CoAP RFC 7252¶

RFC 7252 - The Constrained Application Protocol (CoAP)
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Updated by: 7959 PROPOSED STANDARD
Errata Exist
Internet Engineering Task Force (IETF)                         Z. Shelby
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.

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RFC 7252       The Constrained Application Protocol (CoAP)     June 2014

Copyright (c) 2014 IETF Trust and the persons identified as the

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.

Annotations from the IoT-Testware Team

Conformance Statements

The concept of referenceable conformance statements has proven
to be handy as it facilitates the implementation of Test Purposes.

Table of Conformance Statements

3.  Message Format  . . . . . . . . . . . . . . . . . . . . . . .  15
[CoAP-3.0-1]  . . . . . . . . . . . . . . . . . . . . . . .  15
[CoAP-3.0-2]  . . . . . . . . . . . . . . . . . . . . . . .  15
[CoAP-3.0-3]  . . . . . . . . . . . . . . . . . . . . . . .  15
[CoAP-3.0-4]  . . . . . . . . . . . . . . . . . . . . . . .  16

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

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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

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

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RFC 7252       The Constrained Application Protocol (CoAP)     June 2014

1.  Introduction

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

The work on Constrained RESTful Environments (CoRE) aims at realizing
the REST architecture in a suitable form for the most constrained
nodes (e.g., 8-bit microcontrollers with limited RAM and ROM) and
networks (e.g., 6LoWPAN, [RFC4944]).  Constrained networks such as
6LoWPAN support the fragmentation of IPv6 packets into small link-
layer frames; however, this causes significant reduction in packet
delivery probability.  One design goal of CoAP has been to keep
message overhead small, thus limiting the need for fragmentation.

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

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

1.1.  Features

CoAP has the following main features:

o  Web protocol fulfilling M2M requirements in constrained
environments

o  UDP [RFC0768] binding with optional reliability supporting unicast
and multicast requests.

o  Asynchronous message exchanges.

o  URI and Content-type support.

o  Simple proxy and caching capabilities.

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

o  Security binding to Datagram Transport Layer Security (DTLS)
[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:

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

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

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

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

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

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

Intermediary
A CoAP endpoint that acts both as a server and as a client towards
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.

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

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

Acknowledgement Message
An Acknowledgement message acknowledges that a specific
Confirmable message arrived.  By itself, an Acknowledgement
message does not indicate success or failure of any request
encapsulated in the Confirmable message, but the Acknowledgement
message may also carry a Piggybacked Response (see below).

Reset Message
A Reset message indicates that a specific message (Confirmable or
Non-confirmable) was received, but some context is missing to
properly process it.  This condition is usually caused when the
receiving node has rebooted and has forgotten some state that
would be required to interpret the message.  Provoking a Reset
message (e.g., by sending an Empty Confirmable message) is also
useful as an inexpensive check of the liveness of an endpoint
("CoAP ping").

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

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

Empty Message
A message with a Code of 0.00; neither a request nor a response.
An Empty message only contains the 4-byte header.

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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.

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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

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

Figure 1: Abstract Layering of CoAP

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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).

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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

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Client              Server       Client              Server
|                  |             |                  |
|   CON [0xbc90]   |             |   CON [0xbc91]   |
| GET /temperature |             | GET /temperature |
|   (Token 0x71)   |             |   (Token 0x72)   |
+----------------->|             +----------------->|
|                  |             |                  |
|   ACK [0xbc90]   |             |   ACK [0xbc91]   |
|   (Token 0x71)   |             |   (Token 0x72)   |
|<-----------------+             |<-----------------+
|                  |             |                  |

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

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

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

Figure 6: A Request and a Response Carried in Non-confirmable
Messages

CoAP makes use of GET, PUT, POST, and DELETE methods in a similar
manner to HTTP, with the semantics specified in Section 5.8.  (Note
that the detailed semantics of CoAP methods are "almost, but not
entirely unlike" [HHGTTG] those of HTTP methods: intuition taken from
HTTP experience generally does apply well, but there are enough
differences that make it worthwhile to actually read the present
specification.)

Methods beyond the basic four can be added to CoAP in separate
specifications.  New methods do not necessarily have to use requests
and responses in pairs.  Even for existing methods, a single request
may yield multiple responses, e.g., for a multicast request
(Section 8) or with the Observe option [OBSERVE].

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

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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.

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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. [CoAP-3.0-1] Implementations of this specification MUST
set this field to 1 (01 binary).  Other values are reserved for
future versions.
[CoAP-3.0-2] 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). [CoAP-3.0-3]
Lengths 9-15 are reserved, MUST NOT be sent, and MUST be
processed as a message format error.

Code:  8-bit unsigned integer, split into a 3-bit class (most
significant bits) and a 5-bit detail (least significant bits),
documented as "c.dd" where "c" is a digit from 0 to 7 for the
3-bit subfield and "dd" are two digits from 00 to 31 for the 5-bit
subfield.  The class can indicate a request (0), a success
response (2), a client error response (4), or a server error
response (5).  (All other class values are reserved.)  As a
special case, Code 0.00 indicates an Empty message.  In case of a
request, the Code field indicates the Request Method; in case of a
response, a Response Code.  Possible values are maintained in the
CoAP Code Registries (Section 12.1).  The semantics of requests
and responses are defined in Section 5.

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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

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
[CoAP-3.0-4] 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.

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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.

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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.

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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.

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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

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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.

