CoAP

CoAP, the Constrained Application Protocol, is a specialized web transfer protocol defined in RFC 7252 for constrained devices and low-power, lossy networks. It deliberately mirrors the request/response and resource model of HTTP — GET, POST, PUT, DELETE, URIs, media types — but runs over UDP with a compact binary header so it can operate where full HTTP over TCP would be far too heavy. It is the protocol that lets very small IoT nodes still participate in a RESTful, web-shaped architecture.

• HTTP-like REST semantics - Familiar methods, status codes, and URIs that map cleanly onto HTTP for easy proxying.
• UDP with low overhead - A four-byte base header and optional reliability via confirmable messages, sized for battery- and bandwidth-limited devices.
• Resource discovery - The /.well-known/core interface lets clients discover what a device exposes without prior knowledge.
• Observe and multicast - An observe option for push-style updates and multicast support for addressing groups of devices at once.

In real deployments, CoAP shows up at the constrained edge of API architectures — sensor networks, smart metering, and industrial telemetry — usually fronted by a gateway that translates CoAP to HTTP so the data lands in conventional API pipelines. It commonly rides alongside MQTT in IoT stacks, with CoAP favored where a request/response REST model fits and DTLS providing the security layer. For anyone governing device APIs, CoAP is a reminder that RESTful design scales all the way down, and that the same discovery and observability discipline we expect from web APIs is achievable even on the smallest hardware.