GET
   request with piggy-backed response are shown in Figure 4, one
   successful, one resulting in a 4.04 (Not Found) response.

        Client              Server       Client              Server
           |                  |             |                  |
           |   CON [0xbc90]   |             |   CON [0xbc91]   |
           | GET /temperature |             | GET /temperature |
           |   (Token 0x71)   |             |   (Token 0x72)   |
           +----------------->|             +----------------->|
           |                  |             |                  |
           |   ACK [0xbc90]   |             |   ACK [0xbc91]   |
           |   2.05 Content   |             |  4.04 Not Found  |
           |   (Token 0x71)   |             |   (Token 0x72)   |
           |     "22.5 C"     |             |   "Not found"    |
           |<-----------------+             |<-----------------+
           |                  |             |                  |

          Figure 4: Two GET requests with piggy-backed responses

   If the server is not able to respond immediately to a request carried
   in a Confirmable message, it simply responds with an empty
   Acknowledgement message so that the client can stop retransmitting
   the request.  When the response is ready, the server sends it in a
   new Confirmable message (which then in turn needs to be acknowledged
   by the client).  This is called a separate response, as illustrated
   in Figure 5 and described in more detail in Section 5.2.2.






















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                         Client              Server
                            |                  |
                            |   CON [0x7a10]   |
                            | GET /temperature |
                            |   (Token 0x73)   |
                            +----------------->|
                            |                  |
                            |   ACK [0x7a10]   |
                            |<-----------------+
                            |                  |
                            ... Time Passes  ...
                            |                  |
                            |   CON [0x23bb]   |
                            |   2.05 Content   |
                            |   (Token 0x73)   |
                            |     "22.5 C"     |
                            |<-----------------+
                            |                  |
                            |   ACK [0x23bb]   |
                            +----------------->|
                            |                  |

             Figure 5: A GET request with a separate response

   Likewise, if a request is sent in a Non-Confirmable message, then the
   response is usually sent using a new Non-Confirmable message,
   although the server may send a Confirmable message.  This type of
   exchange is illustrated in Figure 6.

                         Client              Server
                            |                  |
                            |   NON [0x7a11]   |
                            | GET /temperature |
                            |   (Token 0x74)   |
                            +----------------->|
                            |                  |
                            |   NON [0x23bc]   |
                            |   2.05 Content   |
                            |   (Token 0x74)   |
                            |     "22.5 C"     |
                            |<-----------------+
                            |                  |

                   Figure 6: A NON request and response

   CoAP makes use of GET, PUT, POST and DELETE methods in a similar
   manner to HTTP, with the semantics specified in Section 5.8.  (Note
   that the detailed semantics of CoAP methods are "almost, but not



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   entirely unlike" those of HTTP methods: Intuition taken from HTTP
   experience generally does apply well, but there are enough
   differences that make it worthwhile to actually read the present
   specification.)

   URI support in a server is simplified as the client already parses
   the URI and splits it into host, port, path and query components,
   making use of default values for efficiency.  Response codes
   correspond to a small subset of HTTP response codes with a few CoAP
   specific codes added, as defined in Section 5.9.

2.3.  Intermediaries and Caching

   The protocol supports the caching of responses in order to
   efficiently fulfill requests.  Simple caching is enabled using
   freshness and validity information carried with CoAP responses.  A
   cache could be located in an endpoint or an intermediary.  Caching
   functionality is specified in Section 5.6.

   Proxying is useful in constrained networks for several reasons,
   including network traffic limiting, to improve performance, to access
   resources of sleeping devices or for security reasons.  The proxying
   of requests on behalf of another CoAP endpoint is supported in the
   protocol.  When using a proxy, the URI of the resource to request is
   included in the request, while the destination IP address is set to
   the address of the proxy.  See Section 5.7 for more information on
   proxy functionality.

   As CoAP was designed according to the REST architecture and thus
   exhibits functionality similar to that of the HTTP protocol, it is
   quite straightforward to map from CoAP to HTTP and from HTTP to CoAP.
   Such a mapping may be used to realize an HTTP REST interface using
   CoAP, or for converting between HTTP and CoAP.  This conversion can
   be carried out by a cross-protocol proxy ("cross-proxy"), which
   converts the method or response code, media type, and options to the
   corresponding HTTP feature.  Section 10 provides more detail about
   HTTP mapping.

2.4.  Resource Discovery

   Resource discovery is important for machine-to-machine interactions,
   and is supported using the CoRE Link Format [RFC6690] as discussed in
   Section 7.


3.  Message Format

   CoAP is based on the exchange of short messages which, by default,



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   are transported over UDP (i.e. each CoAP message occupies the data
   section of one UDP datagram).  CoAP may also be used over Datagram
   Transport Layer Security (DTLS) (see Section 9.1).  It could also be
   used over other transports such as SMS, TCP or SCTP, the
   specification of which is out of this document's scope.

   CoAP messages are encoded in a simple binary format.  A message
   consists of a fixed-sized CoAP Header followed by options in Type-
   Length-Value (TLV) format and a payload.  The number of options is
   determined by the header.  The payload is made up of the bytes after
   the options, if any; its length is calculated from the datagram
   length.

     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     |          Message ID           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Payload (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 7: Message Format

3.1.  Header Format

   The fields in the header are defined as follows:

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

   Type (T):  2-bit unsigned integer.  Indicates if this message is of
      type Confirmable (0), Non-Confirmable (1), Acknowledgement (2) or
      Reset (3).  See Section 4 for the semantics of these message
      types.

   Option Count (OC):  4-bit unsigned integer.  Indicates the number of
      options after the header (0-14).  If set to 0, there are no
      options and the payload (if any) immediately follows the header.
      If set to 15, then an end-of-options marker is used to indicate
      the end of options and the start of the payload.  The format of
      options is defined below.







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   Code:  8-bit unsigned integer.  Indicates if the message carries a
      request (1-31) or a response (64-191), or is empty (0).  (All
      other code values are reserved.)  In case of a request, the Code
      field indicates the Request Method; in case of a response a
      Response Code.  Possible values are maintained in the CoAP Code
      Registry (Section 12.1).  See Section 5 for the semantics of
      requests and responses.

   Message ID:  16-bit unsigned integer in network byte order.  Used for
      the detection of message duplication, and to match messages of
      type Acknowledgement/Reset and messages of type Confirmable/
      Non-confirmable.  See Section 4 for Message ID generation rules
      and how messages are matched.

3.2.  Option Format

   Options MUST appear in order of their Option Number (see
   Section 5.4.6).  A delta encoding is used between options: The Option
   Number for each Option is calculated as the sum of its Option Delta
   field and the Option Number of the preceding Option in the message,
   if any.  For the first Option in the message, the Option Delta
   becomes the Option Number (i.e., an implementation can simply
   initialize the number variable as zero).  Multiple options with the
   same Option Number can be included by using an Option Delta of zero.
   The Option Jump mechanism (Section 3.3) is used when the delta to the
   next option number is greater than 14.

   Following the Option Delta, each option has a Length field which
   specifies the length of the Option Value, in bytes.  The Length field
   can be extended for options with values longer than 14 bytes by
   adding extension bytes.  The maximum length for an option is 1034
   bytes.  The Option Value immediately follows the Length field.



















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   for 0..14:
     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | Option Delta  |    Length     |
   +---+---+---+---+---+---+---+---+
   |   Option Value ...
   +---+---+---+---+---+---+---+---+

   for 15..269:
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | Option Delta  | 1   1   1   1 |          Length - 15          |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |   Option Value ...
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   for 270..524:
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | Option Delta  | 1   1   1   1 | 1   1   1   1   1   1   1   1 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |         Length - 270          |   Option Value ...
   +---+---+---+---+---+---+---+---+
   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   for 525..779:
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | Option Delta  | 1   1   1   1 | 1   1   1   1   1   1   1   1 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 1   1   1   1   1   1   1   1 |        Length - 525           |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |   Option Value ...
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

   for 780..1034:
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | Option Delta  | 1   1   1   1 | 1   1   1   1   1   1   1   1 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   | 1   1   1   1   1   1   1   1 | 1   1   1   1   1   1   1   1 |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
   |         Length - 780          |   Option Value ...
   +---+---+---+---+---+---+---+---+
   |
   +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+


                          Figure 8: Option Format

   The fields in an option are defined as follows:



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   Option Delta:  4-bit unsigned integer.  Indicates the difference
      between the Option Number of this option and the previous option
      (or zero for the first option).  In other words, the Option Number
      is calculated by simply summing the Option Delta fields of this
      and previous options before it.  The Option Delta 15 is reserved
      for special constructs such as the end-of-options marker (see
      below) and Option Jumps.  The Option Jump mechanism (Section 3.3)
      is used when the delta to the next option number is larger than
      14.

   Length:  Indicates the length of the Option Value, in bytes.
      Normally Length is a 4-bit unsigned integer allowing value lengths
      of 0-14 bytes.  When the Length field is set to 15, another byte
      is added as an 8-bit unsigned integer whose value is added to the
      15, allowing option value lengths of 15-270 bytes.  For option
      lengths beyond 270 bytes, we reserve the value 255 of an extension
      byte to mean "add 255, read another extension byte".  Options that
      are longer than 1034 bytes MUST NOT be sent; an option that has
      255 (all one bits) in the field called "Length - 780" MUST be
      rejected upon reception as an encoding error.

   Value:  The length and format of the Option Value depends on the
      respective option, which MAY define variable length values.  See
      Section 3.4 for the formats the options defined in this document
      make use of; other options MAY make use of other option value
      formats.

   If the Option Count field in the CoAP header is 15 and the Option
   Header byte is 0xf0 (the Option Delta is 15 and the Option Length is
   0), the option is interpreted as the end-of-options marker instead of
   the option with the resulting Option Number.  (In other words, the
   end-of-options marker always is just a single byte valued 0xf0.)
   When this marker is encountered, it is immediately followed by the
   payload (if any).  (Note that, by this special meaning, the Option
   Delta of 15 is made special, not any specific Option Number.)  The
   sender MUST NOT include the end-of-options marker in an Option in a
   message with an Option Count other than 15; recipients MUST treat
   this as an encoding error.

   Option Numbers are maintained in the CoAP Option Number Registry
   (Section 12.2).  See Section 5.10 for the semantics of the options
   defined in this document.

3.3.  Option Jump

   The following construct can occur in front of any Option:





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     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   1   1 | 0   0   0   1 |  0xf1 (Delta = 15)
   +---+---+---+---+---+---+---+---+

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   1   1 | 0   0   1   0 |  0xf2
   +---+---+---+---+---+---+---+---+
   |       Option Jump Value       |  (Delta/8)-2
   +---+---+---+---+---+---+---+---+

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   1   1 | 0   0   1   1 |  0xf3
   +---+---+---+---+---+---+---+---+
   |                               |
   +---    Option Jump Value    ---+  (Delta/8)-258
   |                               |
   +---+---+---+---+---+---+---+---+

                       Figure 9: Option Jump Format

   This construct is not by itself an Option.  It can occur in front of
   any Option to increase the current Option number that then goes into
   its Option number calculation.  The increase is done by 15 or in
   multiples of eight.  For the formats that include an Option Jump
   Value, the actual addition to the current Option number is computed
   as follows:

      Delta = ((Option Jump Value) + N) * 8

   where N is 2 for the one-byte version and N is 258 for the two-byte
   version.

   An Option Jump MUST be followed by an actual Option, i.e., it MUST
   NOT be followed by another Option Jump or an end-of-options
   indicator.  A message violating this MUST be treated as an encoding
   error.

   Option Jumps do NOT count as Options in the Option Count field of the
   header (i.e., they cannot by themselves end the Option sequence).

3.4.  Option Value Formats

   The options defined in this document make use of the following option
   value formats.




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3.4.1.  uint

   A non-negative integer which is represented in network byte order
   using the given number of bytes.  An option definition may specify a
   range of permissible numbers of bytes; if it has a choice, a sender
   SHOULD represent the integer with as few bytes as possible, i.e.,
   without leading zeros.  A recipient MUST be prepared to process
   values with leading zeros.

   Implementation Note:  The exceptional behavior permitted above is for
      highly constrained templated implementations (e.g. hardware
      implementations) that use fixed size options in the templates.


   Length = 0     (implies value of 0)

                   0
                   0 1 2 3 4 5 6 7
                  +-+-+-+-+-+-+-+-+
   Length = 1     |     0-255     |
                  +-+-+-+-+-+-+-+-+

                   0                   1
                   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Length = 2     |            0-65535            |
                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length = 3 is 24 bits, Length = 4 is 32 bits etc.

3.4.2.  string

   A Unicode string which is encoded using UTF-8 [RFC3629] in Net-
   Unicode form [RFC5198].  Note that here and in all other places where
   UTF-8 encoding is used in the CoAP protocol, the intention is that
   the encoded strings can be directly used and compared as opaque byte
   strings by CoAP protocol implementations.  There is no expectation
   and no need to perform normalization within a CoAP implementation
   unless Unicode strings that are not known to be normalized are
   imported from sources outside the CoAP protocol.  Note also that
   ASCII strings (that do not make use of special control characters)
   are always valid UTF-8 Net-Unicode strings.

3.4.3.  opaque

   An opaque sequence of bytes.





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3.4.4.  empty

   A zero-length sequence of bytes.


4.  Message Transmission

   CoAP messages are exchanged asynchronously between CoAP endpoints.
   They are used to transport CoAP requests and responses, the semantics
   of which are defined in Section 5.

   As CoAP is bound to non-reliable transports such as UDP, CoAP
   messages may arrive out of order, appear duplicated, or go missing
   without notice.  For this reason, CoAP implements a lightweight
   reliability mechanism, without trying to re-create the full feature
   set of a transport like TCP.  It has the following features:

   o  Simple stop-and-wait retransmission reliability with exponential
      back-off for Confirmable messages.

   o  Duplicate detection for both Confirmable and Non-confirmable
      messages.

4.1.  Messages and Endpoints

   A CoAP endpoint is the source or destination of a CoAP message.  It
   is identified depending on the security mode used (see Section 9):
   With no security, the endpoint is solely identified by an IP address
   and a UDP port number.  With other security modes, the endpoint is
   identified as defined by the security mode.

   There are different types of messages.  The type of a message is
   specified by the T field of the CoAP header.

   Separate from the message type, a message may carry a request, a
   response, or be empty.  This is signaled by the Code field in the
   CoAP header and is relevant to the request/response model.  Possible
   values for the Code field are maintained by the CoAP Code Registry
   (Section 12.1).

   An empty message has the Code field set to 0.  The OC field SHOULD be
   set to 0 and no bytes SHOULD be present after the Message ID field.
   The OC field and any bytes trailing the header MUST be ignored by any
   recipient.







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4.2.  Messages Transmitted Reliably

   The reliable transmission of a message is initiated by marking the
   message as Confirmable in the CoAP header.  A Confirmable message
   always carries either a request or response and MUST NOT be empty.  A
   recipient MUST acknowledge such a message with an Acknowledgement
   message or, if it lacks context to process the message properly
   (including the case where the message is empty or has an encoding
   error), MUST reject it; rejecting a Confirmable message is effected
   by sending a matching Reset message and otherwise ignoring it.  The
   Acknowledgement message MUST echo the Message ID of the Confirmable
   message, and MUST carry a response or be empty (see Section 5.2.1 and
   Section 5.2.2).  The Reset message MUST echo the Message ID of the
   confirmable message, and MUST be empty.  Rejecting an Acknowledgement
   or Reset message is effected by silently ignoring it.

   The sender retransmits the Confirmable message at exponentially
   increasing intervals, until it receives an acknowledgement (or Reset
   message), or runs out of attempts.

   Retransmission is controlled by two things that a CoAP endpoint MUST
   keep track of for each Confirmable message it sends while waiting for
   an acknowledgement (or reset): a timeout and a retransmission
   counter.  For a new Confirmable message, the initial timeout is set
   to a random number between ACK_TIMEOUT and (ACK_TIMEOUT *
   ACK_RANDOM_FACTOR) (see Section 4.8), and the retransmission counter
   is set to 0.  When the timeout is triggered and the retransmission
   counter is less than MAX_RETRANSMIT, the message is retransmitted,
   the retransmission counter is incremented, and the timeout is
   doubled.  If the retransmission counter reaches MAX_RETRANSMIT on a
   timeout, or if the endpoint receives a Reset message, then the
   attempt to transmit the message is canceled and the application
   process informed of failure.  On the other hand, if the endpoint
   receives an acknowledgement message in time, transmission is
   considered successful.

   A CoAP endpoint that sent a Confirmable message MAY give up in
   attempting to obtain an ACK even before the MAX_RETRANSMIT counter
   value is reached: E.g., the application has canceled the request as
   it no longer needs a response, or there is some other indication that
   the CON message did arrive.  In particular, a CoAP request message
   may have elicited a separate response, in which case it is clear to
   the requester that only the ACK was lost and a retransmission of the
   request would serve no purpose.  However, a responder MUST NOT in
   turn rely on this cross-layer behavior from a requester, i.e. it
   SHOULD retain the state to create the ACK for the request, if needed,
   even if a Confirmable response was already acknowledged by the
   requester.



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4.3.  Messages Transmitted Without Reliability

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

   As a more lightweight alternative, a message can be transmitted less
   reliably by marking the message as Non-confirmable.  A Non-
   confirmable message always carries either a request or response and
   MUST NOT be empty.  A Non-confirmable message MUST NOT be
   acknowledged by the recipient.  If a recipient lacks context to
   process the message properly (including the case where the message is
   empty or has an encoding error), it MUST reject the message;
   rejecting a Non-Confirmable message MAY involve sending a matching
   Reset message, and apart from the Reset message the rejected message
   MUST be silently ignored.

   At the CoAP level, there is no way for the sender to detect if a Non-
   confirmable message was received or not.  A sender MAY choose to
   transmit multiple copies of a Non-confirmable message within
   MAX_TRANSMIT_SPAN, or the network may duplicate the message in
   transit.  To enable the receiver to act only once on the message,
   Non-confirmable messages specify a Message ID as well.  (This Message
   ID is drawn from the same number space as the Message IDs for
   Confirmable messages.)

4.4.  Message Correlation

   An Acknowledgement or Reset message is related to a Confirmable
   message or Non-confirmable message by means of a Message ID along
   with additional address information of the corresponding endpoint.
   The Message ID is a 16-bit unsigned integer that is generated by the
   sender of a Confirmable or Non-confirmable message and included in
   the CoAP header.  The Message ID MUST be echoed in the
   Acknowledgement or Reset message by the recipient.

   The same Message ID MUST NOT be re-used (in communicating with the
   same endpoint) within the EXCHANGE_LIFETIME (Section 4.8.2).

   Implementation Note:  Several implementation strategies can be
      employed for generating Message IDs.  In the simplest case a CoAP
      endpoint generates Message IDs by keeping a single Message ID
      variable, which is changed each time a new confirmable or non-
      confirmable message is sent regardless of the destination address
      or port.  Endpoints dealing with large numbers of transactions
      could keep multiple Message ID variables, for example per prefix
      or destination address.  The initial variable value should be



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      randomized.

