CoAP

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 CoAP

The CoAP protocol is a web transfer protocol which is used in constrained nodes or networks such as WSN, IoT, M2M etc. Hence the name Constrained Application Protocol. The protocol is targeted for Internet of Things (IoT) devices having less memory and less power specifications.

As it is designed for web applications it is also known as "The Web of Things Protocol". It can be used to transport data from few bytes to 1000s of bytes over web applications. It exists between UDP layer and Application layer.

Following are the features of CoAP Protocol:
• It is very efficient RESTful protocol.
• Easy to proxy to/from HTTP.
• It is open IETF standard
• It is Embedded web transfer protocol (coap://)
• It uses asynchronous transaction model.
• UDP is binding with reliability and multicast support.
• GET, POST, PUT and DELETE methods are used.
• URI is supported.
• It uses small and simple 4 byte header.
• Supports binding to UDP, SMS and TCP.
• DTLS based PSK, RPK and certificate security is used.
• uses subset of MIME types and HTTP response codes.
• Uses built in discovery mechanism.

 

 

 

 

 

CoAP Architecture

 

 

The figure-1 depicts CoAP Architecture. As shown it extends normal HTTP clients to clients having resource constraints. These clients are known as CoAP clients. Proxy device bridges gap between constained environment and typical internet environment based on HTTP protocols. Same server takes care of both HTTP and CoAP protocol messages.

CoAP Message Format | CoAP Header

The figure-2 depicts CoAP message format consists of 4 bytes header followed by token value (from 0 to 8 bytes). The table below mentions header which consists of 4 bytes i.e. 32 bits.

 

CoAP message header	Description
Ver	It is 2 bit unsigned integer. It mentions CoAP version number. Set to one.
T	It is 2 bit unsigned integer. Indicates message type viz. confirmable (0), non-confirmable (1), ACK (2) or RESET(3).
TKL	It is 4 bit unsigned integer, Indicates length of token (0 to 8 bytes).
Code	It is 8 bit unsigned integer, It is split into two parts viz. 3 bit class (MSBs) and 5 bit detail (LSBs).
Message ID	16 bit unsigned integer. Used for matching responses. Used to detect message duplication.

 

 

CoAP Protocol Message Exchanges

There are two modes in which CoAP protocol messages get exchanged between CoAP client and CoAP server viz. without separate response and with separate response.

With separate response, server notifies client about receipt of the request message. This will increase processing time but help in avoiding unnecessary retransmissions.

CoAP IoT is unreliable protocol due to use of UDP. Hence CoAP messages reach unordered or will get lost when they arrive at destination.

To make CoAP as reliable protocol, stop and wait with exponential backoff retransmission feature is incorporated in it. Duplicate detection is also introduced.

 

Difference between COAP and MQTT protocols

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1. Constrained Application Protocol (COAP): The constrained application protocol is a client server-based protocol. With this protocol, the COAP packet can be shared between different client nodes which are commanded by the COAP server. The server is responsible to share the information depending on its logic but has not acknowledged it. This is used with the applications which support the state transfer model. 

2. Message Queuing Telemetry Transport (MQTT): The message query telemetry transport protocol is a communication-based protocol that is used for IoT devices. This protocol is based on the publish-subscribe methodology in which clients receive the information through a broker only to the subscribed topic. A broker is a mediator who categorizes messages into labels before being delivered. 

Difference between COAP and MQTT protocols: 
 

Basis of	COAP	MQTT
Abbreviation	Constrained Application Protocol	Message Queuing Telemetry Transport
Communication Type	It uses Request-Response model.	It uses Publish-Subscribe model
Messaging Mode	This uses both Asynchronous and Synchronous.	This uses only Asynchronous
Transport layer protocol	This mainly uses User Datagram protocol(UDP)	This mainly uses Transmission Control protocol(TCP)
Header size	It has 4 bytes sized header	It has 2 bytes sized header
RESTful based	Yes it uses REST principles	No it does not uses REST principles
Persistence support	It does not has such support	It supports and best used for live data communication
Message Labelling	It provides by adding labels to the messages.	It has no such feature.
Usability/Security	It is used in Utility area networks and has secured mechanism.	It is used in IoT applications and is secure
Effectiveness	Effectiveness in LNN is excellent.	Effectiveness in LNN is low.
Communication Model	Communication model is one-one.	Communication model is many-many.

 

Class Notes: