Constrained Application Protocol

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Constrained Application Protocol (CoAP) is a software protocol intended to be used in very simple electronics devices that allows them to communicate interactively over the Internet. It is particularly targeted for small low power sensors, switches, valves and similar components that need to be controlled or supervised remotely, through standard Internet networks. CoAP is an application layer protocol that is intended for use in resource-constrained internet devices, such as WSN nodes. CoAP is designed to easily translate to HTTP for simplified integration with the web, while also meeting specialized requirements such as multicast support, very low overhead, and simplicity.[1][2] Multicast, low overhead, and simplicity are extremely important for Internet of Things (IoT) and Machine-to-Machine (M2M) devices, which tend to be deeply embedded and have much less memory and power supply than traditional internet devices have. Therefore, efficiency is very important. CoAP can run on most devices that support UDP or a UDP analogue.

The Internet Engineering Task Force (IETF) Constrained RESTful environments (CoRE) Working Group has done the major standardization work for this protocol. In order to make the protocol suitable to IoT and M2M applications, various new functionalities have been added.[3] The core of the protocol is specified in RFC 7252, important extensions are in various stages of the standardization process.

Features[edit]

The CoRE group has designed CoAP with the following features in mind:

  • RESTful protocol design minimizing the complexity of mapping with HTTP.
  • Low header overhead and parsing complexity.
  • URI and content-type support.
  • Support for the discovery of resources provided by known CoAP services.
  • Simple subscription for a resource, and resulting push notifications.
  • Simple caching based on max-age.

The mapping of CoAP with HTTP is also defined, allowing proxies to be built providing access to CoAP resources via HTTP in a uniform way.[4]

Message Formats[edit]

CoAP makes use of two message types, requests and responses, using a simple binary base header format. The base header may be followed by options in an optimized Type-Length-Value format. CoAP is by default bound to UDP and optionally to DTLS, providing a high level of communications security.

Any bytes after the headers in the packet are considered the message body if any. The length of the message body is implied by the datagram length. When bound to UDP the entire message MUST fit within a single datagram. When used with 6LoWPAN as defined in RFC 4944, messages SHOULD fit into a single IEEE 802.15.4 frame to minimize fragmentation.

Implementations[edit]

Name Programming Language Implemented CoAP version Client/Server Implemented CoAP features License Link
libcoap C RFC 7252 Client + Server Observe, Blockwise Transfers BSD/GPL http://sourceforge.net/projects/libcoap/develop
iCoAP Objective-C RFC 7252 Client Observe, Blockwise Transfers MIT https://github.com/stuffrabbit/iCoAP
nCoap Java RFC 7252 Client + Server Observe BSD https://github.com/okleine/nCoAP
jCoAP Java RFC 7252 Client + Server Observe, Blockwise Transfers Apache License 2.0 https://code.google.com/p/jcoap/
CoAPthon Python RFC 7252 Client + Server + Forward Proxy + Reverse Proxy Observe, Multicast server discovery, CoRE Link Format parsing BSD https://github.com/Tanganelli/CoAPthon
txThings Python (Twisted) RFC 7252 Client + Server Blockwise Transfers, Observe (partial) MIT https://github.com/siskin/txThings/
TinyOS CoapBlip nesC/C coap-13 Client + Server Observe, Blockwise Transfers BSD http://docs.tinyos.net/tinywiki/index.php/CoAP
Erbium for Contiki C RFC 7252 Client + Server Observe, Blockwise Transfers 3-clause BSD http://www.contiki-os.org/ (er-rest-example)
Californium Java RFC 7252 Client + Server Observe, Blockwise Transfers, DTLS 3-clause BSD https://github.com/mkovatsc/Californium
Copper JavaScript (Browser Plugin) RFC 7252 Client Observe, Blockwise Transfers 3-clause BSD https://github.com/mkovatsc/Copper https://addons.mozilla.org/de/firefox/addon/copper-270430/
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
cantcoap C++/C RFC 7252 Client + Server BSD https://github.com/staropram/cantcoap
microcoap C RFC 7252 Client + Server MIT https://github.com/1248/microcoap
node-coap Javascript RFC 7252 Client + Server Core, Observe, Block MIT https://github.com/mcollina/node-coap
SMCP C RFC 7252 Client + Server Core, Observe, Block MIT https://github.com/darconeous/smcp
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
ETRI CoAP C RFC 7252 Client + Server Core, Observe, Block Commercial http://coap.or.kr/index_en.html

Proxy Implementations[edit]

References[edit]