MessageTypes define the structure and semantics of NetworkMessages. NetworkMessages with different MessageTypes are necessary to support the discovery of Publisher configuration information and the reporting of live data and events.

The defined MessageTypes are listed in Table 134.

Table 134 – PubSub MessageTypes

MessageType	Description
DataSetMessage	Application data or events supplied by the Publisher as DataSet.
DataSetMetaData	Discovery message with content and semantics of a DataSetMessage
ApplicationDescription	Discovery message with OPC UA application description and capabilities of the Publisher.
ServerEndpoints	Discovery message with the OPC UA server endpoints of the Publisher.
Status	Discovery message with the current operational status of a Publisher
PubSubConnection	Discovery message with the PubSubConnections configured in the Publisher.

Each message mapping defines the structure of each supported MessageType. The mechanism for identifying the MessageType associated with a NetworkMessage is specific to the message mapping. The mapping to UADP messages is defined in 7.2.4.2. The mapping to JSON messages is defined in 7.2.5.2.

The discovery messages are required to send discovery information from the Publishers to the Subscribers that do not have out of band knowledge about available Publishers.

A Message Oriented Middleware where retained messages are supported can use a known addressing schema for queues or topics to provide discovery messages to Subscribers. Such a Topic tree is defined for the MQTT transport protocol mapping in 7.3.5.7. The mapping of MessageTypes for Topics is defined in 7.3.5.7.2.

The UADP message mapping defines also discovery probe messages to request discovery information in a Message Oriented Middleware where retained messages are not available.