CoAP

CoAP, short for Constrained Application Protocol, is a specialized web transfer protocol designed for constrained nodes and networks in the Internet of Things (IoT). Developed by the IETF Constrained RESTful Environments (CoRE) working group, it provides a lightweight, RESTful communication model similar to HTTP but optimized for devices with limited power, memory, and bandwidth.

CoAP runs over UDP and uses a compact binary header. It supports common RESTful methods such as GET, POST, PUT, and DELETE. Messages can be either confirmable or non-confirmable, with a server sending acknowledgments for confirmable requests. The protocol uses simple options and supports content negotiation, resource discovery through link formats, and payloads with a concise content type representation. Block-wise transfer allows large payloads to be exchanged in smaller blocks, reducing memory usage on constrained devices.

Key features include native support for multicast, enabling efficient group communication for discovery and control in local networks. The observe extension allows clients to subscribe to changes in resource state, providing asynchronous updates. CoAP can operate with proxies and gateways to bridge to HTTP and other protocols, facilitating interoperability in mixed networks. Caching and proxying mechanisms help optimize network efficiency.

Security is provided through optional DTLS for transport-level protection over UDP, offering confidentiality, integrity, and authenticity. OSCORE provides end-to-end security at the application layer, independent of the underlying transport. The default CoAP port is 5683 and 5684 for secure DTLS communications.

CoAP is widely used in IoT applications such as home automation, smart buildings, and sensor networks, where lightweight, reliable, and scalable communication is required.