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rfc:rfc7252

Internet Engineering Task Force (IETF) Z. Shelby Request for Comments: 7252 ARM Category: Standards Track K. Hartke ISSN: 2070-1721 C. Bormann

                                               Universitaet Bremen TZI
                                                             June 2014
            The Constrained Application Protocol (CoAP)

Abstract

 The Constrained Application Protocol (CoAP) is a specialized web
 transfer protocol for use with constrained nodes and constrained
 (e.g., low-power, lossy) networks.  The nodes often have 8-bit
 microcontrollers with small amounts of ROM and RAM, while constrained
 networks such as IPv6 over Low-Power Wireless Personal Area Networks
 (6LoWPANs) often have high packet error rates and a typical
 throughput of 10s of kbit/s.  The protocol is designed for machine-
 to-machine (M2M) applications such as smart energy and building
 automation.
 CoAP provides a request/response interaction model between
 application endpoints, supports built-in discovery of services and
 resources, and includes key concepts of the Web such as URIs and
 Internet media types.  CoAP is designed to easily interface with HTTP
 for integration with the Web while meeting specialized requirements
 such as multicast support, very low overhead, and simplicity for
 constrained environments.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7252.

Shelby, et al. Standards Track [Page 1] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   1.1.  Features  . . . . . . . . . . . . . . . . . . . . . . . .   5
   1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
 2.  Constrained Application Protocol  . . . . . . . . . . . . . .  10
   2.1.  Messaging Model . . . . . . . . . . . . . . . . . . . . .  11
   2.2.  Request/Response Model  . . . . . . . . . . . . . . . . .  12
   2.3.  Intermediaries and Caching  . . . . . . . . . . . . . . .  15
   2.4.  Resource Discovery  . . . . . . . . . . . . . . . . . . .  15
 3.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .  15
   3.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .  17
   3.2.  Option Value Formats  . . . . . . . . . . . . . . . . . .  19
 4.  Message Transmission  . . . . . . . . . . . . . . . . . . . .  20
   4.1.  Messages and Endpoints  . . . . . . . . . . . . . . . . .  20
   4.2.  Messages Transmitted Reliably . . . . . . . . . . . . . .  21
   4.3.  Messages Transmitted without Reliability  . . . . . . . .  23
   4.4.  Message Correlation . . . . . . . . . . . . . . . . . . .  24
   4.5.  Message Deduplication . . . . . . . . . . . . . . . . . .  24
   4.6.  Message Size  . . . . . . . . . . . . . . . . . . . . . .  25
   4.7.  Congestion Control  . . . . . . . . . . . . . . . . . . .  26
   4.8.  Transmission Parameters . . . . . . . . . . . . . . . . .  27
     4.8.1.  Changing the Parameters . . . . . . . . . . . . . . .  27
     4.8.2.  Time Values Derived from Transmission Parameters  . .  28
 5.  Request/Response Semantics  . . . . . . . . . . . . . . . . .  31
   5.1.  Requests  . . . . . . . . . . . . . . . . . . . . . . . .  31
   5.2.  Responses . . . . . . . . . . . . . . . . . . . . . . . .  31
     5.2.1.  Piggybacked . . . . . . . . . . . . . . . . . . . . .  33
     5.2.2.  Separate  . . . . . . . . . . . . . . . . . . . . . .  33
     5.2.3.  Non-confirmable . . . . . . . . . . . . . . . . . . .  34
   5.3.  Request/Response Matching . . . . . . . . . . . . . . . .  34
     5.3.1.  Token . . . . . . . . . . . . . . . . . . . . . . . .  34
     5.3.2.  Request/Response Matching Rules . . . . . . . . . . .  35

Shelby, et al. Standards Track [Page 2] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

   5.4.  Options . . . . . . . . . . . . . . . . . . . . . . . . .  36
     5.4.1.  Critical/Elective . . . . . . . . . . . . . . . . . .  37
     5.4.2.  Proxy Unsafe or Safe-to-Forward and NoCacheKey  . . .  38
     5.4.3.  Length  . . . . . . . . . . . . . . . . . . . . . . .  38
     5.4.4.  Default Values  . . . . . . . . . . . . . . . . . . .  38
     5.4.5.  Repeatable Options  . . . . . . . . . . . . . . . . .  39
     5.4.6.  Option Numbers  . . . . . . . . . . . . . . . . . . .  39
   5.5.  Payloads and Representations  . . . . . . . . . . . . . .  40
     5.5.1.  Representation  . . . . . . . . . . . . . . . . . . .  40
     5.5.2.  Diagnostic Payload  . . . . . . . . . . . . . . . . .  41
     5.5.3.  Selected Representation . . . . . . . . . . . . . . .  41
     5.5.4.  Content Negotiation . . . . . . . . . . . . . . . . .  41
   5.6.  Caching . . . . . . . . . . . . . . . . . . . . . . . . .  42
     5.6.1.  Freshness Model . . . . . . . . . . . . . . . . . . .  43
     5.6.2.  Validation Model  . . . . . . . . . . . . . . . . . .  43
   5.7.  Proxying  . . . . . . . . . . . . . . . . . . . . . . . .  44
     5.7.1.  Proxy Operation . . . . . . . . . . . . . . . . . . .  44
     5.7.2.  Forward-Proxies . . . . . . . . . . . . . . . . . . .  46
     5.7.3.  Reverse-Proxies . . . . . . . . . . . . . . . . . . .  46
   5.8.  Method Definitions  . . . . . . . . . . . . . . . . . . .  47
     5.8.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  47
     5.8.2.  POST  . . . . . . . . . . . . . . . . . . . . . . . .  47
     5.8.3.  PUT . . . . . . . . . . . . . . . . . . . . . . . . .  48
     5.8.4.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  48
   5.9.  Response Code Definitions . . . . . . . . . . . . . . . .  48
     5.9.1.  Success 2.xx  . . . . . . . . . . . . . . . . . . . .  48
     5.9.2.  Client Error 4.xx . . . . . . . . . . . . . . . . . .  50
     5.9.3.  Server Error 5.xx . . . . . . . . . . . . . . . . . .  51
   5.10. Option Definitions  . . . . . . . . . . . . . . . . . . .  52
     5.10.1.  Uri-Host, Uri-Port, Uri-Path, and Uri-Query  . . . .  53
     5.10.2.  Proxy-Uri and Proxy-Scheme . . . . . . . . . . . . .  54
     5.10.3.  Content-Format . . . . . . . . . . . . . . . . . . .  55
     5.10.4.  Accept . . . . . . . . . . . . . . . . . . . . . . .  55
     5.10.5.  Max-Age  . . . . . . . . . . . . . . . . . . . . . .  55
     5.10.6.  ETag . . . . . . . . . . . . . . . . . . . . . . . .  56
     5.10.7.  Location-Path and Location-Query . . . . . . . . . .  57
     5.10.8.  Conditional Request Options  . . . . . . . . . . . .  57
     5.10.9.  Size1 Option . . . . . . . . . . . . . . . . . . . .  59
 6.  CoAP URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  59
   6.1.  coap URI Scheme . . . . . . . . . . . . . . . . . . . . .  59
   6.2.  coaps URI Scheme  . . . . . . . . . . . . . . . . . . . .  60
   6.3.  Normalization and Comparison Rules  . . . . . . . . . . .  61
   6.4.  Decomposing URIs into Options . . . . . . . . . . . . . .  61
   6.5.  Composing URIs from Options . . . . . . . . . . . . . . .  62
 7.  Discovery . . . . . . . . . . . . . . . . . . . . . . . . . .  64
   7.1.  Service Discovery . . . . . . . . . . . . . . . . . . . .  64
   7.2.  Resource Discovery  . . . . . . . . . . . . . . . . . . .  64
     7.2.1.  'ct' Attribute  . . . . . . . . . . . . . . . . . . .  64

Shelby, et al. Standards Track [Page 3] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 8.  Multicast CoAP  . . . . . . . . . . . . . . . . . . . . . . .  65
   8.1.  Messaging Layer . . . . . . . . . . . . . . . . . . . . .  65
   8.2.  Request/Response Layer  . . . . . . . . . . . . . . . . .  66
     8.2.1.  Caching . . . . . . . . . . . . . . . . . . . . . . .  67
     8.2.2.  Proxying  . . . . . . . . . . . . . . . . . . . . . .  67
 9.  Securing CoAP . . . . . . . . . . . . . . . . . . . . . . . .  68
   9.1.  DTLS-Secured CoAP . . . . . . . . . . . . . . . . . . . .  69
     9.1.1.  Messaging Layer . . . . . . . . . . . . . . . . . . .  70
     9.1.2.  Request/Response Layer  . . . . . . . . . . . . . . .  71
     9.1.3.  Endpoint Identity . . . . . . . . . . . . . . . . . .  71
 10. Cross-Protocol Proxying between CoAP and HTTP . . . . . . . .  74
   10.1.  CoAP-HTTP Proxying . . . . . . . . . . . . . . . . . . .  75
     10.1.1.  GET  . . . . . . . . . . . . . . . . . . . . . . . .  76
     10.1.2.  PUT  . . . . . . . . . . . . . . . . . . . . . . . .  77
     10.1.3.  DELETE . . . . . . . . . . . . . . . . . . . . . . .  77
     10.1.4.  POST . . . . . . . . . . . . . . . . . . . . . . . .  77
   10.2.  HTTP-CoAP Proxying . . . . . . . . . . . . . . . . . . .  77
     10.2.1.  OPTIONS and TRACE  . . . . . . . . . . . . . . . . .  78
     10.2.2.  GET  . . . . . . . . . . . . . . . . . . . . . . . .  78
     10.2.3.  HEAD . . . . . . . . . . . . . . . . . . . . . . . .  79
     10.2.4.  POST . . . . . . . . . . . . . . . . . . . . . . . .  79
     10.2.5.  PUT  . . . . . . . . . . . . . . . . . . . . . . . .  79
     10.2.6.  DELETE . . . . . . . . . . . . . . . . . . . . . . .  80
     10.2.7.  CONNECT  . . . . . . . . . . . . . . . . . . . . . .  80
 11. Security Considerations . . . . . . . . . . . . . . . . . . .  80
   11.1.  Parsing the Protocol and Processing URIs . . . . . . . .  80
   11.2.  Proxying and Caching . . . . . . . . . . . . . . . . . .  81
   11.3.  Risk of Amplification  . . . . . . . . . . . . . . . . .  81
   11.4.  IP Address Spoofing Attacks  . . . . . . . . . . . . . .  83
   11.5.  Cross-Protocol Attacks . . . . . . . . . . . . . . . . .  84
   11.6.  Constrained-Node Considerations  . . . . . . . . . . . .  86
 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  86
   12.1.  CoAP Code Registries . . . . . . . . . . . . . . . . . .  86
     12.1.1.  Method Codes . . . . . . . . . . . . . . . . . . . .  87
     12.1.2.  Response Codes . . . . . . . . . . . . . . . . . . .  88
   12.2.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  89
   12.3.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  91
   12.4.  URI Scheme Registration  . . . . . . . . . . . . . . . .  93
   12.5.  Secure URI Scheme Registration . . . . . . . . . . . . .  94
   12.6.  Service Name and Port Number Registration  . . . . . . .  95
   12.7.  Secure Service Name and Port Number Registration . . . .  96
   12.8.  Multicast Address Registration . . . . . . . . . . . . .  97
 13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  97
 14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  98
   14.1.  Normative References . . . . . . . . . . . . . . . . . .  98
   14.2.  Informative References . . . . . . . . . . . . . . . . . 100
 Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . . 104
 Appendix B.  URI Examples . . . . . . . . . . . . . . . . . . . . 110

Shelby, et al. Standards Track [Page 4] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

1. Introduction

 The use of web services (web APIs) on the Internet has become
 ubiquitous in most applications and depends on the fundamental
 Representational State Transfer [REST] architecture of the Web.
 The work on Constrained RESTful Environments (CoRE) aims at realizing
 the REST architecture in a suitable form for the most constrained
 nodes (e.g., 8-bit microcontrollers with limited RAM and ROM) and
 networks (e.g., 6LoWPAN, [RFC4944]).  Constrained networks such as
 6LoWPAN support the fragmentation of IPv6 packets into small link-
 layer frames; however, this causes significant reduction in packet
 delivery probability.  One design goal of CoAP has been to keep
 message overhead small, thus limiting the need for fragmentation.
 One of the main goals of CoAP is to design a generic web protocol for
 the special requirements of this constrained environment, especially
 considering energy, building automation, and other machine-to-machine
 (M2M) applications.  The goal of CoAP is not to blindly compress HTTP
 [RFC2616], but rather to realize a subset of REST common with HTTP
 but optimized for M2M applications.  Although CoAP could be used for
 refashioning simple HTTP interfaces into a more compact protocol,
 more importantly it also offers features for M2M such as built-in
 discovery, multicast support, and asynchronous message exchanges.
 This document specifies the Constrained Application Protocol (CoAP),
 which easily translates to HTTP for integration with the existing Web
 while meeting specialized requirements such as multicast support,
 very low overhead, and simplicity for constrained environments and
 M2M applications.

1.1. Features

 CoAP has the following main features:
 o  Web protocol fulfilling M2M requirements in constrained
    environments
 o  UDP [RFC0768] binding with optional reliability supporting unicast
    and multicast requests.
 o  Asynchronous message exchanges.
 o  Low header overhead and parsing complexity.
 o  URI and Content-type support.
 o  Simple proxy and caching capabilities.

Shelby, et al. Standards Track [Page 5] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 o  A stateless HTTP mapping, allowing proxies to be built providing
    access to CoAP resources via HTTP in a uniform way or for HTTP
    simple interfaces to be realized alternatively over CoAP.
 o  Security binding to Datagram Transport Layer Security (DTLS)
    [RFC6347].

1.2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119] when they appear in ALL CAPS.  These words may also appear
 in this document in lowercase, absent their normative meanings.
 This specification requires readers to be familiar with all the terms
 and concepts that are discussed in [RFC2616], including "resource",
 "representation", "cache", and "fresh".  (Having been completed
 before the updated set of HTTP RFCs, RFC 7230 to RFC 7235, became
 available, this specification specifically references the predecessor
 version -- RFC 2616.)  In addition, this specification defines the
 following terminology:
 Endpoint
    An entity participating in the CoAP protocol.  Colloquially, an
    endpoint lives on a "Node", although "Host" would be more
    consistent with Internet standards usage, and is further
    identified by transport-layer multiplexing information that can
    include a UDP port number and a security association
    (Section 4.1).
 Sender
    The originating endpoint of a message.  When the aspect of
    identification of the specific sender is in focus, also "source
    endpoint".
 Recipient
    The destination endpoint of a message.  When the aspect of
    identification of the specific recipient is in focus, also
    "destination endpoint".
 Client
    The originating endpoint of a request; the destination endpoint of
    a response.
 Server
    The destination endpoint of a request; the originating endpoint of
    a response.

Shelby, et al. Standards Track [Page 6] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Origin Server
    The server on which a given resource resides or is to be created.
 Intermediary
    A CoAP endpoint that acts both as a server and as a client towards
    an origin server (possibly via further intermediaries).  A common
    form of an intermediary is a proxy; several classes of such
    proxies are discussed in this specification.
 Proxy
    An intermediary that mainly is concerned with forwarding requests
    and relaying back responses, possibly performing caching,
    namespace translation, or protocol translation in the process.  As
    opposed to intermediaries in the general sense, proxies generally
    do not implement specific application semantics.  Based on the
    position in the overall structure of the request forwarding, there
    are two common forms of proxy: forward-proxy and reverse-proxy.
    In some cases, a single endpoint might act as an origin server,
    forward-proxy, or reverse-proxy, switching behavior based on the
    nature of each request.
 Forward-Proxy
    An endpoint selected by a client, usually via local configuration
    rules, to perform requests on behalf of the client, doing any
    necessary translations.  Some translations are minimal, such as
    for proxy requests for "coap" URIs, whereas other requests might
    require translation to and from entirely different application-
    layer protocols.
 Reverse-Proxy
    An endpoint that stands in for one or more other server(s) and
    satisfies requests on behalf of these, doing any necessary
    translations.  Unlike a forward-proxy, the client may not be aware
    that it is communicating with a reverse-proxy; a reverse-proxy
    receives requests as if it were the origin server for the target
    resource.
 CoAP-to-CoAP Proxy
    A proxy that maps from a CoAP request to a CoAP request, i.e.,
    uses the CoAP protocol both on the server and the client side.
    Contrast to cross-proxy.
 Cross-Proxy
    A cross-protocol proxy, or "cross-proxy" for short, is a proxy
    that translates between different protocols, such as a CoAP-to-
    HTTP proxy or an HTTP-to-CoAP proxy.  While this specification
    makes very specific demands of CoAP-to-CoAP proxies, there is more
    variation possible in cross-proxies.

Shelby, et al. Standards Track [Page 7] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Confirmable Message
    Some messages require an acknowledgement.  These messages are
    called "Confirmable".  When no packets are lost, each Confirmable
    message elicits exactly one return message of type Acknowledgement
    or type Reset.
 Non-confirmable Message
    Some other messages do not require an acknowledgement.  This is
    particularly true for messages that are repeated regularly for
    application requirements, such as repeated readings from a sensor.
 Acknowledgement Message
    An Acknowledgement message acknowledges that a specific
    Confirmable message arrived.  By itself, an Acknowledgement
    message does not indicate success or failure of any request
    encapsulated in the Confirmable message, but the Acknowledgement
    message may also carry a Piggybacked Response (see below).
 Reset Message
    A Reset message indicates that a specific message (Confirmable or
    Non-confirmable) was received, but some context is missing to
    properly process it.  This condition is usually caused when the
    receiving node has rebooted and has forgotten some state that
    would be required to interpret the message.  Provoking a Reset
    message (e.g., by sending an Empty Confirmable message) is also
    useful as an inexpensive check of the liveness of an endpoint
    ("CoAP ping").
 Piggybacked Response
    A piggybacked Response is included right in a CoAP Acknowledgement
    (ACK) message that is sent to acknowledge receipt of the Request
    for this Response (Section 5.2.1).
 Separate Response
    When a Confirmable message carrying a request is acknowledged with
    an Empty message (e.g., because the server doesn't have the answer
    right away), a Separate Response is sent in a separate message
    exchange (Section 5.2.2).
 Empty Message
    A message with a Code of 0.00; neither a request nor a response.
    An Empty message only contains the 4-byte header.

Shelby, et al. Standards Track [Page 8] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Critical Option
    An option that would need to be understood by the endpoint
    ultimately receiving the message in order to properly process the
    message (Section 5.4.1).  Note that the implementation of critical
    options is, as the name "Option" implies, generally optional:
    unsupported critical options lead to an error response or summary
    rejection of the message.
 Elective Option
    An option that is intended to be ignored by an endpoint that does
    not understand it.  Processing the message even without
    understanding the option is acceptable (Section 5.4.1).
 Unsafe Option
    An option that would need to be understood by a proxy receiving
    the message in order to safely forward the message
    (Section 5.4.2).  Not every critical option is an unsafe option.
 Safe-to-Forward Option
    An option that is intended to be safe for forwarding by a proxy
    that does not understand it.  Forwarding the message even without
    understanding the option is acceptable (Section 5.4.2).
 Resource Discovery
    The process where a CoAP client queries a server for its list of
    hosted resources (i.e., links as defined in Section 7).
 Content-Format
    The combination of an Internet media type, potentially with
    specific parameters given, and a content-coding (which is often
    the identity content-coding), identified by a numeric identifier
    defined by the "CoAP Content-Formats" registry.  When the focus is
    less on the numeric identifier than on the combination of these
    characteristics of a resource representation, this is also called
    "representation format".
 Additional terminology for constrained nodes and constrained-node
 networks can be found in [RFC7228].
 In this specification, the term "byte" is used in its now customary
 sense as a synonym for "octet".
 All multi-byte integers in this protocol are interpreted in network
 byte order.
 Where arithmetic is used, this specification uses the notation
 familiar from the programming language C, except that the operator
 "**" stands for exponentiation.

Shelby, et al. Standards Track [Page 9] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

2. Constrained Application Protocol

 The interaction model of CoAP is similar to the client/server model
 of HTTP.  However, machine-to-machine interactions typically result
 in a CoAP implementation acting in both client and server roles.  A
 CoAP request is equivalent to that of HTTP and is sent by a client to
 request an action (using a Method Code) on a resource (identified by
 a URI) on a server.  The server then sends a response with a Response
 Code; this response may include a resource representation.
 Unlike HTTP, CoAP deals with these interchanges asynchronously over a
 datagram-oriented transport such as UDP.  This is done logically
 using a layer of messages that supports optional reliability (with
 exponential back-off).  CoAP defines four types of messages:
 Confirmable, Non-confirmable, Acknowledgement, Reset.  Method Codes
 and Response Codes included in some of these messages make them carry
 requests or responses.  The basic exchanges of the four types of
 messages are somewhat orthogonal to the request/response
 interactions; requests can be carried in Confirmable and Non-
 confirmable messages, and responses can be carried in these as well
 as piggybacked in Acknowledgement messages.
 One could think of CoAP logically as using a two-layer approach, a
 CoAP messaging layer used to deal with UDP and the asynchronous
 nature of the interactions, and the request/response interactions
 using Method and Response Codes (see Figure 1).  CoAP is however a
 single protocol, with messaging and request/response as just features
 of the CoAP header.
                      +----------------------+
                      |      Application     |
                      +----------------------+
                      +----------------------+  \
                      |  Requests/Responses  |  |
                      |----------------------|  | CoAP
                      |       Messages       |  |
                      +----------------------+  /
                      +----------------------+
                      |          UDP         |
                      +----------------------+
                  Figure 1: Abstract Layering of CoAP

Shelby, et al. Standards Track [Page 10] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

2.1. Messaging Model

 The CoAP messaging model is based on the exchange of messages over
 UDP between endpoints.
 CoAP uses a short fixed-length binary header (4 bytes) that may be
 followed by compact binary options and a payload.  This message
 format is shared by requests and responses.  The CoAP message format
 is specified in Section 3.  Each message contains a Message ID used
 to detect duplicates and for optional reliability.  (The Message ID
 is compact; its 16-bit size enables up to about 250 messages per
 second from one endpoint to another with default protocol
 parameters.)
 Reliability is provided by marking a message as Confirmable (CON).  A
 Confirmable message is retransmitted using a default timeout and
 exponential back-off between retransmissions, until the recipient
 sends an Acknowledgement message (ACK) with the same Message ID (in
 this example, 0x7d34) from the corresponding endpoint; see Figure 2.
 When a recipient is not at all able to process a Confirmable message
 (i.e., not even able to provide a suitable error response), it
 replies with a Reset message (RST) instead of an Acknowledgement
 (ACK).
                      Client              Server
                         |                  |
                         |   CON [0x7d34]   |
                         +----------------->|
                         |                  |
                         |   ACK [0x7d34]   |
                         |<-----------------+
                         |                  |
                Figure 2: Reliable Message Transmission
 A message that does not require reliable transmission (for example,
 each single measurement out of a stream of sensor data) can be sent
 as a Non-confirmable message (NON).  These are not acknowledged, but
 still have a Message ID for duplicate detection (in this example,
 0x01a0); see Figure 3.  When a recipient is not able to process a
 Non-confirmable message, it may reply with a Reset message (RST).

Shelby, et al. Standards Track [Page 11] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

                      Client              Server
                         |                  |
                         |   NON [0x01a0]   |
                         +----------------->|
                         |                  |
               Figure 3: Unreliable Message Transmission
 See Section 4 for details of CoAP messages.
 As CoAP runs over UDP, it also supports the use of multicast IP
 destination addresses, enabling multicast CoAP requests.  Section 8
 discusses the proper use of CoAP messages with multicast addresses
 and precautions for avoiding response congestion.
 Several security modes are defined for CoAP in Section 9 ranging from
 no security to certificate-based security.  This document specifies a
 binding to DTLS for securing the protocol; the use of IPsec with CoAP
 is discussed in [IPsec-CoAP].