   For an Acknowledgement or Reset message to match a Confirmable or
   Non-confirmable message, the Message ID and source endpoint of the
   Acknowledgement or Reset message MUST match the Message ID and
   destination endpoint of the Confirmable or Non-confirmable message.

4.5.  Message Deduplication

   A recipient MUST be prepared to receive the same Confirmable message
   (as indicated by the Message ID and source endpoint) multiple times
   within the EXCHANGE_LIFETIME (Section 4.8.2), for example, when its
   Acknowledgement went missing or didn't reach the original sender
   before the first timeout.  The recipient SHOULD acknowledge each
   duplicate copy of a Confirmable message using the same
   Acknowledgement or Reset message, but SHOULD process any request or
   response in the message only once.  This rule MAY be relaxed in case
   the Confirmable message transports a request that is idempotent (see
   Section 5.1) or can be handled in an idempotent fashion.  Examples
   for relaxed message deduplication:

   o  A server MAY relax the requirement to answer all retransmissions
      of an idempotent request with the same response (Section 4.2), so
      that it does not have to maintain state for Message IDs.  For
      example, an implementation might want to process duplicate
      transmissions of a GET, PUT or DELETE request as separate requests
      if the effort incurred by duplicate processing is less expensive
      than keeping track of previous responses would be.

   o  A constrained server MAY even want to relax this requirement for
      certain non-idempotent requests if the application semantics make
      this trade-off favorable.  For example, if the result of a POST
      request is just the creation of some short-lived state at the
      server, it may be less expensive to incur this effort multiple
      times for a request than keeping track of whether a previous
      transmission of the same request already was processed.

   A recipient MUST be prepared to receive the same Non-confirmable
   message (as indicated by the Message ID and source endpoint) multiple
   times within NON_LIFETIME (Section 4.8.2).  As a general rule that
   may be relaxed based on the specific semantics of a message, the
   recipient SHOULD silently ignore any duplicated Non-confirmable
   message, and SHOULD process any request or response in the message
   only once.







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4.6.  Message Size

   While specific link layers make it beneficial to keep CoAP messages
   small enough to fit into their link layer packets (see Section 1),
   this is a matter of implementation quality.  The CoAP specification
   itself provides only an upper bound to the message size.  Messages
   larger than an IP fragment result in undesired packet fragmentation.
   A CoAP message, appropriately encapsulated, SHOULD fit within a
   single IP packet (i.e., avoid IP fragmentation) and (by fitting into
   one UDP payload) obviously MUST fit within a single IP datagram.  If
   the Path MTU is not known for a destination, an IP MTU of 1280 bytes
   SHOULD be assumed; if nothing is known about the size of the headers,
   good upper bounds are 1152 bytes for the message size and 1024 bytes
   for the payload size.

   Implementation Note:  CoAP's choice of message size parameters works
      well with IPv6 and with most of today's IPv4 paths.  (However,
      with IPv4, it is harder to absolutely ensure that there is no IP
      fragmentation.  If IPv4 support on unusual networks is a
      consideration, implementations may want to limit themselves to
      more conservative IPv4 datagram sizes such as 576 bytes; worse,
      the absolute minimum value of the IP MTU for IPv4 is as low as 68
      bytes, which would leave only 40 bytes minus security overhead for
      a UDP payload.  Implementations extremely focused on this problem
      set might also set the IPv4 DF bit and perform some form of path
      MTU discovery; this should generally be unnecessary in most
      realistic use cases for CoAP, however.)  A more important kind of
      fragmentation in many constrained networks is that on the
      adaptation layer (e.g., 6LoWPAN L2 packets are limited to 127
      bytes including various overheads); this may motivate
      implementations to be frugal in their packet sizes and to move to
      block-wise transfers [I-D.ietf-core-block] when approaching three-
      digit message sizes.

      Message sizes are also of considerable importance to
      implementations on constrained nodes.  Many implementations will
      need to allocate a buffer for incoming messages.  If an
      implementation is too constrained to allow for allocating the
      above-mentioned upper bound, it could apply the following
      implementation strategy: Implementations receiving a datagram into
      a buffer that is too small are usually able to determine if the
      trailing portion of a datagram was discarded and to retrieve the
      initial portion.  So, if not all of the payload, at least the CoAP
      header and options are likely to fit within the buffer.  A server
      can thus fully interpret a request and return a 4.13 (Request
      Entity Too Large) response code if the payload was truncated.  A
      client sending an idempotent request and receiving a response
      larger than would fit in the buffer can repeat the request with a



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      suitable value for the Block Option [I-D.ietf-core-block].

4.7.  Congestion Control

   Basic congestion control for CoAP is provided by the exponential
   back-off mechanism in Section 4.2.

   In order not to cause congestion, Clients (including proxies) MUST
   strictly limit the number of simultaneous outstanding interactions
   that they maintain to a given server (including proxies) to NSTART.
   An outstanding interaction is either a CON for which an ACK has not
   yet been received but is still expected (message layer) or a request
   for which neither a response nor an Acknowledgment message has yet
   been received but is still expected (which may both occur at the same
   time, counting as one outstanding interaction).  The default value of
   NSTART for this specification is 1.

   Further congestion control optimizations and considerations are
   expected in the future, which may for example provide automatic
   initialization of the CoAP transmission parameters defined in
   Section 4.8, and thus may allow a value for NSTART greater than one.

   A client stops expecting a response to a Confirmable request for
   which no acknowledgment message was received, after
   EXCHANGE_LIFETIME.  The specific algorithm by which a client stops to
   "expect" a response to a Confirmable request that was acknowledged,
   or to a Non-confirmable request, is not defined.  Unless this is
   modified by additional congestion control optimizations, it MUST be
   chosen in such a way that an endpoint does not exceed an average data
   rate of PROBING_RATE in sending to another endpoint that does not
   respond.

   Note:  CoAP places the onus of congestion control mostly on the
      clients.  However, clients may malfunction or actually be
      attackers, e.g. to perform amplification attacks (Section 11.3).
      To limit the damage (to the network and to its own energy
      resources), a server SHOULD implement some rate limiting for its
      response transmission based on reasonable assumptions about
      application requirements.  This is most helpful if the rate limit
      can be made effective for the misbehaving endpoints, only.

4.8.  Transmission Parameters

   Message transmission is controlled by the following parameters:







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                   +-------------------+---------------+
                   | name              | default value |
                   +-------------------+---------------+
                   | ACK_TIMEOUT       | 2 seconds     |
                   | ACK_RANDOM_FACTOR | 1.5           |
                   | MAX_RETRANSMIT    | 4             |
                   | NSTART            | 1             |
                   | DEFAULT_LEISURE   | 5 seconds     |
                   | PROBING_RATE      | 1 Byte/second |
                   +-------------------+---------------+

4.8.1.  Changing The Parameters

   The values for ACK_TIMEOUT, ACK_RANDOM_FACTOR, MAX_RETRANSMIT,
   NSTART, DEFAULT_LEISURE, and PROBING_RATE may be configured to values
   specific to the application environment (including dynamically
   adjusted values), however the configuration method is out of scope of
   this document.  It is recommended that an application environment use
   consistent values for these parameters.

   The transmission parameters have been chosen to achieve a behavior in
   the presence of congestion that is safe in the Internet.  If a
   configuration desires to use different values, the onus is on the
   configuration to ensure these congestion control properties are not
   violated.  In particular, a decrease of ACK_TIMEOUT below 1 second
   would violate the guidelines of [RFC5405].
   ([I-D.allman-tcpm-rto-consider] provides some additional background.)
   CoAP was designed to enable implementations that do not maintain
   round-trip-time (RTT) measurements.  However, where it is desired to
   decrease the ACK_TIMEOUT significantly or increase NSTART, this can
   only be done safely when maintaining such measurements.
   Configurations MUST NOT decrease ACK_TIMEOUT or increase NSTART
   without using mechanisms that ensure congestion control safety,
   either defined in the configuration or in future standards documents.

   ACK_RANDOM_FACTOR MUST NOT be decreased below 1.0, and it SHOULD have
   a value that is sufficiently different from 1.0 to provide some
   protection from synchronization effects.

   MAX_RETRANSMIT can be freely adjusted, but a too small value will
   reduce the probability that a confirmable message is actually
   received, while a larger value than given here will require further
   adjustments in the time values (see discussion below).

   If the choice of transmission parameters leads to an increase of
   derived time values (see below), the configuration mechanism MUST
   ensure the adjusted value is also available to all the endpoints in
   communicating with which these adjusted values are to be used.



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4.8.2.  Time Values derived from Transmission Parameters

   The combination of ACK_TIMEOUT, ACK_RANDOM_FACTOR and MAX_RETRANSMIT
   influences the timing of retransmissions, which in turn influences
   how long certain information items need to be kept by an
   implementation.  To be able to unambiguously reference these derived
   time values, we give them names as follows:

   o  MAX_TRANSMIT_SPAN is the maximum time from the first transmission
      of a confirmable message to its last retransmission.  For the
      default transmission parameters, the value is (2+4+8+16)*1.5 = 45
      seconds, or more generally:

         ACK_TIMEOUT * (2 ** MAX_RETRANSMIT - 1) * ACK_RANDOM_FACTOR

   o  MAX_TRANSMIT_WAIT is the maximum time from the first transmission
      of a confirmable message to the time when the sender gives up on
      receiving an acknowledgement or reset.  For the default
      transmission parameters, the value is (2+4+8+16+32)*1.5 = 93
      seconds, or more generally:

         ACK_TIMEOUT * (2 ** (MAX_RETRANSMIT + 1) - 1) *
         ACK_RANDOM_FACTOR

   In addition, some assumptions need to be made on the characteristics
   of the network and the nodes.

   o  MAX_LATENCY is the maximum time a datagram is expected to take
      from the start of its transmission to the completion of its
      reception.  This constant is related to the MSL (Maximum Segment
      Lifetime) of [RFC0793], which is "arbitrarily defined to be 2
      minutes" ([RFC0793] glossary, page 81).  Note that this is not
      necessarily smaller than MAX_TRANSMIT_WAIT, as MAX_LATENCY is not
      intended to describe a situation when the protocol works well, but
      the worst case situation against which the protocol has to guard.
      We, also arbitrarily, define MAX_LATENCY to be 100 seconds.  Apart
      from being reasonably realistic for the bulk of configurations as
      well as close to the historic choice for TCP, this value also
      allows message ID lifetime timers to be represented in 8 bits
      (when measured in seconds).  In these calculations, there is no
      assumption that the direction of the transmission is irrelevant
      (i.e. that the network is symmetric), just that the same value can
      reasonably be used as a maximum value for both directions.  If
      that is not the case, the following calculations become only
      slightly more complex.

   o  PROCESSING_DELAY is the time a node takes to turn around a
      confirmable message into an acknowledgement.  We assume the node



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      will attempt to send an ACK before having the sender time out, so
      as a conservative assumption we set it equal to ACK_TIMEOUT.

   o  MAX_RTT is the maximum round-trip time, or:

         2 * MAX_LATENCY + PROCESSING_DELAY

   From these values, we can derive the following values relevant to the
   protocol operation:

   o  EXCHANGE_LIFETIME is the time from starting to send a confirmable
      message to the time when an acknowledgement is no longer expected,
      i.e. message layer information about the message exchange can be
      purged.  EXCHANGE_LIFETIME includes a MAX_TRANSMIT_SPAN, a
      MAX_LATENCY forward, PROCESSING_DELAY, and a MAX_LATENCY for the
      way back.  Note that there is no need to consider
      MAX_TRANSMIT_WAIT if the configuration is chosen such that the
      last waiting period (ACK_TIMEOUT * (2 ** MAX_RETRANSMIT) or the
      difference between MAX_TRANSMIT_SPAN and MAX_TRANSMIT_WAIT) is
      less than MAX_LATENCY -- which is a likely choice, as MAX_LATENCY
      is a worst case value unlikely to be met in the real world.  In
      this case, EXCHANGE_LIFETIME simplifies to:

         (ACK_TIMEOUT * (2 ** MAX_RETRANSMIT - 1) * ACK_RANDOM_FACTOR) +
         (2 * MAX_LATENCY) + PROCESSING_DELAY

      or 248 seconds with the default transmission parameters.

   o  NON_LIFETIME is the time from sending a non-confirmable message to
      the time its message-ID can be safely reused.  If multiple
      transmission of a NON message is not used, its value is
      MAX_LATENCY, or 100 seconds.  However, a CoAP sender might send a
      NON message multiple times, in particular for multicast
      applications.  While the period of re-use is not bounded by the
      specification, an expectation of reliable detection of duplication
      at the receiver is in the timescales of MAX_TRANSMIT_SPAN.
      Therefore, for this purpose, it is safer to use the value:

         MAX_TRANSMIT_SPAN + MAX_LATENCY

      or 145 seconds with the default transmission parameters; however,
      an implementation that just wants to use a single timeout value
      for retiring message-IDs can safely use the larger value for
      EXCHANGE_LIFETIME.







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5.  Request/Response Semantics

   CoAP operates under a similar request/response model as HTTP: a CoAP
   endpoint in the role of a "client" sends one or more CoAP requests to
   a "server", which services the requests by sending CoAP responses.
   Unlike HTTP, requests and responses are not sent over a previously
   established connection, but exchanged asynchronously over CoAP
   messages.

5.1.  Requests

   A CoAP request consists of the method to be applied to the resource,
   the identifier of the resource, a payload and Internet media type (if
   any), and optional meta-data about the request.

   CoAP supports the basic methods of GET, POST, PUT, DELETE, which are
   easily mapped to HTTP.  They have the same properties of safe (only
   retrieval) and idempotent (you can invoke it multiple times with the
   same effects) as HTTP (see Section 9.1 of [RFC2616]).  The GET method
   is safe, therefore it MUST NOT take any other action on a resource
   other than retrieval.  The GET, PUT and DELETE methods MUST be
   performed in such a way that they are idempotent.  POST is not
   idempotent, because its effect is determined by the origin server and
   dependent on the target resource; it usually results in a new
   resource being created or the target resource being updated.

   A request is initiated by setting the Code field in the CoAP header
   of a Confirmable or a Non-confirmable message to a Method Code and
   including request information.

   The methods used in requests are described in detail in Section 5.8.

5.2.  Responses

   After receiving and interpreting a request, a server responds with a
   CoAP response, which is matched to the request by means of a client-
   generated token.

   A response is identified by the Code field in the CoAP header being
   set to a Response Code.  Similar to the HTTP Status Code, the CoAP
   Response Code indicates the result of the attempt to understand and
   satisfy the request.  These codes are fully defined in Section 5.9.
   The Response Code numbers to be set in the Code field of the CoAP
   header are maintained in the CoAP Response Code Registry
   (Section 12.1.2).






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                              0
                              0 1 2 3 4 5 6 7
                             +-+-+-+-+-+-+-+-+
                             |class|  detail |
                             +-+-+-+-+-+-+-+-+

                  Figure 10: Structure of a Response Code

   The upper three bits of the 8-bit Response Code number define the
   class of response.  The lower five bits do not have any
   categorization role; they give additional detail to the overall class
   (Figure 10).  There are 3 classes:

   2 - Success:  The request was successfully received, understood, and
      accepted.

   4 - Client Error:  The request contains bad syntax or cannot be
      fulfilled.

   5 - Server Error:  The server failed to fulfill an apparently valid
      request.

   The response codes are designed to be extensible: Response Codes in
   the Client Error and Server Error class that are unrecognized by an
   endpoint MUST be treated as being equivalent to the generic Response
   Code of that class (4.00 and 5.00, respectively).  However, there is
   no generic Response Code indicating success, so a Response Code in
   the Success class that is unrecognized by an endpoint can only be
   used to determine that the request was successful without any further
   details.

   As a human readable notation for specifications and protocol
   diagnostics, the numeric value of a response code is indicated by
   giving the upper three bits in decimal, followed by a dot and then
   the lower five bits in a two-digit decimal.  E.g., "Not Found" is
   written as 4.04 -- indicating a value of hexadecimal 0x84 or decimal
   132.  In other words, the dot "." functions as a short-cut for
   "*32+".

   The possible response codes are described in detail in Section 5.9.

   Responses can be sent in multiple ways, which are defined below.

5.2.1.  Piggy-backed

   In the most basic case, the response is carried directly in the
   Acknowledgement message that acknowledges the request (which requires
   that the request was carried in a Confirmable message).  This is



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   called a "Piggy-backed" Response.

   The response is returned in the Acknowledgement message independent
   of whether the response indicates success or failure.  In effect, the
   response is piggy-backed on the Acknowledgement message, so no
   separate message is required to both acknowledge that the request was
   received and return the response.

   Implementation Note:  The protocol leaves the decision whether to
      piggy-back a response or not (i.e., send a separate response) to
      the server.  The client MUST be prepared to receive either.  On
      the quality of implementation level, there is a strong expectation
      that servers will implement code to piggy-back whenever possible
      -- saving resources in the network and both at the client and at
      the server.

5.2.2.  Separate

   It may not be possible to return a piggy-backed response in all
   cases.  For example, a server might need longer to obtain the
   representation of the resource requested than it can wait sending
   back the Acknowledgement message, without risking the client to
   repeatedly retransmit the request message.  Responses to requests
   carried in a Non-Confirmable message are always sent separately (as
   there is no Acknowledgement message).

   The server maybe initiates the attempt to obtain the resource
   representation and times out an acknowledgement timer, or it
   immediately sends an acknowledgement knowing in advance that there
   will be no piggy-backed response.  The acknowledgement effectively is
   a promise that the request will be acted upon.

   When the server finally has obtained the resource representation, it
   sends the response.  When it is desired that this message is not
   lost, it is sent as a Confirmable message from the server to the
   client and answered by the client with an Acknowledgement, echoing
   the new Message ID chosen by the server.  (It may also be sent as a
   Non-Confirmable message; see Section 5.2.3.)

   Implementation Notes:  Note that, as the underlying datagram
      transport may not be sequence-preserving, the Confirmable message
      carrying the response may actually arrive before or after the
      acknowledgement message for the request.  Note also that, while
      the CoAP protocol itself does not make any specific demands here,
      there is an expectation that the response will come within a time
      frame that is reasonable from an application point of view; as
      there is no underlying transport protocol that could be instructed
      to run a keep-alive mechanism, the requester MAY want to set up a



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      timeout that is unrelated to CoAP's retransmission timers in case
      the server is destroyed or otherwise unable to send the response.)

   An exchange is separate by definition when the Acknowledgement to the
   Confirmable request is an empty message.  The Acknowledgement to the
   Confirmable response MUST also be an empty message, i.e. one that
   carries neither a request nor a response.  However, a server MUST
   stop retransmitting its response on any matching Acknowledgement
   (silently ignoring any response code or payload) or Reset message.

5.2.3.  Non-Confirmable

   If the request message is Non-confirmable, then the response SHOULD
   be returned in a Non-confirmable message as well.  However, an
   endpoint MUST be prepared to receive a Non-confirmable response
   (preceded or followed by an empty acknowledgement message) in reply
   to a Confirmable request, or a Confirmable response in reply to a
   Non-confirmable request.

5.3.  Request/Response Matching

   Regardless of how a response is sent, it is matched to the request by
   means of a token that is included by the client in the request as one
   of the options along with additional address information of the
   corresponding endpoint.  The token MUST be echoed by the server in
   any resulting response without modification.

