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

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. Implementations for constrained devices are typically written in C. CoAP is also used between them and more powerful systems such as cloud servers, home centrals, smartphones:

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 C 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
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 Swift 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

References

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