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Beginners Guide To The MQTT Protocol


What is MQTT?

MQTT is a Machine-to-Machine (M2M) data transfer protocol that allows SCADA systems to access IIoT data. It is considered one of the leading messaging protocols for the Industrial Internet of Things (IIoT).

MQTT is a messaging protocol i.e it was designed for transferring messages, and uses a publish and subscribe model.

How Does MQTT Work ?

MQTT is a publish/subscribe protocol that allows edge-of-network devices to publish to a broker. Clients connect to this broker, which then mediates communication between the two devices. … When another client publishes a message on a subscribed topic, the broker forwards the message to any client that has subscribed.



Term Explanation
Client A program or device that uses MQTT. Client:

  • opens the Network Connection to the Server
  • publishes Application Messages that other Clients
  • subscribes to request Application Messages
  • unsubscribes to remove a request for Application Messages
  • closes the Network Connection to the Server
Server (Broker) A program or device that acts as an intermediary between Clients which publish Application Messages and Clients which have made Subscriptions. Server:

  • accepts Network Connections from Clients
  • accepts Application Messages published by Clients
  • processes Subscribe and Unsubscribe requests from Clients
  • forwards Application Messages that match Client Subscriptions
  • closes the Network Connection from the Client
Connection Connects the Client to the Server. Provides the means to send an ordered, lossless, stream of bytes in both directions.
Session A stateful interaction between a Client and a Server.
Some Sessions last only as long as the Network Connection, others can span multiple consecutive Network Connections between a Client and a Server.
Subscription Subscription comprises a Topic Filter and a maximum QoS. A Subscription is associated with a single Session. A Session can contain more than one Subscription. Each Subscription within a Session has a different Topic Filter.
Topic Filter An expression contained in a Subscription to indicate an interest in one or more topics. A Topic Filter can include wildcard characters.
Topic Name The label attached to an Application Message which is matched against the Subscriptions known to the Server.
Application Message The data carried by the MQTT protocol across the network for the application.
When an Application Message is transported by MQTT it contains:

  • payload data,
  • a Quality of Service(QoS),
  • a collection of Properties,
  • a Topic Name.
Will Message An Application Message which is published by the Server after the Network Connection is broken.
QoS 0 At most once delivery
The message is delivered according to the capabilities of the underlying network. No response is sent by Sever and no retry is performed by the Client. The message arrives at the Server either once or not at all.
QoS 1 At least once devilery
A QoS 1 PUBLISH packet sent by Client is acknowledged by a PUBACK packet sent by Server. The message arrives at the Server once or more times.
QoS 2 Exactly once delivery
The Server PUBLISH packet acknowledges receipt with a two-step acknowledgement process. For use when neither loss nor duplication of messages are acceptable. There is an increased overhead associated with QoS 2.