2.2. Request/Response Model

 CoAP request and response semantics are carried in CoAP messages,
 which include either a Method Code or Response Code, respectively.
 Optional (or default) request and response information, such as the
 URI and payload media type are carried as CoAP options.  A Token is
 used to match responses to requests independently from the underlying
 messages (Section 5.3).  (Note that the Token is a concept separate
 from the Message ID.)
 A request is carried in a Confirmable (CON) or Non-confirmable (NON)
 message, and, if immediately available, the response to a request
 carried in a Confirmable message is carried in the resulting
 Acknowledgement (ACK) message.  This is called a piggybacked
 response, detailed in Section 5.2.1.  (There is no need for
 separately acknowledging a piggybacked response, as the client will
 retransmit the request if the Acknowledgement message carrying the
 piggybacked response is lost.)  Two examples for a basic GET request
 with piggybacked response are shown in Figure 4, one successful, one
 resulting in a 4.04 (Not Found) response.

Shelby, et al. Standards Track [Page 12] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

      Client              Server       Client              Server
         |                  |             |                  |
         |   CON [0xbc90]   |             |   CON [0xbc91]   |
         | GET /temperature |             | GET /temperature |
         |   (Token 0x71)   |             |   (Token 0x72)   |
         +----------------->|             +----------------->|
         |                  |             |                  |
         |   ACK [0xbc90]   |             |   ACK [0xbc91]   |
         |   2.05 Content   |             |  4.04 Not Found  |
         |   (Token 0x71)   |             |   (Token 0x72)   |
         |     "22.5 C"     |             |   "Not found"    |
         |<-----------------+             |<-----------------+
         |                  |             |                  |
         Figure 4: Two GET Requests with Piggybacked Responses
 If the server is not able to respond immediately to a request carried
 in a Confirmable message, it simply responds with an Empty
 Acknowledgement message so that the client can stop retransmitting
 the request.  When the response is ready, the server sends it in a
 new Confirmable message (which then in turn needs to be acknowledged
 by the client).  This is called a "separate response", as illustrated
 in Figure 5 and described in more detail in Section 5.2.2.
                      Client              Server
                         |                  |
                         |   CON [0x7a10]   |
                         | GET /temperature |
                         |   (Token 0x73)   |
                         +----------------->|
                         |                  |
                         |   ACK [0x7a10]   |
                         |<-----------------+
                         |                  |
                         ... Time Passes  ...
                         |                  |
                         |   CON [0x23bb]   |
                         |   2.05 Content   |
                         |   (Token 0x73)   |
                         |     "22.5 C"     |
                         |<-----------------+
                         |                  |
                         |   ACK [0x23bb]   |
                         +----------------->|
                         |                  |
           Figure 5: A GET Request with a Separate Response

Shelby, et al. Standards Track [Page 13] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 If a request is sent in a Non-confirmable message, then the response
 is sent using a new Non-confirmable message, although the server may
 instead send a Confirmable message.  This type of exchange is
 illustrated in Figure 6.
                      Client              Server
                         |                  |
                         |   NON [0x7a11]   |
                         | GET /temperature |
                         |   (Token 0x74)   |
                         +----------------->|
                         |                  |
                         |   NON [0x23bc]   |
                         |   2.05 Content   |
                         |   (Token 0x74)   |
                         |     "22.5 C"     |
                         |<-----------------+
                         |                  |
     Figure 6: A Request and a Response Carried in Non-confirmable
                               Messages
 CoAP makes use of GET, PUT, POST, and DELETE methods in a similar
 manner to HTTP, with the semantics specified in Section 5.8.  (Note
 that the detailed semantics of CoAP methods are "almost, but not
 entirely unlike" [HHGTTG] those of HTTP methods: intuition taken from
 HTTP experience generally does apply well, but there are enough
 differences that make it worthwhile to actually read the present
 specification.)
 Methods beyond the basic four can be added to CoAP in separate
 specifications.  New methods do not necessarily have to use requests
 and responses in pairs.  Even for existing methods, a single request
 may yield multiple responses, e.g., for a multicast request
 (Section 8) or with the Observe option [OBSERVE].
 URI support in a server is simplified as the client already parses
 the URI and splits it into host, port, path, and query components,
 making use of default values for efficiency.  Response Codes relate
 to a small subset of HTTP status codes with a few CoAP-specific codes
 added, as defined in Section 5.9.

Shelby, et al. Standards Track [Page 14] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

2.3. Intermediaries and Caching

 The protocol supports the caching of responses in order to
 efficiently fulfill requests.  Simple caching is enabled using
 freshness and validity information carried with CoAP responses.  A
 cache could be located in an endpoint or an intermediary.  Caching
 functionality is specified in Section 5.6.
 Proxying is useful in constrained networks for several reasons,
 including to limit network traffic, to improve performance, to access
 resources of sleeping devices, and for security reasons.  The
 proxying of requests on behalf of another CoAP endpoint is supported
 in the protocol.  When using a proxy, the URI of the resource to
 request is included in the request, while the destination IP address
 is set to the address of the proxy.  See Section 5.7 for more
 information on proxy functionality.
 As CoAP was designed according to the REST architecture [REST], and
 thus exhibits functionality similar to that of the HTTP protocol, it
 is quite straightforward to map from CoAP to HTTP and from HTTP to
 CoAP.  Such a mapping may be used to realize an HTTP REST interface
 using CoAP or to convert between HTTP and CoAP.  This conversion can
 be carried out by a cross-protocol proxy ("cross-proxy"), which
 converts the Method or Response Code, media type, and options to the
 corresponding HTTP feature.  Section 10 provides more detail about
 HTTP mapping.

2.4. Resource Discovery

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

3. Message Format

 CoAP is based on the exchange of compact messages that, by default,
 are transported over UDP (i.e., each CoAP message occupies the data
 section of one UDP datagram).  CoAP may also be used over Datagram
 Transport Layer Security (DTLS) (see Section 9.1).  It could also be
 used over other transports such as SMS, TCP, or SCTP, the
 specification of which is out of this document's scope.  (UDP-lite
 [RFC3828] and UDP zero checksum [RFC6936] are not supported by CoAP.)
 CoAP messages are encoded in a simple binary format.  The message
 format starts with a fixed-size 4-byte header.  This is followed by a
 variable-length Token value, which can be between 0 and 8 bytes long.

Shelby, et al. Standards Track [Page 15] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Following the Token value comes a sequence of zero or more CoAP
 Options in Type-Length-Value (TLV) format, optionally followed by a
 payload that takes up the rest of the datagram.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |Ver| T |  TKL  |      Code     |          Message ID           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Token (if any, TKL bytes) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Options (if any) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1 1 1 1 1 1 1 1|    Payload (if any) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 7: Message Format
 The fields in the header are defined as follows:
 Version (Ver):  2-bit unsigned integer.  Indicates the CoAP version
    number.  Implementations of this specification MUST set this field
    to 1 (01 binary).  Other values are reserved for future versions.
    Messages with unknown version numbers MUST be silently ignored.
 Type (T):  2-bit unsigned integer.  Indicates if this message is of
    type Confirmable (0), Non-confirmable (1), Acknowledgement (2), or
    Reset (3).  The semantics of these message types are defined in
    Section 4.
 Token Length (TKL):  4-bit unsigned integer.  Indicates the length of
    the variable-length Token field (0-8 bytes).  Lengths 9-15 are
    reserved, MUST NOT be sent, and MUST be processed as a message
    format error.
 Code:  8-bit unsigned integer, split into a 3-bit class (most
    significant bits) and a 5-bit detail (least significant bits),
    documented as "c.dd" where "c" is a digit from 0 to 7 for the
    3-bit subfield and "dd" are two digits from 00 to 31 for the 5-bit
    subfield.  The class can indicate a request (0), a success
    response (2), a client error response (4), or a server error
    response (5).  (All other class values are reserved.)  As a
    special case, Code 0.00 indicates an Empty message.  In case of a
    request, the Code field indicates the Request Method; in case of a
    response, a Response Code.  Possible values are maintained in the
    CoAP Code Registries (Section 12.1).  The semantics of requests
    and responses are defined in Section 5.

Shelby, et al. Standards Track [Page 16] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Message ID:  16-bit unsigned integer in network byte order.  Used to
    detect message duplication and to match messages of type
    Acknowledgement/Reset to messages of type Confirmable/Non-
    confirmable.  The rules for generating a Message ID and matching
    messages are defined in Section 4.
 The header is followed by the Token value, which may be 0 to 8 bytes,
 as given by the Token Length field.  The Token value is used to
 correlate requests and responses.  The rules for generating a Token
 and correlating requests and responses are defined in Section 5.3.1.
 Header and Token are followed by zero or more Options (Section 3.1).
 An Option can be followed by the end of the message, by another
 Option, or by the Payload Marker and the payload.
 Following the header, token, and options, if any, comes the optional
 payload.  If present and of non-zero length, it is prefixed by a
 fixed, one-byte Payload Marker (0xFF), which indicates the end of
 options and the start of the payload.  The payload data extends from
 after the marker to the end of the UDP datagram, i.e., the Payload
 Length is calculated from the datagram size.  The absence of the
 Payload Marker denotes a zero-length payload.  The presence of a
 marker followed by a zero-length payload MUST be processed as a
 message format error.
 Implementation Note:  The byte value 0xFF may also occur within an
    option length or value, so simple byte-wise scanning for 0xFF is
    not a viable technique for finding the payload marker.  The byte
    0xFF has the meaning of a payload marker only where the beginning
    of another option could occur.

3.1. Option Format

 CoAP defines a number of options that can be included in a message.
 Each option instance in a message specifies the Option Number of the
 defined CoAP option, the length of the Option Value, and the Option
 Value itself.
 Instead of specifying the Option Number directly, the instances MUST
 appear in order of their Option Numbers and a delta encoding is used
 between them: the Option Number for each instance is calculated as
 the sum of its delta and the Option Number of the preceding instance
 in the message.  For the first instance in a message, a preceding
 option instance with Option Number zero is assumed.  Multiple
 instances of the same option can be included by using a delta of
 zero.

Shelby, et al. Standards Track [Page 17] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Option Numbers are maintained in the "CoAP Option Numbers" registry
 (Section 12.2).  See Section 5.4 for the semantics of the options
 defined in this document.
   0   1   2   3   4   5   6   7
 +---------------+---------------+
 |               |               |
 |  Option Delta | Option Length |   1 byte
 |               |               |
 +---------------+---------------+
 \                               \
 /         Option Delta          /   0-2 bytes
 \          (extended)           \
 +-------------------------------+
 \                               \
 /         Option Length         /   0-2 bytes
 \          (extended)           \
 +-------------------------------+
 \                               \
 /                               /
 \                               \
 /         Option Value          /   0 or more bytes
 \                               \
 /                               /
 \                               \
 +-------------------------------+
                        Figure 8: Option Format
 The fields in an option are defined as follows:
 Option Delta:  4-bit unsigned integer.  A value between 0 and 12
    indicates the Option Delta.  Three values are reserved for special
    constructs:
    13:  An 8-bit unsigned integer follows the initial byte and
       indicates the Option Delta minus 13.
    14:  A 16-bit unsigned integer in network byte order follows the
       initial byte and indicates the Option Delta minus 269.
    15:  Reserved for the Payload Marker.  If the field is set to this
       value but the entire byte is not the payload marker, this MUST
       be processed as a message format error.

Shelby, et al. Standards Track [Page 18] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

    The resulting Option Delta is used as the difference between the
    Option Number of this option and that of the previous option (or
    zero for the first option).  In other words, the Option Number is
    calculated by simply summing the Option Delta values of this and
    all previous options before it.
 Option Length:  4-bit unsigned integer.  A value between 0 and 12
    indicates the length of the Option Value, in bytes.  Three values
    are reserved for special constructs:
    13:  An 8-bit unsigned integer precedes the Option Value and
       indicates the Option Length minus 13.
    14:  A 16-bit unsigned integer in network byte order precedes the
       Option Value and indicates the Option Length minus 269.
    15:  Reserved for future use.  If the field is set to this value,
       it MUST be processed as a message format error.
 Value:  A sequence of exactly Option Length bytes.  The length and
    format of the Option Value depend on the respective option, which
    MAY define variable-length values.  See Section 3.2 for the
    formats used in this document; options defined in other documents
    MAY make use of other option value formats.

3.2. Option Value Formats

 The options defined in this document make use of the following option
 value formats.
 empty:    A zero-length sequence of bytes.
 opaque:   An opaque sequence of bytes.
 uint:     A non-negative integer that is represented in network byte
           order using the number of bytes given by the Option Length
           field.
           An option definition may specify a range of permissible
           numbers of bytes; if it has a choice, a sender SHOULD
           represent the integer with as few bytes as possible, i.e.,
           without leading zero bytes.  For example, the number 0 is
           represented with an empty option value (a zero-length
           sequence of bytes) and the number 1 by a single byte with
           the numerical value of 1 (bit combination 00000001 in most
           significant bit first notation).  A recipient MUST be
           prepared to process values with leading zero bytes.

Shelby, et al. Standards Track [Page 19] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

           Implementation Note:  The exceptional behavior permitted
              for the sender is intended for highly constrained,
              templated implementations (e.g., hardware
              implementations) that use fixed-size options in the
              templates.
 string:   A Unicode string that is encoded using UTF-8 [RFC3629] in
           Net-Unicode form [RFC5198].
           Note that here, and in all other places where UTF-8
           encoding is used in the CoAP protocol, the intention is
           that the encoded strings can be directly used and compared
           as opaque byte strings by CoAP protocol implementations.
           There is no expectation and no need to perform
           normalization within a CoAP implementation (except where
           Unicode strings that are not known to be normalized are
           imported from sources outside the CoAP protocol).  Note
           also that ASCII strings (that do not make use of special
           control characters) are always valid UTF-8 Net-Unicode
           strings.

4. Message Transmission

 CoAP messages are exchanged asynchronously between CoAP endpoints.
 They are used to transport CoAP requests and responses, the semantics
 of which are defined in Section 5.
 As CoAP is bound to unreliable transports such as UDP, CoAP messages
 may arrive out of order, appear duplicated, or go missing without
 notice.  For this reason, CoAP implements a lightweight reliability
 mechanism, without trying to re-create the full feature set of a
 transport like TCP.  It has the following features:
 o  Simple stop-and-wait retransmission reliability with exponential
    back-off for Confirmable messages.
 o  Duplicate detection for both Confirmable and Non-confirmable
    messages.

4.1. Messages and Endpoints

 A CoAP endpoint is the source or destination of a CoAP message.  The
 specific definition of an endpoint depends on the transport being
 used for CoAP.  For the transports defined in this specification, the
 endpoint is identified depending on the security mode used (see
 Section 9): With no security, the endpoint is solely identified by an
 IP address and a UDP port number.  With other security modes, the
 endpoint is identified as defined by the security mode.

Shelby, et al. Standards Track [Page 20] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 There are different types of messages.  The type of a message is
 specified by the Type field of the CoAP Header.
 Separate from the message type, a message may carry a request, a
 response, or be Empty.  This is signaled by the Request/Response Code
 field in the CoAP Header and is relevant to the request/response
 model.  Possible values for the field are maintained in the CoAP Code
 Registries (Section 12.1).
 An Empty message has the Code field set to 0.00.  The Token Length
 field MUST be set to 0 and bytes of data MUST NOT be present after
 the Message ID field.  If there are any bytes, they MUST be processed
 as a message format error.

4.2. Messages Transmitted Reliably

 The reliable transmission of a message is initiated by marking the
 message as Confirmable in the CoAP header.  A Confirmable message
 always carries either a request or response, unless it is used only
 to elicit a Reset message, in which case it is Empty.  A recipient
 MUST either (a) acknowledge a Confirmable message with an
 Acknowledgement message or (b) reject the message if the recipient
 lacks context to process the message properly, including situations
 where the message is Empty, uses a code with a reserved class (1, 6,
 or 7), or has a message format error.  Rejecting a Confirmable
 message is effected by sending a matching Reset message and otherwise
 ignoring it.  The Acknowledgement message MUST echo the Message ID of
 the Confirmable message and MUST carry a response or be Empty (see
 Sections 5.2.1 and 5.2.2).  The Reset message MUST echo the Message
 ID of the Confirmable message and MUST be Empty.  Rejecting an
 Acknowledgement or Reset message (including the case where the
 Acknowledgement carries a request or a code with a reserved class, or
 the Reset message is not Empty) is effected by silently ignoring it.
 More generally, recipients of Acknowledgement and Reset messages MUST
 NOT respond with either Acknowledgement or Reset messages.
 The sender retransmits the Confirmable message at exponentially
 increasing intervals, until it receives an acknowledgement (or Reset
 message) or runs out of attempts.
 Retransmission is controlled by two things that a CoAP endpoint MUST
 keep track of for each Confirmable message it sends while waiting for
 an acknowledgement (or reset): a timeout and a retransmission
 counter.  For a new Confirmable message, the initial timeout is set
 to a random duration (often not an integral number of seconds)
 between ACK_TIMEOUT and (ACK_TIMEOUT * ACK_RANDOM_FACTOR) (see
 Section 4.8), and the retransmission counter is set to 0.  When the
 timeout is triggered and the retransmission counter is less than

Shelby, et al. Standards Track [Page 21] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 MAX_RETRANSMIT, the message is retransmitted, the retransmission
 counter is incremented, and the timeout is doubled.  If the
 retransmission counter reaches MAX_RETRANSMIT on a timeout, or if the
 endpoint receives a Reset message, then the attempt to transmit the
 message is canceled and the application process informed of failure.
 On the other hand, if the endpoint receives an acknowledgement in
 time, transmission is considered successful.
 This specification makes no strong requirements on the accuracy of
 the clocks used to implement the above binary exponential back-off
 algorithm.  In particular, an endpoint may be late for a specific
 retransmission due to its sleep schedule and may catch up on the next
 one.  However, the minimum spacing before another retransmission is
 ACK_TIMEOUT, and the entire sequence of (re-)transmissions MUST stay
 in the envelope of MAX_TRANSMIT_SPAN (see Section 4.8.2), even if
 that means a sender may miss an opportunity to transmit.
 A CoAP endpoint that sent a Confirmable message MAY give up in
 attempting to obtain an ACK even before the MAX_RETRANSMIT counter
 value is reached.  For example, the application has canceled the
 request as it no longer needs a response, or there is some other
 indication that the CON message did arrive.  In particular, a CoAP
 request message may have elicited a separate response, in which case
 it is clear to the requester that only the ACK was lost and a
 retransmission of the request would serve no purpose.  However, a
 responder MUST NOT in turn rely on this cross-layer behavior from a
 requester, i.e., it MUST retain the state to create the ACK for the
 request, if needed, even if a Confirmable response was already
 acknowledged by the requester.
 Another reason for giving up retransmission MAY be the receipt of
 ICMP errors.  If it is desired to take account of ICMP errors, to
 mitigate potential spoofing attacks, implementations SHOULD take care
 to check the information about the original datagram in the ICMP
 message, including port numbers and CoAP header information such as
 message type and code, Message ID, and Token; if this is not possible
 due to limitations of the UDP service API, ICMP errors SHOULD be
 ignored.  Packet Too Big errors [RFC4443] ("fragmentation needed and
 DF set" for IPv4 [RFC0792]) cannot properly occur and SHOULD be
 ignored if the implementation note in Section 4.6 is followed;
 otherwise, they SHOULD feed into a path MTU discovery algorithm
 [RFC4821].  Source Quench and Time Exceeded ICMP messages SHOULD be
 ignored.  Host, network, port, or protocol unreachable errors or
 parameter problem errors MAY, after appropriate vetting, be used to
 inform the application of a failure in sending.

Shelby, et al. Standards Track [Page 22] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

4.3. Messages Transmitted without Reliability

 Some messages do not require an acknowledgement.  This is
 particularly true for messages that are repeated regularly for
 application requirements, such as repeated readings from a sensor
 where eventual success is sufficient.
 As a more lightweight alternative, a message can be transmitted less
 reliably by marking the message as Non-confirmable.  A Non-
 confirmable message always carries either a request or response and
 MUST NOT be Empty.  A Non-confirmable message MUST NOT be
 acknowledged by the recipient.  A recipient MUST reject the message
 if it lacks context to process the message properly, including the
 case where the message is Empty, uses a code with a reserved class
 (1, 6, or 7), or has a message format error.  Rejecting a Non-
 confirmable message MAY involve sending a matching Reset message, and
 apart from the Reset message the rejected message MUST be silently
 ignored.
 At the CoAP level, there is no way for the sender to detect if a Non-
 confirmable message was received or not.  A sender MAY choose to
 transmit multiple copies of a Non-confirmable message within
 MAX_TRANSMIT_SPAN (limited by the provisions of Section 4.7, in
 particular, by PROBING_RATE if no response is received), or the
 network may duplicate the message in transit.  To enable the receiver
 to act only once on the message, Non-confirmable messages specify a
 Message ID as well.  (This Message ID is drawn from the same number
 space as the Message IDs for Confirmable messages.)
 Summarizing Sections 4.2 and 4.3, the four message types can be used
 as in Table 1.  "*" means that the combination is not used in normal
 operation but only to elicit a Reset message ("CoAP ping").
                 +----------+-----+-----+-----+-----+
                 |          | CON | NON | ACK | RST |
                 +----------+-----+-----+-----+-----+
                 | Request  | X   | X   | -   | -   |
                 | Response | X   | X   | X   | -   |
                 | Empty    | *   | -   | X   | X   |
                 +----------+-----+-----+-----+-----+
                    Table 1: Usage of Message Types

Shelby, et al. Standards Track [Page 23] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

4.4. Message Correlation

 An Acknowledgement or Reset message is related to a Confirmable
 message or Non-confirmable message by means of a Message ID along
 with additional address information of the corresponding endpoint.
 The Message ID is a 16-bit unsigned integer that is generated by the
 sender of a Confirmable or Non-confirmable message and included in
 the CoAP header.  The Message ID MUST be echoed in the
 Acknowledgement or Reset message by the recipient.
 The same Message ID MUST NOT be reused (in communicating with the
 same endpoint) within the EXCHANGE_LIFETIME (Section 4.8.2).
 Implementation Note:  Several implementation strategies can be
    employed for generating Message IDs.  In the simplest case, a CoAP
    endpoint generates Message IDs by keeping a single Message ID
    variable, which is changed each time a new Confirmable or Non-
    confirmable message is sent, regardless of the destination address
    or port.  Endpoints dealing with large numbers of transactions
    could keep multiple Message ID variables, for example, per prefix
    or destination address.  (Note that some receiving endpoints may
    not be able to distinguish unicast and multicast packets addressed
    to it, so endpoints generating Message IDs need to make sure these
    do not overlap.)  It is strongly recommended that the initial
    value of the variable (e.g., on startup) be randomized, in order
    to make successful off-path attacks on the protocol less likely.
 For an Acknowledgement or Reset message to match a Confirmable or
 Non-confirmable message, the Message ID and source endpoint of the
 Acknowledgement or Reset message MUST match the Message ID and
 destination endpoint of the Confirmable or Non-confirmable message.

4.5. Message Deduplication

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

Shelby, et al. Standards Track [Page 24] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 o  A server might relax the requirement to answer all retransmissions
    of an idempotent request with the same response (Section 4.2), so
    that it does not have to maintain state for Message IDs.  For
    example, an implementation might want to process duplicate
    transmissions of a GET, PUT, or DELETE request as separate
    requests if the effort incurred by duplicate processing is less
    expensive than keeping track of previous responses would be.
 o  A constrained server might even want to relax this requirement for
    certain non-idempotent requests if the application semantics make
    this trade-off favorable.  For example, if the result of a POST
    request is just the creation of some short-lived state at the
    server, it may be less expensive to incur this effort multiple
    times for a request than keeping track of whether a previous
    transmission of the same request already was processed.
 A recipient might receive the same Non-confirmable message (as
 indicated by the Message ID and source endpoint) multiple times
 within NON_LIFETIME (Section 4.8.2).  As a general rule that MAY be
 relaxed based on the specific semantics of a message, the recipient
 SHOULD silently ignore any duplicated Non-confirmable message and
 SHOULD process any request or response in the message only once.

