MQTT ( Message Queue Telemetry Transport ) is a messaging protocol based on the publish/subscribe paradigm under the ISO standard (ISO/IEC PRF 20922). It operates on the TCP/IP family of protocols and is a publish/subscribe messaging protocol designed for remote devices with low hardware performance and poor network conditions, for which it requires a message middleware.
MQTT is a client-server based publish/subscribe transport protocol. the MQTT protocol is lightweight, simple, open, and easy to implement, characteristics that make it suitable for a wide range of applications. In many cases, including in constrained environments such as: machine-to-machine (M2M) communication and the Internet of Things (IoT). Its use has been widespread in, communicating sensors via satellite links, medical devices with occasional dialing, smart homes, and some miniaturized devices.
The MQTT protocol is a protocol designed to communicate with a large number of remote sensors and control devices that have limited computational power and work in low-bandwidth, unreliable networks, and has the following key features:
Implementing the MQTT protocol requires that the client-side and server-side communication is complete, and there are three identities in the MQTT protocol in the process of communicating: Publisher, Broker (server), Subscriber. In this case, the publisher and subscriber of a message are clients, the message broker is a server, and the publisher of a message can be a subscriber at the same time.
The messages transported by MQTT are divided into topic and payload parts:
An MQTT server, called a broker, can be an application or a device. It sits between the message publisher and the subscriber and can:
In the MQTT protocol, an MQTT packet consists of: a Fixed header, a Variable header, and a payload. The structure of an MQTT packet is as follows.
(1) Fixed header. Existing in all MQTT packets, indicating the packet type and packet class identifier.
(2) Variable header. Present in some MQTT packets, the packet type determines whether a variable header exists and what it contains.
(3) Payload. Present in some MQTT packets, it indicates the specific content received by the client.
The fixed header is present in all MQTT packets and is structured as follows:
As opposed to a 4-bit unsigned value, the type, value, and description are as follows
In message types that do not use the sign bit, the sign bit is used as a reserved bit. If an invalid flag is received, the receiver must close the network connection:
The Payload message body is the third part of the MQTT packet, and contains four types of messages: CONNECT, SUBSCRIBE, SUBACK, and UNSUBSCRIBE: