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 messages are encoded in a simple binary format

   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) ...
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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/