   The exact rules for matching a response to a request are as follows:

   1.  The source endpoint of the response MUST be the same as the
       destination endpoint of the original request.

   2.  In a piggy-backed response, both the Message ID of the
       Confirmable request and the Acknowledgement, and the token of the
       response and original request MUST match.  In a separate
       response, just the token of the response and original request
       MUST match.

   The client SHOULD generate tokens in a way that tokens currently in
   use for a given source/destination pair are unique.  (Note that a
   client can use the same token for any request if it uses a different
   source port number each time.)

   An endpoint that did not generate a token MUST treat it as opaque and
   make no assumptions about its format.  (Note that there is a default
   value for the Token Option, so every message carries a token, even if
   it is not explicitly expressed in a CoAP option.)




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   In case a message carrying a response is unexpected (i.e. the client
   is not waiting for a response at the endpoint addressed and/or with
   the given token), the response is rejected (Section 4.2,
   Section 4.3).

   Implementation Note:  A client that receives a response in a CON
      message may want to clean up the message state right after sending
      the ACK.  If that ACK is lost and the server retransmits the CON,
      the client may no longer have any state to correlate this response
      to, making the retransmission an unexpected message; the client
      may send a Reset message so it does not receive any more
      retransmissions.  This behavior is normal and not an indication of
      an error.  (Clients that are not aggressively optimized in their
      state memory usage will still have message state that will
      identify the second CON as a retransmission.  Clients that
      actually expect more messages from the server
      [I-D.ietf-core-observe] will have to keep state in any case.)

5.4.  Options

   Both requests and responses may include a list of one or more
   options.  For example, the URI in a request is transported in several
   options, and meta-data that would be carried in an HTTP header in
   HTTP is supplied as options as well.

   CoAP defines a single set of options that are used in both requests
   and responses:

   o  Content-Format

   o  ETag

   o  Location-Path

   o  Location-Query

   o  Max-Age

   o  Proxy-Uri

   o  Token

   o  Uri-Host

   o  Uri-Path

   o  Uri-Port




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   o  Uri-Query

   o  Accept

   o  If-Match

   o  If-None-Match

   The semantics of these options along with their properties are
   defined in detail in Section 5.10.

   Not all options are defined for use with all methods and response
   codes.  The possible options for methods and response codes are
   defined in Section 5.8 and Section 5.9 respectively.  In case an
   option is not defined for a method or response code, it MUST NOT be
   included by a sender and MUST be treated like an unrecognized option
   by a recipient.

   An Option number is constructed with a bit mask to indicate if an
   option is Critical/Elective, Unsafe/Safe and in the case of Safe,
   also a Cache-Key as indicated by the following figure.  When bit 7
   (the least significant bit) is 1, an option is Critical (and likewise
   Elective when 0).  When bit 6 is 1, an option is Unsafe (and likewise
   Safe when 0).  When an option is not Unsafe, it is not a Cache-Key
   (NoCacheKey) if and only if bits 3-5 are all set to 1; all other bit
   combinations mean that it indeed is a Cache-Key.  These classes of
   options are explained in the next sections.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   |           | NoCacheKey| U | C |
   +---+---+---+---+---+---+---+---+


                       Figure 11: Option Number Mask

   An endpoint may use an equivalent of the following C code to derive
   the characteristics of an option number "onum":

   Critical = (onum & 1);
   UnSafe = (onum & 2);
   NoCacheKey = ((onum & 0x1e) == 0x1c);

                                 Figure 12







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5.4.1.  Critical/Elective

   Options fall into one of two classes: "critical" or "elective".  The
   difference between these is how an option unrecognized by an endpoint
   is handled:

   o  Upon reception, unrecognized options of class "elective" MUST be
      silently ignored.

   o  Unrecognized options of class "critical" that occur in a
      confirmable request MUST cause the return of a 4.02 (Bad Option)
      response.  This response SHOULD include a diagnostic message
      describing the unrecognized option(s) (see Section 5.5.2).

   o  Unrecognized options of class "critical" that occur in a
      confirmable response, or piggy-backed in an acknowledgement, MUST
      cause the response to be rejected (Section 4.2).

   o  Unrecognized options of class "critical" that occur in a non-
      confirmable message MUST cause the message to be rejected
      (Section 4.3).

   Note that, whether critical or elective, an option is never
   "mandatory" (it is always optional): These rules are defined in order
   to enable implementations to stop processing options they do not
   understand or implement.

   Critical/Elective rules apply to non-proxying endpoints.  A proxy
   processes options based on Unsafe/Safe classes as defined in
   Section 5.7.

5.4.2.  Proxy Unsafe/Safe and Cache-Key

   In addition to an option being marked as Critical or Elective,
   options are also classified based on how a proxy is to deal with the
   option if it does not recognize it.  For this purpose, an option can
   either be considered Unsafe to Forward (UnSafe is set) or Safe to
   Forward (UnSafe is clear).

   In addition, for options that are marked Safe to Forward, the option
   indicates whether it is intended to be part of the Cache-Key in a
   request (NoCacheKey is not all set) or not (NoCacheKey is set).

   Note:  The Cache-Key indication is relevant only for proxies that do
      not implement the given option as a request option and instead
      rely on the Safe/Unsafe indication only.  E.g., for ETag, actually
      using the request option as a cache key is grossly inefficient,
      but it is the best thing one can do if ETag is not implemented by



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      a proxy, as the reponse is going to differ based on the presence
      of the request option.  A more useful proxy that does implement
      the ETag request option is not using ETag as a cache key.

   Proxy behavior with regard to these classes is defined in
   Section 5.7.

5.4.3.  Length

   Option values are defined to have a specific length, often in the
   form of an upper and lower bound.  If the length of an option value
   in a request is outside the defined range, that option MUST be
   treated like an unrecognized option (see Section 5.4.1).

5.4.4.  Default Values

   Options may be defined to have a default value.  If the value of
   option is intended to be this default value, the option SHOULD NOT be
   included in the message.  If the option is not present, the default
   value MUST be assumed.

   Where a critical option has a default value, this is chosen in such a
   way that the absence of the option in a message can be processed
   properly both by implementations unaware of the critical option and
   by implementations that interpret this absence as the presence of the
   default value for the option.

5.4.5.  Repeatable Options

   The definition of an option MAY specify the option to be repeatable.
   An option that is repeatable MAY be included one or more times in a
   message.  An option that is not repeatable MUST NOT be included more
   than once in a message.

   If a message includes an option with more occurrences than the option
   is defined for, the additional option occurrences MUST be treated
   like an unrecognized option (see Section 5.4.1).

5.4.6.  Option Numbers

   Options are identified by an option number.  Odd numbers indicate a
   critical option, while even numbers indicate an elective option.
   (Note that this is not just a convention, it is a feature of the
   protocol: Whether an option is elective or critical is entirely
   determined by whether its option number is even or odd.)

   The option numbers for the options defined in this document are
   listed in the CoAP Option Number Registry (Section 12.2).



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5.5.  Payload

   Both requests and responses may include payload, depending on the
   method or response code respectively.  If a method or response code
   is not defined to have a payload, then a sender MUST NOT include one,
   and a recipient MUST ignore it.

5.5.1.  Representation

   The payload of requests or of responses indicating success is
   typically a representation of a resource or the result of the
   requested action.  Its format is specified by the Internet media type
   and content coding given by the Content-Format Option.  In the
   absence of this option, no default value is assumed and the format
   must be inferred by the application (e.g., from the application
   context or by "sniffing" the payload).

5.5.2.  Diagnostic Message

   The payload of responses indicating a client or server error is a
   brief human-readable diagnostic message, explaining the error
   situation.  This diagnostic message MUST be encoded using UTF-8
   [RFC3629], more specifically using Net-Unicode form [RFC5198].  The
   Content-Format Option MUST NOT be included by the sender and MUST be
   treated like an unrecognized option by the recipient.

   The message is similar to the Reason-Phrase on an HTTP status line.
   It is not intended for end-users but for software engineers that
   during debugging need to interpret it in the context of the present,
   English-language specification; therefore no mechanism for language
   tagging is needed or provided.  In contrast to what is usual in HTTP,
   the message SHOULD be empty if there is no additional information
   beyond the response code.

5.6.  Caching

   CoAP endpoints MAY cache responses in order to reduce the response
   time and network bandwidth consumption on future, equivalent
   requests.

   The goal of caching in CoAP is to reuse a prior response message to
   satisfy a current request.  In some cases, a stored response can be
   reused without the need for a network request, reducing latency and
   network round-trips; a "freshness" mechanism is used for this purpose
   (see Section 5.6.1).  Even when a new request is required, it is
   often possible to reuse the payload of a prior response to satisfy
   the request, thereby reducing network bandwidth usage; a "validation"
   mechanism is used for this purpose (see Section 5.6.2).



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   Unlike HTTP, the cacheability of CoAP responses does not depend on
   the request method, but the Response Code.  The cacheability of each
   Response Code is defined along the Response Code definitions in
   Section 5.9.  Response Codes that indicate success and are
   unrecognized by an endpoint MUST NOT be cached.

   For a presented request, a CoAP endpoint MUST NOT use a stored
   response, unless:

   o  the presented request method and that used to obtain the stored
      response match,

   o  all options match between those in the presented request and those
      of the request used to obtain the stored response (which includes
      the request URI), except that there is no need for a match of the
      Token, Max-Age, or ETag request option(s), or any request options
      marked as NoCacheKey (Section 5.4), and

   o  the stored response is either fresh or successfully validated as
      defined below.

5.6.1.  Freshness Model

   When a response is "fresh" in the cache, it can be used to satisfy
   subsequent requests without contacting the origin server, thereby
   improving efficiency.

   The mechanism for determining freshness is for an origin server to
   provide an explicit expiration time in the future, using the Max-Age
   Option (see Section 5.10.6).  The Max-Age Option indicates that the
   response is to be considered not fresh after its age is greater than
   the specified number of seconds.

   The Max-Age Option defaults to a value of 60.  Thus, if it is not
   present in a cacheable response, then the response is considered not
   fresh after its age is greater than 60 seconds.  If an origin server
   wishes to prevent caching, it MUST explicitly include a Max-Age
   Option with a value of zero seconds.

   If a client has a fresh stored response and makes a new request
   matching the request for that stored response, the new response
   invalidates the old response.

5.6.2.  Validation Model

   When an endpoint has one or more stored responses for a GET request,
   but cannot use any of them (e.g., because they are not fresh), it can
   use the ETag Option (Section 5.10.7) in the GET request to give the



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   origin server an opportunity to both select a stored response to be
   used, and to update its freshness.  This process is known as
   "validating" or "revalidating" the stored response.

   When sending such a request, the endpoint SHOULD add an ETag Option
   specifying the entity-tag of each stored response that is applicable.

   A 2.03 (Valid) response indicates the stored response identified by
   the entity-tag given in the response's ETag Option can be reused,
   after updating its freshness with the value of the Max-Age Option
   that is included (explicitly, or implicitly as a default value) with
   the response (see Section 5.9.1.3).

   Any other response code indicates that none of the stored responses
   nominated in the request is suitable.  Instead, the response SHOULD
   be used to satisfy the request and MAY replace the stored response.

5.7.  Proxying

   A proxy is a CoAP endpoint that can be tasked by CoAP clients to
   perform requests on their behalf.  This may be useful, for example,
   when the request could otherwise not be made, or to service the
   response from a cache in order to reduce response time and network
   bandwidth or energy consumption.

   In an overall architecture for a Constrained RESTful Environment,
   proxies can serve quite different purposes.  Proxies can be
   explicitly selected by clients, a role that we term "forward-proxy".
   Proxies can also be inserted to stand in for origin servers, a role
   that we term "reverse-proxy".  Orthogonal to this distinction, a
   proxy can map from a CoAP request to a CoAP request (CoAP-to-CoAP
   proxy) or translate from or to a different protocol ("cross-proxy").
   Full definitions of these terms are provided in Section 1.2.

   Notes:  The terminology in this specification has been selected to be
      culturally compatible with the terminology used in the wider Web
      application environments, without necessarily matching it in every
      detail (which may not even be relevant to Constrained RESTful
      Environments).  Not too much semantics should be ascribed to the
      components of the terms (such as "forward", "reverse", or
      "cross").

      HTTP proxies, besides acting as HTTP proxies, often offer a
      transport protocol proxying function ("CONNECT") to enable end-to-
      end transport layer security through the proxy.  No such function
      is defined for CoAP-to-CoAP proxies in this specification, as
      forwarding of UDP packets is unlikely to be of much value in
      Constrained RESTful environments.  See also Section 10.2.7 for the



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      cross-proxy case.

5.7.1.  Proxy Operation

   A proxy generally needs a way to determine potential request
   parameters for a request to a destination based on the request it
   received.  This way is fully specified for a forward-proxy, but may
   depend on the specific configuration for a reverse-proxy.  In
   particular, the client of a reverse-proxy generally does not indicate
   a locator for the destination, necessitating some form of namespace
   translation in the reverse-proxy.  However, some aspects of the
   operation of proxies are common to all its forms.

   If a proxy does not employ a cache, then it simply forwards the
   translated request to the determined destination.  Otherwise, if it
   does employ a cache but does not have a stored response that matches
   the translated request and is considered fresh, then it needs to
   refresh its cache according to Section 5.6.  For options in the
   request that the proxy recognizes, it knows whether the option is
   intended to act as part of the key used in looking up the cached
   value or not.  E.g., since requests for different Uri-Path values
   address different resources, Uri-Path values are always parts of the
   cache key, while, e.g., Token values are never part of the cache key.
   For options that the proxy does not recognize but that are marked
   Safe in the option number, the option also indicates whether it is to
   be included in the cache key (NoCacheKey is not all set) or not
   (NoCacheKey is all set).  (Options that are unrecognized and marked
   Unsafe lead to 4.02 Bad Option.)

   If the request to the destination times out, then a 5.04 (Gateway
   Timeout) response MUST be returned.  If the request to the
   destination returns a response that cannot be processed by the proxy
   (e.g, due to unrecognized critical options, encoding errors), then a
   5.02 (Bad Gateway) response MUST be returned.  Otherwise, the proxy
   returns the response to the client.

   If a response is generated out of a cache, it MUST be generated with
   a Max-Age Option that does not extend the max-age originally set by
   the server, considering the time the resource representation spent in
   the cache.  E.g., the Max-Age Option could be adjusted by the proxy
   for each response using the formula:

      proxy-max-age = original-max-age - cache-age

   For example if a request is made to a proxied resource that was
   refreshed 20 seconds ago and had an original Max-Age of 60 seconds,
   then that resource's proxied max-age is now 40 seconds.  Considering
   potential network delays on the way from the origin server, a proxy



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   SHOULD be conservative in the max-age values offered.

   All options present in a proxy request MUST be processed at the
   proxy.  Unsafe options in a request that are not recognized by the
   proxy MUST lead to a 4.02 (Bad Option) response being returned by the
   proxy.  A CoAP-to-CoAP proxy MUST forward to the origin server all
   Safe options that it does not recognize.  Similarly, Unsafe options
   in a response that are not recognized by the CoAP-to-CoAP proxy
   server MUST lead to a 5.02 (Bad Gateway) response.  Again, Safe
   options that are not recognized MUST be forwarded.

   Additional considerations for cross-protocol proxying between CoAP
   and HTTP are discussed in Section 10.

5.7.2.  Forward-Proxies

   CoAP distinguishes between requests made (as if) to an origin server
   and a request made through a forward-proxy.  CoAP requests to a
   forward-proxy are made as normal confirmable or non-confirmable
   requests to the forward-proxy endpoint, but specify the request URI
   in a different way: The request URI in a proxy request is specified
   as a string in the Proxy-Uri Option (see Section 5.10.3), while the
   request URI in a request to an origin server is split into the Uri-
   Host, Uri-Port, Uri-Path and Uri-Query Options (see Section 5.10.2).

   When a proxy request is made to an endpoint and the endpoint is
   unwilling or unable to act as proxy for the request URI, it MUST
   return a 5.05 (Proxying Not Supported) response.  If the authority
   (host and port) is recognized as identifying the proxy endpoint
   itself (see Section 5.10.3), then the request MUST be treated as a
   local (non-proxied) request.

   Unless a proxy is configured to forward the proxy request to another
   proxy, it MUST translate the request as follows: The scheme of the
   request URI defines the outgoing protocol and its details (e.g., CoAP
   is used over UDP for the "coap" scheme and over DTLS for the "coaps"
   scheme.)  For a CoAP-to-CoAP proxy, the origin server's IP address
   and port are determined by the authority component of the request
   URI, and the request URI is decoded and split into the Uri-Host, Uri-
   Port, Uri-Path and Uri-Query Options.  This consumes the Proxy-URI
   option, which is therefore not forwarded to the origin server.

5.7.3.  Reverse-Proxies

   Reverse-proxies do not make use of the Proxy-Uri option, but need to
   determine the destination (next hop) of a request from information in
   the request and information in their configuration.  E.g., a reverse-
   proxy might offer various resources the existence of which it has



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   learned through resource discovery as if they were its own resources.
   The reverse-proxy is free to build a namespace for the URIs that
   identify these resources.  A reverse-proxy may also build a namespace
   that gives the client more control over where the request goes, e.g.
   by embedding host identifiers and port numbers into the URI path of
   the resources offered.

   In processing the response, a reverse-proxy has to be careful about
   namespacing the ETag option.  In many cases, it can be forwarded
   unchanged.  If the mapping from a resource offered by the reverse-
   proxy to resources offered by its various origin servers is not
   unique, the reverse-proxy may need to generate a new ETag, making
   sure the semantics of this option are properly preserved.

5.8.  Method Definitions

   In this section each method is defined along with its behavior.  A
   request with an unrecognized or unsupported Method Code MUST generate
   a 4.05 (Method Not Allowed) piggy-backed response.

5.8.1.  GET

   The GET method retrieves a representation for the information that
   currently corresponds to the resource identified by the request URI.
   If the request includes one or more Accept Options, they indicate the
   preferred content-format of a response.  If the request includes an
   ETag Option, the GET method requests that ETag be validated and that
   the representation be transferred only if validation failed.  Upon
   success a 2.05 (Content) or 2.03 (Valid) response code SHOULD be
   present in the response.

   The GET method is safe and idempotent.

5.8.2.  POST

   The POST method requests that the representation enclosed in the
   request be processed.  The actual function performed by the POST
   method is determined by the origin server and dependent on the target
   resource.  It usually results in a new resource being created or the
   target resource being updated.

   If a resource has been created on the server, the response returned
   by the server SHOULD have a 2.01 (Created) response code and SHOULD
   include the URI of the new resource in a sequence of one or more
   Location-Path and/or Location-Query Options (Section 5.10.8).  If the
   POST succeeds but does not result in a new resource being created on
   the server, the response SHOULD have a 2.04 (Changed) response code.
   If the POST succeeds and results in the target resource being



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   deleted, the response SHOULD have a 2.02 (Deleted) response code.

   POST is neither safe nor idempotent.

5.8.3.  PUT

   The PUT method requests that the resource identified by the request
   URI be updated or created with the enclosed representation.  The
   representation format is specified by the media type and content
   coding given in the Content-Format Option, if provided.

