MQTT (Message Queuing Telemetry Transport) is an application layer protocol typically run over TCP, commonly used as a backend for cloud platforms.

MQTT is a publish-subscribe network protocol that consists of a message broker and one or more clients. Information is organized via topics; clients can publish and subscribe to specific topics through the MQTT broker. When publishing to a topic, all clients subscribed to that topic will receive the message from the MQTT broker.

MQTT broker

An MQTT broker is essentially a server that receives messages from connected clients and routes them to the applicable destination clients. Multiple clients can be subscribed to a single topic, and multiple clients can publish to the same topic as well.

MQTT allows for the decoupling of the client-side and server-side, so connected clients are unaware of each other’s information.

The MQTT protocol has a keep-alive function that ensures the connection between the broker and the client stays alive. When connecting to a broker, the client specifies a keep-alive interval, the maximum time the broker and client can go without communicating before the connection is closed. The broker can also set a keep-alive time; if the broker’s keep-alive time is shorter, this can close the connection on the broker-side.

Quality of service

When connecting to the broker, the client can specify a quality of service (QoS) level.

• At most once (0) – The message is sent only once, with no acknowledgment receipt.
• At least once (1) – The message is re-tried by the sender multiple times until acknowledgment is received.
• Exactly once (2) – The sender and receiver engage in a two-level handshake to ensure the message is received only once.

Security

MQTT supports numerous authentication and data security mechanisms. In this course, we will focus on connection to the MQTT broker via TLS. Adding TLS will encrypt the communication between the MQTT client and broker. We will take a look at how to implement TLS in an MQTT connection in Exercise 2.