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

Generic implementations are becoming availible for a variety of platforms.

Constrained Devices

Name	Programming Language	Client/Server	License	Link
Erbium for Contiki	C	Client + Server	3-clause BSD	http://www.contiki-os.org/
libcoap	C	Client + Server	BSD/GPL	https://libcoap.net/
Eclipse tinydtls	Client + Server	EPL+EDL	https://projects.eclipse.org/projects/iot.tinydtls
LibNyoci	C	Client + Server	MIT	https://github.com/darconeous/libnyoci
microcoap	C	Client + Server	MIT	https://github.com/1248/microcoap
cantcoap	C++/C	Client + Server	BSD	https://github.com/staropram/cantcoap
Lobaro CoAP	C	Client + Server	MIT	https://www.lobaro.com/portfolio/lobaro-coap/
MR-CoAP	Java		BSD	https://github.com/MR-CoAP/CoAP
Wakaama	C	Client + Server	EPL+EDL	https://www.eclipse.org/wakaama/index.html
coap-node	Javascript	Client	MIT	https://github.com/PeterEB/coap-node
coap-shepherd	Javascript	Server	MIT	https://github.com/PeterEB/coap-shepherd
Californium	Java	Client + Server	EPL+EDL	https://www.eclipse.org/californium
nCoap	Java	Client + Server	BSD	https://github.com/okleine/nCoAP
leshan	Java	Client + Server	EPL+EDL	https://github.com/eclipse/leshan
CoAP.NET	C#	Client + Server	3-clause BSD	https://github.com/smeshlink/CoAP.NET
CoAPSharp	C#, .NET	Client + Server	LGPL	http://www.coapsharp.com
Waher.Networking.CoAP	C#	Server		https://github.com/PeterWaher/IoTGateway/tree/master/Networking/Waher.Networking.CoAP
gen_coap	Erlang	Client + Server	MPL v1.1	https://github.com/gotthardp/gen_coap
go-coap	Go	Client + Server	MIT	https://github.com/go-ocf/go-coap
node-coap	Javascript	Client + Server	MIT	https://github.com/mcollina/node-coap
coap-cli	Javascript		MIT	https://github.com/mcollina/coap-cli
txThings	Python (Twisted)	Client + Server	MIT	https://github.com/mwasilak/txThings
aiocoap	Python 3	Client + Server	MIT	https://github.com/chrysn/aiocoap
CoAPthon	Python	Client + Server	MIT	https://github.com/Tanganelli/CoAPthon
Ruby coap	Ruby	Client	MIT	https://github.com/nning/coap
Ruby david	Ruby	Server	MIT	https://github.com/nning/david
coap-rs	Rust	Client + Server	MIT	https://github.com/Covertness/coap-rs
Copper	JavaScript (Browser Plugin)	Client	BSD	https://github.com/mkovatsc/Copper
iCoAP	Objective-C	Client	MIT	https://github.com/stuffrabbit/iCoAP
SwiftCoAP	Client + Server	MIT	https://github.com/stuffrabbit/SwiftCoAP
nCoap	Java	Client + Server	BSD	https://github.com/okleine/nCoAP
txThings	Python (Twisted)	Client + Server	MIT	https://github.com/mwasilak/txThings

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/