The Constrained Application Protocol (CoAP)
RFC 7252

Internet Engineering Task Force (IETF)                         Z. Shelby
Request for Comments: 7252                                           ARM
Category: Standards Track                                      K. Hartke
ISSN: 2070-1721                                               C. Bormann
                                                 Universitaet Bremen TZI
                                                               June 2014

              The Constrained Application Protocol (CoAP)

Abstract

   The Constrained Application Protocol (CoAP) is a specialized web
   transfer protocol for use with constrained nodes and constrained
   (e.g., low-power, lossy) networks.  The nodes often have 8-bit
   microcontrollers with small amounts of ROM and RAM, while constrained
   networks such as IPv6 over Low-Power Wireless Personal Area Networks
   (6LoWPANs) often have high packet error rates and a typical
   throughput of 10s of kbit/s.  The protocol is designed for machine-
   to-machine (M2M) applications such as smart energy and building
   automation.

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7252.

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

Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

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