Constrained Application Protocol

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The Constrained Application Protocol (CoAP) is a specialized web transfer protocol, as defined in RFC 7252, for use with constrained nodes and constrained networks in the Internet of Things. The protocol is designed for machine-to-machine (M2M) applications such as smart energy and building automation.

Features

The work on Constrained Environments aims at realizing the REST architecture in a suitable form for the most constrained nodes and networks. The nodes usually consist of 8-bit microcontrollers with limited amounts of RAM and ROM.


CoAP has the following main features:

  • Web protocol fulfilling M2M requirements in constrained environments
  • UDP [RFC0768] binding with optional reliability supporting unicast and multicast requests
  • Asynchronous message exchanges
  • Low header overhead and parsing complexity
  • URI and Content-type support
  • Simple proxy and caching capabilities
  • Stateless HTTP mapping
  • Security binding to Datagram Transport Layer Security (DTLS)

The Protocol

CoAP is similar to the client/server model of HTTP. M2M interactions typically result in a CoAP implementation acting in both client and server roles. A request is sent by a client to request an action on a resource (identified by a URI) on a server. The server then sends a response with a response code (equivalent to that of HTTP). Therefore, efficiency is very important, so CoAP uses UDP, a datagram-oriented transport.

CoAP is however a single protocol, with messaging and request/response as just features of the CoAP header 

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

Abstract Layering of CoAP.JPG

Message Format

By default, the messages are encoded in a simple binary format and transported over UDP. The message format starts with a fixed-size 4-byte header, followed by a variable-length Token value, which can be between 0 and 8 bytes long. 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) ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version (Ver) (2 bits)
Indicates the CoAP Version number. This must set this field to 1 (binary 01). Other values are reserved for future versions.
Type (T) (2 bits)
Indicates if this message is of type Confirmable (0), Non-confirmable (1), Acknowledgement (2), or Reset (3).
Token Length (TKL) (4 bits)
Indicates the length of the variable-length Token field, which may be 0-8 bytes in length.
CoAP Request/Response Code (8 bits)
Splits into a 3-bit class and 5-bit detail.
Message ID (16 bits)
Used to detect message duplication and to match messages of type Acknowledgement/Reset to messages of type Confirmable/Non-confirmable.

Implementations

Generic implementations are becoming availible for a variety of platforms.

Constrained Devices

Name Programming Language Implemented CoAP version Client/Server License Link
Erbium for Contiki C RFC 7252 Client + Server 3-clause BSD http://www.contiki-os.org/
libcoap C RFC 7252 Client + Server BSD/GPL https://libcoap.net/
Eclipse tinydtls C Client + Server EPL+EDL https://projects.eclipse.org/projects/iot.tinydtls
LibNyoci C RFC 7252 Client + Server MIT https://github.com/darconeous/libnyoci
microcoap C RFC 7252 Client + Server MIT https://github.com/1248/microcoap
cantcoap C++/C RFC 7252 Client + Server BSD https://github.com/staropram/cantcoap
Lobaro CoAP C RFC 7252 Client + Server MIT https://www.lobaro.com/portfolio/lobaro-coap/
MR-CoAP Java RFC 7252 BSD https://github.com/MR-CoAP/CoAP


aiocoap Python 3 RFC 7252 Client + Server MIT https://pypi.python.org/pypi/aiocoap
Californium Java RFC 7252 Client + Server Observe, Blockwise Transfers, DTLS EPL+EDL https://www.eclipse.org/californium
cantcoap C++/C RFC 7252 Client + Server BSD https://github.com/staropram/cantcoap
Canopus Go RFC 7252 Client + Server Core Apache License 2.0 https://github.com/zubairhamed/canopus
CoAP implementation for Go Go RFC 7252 Client + Server Core + Draft Subscribe MIT https://github.com/dustin/go-coap
CoAP.NET C# RFC 7252, coap-13, coap-08, coap-03 Client + Server Core, Observe, Blockwise Transfers 3-clause BSD https://github.com/smeshlink/CoAP.NET
CoAPSharp C#, .NET RFC 7252 Client + Server Core, Observe, Block, RD LGPL http://www.coapsharp.com
CoAPthon Python RFC 7252 Client + Server + Forward Proxy + Reverse Proxy Observe, Multicast server discovery, CoRE Link Format parsing, Block-wise MIT https://github.com/Tanganelli/CoAPthon
CoAP Shell Java RFC 7252 Client Observe, Blockwise Transfers, DTLS Apache License 2.0 https://github.com/tzolov/coap-shell
Copper JavaScript (Browser Plugin) RFC 7252 Client Observe, Blockwise Transfers 3-clause BSD https://github.com/mkovatsc/Copper https://addons.mozilla.org/firefox/addon/copper-270430/
eCoAP C RFC 7252 Client + Server Core MIT https://gitlab.com/jobol/ecoap
Erbium for Contiki C RFC 7252 Client + Server Observe, Blockwise Transfers 3-clause BSD http://www.contiki-os.org/ (er-rest-example)
iCoAP Objective-C RFC 7252 Client Core, Observe, Blockwise Transfers MIT https://github.com/stuffrabbit/iCoAP
jCoAP Java RFC 7252 Client + Server Observe, Blockwise Transfers Apache License 2.0 https://code.google.com/p/jcoap/
lobaro-coap C RFC 7252 Client + Server Observe, Blockwise Transfers MIT http://www.lobaro.com/lobaro-coap
nCoap Java RFC 7252 Client + Server Observe, Blockwise Transfers, CoRE Link Format, Endpoint-ID-Draft BSD https://github.com/okleine/nCoAP
node-coap Javascript RFC 7252 Client + Server Core, Observe, Block MIT https://github.com/mcollina/node-coap
Ruby coap Ruby RFC 7252 Client + Server (david) Core, Observe, Block, RD MIT, GPL https://github.com/nning/coap
https://github.com/nning/david
Sensinode C Device Library C RFC 7252 Client + Server Core, Observe, Block, RD Commercial https://silver.arm.com/browse/SEN00
Sensinode Java Device Library Java SE RFC 7252 Client + Server Core, Observe, Block, RD Commercial https://silver.arm.com/browse/SEN00
Sensinode NanoService Platform Java SE RFC 7252 Cloud Server Core, Observe, Block, RD Commercial https://silver.arm.com/browse/SEN00
SwiftCoAP Swift RFC 7252 Client + Server Core, Observe, Blockwise Transfers MIT https://github.com/stuffrabbit/SwiftCoAP
TinyOS CoapBlip nesC/C coap-13 Client + Server Observe, Blockwise Transfers BSD http://docs.tinyos.net/tinywiki/index.php/CoAP
txThings Python (Twisted) RFC 7252 Client + Server Blockwise Transfers, Observe (partial) MIT https://github.com/mwasilak/txThings/
FreeCoAP C RFC 7252 Client + Server + HTTP/CoAP Proxy Core, DTLS, Blockwise Transfers BSD https://github.com/keith-cullen/FreeCoAP
coap-rs Rust RFC 7252 Client + Server MIT https://github.com/Covertness/coap-rs
YaCoAP C MIT https://github.com/RIOT-Makers/YaCoAP

Courses

References

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