4.6. Message Size

 While specific link layers make it beneficial to keep CoAP messages
 small enough to fit into their link-layer packets (see Section 1),
 this is a matter of implementation quality.  The CoAP specification
 itself provides only an upper bound to the message size.  Messages
 larger than an IP packet result in undesirable packet fragmentation.
 A CoAP message, appropriately encapsulated, SHOULD fit within a
 single IP packet (i.e., avoid IP fragmentation) and (by fitting into
 one UDP payload) obviously needs to fit within a single IP datagram.
 If the Path MTU is not known for a destination, an IP MTU of 1280
 bytes SHOULD be assumed; if nothing is known about the size of the
 headers, good upper bounds are 1152 bytes for the message size and
 1024 bytes for the payload size.
 Implementation Note:  CoAP's choice of message size parameters works
    well with IPv6 and with most of today's IPv4 paths.  (However,
    with IPv4, it is harder to absolutely ensure that there is no IP
    fragmentation.  If IPv4 support on unusual networks is a
    consideration, implementations may want to limit themselves to
    more conservative IPv4 datagram sizes such as 576 bytes; per
    [RFC0791], the absolute minimum value of the IP MTU for IPv4 is as
    low as 68 bytes, which would leave only 40 bytes minus security
    overhead for a UDP payload.  Implementations extremely focused on
    this problem set might also set the IPv4 DF bit and perform some

Shelby, et al. Standards Track [Page 25] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

    form of path MTU discovery [RFC4821]; this should generally be
    unnecessary in realistic use cases for CoAP, however.)  A more
    important kind of fragmentation in many constrained networks is
    that on the adaptation layer (e.g., 6LoWPAN L2 packets are limited
    to 127 bytes including various overheads); this may motivate
    implementations to be frugal in their packet sizes and to move to
    block-wise transfers [BLOCK] when approaching three-digit message
    sizes.
    Message sizes are also of considerable importance to
    implementations on constrained nodes.  Many implementations will
    need to allocate a buffer for incoming messages.  If an
    implementation is too constrained to allow for allocating the
    above-mentioned upper bound, it could apply the following
    implementation strategy for messages not using DTLS security:
    Implementations receiving a datagram into a buffer that is too
    small are usually able to determine if the trailing portion of a
    datagram was discarded and to retrieve the initial portion.  So,
    at least the CoAP header and options, if not all of the payload,
    are likely to fit within the buffer.  A server can thus fully
    interpret a request and return a 4.13 (Request Entity Too Large;
    see Section 5.9.2.9) Response Code if the payload was truncated.
    A client sending an idempotent request and receiving a response
    larger than would fit in the buffer can repeat the request with a
    suitable value for the Block Option [BLOCK].

4.7. Congestion Control

 Basic congestion control for CoAP is provided by the exponential
 back-off mechanism in Section 4.2.
 In order not to cause congestion, clients (including proxies) MUST
 strictly limit the number of simultaneous outstanding interactions
 that they maintain to a given server (including proxies) to NSTART.
 An outstanding interaction is either a CON for which an ACK has not
 yet been received but is still expected (message layer) or a request
 for which neither a response nor an Acknowledgment message has yet
 been received but is still expected (which may both occur at the same
 time, counting as one outstanding interaction).  The default value of
 NSTART for this specification is 1.
 Further congestion control optimizations and considerations are
 expected in the future, may for example provide automatic
 initialization of the CoAP transmission parameters defined in
 Section 4.8, and thus may allow a value for NSTART greater than one.
 After EXCHANGE_LIFETIME, a client stops expecting a response to a
 Confirmable request for which no acknowledgment message was received.

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 The specific algorithm by which a client stops to "expect" a response
 to a Confirmable request that was acknowledged, or to a Non-
 confirmable request, is not defined.  Unless this is modified by
 additional congestion control optimizations, it MUST be chosen in
 such a way that an endpoint does not exceed an average data rate of
 PROBING_RATE in sending to another endpoint that does not respond.
 Note:  CoAP places the onus of congestion control mostly on the
    clients.  However, clients may malfunction or actually be
    attackers, e.g., to perform amplification attacks (Section 11.3).
    To limit the damage (to the network and to its own energy
    resources), a server SHOULD implement some rate limiting for its
    response transmission based on reasonable assumptions about
    application requirements.  This is most helpful if the rate limit
    can be made effective for the misbehaving endpoints, only.

4.8. Transmission Parameters

 Message transmission is controlled by the following parameters:
                 +-------------------+---------------+
                 | name              | default value |
                 +-------------------+---------------+
                 | ACK_TIMEOUT       | 2 seconds     |
                 | ACK_RANDOM_FACTOR | 1.5           |
                 | MAX_RETRANSMIT    | 4             |
                 | NSTART            | 1             |
                 | DEFAULT_LEISURE   | 5 seconds     |
                 | PROBING_RATE      | 1 byte/second |
                 +-------------------+---------------+
                   Table 2: CoAP Protocol Parameters

4.8.1. Changing the Parameters

 The values for ACK_TIMEOUT, ACK_RANDOM_FACTOR, MAX_RETRANSMIT,
 NSTART, DEFAULT_LEISURE (Section 8.2), and PROBING_RATE may be
 configured to values specific to the application environment
 (including dynamically adjusted values); however, the configuration
 method is out of scope of this document.  It is RECOMMENDED that an
 application environment use consistent values for these parameters;
 the specific effects of operating with inconsistent values in an
 application environment are outside the scope of the present
 specification.
 The transmission parameters have been chosen to achieve a behavior in
 the presence of congestion that is safe in the Internet.  If a
 configuration desires to use different values, the onus is on the

Shelby, et al. Standards Track [Page 27] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 configuration to ensure these congestion control properties are not
 violated.  In particular, a decrease of ACK_TIMEOUT below 1 second
 would violate the guidelines of [RFC5405].  ([RTO-CONSIDER] provides
 some additional background.)  CoAP was designed to enable
 implementations that do not maintain round-trip-time (RTT)
 measurements.  However, where it is desired to decrease the
 ACK_TIMEOUT significantly or increase NSTART, this can only be done
 safely when maintaining such measurements.  Configurations MUST NOT
 decrease ACK_TIMEOUT or increase NSTART without using mechanisms that
 ensure congestion control safety, either defined in the configuration
 or in future standards documents.
 ACK_RANDOM_FACTOR MUST NOT be decreased below 1.0, and it SHOULD have
 a value that is sufficiently different from 1.0 to provide some
 protection from synchronization effects.
 MAX_RETRANSMIT can be freely adjusted, but a value that is too small
 will reduce the probability that a Confirmable message is actually
 received, while a larger value than given here will require further
 adjustments in the time values (see Section 4.8.2).
 If the choice of transmission parameters leads to an increase of
 derived time values (see Section 4.8.2), the configuration mechanism
 MUST ensure the adjusted value is also available to all the endpoints
 with which these adjusted values are to be used to communicate.

4.8.2. Time Values Derived from Transmission Parameters

 The combination of ACK_TIMEOUT, ACK_RANDOM_FACTOR, and MAX_RETRANSMIT
 influences the timing of retransmissions, which in turn influences
 how long certain information items need to be kept by an
 implementation.  To be able to unambiguously reference these derived
 time values, we give them names as follows:
 o  MAX_TRANSMIT_SPAN is the maximum time from the first transmission
    of a Confirmable message to its last retransmission.  For the
    default transmission parameters, the value is (2+4+8+16)*1.5 = 45
    seconds, or more generally:
       ACK_TIMEOUT * ((2 ** MAX_RETRANSMIT) - 1) * ACK_RANDOM_FACTOR

Shelby, et al. Standards Track [Page 28] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 o  MAX_TRANSMIT_WAIT is the maximum time from the first transmission
    of a Confirmable message to the time when the sender gives up on
    receiving an acknowledgement or reset.  For the default
    transmission parameters, the value is (2+4+8+16+32)*1.5 = 93
    seconds, or more generally:
       ACK_TIMEOUT * ((2 ** (MAX_RETRANSMIT + 1)) - 1) *
       ACK_RANDOM_FACTOR
 In addition, some assumptions need to be made on the characteristics
 of the network and the nodes.
 o  MAX_LATENCY is the maximum time a datagram is expected to take
    from the start of its transmission to the completion of its
    reception.  This constant is related to the MSL (Maximum Segment
    Lifetime) of [RFC0793], which is "arbitrarily defined to be 2
    minutes" ([RFC0793] glossary, page 81).  Note that this is not
    necessarily smaller than MAX_TRANSMIT_WAIT, as MAX_LATENCY is not
    intended to describe a situation when the protocol works well, but
    the worst-case situation against which the protocol has to guard.
    We, also arbitrarily, define MAX_LATENCY to be 100 seconds.  Apart
    from being reasonably realistic for the bulk of configurations as
    well as close to the historic choice for TCP, this value also
    allows Message ID lifetime timers to be represented in 8 bits
    (when measured in seconds).  In these calculations, there is no
    assumption that the direction of the transmission is irrelevant
    (i.e., that the network is symmetric); there is just the
    assumption that the same value can reasonably be used as a maximum
    value for both directions.  If that is not the case, the following
    calculations become only slightly more complex.
 o  PROCESSING_DELAY is the time a node takes to turn around a
    Confirmable message into an acknowledgement.  We assume the node
    will attempt to send an ACK before having the sender time out, so
    as a conservative assumption we set it equal to ACK_TIMEOUT.
 o  MAX_RTT is the maximum round-trip time, or:
       (2 * MAX_LATENCY) + PROCESSING_DELAY
 From these values, we can derive the following values relevant to the
 protocol operation:
 o  EXCHANGE_LIFETIME is the time from starting to send a Confirmable
    message to the time when an acknowledgement is no longer expected,
    i.e., message-layer information about the message exchange can be
    purged.  EXCHANGE_LIFETIME includes a MAX_TRANSMIT_SPAN, a
    MAX_LATENCY forward, PROCESSING_DELAY, and a MAX_LATENCY for the

Shelby, et al. Standards Track [Page 29] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

    way back.  Note that there is no need to consider
    MAX_TRANSMIT_WAIT if the configuration is chosen such that the
    last waiting period (ACK_TIMEOUT * (2 ** MAX_RETRANSMIT) or the
    difference between MAX_TRANSMIT_SPAN and MAX_TRANSMIT_WAIT) is
    less than MAX_LATENCY -- which is a likely choice, as MAX_LATENCY
    is a worst-case value unlikely to be met in the real world.  In
    this case, EXCHANGE_LIFETIME simplifies to:
       MAX_TRANSMIT_SPAN + (2 * MAX_LATENCY) + PROCESSING_DELAY
    or 247 seconds with the default transmission parameters.
 o  NON_LIFETIME is the time from sending a Non-confirmable message to
    the time its Message ID can be safely reused.  If multiple
    transmission of a NON message is not used, its value is
    MAX_LATENCY, or 100 seconds.  However, a CoAP sender might send a
    NON message multiple times, in particular for multicast
    applications.  While the period of reuse is not bounded by the
    specification, an expectation of reliable detection of duplication
    at the receiver is on the timescales of MAX_TRANSMIT_SPAN.
    Therefore, for this purpose, it is safer to use the value:
       MAX_TRANSMIT_SPAN + MAX_LATENCY
    or 145 seconds with the default transmission parameters; however,
    an implementation that just wants to use a single timeout value
    for retiring Message IDs can safely use the larger value for
    EXCHANGE_LIFETIME.
 Table 3 lists the derived parameters introduced in this subsection
 with their default values.
                 +-------------------+---------------+
                 | name              | default value |
                 +-------------------+---------------+
                 | MAX_TRANSMIT_SPAN |          45 s |
                 | MAX_TRANSMIT_WAIT |          93 s |
                 | MAX_LATENCY       |         100 s |
                 | PROCESSING_DELAY  |           2 s |
                 | MAX_RTT           |         202 s |
                 | EXCHANGE_LIFETIME |         247 s |
                 | NON_LIFETIME      |         145 s |
                 +-------------------+---------------+
                 Table 3: Derived Protocol Parameters

Shelby, et al. Standards Track [Page 30] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

5. Request/Response Semantics

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

5.1. Requests

 A CoAP request consists of the method to be applied to the resource,
 the identifier of the resource, a payload and Internet media type (if
 any), and optional metadata about the request.
 CoAP supports the basic methods of GET, POST, PUT, and DELETE, which
 are easily mapped to HTTP.  They have the same properties of safe
 (only retrieval) and idempotent (you can invoke it multiple times
 with the same effects) as HTTP (see Section 9.1 of [RFC2616]).  The
 GET method is safe; therefore, it MUST NOT take any other action on a
 resource other than retrieval.  The GET, PUT, and DELETE methods MUST
 be performed in such a way that they are idempotent.  POST is not
 idempotent, because its effect is determined by the origin server and
 dependent on the target resource; it usually results in a new
 resource being created or the target resource being updated.
 A request is initiated by setting the Code field in the CoAP header
 of a Confirmable or a Non-confirmable message to a Method Code and
 including request information.
 The methods used in requests are described in detail in Section 5.8.

5.2. Responses

 After receiving and interpreting a request, a server responds with a
 CoAP response that is matched to the request by means of a client-
 generated token (Section 5.3); note that this is different from the
 Message ID that matches a Confirmable message to its Acknowledgement.
 A response is identified by the Code field in the CoAP header being
 set to a Response Code.  Similar to the HTTP Status Code, the CoAP
 Response Code indicates the result of the attempt to understand and
 satisfy the request.  These codes are fully defined in Section 5.9.
 The Response Code numbers to be set in the Code field of the CoAP
 header are maintained in the CoAP Response Code Registry
 (Section 12.1.2).

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                            0
                            0 1 2 3 4 5 6 7
                           +-+-+-+-+-+-+-+-+
                           |class|  detail |
                           +-+-+-+-+-+-+-+-+
                Figure 9: Structure of a Response Code
 The upper three bits of the 8-bit Response Code number define the
 class of response.  The lower five bits do not have any
 categorization role; they give additional detail to the overall class
 (Figure 9).
 As a human-readable notation for specifications and protocol
 diagnostics, CoAP code numbers including the Response Code are
 documented in the format "c.dd", where "c" is the class in decimal,
 and "dd" is the detail as a two-digit decimal.  For example,
 "Forbidden" is written as 4.03 -- indicating an 8-bit code value of
 hexadecimal 0x83 (4*0x20+3) or decimal 131 (4*32+3).
 There are 3 classes of Response Codes:
 2 - Success:  The request was successfully received, understood, and
    accepted.
 4 - Client Error:  The request contains bad syntax or cannot be
    fulfilled.
 5 - Server Error:  The server failed to fulfill an apparently valid
    request.
 The Response Codes are designed to be extensible: Response Codes in
 the Client Error or Server Error class that are unrecognized by an
 endpoint are treated as being equivalent to the generic Response Code
 of that class (4.00 and 5.00, respectively).  However, there is no
 generic Response Code indicating success, so a Response Code in the
 Success class that is unrecognized by an endpoint can only be used to
 determine that the request was successful without any further
 details.
 The possible Response Codes are described in detail in Section 5.9.
 Responses can be sent in multiple ways, which are defined in the
 following subsections.

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5.2.1. Piggybacked

 In the most basic case, the response is carried directly in the
 Acknowledgement message that acknowledges the request (which requires
 that the request was carried in a Confirmable message).  This is
 called a "Piggybacked Response".
 The response is returned in the Acknowledgement message, independent
 of whether the response indicates success or failure.  In effect, the
 response is piggybacked on the Acknowledgement message, and no
 separate message is required to return the response.
 Implementation Note:  The protocol leaves the decision whether to
    piggyback a response or not (i.e., send a separate response) to
    the server.  The client MUST be prepared to receive either.  On
    the quality-of-implementation level, there is a strong expectation
    that servers will implement code to piggyback whenever possible --
    saving resources in the network and both at the client and at the
    server.

5.2.2. Separate

 It may not be possible to return a piggybacked response in all cases.
 For example, a server might need longer to obtain the representation
 of the resource requested than it can wait to send back the
 Acknowledgement message, without risking the client repeatedly
 retransmitting the request message (see also the discussion of
 PROCESSING_DELAY in Section 4.8.2).  The response to a request
 carried in a Non-confirmable message is always sent separately (as
 there is no Acknowledgement message).
 One way to implement this in a server is to initiate the attempt to
 obtain the resource representation and, while that is in progress,
 time out an acknowledgement timer.  A server may also immediately
 send an acknowledgement if it knows in advance that there will be no
 piggybacked response.  In both cases, the acknowledgement effectively
 is a promise that the request will be acted upon later.
 When the server finally has obtained the resource representation, it
 sends the response.  When it is desired that this message is not
 lost, it is sent as a Confirmable message from the server to the
 client and answered by the client with an Acknowledgement, echoing
 the new Message ID chosen by the server.  (It may also be sent as a
 Non-confirmable message; see Section 5.2.3.)
 When the server chooses to use a separate response, it sends the
 Acknowledgement to the Confirmable request as an Empty message.  Once
 the server sends back an Empty Acknowledgement, it MUST NOT send back

Shelby, et al. Standards Track [Page 33] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 the response in another Acknowledgement, even if the client
 retransmits another identical request.  If a retransmitted request is
 received (perhaps because the original Acknowledgement was delayed),
 another Empty Acknowledgement is sent, and any response MUST be sent
 as a separate response.
 If the server then sends a Confirmable response, the client's
 Acknowledgement to that response MUST also be an Empty message (one
 that carries neither a request nor a response).  The server MUST stop
 retransmitting its response on any matching Acknowledgement (silently
 ignoring any Response Code or payload) or Reset message.
 Implementation Notes:  Note that, as the underlying datagram
    transport may not be sequence-preserving, the Confirmable message
    carrying the response may actually arrive before or after the
    Acknowledgement message for the request; for the purposes of
    terminating the retransmission sequence, this also serves as an
    acknowledgement.  Note also that, while the CoAP protocol itself
    does not make any specific demands here, there is an expectation
    that the response will come within a time frame that is reasonable
    from an application point of view.  As there is no underlying
    transport protocol that could be instructed to run a keep-alive
    mechanism, the requester may want to set up a timeout that is
    unrelated to CoAP's retransmission timers in case the server is
    destroyed or otherwise unable to send the response.

5.2.3. Non-confirmable

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

5.3. Request/Response Matching

 Regardless of how a response is sent, it is matched to the request by
 means of a token that is included by the client in the request, along
 with additional address information of the corresponding endpoint.

5.3.1. Token

 The Token is used to match a response with a request.  The token
 value is a sequence of 0 to 8 bytes.  (Note that every message
 carries a token, even if it is of zero length.)  Every request
 carries a client-generated token that the server MUST echo (without
 modification) in any resulting response.

Shelby, et al. Standards Track [Page 34] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 A token is intended for use as a client-local identifier for
 differentiating between concurrent requests (see Section 5.3); it
 could have been called a "request ID".
 The client SHOULD generate tokens in such a way that tokens currently
 in use for a given source/destination endpoint pair are unique.
 (Note that a client implementation can use the same token for any
 request if it uses a different endpoint each time, e.g., a different
 source port number.)  An empty token value is appropriate e.g., when
 no other tokens are in use to a destination, or when requests are
 made serially per destination and receive piggybacked responses.
 There are, however, multiple possible implementation strategies to
 fulfill this.
 A client sending a request without using Transport Layer Security
 (Section 9) SHOULD use a nontrivial, randomized token to guard
 against spoofing of responses (Section 11.4).  This protective use of
 tokens is the reason they are allowed to be up to 8 bytes in size.
 The actual size of the random component to be used for the Token
 depends on the security requirements of the client and the level of
 threat posed by spoofing of responses.  A client that is connected to
 the general Internet SHOULD use at least 32 bits of randomness,
 keeping in mind that not being directly connected to the Internet is
 not necessarily sufficient protection against spoofing.  (Note that
 the Message ID adds little in protection as it is usually
 sequentially assigned, i.e., guessable, and can be circumvented by
 spoofing a separate response.)  Clients that want to optimize the
 Token length may further want to detect the level of ongoing attacks
 (e.g., by tallying recent Token mismatches in incoming messages) and
 adjust the Token length upwards appropriately.  [RFC4086] discusses
 randomness requirements for security.
 An endpoint receiving a token it did not generate MUST treat the
 token as opaque and make no assumptions about its content or
 structure.

5.3.2. Request/Response Matching Rules

 The exact rules for matching a response to a request are as follows:
 1.  The source endpoint of the response MUST be the same as the
     destination endpoint of the original request.
 2.  In a piggybacked response, the Message ID of the Confirmable
     request and the Acknowledgement MUST match, and the tokens of the
     response and original request MUST match.  In a separate
     response, just the tokens of the response and original request
     MUST match.

Shelby, et al. Standards Track [Page 35] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 In case a message carrying a response is unexpected (the client is
 not waiting for a response from the identified endpoint, at the
 endpoint addressed, and/or with the given token), the response is
 rejected (Sections 4.2 and 4.3).
 Implementation Note:  A client that receives a response in a CON
    message may want to clean up the message state right after sending
    the ACK.  If that ACK is lost and the server retransmits the CON,
    the client may no longer have any state to which to correlate this
    response, making the retransmission an unexpected message; the
    client will likely send a Reset message so it does not receive any
    more retransmissions.  This behavior is normal and not an
    indication of an error.  (Clients that are not aggressively
    optimized in their state memory usage will still have message
    state that will identify the second CON as a retransmission.
    Clients that actually expect more messages from the server
    [OBSERVE] will have to keep state in any case.)

5.4. Options

 Both requests and responses may include a list of one or more
 options.  For example, the URI in a request is transported in several
 options, and metadata that would be carried in an HTTP header in HTTP
 is supplied as options as well.
 CoAP defines a single set of options that are used in both requests
 and responses:
 o  Content-Format
 o  ETag
 o  Location-Path
 o  Location-Query
 o  Max-Age
 o  Proxy-Uri
 o  Proxy-Scheme
 o  Uri-Host
 o  Uri-Path
 o  Uri-Port

Shelby, et al. Standards Track [Page 36] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 o  Uri-Query
 o  Accept
 o  If-Match
 o  If-None-Match
 o  Size1
 The semantics of these options along with their properties are
 defined in detail in Section 5.10.
 Not all options are defined for use with all methods and Response
 Codes.  The possible options for methods and Response Codes are
 defined in Sections 5.8 and 5.9, respectively.  In case an option is
 not defined for a Method or Response Code, it MUST NOT be included by
 a sender and MUST be treated like an unrecognized option by a
 recipient.

5.4.1. Critical/Elective

 Options fall into one of two classes: "critical" or "elective".  The
 difference between these is how an option unrecognized by an endpoint
 is handled:
 o  Upon reception, unrecognized options of class "elective" MUST be
    silently ignored.
 o  Unrecognized options of class "critical" that occur in a
    Confirmable request MUST cause the return of a 4.02 (Bad Option)
    response.  This response SHOULD include a diagnostic payload
    describing the unrecognized option(s) (see Section 5.5.2).
 o  Unrecognized options of class "critical" that occur in a
    Confirmable response, or piggybacked in an Acknowledgement, MUST
    cause the response to be rejected (Section 4.2).
 o  Unrecognized options of class "critical" that occur in a Non-
    confirmable message MUST cause the message to be rejected
    (Section 4.3).
 Note that, whether critical or elective, an option is never
 "mandatory" (it is always optional): these rules are defined in order
 to enable implementations to stop processing options they do not
 understand or implement.

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

5.4.2. Proxy Unsafe or Safe-to-Forward and NoCacheKey

 In addition to an option being marked as critical or elective,
 options are also classified based on how a proxy is to deal with the
 option if it does not recognize it.  For this purpose, an option can
 either be considered Unsafe to forward (UnSafe is set) or Safe-to-
 Forward (UnSafe is clear).
 In addition, for an option that is marked Safe-to-Forward, the option
 number indicates whether or not it is intended to be part of the
 Cache-Key (Section 5.6) in a request.  If some of the NoCacheKey bits
 are 0, it is; if all NoCacheKey bits are 1, it is not (see
 Section 5.4.6).
 Note:  The Cache-Key indication is relevant only for proxies that do
    not implement the given option as a request option and instead
    rely on the Unsafe/Safe-to-Forward indication only.  For example,
    for ETag, actually using the request option as a part of the
    Cache-Key is grossly inefficient, but it is the best thing one can
    do if ETag is not implemented by a proxy, as the response is going
    to differ based on the presence of the request option.  A more
    useful proxy that does implement the ETag request option is not
    using ETag as a part of the Cache-Key.
    NoCacheKey is indicated in three bits so that only one out of
    eight codepoints is qualified as NoCacheKey, leaving seven out of
    eight codepoints for what appears to be the more likely case.
 Proxy behavior with regard to these classes is defined in
 Section 5.7.

5.4.3. Length

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

5.4.4. Default Values

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

Shelby, et al. Standards Track [Page 38] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

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

5.4.5. Repeatable Options

 The definition of some options specifies that those options are
 repeatable.  An option that is repeatable MAY be included one or more
 times in a message.  An option that is not repeatable MUST NOT be
 included more than once in a message.
 If a message includes an option with more occurrences than the option
 is defined for, each supernumerary option occurrence that appears
 subsequently in the message MUST be treated like an unrecognized
 option (see Section 5.4.1).

5.4.6. Option Numbers

 An Option is identified by an option number, which also provides some
 additional semantics information, e.g., odd numbers indicate a
 critical option, while even numbers indicate an elective option.
 Note that this is not just a convention, it is a feature of the
 protocol: Whether an option is elective or critical is entirely
 determined by whether its option number is even or odd.
 More generally speaking, an Option number is constructed with a bit
 mask to indicate if an option is Critical or Elective, Unsafe or
 Safe-to-Forward, and, in the case of Safe-to-Forward, to provide a
 Cache-Key indication as shown by the following figure.  In the
 following text, the bit mask is expressed as a single byte that is
 applied to the least significant byte of the option number in
 unsigned integer representation.  When bit 7 (the least significant
 bit) is 1, an option is Critical (and likewise Elective when 0).
 When bit 6 is 1, an option is Unsafe (and likewise Safe-to-Forward
 when 0).  When bit 6 is 0, i.e., the option is not Unsafe, it is not
 a Cache-Key (NoCacheKey) if and only if bits 3-5 are all set to 1;
 all other bit combinations mean that it indeed is a Cache-Key.  These
 classes of options are explained in the next sections.
                     0   1   2   3   4   5   6   7
                   +---+---+---+---+---+---+---+---+
                   |           | NoCacheKey| U | C |
                   +---+---+---+---+---+---+---+---+
        Figure 10: Option Number Mask (Least Significant Byte)

Shelby, et al. Standards Track [Page 39] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 An endpoint may use an equivalent of the C code in Figure 11 to
 derive the characteristics of an option number "onum".
 Critical = (onum & 1);
 UnSafe = (onum & 2);
 NoCacheKey = ((onum & 0x1e) == 0x1c);
     Figure 11: Determining Characteristics from an Option Number
 The option numbers for the options defined in this document are
 listed in the "CoAP Option Numbers" registry (Section 12.2).

5.5. Payloads and Representations

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

5.5.1. Representation

 The payload of requests or of responses indicating success is
 typically a representation of a resource ("resource representation")
 or the result of the requested action ("action result").  Its format
 is specified by the Internet media type and content coding given by
 the Content-Format Option.  In the absence of this option, no default
 value is assumed, and the format will need to be inferred by the
 application (e.g., from the application context).  Payload "sniffing"
 SHOULD only be attempted if no content type is given.
 Implementation Note:  On a quality-of-implementation level, there is
    a strong expectation that a Content-Format indication will be
    provided with resource representations whenever possible.  This is
    not a "SHOULD" level requirement solely because it is not a
    protocol requirement, and it also would be difficult to outline
    exactly in what cases this expectation can be violated.
 For responses indicating a client or server error, the payload is
 considered a representation of the result of the requested action
 only if a Content-Format Option is given.  In the absence of this
 option, the payload is a Diagnostic Payload (Section 5.5.2).

Shelby, et al. Standards Track [Page 40] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

5.5.2. Diagnostic Payload

 If no Content-Format option is given, the payload of responses
 indicating a client or server error is a brief human-readable
 diagnostic message, explaining the error situation.  This diagnostic
 message MUST be encoded using UTF-8 [RFC3629], more specifically
 using Net-Unicode form [RFC5198].
 The message is similar to the Reason-Phrase on an HTTP status line.
 It is not intended for end users but for software engineers that
 during debugging need to interpret it in the context of the present,
 English-language specification; therefore, no mechanism for language
 tagging is needed or provided.  In contrast to what is usual in HTTP,
 the payload SHOULD be empty if there is no additional information
 beyond the Response Code.

5.5.3. Selected Representation

 Not all responses carry a payload that provides a representation of
 the resource addressed by the request.  It is, however, sometimes
 useful to be able to refer to such a representation in relation to a
 response, independent of whether it actually was enclosed.
 We use the term "selected representation" to refer to the current
 representation of a target resource that would have been selected in
 a successful response if the corresponding request had used the
 method GET and excluded any conditional request options
 (Section 5.10.8).
 Certain response options provide metadata about the selected
 representation, which might differ from the representation included
 in the message for responses to some state-changing methods.  Of the
 response options defined in this specification, only the ETag
 response option (Section 5.10.6) is defined as metadata about the
 selected representation.

5.5.4. Content Negotiation

 A server may be able to supply a representation for a resource in one
 of multiple representation formats.  Without further information from
 the client, it will provide the representation in the format it
 prefers.
 By using the Accept Option (Section 5.10.4) in a request, the client
 can indicate which content-format it prefers to receive.

Shelby, et al. Standards Track [Page 41] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

5.6. Caching

 CoAP endpoints MAY cache responses in order to reduce the response
 time and network bandwidth consumption on future, equivalent
 requests.
 The goal of caching in CoAP is to reuse a prior response message to
 satisfy a current request.  In some cases, a stored response can be
 reused without the need for a network request, reducing latency and
 network round-trips; a "freshness" mechanism is used for this purpose
 (see Section 5.6.1).  Even when a new request is required, it is
 often possible to reuse the payload of a prior response to satisfy
 the request, thereby reducing network bandwidth usage; a "validation"
 mechanism is used for this purpose (see Section 5.6.2).
 Unlike HTTP, the cacheability of CoAP responses does not depend on
 the request method, but it depends on the Response Code.  The
 cacheability of each Response Code is defined along the Response Code
 definitions in Section 5.9.  Response Codes that indicate success and
 are unrecognized by an endpoint MUST NOT be cached.
 For a presented request, a CoAP endpoint MUST NOT use a stored
 response, unless:
 o  the presented request method and that used to obtain the stored
    response match,
 o  all options match between those in the presented request and those
    of the request used to obtain the stored response (which includes
    the request URI), except that there is no need for a match of any
    request options marked as NoCacheKey (Section 5.4) or recognized
    by the Cache and fully interpreted with respect to its specified
    cache behavior (such as the ETag request option described in
    Section 5.10.6; see also Section 5.4.2), and
 o  the stored response is either fresh or successfully validated as
    defined below.
 The set of request options that is used for matching the cache entry
 is also collectively referred to as the "Cache-Key".  For URI schemes
 other than coap and coaps, matching of those options that constitute
 the request URI may be performed under rules specific to the URI
 scheme.

Shelby, et al. Standards Track [Page 42] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

5.6.1. Freshness Model

 When a response is "fresh" in the cache, it can be used to satisfy
 subsequent requests without contacting the origin server, thereby
 improving efficiency.
 The mechanism for determining freshness is for an origin server to
 provide an explicit expiration time in the future, using the Max-Age
 Option (see Section 5.10.5).  The Max-Age Option indicates that the
 response is to be considered not fresh after its age is greater than
 the specified number of seconds.
 The Max-Age Option defaults to a value of 60.  Thus, if it is not
 present in a cacheable response, then the response is considered not
 fresh after its age is greater than 60 seconds.  If an origin server
 wishes to prevent caching, it MUST explicitly include a Max-Age
 Option with a value of zero seconds.
 If a client has a fresh stored response and makes a new request
 matching the request for that stored response, the new response
 invalidates the old response.

5.6.2. Validation Model

 When an endpoint has one or more stored responses for a GET request,
 but cannot use any of them (e.g., because they are not fresh), it can
 use the ETag Option (Section 5.10.6) in the GET request to give the
 origin server an opportunity both to select a stored response to be
 used, and to update its freshness.  This process is known as
 "validating" or "revalidating" the stored response.
 When sending such a request, the endpoint SHOULD add an ETag Option
 specifying the entity-tag of each stored response that is applicable.
 A 2.03 (Valid) response indicates the stored response identified by
 the entity-tag given in the response's ETag Option can be reused
 after updating it as described in Section 5.9.1.3.
 Any other Response Code indicates that none of the stored responses
 nominated in the request is suitable.  Instead, the response SHOULD
 be used to satisfy the request and MAY replace the stored response.

Shelby, et al. Standards Track [Page 43] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

5.7. Proxying

 A proxy is a CoAP endpoint that can be tasked by CoAP clients to
 perform requests on their behalf.  This may be useful, for example,
 when the request could otherwise not be made, or to service the
 response from a cache in order to reduce response time and network
 bandwidth or energy consumption.
 In an overall architecture for a Constrained RESTful Environment,
 proxies can serve quite different purposes.  Proxies can be
 explicitly selected by clients, a role that we term "forward-proxy".
 Proxies can also be inserted to stand in for origin servers, a role
 that we term "reverse-proxy".  Orthogonal to this distinction, a
 proxy can map from a CoAP request to a CoAP request (CoAP-to-CoAP
 proxy) or translate from or to a different protocol ("cross-proxy").
 Full definitions of these terms are provided in Section 1.2.
 Notes:  The terminology in this specification has been selected to be
    culturally compatible with the terminology used in the wider web
    application environments, without necessarily matching it in every
    detail (which may not even be relevant to Constrained RESTful
    Environments).  Not too much semantics should be ascribed to the
    components of the terms (such as "forward", "reverse", or
    "cross").
    HTTP proxies, besides acting as HTTP proxies, often offer a
    transport-protocol proxying function ("CONNECT") to enable end-to-
    end transport layer security through the proxy.  No such function
    is defined for CoAP-to-CoAP proxies in this specification, as
    forwarding of UDP packets is unlikely to be of much value in
    Constrained RESTful Environments.  See also Section 10.2.7 for the
    cross-proxy case.
 When a client uses a proxy to make a request that will use a secure
 URI scheme (e.g., "coaps" or "https"), the request towards the proxy
 SHOULD be sent using DTLS except where equivalent lower-layer
 security is used for the leg between the client and the proxy.

5.7.1. Proxy Operation

 A proxy generally needs a way to determine potential request
 parameters for a request it places to a destination, based on the
 request it received from its client.  This way is fully specified for
 a forward-proxy but may depend on the specific configuration for a
 reverse-proxy.  In particular, the client of a reverse-proxy
 generally does not indicate a locator for the destination,

Shelby, et al. Standards Track [Page 44] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 necessitating some form of namespace translation in the reverse-
 proxy.  However, some aspects of the operation of proxies are common
 to all its forms.
 If a proxy does not employ a cache, then it simply forwards the
 translated request to the determined destination.  Otherwise, if it
 does employ a cache but does not have a stored response that matches
 the translated request and is considered fresh, then it needs to
 refresh its cache according to Section 5.6.  For options in the
 request that the proxy recognizes, it knows whether the option is
 intended to act as part of the key used in looking up the cached
 value or not.  For example, since requests for different Uri-Path
 values address different resources, Uri-Path values are always part
 of the Cache-Key, while, e.g., Token values are never part of the
 Cache-Key.  For options that the proxy does not recognize but that
 are marked Safe-to-Forward in the option number, the option also
 indicates whether it is to be included in the Cache-Key (NoCacheKey
 is not all set) or not (NoCacheKey is all set).  (Options that are
 unrecognized and marked Unsafe lead to 4.02 Bad Option.)
 If the request to the destination times out, then a 5.04 (Gateway
 Timeout) response MUST be returned.  If the request to the
 destination returns a response that cannot be processed by the proxy
 (e.g, due to unrecognized critical options or message format errors),
 then a 5.02 (Bad Gateway) response MUST be returned.  Otherwise, the
 proxy returns the response to the client.
 If a response is generated out of a cache, the generated (or implied)
 Max-Age Option MUST NOT extend the max-age originally set by the
 server, considering the time the resource representation spent in the
 cache.  For example, the Max-Age Option could be adjusted by the
 proxy for each response using the formula:
    proxy-max-age = original-max-age - cache-age
 For example, if a request is made to a proxied resource that was
 refreshed 20 seconds ago and had an original Max-Age of 60 seconds,
 then that resource's proxied max-age is now 40 seconds.  Considering
 potential network delays on the way from the origin server, a proxy
 should be conservative in the max-age values offered.
 All options present in a proxy request MUST be processed at the
 proxy.  Unsafe options in a request that are not recognized by the
 proxy MUST lead to a 4.02 (Bad Option) response being returned by the
 proxy.  A CoAP-to-CoAP proxy MUST forward to the origin server all
 Safe-to-Forward options that it does not recognize.  Similarly,

Shelby, et al. Standards Track [Page 45] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Unsafe options in a response that are not recognized by the CoAP-to-
 CoAP proxy server MUST lead to a 5.02 (Bad Gateway) response.  Again,
 Safe-to-Forward options that are not recognized MUST be forwarded.
 Additional considerations for cross-protocol proxying between CoAP
 and HTTP are discussed in Section 10.

5.7.2. Forward-Proxies

 CoAP distinguishes between requests made (as if) to an origin server
 and requests made through a forward-proxy.  CoAP requests to a
 forward-proxy are made as normal Confirmable or Non-confirmable
 requests to the forward-proxy endpoint, but they specify the request
 URI in a different way: The request URI in a proxy request is
 specified as a string in the Proxy-Uri Option (see Section 5.10.2),
 while the request URI in a request to an origin server is split into
 the Uri-Host, Uri-Port, Uri-Path, and Uri-Query Options (see
 Section 5.10.1).  Alternatively, the URI in a proxy request can be
 assembled from a Proxy-Scheme option and the split options mentioned.
 When a proxy request is made to an endpoint and the endpoint is
 unwilling or unable to act as proxy for the request URI, it MUST
 return a 5.05 (Proxying Not Supported) response.  If the authority
 (host and port) is recognized as identifying the proxy endpoint
 itself (see Section 5.10.2), then the request MUST be treated as a
 local (non-proxied) request.
 Unless a proxy is configured to forward the proxy request to another
 proxy, it MUST translate the request as follows: the scheme of the
 request URI defines the outgoing protocol and its details (e.g., CoAP
 is used over UDP for the "coap" scheme and over DTLS for the "coaps"
 scheme.)  For a CoAP-to-CoAP proxy, the origin server's IP address
 and port are determined by the authority component of the request
 URI, and the request URI is decoded and split into the Uri-Host, Uri-
 Port, Uri-Path and Uri-Query Options.  This consumes the Proxy-Uri or
 Proxy-Scheme option, which is therefore not forwarded to the origin
 server.

5.7.3. Reverse-Proxies

 Reverse-proxies do not make use of the Proxy-Uri or Proxy-Scheme
 options but need to determine the destination (next hop) of a request
 from information in the request and information in their
 configuration.  For example, a reverse-proxy might offer various
 resources as if they were its own resources, after having learned of
 their existence through resource discovery.  The reverse-proxy is
 free to build a namespace for the URIs that identify these resources.
 A reverse-proxy may also build a namespace that gives the client more

Shelby, et al. Standards Track [Page 46] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 control over where the request goes, e.g., by embedding host
 identifiers and port numbers into the URI path of the resources
 offered.
 In processing the response, a reverse-proxy has to be careful that
 ETag option values from different sources are not mixed up on one
 resource offered to its clients.  In many cases, the ETag can be
 forwarded unchanged.  If the mapping from a resource offered by the
 reverse-proxy to resources offered by its various origin servers is
 not unique, the reverse-proxy may need to generate a new ETag, making
 sure the semantics of this option are properly preserved.

5.8. Method Definitions

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

5.8.1. GET

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

5.8.2. POST

 The POST method requests that the representation enclosed in the
 request be processed.  The actual function performed by the POST
 method is determined by the origin server and dependent on the target
 resource.  It usually results in a new resource being created or the
 target resource being updated.
 If a resource has been created on the server, the response returned
 by the server SHOULD have a 2.01 (Created) Response Code and SHOULD
 include the URI of the new resource in a sequence of one or more
 Location-Path and/or Location-Query Options (Section 5.10.7).  If the
 POST succeeds but does not result in a new resource being created on
 the server, the response SHOULD have a 2.04 (Changed) Response Code.
 If the POST succeeds and results in the target resource being
 deleted, the response SHOULD have a 2.02 (Deleted) Response Code.
 POST is neither safe nor idempotent.

Shelby, et al. Standards Track [Page 47] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

5.8.3. PUT

 The PUT method requests that the resource identified by the request
 URI be updated or created with the enclosed representation.  The
 representation format is specified by the media type and content
 coding given in the Content-Format Option, if provided.
 If a resource exists at the request URI, the enclosed representation
 SHOULD be considered a modified version of that resource, and a 2.04
 (Changed) Response Code SHOULD be returned.  If no resource exists,
 then the server MAY create a new resource with that URI, resulting in
 a 2.01 (Created) Response Code.  If the resource could not be created
 or modified, then an appropriate error Response Code SHOULD be sent.
 Further restrictions to a PUT can be made by including the If-Match
 (see Section 5.10.8.1) or If-None-Match (see Section 5.10.8.2)
 options in the request.
 PUT is not safe but is idempotent.

5.8.4. DELETE

 The DELETE method requests that the resource identified by the
 request URI be deleted.  A 2.02 (Deleted) Response Code SHOULD be
 used on success or in case the resource did not exist before the
 request.
 DELETE is not safe but is idempotent.

5.9. Response Code Definitions

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

5.9.1. Success 2.xx

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

5.9.1.1. 2.01 Created

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

Shelby, et al. Standards Track [Page 48] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

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

5.9.1.2. 2.02 Deleted

 This Response Code is like HTTP 204 "No Content" but only used in
 response to requests that cause the resource to cease being
 available, such as DELETE and, in certain circumstances, POST.  The
 payload returned with the response, if any, is a representation of
 the action result.
 This response is not cacheable.  However, a cache MUST mark any
 stored response for the deleted resource as not fresh.

5.9.1.3. 2.03 Valid

 This Response Code is related to HTTP 304 "Not Modified" but only
 used to indicate that the response identified by the entity-tag
 identified by the included ETag Option is valid.  Accordingly, the
 response MUST include an ETag Option and MUST NOT include a payload.
 When a cache that recognizes and processes the ETag response option
 receives a 2.03 (Valid) response, it MUST update the stored response
 with the value of the Max-Age Option included in the response
 (explicitly, or implicitly as a default value; see also
 Section 5.6.2).  For each type of Safe-to-Forward option present in
 the response, the (possibly empty) set of options of this type that
 are present in the stored response MUST be replaced with the set of
 options of this type in the response received.  (Unsafe options may
 trigger similar option-specific processing as defined by the option.)

5.9.1.4. 2.04 Changed

 This Response Code is like HTTP 204 "No Content" but only used in
 response to POST and PUT requests.  The payload returned with the
 response, if any, is a representation of the action result.
 This response is not cacheable.  However, a cache MUST mark any
 stored response for the changed resource as not fresh.

Shelby, et al. Standards Track [Page 49] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

5.9.1.5. 2.05 Content

 This Response Code is like HTTP 200 "OK" but only used in response to
 GET requests.
 The payload returned with the response is a representation of the
 target resource.
 This response is cacheable: Caches can use the Max-Age Option to
 determine freshness (see Section 5.6.1) and (if present) the ETag
 Option for validation (see Section 5.6.2).

5.9.2. Client Error 4.xx

 This class of Response Code is intended for cases in which the client
 seems to have erred.  These Response Codes are applicable to any
 request method.
 The server SHOULD include a diagnostic payload under the conditions
 detailed in Section 5.5.2.
 Responses of this class are cacheable: Caches can use the Max-Age
 Option to determine freshness (see Section 5.6.1).  They cannot be
 validated.

5.9.2.1. 4.00 Bad Request

 This Response Code is Like HTTP 400 "Bad Request".

5.9.2.2. 4.01 Unauthorized

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

5.9.2.3. 4.02 Bad Option

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

5.9.2.4. 4.03 Forbidden

 This Response Code is like HTTP 403 "Forbidden".

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5.9.2.5. 4.04 Not Found

 This Response Code is like HTTP 404 "Not Found".

5.9.2.6. 4.05 Method Not Allowed

 This Response Code is like HTTP 405 "Method Not Allowed" but with no
 parallel to the "Allow" header field.

5.9.2.7. 4.06 Not Acceptable

 This Response Code is like HTTP 406 "Not Acceptable", but with no
 response entity.

5.9.2.8. 4.12 Precondition Failed

 This Response Code is like HTTP 412 "Precondition Failed".

5.9.2.9. 4.13 Request Entity Too Large

 This Response Code is like HTTP 413 "Request Entity Too Large".
 The response SHOULD include a Size1 Option (Section 5.10.9) to
 indicate the maximum size of request entity the server is able and
 willing to handle, unless the server is not in a position to make
 this information available.

5.9.2.10. 4.15 Unsupported Content-Format

 This Response Code is like HTTP 415 "Unsupported Media Type".

5.9.3. Server Error 5.xx

 This class of Response Code indicates cases in which the server is
 aware that it has erred or is incapable of performing the request.
 These Response Codes are applicable to any request method.
 The server SHOULD include a diagnostic payload under the conditions
 detailed in Section 5.5.2.
 Responses of this class are cacheable: Caches can use the Max-Age
 Option to determine freshness (see Section 5.6.1).  They cannot be
 validated.

5.9.3.1. 5.00 Internal Server Error

 This Response Code is like HTTP 500 "Internal Server Error".

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5.9.3.2. 5.01 Not Implemented

 This Response Code is like HTTP 501 "Not Implemented".

5.9.3.3. 5.02 Bad Gateway

 This Response Code is like HTTP 502 "Bad Gateway".

5.9.3.4. 5.03 Service Unavailable

 This Response Code is like HTTP 503 "Service Unavailable" but uses
 the Max-Age Option in place of the "Retry-After" header field to
 indicate the number of seconds after which to retry.

5.9.3.5. 5.04 Gateway Timeout

 This Response Code is like HTTP 504 "Gateway Timeout".

5.9.3.6. 5.05 Proxying Not Supported

 The server is unable or unwilling to act as a forward-proxy for the
 URI specified in the Proxy-Uri Option or using Proxy-Scheme (see
 Section 5.10.2).

5.10. Option Definitions

 The individual CoAP options are summarized in Table 4 and explained
 in the subsections of this section.
 In this table, the C, U, and N columns indicate the properties
 Critical, UnSafe, and NoCacheKey, respectively.  Since NoCacheKey
 only has a meaning for options that are Safe-to-Forward (not marked
 Unsafe), the column is filled with a dash for UnSafe options.

