CoAP, pronounced “co-app” Constrained Application Protocol,  a specification under IETF.

Speaking generally, it is the restful protocol in constrained sensor networks with IP support, aims at realizing the REST architecture in a suitable form for the most constrained nodes (e.g. 8-bit microcontrollers with limited RAM and ROM) and networks (e.g. 6LoWPAN).

Low-Power Wireless Personal Area Networks (LoWPANs) are an example of this type of network. Constrained networks can occur as part of home and building automation, energy management, and the Internet of Things.

Applications to monitor simple sensors (e.g. temperature sensors, light switches, and power meters), to control actuators (e.g. light switches, heating controllers, and door locks), and to manage devices, will become more easier.   The use of web services on the Internet has become ubiquitous in most applications, and depends on the fundamental Representational State Transfer (REST) architecture of the web.

RESTful protocol for use with constrained networks and nodes.  CoAP easily translates to HTTP for integration with the web while meeting
specialized requirements such as multicast support, very low overhead
and simplicity for constrained environments.

CoAP has the following main features:

   o  RESTful protocol design minimizing the complexity of mapping with
      HTTP.

   o  Low header overhead and parsing complexity.

   o  URI and content-type support.

   o  Support for the discovery of resources provided by known CoAP
      services.

   o  Simple subscription for a resource, and resulting push
      notifications.

   o  Simple caching based on max-age.

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

The draft document is

http://tools.ietf.org/id/draft-shelby-core-coap-00.txt

http://tools.ietf.org/html/draft-ietf-core-coap-01

http://tools.ietf.org/html/draft-ietf-core-coap-03

http://tools.ietf.org/html/draft-ietf-core-link-format-00

http://tools.ietf.org/html/draft-ietf-core-block-00

http://tools.ietf.org/html/draft-ietf-core-observe-00

http://xml.coverpages.org/draft-shelby-core-coap-req-00.txt

http://trac.tools.ietf.org/wg/core/trac/wiki/PlugFest

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