Understanding CoAP Protocol Architecture in IoT

Introduction

The Constrained Application Protocol (CoAP) is a specialized web transfer protocol designed for use in IoT (Internet of Things) and constrained environments. It operates on a lightweight architecture to enable efficient communication between devices with limited resources. This article delves into the CoAP protocol, its architecture, layers, and working model.

The diagrams provided illustrate the message format and messages exchanged between a CoAP client and server. The CoAP protocol, as specified in RFC 7252, is a web transfer protocol utilized in constrained nodes or networks such as WSN, IoT, and M2M. Hence the name “Constrained Application Protocol.”

The protocol targets Internet of Things (IoT) devices with limited memory and power specifications. As it’s designed for web applications, it’s also known as “The Web of Things Protocol.” It can transport data from a few bytes to thousands of bytes over web applications, residing between the UDP and Application layers.

Here are some key features of the CoAP Protocol:

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

CoAP Architecture

The figure above (Figure 1) depicts the CoAP Architecture. It extends normal HTTP clients to clients with resource constraints, known as CoAP clients. A proxy device bridges the gap between the constrained environment and the typical internet environment based on HTTP protocols. The same server handles both HTTP and CoAP protocol messages.

CoAP Message Format | CoAP Header

Figure 2 illustrates the CoAP message format, which consists of a 4-byte header followed by a token value (from 0 to 8 bytes).

The table below describes the header, which consists of 4 bytes (32 bits).

CoAP Protocol Message Exchanges

There are two modes in which CoAP protocol messages get exchanged between a CoAP client and CoAP server: without a separate response and with a separate response. With a separate response, the server notifies the client about the receipt of the request message. This increases processing time but helps avoid unnecessary retransmissions.

CoAP in IoT is inherently unreliable due to the use of UDP. Consequently, CoAP messages may arrive out of order or be lost entirely upon reaching their destination. To enhance CoAP’s reliability, a stop-and-wait mechanism with exponential backoff retransmission is implemented, along with duplicate detection.

Conclusion

CoAP stands as a cornerstone in the IoT communication landscape due to its simplicity and efficiency. By understanding its architecture and operation, developers can design robust and scalable IoT systems. Its ability to work with constrained devices makes it ideal for IoT applications ranging from smart homes to industrial automation.