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 +-----+---+---+---+---+----------------+--------+--------+----------+
 | No. | C | U | N | R | Name           | Format | Length | Default  |
 +-----+---+---+---+---+----------------+--------+--------+----------+
 |   1 | x |   |   | x | If-Match       | opaque | 0-8    | (none)   |
 |   3 | x | x | - |   | Uri-Host       | string | 1-255  | (see     |
 |     |   |   |   |   |                |        |        | below)   |
 |   4 |   |   |   | x | ETag           | opaque | 1-8    | (none)   |
 |   5 | x |   |   |   | If-None-Match  | empty  | 0      | (none)   |
 |   7 | x | x | - |   | Uri-Port       | uint   | 0-2    | (see     |
 |     |   |   |   |   |                |        |        | below)   |
 |   8 |   |   |   | x | Location-Path  | string | 0-255  | (none)   |
 |  11 | x | x | - | x | Uri-Path       | string | 0-255  | (none)   |
 |  12 |   |   |   |   | Content-Format | uint   | 0-2    | (none)   |
 |  14 |   | x | - |   | Max-Age        | uint   | 0-4    | 60       |
 |  15 | x | x | - | x | Uri-Query      | string | 0-255  | (none)   |
 |  17 | x |   |   |   | Accept         | uint   | 0-2    | (none)   |
 |  20 |   |   |   | x | Location-Query | string | 0-255  | (none)   |
 |  35 | x | x | - |   | Proxy-Uri      | string | 1-1034 | (none)   |
 |  39 | x | x | - |   | Proxy-Scheme   | string | 1-255  | (none)   |
 |  60 |   |   | x |   | Size1          | uint   | 0-4    | (none)   |
 +-----+---+---+---+---+----------------+--------+--------+----------+
           C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
                           Table 4: Options

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

 The Uri-Host, Uri-Port, Uri-Path, and Uri-Query Options are used to
 specify the target resource of a request to a CoAP origin server.
 The options encode the different components of the request URI in a
 way that no percent-encoding is visible in the option values and that
 the full URI can be reconstructed at any involved endpoint.  The
 syntax of CoAP URIs is defined in Section 6.
 The steps for parsing URIs into options is defined in Section 6.4.
 These steps result in zero or more Uri-Host, Uri-Port, Uri-Path, and
 Uri-Query Options being included in a request, where each option
 holds the following values:
 o  the Uri-Host Option specifies the Internet host of the resource
    being requested,
 o  the Uri-Port Option specifies the transport-layer port number of
    the resource,
 o  each Uri-Path Option specifies one segment of the absolute path to
    the resource, and

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 o  each Uri-Query Option specifies one argument parameterizing the
    resource.
 Note: Fragments ([RFC3986], Section 3.5) are not part of the request
 URI and thus will not be transmitted in a CoAP request.
 The default value of the Uri-Host Option is the IP literal
 representing the destination IP address of the request message.
 Likewise, the default value of the Uri-Port Option is the destination
 UDP port.  The default values for the Uri-Host and Uri-Port Options
 are sufficient for requests to most servers.  Explicit Uri-Host and
 Uri-Port Options are typically used when an endpoint hosts multiple
 virtual servers.
 The Uri-Path and Uri-Query Option can contain any character sequence.
 No percent-encoding is performed.  The value of a Uri-Path Option
 MUST NOT be "." or ".." (as the request URI must be resolved before
 parsing it into options).
 The steps for constructing the request URI from the options are
 defined in Section 6.5.  Note that an implementation does not
 necessarily have to construct the URI; it can simply look up the
 target resource by examining the individual options.
 Examples can be found in Appendix B.

5.10.2. Proxy-Uri and Proxy-Scheme

 The Proxy-Uri Option is used to make a request to a forward-proxy
 (see Section 5.7).  The forward-proxy is requested to forward the
 request or service it from a valid cache and return the response.
 The option value is an absolute-URI ([RFC3986], Section 4.3).
 Note that the forward-proxy MAY forward the request on to another
 proxy or directly to the server specified by the absolute-URI.  In
 order to avoid request loops, a proxy MUST be able to recognize all
 of its server names, including any aliases, local variations, and the
 numeric IP addresses.
 An endpoint receiving a request with a Proxy-Uri Option that is
 unable or unwilling to act as a forward-proxy for the request MUST
 cause the return of a 5.05 (Proxying Not Supported) response.
 The Proxy-Uri Option MUST take precedence over any of the Uri-Host,
 Uri-Port, Uri-Path or Uri-Query options (each of which MUST NOT be
 included in a request containing the Proxy-Uri Option).

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 As a special case to simplify many proxy clients, the absolute-URI
 can be constructed from the Uri-* options.  When a Proxy-Scheme
 Option is present, the absolute-URI is constructed as follows: a CoAP
 URI is constructed from the Uri-* options as defined in Section 6.5.
 In the resulting URI, the initial scheme up to, but not including,
 the following colon is then replaced by the content of the Proxy-
 Scheme Option.  Note that this case is only applicable if the
 components of the desired URI other than the scheme component
 actually can be expressed using Uri-* options; for example, to
 represent a URI with a userinfo component in the authority, only
 Proxy-Uri can be used.

5.10.3. Content-Format

 The Content-Format Option indicates the representation format of the
 message payload.  The representation format is given as a numeric
 Content-Format identifier that is defined in the "CoAP Content-
 Formats" registry (Section 12.3).  In the absence of the option, no
 default value is assumed, i.e., the representation format of any
 representation message payload is indeterminate (Section 5.5).

5.10.4. Accept

 The CoAP Accept option can be used to indicate which Content-Format
 is acceptable to the client.  The representation format is given as a
 numeric Content-Format identifier that is defined in the "CoAP
 Content-Formats" registry (Section 12.3).  If no Accept option is
 given, the client does not express a preference (thus no default
 value is assumed).  The client prefers the representation returned by
 the server to be in the Content-Format indicated.  The server returns
 the preferred Content-Format if available.  If the preferred Content-
 Format cannot be returned, then a 4.06 "Not Acceptable" MUST be sent
 as a response, unless another error code takes precedence for this
 response.

5.10.5. Max-Age

 The Max-Age Option indicates the maximum time a response may be
 cached before it is considered not fresh (see Section 5.6.1).
 The option value is an integer number of seconds between 0 and
 2**32-1 inclusive (about 136.1 years).  A default value of 60 seconds
 is assumed in the absence of the option in a response.
 The value is intended to be current at the time of transmission.
 Servers that provide resources with strict tolerances on the value of
 Max-Age SHOULD update the value before each retransmission.  (See
 also Section 5.7.1.)

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5.10.6. ETag

 An entity-tag is intended for use as a resource-local identifier for
 differentiating between representations of the same resource that
 vary over time.  It is generated by the server providing the
 resource, which may generate it in any number of ways including a
 version, checksum, hash, or time.  An endpoint receiving an entity-
 tag MUST treat it as opaque and make no assumptions about its content
 or structure.  (Endpoints that generate an entity-tag are encouraged
 to use the most compact representation possible, in particular in
 regards to clients and intermediaries that may want to store multiple
 ETag values.)

5.10.6.1. ETag as a Response Option

 The ETag Option in a response provides the current value (i.e., after
 the request was processed) of the entity-tag for the "tagged
 representation".  If no Location-* options are present, the tagged
 representation is the selected representation (Section 5.5.3) of the
 target resource.  If one or more Location-* options are present and
 thus a location URI is indicated (Section 5.10.7), the tagged
 representation is the representation that would be retrieved by a GET
 request to the location URI.
 An ETag response option can be included with any response for which
 there is a tagged representation (e.g., it would not be meaningful in
 a 4.04 or 4.00 response).  The ETag Option MUST NOT occur more than
 once in a response.
 There is no default value for the ETag Option; if it is not present
 in a response, the server makes no statement about the entity-tag for
 the tagged representation.

5.10.6.2. ETag as a Request Option

 In a GET request, an endpoint that has one or more representations
 previously obtained from the resource, and has obtained ETag response
 options with these, can specify an instance of the ETag Option for
 one or more of these stored responses.
 A server can issue a 2.03 Valid response (Section 5.9.1.3) in place
 of a 2.05 Content response if one of the ETags given is the entity-
 tag for the current representation, i.e., is valid; the 2.03 Valid
 response then echoes this specific ETag in a response option.
 In effect, a client can determine if any of the stored
 representations is current (see Section 5.6.2) without needing to
 transfer them again.

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 The ETag Option MAY occur zero, one, or multiple times in a request.

5.10.7. Location-Path and Location-Query

 The Location-Path and Location-Query Options together indicate a
 relative URI that consists either of an absolute path, a query
 string, or both.  A combination of these options is included in a
 2.01 (Created) response to indicate the location of the resource
 created as the result of a POST request (see Section 5.8.2).  The
 location is resolved relative to the request URI.
 If a response with one or more Location-Path and/or Location-Query
 Options passes through a cache that interprets these options and the
 implied URI identifies one or more currently stored responses, those
 entries MUST be marked as not fresh.
 Each Location-Path Option specifies one segment of the absolute path
 to the resource, and each Location-Query Option specifies one
 argument parameterizing the resource.  The Location-Path and
 Location-Query Option can contain any character sequence.  No
 percent-encoding is performed.  The value of a Location-Path Option
 MUST NOT be "." or "..".
 The steps for constructing the location URI from the options are
 analogous to Section 6.5, except that the first five steps are
 skipped and the result is a relative URI-reference, which is then
 interpreted relative to the request URI.  Note that the relative URI-
 reference constructed this way always includes an absolute path
 (e.g., leaving out Location-Path but supplying Location-Query means
 the path component in the URI is "/").
 The options that are used to compute the relative URI-reference are
 collectively called Location-* options.  Beyond Location-Path and
 Location-Query, more Location-* options may be defined in the future
 and have been reserved option numbers 128, 132, 136, and 140.  If any
 of these reserved option numbers occurs in addition to Location-Path
 and/or Location-Query and are not supported, then a 4.02 (Bad Option)
 error MUST be returned.

5.10.8. Conditional Request Options

 Conditional request options enable a client to ask the server to
 perform the request only if certain conditions specified by the
 option are fulfilled.

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 For each of these options, if the condition given is not fulfilled,
 then the server MUST NOT perform the requested method.  Instead, the
 server MUST respond with the 4.12 (Precondition Failed) Response
 Code.
 If the condition is fulfilled, the server performs the request method
 as if the conditional request options were not present.
 If the request would, without the conditional request options, result
 in anything other than a 2.xx or 4.12 Response Code, then any
 conditional request options MAY be ignored.

5.10.8.1. If-Match

 The If-Match Option MAY be used to make a request conditional on the
 current existence or value of an ETag for one or more representations
 of the target resource.  If-Match is generally useful for resource
 update requests, such as PUT requests, as a means for protecting
 against accidental overwrites when multiple clients are acting in
 parallel on the same resource (i.e., the "lost update" problem).
 The value of an If-Match option is either an ETag or the empty
 string.  An If-Match option with an ETag matches a representation
 with that exact ETag.  An If-Match option with an empty value matches
 any existing representation (i.e., it places the precondition on the
 existence of any current representation for the target resource).
 The If-Match Option can occur multiple times.  If any of the options
 match, then the condition is fulfilled.
 If there is one or more If-Match Options, but none of the options
 match, then the condition is not fulfilled.

5.10.8.2. If-None-Match

 The If-None-Match Option MAY be used to make a request conditional on
 the nonexistence of the target resource.  If-None-Match is useful for
 resource creation requests, such as PUT requests, as a means for
 protecting against accidental overwrites when multiple clients are
 acting in parallel on the same resource.  The If-None-Match Option
 carries no value.
 If the target resource does exist, then the condition is not
 fulfilled.
 (It is not very useful to combine If-Match and If-None-Match options
 in one request, because the condition will then never be fulfilled.)

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5.10.9. Size1 Option

 The Size1 option provides size information about the resource
 representation in a request.  The option value is an integer number
 of bytes.  Its main use is with block-wise transfers [BLOCK].  In the
 present specification, it is used in 4.13 responses (Section 5.9.2.9)
 to indicate the maximum size of request entity that the server is
 able and willing to handle.

6. CoAP URIs

 CoAP uses the "coap" and "coaps" URI schemes for identifying CoAP
 resources and providing a means of locating the resource.  Resources
 are organized hierarchically and governed by a potential CoAP origin
 server listening for CoAP requests ("coap") or DTLS-secured CoAP
 requests ("coaps") on a given UDP port.  The CoAP server is
 identified via the generic syntax's authority component, which
 includes a host component and optional UDP port number.  The
 remainder of the URI is considered to be identifying a resource that
 can be operated on by the methods defined by the CoAP protocol.  The
 "coap" and "coaps" URI schemes can thus be compared to the "http" and
 "https" URI schemes, respectively.
 The syntax of the "coap" and "coaps" URI schemes is specified in this
 section in Augmented Backus-Naur Form (ABNF) [RFC5234].  The
 definitions of "host", "port", "path-abempty", "query", "segment",
 "IP-literal", "IPv4address", and "reg-name" are adopted from
 [RFC3986].
 Implementation Note:  Unfortunately, over time, the URI format has
    acquired significant complexity.  Implementers are encouraged to
    examine [RFC3986] closely.  For example, the ABNF for IPv6
    addresses is more complicated than maybe expected.  Also,
    implementers should take care to perform the processing of
    percent-decoding or percent-encoding exactly once on the way from
    a URI to its decoded components or back.  Percent-encoding is
    crucial for data transparency but may lead to unusual results such
    as a slash character in a path component.

6.1. coap URI Scheme

 coap-URI = "coap:" "//" host [ ":" port ] path-abempty [ "?" query ]
 If the host component is provided as an IP-literal or IPv4address,
 then the CoAP server can be reached at that IP address.  If host is a
 registered name, then that name is considered an indirect identifier
 and the endpoint might use a name resolution service, such as DNS, to
 find the address of that host.  The host MUST NOT be empty; if a URI

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 is received with a missing authority or an empty host, then it MUST
 be considered invalid.  The port subcomponent indicates the UDP port
 at which the CoAP server is located.  If it is empty or not given,
 then the default port 5683 is assumed.
 The path identifies a resource within the scope of the host and port.
 It consists of a sequence of path segments separated by a slash
 character (U+002F SOLIDUS "/").
 The query serves to further parameterize the resource.  It consists
 of a sequence of arguments separated by an ampersand character
 (U+0026 AMPERSAND "&").  An argument is often in the form of a
 "key=value" pair.
 The "coap" URI scheme supports the path prefix "/.well-known/"
 defined by [RFC5785] for "well-known locations" in the namespace of a
 host.  This enables discovery of policy or other information about a
 host ("site-wide metadata"), such as hosted resources (see
 Section 7).
 Application designers are encouraged to make use of short but
 descriptive URIs.  As the environments that CoAP is used in are
 usually constrained for bandwidth and energy, the trade-off between
 these two qualities should lean towards the shortness, without
 ignoring descriptiveness.

6.2. coaps URI Scheme

 coaps-URI = "coaps:" "//" host [ ":" port ] path-abempty
             [ "?" query ]
 All of the requirements listed above for the "coap" scheme are also
 requirements for the "coaps" scheme, except that a default UDP port
 of 5684 is assumed if the port subcomponent is empty or not given,
 and the UDP datagrams MUST be secured through the use of DTLS as
 described in Section 9.1.
 Considerations for caching of responses to "coaps" identified
 requests are discussed in Section 11.2.
 Resources made available via the "coaps" scheme have no shared
 identity with the "coap" scheme even if their resource identifiers
 indicate the same authority (the same host listening to the same UDP
 port).  They are distinct namespaces and are considered to be
 distinct origin servers.

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6.3. Normalization and Comparison Rules

 Since the "coap" and "coaps" schemes conform to the URI generic
 syntax, such URIs are normalized and compared according to the
 algorithm defined in [RFC3986], Section 6, using the defaults
 described above for each scheme.
 If the port is equal to the default port for a scheme, the normal
 form is to elide the port subcomponent.  Likewise, an empty path
 component is equivalent to an absolute path of "/", so the normal
 form is to provide a path of "/" instead.  The scheme and host are
 case insensitive and normally provided in lowercase; IP-literals are
 in recommended form [RFC5952]; all other components are compared in a
 case-sensitive manner.  Characters other than those in the "reserved"
 set are equivalent to their percent-encoded bytes (see [RFC3986],
 Section 2.1): the normal form is to not encode them.
 For example, the following three URIs are equivalent and cause the
 same options and option values to appear in the CoAP messages:
 coap://example.com:5683/~sensors/temp.xml
 coap://EXAMPLE.com/%7Esensors/temp.xml
 coap://EXAMPLE.com:/%7esensors/temp.xml

6.4. Decomposing URIs into Options

 The steps to parse a request's options from a string |url| are as
 follows.  These steps either result in zero or more of the Uri-Host,
 Uri-Port, Uri-Path, and Uri-Query Options being included in the
 request or they fail.
 1.  If the |url| string is not an absolute URI ([RFC3986]), then fail
     this algorithm.
 2.  Resolve the |url| string using the process of reference
     resolution defined by [RFC3986].  At this stage, the URL is in
     ASCII encoding [RFC0020], even though the decoded components will
     be interpreted in UTF-8 [RFC3629] after steps 5, 8, and 9.
     NOTE: It doesn't matter what it is resolved relative to, since we
     already know it is an absolute URL at this point.
 3.  If |url| does not have a <scheme> component whose value, when
     converted to ASCII lowercase, is "coap" or "coaps", then fail
     this algorithm.
 4.  If |url| has a <fragment> component, then fail this algorithm.

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 5.  If the <host> component of |url| does not represent the request's
     destination IP address as an IP-literal or IPv4address, include a
     Uri-Host Option and let that option's value be the value of the
     <host> component of |url|, converted to ASCII lowercase, and then
     convert all percent-encodings ("%" followed by two hexadecimal
     digits) to the corresponding characters.
     NOTE: In the usual case where the request's destination IP
     address is derived from the host part, this ensures that a Uri-
     Host Option is only used for a <host> component of the form reg-
     name.
 6.  If |url| has a <port> component, then let |port| be that
     component's value interpreted as a decimal integer; otherwise,
     let |port| be the default port for the scheme.
 7.  If |port| does not equal the request's destination UDP port,
     include a Uri-Port Option and let that option's value be |port|.
 8.  If the value of the <path> component of |url| is empty or
     consists of a single slash character (U+002F SOLIDUS "/"), then
     move to the next step.
     Otherwise, for each segment in the <path> component, include a
     Uri-Path Option and let that option's value be the segment (not
     including the delimiting slash characters) after converting each
     percent-encoding ("%" followed by two hexadecimal digits) to the
     corresponding byte.
 9.  If |url| has a <query> component, then, for each argument in the
     <query> component, include a Uri-Query Option and let that
     option's value be the argument (not including the question mark
     and the delimiting ampersand characters) after converting each
     percent-encoding to the corresponding byte.
 Note that these rules completely resolve any percent-encoding.

6.5. Composing URIs from Options

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

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 1.   If the request is secured using DTLS, let |url| be the string
      "coaps://".  Otherwise, let |url| be the string "coap://".
 2.   If the request includes a Uri-Host Option, let |host| be that
      option's value, where any non-ASCII characters are replaced by
      their corresponding percent-encoding.  If |host| is not a valid
      reg-name or IP-literal or IPv4address, fail the algorithm.  If
      the request does not include a Uri-Host Option, let |host| be
      the IP-literal (making use of the conventions of [RFC5952]) or
      IPv4address representing the request's destination IP address.
 3.   Append |host| to |url|.
 4.   If the request includes a Uri-Port Option, let |port| be that
      option's value.  Otherwise, let |port| be the request's
      destination UDP port.
 5.   If |port| is not the default port for the scheme, then append a
      single U+003A COLON character (:) followed by the decimal
      representation of |port| to |url|.
 6.   Let |resource name| be the empty string.  For each Uri-Path
      Option in the request, append a single character U+002F SOLIDUS
      (/) followed by the option's value to |resource name|, after
      converting any character that is not either in the "unreserved"
      set, in the "sub-delims" set, a U+003A COLON (:) character, or a
      U+0040 COMMERCIAL AT (@) character to its percent-encoded form.
 7.   If |resource name| is the empty string, set it to a single
      character U+002F SOLIDUS (/).
 8.   For each Uri-Query Option in the request, append a single
      character U+003F QUESTION MARK (?) (first option) or U+0026
      AMPERSAND (&) (subsequent options) followed by the option's
      value to |resource name|, after converting any character that is
      not either in the "unreserved" set, in the "sub-delims" set
      (except U+0026 AMPERSAND (&)), a U+003A COLON (:), a U+0040
      COMMERCIAL AT (@), a U+002F SOLIDUS (/), or a U+003F QUESTION
      MARK (?) character to its percent-encoded form.
 9.   Append |resource name| to |url|.
 10.  Return |url|.
 Note that these steps have been designed to lead to a URI in normal
 form (see Section 6.3).

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7. Discovery

7.1. Service Discovery

 As a part of discovering the services offered by a CoAP server, a
 client has to learn about the endpoint used by a server.
 A server is discovered by a client (knowing or) learning a URI that
 references a resource in the namespace of the server.  Alternatively,
 clients can use multicast CoAP (see Section 8) and the "All CoAP
 Nodes" multicast address to find CoAP servers.
 Unless the port subcomponent in a "coap" or "coaps" URI indicates the
 UDP port at which the CoAP server is located, the server is assumed
 to be reachable at the default port.
 The CoAP default port number 5683 MUST be supported by a server that
 offers resources for resource discovery (see Section 7.2 below) and
 SHOULD be supported for providing access to other resources.  The
 default port number 5684 for DTLS-secured CoAP MAY be supported by a
 server for resource discovery and for providing access to other
 resources.  In addition, other endpoints may be hosted at other
 ports, e.g., in the dynamic port space.
 Implementation Note:  When a CoAP server is hosted by a 6LoWPAN node,
    header compression efficiency is improved when it also supports a
    port number in the 61616-61631 compressed UDP port space defined
    in [RFC4944] and [RFC6282].  (Note that, as its UDP port differs
    from the default port, it is a different endpoint from the server
    at the default port.)

7.2. Resource Discovery

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

7.2.1. 'ct' Attribute

 This section defines a new Web Linking [RFC5988] attribute for use
 with [RFC6690].  The Content-Format code "ct" attribute provides a
 hint about the Content-Formats this resource returns.  Note that this
 is only a hint, and it does not override the Content-Format Option of

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 a CoAP response obtained by actually requesting the representation of
 the resource.  The value is in the CoAP identifier code format as a
 decimal ASCII integer and MUST be in the range of 0-65535 (16-bit
 unsigned integer).  For example, "application/xml" would be indicated
 as "ct=41".  If no Content-Format code attribute is present, then
 nothing about the type can be assumed.  The Content-Format code
 attribute MAY include a space-separated sequence of Content-Format
 codes, indicating that multiple content-formats are available.  The
 syntax of the attribute value is summarized in the production "ct-
 value" in Figure 12, where "cardinal", "SP", and "DQUOTE" are defined
 as in [RFC6690].
    ct-value =  cardinal
             /  DQUOTE cardinal *( 1*SP cardinal ) DQUOTE
                               Figure 12

8. Multicast CoAP

 CoAP supports making requests to an IP multicast group.  This is
 defined by a series of deltas to unicast CoAP.  A more general
 discussion of group communication with CoAP is in [GROUPCOMM].
 CoAP endpoints that offer services that they want other endpoints to
 be able to find using multicast service discovery join one or more of
 the appropriate all-CoAP-node multicast addresses (Section 12.8) and
 listen on the default CoAP port.  Note that an endpoint might receive
 multicast requests on other multicast addresses, including the all-
 nodes IPv6 address (or via broadcast on IPv4); an endpoint MUST
 therefore be prepared to receive such messages but MAY ignore them if
 multicast service discovery is not desired.

8.1. Messaging Layer

 A multicast request is characterized by being transported in a CoAP
 message that is addressed to an IP multicast address instead of a
 CoAP endpoint.  Such multicast requests MUST be Non-confirmable.
 A server SHOULD be aware that a request arrived via multicast, e.g.,
 by making use of modern APIs such as IPV6_RECVPKTINFO [RFC3542], if
 available.
 To avoid an implosion of error responses, when a server is aware that
 a request arrived via multicast, it MUST NOT return a Reset message
 in reply to a Non-confirmable message.  If it is not aware, it MAY
 return a Reset message in reply to a Non-confirmable message as
 usual.  Because such a Reset message will look identical to one for a

Shelby, et al. Standards Track [Page 65] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 unicast message from the sender, the sender MUST avoid using a
 Message ID that is also still active from this endpoint with any
 unicast endpoint that might receive the multicast message.
 At the time of writing, multicast messages can only be carried in UDP
 not in DTLS.  This means that the security modes defined for CoAP in
 this document are not applicable to multicast.

