Explain MQTT protocol and its use in IoT.

MQTT (Message Queuing Telemetry Transport) is a lightweight publish-subscribe messaging protocol designed for constrained devices and low-bandwidth networks. Architecture: Broker (server) manages message routing. Clients publish messages to topics. Clients subscribe to topics of interest. Key features: Small overhead (2-byte header minimum). QoS levels: 0 (at most once), 1 (at least once), 2 (exactly once). Retained messages for last known value. Last Will and Testament (LWT) for disconnect notification. Keep-alive and session persistence. Port 1883 (unencrypted), 8883 (TLS). Ideal for IoT because: Low bandwidth and power requirements. Works well on unreliable networks. Simple implementation on constrained devices. Widely supported by cloud platforms (AWS IoT, Azure IoT, Google Cloud IoT).