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CoAP in IoT (Basic of IoT Networking) part III

Introduction
  • CoAP - Constrained Application Protocol.
  • Web transfer protocol for use with constrained nodes and networks.
  • Designed for Machine to Machine (M2M) applications such as smart energy and building automation.
  • Based on Request-Response model between end-points
  • Client-Server interaction is asynchronous over a datagram oriented transport protocol such as UDP
  • The Constrained Application Protocol (CoAP) is a session layer protocol designed by IETF COnstrained RESTful Environment (CoRE) working to provide lightweight RESTful (HTTP) interface.
  • Representational State Transfer (REST) is the standard interface between HTTP client and servers.
  • Lightweight applications such as those in IoT, could result signification overhead and power consumption by REST.
  • CoAP is designed to enable low-power sensors to use RESTful services while meeting their power constraints.
  • Built over UDP, instead of TCP (which is commonly used with HTTP) and has a light mechanism to provide reliability.
  • CoAP architecture is devided into two main sub-layers:
                  Messsaging
             Request/response.
  • The messaging sub-layer is responsible for reliability and duplication of messages, while the request/response sub-layer is responsible for communication.
  • CoAP has four messaging modes:
                   Confirmable
                   Non-confirmable
                   Piggyback
                   Separate

CoAP Position


CoAp Message Types


 CoAP Request-Response Model

  •  Confirmable and non-confirmable modes represent the reliable and unreliable transmissions, respectively, while the other modes are used for request/response.
  • Piggyback is used for client/server direct communication where the server sends its response directly after receiving the message, i.e., within the acknowledgment message.
  • On the other hand, the separate mode is used when the server response comes in a message seperate from the acknowledgment, and may take some time to be sent by the server.
  • Similar to HTTP, CoAP utilizes GET, PUT,PUSH,DELETE messages requests to retrieve,create, update, and delete respectively.


Features
  • Reduced overheads and parsing complexity.
  • URL 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 maximum message age.

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