8.2. Request/Response Layer

 When a server is aware that a request arrived via multicast, the
 server MAY always ignore the request, in particular if it doesn't
 have anything useful to respond (e.g., if it only has an empty
 payload or an error response).  The decision for this may depend on
 the application.  (For example, in query filtering as described in
 [RFC6690], a server should not respond to a multicast request if the
 filter does not match.  More examples are in [GROUPCOMM].)
 If a server does decide to respond to a multicast request, it should
 not respond immediately.  Instead, it should pick a duration for the
 period of time during which it intends to respond.  For the purposes
 of this exposition, we call the length of this period the Leisure.
 The specific value of this Leisure may depend on the application or
 MAY be derived as described below.  The server SHOULD then pick a
 random point of time within the chosen leisure period to send back
 the unicast response to the multicast request.  If further responses
 need to be sent based on the same multicast address membership, a new
 leisure period starts at the earliest after the previous one
 finishes.
 To compute a value for Leisure, the server should have a group size
 estimate G, a target data transfer rate R (which both should be
 chosen conservatively), and an estimated response size S; a rough
 lower bound for Leisure can then be computed as
                        lb_Leisure = S * G / R
 For example, for a multicast request with link-local scope on a 2.4
 GHz IEEE 802.15.4 (6LoWPAN) network, G could be (relatively
 conservatively) set to 100, S to 100 bytes, and the target rate to 8
 kbit/s = 1 kB/s.  The resulting lower bound for the Leisure is 10
 seconds.
 If a CoAP endpoint does not have suitable data to compute a value for
 Leisure, it MAY resort to DEFAULT_LEISURE.

Shelby, et al. Standards Track [Page 66] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 When matching a response to a multicast request, only the token MUST
 match; the source endpoint of the response does not need to (and will
 not) be the same as the destination endpoint of the original request.
 For the purposes of interpreting the Location-* options and any links
 embedded in the representation, the request URI (i.e., the base URI
 relative to which the response is interpreted) is formed by replacing
 the multicast address in the Host component of the original request
 URI by the literal IP address of the endpoint actually responding.

8.2.1. Caching

 When a client makes a multicast request, it always makes a new
 request to the multicast group (since there may be new group members
 that joined meanwhile or ones that did not get the previous request).
 It MAY update a cache with the received responses.  Then, it uses
 both cached-still-fresh and new responses as the result of the
 request.
 A response received in reply to a GET request to a multicast group
 MAY be used to satisfy a subsequent request on the related unicast
 request URI.  The unicast request URI is obtained by replacing the
 authority part of the request URI with the transport-layer source
 address of the response message.
 A cache MAY revalidate a response by making a GET request on the
 related unicast request URI.
 A GET request to a multicast group MUST NOT contain an ETag option.
 A mechanism to suppress responses the client already has is left for
 further study.

8.2.2. Proxying

 When a forward-proxy receives a request with a Proxy-Uri or URI
 constructed from Proxy-Scheme that indicates a multicast address, the
 proxy obtains a set of responses as described above and sends all
 responses (both cached-still-fresh and new) back to the original
 client.
 This specification does not provide a way to indicate the unicast-
 modified request URI (base URI) in responses thus forwarded.
 Proxying multicast requests is discussed in more detail in
 [GROUPCOMM]; one proposal to address the base URI issue can be found
 in Section 3 of [CoAP-MISC].

Shelby, et al. Standards Track [Page 67] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

9. Securing CoAP

 This section defines the DTLS binding for CoAP.
 During the provisioning phase, a CoAP device is provided with the
 security information that it needs, including keying materials and
 access control lists.  This specification defines provisioning for
 the RawPublicKey mode in Section 9.1.3.2.1.  At the end of the
 provisioning phase, the device will be in one of four security modes
 with the following information for the given mode.  The NoSec and
 RawPublicKey modes are mandatory to implement for this specification.
 NoSec:  There is no protocol-level security (DTLS is disabled).
    Alternative techniques to provide lower-layer security SHOULD be
    used when appropriate.  The use of IPsec is discussed in
    [IPsec-CoAP].  Certain link layers in use with constrained nodes
    also provide link-layer security, which may be appropriate with
    proper key management.
 PreSharedKey:  DTLS is enabled, there is a list of pre-shared keys
    [RFC4279], and each key includes a list of which nodes it can be
    used to communicate with as described in Section 9.1.3.1.  At the
    extreme, there may be one key for each node this CoAP node needs
    to communicate with (1:1 node/key ratio).  Conversely, if more
    than two entities share a specific pre-shared key, this key only
    enables the entities to authenticate as a member of that group and
    not as a specific peer.
 RawPublicKey:  DTLS is enabled and the device has an asymmetric key
    pair without a certificate (a raw public key) that is validated
    using an out-of-band mechanism [RFC7250] as described in
    Section 9.1.3.2.  The device also has an identity calculated from
    the public key and a list of identities of the nodes it can
    communicate with.
 Certificate:  DTLS is enabled and the device has an asymmetric key
    pair with an X.509 certificate [RFC5280] that binds it to its
    subject and is signed by some common trust root as described in
    Section 9.1.3.3.  The device also has a list of root trust anchors
    that can be used for validating a certificate.
 In the "NoSec" mode, the system simply sends the packets over normal
 UDP over IP and is indicated by the "coap" scheme and the CoAP
 default port.  The system is secured only by keeping attackers from
 being able to send or receive packets from the network with the CoAP
 nodes; see Section 11.5 for an additional complication with this
 approach.

Shelby, et al. Standards Track [Page 68] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 The other three security modes are achieved using DTLS and are
 indicated by the "coaps" scheme and DTLS-secured CoAP default port.
 The result is a security association that can be used to authenticate
 (within the limits of the security model) and, based on this
 authentication, authorize the communication partner.  CoAP itself
 does not provide protocol primitives for authentication or
 authorization; where this is required, it can either be provided by
 communication security (i.e., IPsec or DTLS) or by object security
 (within the payload).  Devices that require authorization for certain
 operations are expected to require one of these two forms of
 security.  Necessarily, where an intermediary is involved,
 communication security only works when that intermediary is part of
 the trust relationships.  CoAP does not provide a way to forward
 different levels of authorization that clients may have with an
 intermediary to further intermediaries or origin servers -- it
 therefore may be required to perform all authorization at the first
 intermediary.

9.1. DTLS-Secured CoAP

 Just as HTTP is secured using Transport Layer Security (TLS) over
 TCP, CoAP is secured using Datagram TLS (DTLS) [RFC6347] over UDP
 (see Figure 13).  This section defines the CoAP binding to DTLS,
 along with the minimal mandatory-to-implement configurations
 appropriate for constrained environments.  The binding is defined by
 a series of deltas to unicast CoAP.  In practice, DTLS is TLS with
 added features to deal with the unreliable nature of the UDP
 transport.
                       +----------------------+
                       |      Application     |
                       +----------------------+
                       +----------------------+
                       |  Requests/Responses  |
                       |----------------------|  CoAP
                       |       Messages       |
                       +----------------------+
                       +----------------------+
                       |         DTLS         |
                       +----------------------+
                       +----------------------+
                       |          UDP         |
                       +----------------------+
           Figure 13: Abstract Layering of DTLS-Secured CoAP

Shelby, et al. Standards Track [Page 69] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 In some constrained nodes (limited flash and/or RAM) and networks
 (limited bandwidth or high scalability requirements), and depending
 on the specific cipher suites in use, all modes of DTLS may not be
 applicable.  Some DTLS cipher suites can add significant
 implementation complexity as well as some initial handshake overhead
 needed when setting up the security association.  Once the initial
 handshake is completed, DTLS adds a limited per-datagram overhead of
 approximately 13 bytes, not including any initialization vectors/
 nonces (e.g., 8 bytes with TLS_PSK_WITH_AES_128_CCM_8 [RFC6655]),
 integrity check values (e.g., 8 bytes with TLS_PSK_WITH_AES_128_CCM_8
 [RFC6655]), and padding required by the cipher suite.  Whether the
 use of a given mode of DTLS is applicable for a CoAP-based
 application should be carefully weighed considering the specific
 cipher suites that may be applicable, whether the session maintenance
 makes it compatible with application flows, and whether sufficient
 resources are available on the constrained nodes and for the added
 network overhead.  (For some modes of using DTLS, this specification
 identifies a mandatory-to-implement cipher suite.  This is an
 implementation requirement to maximize interoperability in those
 cases where these cipher suites are indeed appropriate.  The specific
 security policies of an application may determine the actual set of
 cipher suites that can be used.)  DTLS is not applicable to group
 keying (multicast communication); however, it may be a component in a
 future group key management protocol.

9.1.1. Messaging Layer

 The endpoint acting as the CoAP client should also act as the DTLS
 client.  It should initiate a session to the server on the
 appropriate port.  When the DTLS handshake has finished, the client
 may initiate the first CoAP request.  All CoAP messages MUST be sent
 as DTLS "application data".
 The following rules are added for matching an Acknowledgement message
 or Reset message to a Confirmable message, or a Reset message to a
 Non-confirmable message: The DTLS session MUST be the same, and the
 epoch MUST be the same.
 A message is the same when it is sent within the same DTLS session
 and same epoch and has the same Message ID.
 Note: When a Confirmable message is retransmitted, a new DTLS
 sequence_number is used for each attempt, even though the CoAP
 Message ID stays the same.  So a recipient still has to perform
 deduplication as described in Section 4.5.  Retransmissions MUST NOT
 be performed across epochs.

Shelby, et al. Standards Track [Page 70] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

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

9.1.2. Request/Response Layer

 The following rules are added for matching a response to a request:
 The DTLS session MUST be the same, and the epoch MUST be the same.
 This means the response to a DTLS secured request MUST always be DTLS
 secured using the same security session and epoch.  Any attempt to
 supply a NoSec response to a DTLS request simply does not match the
 request and therefore MUST be rejected (unless it does match an
 unrelated NoSec request).

9.1.3. Endpoint Identity

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

9.1.3.1. Pre-Shared Keys

 When forming a connection to a new node, the system selects an
 appropriate key based on which nodes it is trying to reach and then
 forms a DTLS session using a PSK (Pre-Shared Key) mode of DTLS.
 Implementations in these modes MUST support the mandatory-to-
 implement cipher suite TLS_PSK_WITH_AES_128_CCM_8 as specified in
 [RFC6655].
 Depending on the commissioning model, applications may need to define
 an application profile for identity hints (as required and detailed
 in Section 5.2 of [RFC4279]) to enable the use of PSK identity hints.
 The security considerations of Section 7 of [RFC4279] apply.  In
 particular, applications should carefully weigh whether or not they
 need Perfect Forward Secrecy (PFS) and select an appropriate cipher
 suite (Section 7.1 of [RFC4279]).  The entropy of the PSK must be
 sufficient to mitigate against brute-force and (where the PSK is not
 chosen randomly but by a human) dictionary attacks (Section 7.2 of
 [RFC4279]).  The cleartext communication of client identities may
 leak data or compromise privacy (Section 7.3 of [RFC4279]).

Shelby, et al. Standards Track [Page 71] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

9.1.3.2. Raw Public Key Certificates

 In this mode, the device has an asymmetric key pair but without an
 X.509 certificate (called a raw public key); for example, the
 asymmetric key pair is generated by the manufacturer and installed on
 the device (see also Section 11.6).  A device MAY be configured with
 multiple raw public keys.  The type and length of the raw public key
 depends on the cipher suite used.  Implementations in RawPublicKey
 mode MUST support the mandatory-to-implement cipher suite
 TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as specified in [RFC7251],
 [RFC5246], and [RFC4492].  The key used MUST be ECDSA capable.  The
 curve secp256r1 MUST be supported [RFC4492]; this curve is equivalent
 to the NIST P-256 curve.  The hash algorithm is SHA-256.
 Implementations MUST use the Supported Elliptic Curves and Supported
 Point Formats Extensions [RFC4492]; the uncompressed point format
 MUST be supported; [RFC6090] can be used as an implementation method.
 Some guidance relevant to the implementation of this cipher suite can
 be found in [W3CXMLSEC].  The mechanism for using raw public keys
 with TLS is specified in [RFC7250].
 Implementation Note:  Specifically, this means the extensions listed
    in Figure 14 with at least the values listed will be present in
    the DTLS handshake.
 Extension: elliptic_curves
  Type: elliptic_curves (0x000a)
  Length: 4
  Elliptic Curves Length: 2
  Elliptic curves (1 curve)
    Elliptic curve: secp256r1 (0x0017)
 Extension: ec_point_formats
  Type: ec_point_formats (0x000b)
  Length: 2
  EC point formats Length: 1
  Elliptic curves point formats (1)
    EC point format: uncompressed (0)
 Extension: signature_algorithms
  Type: signature_algorithms (0x000d)
  Length: 4
  Data (4 bytes): 00 02 04 03
    HashAlgorithm: sha256 (4)
    SignatureAlgorithm: ecdsa (3)
                Figure 14: DTLS Extensions Present for
                  TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8

Shelby, et al. Standards Track [Page 72] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

9.1.3.2.1. Provisioning

 The RawPublicKey mode was designed to be easily provisioned in M2M
 deployments.  It is assumed that each device has an appropriate
 asymmetric public key pair installed.  An identifier is calculated by
 the endpoint from the public key as described in Section 2 of
 [RFC6920].  All implementations that support checking RawPublicKey
 identities MUST support at least the sha-256-120 mode (SHA-256
 truncated to 120 bits).  Implementations SHOULD also support longer
 length identifiers and MAY support shorter lengths.  Note that the
 shorter lengths provide less security against attacks, and their use
 is NOT RECOMMENDED.
 Depending on how identifiers are given to the system that verifies
 them, support for URI, binary, and/or human-speakable format
 [RFC6920] needs to be implemented.  All implementations SHOULD
 support the binary mode, and implementations that have a user
 interface SHOULD also support the human-speakable format.
 During provisioning, the identifier of each node is collected, for
 example, by reading a barcode on the outside of the device or by
 obtaining a pre-compiled list of the identifiers.  These identifiers
 are then installed in the corresponding endpoint, for example, an M2M
 data collection server.  The identifier is used for two purposes, to
 associate the endpoint with further device information and to perform
 access control.  During (initial and ongoing) provisioning, an access
 control list of identifiers with which the device may start DTLS
 sessions SHOULD also be installed and maintained.

9.1.3.3. X.509 Certificates

 Implementations in Certificate Mode MUST support the mandatory-to-
 implement cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as
 specified in [RFC7251], [RFC5246], and [RFC4492].  Namely, the
 certificate includes a SubjectPublicKeyInfo that indicates an
 algorithm of id-ecPublicKey with namedCurves secp256r1 [RFC5480]; the
 public key format is uncompressed [RFC5480]; the hash algorithm is
 SHA-256; if included, the key usage extension indicates
 digitalSignature.  Certificates MUST be signed with ECDSA using
 secp256r1, and the signature MUST use SHA-256.  The key used MUST be
 ECDSA capable.  The curve secp256r1 MUST be supported [RFC4492]; this
 curve is equivalent to the NIST P-256 curve.  The hash algorithm is
 SHA-256.  Implementations MUST use the Supported Elliptic Curves and
 Supported Point Formats Extensions [RFC4492]; the uncompressed point
 format MUST be supported; [RFC6090] can be used as an implementation
 method.

Shelby, et al. Standards Track [Page 73] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 The subject in the certificate would be built out of a long-term
 unique identifier for the device such as the EUI-64 [EUI64].  The
 subject could also be based on the Fully Qualified Domain Name (FQDN)
 that was used as the Host part of the CoAP URI.  However, the
 device's IP address should not typically be used as the subject, as
 it would change over time.  The discovery process used in the system
 would build up the mapping between IP addresses of the given devices
 and the subject for each device.  Some devices could have more than
 one subject and would need more than a single certificate.
 When a new connection is formed, the certificate from the remote
 device needs to be verified.  If the CoAP node has a source of
 absolute time, then the node SHOULD check that the validity dates of
 the certificate are within range.  The certificate MUST be validated
 as appropriate for the security requirements, using functionality
 equivalent to the algorithm specified in Section 6 of [RFC5280].  If
 the certificate contains a SubjectAltName, then the authority of the
 request URI MUST match at least one of the authorities of any CoAP
 URI found in a field of URI type in the SubjectAltName set.  If there
 is no SubjectAltName in the certificate, then the authority of the
 request URI MUST match the Common Name (CN) found in the certificate
 using the matching rules defined in [RFC3280] with the exception that
 certificates with wildcards are not allowed.
 CoRE support for certificate status checking requires further study.
 As a mapping of the Online Certificate Status Protocol (OCSP)
 [RFC6960] onto CoAP is not currently defined and OCSP may also not be
 easily applicable in all environments, an alternative approach may be
 using the TLS Certificate Status Request extension (Section 8 of
 [RFC6066]; also known as "OCSP stapling") or preferably the Multiple
 Certificate Status Extension ([RFC6961]), if available.
 If the system has a shared key in addition to the certificate, then a
 cipher suite that includes the shared key such as
 TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA [RFC5489] SHOULD be used.

10. Cross-Protocol Proxying between CoAP and HTTP

 CoAP supports a limited subset of HTTP functionality, and thus cross-
 protocol proxying to HTTP is straightforward.  There might be several
 reasons for proxying between CoAP and HTTP, for example, when
 designing a web interface for use over either protocol or when
 realizing a CoAP-HTTP proxy.  Likewise, CoAP could equally be proxied
 to other protocols such as XMPP [RFC6120] or SIP [RFC3264]; the
 definition of these mechanisms is out of scope for this
 specification.

Shelby, et al. Standards Track [Page 74] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 There are two possible directions to access a resource via a forward-
 proxy:
 CoAP-HTTP Proxying:  Enables CoAP clients to access resources on HTTP
    servers through an intermediary.  This is initiated by including
    the Proxy-Uri or Proxy-Scheme Option with an "http" or "https" URI
    in a CoAP request to a CoAP-HTTP proxy.
 HTTP-CoAP Proxying:  Enables HTTP clients to access resources on CoAP
    servers through an intermediary.  This is initiated by specifying
    a "coap" or "coaps" URI in the Request-Line of an HTTP request to
    an HTTP-CoAP proxy.
 Either way, only the request/response model of CoAP is mapped to
 HTTP.  The underlying model of Confirmable or Non-confirmable
 messages, etc., is invisible and MUST have no effect on a proxy
 function.  The following sections describe the handling of requests
 to a forward-proxy.  Reverse-proxies are not specified, as the proxy
 function is transparent to the client with the proxy acting as if it
 were the origin server.  However, similar considerations apply to
 reverse-proxies as to forward-proxies, and there generally will be an
 expectation that reverse-proxies operate in a similar way forward-
 proxies would.  As an implementation note, HTTP client libraries may
 make it hard to operate an HTTP-CoAP forward-proxy by not providing a
 way to put a CoAP URI on the HTTP Request-Line; reverse-proxying may
 therefore lead to wider applicability of a proxy.  A separate
 specification may define a convention for URIs operating such an
 HTTP-CoAP reverse-proxy [MAPPING].

10.1. CoAP-HTTP Proxying

 If a request contains a Proxy-Uri or Proxy-Scheme Option with an
 'http' or 'https' URI [RFC2616], then the receiving CoAP endpoint
 (called "the proxy" henceforth) is requested to perform the operation
 specified by the request method on the indicated HTTP resource and
 return the result to the client.  (See also Section 5.7 for how the
 request to the proxy is formulated, including security requirements.)
 This section specifies for any CoAP request the CoAP response that
 the proxy should return to the client.  How the proxy actually
 satisfies the request is an implementation detail, although the
 typical case is expected to be that the proxy translates and forwards
 the request to an HTTP origin server.

Shelby, et al. Standards Track [Page 75] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Since HTTP and CoAP share the basic set of request methods,
 performing a CoAP request on an HTTP resource is not so different
 from performing it on a CoAP resource.  The meanings of the
 individual CoAP methods when performed on HTTP resources are
 explained in the subsections of this section.
 If the proxy is unable or unwilling to service a request with an HTTP
 URI, a 5.05 (Proxying Not Supported) response is returned to the
 client.  If the proxy services the request by interacting with a
 third party (such as the HTTP origin server) and is unable to obtain
 a result within a reasonable time frame, a 5.04 (Gateway Timeout)
 response is returned; if a result can be obtained but is not
 understood, a 5.02 (Bad Gateway) response is returned.

10.1.1. GET

 The GET method requests the proxy to return a representation of the
 HTTP resource identified by the request URI.
 Upon success, a 2.05 (Content) Response Code SHOULD be returned.  The
 payload of the response MUST be a representation of the target HTTP
 resource, and the Content-Format Option MUST be set accordingly.  The
 response MUST indicate a Max-Age value that is no greater than the
 remaining time the representation can be considered fresh.  If the
 HTTP entity has an entity-tag, the proxy SHOULD include an ETag
 Option in the response and process ETag Options in requests as
 described below.
 A client can influence the processing of a GET request by including
 the following option:
 Accept:  The request MAY include an Accept Option, identifying the
    preferred response content-format.
 ETag:  The request MAY include one or more ETag Options, identifying
    responses that the client has stored.  This requests the proxy to
    send a 2.03 (Valid) response whenever it would send a 2.05
    (Content) response with an entity-tag in the requested set
    otherwise.  Note that CoAP ETags are always strong ETags in the
    HTTP sense; CoAP does not have the equivalent of HTTP weak ETags,
    and there is no good way to make use of these in a cross-proxy.

Shelby, et al. Standards Track [Page 76] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

10.1.2. PUT

 The PUT method requests the proxy to update or create the HTTP
 resource identified by the request URI with the enclosed
 representation.
 If a new resource is created at the request URI, a 2.01 (Created)
 response MUST be returned to the client.  If an existing resource is
 modified, a 2.04 (Changed) response MUST be returned to indicate
 successful completion of the request.

10.1.3. DELETE

 The DELETE method requests the proxy to delete the HTTP resource
 identified by the request URI at the HTTP origin server.
 A 2.02 (Deleted) response MUST be returned to the client upon success
 or if the resource does not exist at the time of the request.

10.1.4. POST

 The POST method requests the proxy to have the representation
 enclosed in the request be processed by the HTTP origin server.  The
 actual function performed by the POST method is determined by the
 origin server and dependent on the resource identified by the request
 URI.
 If the action performed by the POST method does not result in a
 resource that can be identified by a URI, a 2.04 (Changed) response
 MUST be returned to the client.  If a resource has been created on
 the origin server, a 2.01 (Created) response MUST be returned.

10.2. HTTP-CoAP Proxying

 If an HTTP request contains a Request-URI with a "coap" or "coaps"
 URI, then the receiving HTTP endpoint (called "the proxy" henceforth)
 is requested to perform the operation specified by the request method
 on the indicated CoAP resource and return the result to the client.
 This section specifies for any HTTP request the HTTP response that
 the proxy should return to the client.  Unless otherwise specified,
 all the statements made are RECOMMENDED behavior; some highly
 constrained implementations may need to resort to shortcuts.  How the
 proxy actually satisfies the request is an implementation detail,
 although the typical case is expected to be that the proxy translates
 and forwards the request to a CoAP origin server.  The meanings of
 the individual HTTP methods when performed on CoAP resources are
 explained in the subsections of this section.

Shelby, et al. Standards Track [Page 77] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

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

10.2.1. OPTIONS and TRACE

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

10.2.2. GET

 The GET method requests the proxy to return a representation of the
 CoAP resource identified by the Request-URI.
 Upon success, a 200 (OK) response is returned.  The payload of the
 response MUST be a representation of the target CoAP resource, and
 the Content-Type and Content-Encoding header fields MUST be set
 accordingly.  The response MUST indicate a max-age directive that
 indicates a value no greater than the remaining time the
 representation can be considered fresh.  If the CoAP response has an
 ETag option, the proxy should include an ETag header field in the
 response.
 A client can influence the processing of a GET request by including
 the following options:
 Accept:  The most-preferred media type of the HTTP Accept header
    field in a request is mapped to a CoAP Accept option.  HTTP Accept
    media-type ranges, parameters, and extensions are not supported by
    the CoAP Accept option.  If the proxy cannot send a response that
    is acceptable according to the combined Accept field value, then
    the proxy sends a 406 (Not Acceptable) response.  The proxy MAY
    then retry the request with further media types from the HTTP
    Accept header field.
 Conditional GETs:  Conditional HTTP GET requests that include an "If-
    Match" or "If-None-Match" request-header field can be mapped to a
    corresponding CoAP request.  The "If-Modified-Since" and "If-
    Unmodified-Since" request-header fields are not directly supported
    by CoAP but are implemented locally by a caching proxy.

Shelby, et al. Standards Track [Page 78] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

10.2.3. HEAD

 The HEAD method is identical to GET except that the server MUST NOT
 return a message-body in the response.
 Although there is no direct equivalent of HTTP's HEAD method in CoAP,
 an HTTP-CoAP proxy responds to HEAD requests for CoAP resources, and
 the HTTP headers are returned without a message-body.
 Implementation Note:  An HTTP-CoAP proxy may want to try using a
    block-wise transfer option [BLOCK] to minimize the amount of data
    actually transferred, but it needs to be prepared for the case
    that the origin server does not support block-wise transfers.