   If a resource exists at the request URI the enclosed representation
   SHOULD be considered a modified version of that resource, and a 2.04
   (Changed) response code SHOULD be returned.  If no resource exists
   then the server MAY create a new resource with that URI, resulting in
   a 2.01 (Created) response code.  If the resource could not be created
   or modified, then an appropriate error response code SHOULD be sent.

   Further restrictions to a PUT can be made by including the If-Match
   (see Section 5.10.9) or If-None-Match (see Section 5.10.10) options
   in the request.

   PUT is not safe, but is idempotent.

5.8.4.  DELETE

   The DELETE method requests that the resource identified by the
   request URI be deleted.  A 2.02 (Deleted) response code SHOULD be
   used on success or in case the resource did not exist before the
   request.

   DELETE is not safe, but is idempotent.

5.9.  Response Code Definitions

   Each response code is described below, including any options required
   in the response.  Where appropriate, some of the codes will be
   specified in regards to related response codes in HTTP [RFC2616];
   this does not mean that any such relationship modifies the HTTP
   mapping specified in Section 10.

5.9.1.  Success 2.xx

   This class of status code indicates that the clients request was
   successfully received, understood, and accepted.






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5.9.1.1.  2.01 Created

   Like HTTP 201 "Created", but only used in response to POST and PUT
   requests.  The payload returned with the response, if any, is a
   representation of the action result.

   If the response includes one or more Location-Path and/or Location-
   Query Options, the values of these options specify the location at
   which the resource was created.  Otherwise, the resource was created
   at the request URI.  A cache receiving this response MUST mark any
   stored response for the created resource as not fresh.

   This response is not cacheable.

5.9.1.2.  2.02 Deleted

   Like HTTP 204 "No Content", but only used in response to DELETE
   requests.  The payload returned with the response, if any, is a
   representation of the action result.

   This response is not cacheable.  However, a cache SHOULD mark any
   stored response for the deleted resource as not fresh.

5.9.1.3.  2.03 Valid

   Related to HTTP 304 "Not Modified", but only used to indicate that
   the response identified by the entity-tag identified by the included
   ETag Option is valid.  Accordingly, the response MUST include an ETag
   Option.

   When a cache receives a 2.03 (Valid) response, it MUST update the
   stored response with the value of the Max-Age Option included in the
   response (see Section 5.6.2).

5.9.1.4.  2.04 Changed

   Like HTTP 204 "No Content", but only used in response to POST and PUT
   requests.  The payload returned with the response, if any, is a
   representation of the action result.

   This response is not cacheable.  However, a cache MUST mark any
   stored response for the changed resource as not fresh.

5.9.1.5.  2.05 Content

   Like HTTP 200 "OK", but only used in response to GET requests.

   The payload returned with the response is a representation of the



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   target resource.

   This response is cacheable: Caches can use the Max-Age Option to
   determine freshness (see Section 5.6.1) and (if present) the ETag
   Option for validation (see Section 5.6.2).

5.9.2.  Client Error 4.xx

   This class of response code is intended for cases in which the client
   seems to have erred.  These response codes are applicable to any
   request method.

   The server SHOULD include a diagnostic message as detailed in
   Section 5.5.2.

   Responses of this class are cacheable: Caches can use the Max-Age
   Option to determine freshness (see Section 5.6.1).  They cannot be
   validated.

5.9.2.1.  4.00 Bad Request

   Like HTTP 400 "Bad Request".

5.9.2.2.  4.01 Unauthorized

   The client is not authorized to perform the requested action.  The
   client SHOULD NOT repeat the request without previously improving its
   authentication status to the server.  Which specific mechanism can be
   used for this is outside this document's scope; see also Section 9.

5.9.2.3.  4.02 Bad Option

   The request could not be understood by the server due to one or more
   unrecognized or malformed options.  The client SHOULD NOT repeat the
   request without modification.

5.9.2.4.  4.03 Forbidden

   Like HTTP 403 "Forbidden".

5.9.2.5.  4.04 Not Found

   Like HTTP 404 "Not Found".

5.9.2.6.  4.05 Method Not Allowed

   Like HTTP 405 "Method Not Allowed", but with no parallel to the
   "Allow" header field.



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5.9.2.7.  4.06 Not Acceptable

   Like HTTP 406 "Not Acceptable", but with no response entity.

5.9.2.8.  4.12 Precondition Failed

   Like HTTP 412 "Precondition Failed".

5.9.2.9.  4.13 Request Entity Too Large

   Like HTTP 413 "Request Entity Too Large".

5.9.2.10.  4.15 Unsupported Content-Format

   Like HTTP 415 "Unsupported Media Type".

5.9.3.  Server Error 5.xx

   This class of response code indicates cases in which the server is
   aware that it has erred or is incapable of performing the request.
   These response codes are applicable to any request method.

   The server SHOULD include a diagnostic message as detailed in
   Section 5.5.2.

   Responses of this class are cacheable: Caches can use the Max-Age
   Option to determine freshness (see Section 5.6.1).  They cannot be
   validated.

5.9.3.1.  5.00 Internal Server Error

   Like HTTP 500 "Internal Server Error".

5.9.3.2.  5.01 Not Implemented

   Like HTTP 501 "Not Implemented".

5.9.3.3.  5.02 Bad Gateway

   Like HTTP 502 "Bad Gateway".

5.9.3.4.  5.03 Service Unavailable

   Like HTTP 503 "Service Unavailable", but using the Max-Age Option in
   place of the "Retry-After" header field to indicate the number of
   seconds after which to retry.





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5.9.3.5.  5.04 Gateway Timeout

   Like HTTP 504 "Gateway Timeout".

5.9.3.6.  5.05 Proxying Not Supported

   The server is unable or unwilling to act as a forward-proxy for the
   URI specified in the Proxy-Uri Option (see Section 5.10.3).

5.10.  Option Definitions

   The individual CoAP options are summarized in Table 1 and explained
   below.

   +-----+---+---+---+---+----------------+--------+--------+----------+
   | No. | C | U | N | R | Name           | Format | Length | Default  |
   +-----+---+---+---+---+----------------+--------+--------+----------+
   |   1 | x |   |   | x | If-Match       | opaque | 0-8    | (none)   |
   |   3 | x | x |   |   | Uri-Host       | string | 1-255  | (see     |
   |     |   |   |   |   |                |        |        | below)   |
   |   4 |   |   |   | x | ETag           | opaque | 1-8    | (none)   |
   |   5 | x |   |   |   | If-None-Match  | empty  | 0      | (none)   |
   |   7 | x | x |   |   | Uri-Port       | uint   | 0-2    | (see     |
   |     |   |   |   |   |                |        |        | below)   |
   |   8 |   |   |   | x | Location-Path  | string | 0-255  | (none)   |
   |  11 | x | x |   | x | Uri-Path       | string | 0-255  | (none)   |
   |  12 |   |   |   |   | Content-Format | uint   | 0-2    | (none)   |
   |  14 |   | x |   |   | Max-Age        | uint   | 0-4    | 60       |
   |  15 | x | x |   | x | Uri-Query      | string | 1-255  | (none)   |
   |  16 |   |   |   | x | Accept         | uint   | 0-2    | (none)   |
   |  19 | x | x |   |   | Token          | opaque | 1-8    | (empty)  |
   |  20 |   |   |   | x | Location-Query | string | 0-255  | (none)   |
   |  35 | x | x |   |   | Proxy-Uri      | string | 1-1034 | (none)   |
   +-----+---+---+---+---+----------------+--------+--------+----------+

            C=Critical, U=Unsafe, N=No-Cache-Key, R=Repeatable

                             Table 1: Options

   Temporary Note:  At the time of submission, the great renumbering is
      not yet reflected in [I-D.ietf-core-block] and
      [I-D.ietf-core-observe].  The new numbers are: 6 for Observe, 23
      for Block2, 27 for Block1, and 28 for Size.  This note to be
      removed when the satellite drafts are updated.







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5.10.1.  Token

   The Token Option is used to match a response with a request.  Every
   request has a client-generated token which the server MUST echo in
   any response.  The token value is a sequence of 0 to 8 bytes.  A
   default value of the zero-length token is assumed in the absence of
   the option.  A value of 1 to 8 bytes can be sent as an option value.
   Thus when the token value is empty, the Token Option MUST be elided.

   A token is intended for use as a client-local identifier for
   differentiating between concurrent requests (see Section 5.3).  A
   client SHOULD generate tokens in a way that tokens currently in use
   for a given source/destination pair are unique.  An empty token value
   is appropriate e.g. when no other tokens are in use to a destination,
   or when requests are made serially per destination.  There are
   however multiple possible implementation strategies to fulfill this.
   An endpoint receiving a token MUST treat it as opaque and make no
   assumptions about its format.

5.10.2.  Uri-Host, Uri-Port, Uri-Path and Uri-Query

   The Uri-Host, Uri-Port, Uri-Path and Uri-Query Options are used to
   specify the target resource of a request to a CoAP origin server.
   The options encode the different components of the request URI in a
   way that no percent-encoding is visible in the option values and that
   the full URI can be reconstructed at any involved endpoint.  The
   syntax of CoAP URIs is defined in Section 6.

   The steps for parsing URIs into options is defined in Section 6.4.
   These steps result in zero or more Uri-Host, Uri-Port, Uri-Path and
   Uri-Query Options being included in a request, where each option
   holds the following values:

   o  the Uri-Host Option specifies the Internet host of the resource
      being requested,

   o  the Uri-Port Option specifies the transport layer port number of
      the resource,

   o  each Uri-Path Option specifies one segment of the absolute path to
      the resource, and

   o  each Uri-Query Option specifies one argument parameterizing the
      resource.

   Note: Fragments ([RFC3986], Section 3.5) are not part of the request
   URI and thus will not be transmitted in a CoAP request.




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   The default value of the Uri-Host Option is the IP literal
   representing the destination IP address of the request message.
   Likewise, the default value of the Uri-Port Option is the destination
   UDP port.  The default values for the Uri-Host and Uri-Port Options
   are sufficient for requests to most servers.  Explicit Uri-Host and
   Uri-Port Options are typically used when an endpoint hosts multiple
   virtual servers.

   The Uri-Path and Uri-Query Option can contain any character sequence.
   No percent-encoding is performed.  The value of a Uri-Path Option
   MUST NOT be "." or ".." (as the request URI must be resolved before
   parsing it into options).

   The steps for constructing the request URI from the options are
   defined in Section 6.5.  Note that an implementation does not
   necessarily have to construct the URI; it can simply look up the
   target resource by looking at the individual options.

   Examples can be found in Appendix B.

5.10.3.  Proxy-Uri

   The Proxy-Uri Option is used to make a request to a forward-proxy
   (see Section 5.7).  The forward-proxy is requested to forward the
   request or service it from a valid cache, and return the response.

   The option value is an absolute-URI ([RFC3986], Section 4.3).

   Note that the forward-proxy MAY forward the request on to another
   proxy or directly to the server specified by the absolute-URI.  In
   order to avoid request loops, a proxy MUST be able to recognize all
   of its server names, including any aliases, local variations, and the
   numeric IP addresses.

   An endpoint receiving a request with a Proxy-Uri Option that is
   unable or unwilling to act as a forward-proxy for the request MUST
   cause the return of a 5.05 (Proxying Not Supported) response.

   The Proxy-Uri Option MUST take precedence over any of the Uri-Host,
   Uri-Port, Uri-Path or Uri-Query options (which MUST NOT be included
   at the same time in a request containing the Proxy-Uri Option).

5.10.4.  Content-Format

   The Content-Format Option indicates the representation format of the
   message payload.  The representation format is given as a numeric
   content format identifier that is defined in the CoAP Content Format
   registry (Section 12.3).  In the absence of the option, no default



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   value is assumed, i.e. the representation format of any
   representation message payload is indeterminate (Section 5.5).

5.10.5.  Accept

   The CoAP Accept option indicates when included one or more times in a
   request, one or more Content-Formats, each of which is an acceptable
   Content-Format for the client, in the order of preference (most
   preferred first).  The representation format is given as a numeric
   Content-Format identifier that is defined in the CoAP Content-Format
   registry (Section 12.3).  If no Accept options are given, the client
   does not express a preference (thus no default value is assumed).
   The client prefers the representation returned by the server to be in
   one of the Content-Formats indicated.  The server SHOULD return one
   of the preferred Content-Formats if available.  If none of the
   preferred Content-Formats can be returned, then a 4.06 "Not
   Acceptable" SHOULD be sent as a response.

   Note that as a server might not support the Accept option (and thus
   would ignore it as it is elective), the client needs to be prepared
   to receive a representation in a different Content-Format.  The
   client can simply discard a representation it can not make use of.

5.10.6.  Max-Age

   The Max-Age Option indicates the maximum time a response may be
   cached before it MUST be considered not fresh (see Section 5.6.1).

   The option value is an integer number of seconds between 0 and
   2**32-1 inclusive (about 136.1 years).  A default value of 60 seconds
   is assumed in the absence of the option in a response.

   The value is intended to be current at the time of transmission.
   Servers that provide resources with strict tolerances on the value of
   Max-Age SHOULD update the value before each retransmission.  (See
   also Section 5.7.1.)

5.10.7.  ETag

   The ETag Option in a response provides the current value of the
   entity-tag for the enclosed representation of the target resource.

   An entity-tag is intended for use as a resource-local identifier for
   differentiating between representations of the same resource that
   vary over time.  It may be generated in any number of ways including
   a version, checksum, hash or time.  An endpoint receiving an entity-
   tag MUST treat it as opaque and make no assumptions about its format.
   (Endpoints generating an entity-tag are encouraged to use the most



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   compact representation possible, in particular in regards to clients
   and intermediaries that may want to store multiple ETag values.)

   An endpoint that has one or more representations previously obtained
   from the resource can specify the ETag Option in a request for each
   stored response to determine if any of those representations is
   current (see Section 5.6.2).

   The ETag Option MUST NOT occur more than once in a response, and MAY
   occur one or more times in a request.

5.10.8.  Location-Path and Location-Query

   The Location-Path and Location-Query Options together indicate a
   relative URI that consists either of an absolute path, a query string
   or both.  A combination of these options is included in a 2.01
   (Created) response to indicate the location of the resource created
   as the result of a POST request (see Section 5.8.2).  The location is
   resolved relative to the request URI.

   If a response with one or more Location-Path and/or Location-Query
   Options passes through a cache and the implied URI identifies one or
   more currently stored responses, those entries SHOULD be marked as
   not fresh.

   Each Location-Path Option specifies one segment of the absolute path
   to the resource, and each Location-Query Option specifies one
   argument parameterizing the resource.  The Location-Path and
   Location-Query Option can contain any character sequence.  No
   percent-encoding is performed.  The value of a Location-Path Option
   MUST NOT be "." or "..".

   The steps for constructing the location URI from the options are
   analogous to Section 6.5, except that the first five steps are
   skipped and the result is a relative URI-reference.

   More Location-* options may be defined in the future, and have been
   reserved option numbers 128, 132 and 136.  If any of these reserved
   option numbers occurs in addition to Location-Path and/or Location-
   Query and are not supported, then a 4.02 (Bad Option) error MUST be
   returned.

5.10.9.  If-Match

   The If-Match Option MAY be used to make a request conditional on the
   current existence or value of an ETag for one or more representations
   of the target resource.  If-Match is generally useful for resource
   update requests, such as PUT requests, as a means for protecting



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   against accidental overwrites when multiple clients are acting in
   parallel on the same resource (i.e., the "lost update" problem).

   The value of an If-Match option is either an ETag or the empty
   string.  An If-Match option with an ETag matches a representation
   with that exact ETag.  An If-Match option with an empty value matches
   any existing representation (i.e., it places the precondition on the
   existence of any current representation for the target resource).

   The If-Match Option can occur multiple times.  If any of the options
   match, then the server performs the request method as if the set of
   If-Match Options were not present.

   If there is one or more If-Match Option, but none of the options
   match, the server MUST NOT perform the requested method.  Instead,
   the server MUST respond with the 4.12 (Precondition Failed) response
   code.

   If the request would, without the If-Match Options, result in
   anything other than a 2.xx or 4.12 response code, then any If-Match
   Options MUST be ignored.

5.10.10.  If-None-Match

   The If-None-Match Option MAY be used to make a request conditional on
   the non-existence of the target resource.  If-None-Match is useful
   for resource creation requests, such as PUT requests, as a means for
   protecting against accidental overwrites when multiple clients are
   acting in parallel on the same resource.  The If-None-Match Option
   carries no value.

   If the target resource does exist, then the server MUST NOT perform
   the requested method.  Instead, the server MUST respond with the 4.12
   (Precondition Failed) response code.


6.  CoAP URIs

   CoAP uses the "coap" and "coaps" URI schemes for identifying CoAP
   resources and providing a means of locating the resource.  Resources
   are organized hierarchically and governed by a potential CoAP origin
   server listening for CoAP requests ("coap") or DTLS-secured CoAP
   requests ("coaps") on a given UDP port.  The CoAP server is
   identified via the generic syntax's authority component, which
   includes a host component and optional UDP port number.  The
   remainder of the URI is considered to be identifying a resource which
   can be operated on by the methods defined by the CoAP protocol.  The
   "coap" and "coaps" URI schemes can thus be compared to the "http" and



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   "https" URI schemes respectively.

   The syntax of the "coap" and "coaps" URI schemes is specified below
   in Augmented Backus-Naur Form (ABNF) [RFC5234].  The definitions of
   "host", "port", "path-abempty", "query", "segment", "IP-literal",
   "IPv4address" and "reg-name" are adopted from [RFC3986].

   Implementation Note:  Unfortunately, over time the URI format has
      acquired significant complexity.  Implementers are encouraged to
      examine [RFC3986] closely.  E.g., the ABNF for IPv6 addresses is
      more complicated than maybe expected.  Also, implementers should
      take care to perform the processing of percent decoding/encoding
      exactly once on the way from a URI to its decoded components or
      back.  Percent encoding is crucial for data transparency, but may
      lead to unusual results such as a slash in a path component.

6.1.  coap URI Scheme

   coap-URI = "coap:" "//" host [ ":" port ] path-abempty [ "?" query ]

   If the host component is provided as an IP-literal or IPv4address,
   then the CoAP server can be reached at that IP address.  If host is a
   registered name, then that name is considered an indirect identifier
   and the endpoint might use a name resolution service, such as DNS, to
   find the address of that host.  The host MUST NOT be empty; if a URI
   is received with a missing authority or an empty host, then it MUST
   be considered invalid.  The port subcomponent indicates the UDP port
   at which the CoAP server is located.  If it is empty or not given,
   then the default port 5683 is assumed.

   The path identifies a resource within the scope of the host and port.
   It consists of a sequence of path segments separated by a slash
   character (U+002F SOLIDUS "/").

   The query serves to further parameterize the resource.  It consists
   of a sequence of arguments separated by an ampersand character
   (U+0026 AMPERSAND "&").  An argument is often in the form of a
   "key=value" pair.

   The "coap" URI scheme supports the path prefix "/.well-known/"
   defined by [RFC5785] for "well-known locations" in the name-space of
   a host.  This enables discovery of policy or other information about
   a host ("site-wide metadata"), such as hosted resources (see
   Section 7).

