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. 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/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 | 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
- 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/