10.2.4. POST

 The POST method requests the proxy to have the representation
 enclosed in the request be processed by the CoAP origin server.  The
 actual function performed by the POST method is determined by the
 origin server and dependent on the resource identified by the request
 URI.
 If the action performed by the POST method does not result in a
 resource that can be identified by a URI, a 200 (OK) or 204 (No
 Content) response MUST be returned to the client.  If a resource has
 been created on the origin server, a 201 (Created) response MUST be
 returned.
 If any of the Location-* Options are present in the CoAP response, a
 Location header field constructed from the values of these options is
 returned.

10.2.5. PUT

 The PUT method requests the proxy to update or create the CoAP
 resource identified by the Request-URI with the enclosed
 representation.
 If a new resource is created at the Request-URI, a 201 (Created)
 response is returned to the client.  If an existing resource is
 modified, either the 200 (OK) or 204 (No Content) Response Codes is
 sent to indicate successful completion of the request.

Shelby, et al. Standards Track [Page 79] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

10.2.6. DELETE

 The DELETE method requests the proxy to delete the CoAP resource
 identified by the Request-URI at the CoAP origin server.
 A successful response is 200 (OK) if the response includes an entity
 describing the status or 204 (No Content) if the action has been
 enacted but the response does not include an entity.

10.2.7. CONNECT

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

11. Security Considerations

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

11.1. Parsing the Protocol and Processing URIs

 A network-facing application can exhibit vulnerabilities in its
 processing logic for incoming packets.  Complex parsers are well-
 known as a likely source of such vulnerabilities, such as the ability
 to remotely crash a node, or even remotely execute arbitrary code on
 it.  CoAP attempts to narrow the opportunities for introducing such
 vulnerabilities by reducing parser complexity, by giving the entire
 range of encodable values a meaning where possible, and by
 aggressively reducing complexity that is often caused by unnecessary
 choice between multiple representations that mean the same thing.
 Much of the URI processing has been moved to the clients, further
 reducing the opportunities for introducing vulnerabilities into the
 servers.  Even so, the URI processing code in CoAP implementations is
 likely to be a large source of remaining vulnerabilities and should
 be implemented with special care.  CoAP access control
 implementations need to ensure they don't introduce vulnerabilities
 through discrepancies between the code deriving access control
 decisions from a URI and the code finally serving up the resource
 addressed by the URI.  The most complex parser remaining could be the
 one for the CoRE Link Format, although this also has been designed
 with a goal of reduced implementation complexity [RFC6690].  (See
 also Section 15.2 of [RFC2616].)

Shelby, et al. Standards Track [Page 80] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

11.2. Proxying and Caching

 As mentioned in Section 15.7 of [RFC2616], proxies are by their very
 nature men-in-the-middle, breaking any IPsec or DTLS protection that
 a direct CoAP message exchange might have.  They are therefore
 interesting targets for breaking confidentiality or integrity of CoAP
 message exchanges.  As noted in [RFC2616], they are also interesting
 targets for breaking availability.
 The threat to confidentiality and integrity of request/response data
 is amplified where proxies also cache.  Note that CoAP does not
 define any of the cache-suppressing Cache-Control options that
 HTTP/1.1 provides to better protect sensitive data.
 For a caching implementation, any access control considerations that
 would apply to making the request that generated the cache entry also
 need to be applied to the value in the cache.  This is relevant for
 clients that implement multiple security domains, as well as for
 proxies that may serve multiple clients.  Also, a caching proxy MUST
 NOT make cached values available to requests that have lesser
 transport-security properties than those the proxy would require to
 perform request forwarding in the first place.
 Unlike the "coap" scheme, responses to "coaps" identified requests
 are never "public" and thus MUST NOT be reused for shared caching,
 unless the cache is able to make equivalent access control decisions
 to the ones that led to the cached entry.  They can, however, be
 reused in a private cache if the message is cacheable by default in
 CoAP.
 Finally, a proxy that fans out Separate Responses (as opposed to
 piggybacked Responses) to multiple original requesters may provide
 additional amplification (see Section 11.3).

11.3. Risk of Amplification

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

Shelby, et al. Standards Track [Page 81] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 This is particularly a problem in nodes that enable NoSec access, are
 accessible from an attacker, and can access potential victims (e.g.,
 on the general Internet), as the UDP protocol provides no way to
 verify the source address given in the request packet.  An attacker
 need only place the IP address of the victim in the source address of
 a suitable request packet to generate a larger packet directed at the
 victim.
 As a mitigating factor, many constrained networks will only be able
 to generate a small amount of traffic, which may make CoAP nodes less
 attractive for this attack.  However, the limited capacity of the
 constrained network makes the network itself a likely victim of an
 amplification attack.
 Therefore, large amplification factors SHOULD NOT be provided in the
 response if the request is not authenticated.  A CoAP server can
 reduce the amount of amplification it provides to an attacker by
 using slicing/blocking modes of CoAP [BLOCK] and offering large
 resource representations only in relatively small slices.  For
 example, for a 1000-byte resource, a 10-byte request might result in
 an 80-byte response (with a 64-byte block) instead of a 1016-byte
 response, considerably reducing the amplification provided.
 CoAP also supports the use of multicast IP addresses in requests, an
 important requirement for M2M.  Multicast CoAP requests may be the
 source of accidental or deliberate DoS attacks, especially over
 constrained networks.  This specification attempts to reduce the
 amplification effects of multicast requests by limiting when a
 response is returned.  To limit the possibility of malicious use,
 CoAP servers SHOULD NOT accept multicast requests that can not be
 authenticated in some way, cryptographically or by some multicast
 boundary limiting the potential sources.  If possible, a CoAP server
 SHOULD limit the support for multicast requests to the specific
 resources where the feature is required.
 On some general-purpose operating systems providing a POSIX-style API
 [IEEE1003.1], it is not straightforward to find out whether a packet
 received was addressed to a multicast address.  While many
 implementations will know whether they have joined a multicast group,
 this creates a problem for packets addressed to multicast addresses
 of the form FF0x::1, which are received by every IPv6 node.
 Implementations SHOULD make use of modern APIs such as
 IPV6_RECVPKTINFO [RFC3542], if available, to make this determination.

Shelby, et al. Standards Track [Page 82] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

11.4. IP Address Spoofing Attacks

 Due to the lack of a handshake in UDP, a rogue endpoint that is free
 to read and write messages carried by the constrained network (i.e.,
 NoSec or PreSharedKey deployments with a nodes/key ratio > 1:1), may
 easily attack a single endpoint, a group of endpoints, as well as the
 whole network, e.g., by:
 1.  spoofing a Reset message in response to a Confirmable message or
     Non-confirmable message, thus making an endpoint "deaf"; or
 2.  spoofing an ACK in response to a CON message, thus potentially
     preventing the sender of the CON message from retransmitting, and
     drowning out the actual response; or
 3.  spoofing the entire response with forged payload/options (this
     has different levels of impact: from single-response disruption,
     to much bolder attacks on the supporting infrastructure, e.g.,
     poisoning proxy caches, or tricking validation/lookup interfaces
     in resource directories and, more generally, any component that
     stores global network state and uses CoAP as the messaging
     facility to handle setting or updating state is a potential
     target.); or
 4.  spoofing a multicast request for a target node; this may result
     in network congestion/collapse, a DoS attack on the victim, or
     forced wake-up from sleeping; or
 5.  spoofing observe messages, etc.
 Response spoofing by off-path attackers can be detected and mitigated
 even without transport layer security by choosing a nontrivial,
 randomized token in the request (Section 5.3.1).  [RFC4086] discusses
 randomness requirements for security.
 In principle, other kinds of spoofing can be detected by CoAP only in
 case Confirmable message semantics is used, because of unexpected
 Acknowledgement or Reset messages coming from the deceived endpoint.
 But this imposes keeping track of the used Message IDs, which is not
 always possible, and moreover detection becomes available usually
 after the damage is already done.  This kind of attack can be
 prevented using security modes other than NoSec.
 With or without source address spoofing, a client can attempt to
 overload a server by sending requests, preferably complex ones, to a
 server; address spoofing makes tracing back, and blocking, this
 attack harder.  Given that the cost of a CON request is small, this
 attack can easily be executed.  Under this attack, a constrained node

Shelby, et al. Standards Track [Page 83] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 with limited total energy available may exhaust that energy much more
 quickly than planned (battery depletion attack).  Also, if the client
 uses a Confirmable message and the server responds with a Confirmable
 separate response to a (possibly spoofed) address that does not
 respond, the server will have to allocate buffer and retransmission
 logic for each response up to the exhaustion of MAX_TRANSMIT_SPAN,
 making it more likely that it runs out of resources for processing
 legitimate traffic.  The latter problem can be mitigated somewhat by
 limiting the rate of responses as discussed in Section 4.7.  An
 attacker could also spoof the address of a legitimate client; this
 might cause the server, if it uses separate responses, to block
 legitimate responses to that client because of NSTART=1.  All these
 attacks can be prevented using a security mode other than NoSec, thus
 leaving only attacks on the security protocol.

11.5. Cross-Protocol Attacks

 The ability to incite a CoAP endpoint to send packets to a fake
 source address can be used not only for amplification, but also for
 cross-protocol attacks against a victim listening to UDP packets at a
 given address (IP address and port).  This would occur as follows:
 o  The attacker sends a message to a CoAP endpoint with the given
    address as the fake source address.
 o  The CoAP endpoint replies with a message to the given source
    address.
 o  The victim at the given address receives a UDP packet that it
    interprets according to the rules of a different protocol.
 This may be used to circumvent firewall rules that prevent direct
 communication from the attacker to the victim but happen to allow
 communication from the CoAP endpoint (which may also host a valid
 role in the other protocol) to the victim.
 Also, CoAP endpoints may be the victim of a cross-protocol attack
 generated through an endpoint of another UDP-based protocol such as
 DNS.  In both cases, attacks are possible if the security properties
 of the endpoints rely on checking IP addresses (and firewalling off
 direct attacks sent from outside using fake IP addresses).  In
 general, because of their lack of context, UDP-based protocols are
 relatively easy targets for cross-protocol attacks.
 Finally, CoAP URIs transported by other means could be used to incite
 clients to send messages to endpoints of other protocols.

Shelby, et al. Standards Track [Page 84] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 One mitigation against cross-protocol attacks is strict checking of
 the syntax of packets received, combined with sufficient difference
 in syntax.  As an example, it might help if it were difficult to
 incite a DNS server to send a DNS response that would pass the checks
 of a CoAP endpoint.  Unfortunately, the first two bytes of a DNS
 reply are an ID that can be chosen by the attacker and that map into
 the interesting part of the CoAP header, and the next two bytes are
 then interpreted as CoAP's Message ID (i.e., any value is
 acceptable).  The DNS count words may be interpreted as multiple
 instances of a (nonexistent but elective) CoAP option 0, or possibly
 as a Token.  The echoed query finally may be manufactured by the
 attacker to achieve a desired effect on the CoAP endpoint; the
 response added by the server (if any) might then just be interpreted
 as added payload.
                                 1  1  1  1  1  1
   0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 |                      ID                       | T, TKL, code
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 |QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   | Message ID
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 |                    QDCOUNT                    | (options 0)
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 |                    ANCOUNT                    | (options 0)
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 |                    NSCOUNT                    | (options 0)
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 |                    ARCOUNT                    | (options 0)
 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
   Figure 15: DNS Header ([RFC1035], Section 4.1.1) vs. CoAP Message
 In general, for any pair of protocols, one of the protocols can very
 well have been designed in a way that enables an attacker to cause
 the generation of replies that look like messages of the other
 protocol.  It is often much harder to ensure or prove the absence of
 viable attacks than to generate examples that may not yet completely
 enable an attack but might be further developed by more creative
 minds.  Cross-protocol attacks can therefore only be completely
 mitigated if endpoints don't authorize actions desired by an attacker
 just based on trusting the source IP address of a packet.
 Conversely, a NoSec environment that completely relies on a firewall
 for CoAP security not only needs to firewall off the CoAP endpoints
 but also all other endpoints that might be incited to send UDP
 messages to CoAP endpoints using some other UDP-based protocol.

Shelby, et al. Standards Track [Page 85] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

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

11.6. Constrained-Node Considerations

 Implementers on constrained nodes often find themselves without a
 good source of entropy [RFC4086].  If that is the case, the node MUST
 NOT be used for processes that require good entropy, such as key
 generation.  Instead, keys should be generated externally and added
 to the device during manufacturing or commissioning.
 Due to their low processing power, constrained nodes are particularly
 susceptible to timing attacks.  Special care must be taken in
 implementation of cryptographic primitives.
 Large numbers of constrained nodes will be installed in exposed
 environments and will have little resistance to tampering, including
 recovery of keying materials.  This needs to be considered when
 defining the scope of credentials assigned to them.  In particular,
 assigning a shared key to a group of nodes may make any single
 constrained node a target for subverting the entire group.

12. IANA Considerations

12.1. CoAP Code Registries

 This document defines two sub-registries for the values of the Code
 field in the CoAP header within the "Constrained RESTful Environments
 (CoRE) Parameters" registry, hereafter referred to as the "CoRE
 Parameters" registry.
 Values in the two sub-registries are eight-bit values notated as
 three decimal digits c.dd separated by a period between the first and
 the second digit; the first digit c is between 0 and 7 and denotes
 the code class; the second and third digits dd denote a decimal
 number between 00 and 31 for the detail.

Shelby, et al. Standards Track [Page 86] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 All Code values are assigned by sub-registries according to the
 following ranges:
 0.00      Indicates an Empty message (see Section 4.1).
 0.01-0.31 Indicates a request.  Values in this range are assigned by
           the "CoAP Method Codes" sub-registry (see Section 12.1.1).
 1.00-1.31 Reserved
 2.00-5.31 Indicates a response.  Values in this range are assigned by
           the "CoAP Response Codes" sub-registry (see
           Section 12.1.2).
 6.00-7.31 Reserved

12.1.1. Method Codes

 The name of the sub-registry is "CoAP Method Codes".
 Each entry in the sub-registry must include the Method Code in the
 range 0.01-0.31, the name of the method, and a reference to the
 method's documentation.
 Initial entries in this sub-registry are as follows:
                     +------+--------+-----------+
                     | Code | Name   | Reference |
                     +------+--------+-----------+
                     | 0.01 | GET    | [RFC7252] |
                     | 0.02 | POST   | [RFC7252] |
                     | 0.03 | PUT    | [RFC7252] |
                     | 0.04 | DELETE | [RFC7252] |
                     +------+--------+-----------+
                      Table 5: CoAP Method Codes
 All other Method Codes are Unassigned.
 The IANA policy for future additions to this sub-registry is "IETF
 Review or IESG Approval" as described in [RFC5226].
 The documentation of a Method Code should specify the semantics of a
 request with that code, including the following properties:
 o  The Response Codes the method returns in the success case.
 o  Whether the method is idempotent, safe, or both.

Shelby, et al. Standards Track [Page 87] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

12.1.2. Response Codes

 The name of the sub-registry is "CoAP Response Codes".
 Each entry in the sub-registry must include the Response Code in the
 range 2.00-5.31, a description of the Response Code, and a reference
 to the Response Code's documentation.
 Initial entries in this sub-registry are as follows:
          +------+------------------------------+-----------+
          | Code | Description                  | Reference |
          +------+------------------------------+-----------+
          | 2.01 | Created                      | [RFC7252] |
          | 2.02 | Deleted                      | [RFC7252] |
          | 2.03 | Valid                        | [RFC7252] |
          | 2.04 | Changed                      | [RFC7252] |
          | 2.05 | Content                      | [RFC7252] |
          | 4.00 | Bad Request                  | [RFC7252] |
          | 4.01 | Unauthorized                 | [RFC7252] |
          | 4.02 | Bad Option                   | [RFC7252] |
          | 4.03 | Forbidden                    | [RFC7252] |
          | 4.04 | Not Found                    | [RFC7252] |
          | 4.05 | Method Not Allowed           | [RFC7252] |
          | 4.06 | Not Acceptable               | [RFC7252] |
          | 4.12 | Precondition Failed          | [RFC7252] |
          | 4.13 | Request Entity Too Large     | [RFC7252] |
          | 4.15 | Unsupported Content-Format   | [RFC7252] |
          | 5.00 | Internal Server Error        | [RFC7252] |
          | 5.01 | Not Implemented              | [RFC7252] |
          | 5.02 | Bad Gateway                  | [RFC7252] |
          | 5.03 | Service Unavailable          | [RFC7252] |
          | 5.04 | Gateway Timeout              | [RFC7252] |
          | 5.05 | Proxying Not Supported       | [RFC7252] |
          +------+------------------------------+-----------+
                     Table 6: CoAP Response Codes
 The Response Codes 3.00-3.31 are Reserved for future use.  All other
 Response Codes are Unassigned.
 The IANA policy for future additions to this sub-registry is "IETF
 Review or IESG Approval" as described in [RFC5226].

Shelby, et al. Standards Track [Page 88] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 The documentation of a Response Code should specify the semantics of
 a response with that code, including the following properties:
 o  The methods the Response Code applies to.
 o  Whether payload is required, optional, or not allowed.
 o  The semantics of the payload.  For example, the payload of a 2.05
    (Content) response is a representation of the target resource; the
    payload in an error response is a human-readable diagnostic
    payload.
 o  The format of the payload.  For example, the format in a 2.05
    (Content) response is indicated by the Content-Format Option; the
    format of the payload in an error response is always Net-Unicode
    text.
 o  Whether the response is cacheable according to the freshness
    model.
 o  Whether the response is validatable according to the validation
    model.
 o  Whether the response causes a cache to mark responses stored for
    the request URI as not fresh.

12.2. CoAP Option Numbers Registry

 This document defines a sub-registry for the Option Numbers used in
 CoAP options within the "CoRE Parameters" registry.  The name of the
 sub-registry is "CoAP Option Numbers".
 Each entry in the sub-registry must include the Option Number, the
 name of the option, and a reference to the option's documentation.

Shelby, et al. Standards Track [Page 89] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Initial entries in this sub-registry are as follows:
               +--------+------------------+-----------+
               | Number | Name             | Reference |
               +--------+------------------+-----------+
               |      0 | (Reserved)       | [RFC7252] |
               |      1 | If-Match         | [RFC7252] |
               |      3 | Uri-Host         | [RFC7252] |
               |      4 | ETag             | [RFC7252] |
               |      5 | If-None-Match    | [RFC7252] |
               |      7 | Uri-Port         | [RFC7252] |
               |      8 | Location-Path    | [RFC7252] |
               |     11 | Uri-Path         | [RFC7252] |
               |     12 | Content-Format   | [RFC7252] |
               |     14 | Max-Age          | [RFC7252] |
               |     15 | Uri-Query        | [RFC7252] |
               |     17 | Accept           | [RFC7252] |
               |     20 | Location-Query   | [RFC7252] |
               |     35 | Proxy-Uri        | [RFC7252] |
               |     39 | Proxy-Scheme     | [RFC7252] |
               |     60 | Size1            | [RFC7252] |
               |    128 | (Reserved)       | [RFC7252] |
               |    132 | (Reserved)       | [RFC7252] |
               |    136 | (Reserved)       | [RFC7252] |
               |    140 | (Reserved)       | [RFC7252] |
               +--------+------------------+-----------+
                     Table 7: CoAP Option Numbers
 The IANA policy for future additions to this sub-registry is split
 into three tiers as follows.  The range of 0..255 is reserved for
 options defined by the IETF (IETF Review or IESG Approval).  The
 range of 256..2047 is reserved for commonly used options with public
 specifications (Specification Required).  The range of 2048..64999 is
 for all other options including private or vendor-specific ones,
 which undergo a Designated Expert review to help ensure that the
 option semantics are defined correctly.  The option numbers between
 65000 and 65535 inclusive are reserved for experiments.  They are not
 meant for vendor-specific use of any kind and MUST NOT be used in
 operational deployments.

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        +-------------+---------------------------------------+
        |       Range | Registration Procedures               |
        +-------------+---------------------------------------+
        |       0-255 | IETF Review or IESG Approval          |
        |    256-2047 | Specification Required                |
        |  2048-64999 | Expert Review                         |
        | 65000-65535 | Experimental use (no operational use) |
        +-------------+---------------------------------------+
         Table 8: CoAP Option Numbers: Registration Procedures
 The documentation of an Option Number should specify the semantics of
 an option with that number, including the following properties:
 o  The meaning of the option in a request.
 o  The meaning of the option in a response.
 o  Whether the option is critical or elective, as determined by the
    Option Number.
 o  Whether the option is Safe-to-Forward, and, if yes, whether it is
    part of the Cache-Key, as determined by the Option Number (see
    Section 5.4.2).
 o  The format and length of the option's value.
 o  Whether the option must occur at most once or whether it can occur
    multiple times.
 o  The default value, if any.  For a critical option with a default
    value, a discussion on how the default value enables processing by
    implementations that do not support the critical option
    (Section 5.4.4).

12.3. CoAP Content-Formats Registry

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

Shelby, et al. Standards Track [Page 91] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Each entry in the sub-registry must include the media type registered
 with IANA, the numeric identifier in the range 0-65535 to be used for
 that media type in CoAP, the content-coding associated with this
 identifier, and a reference to a document describing what a payload
 with that media type means semantically.
 CoAP does not include a separate way to convey content-encoding
 information with a request or response, and for that reason the
 content-encoding is also specified for each identifier (if any).  If
 multiple content-encodings will be used with a media type, then a
 separate Content-Format identifier for each is to be registered.
 Similarly, other parameters related to an Internet media type, such
 as level, can be defined for a CoAP Content-Format entry.
 Initial entries in this sub-registry are as follows:
 +--------------------------+----------+----+------------------------+
 | Media type               | Encoding | ID | Reference              |
 +--------------------------+----------+----+------------------------+
 | text/plain;              | -        |  0 | [RFC2046] [RFC3676]    |
 | charset=utf-8            |          |    | [RFC5147]              |
 | application/link-format  | -        | 40 | [RFC6690]              |
 | application/xml          | -        | 41 | [RFC3023]              |
 | application/octet-stream | -        | 42 | [RFC2045] [RFC2046]    |
 | application/exi          | -        | 47 | [REC-exi-20140211]     |
 | application/json         | -        | 50 | [RFC7159]              |
 +--------------------------+----------+----+------------------------+
                     Table 9: CoAP Content-Formats
 The identifiers between 65000 and 65535 inclusive are reserved for
 experiments.  They are not meant for vendor-specific use of any kind
 and MUST NOT be used in operational deployments.  The identifiers
 between 256 and 9999 are reserved for future use in IETF
 specifications (IETF Review or IESG Approval).  All other identifiers
 are Unassigned.
 Because the namespace of single-byte identifiers is so small, the
 IANA policy for future additions in the range 0-255 inclusive to the
 sub-registry is "Expert Review" as described in [RFC5226].  The IANA
 policy for additions in the range 10000-64999 inclusive is "First
 Come First Served" as described in [RFC5226].  This is summarized in
 the following table.

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        +-------------+---------------------------------------+
        |       Range | Registration Procedures               |
        +-------------+---------------------------------------+
        |       0-255 | Expert Review                         |
        |    256-9999 | IETF Review or IESG Approval          |
        | 10000-64999 | First Come First Served               |
        | 65000-65535 | Experimental use (no operational use) |
        +-------------+---------------------------------------+
        Table 10: CoAP Content-Formats: Registration Procedures
 In machine-to-machine applications, it is not expected that generic
 Internet media types such as text/plain, application/xml or
 application/octet-stream are useful for real applications in the long
 term.  It is recommended that M2M applications making use of CoAP
 request new Internet media types from IANA indicating semantic
 information about how to create or parse a payload.  For example, a
 Smart Energy application payload carried as XML might request a more
 specific type like application/se+xml or application/se-exi.

