The term message is used with various intentions in the messaging world. It sometimes only refers to the payload (the application data) and sometimes to the network packet that also includes protocol-, security-, or encoding-specific data. To avoid confusion, this specification formally defines the term DataSetMessage to mean the application data (the payload) supplied by the Publisher and the term NetworkMessage to mean the message handed off and received from a specific Message Oriented Middleware. DataSetMessages are embedded in NetworkMessages. Figure 4 shows the relationship of these message types.
Figure 4 – OPC UA PubSub Message Layers
The transport protocol-specific headers and definitions are described in 7.3.
Following is an abstract definition of DataSetMessage and NetworkMessage. The concrete structure depends on the message mapping and is described in 7.2.