CoAP Constrained Application Protocol
HTTP's ideas shrunk to fit a sensor running off a coin cell. Standardized in RFC 7252. The same methods — GET, POST, PUT, DELETE — and the same resource thinking, but over UDP and with a four-byte binary header.
The target is devices with kilobytes of memory on networks where every packet sent costs battery life. Reliability is handled inside the protocol with confirmable messages rather than by pulling in TCP. The Observe extension inverts the model: a client subscribes to a resource and the server pushes updates when the value changes, so no polling. Security via DTLS or OSCORE. Because the model is REST-like, a proxy can translate straight between CoAP and HTTP. Compared with MQTT the difference is architectural: CoAP is direct client–server, MQTT always goes through a broker.