   Application designers are encouraged to make use of short, but
   descriptive URIs.  As the environments that CoAP is used in are
   usually constrained for bandwidth and energy, the trade-off between



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   these two qualities should lean towards the shortness, without
   ignoring descriptiveness.

6.2.  coaps URI Scheme

   coaps-URI = "coaps:" "//" host [ ":" port ] path-abempty
               [ "?" query ]

   All of the requirements listed above for the "coap" scheme are also
   requirements for the "coaps" scheme, except that a default UDP port
   of [IANA_TBD_PORT] is assumed if the port subcomponent is empty or
   not given, and the UDP datagrams MUST be secured for privacy through
   the use of DTLS as described in Section 9.1.

   Unlike the "coap" scheme, responses to "coaps" identified requests
   are never "public" and thus MUST NOT be reused for shared caching
   unless the cache is able to make equivalent access control decisions
   to the ones that led to the cached entry (Section 11.2).  They can,
   however, be reused in a private cache if the message is cacheable by
   default in CoAP.

   Resources made available via the "coaps" scheme have no shared
   identity with the "coap" scheme even if their resource identifiers
   indicate the same authority (the same host listening to the same UDP
   port).  They are distinct name spaces and are considered to be
   distinct origin servers.

6.3.  Normalization and Comparison Rules

   Since the "coap" and "coaps" schemes conform to the URI generic
   syntax, such URIs are normalized and compared according to the
   algorithm defined in [RFC3986], Section 6, using the defaults
   described above for each scheme.

   If the port is equal to the default port for a scheme, the normal
   form is to elide the port subcomponent.  Likewise, an empty path
   component is equivalent to an absolute path of "/", so the normal
   form is to provide a path of "/" instead.  The scheme and host are
   case-insensitive and normally provided in lowercase; IP-literals are
   in recommended form [RFC5952]; all other components are compared in a
   case-sensitive manner.  Characters other than those in the "reserved"
   set are equivalent to their percent-encoded octets (see [RFC3986],
   Section 2.1): the normal form is to not encode them.

   For example, the following three URIs are equivalent, and cause the
   same options and option values to appear in the CoAP messages:





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   coap://example.com:5683/~sensors/temp.xml
   coap://EXAMPLE.com/%7Esensors/temp.xml
   coap://EXAMPLE.com:/%7esensors/temp.xml

6.4.  Decomposing URIs into Options

   The steps to parse a request's options from a string /url/ are as
   follows.  These steps either result in zero or more of the Uri-Host,
   Uri-Port, Uri-Path and Uri-Query Options being included in the
   request, or they fail.

   1.  If the /url/ string is not an absolute URI ([RFC3986]), then fail
       this algorithm.

   2.  Resolve the /url/ string using the process of reference
       resolution defined by [RFC3986], with the URL character encoding
       set to UTF-8 [RFC3629].

       NOTE: It doesn't matter what it is resolved relative to, since we
       already know it is an absolute URL at this point.

   3.  If /url/ does not have a <scheme> component whose value, when
       converted to ASCII lowercase, is "coap" or "coaps", then fail
       this algorithm.

   4.  If /url/ has a <fragment> component, then fail this algorithm.

   5.  If the <host> component of /url/ does not represent the request's
       destination IP address as an IP-literal or IPv4address, include a
       Uri-Host Option and let that option's value be the value of the
       <host> component of /url/, converted to ASCII lowercase, and then
       converting all percent-encodings ("%" followed by two hexadecimal
       digits) to the corresponding characters.

       NOTE: In the usual case where the request's destination IP
       address is derived from the host part, this ensures that a Uri-
       Host Option is only used for a <host> component of the form reg-
       name.

   6.  If /url/ has a <port> component, then let /port/ be that
       component's value interpreted as a decimal integer; otherwise,
       let /port/ be the default port for the scheme.

   7.  If /port/ does not equal the request's destination UDP port,
       include a Uri-Port Option and let that option's value be /port/.

   8.  If the value of the <path> component of /url/ is empty or
       consists of a single slash character (U+002F SOLIDUS "/"), then



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       move to the next step.

       Otherwise, for each segment in the <path> component, include a
       Uri-Path Option and let that option's value be the segment (not
       including the delimiting slash characters) after converting all
       percent-encodings ("%" followed by two hexadecimal digits) to the
       corresponding characters.

   9.  If /url/ has a <query> component, then, for each argument in the
       <query> component, include a Uri-Query Option and let that
       option's value be the argument (not including the question mark
       and the delimiting ampersand characters) after converting all
       percent-encodings to the corresponding characters.

   Note that these rules completely resolve any percent-encoding.

6.5.  Composing URIs from Options

   The steps to construct a URI from a request's options are as follows.
   These steps either result in a URI, or they fail.  In these steps,
   percent-encoding a character means replacing each of its (UTF-8
   encoded) bytes by a "%" character followed by two hexadecimal digits
   representing the byte, where the digits A-F are in upper case (as
   defined in [RFC3986] Section 2.1; to reduce variability, the
   hexadecimal notation for percent-encoding in CoAP URIs MUST use
   uppercase letters).  The definitions of "unreserved" and "sub-delims"
   are adopted from [RFC3986].

   1.   If the request is secured using DTLS, let /url/ be the string
        "coaps://".  Otherwise, let /url/ be the string "coap://".

   2.   If the request includes a Uri-Host Option, let /host/ be that
        option's value, where any non-ASCII characters are replaced by
        their corresponding percent-encoding.  If /host/ is not a valid
        reg-name or IP-literal or IPv4address, fail the algorithm.  If
        the request does not include a Uri-Host Option, let /host/ be
        the IP-literal (making use of the conventions of [RFC5952]) or
        IPv4address representing the request's destination IP address.

   3.   Append /host/ to /url/.

   4.   If the request includes a Uri-Port Option, let /port/ be that
        option's value.  Otherwise, let /port/ be the request's
        destination UDP port.

   5.   If /port/ is not the default port for the scheme, then append a
        single U+003A COLON character (:) followed by the decimal
        representation of /port/ to /url/.



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   6.   Let /resource name/ be the empty string.  For each Uri-Path
        Option in the request, append a single character U+002F SOLIDUS
        (/) followed by the option's value to /resource name/, after
        converting any character that is not either in the "unreserved"
        set, "sub-delims" set, a U+003A COLON (:) or U+0040 COMMERCIAL
        AT (@) character, to its percent-encoded form.

   7.   If /resource name/ is the empty string, set it to a single
        character U+002F SOLIDUS (/).

   8.   For each Uri-Query Option in the request, append a single
        character U+003F QUESTION MARK (?) (first option) or U+0026
        AMPERSAND (&) (subsequent options) followed by the option's
        value to /resource name/, after converting any character that is
        not either in the "unreserved" set, "sub-delims" set (except
        U+0026 AMPERSAND (&)), a U+003A COLON (:), U+0040 COMMERCIAL AT
        (@), U+002F SOLIDUS (/) or U+003F QUESTION MARK (?) character,
        to its percent-encoded form.

   9.   Append /resource name/ to /url/.

   10.  Return /url/.

   Note that these steps have been designed to lead to a URI in normal
   form (see Section 6.3).


7.  Discovery

7.1.  Service Discovery

   A server is discovered by a client by the client knowing or learning
   a URI that references a resource in the namespace of the server.
   Alternatively, clients can use Multicast CoAP (see Section 8) and the
   "All CoAP Nodes" multicast address to find CoAP servers.

   Unless the port subcomponent in a "coap" or "coaps" URI indicates the
   UDP port at which the CoAP server is located, the server is assumed
   to be reachable at the default port.

   The CoAP default port number 5683 MUST be supported by a server that
   offers resources for resource discovery (see Section 7.2 below) and
   SHOULD be supported for providing access to other resources.  The
   default port number [IANA_TBD_PORT] for DTLS-secured CoAP MAY be
   supported by a server for resource discovery and for providing access
   to other resources.  In addition other endpoints may be hosted at
   other ports, e.g. in the dynamic port space.




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   Implementation Note:  When a CoAP server is hosted by a 6LoWPAN node,
      header compression efficiency is improved when it also supports a
      port number in the 61616-61631 compressed UDP port space defined
      in [RFC4944] (note that, as its UDP port differs from the default
      port, it is a different endpoint from the server at the default
      port).

7.2.  Resource Discovery

   The discovery of resources offered by a CoAP endpoint is extremely
   important in machine-to-machine applications where there are no
   humans in the loop and static interfaces result in fragility.  A CoAP
   endpoint SHOULD support the CoRE Link Format of discoverable
   resources as described in [RFC6690].  It is up to the server which
   resources are made discoverable (if any).

7.2.1.  'ct' Attribute

   This section defines a new Web Linking [RFC5988] attribute for use
   with [RFC6690].  The Content-Format code "ct" attribute provides a
   hint about the Content-Formats this resource returns.  Note that this
   is only a hint, and does not override the Content-Format Option of a
   CoAP response obtained by actually requesting the representation of
   the resource.  The value is in the CoAP identifier code format as a
   decimal ASCII integer and MUST be in the range of 0-65535 (16-bit
   unsigned integer).  For example application/xml would be indicated as
   "ct=41".  If no Content-Format code attribute is present then nothing
   about the type can be assumed.  The Content-Format code attribute MAY
   include a space-separated sequence of Content-Format codes,
   indicating that multiple content-formats are available.  The syntax
   of the attribute value is summarized in the production ct-value in
   Figure 13, where cardinal, SP and DQUOTE are defined as in [RFC6690].

      ct-value =  cardinal
               /  DQUOTE cardinal *( 1*SP cardinal ) DQUOTE

                                 Figure 13


8.  Multicast CoAP

   CoAP supports making requests to a IP multicast group.  This is
   defined by a series of deltas to Unicast CoAP.

   CoAP endpoints that offer services that they want other endpoints to
   be able to find using multicast service discovery, join one or more
   of the appropriate all-CoAP-nodes multicast addresses Section 12.8
   and listen on the default CoAP port.  Note that an endpoint might



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   receive multicast requests on other multicast addresses, including
   the all-nodes IPv6 address (or via broadcast on IPv4); an endpoint
   MUST therefore be prepared to receive such messages but MAY ignore
   them if multicast service discovery is not desired.

8.1.  Messaging Layer

   A multicast request is characterized by being transported in a CoAP
   message that is addressed to an IP multicast address instead of a
   CoAP endpoint.  Such multicast requests MUST be Non-Confirmable.

   A server SHOULD be aware that a request arrived via multicast, e.g.
   by making use of modern APIs such as IPV6_RECVPKTINFO [RFC3542], if
   available.

   When a server is aware that a request arrived via multicast, it MUST
   NOT return a RST in reply to NON.  If it is not aware, it MAY return
   a RST in reply to NON as usual.  Because such a Reset message will
   look identical to an RST for a unicast message from the sender, the
   sender MUST avoid using a Message ID that is also still active from
   this endpoint with any unicast endpoint that might receive the
   multicast message.

8.2.  Request/Response Layer

   When a server is aware that a request arrived via multicast, the
   server MAY always pretend it did not receive the request, in
   particular if it doesn't have anything useful to respond (e.g., if it
   only has an empty payload or an error response).  The decision for
   this may depend on the application.  (For example, in [RFC6690] query
   filtering, a server should not respond to a multicast request if the
   filter does not match.)

   If a server does decide to respond to a multicast request, it should
   not respond immediately.  Instead, it should pick a duration for the
   period of time during which it intends to respond.  For purposes of
   this exposition, we call the length of this period the Leisure.  The
   specific value of this Leisure may depend on the application, or MAY
   be derived as described below.  The server SHOULD then pick a random
   point of time within the chosen Leisure period to send back the
   unicast response to the multicast request.  If further responses need
   to be sent based on the same multicast address membership, a new
   leisure period starts at the earliest after the previous one
   finishes.

   To compute a value for Leisure, the server should have a group size
   estimate G, a target data transfer rate R (which both should be
   chosen conservatively) and an estimated response size S; a rough



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   lower bound for Leisure can then be computed as
                           lb_Leisure = S * G / R

   E.g., for a multicast request with link-local scope on an 2.4 GHz
   IEEE 802.15.4 (6LoWPAN) network, G could be (relatively
   conservatively) set to 100, S to 100 bytes, and the target rate to a
   conservative 8 kbit/s = 1 kB/s.  The resulting lower bound for the
   Leisure is 10 seconds.

   If a CoAP endpoint does not have suitable data to compute a value for
   Leisure, it MAY resort to DEFAULT_LEISURE.

   When matching a response to a multicast request, only the token MUST
   match; the source endpoint of the response does not need to (and will
   not) be the same as the destination endpoint of the original request.

8.2.1.  Caching

   When a client makes a multicast request, it always makes a new
   request to the multicast group (since there may be new group members
   that joined meanwhile or ones that did not get the previous request).
   It MAY update the cache with the received responses.  Then it uses
   both cached-still-fresh and 'new' responses as the result of the
   request.

   A response received in reply to a GET request to a multicast group
   MAY be used to satisfy a subsequent request on the related unicast
   request URI.  The unicast request URI is obtained by replacing the
   authority part of the request URI with the transport layer source
   address of the response message.

   A cache MAY revalidate a response by making a GET request on the
   related unicast request URI.

   A GET request to a multicast group MUST NOT contain an ETag option.
   A mechanism to suppress responses the client already has is left for
   further study.

8.2.2.  Proxying

   When a forward-proxy receives a request with a Proxy-Uri that
   indicates a multicast address, the proxy obtains a set of responses
   as described above and sends all responses (both cached-still-fresh
   and new) back to the original client.







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9.  Securing CoAP

   This section defines the DTLS binding for CoAP, and the alternative
   use of IPsec.

   During the provisioning phase, a CoAP device is provided with the
   security information that it needs, including keying materials and
   access control lists.  This specification defines provisioning for
   the RawPublicKey mode in Section 9.1.3.2.1.  At the end of the
   provisioning phase, the device will be in one of four security modes
   with the following information for the given mode.  The NoSec and
   RawPublicKey modes are mandatory to implement for this specification.

   NoSec:  There is no protocol level security (DTLS is disabled).
      Alternative techniques to provide lower layer security SHOULD be
      used when appropriate.  The use of IPsec is discussed in
      Section 9.2.

   PreSharedKey:  DTLS is enabled and there is a list of pre-shared keys
      [RFC4279] and each key includes a list of which nodes it can be
      used to communicate with as described in Section 9.1.3.1.  At the
      extreme there may be one key for each node this CoAP node needs to
      communicate with (1:1 node/key ratio).

   RawPublicKey:  DTLS is enabled and the device has an asymmetric key
      pair without a certificate (a raw public key) that is validated
      using an out-of-band mechanism [I-D.ietf-tls-oob-pubkey] as
      described in Section 9.1.3.2.  The device also has an identity
      calculated from the public key and a list of identities of the
      nodes it can communicate with.

   Certificate:  DTLS is enabled and the device has an asymmetric key
      pair with an X.509 certificate [RFC5280] that binds it to its
      Authority Name and is signed by some common trust root as
      described in Section 9.1.3.3.  The device also has a list of root
      trust anchors that can be used for validating a certificate.

   In the "NoSec" mode, the system simply sends the packets over normal
   UDP over IP and is indicated by the "coap" scheme and the CoAP
   default port.  The system is secured only by keeping attackers from
   being able to send or receive packets from the network with the CoAP
   nodes; see Section 11.5 for an additional complication with this
   approach.

   The other three security modes are achieved using DTLS and are
   indicated by the "coaps" scheme and DTLS-secured CoAP default port.
   The result is a security association that can be used to authenticate
   (within the limits of the security model) and, based on this



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   authentication, authorize the communication partner.  CoAP itself
   does not provide protocol primitives for authentication or
   authorization; where this is required, it can either be provided by
   communication security (i.e., IPsec or DTLS) or by object security
   (within the payload).  Devices that require authorization for certain
   operations are expected to require one of these two forms of
   security.  Necessarily, where an intermediary is involved,
   communication security only works when that intermediary is part of
   the trust relationships; CoAP does not provide a way to forward
   different levels of authorization that clients may have with an
   intermediary to further intermediaries or origin servers -- it
   therefore may be required to perform all authorization at the first
   intermediary.

9.1.  DTLS-secured CoAP

   Just as HTTP is secured using Transport Layer Security (TLS) over
   TCP, CoAP is secured using Datagram TLS (DTLS) [RFC6347] over UDP
   (see Figure 14).  This section defines the CoAP binding to DTLS,
   along with the minimal mandatory-to-implement configurations
   appropriate for constrained environments.  The binding is defined by
   a series of deltas to Unicast CoAP.  DTLS is in practice TLS with
   added features to deal with the unreliable nature of the UDP
   transport.

                          +----------------------+
                          |      Application     |
                          +----------------------+
                          +----------------------+
                          |  Requests/Responses  |
                          |----------------------|  CoAP
                          |       Messages       |
                          +----------------------+
                          +----------------------+
                          |         DTLS         |
                          +----------------------+
                          +----------------------+
                          |          UDP         |
                          +----------------------+

             Figure 14: Abstract layering of DTLS-secured CoAP

   In some constrained nodes (limited flash and/or RAM) and networks
   (limited bandwidth or high scalability requirements), and depending
   on the specific cipher suites in use, all modes of DTLS may not be
   applicable.  Some DTLS cipher suites can add significant
   implementation complexity as well as some initial handshake overhead
   needed when setting up the security association.  Once the initial



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   handshake is completed, DTLS adds a limited per-datagram overhead of
   approximately 13 bytes, not including any initialization vectors/
   nonces (e.g., 8 bytes with TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]),
   integrity check values (e.g., 8 bytes with TLS_PSK_WITH_AES_128_CCM_8
   [RFC6655]) and padding required by the cipher suite.  Whether and
   which mode of using DTLS is applicable for a CoAP-based application
   should be carefully weighed considering the specific cipher suites
   that may be applicable, and whether the session maintenance makes it
   compatible with application flows and sufficient resources are
   available on the constrained nodes and for the added network
   overhead.  DTLS is not applicable to group keying (multicast
   communication); however, it may be a component in a future group key
   management protocol.

9.1.1.  Messaging Layer

   The endpoint acting as the CoAP client should also act as the DTLS
   client.  It should initiate a session to the server on the
   appropriate port.  When the DTLS handshake has finished, the client
   may initiate the first CoAP request.  All CoAP messages MUST be sent
   as DTLS "application data".

   The following rules are added for matching an ACK or RST to a CON
   message or a RST to a NON message: The DTLS session MUST be the same
   and the epoch MUST be the same.

   A message is the same when it is sent within the same DTLS session
   and same epoch and has the same Message ID.

   Note: When a confirmable message is retransmitted, a new DTLS
   sequence_number is used for each attempt, even though the CoAP
   Message ID stays the same.  So a recipient still has to perform
   deduplication as described in Section 4.5.  Retransmissions MUST NOT
   be performed across epochs.

   DTLS connections in RawPublicKey and Certificate mode are set up
   using mutual authentication so they can remain up and be reused for
   future message exchanges in either direction.  Devices can close a
   DTLS connection when they need to recover resources but in general
   they should keep the connection up for as long as possible.  Closing
   the DTLS connection after every CoAP message exchange is very
   inefficient.

9.1.2.  Request/Response Layer

   The following rules are added for matching a response to a request:
   The DTLS session MUST be the same and the epoch MUST be the same.




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9.1.3.  Endpoint Identity

   Devices SHOULD support the Server Name Indication (SNI) to indicate
   their Authority Name in the SNI HostName field as defined in Section
   3 of [RFC6066].  This is needed so that when a host that acts as a
   virtual server for multiple Authorities receives a new DTLS
   connection, it knows which keys to use for the DTLS session.

9.1.3.1.  Pre-Shared Keys

   When forming a connection to a new node, the system selects an
   appropriate key based on which nodes it is trying to reach and then
   forms a DTLS session using a PSK (Pre-Shared Key) mode of DTLS.
   Implementations in these modes MUST support the mandatory to
   implement cipher suite TLS_PSK_WITH_AES_128_CCM_8 as specified in
   [RFC6655].

   The security considerations of [RFC4279] (Section 7) apply.  In
   particular, applications should carefully weigh whether they need
   Perfect Forward Secrecy (PFS) or not and select an appropriate cipher
   suite (7.1).  The entropy of the PSK must be sufficient to mitigate
   against brute-force and (where the PSK is not chosen randomly but by
   a human) dictionary attacks (7.2).  The cleartext communication of
   client identities may leak data or compromise privacy (7.3).

9.1.3.2.  Raw Public Key Certificates

   In this mode the device has an asymmetric key pair but without an
   X.509 certificate (called a raw public key).  A device MAY be
   configured with multiple raw public keys.  The type and length of the
   raw public key depends on the cipher suite used.  Implementations in
   RawPublicKey mode MUST support the mandatory to implement cipher
   suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as specified in
   [I-D.mcgrew-tls-aes-ccm-ecc], [RFC5246], [RFC4492].  The mechanism
   for using raw public keys with TLS is specified in
   [I-D.ietf-tls-oob-pubkey].

9.1.3.2.1.  Provisioning

   The RawPublicKey mode was designed to be easily provisioned in M2M
   deployments.  It is assumed that each device has an appropriate
   asymmetric public key pair installed.  An identifier is calculated
   from the public key as described in Section 2 of
   [I-D.farrell-decade-ni].  All implementations that support checking
   RawPublicKey identities MUST support at least the sha-256-120 mode
   (SHA-256 truncated to 120 bits).  Implementations SHOULD support also
   longer length identifiers and MAY support shorter lengths.  Note that
   the shorter lengths provide less security against attacks and their



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   use is NOT RECOMMENDED.

   Depending on how identifiers are given to the system that verifies
   them, support for URI, binary, and/or human-speakable format
   [I-D.farrell-decade-ni] needs to be implemented.  All implementations
   SHOULD support the binary mode and implementations that have a user
   interface SHOULD also support the human-speakable format.

   During provisioning, the identifier of each node is collected, for
   example by reading a barcode on the outside of the device or by
   obtaining a pre-compiled list of the identifiers.  These identifiers
   are then installed in the corresponding endpoint, for example an M2M
   data collection server.  The identifier is used for two purposes, to
   associate the endpoint with further device information and to perform
   access control.  During provisioning, an access control list of
   identifiers the device may start DTLS sessions with SHOULD also be
   installed.

9.1.3.3.  X.509 Certificates

   Implementations in Certificate Mode MUST support the mandatory to
   implement cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as
   specified in [RFC5246].

   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 [EUI64].  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.

   When a new connection is formed, the certificate from the remote
   device needs to be verified.  If the CoAP node has a source of
   absolute time, then the node SHOULD check that the validity dates of
   the certificate are within range.  The certificate MUST 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 field 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 [RFC4279] SHOULD be used.



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9.2.  Using CoAP with IPsec

   One mechanism to secure CoAP in constrained environments is the IPsec
   Encapsulating Security Payload (ESP) [RFC4303] when CoAP is used
   without DTLS in NoSec Mode.  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, some 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, particularly on more common IEEE 802.15.4
   hardware that supports AES encryption but not decryption, 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.

   Necessarily for AES-CCM, but much preferably also for AES-CBC, static
   keying should be avoided and the initial keying material be derived
   into transient session keys, e.g. using a low-overhead mode of IKEv2
   [RFC5996] as described in [I-D.kivinen-ipsecme-ikev2-minimal]; such a
   protocol for managing keys and sequence numbers is also the only way
   to achieve anti-replay capabilities.  However, no recommendation can
   be made at this point on how to manage group keys (i.e., for
   multicast) in a constrained environment.  Once any initial setup is
   completed, IPsec ESP adds a limited overhead of approximately 10
   bytes per packet, not including initialization vectors, integrity
   check values and padding required by the cipher suite.

   When using IPsec to secure CoAP, both authentication and
   confidentiality SHOULD be applied as recommended in [RFC4303].  The
   use of IPsec between CoAP endpoints 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
   in full PC operating systems or on back-end 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.  Cross-Protocol Proxying between CoAP and HTTP

   CoAP supports a limited subset of HTTP functionality, and thus cross-
   protocol proxying to HTTP is straightforward.  There might be several
   reasons for proxying between CoAP and HTTP, for example when
   designing a web interface for use over either protocol or when



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   realizing a CoAP-HTTP proxy.  Likewise, CoAP could equally be proxied
   to other protocols such as XMPP [RFC6120] or SIP [RFC3264]; the
   definition of these mechanisms is out of scope of this specification.

   There are two possible directions to access a resource via a forward-
   proxy:

   CoAP-HTTP Proxying:  Enables CoAP clients to access resources on HTTP
      servers through an intermediary.  This is initiated by including
      the Proxy-Uri Option with an "http" or "https" URI in a CoAP
      request to a CoAP-HTTP proxy.

   HTTP-CoAP Proxying:  Enables HTTP clients to access resources on CoAP
      servers through an intermediary.  This is initiated by specifying
      a "coap" or "coaps" URI in the Request-Line of an HTTP request to
      an HTTP-CoAP proxy.

   Either way, only the Request/Response model of CoAP is mapped to
   HTTP.  The underlying model of confirmable or non-confirmable
   messages, etc., is invisible and MUST have no effect on a proxy
   function.  The following sections describe the handling of requests
   to a forward-proxy.  Reverse proxies are not specified as the proxy
   function is transparent to the client with the proxy acting as if it
   was the origin server.

10.1.  CoAP-HTTP Proxying

   If a request contains a Proxy-URI Option with an 'http' or 'https'
   URI [RFC2616], then the receiving CoAP endpoint (called "the proxy"
   henceforth) is requested to perform the operation specified by the
   request method on the indicated HTTP resource and return the result
   to the client.

   This section specifies for any CoAP request the CoAP response that
   the proxy should return to the client.  How the proxy actually
   satisfies the request is an implementation detail, although the
   typical case is expected to be the proxy translating and forwarding
   the request to an HTTP origin server.

   Since HTTP and CoAP share the basic set of request methods,
   performing a CoAP request on an HTTP resource is not so different
   from performing it on a CoAP resource.  The meanings of the
   individual CoAP methods when performed on HTTP resources are
   explained below.

   If the proxy is unable or unwilling to service a request with an HTTP
   URI, a 5.05 (Proxying Not Supported) response is returned to the
   client.  If the proxy services the request by interacting with a



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   third party (such as the HTTP origin server) and is unable to obtain
   a result within a reasonable time frame, a 5.04 (Gateway Timeout)
   response is returned; if a result can be obtained but is not
   understood, a 5.02 (Bad Gateway) response is returned.

10.1.1.  GET

   The GET method requests the proxy to return a representation of the
   HTTP resource identified by the request URI.

   Upon success, a 2.05 (Content) response code SHOULD be returned.  The
   payload of the response MUST be a representation of the target HTTP
   resource, and the Content-Format Option be set accordingly.  The
   response MUST indicate a Max-Age value that is no greater than the
   remaining time the representation can be considered fresh.  If the
   HTTP entity has an entity tag, the proxy SHOULD include an ETag
   Option in the response and process ETag Options in requests as
   described below.

   A client can influence the processing of a GET request by including
   the following option:

   Accept:  The request MAY include one or more Accept Options,
      identifying the preferred response content-format.

   ETag:  The request MAY include one or more ETag Options, identifying
      responses that the client has stored.  This requests the proxy to
      send a 2.03 (Valid) response whenever it would send a 2.05
      (Content) response with an entity tag in the requested set
      otherwise.  Note that CoAP ETags are always strong ETags in the
      HTTP sense; CoAP does not have the equivalent of HTTP weak ETags,
      and there is no good way to make use of these in a cross-proxy.

10.1.2.  PUT

   The PUT method requests the proxy to update or create the HTTP
   resource identified by the request URI with the enclosed
   representation.

   If a new resource is created at the request URI, a 2.01 (Created)
   response MUST be returned to the client.  If an existing resource is
   modified, a 2.04 (Changed) response MUST be returned to indicate
   successful completion of the request.

10.1.3.  DELETE

   The DELETE method requests the proxy to delete the HTTP resource
   identified by the request URI at the HTTP origin server.



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   A 2.02 (Deleted) response MUST be returned to client upon success or
   if the resource does not exist at the time of the request.

10.1.4.  POST

   The POST method requests the proxy to have the representation
   enclosed in the request be processed by the HTTP origin server.  The
   actual function performed by the POST method is determined by the
   origin server and dependent on the resource identified by the request
   URI.

   If the action performed by the POST method does not result in a
   resource that can be identified by a URI, a 2.04 (Changed) response
   MUST be returned to the client.  If a resource has been created on
   the origin server, a 2.01 (Created) response MUST be returned.

10.2.  HTTP-CoAP Proxying

   If an HTTP request contains a Request-URI with a 'coap' or 'coaps'
   URI, then the receiving HTTP endpoint (called "the proxy" henceforth)
   is requested to perform the operation specified by the request method
   on the indicated CoAP resource and return the result to the client.

   This section specifies for any HTTP request the HTTP response that
   the proxy should return to the client.  How the proxy actually
   satisfies the request is an implementation detail, although the
   typical case is expected to be the proxy translating and forwarding
   the request to a CoAP origin server.  The meanings of the individual
   HTTP methods when performed on CoAP resources are explained below.

   If the proxy is unable or unwilling to service a request with a CoAP
   URI, a 501 (Not Implemented) response SHOULD be returned to the
   client.  If the proxy services the request by interacting with a
   third party (such as the CoAP origin server) and is unable to obtain
   a result within a reasonable time frame, a 504 (Gateway Timeout)
   response SHOULD be returned; if a result can be obtained but is not
   understood, a 502 (Bad Gateway) response SHOULD be returned.

10.2.1.  OPTIONS and TRACE

   As the OPTIONS and TRACE methods are not supported in CoAP a 501 (Not
   Implemented) error MUST be returned to the client.

10.2.2.  GET

   The GET method requests the proxy to return a representation of the
   CoAP resource identified by the Request-URI.




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   Upon success, a 200 (OK) response SHOULD be returned.  The payload of
   the response MUST be a representation of the target CoAP resource,
   and the Content-Type and Content-Encoding header fields be set
   accordingly.  The response MUST indicate a max-age directive that
   indicates a value no greater than the remaining time the
   representation can be considered fresh.  If the CoAP response has an
   ETag option, the proxy SHOULD include an ETag header field in the
   response.

   A client can influence the processing of a GET request by including
   the following options:

   Accept:  Each individual Media-type of the HTTP Accept header in a
      request is mapped to a CoAP Accept option.  HTTP Accept Media-type
      ranges, parameters and extensions are not supported by the CoAP
      Accept option.  If the proxy cannot send a response which is
      acceptable according to the combined Accept field value, then the
      proxy SHOULD send a 406 (not acceptable) response.

   Conditional GETs:  Conditional HTTP GET requests that include an "If-
      Match" or "If-None-Match" request-header field can be mapped to a
      corresponding CoAP request.  The "If-Modified-Since" and "If-
      Unmodified-Since" request-header fields are not directly supported
      by CoAP, but SHOULD be implemented locally by a caching proxy.

10.2.3.  HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   return a message-body in the response.

   Although there is no direct equivalent of HTTP's HEAD method in CoAP,
   an HTTP-CoAP proxy responds to HEAD requests for CoAP resources, and
   the HTTP headers are returned without a message-body.

   Implementation Note:  An HTTP-CoAP proxy may want to try using a
      block-wise transfer [I-D.ietf-core-block] option to minimize the
      amount of data actually transferred, but needs to be prepared for
      the case that the origin server does not support block-wise
      transfers.

10.2.4.  POST

   The POST method requests the proxy to have the representation
   enclosed in the request be processed by the CoAP origin server.  The
   actual function performed by the POST method is determined by the
   origin server and dependent on the resource identified by the request
   URI.




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   If the action performed by the POST method does not result in a
   resource that can be identified by a URI, a 200 (OK) or 204 (No
   Content) response MUST be returned to the client.  If a resource has
   been created on the origin server, a 201 (Created) response MUST be
   returned.

   If any of the Location-* Options are present in the CoAP response, a
   Location header field constructed from the values of these options
   SHOULD be returned.

10.2.5.  PUT

   The PUT method requests the proxy to update or create the CoAP
   resource identified by the Request-URI with the enclosed
   representation.

   If a new resource is created at the Request-URI, a 201 (Created)
   response MUST be returned to the client.  If an existing resource is
   modified, either the 200 (OK) or 204 (No Content) response codes
   SHOULD be sent to indicate successful completion of the request.

10.2.6.  DELETE

   The DELETE method requests the proxy to delete the CoAP resource
   identified by the Request-URI at the CoAP origin server.

   A successful response SHOULD be 200 (OK) if the response includes an
   entity describing the status or 204 (No Content) if the action has
   been enacted but the response does not include an entity.

10.2.7.  CONNECT

   This method can not currently be satisfied by an HTTP-CoAP proxy
   function as TLS to DTLS tunneling has not yet been specified.  For
   now, a 501 (Not Implemented) error SHOULD be returned to the client.


11.  Security Considerations

   This section analyzes the possible threats to the protocol.  It is
   meant to inform protocol and application developers about the
   security limitations of CoAP as described in this document.  As CoAP
   realizes a subset of the features in HTTP/1.1, the security
   considerations in Section 15 of [RFC2616] are also pertinent to CoAP.
   This section concentrates on describing limitations specific to CoAP.






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11.1.  Protocol Parsing, Processing URIs

   A network-facing application can exhibit vulnerabilities in its
   processing logic for incoming packets.  Complex parsers are well-
   known as a likely source of such vulnerabilities, such as the ability
   to remotely crash a node, or even remotely execute arbitrary code on
   it.  CoAP attempts to narrow the opportunities for introducing such
   vulnerabilities by reducing parser complexity, by giving the entire
   range of encodable values a meaning where possible, and by
   aggressively reducing complexity that is often caused by unnecessary
   choice between multiple representations that mean the same thing.
   Much of the URI processing has been moved to the clients, further
   reducing the opportunities for introducing vulnerabilities into the
   servers.  Even so, the URI processing code in CoAP implementations is
   likely to be a large source of remaining vulnerabilities and should
   be implemented with special care.  The most complex parser remaining
   could be the one for the CoRE Link Format, although this also has
   been designed with a goal of reduced implementation complexity
   [RFC6690].  (See also section 15.2 of [RFC2616].)

11.2.  Proxying and Caching

   As mentioned in 15.7 of [RFC2616], proxies are by their very nature
   men-in-the-middle, breaking any IPsec or DTLS protection that a
   direct CoAP message exchange might have.  They are therefore
   interesting targets for breaking confidentiality or integrity of CoAP
   message exchanges.  As noted in [RFC2616], they are also interesting
   targets for breaking availability.

   The threat to confidentiality and integrity of request/response data
   is amplified where proxies also cache.  Note that CoAP does not
   define any of the cache-suppressing Cache-Control options that
   HTTP/1.1 provides to better protect sensitive data.

   For a caching implementation, any access control considerations that
   would apply to making the request that generated the cache entry also
   need to be applied to the value in the cache.  This is relevant for
   clients that implement multiple security domains, as well as for
   proxies that may serve multiple clients.  Also, a caching proxy MUST
   NOT make cached values available to requests that have lesser
   transport security properties than to which it would make available
   the process of forwarding the request in the first place.

   Finally, a proxy that fans out Separate Responses (as opposed to
   Piggy-backed Responses) to multiple original requesters may provide
   additional amplification (see below).





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11.3.  Risk of amplification

   CoAP servers generally reply to a request packet with a response
   packet.  This response packet may be significantly larger than the
   request packet.  An attacker might use CoAP nodes to turn a small
   attack packet into a larger attack packet, an approach known as
   amplification.  There is therefore a danger that CoAP nodes could
   become implicated in denial of service (DoS) attacks by using the
   amplifying properties of the protocol: An attacker that is attempting
   to overload a victim but is limited in the amount of traffic it can
   generate, can use amplification to generate a larger amount of
   traffic.

   This is particularly a problem in nodes that enable NoSec access,
   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 networks will only be able
   to generate a small amount of traffic, which may make CoAP nodes less
   attractive for this attack.  However, the limited capacity of the
   constrained network makes the network itself a likely victim of an
   amplification attack.

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



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   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.

11.4.  IP Address Spoofing Attacks

   Due to the lack of a handshake in UDP, a rogue endpoint which is free
   to read and write messages carried by the constrained network (i.e.
   NoSec or PreSharedKey deployments with nodes/key ratio > 1:1), may
   easily attack a single endpoint, a group of endpoints, as well as the
   whole network e.g. by:

   1.  spoofing RST in response to a CON or NON message, thus making an
       endpoint "deaf"; or

   2.  spoofing the entire response with forged payload/options (this
       has different levels of impact: from single response disruption,
       to much bolder attacks on the supporting infrastructure, e.g.
       poisoning proxy caches, or tricking validation / lookup
       interfaces in resource directories and, more generally, any
       component that stores global network state and uses CoAP as the
       messaging facility to handle state set/update's is a potential
       target.); or

   3.  spoofing a multicast request for a target node which may result
       in both network congestion/collapse and victim DoS'ing / forced
       wakeup from sleeping; or

   4.  spoofing observe messages, etc.

   In principle, spoofing can be detected by CoAP only in case CON
   semantics is used, because of unexpected ACK/RSTs coming from the
   deceived endpoint.  But this imposes keeping track of the used
   Message IDs which is not always possible, and moreover detection
   becomes available usually after the damage is already done.  This
   kind of attack can be prevented using security modes other than
   NoSec.

11.5.  Cross-Protocol Attacks

   The ability to incite a CoAP endpoint to send packets to a fake
   source address can be used not only for amplification, but also for
   cross-protocol attacks against a victim listening to UDP packets at a
   given address (IP address and port):




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   o  the attacker sends a message to a CoAP endpoint with the given
      address as the fake source address,

   o  the CoAP endpoint replies with a message to the given source
      address,

   o  the victim at the given address receives a UDP packet that it
      interprets according to the rules of a different protocol.

