MQTT-SN
MQTT for Sensor Networks — a lightweight variant of MQTT adapted for very small, battery-powered devices and networks without TCP (e.g. Zigbee, UDP). Short messages, numeric topic IDs instead of long topic strings, and support for sleeping nodes.
Difference from regular MQTT: MQTT requires a TCP connection, which is too heavy for the smallest sensors. MQTT-SN runs over UDP/non-IP, replaces long topic strings with 2-byte topic IDs (registered once), and has "sleep" support where a gateway buffers messages for a sleeping node. An MQTT-SN gateway translates to regular MQTT against a broker. Use case: extremely resource-constrained wireless sensor networks. Distinct from CoAP (REST-like, request/response) — MQTT-SN is publish/subscribe. Part of the IoT protocol flora for the very smallest devices. Less common than full MQTT but relevant in deeply embedded systems.