MQTT is an open standard application layer protocol for Message Oriented Middleware. MQTT is often used with a Broker that relays messages between applications that cannot communicate directly.

Publishers send MQTT messages to MQTT brokers. Subscribers subscribe to MQTT brokers for messages. A Broker may persist messages so they can be delivered even if the Subscriber is not online. Brokers may also allow messages to be sent to multiple Subscribers.

The MQTT protocol defines a binary protocol used to send and receive messages from and to topics. The body is an opaque binary blob that can contain any data serialized using an encoding chosen by the application.

There are currently two versions of the MQTT protocol in use, version 3.1.1 and version 5.0. Version 5.0 expands on version 3.1.1 by adding support for connection and message properties. This enables advanced routing scenarios at the broker level in particular when using encrypted payloads.

This document defines two possible encodings for the message body: the binary encoded DataSetMessage defined in 7.2.3 and a JSON encoded DataSetMessage defined in

MQTT version 3.1.1 does not provide a mechanism for specifying the encoding of the MQTT message which means the Subscribers need to be configured in advance with knowledge of the expected encoding. As a consequence, Publishers should only publish NetworkMessages using a single encoding to a unique MQTT topic name.

MQTT version 5.0 adds the encoding and the message type information to the message and connection header and therefore allows Subscribers to detect the encoding and the message mapping. No additional information is added to the meta data messages.

MQTT Publisher and Subscriber transport profiles for full and minimal support are defined in OPC 10000-7.

Message security is primarily provided by a TLS connection between the Publisher or Subscriber and the MQTT server; however, this requires that the MQTT server be trusted. For that reason, it may be necessary to provide end-to-end message security. Applications that require end-to-end message security with MQTT need to use the UADP NetworkMessages and binary message encoding defined in 7.2.3. JSON encoded message bodies need to rely on the security mechanisms provided by MQTT and the MQTT broker.