MQTT
Lightweight pub/sub protocol for IoT — minimal overhead (2-byte header), TLS support, designed for unreliable networks with limited bandwidth.
Andy Stanford-Clark + Arlen Nipper (IBM, 1999) created it for oil-pipeline telemetry. ISO standard in 2016 (v3.1.1) and 2019 (v5.0). Model: clients publish/subscribe to "topics" (slash-separated strings, home/lr/temp) via a central broker. QoS levels: 0 (fire-and-forget), 1 (at-least-once), 2 (exactly-once). Brokers: Mosquitto (open), HiveMQ, AWS IoT Core, EMQX. De facto standard for smart home (Home Assistant, OpenHAB) and industrial IoT.