An Overview of the MQTT Protocol: History, Requirements, and Message Format

Introduction The MQTT protocol is a lightweight, publish‑subscribe messaging system designed for low‑bandwidth, high‑latency networks, commonly used in IoT scenarios such as home automation, HVAC, lighting, and security.

History Invented in 1999 by Andy Stanford‑Clark (IBM) and Arlen Nipper (Arcom) to minimize battery consumption and bandwidth for satellite‑linked oil‑pipeline monitoring, MQTT was defined with five core requirements: simple implementation, QoS‑aware data transfer, lightweight and bandwidth‑efficient design, data agnosticism, and persistent session awareness.

Protocol Format An MQTT control packet consists of a fixed header, an optional variable header, and an optional payload. The fixed header is always two bytes, where the first byte encodes the packet type (bits 7‑4) and specific flags (bits 3‑0). The second byte (and up to three additional bytes) encodes the remaining length using a variable‑length scheme.

Fixed Header Details The packet type values include CONNECT, CONNACK, PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, SUBACK, UNSUBSCRIBE, UNSUBACK, PINGREQ, PINGRESP, and DISCONNECT. Flags are packet‑type‑specific and must be set correctly; otherwise the receiver must close the connection.

Variable Header Present only in certain packet types, the variable header may contain fields such as Packet Identifier (two bytes) used by PUBLISH (QoS > 0), PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, SUBACK, UNSUBSCRIBE, and UNSUBACK. The article lists the specific packets that include this header.

Payload The payload carries the application message. In MQTT 3.1.1 it appears in packets like CONNECT, CONNACK, PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, SUBACK, UNSUBSCRIBE, UNSUBACK, PINGREQ, PINGRESP, and DISCONNECT. Detailed formats for each will be covered in future articles.

Considerations MQTT’s binary, bit‑level precision makes it well‑suited for IoT, but its lack of packet fragmentation limits suitability for large data transfers such as video streaming.

Conclusion MQTT’s efficient, bit‑accurate design secures its place in the IoT ecosystem, though it is not ideal for scenarios requiring large payloads or packet segmentation.