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4.3.  Messages Transmitted without Reliability

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

As a more lightweight alternative, a message can be transmitted less
reliably by marking the message as Non-confirmable.  A Non-
confirmable message always carries either a request or response and
MUST NOT be Empty.  A Non-confirmable message MUST NOT be
acknowledged by the recipient.  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

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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
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:

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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
this problem set might also set the IPv4 DF bit and perform some

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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

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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
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

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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

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

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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

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

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

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5.  Request/Response Semantics

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

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
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

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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

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.

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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
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.

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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

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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.

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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)

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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).

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

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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,
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).

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
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.

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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

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

The set of request options that is used for matching the cache entry
is also collectively referred to as the "Cache-Key".  For URI schemes
other than coap and coaps, matching of those options that constitute
the request URI may be performed under rules specific to the URI
scheme.

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5.6.1.  Freshness Model

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

The mechanism for determining freshness is for an origin server to
provide an explicit expiration time in the future, using the Max-Age
Option (see Section 5.10.5).  The Max-Age Option indicates that the
response is to be considered not fresh after its age is greater than
the specified number of seconds.

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

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

5.6.2.  Validation Model

When an endpoint has one or more stored responses for a GET request,
but cannot use any of them (e.g., because they are not fresh), it can
use the ETag Option (Section 5.10.6) in the GET request to give the
origin server an opportunity both to select a stored response to be
used, and to update its freshness.  This process is known as
"validating" or "revalidating" the stored response.

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

A 2.03 (Valid) response indicates the stored response identified by
the entity-tag given in the response's ETag Option can be reused
after updating it as described in Section 5.9.1.3.

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

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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
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,

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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

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,

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Unsafe options in a response that are not recognized by the CoAP-to-
CoAP proxy server MUST lead to a 5.02 (Bad Gateway) response.  Again,
Safe-to-Forward options that are not recognized MUST be forwarded.

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

5.7.2.  Forward-Proxies

CoAP distinguishes between requests made (as if) to an origin server
and requests made through a forward-proxy.  CoAP requests to a
forward-proxy are made as normal Confirmable or Non-confirmable
requests to the forward-proxy endpoint, but they specify the request
URI in a different way: The request URI in a proxy request is
specified as a string in the Proxy-Uri Option (see Section 5.10.2),
while the request URI in a request to an origin server is split into
the Uri-Host, Uri-Port, Uri-Path, and Uri-Query Options (see
Section 5.10.1).  Alternatively, the URI in a proxy request can be
assembled from a Proxy-Scheme option and the split options mentioned.

When a proxy request is made to an endpoint and the endpoint is
unwilling or unable to act as proxy for the request URI, it MUST
return a 5.05 (Proxying Not Supported) response.  If the authority
(host and port) is recognized as identifying the proxy endpoint
itself (see Section 5.10.2), then the request MUST be treated as a
local (non-proxied) request.

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

5.7.3.  Reverse-Proxies

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

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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.

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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

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.

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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
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.

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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.

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.

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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5.9.2.6.  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".

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

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",
[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
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"

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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

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
default port number 5684 for DTLS-secured CoAP MAY be supported by a
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-
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
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

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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.

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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

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].

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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.

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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

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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
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.

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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]).

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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

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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.

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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.

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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
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.

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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.

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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
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.

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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

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

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.

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

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

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

10.2.4.  POST

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

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

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

10.2.5.  PUT

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

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

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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].)

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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

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.

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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
implementations will know whether they have joined a multicast group,
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.

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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

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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

o  The attacker sends a message to a CoAP endpoint with the given

o  The CoAP endpoint replies with a message to the given source

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.

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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

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.

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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
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.

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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.

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12.1.2.  Response Codes

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

Each entry in the sub-registry must include the Response Code in the
range 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] |
| 4.05 | 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].

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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

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.

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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.

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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.

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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.

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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)

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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

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RFC 7252       The Constrained Application Protocol (CoAP)     June 2014

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
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
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.

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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.

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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.

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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/>.

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

[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.

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[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.

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[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>.

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Appendix A.  Examples

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

Figure 16 shows a basic GET request causing a 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

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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
|      |

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

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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
|      |

Figure 18: Confirmable Request (Retransmitted); Piggybacked Response

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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
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
|      |

Figure 19: Confirmable Request; Piggybacked Response (Retransmitted)

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

Client  Server
|      |
|      |
+----->|     Header: GET (T=CON, Code=0.01, MID=0x7d38)
| GET  |      Token: 0x53
|      |   Uri-Path: "temperature"
|      |
|      |
|<- - -+     Header: (T=ACK, Code=0.00, MID=0x7d38)
|      |
|      |
| 2.05 |      Token: 0x53
|      |
|      |
|      |

Figure 20: Confirmable Request; Separate Response

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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)
|      |
|      |
| 2.05 |      Token: 0x64
|      |
|      |
|      |

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"
|      |
|      |
| 2.05 |      Token: 0x75
|      |

Figure 22: Non-confirmable Request; Non-confirmable Response

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In Figure 23, the client sends a Non-confirmable GET request to a
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.

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 UDP Port = 5683

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RFC 7252       The Constrained Application Protocol (CoAP)     June 2014

Output:

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

o  Input:

Destination UDP Port = 5683
Uri-Host = "example.net"

Output:

coap://example.net/

o  Input:

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 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

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.)

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RFC 7252       The Constrained Application Protocol (CoAP)     June 2014

o  Input:

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

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

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