   This may be used to circumvent firewall rules that prevent direct
   communication from the attacker to the victim, but happen to allow
   communication from the CoAP endpoint (which may also host a valid
   role in the other protocol) to the victim.

   Also, CoAP endpoints may be the victim of a cross-protocol attack
   generated through an endpoint of another UDP-based protocol such as
   DNS.  In both cases, attacks are possible if the security properties
   of the endpoints rely on checking IP addresses (and firewalling off
   direct attacks sent from outside using fake IP addresses).  In
   general, because of their lack of context, UDP-based protocols are
   relatively easy targets for cross-protocol attacks.

   Finally, CoAP URIs transported by other means could be used to incite
   clients to send messages to endpoints of other protocols.

   One mitigation against cross-protocol attacks is strict checking of
   the syntax of packets received, combined with sufficient difference
   in syntax.  As an example, it might help if it were difficult to
   incite a DNS server to send a DNS response that would pass the checks
   of a CoAP endpoint.  Unfortunately, the first two bytes of a DNS
   reply are an ID that can be chosen by the attacker, which map into
   the interesting part of the CoAP header, and the next two bytes are
   then interpreted as CoAP's Message ID (i.e., any value is
   acceptable).  The DNS count words may be interpreted as multiple
   instances of a (non-existent, but elective) CoAP option 0.  The
   echoed query finally may be manufactured by the attacker to achieve a
   desired effect on the CoAP endpoint; the response added by the server
   (if any) might then just be interpreted as added payload.













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                                   1  1  1  1  1  1
     0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |                      ID                       | T, OC, code
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   | message id
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |                    QDCOUNT                    | (options 0)
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |                    ANCOUNT                    | (options 0)
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |                    NSCOUNT                    | (options 0)
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   |                    ARCOUNT                    | (options 0)
   +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                  Figure 15: DNS Header vs. CoAP Message

   In general, for any pair of protocols, one of the protocols can very
   well have been designed in a way that enables an attacker to cause
   the generation of replies that look like messages of the other
   protocol.  It is often much harder to ensure or prove the absence of
   viable attacks than to generate examples that may not yet completely
   enable an attack but might be further developed by more creative
   minds.  Cross-protocol attacks can therefore only be completely
   mitigated if endpoints don't authorize actions desired by an attacker
   just based on trusting the source IP address of a packet.
   Conversely, a NoSec environment that completely relies on a firewall
   for CoAP security not only needs to firewall off the CoAP endpoints
   but also all other endpoints that might be incited to send UDP
   messages to CoAP endpoints using some other UDP-based protocol.

   In addition to the considerations above, the security considerations
   for DTLS with respect to cross-protocol attacks apply.  E.g., if the
   same DTLS security association ("connection") is used to carry data
   of multiple protocols, DTLS no longer provides protection against
   cross-protocol attacks between these protocols.


12.  IANA Considerations

12.1.  CoAP Code Registry

   This document defines a registry for the values of the Code field in
   the CoAP header.  The name of the registry is "CoAP Codes".

   All values are assigned by sub-registries according to the following
   ranges:



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   0         Indicates an empty message (see Section 4.1).

   1-31      Indicates a request.  Values in this range are assigned by
             the "CoAP Method Codes" sub-registry (see Section 12.1.1).

   32-63     Reserved

   64-191    Indicates a response.  Values in this range are assigned by
             the "CoAP Response Codes" sub-registry (see
             Section 12.1.2).

   192-255   Reserved

12.1.1.  Method Codes

   The name of the sub-registry is "CoAP Method Codes".

   Each entry in the sub-registry must include the Method Code in the
   range 1-31, the name of the method, and a reference to the method's
   documentation.

   Initial entries in this sub-registry are as follows:

                       +------+--------+-----------+
                       | Code | Name   | Reference |
                       +------+--------+-----------+
                       |    1 | GET    | [RFCXXXX] |
                       |    2 | POST   | [RFCXXXX] |
                       |    3 | PUT    | [RFCXXXX] |
                       |    4 | DELETE | [RFCXXXX] |
                       +------+--------+-----------+

                        Table 2: CoAP Method Codes

   All other Method Codes are Unassigned.

   The IANA policy for future additions to this registry is "IETF
   Review" as described in [RFC5226].

   The documentation of a method code should specify the semantics of a
   request with that code, including the following properties:

   o  The response codes the method returns in the success case.

   o  Whether the method is idempotent, safe, or both.






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12.1.2.  Response Codes

   The name of the sub-registry is "CoAP Response Codes".

   Each entry in the sub-registry must include the Response Code in the
   range 64-191, a description of the Response Code, and a reference to
   the Response Code's documentation.

   Initial entries in this sub-registry are as follows:

          +------+---------------------------------+-----------+
          | Code | Description                     | Reference |
          +------+---------------------------------+-----------+
          |   65 | 2.01 Created                    | [RFCXXXX] |
          |   66 | 2.02 Deleted                    | [RFCXXXX] |
          |   67 | 2.03 Valid                      | [RFCXXXX] |
          |   68 | 2.04 Changed                    | [RFCXXXX] |
          |   69 | 2.05 Content                    | [RFCXXXX] |
          |  128 | 4.00 Bad Request                | [RFCXXXX] |
          |  129 | 4.01 Unauthorized               | [RFCXXXX] |
          |  130 | 4.02 Bad Option                 | [RFCXXXX] |
          |  131 | 4.03 Forbidden                  | [RFCXXXX] |
          |  132 | 4.04 Not Found                  | [RFCXXXX] |
          |  133 | 4.05 Method Not Allowed         | [RFCXXXX] |
          |  134 | 4.06 Not Acceptable             | [RFCXXXX] |
          |  140 | 4.12 Precondition Failed        | [RFCXXXX] |
          |  141 | 4.13 Request Entity Too Large   | [RFCXXXX] |
          |  143 | 4.15 Unsupported Content-Format | [RFCXXXX] |
          |  160 | 5.00 Internal Server Error      | [RFCXXXX] |
          |  161 | 5.01 Not Implemented            | [RFCXXXX] |
          |  162 | 5.02 Bad Gateway                | [RFCXXXX] |
          |  163 | 5.03 Service Unavailable        | [RFCXXXX] |
          |  164 | 5.04 Gateway Timeout            | [RFCXXXX] |
          |  165 | 5.05 Proxying Not Supported     | [RFCXXXX] |
          +------+---------------------------------+-----------+

                       Table 3: CoAP Response Codes

   The Response Codes 96-127 are Reserved for future use.  All other
   Response Codes are Unassigned.

   The IANA policy for future additions to this registry is "IETF
   Review" as described in [RFC5226].

   The documentation of a response code should specify the semantics of
   a response with that code, including the following properties:





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   o  The methods the response code applies to.

   o  Whether payload is required, optional or not allowed.

   o  The semantics of the payload.  For example, the payload of a 2.05
      (Content) response is a representation of the target resource; the
      payload in an error response is a human-readable diagnostic
      message.

   o  The format of the payload.  For example, the format in a 2.05
      (Content) response is indicated by the Content-Format Option; the
      format of the payload in an error response is always Net-Unicode
      text.

   o  Whether the response is cacheable according to the freshness
      model.

   o  Whether the response is validatable according to the validation
      model.

   o  Whether the response causes a cache to mark responses stored for
      the request URI as not fresh.

12.2.  Option Number Registry

   This document defines a registry for the Option Numbers used in CoAP
   options.  The name of the registry is "CoAP Option Numbers".

   Each entry in the registry must include the Option Number, the name
   of the option and a reference to the option's documentation.

   Initial entries in this registry are as follows:



















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                  +--------+----------------+-----------+
                  | Number | Name           | Reference |
                  +--------+----------------+-----------+
                  |      0 | (Reserved)     |           |
                  |      1 | If-Match       | [RFCXXXX] |
                  |      3 | Uri-Host       | [RFCXXXX] |
                  |      4 | ETag           | [RFCXXXX] |
                  |      5 | If-None-Match  | [RFCXXXX] |
                  |      7 | Uri-Port       | [RFCXXXX] |
                  |      8 | Location-Path  | [RFCXXXX] |
                  |     11 | Uri-Path       | [RFCXXXX] |
                  |     12 | Content-Format | [RFCXXXX] |
                  |     14 | Max-Age        | [RFCXXXX] |
                  |     15 | Uri-Query      | [RFCXXXX] |
                  |     16 | Accept         | [RFCXXXX] |
                  |     19 | Token          | [RFCXXXX] |
                  |     20 | Location-Query | [RFCXXXX] |
                  |     35 | Proxy-Uri      | [RFCXXXX] |
                  |    128 | (Reserved)     | [RFCXXXX] |
                  |    132 | (Reserved)     | [RFCXXXX] |
                  |    136 | (Reserved)     | [RFCXXXX] |
                  +--------+----------------+-----------+

                       Table 4: CoAP Option Numbers

   The IANA policy for future additions to this registry is split into
   three tiers as follows.  The range of 0..255 is reserved for options
   defined by the IETF (IETF Review).  The range of 256..2047 is
   reserved for commonly used options with public specifications
   (Specification Required).  The range of 2048..65535 is for all other
   options including private or vendor specific ones, which undergo a
   Designated Expert review to help ensure that the option semantics are
   defined correctly.

                +---------------+------------------------+
                | Option Number | Policy [RFC5226]       |
                +---------------+------------------------+
                |        0..255 | IETF Review            |
                |     256..2047 | Specification Required |
                |   2048..65535 | Designated Expert      |
                +---------------+------------------------+

   The documentation of an Option Number should specify the semantics of
   an option with that number, including the following properties:

   o  The meaning of the option in a request.





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   o  The meaning of the option in a response.

   o  Whether the option is critical or elective, as determined by the
      Option Number.

   o  Whether the option is Safe, and whether it is part of the Cache-
      Key, as determined by the Option Number (see Section 5.4.2).

   o  The format and length of the option's value.

   o  Whether the option must occur at most once or whether it can occur
      multiple times.

   o  The default value, if any.  For a critical option with a default
      value, a discussion on how the default value enables processing by
      implementations not implementing the critical option
      (Section 5.4.4).

12.3.  Content-Format Registry

   Internet media types are identified by a string, such as
   "application/xml" [RFC2046].  In order to minimize the overhead of
   using these media types to indicate the format of payloads, this
   document defines a registry for a subset of Internet media types to
   be used in CoAP and assigns each, in combination with a content-
   coding, a numeric identifier.  The name of the registry is "CoAP
   Content-Formats".

   Each entry in the registry must include the media type registered
   with IANA, the numeric identifier in the range 0-65535 to be used for
   that media type in CoAP, the content-coding associated with this
   identifier, and a reference to a document describing what a payload
   with that media type means semantically.

   CoAP does not include a way to convey content-encoding information
   with a request or response, and for that reason the content-encoding
   is also specified for each identifier (if any).  If multiple content-
   encodings will be used with a media type, then a separate Content-
   Format identifier for each is to be registered.  Similarly, other
   parameters related to an Internet media type, such as level, can be
   defined for a CoAP Content-Format entry.

   Initial entries in this registry are as follows:








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   +--------------------+----------+-----+-----------------------------+
   | Media type         | Encoding | Id. | Reference                   |
   +--------------------+----------+-----+-----------------------------+
   | text/plain;        | -        |   0 | [RFC2046][RFC3676][RFC5147] |
   | charset=utf-8      |          |     |                             |
   | application/       | -        |  40 | [RFC6690]                   |
   | link-format        |          |     |                             |
   | application/xml    | -        |  41 | [RFC3023]                   |
   | application/       | -        |  42 | [RFC2045][RFC2046]          |
   | octet-stream       |          |     |                             |
   | application/exi    | -        |  47 | [EXIMIME]                   |
   | application/json   | -        |  50 | [RFC4627]                   |
   +--------------------+----------+-----+-----------------------------+

                       Table 5: CoAP Content-Formats

   The identifiers between 201 and 255 inclusive are reserved for
   Private Use. All other identifiers are Unassigned.

   Because the name space of single-byte identifiers is so small, the
   IANA policy for future additions in the range 0-200 inclusive to the
   registry is "Expert Review" as described in [RFC5226].  The IANA
   policy for additions in the range 256-65535 inclusive is "First Come
   First Served" as described in [RFC5226].

   In machine to machine applications, it is not expected that generic
   Internet media types such as text/plain, application/xml or
   application/octet-stream are useful for real applications in the long
   term.  It is recommended that M2M applications making use of CoAP
   will request new Internet media types from IANA indicating semantic
   information about how to create or parse a payload.  For example, a
   Smart Energy application payload carried as XML might request a more
   specific type like application/se+xml or application/se-exi.

12.4.  URI Scheme Registration

   This document requests the registration of the Uniform Resource
   Identifier (URI) scheme "coap".  The registration request complies
   with [RFC4395].

   URI scheme name.
      coap

   Status.
      Permanent.






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   URI scheme syntax.
      Defined in Section 6.1 of [RFCXXXX].

   URI scheme semantics.
      The "coap" URI scheme provides a way to identify resources that
      are potentially accessible over the Constrained Application
      Protocol (CoAP).  The resources can be located by contacting the
      governing CoAP server and operated on by sending CoAP requests to
      the server.  This scheme can thus be compared to the "http" URI
      scheme [RFC2616].  See Section 6 of [RFCXXXX] for the details of
      operation.

   Encoding considerations.
      The scheme encoding conforms to the encoding rules established for
      URIs in [RFC3986], i.e. internationalized and reserved characters
      are expressed using UTF-8-based percent-encoding.

   Applications/protocols that use this URI scheme name.
      The scheme is used by CoAP endpoints to access CoAP resources.

   Interoperability considerations.
      None.

   Security considerations.
      See Section 11.1 of [RFCXXXX].

   Contact.
      IETF Chair <chair@ietf.org>

   Author/Change controller.
      IESG <iesg@ietf.org>

   References.
      [RFCXXXX]

12.5.  Secure URI Scheme Registration

   This document requests the registration of the Uniform Resource
   Identifier (URI) scheme "coaps".  The registration request complies
   with [RFC4395].

   URI scheme name.
      coaps

   Status.
      Permanent.





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   URI scheme syntax.
      Defined in Section 6.2 of [RFCXXXX].

   URI scheme semantics.
      The "coaps" URI scheme provides a way to identify resources that
      are potentially accessible over the Constrained Application
      Protocol (CoAP) using Datagram Transport Layer Security (DTLS) for
      transport security.  The resources can be located by contacting
      the governing CoAP server and operated on by sending CoAP requests
      to the server.  This scheme can thus be compared to the "https"
      URI scheme [RFC2616].  See Section 6 of [RFCXXXX] for the details
      of operation.

   Encoding considerations.
      The scheme encoding conforms to the encoding rules established for
      URIs in [RFC3986], i.e. internationalized and reserved characters
      are expressed using UTF-8-based percent-encoding.

   Applications/protocols that use this URI scheme name.
      The scheme is used by CoAP endpoints to access CoAP resources
      using DTLS.

   Interoperability considerations.
      None.

   Security considerations.
      See Section 11.1 of [RFCXXXX].

   Contact.
      IETF Chair <chair@ietf.org>

   Author/Change controller.
      IESG <iesg@ietf.org>

   References.
      [RFCXXXX]

12.6.  Service Name and Port Number Registration

   One of the functions of CoAP is resource discovery: a CoAP client can
   ask a CoAP server about the resources offered by it (see Section 7).
   To enable resource discovery just based on the knowledge of an IP
   address, the CoAP port for resource discovery needs to be
   standardized.

   IANA has assigned the port number 5683 and the service name "coap",
   in accordance with [RFC6335].




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   Besides unicast, CoAP can be used with both multicast and anycast.

   Service Name.
      coap

   Transport Protocol.
      UDP

   Assignee.
      IESG <iesg@ietf.org>

   Contact.
      IETF Chair <chair@ietf.org>

   Description.
      Constrained Application Protocol (CoAP)

   Reference.
      [RFCXXXX]

   Port Number.
      5683

12.7.  Secure Service Name and Port Number Registration

   CoAP resource discovery may also be provided using the DTLS-secured
   CoAP "coaps" scheme.  Thus the CoAP port for secure resource
   discovery needs to be standardized.

   This document requests the assignment of the port number
   [IANA_TBD_PORT] and the service name "coaps", in accordance with
   [RFC6335].

   Besides unicast, DTLS-secured CoAP can be used with anycast.

   Service Name.
      coaps

   Transport Protocol.
      UDP

   Assignee.
      IESG <iesg@ietf.org>

   Contact.
      IETF Chair <chair@ietf.org>





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   Description.
      DTLS-secured CoAP

   Reference.
      [RFCXXXX]

   Port Number.
      [IANA_TBD_PORT]

12.8.  Multicast Address Registration

   Section 8, "Multicast CoAP", defines the use of multicast.  This
   document requests the assignment of the following multicast addresses
   for use by CoAP nodes:

   IPv4  -- "All CoAP Nodes" address [TBD1], from the IPv4 Multicast
      Address Space Registry.  As the address is used for discovery that
      may span beyond a single network, it should come from the
      Internetwork Control Block (224.0.1.x, RFC 5771).

   IPv6  -- "All CoAP Nodes" address [TBD2], from the IPv6 Multicast
      Address Space Registry, in the Variable Scope Multicast Addresses
      space (RFC3307).  Note that there is a distinct multicast address
      for each scope that interested CoAP nodes should listen to.

   [The explanatory text to be removed upon allocation of the addresses,
   except for the note about the distinct multicast addresses.]


13.  Acknowledgements

   Special thanks to Peter Bigot, Esko Dijk and Cullen Jennings for
   substantial contributions to the ideas and text in the document,
   along with countless detailed reviews and discussions.

   Thanks to Ed Beroset, Angelo P. Castellani, Gilbert Clark, Robert
   Cragie, Esko Dijk, Lisa Dussealt, Thomas Fossati, Tom Herbst, Richard
   Kelsey, Ari Keranen, Matthias Kovatsch, Salvatore Loreto, Kerry Lynn,
   Alexey Melnikov, Guido Moritz, Petri Mutka, Colin O'Flynn, Charles
   Palmer, Adriano Pezzuto, Robert Quattlebaum, Akbar Rahman, Eric
   Rescorla, David Ryan, Szymon Sasin, Michael Scharf, Dale Seed, Robby
   Simpson, Peter van der Stok, Michael Stuber, Linyi Tian, Gilman
   Tolle, Matthieu Vial and Alper Yegin for helpful comments and
   discussions that have shaped the document.

   Some of the text has been borrowed from the working documents of the
   IETF httpbis working group.




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14.  References

14.1.  Normative References

   [I-D.farrell-decade-ni]
              Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
              Keranen, A., and P. Hallam-Baker, "Naming Things with
              Hashes", draft-farrell-decade-ni-10 (work in progress),
              August 2012.

   [I-D.ietf-tls-oob-pubkey]
              Wouters, P., Gilmore, J., Weiler, S., Kivinen, T., and H.
              Tschofenig, "Out-of-Band Public Key Validation for
              Transport Layer Security", draft-ietf-tls-oob-pubkey-04
              (work in progress), July 2012.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

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

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3676]  Gellens, R., "The Text/Plain Format and DelSp Parameters",
              RFC 3676, February 2004.

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




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   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [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.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, February 2006.

