Constrained Application Protocol

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         |
             +----------------------+

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

Name	Programming Language	Implemented CoAP version	Client/Server	Implemented CoAP features	License	Link
aiocoap	Python 3	RFC 7252	Client + Server	Blockwise Transfers, Observe (partial)	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/
libcoap	C	RFC 7252	Client + Server	Observe, Blockwise Transfers, DTLS	BSD/GPL	https://github.com/obgm/libcoap
LibNyoci	C	RFC 7252	Client + Server	Core, Observe, Block, DTLS	MIT	https://github.com/darconeous/libnyoci
lobaro-coap	C	RFC 7252	Client + Server	Observe, Blockwise Transfers	MIT	http://www.lobaro.com/lobaro-coap
microcoap	C	RFC 7252	Client + Server		MIT	https://github.com/1248/microcoap
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

• Vertiefendes Wahlfach-Projekt (2018)

References

• RFC 7252: https://tools.ietf.org/html/rfc7252
• CoAP: RFC 7252 Constrained Application Protocol https://coap.technology/
• Learning Internet of Things: https://www.oreilly.com/library/view/learning-internet-of/9781783553532/