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.