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

   [RFC5147]  Wilde, E. and M. Duerst, "URI Fragment Identifiers for the
              text/plain Media Type", RFC 5147, April 2008.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

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

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952, August 2010.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.



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   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, August 2012.

14.2.  Informative References

   [EUI64]    "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64)
              REGISTRATION AUTHORITY", April 2010, <http://
              standards.ieee.org/regauth/oui/tutorials/EUI64.html>.

   [EXIMIME]  "Efficient XML Interchange (EXI) Format 1.0",
              December 2009, <http://www.w3.org/TR/2009/
              CR-exi-20091208/#mediaTypeRegistration>.

   [I-D.allman-tcpm-rto-consider]
              Allman, M., "Retransmission Timeout Considerations",
              draft-allman-tcpm-rto-consider-01 (work in progress),
              May 2012.

   [I-D.ietf-core-block]
              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-09 (work in progress), August 2012.

   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP",
              draft-ietf-core-observe-06 (work in progress),
              September 2012.

   [I-D.kivinen-ipsecme-ikev2-minimal]
              Kivinen, T., "Minimal IKEv2",
              draft-kivinen-ipsecme-ikev2-minimal-00 (work in progress),
              February 2011.

   [I-D.mcgrew-tls-aes-ccm-ecc]
              McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
              CCM ECC Cipher Suites for TLS",
              draft-mcgrew-tls-aes-ccm-ecc-05 (work in progress),
              July 2012.




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   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", Ph.D. Dissertation,
              University of California, Irvine, 2000, <http://
              www.ics.uci.edu/~fielding/pubs/dissertation/
              fielding_dissertation.pdf>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

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

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

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

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492, May 2006.

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

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

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405,
              November 2008.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, August 2011.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655, July 2012.






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Appendix A.  Examples

   This section gives a number of short examples with message flows for
   GET requests.  These examples demonstrate the basic operation, the
   operation in the presence of retransmissions, and multicast.

   Figure 16 shows a basic GET request causing a piggy-backed response:
   The client sends a Confirmable GET request for the resource
   coap://server/temperature to the server with a Message ID of 0x7d34.
   The request includes one Uri-Path Option (Delta 0 + 11 = 11, Length
   11, Value "temperature"); the Token is left at its default value
   (empty).  This request is a total of 16 bytes long.  A 2.05 (Content)
   response is returned in the Acknowledgement message that acknowledges
   the Confirmable request, echoing both the Message ID 0x7d34 and the
   (implicitly empty) Token value.  The response includes a Payload of
   "22.3 C" and is 10 bytes long.

   Client  Server
      |      |
      |      |
      +----->|     Header: GET (T=CON, Code=1, MID=0x7d34)
      | GET  |   Uri-Path: "temperature"
      |      |
      |      |
      |<-----+     Header: 2.05 Content (T=ACK, Code=69, MID=0x7d34)
      | 2.05 |    Payload: "22.3 C"
      |      |


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1 | 0 |   1   |     GET=1     |          MID=0x7d34           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  11   |  11   |      "temperature" (11 B) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1 | 2 |   0   |    2.05=69    |          MID=0x7d34           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      "22.3 C" (6 B) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 16: Confirmable request; piggy-backed response

   Figure 17 shows a similar example, but with the inclusion of an



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   explicit Token Option (Delta 11 + 8 = 19, Length 1, Value 0x20) in
   the request and (Jump 15 + 4 = 19) in the response, increasing the
   sizes to 18 and 12 bytes, respectively.

   Client  Server
      |      |
      |      |
      +----->|     Header: GET (T=CON, Code=1, MID=0x7d35)
      | GET  |      Token: 0x20
      |      |   Uri-Path: "temperature"
      |      |
      |      |
      |<-----+     Header: 2.05 Content (T=ACK, Code=69, MID=0x7d35)
      | 2.05 |      Token: 0x20
      |      |    Payload: "22.3 C"
      |      |


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1 | 0 |   2   |     GET=1     |          MID=0x7d35           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   11   |  11   |      "temperature" (11 B) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   8    |   1   |     0x20      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 1 | 2 |   1   |    2.05=69    |          MID=0x7d35           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Jump 15 = 0xf1 |  4    |   1   |     0x20      | "22.3 C" (6 B) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


           Figure 17: Confirmable request; piggy-backed response

   In Figure 18, the Confirmable GET request is lost.  After ACK_TIMEOUT
   seconds, the client retransmits the request, resulting in a piggy-
   backed response as in the previous example.








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   Client  Server
      |      |
      |      |
      +----X |     Header: GET (T=CON, Code=1, MID=0x7d36)
      | GET  |      Token: 0x31
      |      |   Uri-Path: "temperature"
   TIMEOUT   |
      |      |
      +----->|     Header: GET (T=CON, Code=1, MID=0x7d36)
      | GET  |      Token: 0x31
      |      |   Uri-Path: "temperature"
      |      |
      |      |
      |<-----+     Header: 2.05 Content (T=ACK, Code=69, MID=0x7d36)
      | 2.05 |      Token: 0x31
      |      |    Payload: "22.3 C"
      |      |

   Figure 18: Confirmable request (retransmitted); piggy-backed response

   In Figure 19, the first Acknowledgement message from the server to
   the client is lost.  After ACK_TIMEOUT seconds, the client
   retransmits the request.

   Client  Server
      |      |
      |      |
      +----->|     Header: GET (T=CON, Code=1, MID=0x7d37)
      | GET  |      Token: 0x42
      |      |   Uri-Path: "temperature"
      |      |
      |      |
      | X----+     Header: 2.05 Content (T=ACK, Code=69, MID=0x7d37)
      | 2.05 |      Token: 0x42
      |      |    Payload: "22.3 C"
   TIMEOUT   |
      |      |
      +----->|     Header: GET (T=CON, Code=1, MID=0x7d37)
      | GET  |      Token: 0x42
      |      |   Uri-Path: "temperature"
      |      |
      |      |
      |<-----+     Header: 2.05 Content (T=ACK, Code=69, MID=0x7d37)
      | 2.05 |      Token: 0x42
      |      |    Payload: "22.3 C"
      |      |

   Figure 19: Confirmable request; piggy-backed response (retransmitted)



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   In Figure 20, the server acknowledges the Confirmable request and
   sends a 2.05 (Content) response separately in a Confirmable message.
   Note that the Acknowledgement message and the Confirmable response do
   not necessarily arrive in the same order as they were sent.  The
   client acknowledges the Confirmable response.

   Client  Server
      |      |
      |      |
      +----->|     Header: GET (T=CON, Code=1, MID=0x7d38)
      | GET  |      Token: 0x53
      |      |   Uri-Path: "temperature"
      |      |
      |      |
      |<- - -+     Header: (T=ACK, Code=0, MID=0x7d38)
      |      |
      |      |
      |<-----+     Header: 2.05 Content (T=CON, Code=69, MID=0xad7b)
      | 2.05 |      Token: 0x53
      |      |    Payload: "22.3 C"
      |      |
      |      |
      +- - ->|     Header: (T=ACK, Code=0, MID=0xad7b)
      |      |

             Figure 20: Confirmable request; separate response

   Figure 21 shows an example where the client loses its state (e.g.,
   crashes and is rebooted) right after sending a Confirmable request,
   so the separate response arriving some time later comes unexpected.
   In this case, the client rejects the Confirmable response with a
   Reset message.  Note that the unexpected ACK is silently ignored.



















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   Client  Server
      |      |
      |      |
      +----->|     Header: GET (T=CON, Code=1, MID=0x7d39)
      | GET  |      Token: 0x64
      |      |   Uri-Path: "temperature"
    CRASH    |
      |      |
      |<- - -+     Header: (T=ACK, Code=0, MID=0x7d39)
      |      |
      |      |
      |<-----+     Header: 2.05 Content (T=CON, Code=69, MID=0xad7c)
      | 2.05 |      Token: 0x64
      |      |    Payload: "22.3 C"
      |      |
      |      |
      +- - ->|     Header: (T=RST, Code=0, MID=0xad7c)
      |      |

      Figure 21: Confirmable request; separate response (unexpected)

   Figure 22 shows a basic GET request where the request and the
   response are non-confirmable, so both may be lost without notice.

   Client  Server
      |      |
      |      |
      +----->|     Header: GET (T=NON, Code=1, MID=0x7d40)
      | GET  |      Token: 0x75
      |      |   Uri-Path: "temperature"
      |      |
      |      |
      |<-----+     Header: 2.05 Content (T=NON, Code=69, MID=0xad7d)
      | 2.05 |      Token: 0x75
      |      |    Payload: "22.3 C"
      |      |

       Figure 22: Non-confirmable request; Non-confirmable response

   In Figure 23, the client sends a Non-confirmable GET request to a
   multicast address: all nodes in link-local scope.  There are 3
   servers on the link: A, B and C. Servers A and B have a matching
   resource, therefore they send back a Non-confirmable 2.05 (Content)
   response.  The response sent by B is lost.  C does not have matching
   response, therefore it sends a Non-confirmable 4.04 (Not Found)
   response.





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   Client  ff02::1  A  B  C
      |       |     |  |  |
      |       |     |  |  |
      +------>|     |  |  |   Header: GET (T=NON, Code=1, MID=0x7d41)
      |  GET  |     |  |  |    Token: 0x86
      |             |  |  |    Uri-Path: "temperature"
      |             |  |  |
      |             |  |  |
      |<------------+  |  |   Header: 2.05 (T=NON, Code=69, MID=0x60b1)
      |      2.05   |  |  |    Token: 0x86
      |             |  |  |    Payload: "22.3 C"
      |             |  |  |
      |             |  |  |
      |   X------------+  |   Header: 2.05 (T=NON, Code=69, MID=0x01a0)
      |      2.05   |  |  |    Token: 0x86
      |             |  |  |    Payload: "20.9 C"
      |             |  |  |
      |             |  |  |
      |<------------------+   Header: 4.04 (T=NON, Code=132, MID=0x952a)
      |      4.04   |  |  |    Token: 0x86
      |             |  |  |

      Figure 23: Non-confirmable request (multicast); Non-confirmable
                                 response


Appendix B.  URI Examples

   The following examples demonstrate different sets of Uri options, and
   the result after constructing an URI from them.

   o  coap://[2001:db8::2:1]/

         Destination IP Address = [2001:db8::2:1]

         Destination UDP Port = 5683

   o  coap://example.net/

         Destination IP Address = [2001:db8::2:1]

         Destination UDP Port = 5683

         Uri-Host = "example.net"

   o  coap://example.net/.well-known/core





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         Destination IP Address = [2001:db8::2:1]

         Destination UDP Port = 5683

         Uri-Host = "example.net"

         Uri-Path = ".well-known"

         Uri-Path = "core"

   o  coap://
      xn--18j4d.example/%E3%81%93%E3%82%93%E3%81%AB%E3%81%A1%E3%81%AF

         Destination IP Address = [2001:db8::2:1]

         Destination UDP Port = 5683

         Uri-Host = "xn--18j4d.example"

         Uri-Path = the string composed of the Unicode characters U+3053
         U+3093 U+306b U+3061 U+306f, usually represented in UTF-8 as
         E38193E38293E381ABE381A1E381AF hexadecimal

   o  coap://198.51.100.1:61616//%2F//?%2F%2F&?%26

         Destination IP Address = 198.51.100.1

         Destination UDP Port = 61616

         Uri-Path = ""

         Uri-Path = "/"

         Uri-Path = ""

         Uri-Path = ""

         Uri-Query = "//"

         Uri-Query = "?&"


Appendix C.  Changelog

   Changed from ietf-11 to ietf-12:

   o  Extended options to support lengths of up to 1034 bytes (#202).




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   o  Added new Jump mechanism for options and removed Fenceposting
      (#214).

   o  Added new IANA option number registration policy (#214).

   o  Added Proxy Unsafe/Safe and Cache-Key masking to option numbers
      (#241).

   o  Re-numbered option numbers to use Unsafe/Safe and Cache-Key
      compliant numbers (#241).

   o  Defined NSTART and restricted the value to 1 with a MUST (#215).

   o  Defined PROBING_RATE and set it to 1 Byte/second (#215).

   o  Defined DEFAULT_LEISURE (#246).

   o  Renamed Content-Type into Content-Format, and Media Type registry
      into Content-Format registry.

   o  A large number of small editorial changes, clarifications and
      improvements have been made.

   Changed from ietf-10 to ietf-11:

   o  Expanded section 4.8 on Transmission Parameters, and used the
      derived values defined there (#201).  Changed parameter names to
      be shorter and more to the point.

   o  Several more small editorial changes, clarifications and
      improvements have been made.

   Changed from ietf-09 to ietf-10:

   o  Option deltas are restricted to 0 to 14; the option delta 15 is
      used exclusively for the end-of-options marker (#239).

   o  Option numbers that are a multiple of 14 are not reserved, but are
      required to have an empty default value (#212).

   o  Fixed misleading language that was introduced in 5.10.2 in coap-07
      re Uri-Host and Uri-Port (#208).

   o  Segments and arguments can have a length of zero characters
      (#213).

   o  The Location-* options describe together describe one location.
      The location is a relative URI, not an "absolute path URI" (#218).



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   o  The value of the Location-Path Option must not be '.' or '..'
      (#218).

   o  Added a sentence on constructing URIs from Location-* options
      (#231).

   o  Reserved option numbers for future Location-* options (#230).

   o  Fixed response codes with payload inconsistency (#233).

   o  Added advice on default values for critical options (#207).

   o  Clarified use of identifiers in RawPublicKey Mode Provisioning
      (#222).

   o  Moved "Securing CoAP" out of the "Security Considerations" (#229).

   o  Added "All CoAP Nodes" multicast addresses to "IANA
      Considerations" (#216).

   o  Over 100 small editorial changes, clarifications and improvements
      have been made.

   Changed from ietf-08 to ietf-09:

   o  Improved consistency of statements about RST on NON: RST is a
      valid response to a NON message (#183).

   o  Clarified that the protocol constants can be configured for
      specific application environments.

   o  Added implementation note recommending piggy-backing whenever
      possible (#182).

   o  Added a content-encoding column to the media type registry (#181).

   o  Minor improvements to Appendix D.

   o  Added text about multicast response suppression (#177).

   o  Included the new End-of-options Marker (#176).

   o  Added a reference to draft-ietf-tls-oob-pubkey and updated the RPK
      text accordingly.

   Changed from ietf-07 to ietf-08:





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   o  Clarified matching rules for messages (#175)

   o  Fixed a bug in Section 8.2.2 on Etags (#168)

   o  Added an IP address spoofing threat analysis contribution (#167)

   o  Re-focused the security section on raw public keys (#166)

   o  Added an 4.06 error to Accept (#165)

   Changed from ietf-06 to ietf-07:

   o  application/link-format added to Media types registration (#160)

   o  Moved content-type attribute to the document from link-format.

   o  Added coaps scheme and DTLS-secured CoAP default port (#154)

   o  Allowed 0-length Content-type options (#150)

   o  Added congestion control recommendations (#153)

   o  Improved text on PUT/POST response payloads (#149)

   o  Added an Accept option for content-negotiation (#163)

   o  Added If-Match and If-None-Match options (#155)

   o  Improved Token Option explanation (#147)

   o  Clarified mandatory to implement security (#156)

   o  Added first come first server policy for 2-byte Media type codes
      (#161)

   o  Clarify matching rules for messages and tokens (#151)

   o  Changed OPTIONS and TRACE to always return 501 in HTTP-CoAP
      mapping (#164)

   Changed from ietf-05 to ietf-06:

   o  HTTP mapping section improved with the minimal protocol standard
      text for CoAP-HTTP and HTTP-CoAP forward proxying (#137).

   o  Eradicated percent-encoding by including one Uri-Query Option per
      &-delimited argument in a query.




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   o  Allowed RST message in reply to a NON message with unexpected
      token (#135).

   o  Cache Invalidation only happens upon successful responses (#134).

   o  50% jitter added to the initial retransmit timer (#142).

   o  DTLS cipher suites aligned with ZigBee IP, DTLS clarified as
      default CoAP security mechanism (#138, #139)

   o  Added a minimal reference to draft-kivinen-ipsecme-ikev2-minimal
      (#140).

   o  Clarified the comparison of UTF-8s (#136).

   o  Minimized the initial media type registry (#101).

   Changed from ietf-04 to ietf-05:

   o  Renamed Immediate into Piggy-backed and Deferred into Separate --
      should finally end the confusion on what this is about.

   o  GET requests now return a 2.05 (Content) response instead of 2.00
      (OK) response (#104).

   o  Added text to allow 2.02 (Deleted) responses in reply to POST
      requests (#105).

   o  Improved message deduplication rules (#106).

   o  Section added on message size implementation considerations
      (#103).

   o  Clarification made on human readable error payloads (#109).

   o  Definition of CoAP methods improved (#108).

   o  Max-Age removed from requests (#107).

   o  Clarified uniqueness of tokens (#112).

   o  Location-Query Option added (#113).

   o  ETag length set to 1-8 bytes (#123).

   o  Clarified relation between elective/critical and option numbers
      (#110).




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   o  Defined when to update Version header field (#111).

   o  URI scheme registration improved (#102).

   o  Added review guidelines for new CoAP codes and numbers.

   Changes from ietf-03 to ietf-04:

   o  Major document reorganization (#51, #63, #71, #81).

   o  Max-age length set to 0-4 bytes (#30).

   o  Added variable unsigned integer definition (#31).

   o  Clarification made on human readable error payloads (#50).

   o  Definition of POST improved (#52).

   o  Token length changed to 0-8 bytes (#53).

   o  Section added on multiplexing CoAP, DTLS and STUN (#56).

   o  Added cross-protocol attack considerations (#61).

   o  Used new Immediate/Deferred response definitions (#73).

   o  Improved request/response matching rules (#74).

   o  Removed unnecessary media types and added recommendations for
      their use in M2M (#76).

   o  Response codes changed to base 32 coding, new Y.XX naming (#77).

   o  References updated as per AD review (#79).

   o  IANA section completed (#80).

   o  Proxy-Uri Option added to disambiguate between proxy and non-proxy
      requests (#82).

   o  Added text on critical options in cached states (#83).

   o  HTTP mapping sections improved (#88).

   o  Added text on reverse proxies (#72).

   o  Some security text on multicast added (#54).




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   o  Trust model text added to introduction (#58, #60).

   o  AES-CCM vs. AES-CCB text added (#55).

   o  Text added about device capabilities (#59).

   o  DTLS section improvements (#87).

   o  Caching semantics aligned with RFC2616 (#78).

   o  Uri-Path Option split into multiple path segments.

   o  MAX_RETRANSMIT changed to 4 to adjust for RESPONSE_TIME = 2.

   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).




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   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.

   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.



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   o  Made the URI slash optimization and method idempotence 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.


Authors' Addresses

   Zach Shelby
   Sensinode
   Kidekuja 2
   Vuokatti  88600
   Finland

   Phone: +358407796297
   Email: zach@sensinode.com








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   Klaus Hartke
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63905
   Email: hartke@tzi.org


   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org


   Brian Frank
   SkyFoundry
   Richmond, VA
   USA

   Phone:
   Email: brian@skyfoundry.com
























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