12.4. URI Scheme Registration

 This document contains the request for the registration of the
 Uniform Resource Identifier (URI) scheme "coap".  The registration
 request complies with [RFC4395].
 URI scheme name.
    coap
 Status.
    Permanent.
 URI scheme syntax.
    Defined in Section 6.1 of [RFC7252].
 URI scheme semantics.
    The "coap" URI scheme provides a way to identify resources that
    are potentially accessible over the Constrained Application
    Protocol (CoAP).  The resources can be located by contacting the
    governing CoAP server and operated on by sending CoAP requests to
    the server.  This scheme can thus be compared to the "http" URI
    scheme [RFC2616].  See Section 6 of [RFC7252] for the details of
    operation.
 Encoding considerations.
    The scheme encoding conforms to the encoding rules established for
    URIs in [RFC3986], i.e., internationalized and reserved characters
    are expressed using UTF-8-based percent-encoding.

Shelby, et al. Standards Track [Page 93] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Applications/protocols that use this URI scheme name.
    The scheme is used by CoAP endpoints to access CoAP resources.
 Interoperability considerations.
    None.
 Security considerations.
    See Section 11.1 of [RFC7252].
 Contact.
    IETF Chair <chair@ietf.org>
 Author/Change controller.
    IESG <iesg@ietf.org>
 References.
    [RFC7252]

12.5. Secure URI Scheme Registration

 This document contains the request for the registration of the
 Uniform Resource Identifier (URI) scheme "coaps".  The registration
 request complies with [RFC4395].
 URI scheme name.
    coaps
 Status.
    Permanent.
 URI scheme syntax.
    Defined in Section 6.2 of [RFC7252].
 URI scheme semantics.
    The "coaps" URI scheme provides a way to identify resources that
    are potentially accessible over the Constrained Application
    Protocol (CoAP) using Datagram Transport Layer Security (DTLS) for
    transport security.  The resources can be located by contacting
    the governing CoAP server and operated on by sending CoAP requests
    to the server.  This scheme can thus be compared to the "https"
    URI scheme [RFC2616].  See Section 6 of [RFC7252] for the details
    of operation.
 Encoding considerations.
    The scheme encoding conforms to the encoding rules established for
    URIs in [RFC3986], i.e., internationalized and reserved characters
    are expressed using UTF-8-based percent-encoding.

Shelby, et al. Standards Track [Page 94] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Applications/protocols that use this URI scheme name.
    The scheme is used by CoAP endpoints to access CoAP resources
    using DTLS.
 Interoperability considerations.
    None.
 Security considerations.
    See Section 11.1 of [RFC7252].
 Contact.
    IETF Chair <chair@ietf.org>
 Author/Change controller.
    IESG <iesg@ietf.org>
 References.
    [RFC7252]

12.6. Service Name and Port Number Registration

 One of the functions of CoAP is resource discovery: a CoAP client can
 ask a CoAP server about the resources offered by it (see Section 7).
 To enable resource discovery just based on the knowledge of an IP
 address, the CoAP port for resource discovery needs to be
 standardized.
 IANA has assigned the port number 5683 and the service name "coap",
 in accordance with [RFC6335].
 Besides unicast, CoAP can be used with both multicast and anycast.
 Service Name.
    coap
 Transport Protocol.
    udp
 Assignee.
    IESG <iesg@ietf.org>
 Contact.
    IETF Chair <chair@ietf.org>
 Description.
    Constrained Application Protocol (CoAP)

Shelby, et al. Standards Track [Page 95] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Reference.
    [RFC7252]
 Port Number.
    5683

12.7. Secure Service Name and Port Number Registration

 CoAP resource discovery may also be provided using the DTLS-secured
 CoAP "coaps" scheme.  Thus, the CoAP port for secure resource
 discovery needs to be standardized.
 IANA has assigned the port number 5684 and the service name "coaps",
 in accordance with [RFC6335].
 Besides unicast, DTLS-secured CoAP can be used with anycast.
 Service Name.
    coaps
 Transport Protocol.
    udp
 Assignee.
    IESG <iesg@ietf.org>
 Contact.
    IETF Chair <chair@ietf.org>
 Description.
    DTLS-secured CoAP
 Reference.
    [RFC7252]
 Port Number.
    5684

Shelby, et al. Standards Track [Page 96] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

12.8. Multicast Address Registration

 Section 8, "Multicast CoAP", defines the use of multicast.  IANA has
 assigned the following multicast addresses for use by CoAP nodes:
 IPv4  -- "All CoAP Nodes" address 224.0.1.187, from the "IPv4
    Multicast Address Space Registry".  As the address is used for
    discovery that may span beyond a single network, it has come from
    the Internetwork Control Block (224.0.1.x, RFC 5771).
 IPv6  -- "All CoAP Nodes" address FF0X::FD, from the "IPv6 Multicast
    Address Space Registry", in the "Variable Scope Multicast
    Addresses" space (RFC 3307).  Note that there is a distinct
    multicast address for each scope that interested CoAP nodes should
    listen to; CoAP needs the Link-Local and Site-Local scopes only.

13. Acknowledgements

 Brian Frank was a contributor to and coauthor of early versions of
 this specification.
 Special thanks to Peter Bigot, Esko Dijk, and Cullen Jennings for
 substantial contributions to the ideas and text in the document,
 along with countless detailed reviews and discussions.
 Thanks to Floris Van den Abeele, Anthony Baire, Ed Beroset, Berta
 Carballido, Angelo P. Castellani, Gilbert Clark, Robert Cragie,
 Pierre David, Esko Dijk, Lisa Dusseault, Mehmet Ersue, Thomas
 Fossati, Tobias Gondrom, Bert Greevenbosch, Tom Herbst, Jeroen
 Hoebeke, Richard Kelsey, Sye Loong Keoh, Ari Keranen, Matthias
 Kovatsch, Avi Lior, Stephan Lohse, Salvatore Loreto, Kerry Lynn,
 Andrew McGregor, Alexey Melnikov, Guido Moritz, Petri Mutka, Colin
 O'Flynn, Charles Palmer, Adriano Pezzuto, Thomas Poetsch, Robert
 Quattlebaum, Akbar Rahman, Eric Rescorla, Dan Romascanu, David Ryan,
 Peter Saint-Andre, Szymon Sasin, Michael Scharf, Dale Seed, Robby
 Simpson, Peter van der Stok, Michael Stuber, Linyi Tian, Gilman
 Tolle, Matthieu Vial, Maciej Wasilak, Fan Xianyou, and Alper Yegin
 for helpful comments and discussions that have shaped the document.
 Special thanks also to the responsible IETF area director at the time
 of completion, Barry Leiba, and the IESG reviewers, Adrian Farrel,
 Martin Stiemerling, Pete Resnick, Richard Barnes, Sean Turner,
 Spencer Dawkins, Stephen Farrell, and Ted Lemon, who contributed in-
 depth reviews.
 Some of the text has been borrowed from the working documents of the
 IETF HTTPBIS working group.

Shelby, et al. Standards Track [Page 97] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

14. References

14.1. Normative References

 [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            August 1980.
 [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part One: Format of Internet Message
            Bodies", RFC 2045, November 1996.
 [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Two: Media Types", RFC 2046,
            November 1996.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
            Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
            Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
 [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
            Types", RFC 3023, January 2001.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003.
 [RFC3676]  Gellens, R., "The Text/Plain Format and DelSp Parameters",
            RFC 3676, February 2004.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66, RFC
            3986, January 2005.
 [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
            for Transport Layer Security (TLS)", RFC 4279, December
            2005.
 [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
            Registration Procedures for New URI Schemes", BCP 35, RFC
            4395, February 2006.
 [RFC5147]  Wilde, E. and M. Duerst, "URI Fragment Identifiers for the
            text/plain Media Type", RFC 5147, April 2008.
 [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
            Interchange", RFC 5198, March 2008.

Shelby, et al. Standards Track [Page 98] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008.
 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, May 2008.
 [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
            "Elliptic Curve Cryptography Subject Public Key
            Information", RFC 5480, March 2009.
 [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
            Uniform Resource Identifiers (URIs)", RFC 5785, April
            2010.
 [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
            Address Text Representation", RFC 5952, August 2010.
 [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.
 [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
            Extension Definitions", RFC 6066, January 2011.
 [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security Version 1.2", RFC 6347, January 2012.
 [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
            Format", RFC 6690, August 2012.
 [RFC6920]  Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B.,
            Keranen, A., and P. Hallam-Baker, "Naming Things with
            Hashes", RFC 6920, April 2013.
 [RFC7250]  Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
            T. Kivinen, "Using Raw Public Keys in Transport Layer
            Security (TLS) and Datagram Transport Layer Security
            (DTLS)", RFC 7250, June 2014.

Shelby, et al. Standards Track [Page 99] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
            CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
            Transport Layer Security (TLS)", RFC 7251, June 2014.

14.2. Informative References

 [BLOCK]    Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
            Work in Progress, October 2013.
 [CoAP-MISC]
            Bormann, C. and K. Hartke, "Miscellaneous additions to
            CoAP", Work in Progress, December 2013.
 [EUI64]    IEEE Standards Association, "Guidelines for 64-bit Global
            Identifier (EUI-64 (TM))", Registration Authority
            Tutorials, April 2010, <http://standards.ieee.org/regauth/
            oui/tutorials/EUI64.html>.
 [GROUPCOMM]
            Rahman, A. and E. Dijk, "Group Communication for CoAP",
            Work in Progress, December 2013.
 [HHGTTG]   Adams, D., "The Hitchhiker's Guide to the Galaxy", Pan
            Books ISBN 3320258648, 1979.
 [IEEE1003.1]
            IEEE and The Open Group, "Portable Operating System
            Interface (POSIX)", The Open Group Base Specifications
            Issue 7, IEEE 1003.1, 2013 Edition,
            <http://pubs.opengroup.org/onlinepubs/9699919799/>.
 [IPsec-CoAP]
            Bormann, C., "Using CoAP with IPsec", Work in Progress,
            December 2012.
 [MAPPING]  Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
            E. Dijk, "Guidelines for HTTP-CoAP Mapping
            Implementations", Work in Progress, February 2014.
 [OBSERVE]  Hartke, K., "Observing Resources in CoAP", Work in
            Progress, April 2014.
 [REC-exi-20140211]
            Schneider, J., Kamiya, T., Peintner, D., and R. Kyusakov,
            "Efficient XML Interchange (EXI) Format 1.0 (Second
            Edition)", W3C Recommendation REC-exi-20140211, February
            2014, <http://www.w3.org/TR/2014/REC-exi-20140211/>.

Shelby, et al. Standards Track [Page 100] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 [REST]     Fielding, R., "Architectural Styles and the Design of
            Network-based Software Architectures", Ph.D. Dissertation,
            University of California, Irvine, 2000,
            <http://www.ics.uci.edu/~fielding/pubs/dissertation/
            fielding_dissertation.pdf>.
 [RFC0020]  Cerf, V., "ASCII format for network interchange", RFC 20,
            October 1969.
 [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.
 [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
            RFC 792, September 1981.
 [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
            793, September 1981.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, November 1987.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264, June
            2002.
 [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
            X.509 Public Key Infrastructure Certificate and
            Certificate Revocation List (CRL) Profile", RFC 3280,
            April 2002.
 [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
            "Advanced Sockets Application Program Interface (API) for
            IPv6", RFC 3542, May 2003.
 [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
            G. Fairhurst, "The Lightweight User Datagram Protocol
            (UDP-Lite)", RFC 3828, July 2004.
 [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
            Requirements for Security", BCP 106, RFC 4086, June 2005.
 [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
            Message Protocol (ICMPv6) for the Internet Protocol
            Version 6 (IPv6) Specification", RFC 4443, March 2006.
 [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
            Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
            for Transport Layer Security (TLS)", RFC 4492, May 2006.

Shelby, et al. Standards Track [Page 101] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
            Discovery", RFC 4821, March 2007.
 [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
            "Transmission of IPv6 Packets over IEEE 802.15.4
            Networks", RFC 4944, September 2007.
 [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
            for Application Designers", BCP 145, RFC 5405, November
            2008.
 [RFC5489]  Badra, M. and I. Hajjeh, "ECDHE_PSK Cipher Suites for
            Transport Layer Security (TLS)", RFC 5489, March 2009.
 [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
            Curve Cryptography Algorithms", RFC 6090, February 2011.
 [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Core", RFC 6120, March 2011.
 [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
            Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
            September 2011.
 [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
            Cheshire, "Internet Assigned Numbers Authority (IANA)
            Procedures for the Management of the Service Name and
            Transport Protocol Port Number Registry", BCP 165, RFC
            6335, August 2011.
 [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
            Transport Layer Security (TLS)", RFC 6655, July 2012.
 [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
            for the Use of IPv6 UDP Datagrams with Zero Checksums",
            RFC 6936, April 2013.
 [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
            Galperin, S., and C. Adams, "X.509 Internet Public Key
            Infrastructure Online Certificate Status Protocol - OCSP",
            RFC 6960, June 2013.
 [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
            Multiple Certificate Status Request Extension", RFC 6961,
            June 2013.
 [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
            Interchange Format", RFC 7159, March 2014.

Shelby, et al. Standards Track [Page 102] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
            Constrained-Node Networks", RFC 7228, May 2014.
 [RTO-CONSIDER]
            Allman, M., "Retransmission Timeout Considerations", Work
            in Progress, May 2012.
 [W3CXMLSEC]
            Wenning, R., "Report of the XML Security PAG", W3C XML
            Security PAG, October 2012,
            <http://www.w3.org/2011/xmlsec-pag/pagreport.html>.

Shelby, et al. Standards Track [Page 103] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

Appendix A. Examples

 This section gives a number of short examples with message flows for
 GET requests.  These examples demonstrate the basic operation, the
 operation in the presence of retransmissions, and multicast.
 Figure 16 shows a basic GET request causing a piggybacked response:
 The client sends a Confirmable GET request for the resource
 coap://server/temperature to the server with a Message ID of 0x7d34.
 The request includes one Uri-Path Option (Delta 0 + 11 = 11, Length
 11, Value "temperature"); the Token is left empty.  This request is a
 total of 16 bytes long.  A 2.05 (Content) response is returned in the
 Acknowledgement message that acknowledges the Confirmable request,
 echoing both the Message ID 0x7d34 and the empty Token value.  The
 response includes a Payload of "22.3 C" and is 11 bytes long.
 Client  Server
    |      |
    |      |
    +----->|     Header: GET (T=CON, Code=0.01, MID=0x7d34)
    | GET  |   Uri-Path: "temperature"
    |      |
    |      |
    |<-----+     Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d34)
    | 2.05 |    Payload: "22.3 C"
    |      |
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1 | 0 |   0   |     GET=1     |          MID=0x7d34           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  11   |  11   |      "temperature" (11 B) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1 | 2 |   0   |    2.05=69    |          MID=0x7d34           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1 1 1 1 1 1 1 1|      "22.3 C" (6 B) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Figure 16: Confirmable Request; Piggybacked Response

Shelby, et al. Standards Track [Page 104] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Figure 17 shows a similar example, but with the inclusion of an non-
 empty Token (Value 0x20) in the request and the response, increasing
 the sizes to 17 and 12 bytes, respectively.
 Client  Server
    |      |
    |      |
    +----->|     Header: GET (T=CON, Code=0.01, MID=0x7d35)
    | GET  |      Token: 0x20
    |      |   Uri-Path: "temperature"
    |      |
    |      |
    |<-----+     Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d35)
    | 2.05 |      Token: 0x20
    |      |    Payload: "22.3 C"
    |      |
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1 | 0 |   1   |     GET=1     |          MID=0x7d35           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     0x20      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  11   |  11   |      "temperature" (11 B) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | 1 | 2 |   1   |    2.05=69    |          MID=0x7d35           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     0x20      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1 1 1 1 1 1 1 1| "22.3 C" (6 B) ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Figure 17: Confirmable Request; Piggybacked Response

Shelby, et al. Standards Track [Page 105] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 In Figure 18, the Confirmable GET request is lost.  After ACK_TIMEOUT
 seconds, the client retransmits the request, resulting in a
 piggybacked response as in the previous example.
 Client  Server
    |      |
    |      |
    +----X |     Header: GET (T=CON, Code=0.01, MID=0x7d36)
    | GET  |      Token: 0x31
    |      |   Uri-Path: "temperature"
 TIMEOUT   |
    |      |
    +----->|     Header: GET (T=CON, Code=0.01, MID=0x7d36)
    | GET  |      Token: 0x31
    |      |   Uri-Path: "temperature"
    |      |
    |      |
    |<-----+     Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d36)
    | 2.05 |      Token: 0x31
    |      |    Payload: "22.3 C"
    |      |
 Figure 18: Confirmable Request (Retransmitted); Piggybacked Response

Shelby, et al. Standards Track [Page 106] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 In Figure 19, the first Acknowledgement message from the server to
 the client is lost.  After ACK_TIMEOUT seconds, the client
 retransmits the request.
 Client  Server
    |      |
    |      |
    +----->|     Header: GET (T=CON, Code=0.01, MID=0x7d37)
    | GET  |      Token: 0x42
    |      |   Uri-Path: "temperature"
    |      |
    |      |
    | X----+     Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d37)
    | 2.05 |      Token: 0x42
    |      |    Payload: "22.3 C"
 TIMEOUT   |
    |      |
    +----->|     Header: GET (T=CON, Code=0.01, MID=0x7d37)
    | GET  |      Token: 0x42
    |      |   Uri-Path: "temperature"
    |      |
    |      |
    |<-----+     Header: 2.05 Content (T=ACK, Code=2.05, MID=0x7d37)
    | 2.05 |      Token: 0x42
    |      |    Payload: "22.3 C"
    |      |
 Figure 19: Confirmable Request; Piggybacked Response (Retransmitted)

Shelby, et al. Standards Track [Page 107] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 In Figure 20, the server acknowledges the Confirmable request and
 sends a 2.05 (Content) response separately in a Confirmable message.
 Note that the Acknowledgement message and the Confirmable response do
 not necessarily arrive in the same order as they were sent.  The
 client acknowledges the Confirmable response.
 Client  Server
    |      |
    |      |
    +----->|     Header: GET (T=CON, Code=0.01, MID=0x7d38)
    | GET  |      Token: 0x53
    |      |   Uri-Path: "temperature"
    |      |
    |      |
    |<- - -+     Header: (T=ACK, Code=0.00, MID=0x7d38)
    |      |
    |      |
    |<-----+     Header: 2.05 Content (T=CON, Code=2.05, MID=0xad7b)
    | 2.05 |      Token: 0x53
    |      |    Payload: "22.3 C"
    |      |
    |      |
    +- - ->|     Header: (T=ACK, Code=0.00, MID=0xad7b)
    |      |
           Figure 20: Confirmable Request; Separate Response

Shelby, et al. Standards Track [Page 108] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 Figure 21 shows an example where the client loses its state (e.g.,
 crashes and is rebooted) right after sending a Confirmable request,
 so the separate response arriving some time later comes unexpected.
 In this case, the client rejects the Confirmable response with a
 Reset message.  Note that the unexpected ACK is silently ignored.
 Client  Server
    |      |
    |      |
    +----->|     Header: GET (T=CON, Code=0.01, MID=0x7d39)
    | GET  |      Token: 0x64
    |      |   Uri-Path: "temperature"
  CRASH    |
    |      |
    |<- - -+     Header: (T=ACK, Code=0.00, MID=0x7d39)
    |      |
    |      |
    |<-----+     Header: 2.05 Content (T=CON, Code=2.05, MID=0xad7c)
    | 2.05 |      Token: 0x64
    |      |    Payload: "22.3 C"
    |      |
    |      |
    +- - ->|     Header: (T=RST, Code=0.00, MID=0xad7c)
    |      |
    Figure 21: Confirmable Request; Separate Response (Unexpected)
 Figure 22 shows a basic GET request where the request and the
 response are Non-confirmable, so both may be lost without notice.
 Client  Server
    |      |
    |      |
    +----->|     Header: GET (T=NON, Code=0.01, MID=0x7d40)
    | GET  |      Token: 0x75
    |      |   Uri-Path: "temperature"
    |      |
    |      |
    |<-----+     Header: 2.05 Content (T=NON, Code=2.05, MID=0xad7d)
    | 2.05 |      Token: 0x75
    |      |    Payload: "22.3 C"
    |      |
     Figure 22: Non-confirmable Request; Non-confirmable Response

Shelby, et al. Standards Track [Page 109] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 In Figure 23, the client sends a Non-confirmable GET request to a
 multicast address: all nodes in link-local scope.  There are 3
 servers on the link: A, B and C.  Servers A and B have a matching
 resource, therefore they send back a Non-confirmable 2.05 (Content)
 response.  The response sent by B is lost.  C does not have matching
 response, therefore it sends a Non-confirmable 4.04 (Not Found)
 response.
 Client  ff02::1  A  B  C
    |       |     |  |  |
    |       |     |  |  |
    +------>|     |  |  |  Header: GET (T=NON, Code=0.01, MID=0x7d41)
    |  GET  |     |  |  |   Token: 0x86
    |             |  |  |   Uri-Path: "temperature"
    |             |  |  |
    |             |  |  |
    |<------------+  |  |  Header: 2.05 (T=NON, Code=2.05, MID=0x60b1)
    |      2.05   |  |  |   Token: 0x86
    |             |  |  |   Payload: "22.3 C"
    |             |  |  |
    |             |  |  |
    |   X------------+  |  Header: 2.05 (T=NON, Code=2.05, MID=0x01a0)
    |      2.05   |  |  |   Token: 0x86
    |             |  |  |   Payload: "20.9 C"
    |             |  |  |
    |             |  |  |
    |<------------------+  Header: 4.04 (T=NON, Code=4.04, MID=0x952a)
    |      4.04   |  |  |   Token: 0x86
    |             |  |  |
    Figure 23: Non-confirmable Request (Multicast); Non-confirmable
                               Response

Appendix B. URI Examples

 The following examples demonstrate different sets of Uri options, and
 the result after constructing an URI from them.  In addition to the
 options, Section 6.5 refers to the destination IP address and port,
 but not all paths of the algorithm cause the destination IP address
 and port to be included in the URI.
 o  Input:
       Destination IP Address = [2001:db8::2:1]
       Destination UDP Port = 5683

Shelby, et al. Standards Track [Page 110] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

    Output:
       coap://[2001:db8::2:1]/
 o  Input:
       Destination IP Address = [2001:db8::2:1]
       Destination UDP Port = 5683
       Uri-Host = "example.net"
    Output:
       coap://example.net/
 o  Input:
       Destination IP Address = [2001:db8::2:1]
       Destination UDP Port = 5683
       Uri-Host = "example.net"
       Uri-Path = ".well-known"
       Uri-Path = "core"
    Output:
       coap://example.net/.well-known/core
 o  Input:
       Destination IP Address = [2001:db8::2:1]
       Destination UDP Port = 5683
       Uri-Host = "xn--18j4d.example"
       Uri-Path = the string composed of the Unicode characters U+3053
       U+3093 U+306b U+3061 U+306f, usually represented in UTF-8 as
       E38193E38293E381ABE381A1E381AF hexadecimal
    Output:
       coap://xn--18j4d.example/
       %E3%81%93%E3%82%93%E3%81%AB%E3%81%A1%E3%81%AF
       (The line break has been inserted for readability; it is not
       part of the URI.)

Shelby, et al. Standards Track [Page 111] RFC 7252 The Constrained Application Protocol (CoAP) June 2014

 o  Input:
       Destination IP Address = 198.51.100.1
       Destination UDP Port = 61616
       Uri-Path = ""
       Uri-Path = "/"
       Uri-Path = ""
       Uri-Path = ""
       Uri-Query = "//"
       Uri-Query = "?&"
    Output:
       coap://198.51.100.1:61616//%2F//?%2F%2F&?%26

Authors' Addresses

 Zach Shelby
 ARM
 150 Rose Orchard
 San Jose, CA  95134
 USA
 Phone: +1-408-203-9434
 EMail: zach.shelby@arm.com
 Klaus Hartke
 Universitaet Bremen TZI
 Postfach 330440
 Bremen  D-28359
 Germany
 Phone: +49-421-218-63905
 EMail: hartke@tzi.org
 Carsten Bormann
 Universitaet Bremen TZI
 Postfach 330440
 Bremen  D-28359
 Germany
 Phone: +49-421-218-63921
 EMail: cabo@tzi.org

Shelby, et al. Standards Track [Page 112]

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