The default setting for the MQTT RETAIN flag are defined in Table 204. Table 208 defines an option to change the RETAIN flag setting for DataSetMessages.

A Publisher shall send all RETAIN discovery messages at start up of the Publisher. A Publisher shall update affected RETAIN topics if the Publisher configuration changes. A Publisher shall clear RETAIN topics if the discovery element is deleted from the Publisher configuration like reset the metadata topic if the related DataSetWriter is removed. A Publisher shall subscribe to its own discovery message topics at start-up and clear all topics that do not match the current Publisher configuration.

Publishers using MQTT version 3.1.1 shall clear RETAIN topics when they shut down.

Publishers using MQTT version 5.0 shall set the Message Expiry Interval on RETAIN topics and shall send a new RETAIN message before the interval expires.

The MQTT version 3.1.1 protocol does not support message headers. Any promoted field or additional fields defined on the WriterGroup or DataSetWriter other than RETAIN are not sent as MQTT message properties.

MQTT version 5.0 defines a number of standard message properties. These include properties explicitly defined in the MQTT specification, as well as the MQTT User Property which is a key-value pair of UTF-8 strings. The MQTT User Property is intended to provide a means of transferring application layer name-value tags whose meaning and interpretation are known only by the application programs responsible for sending and receiving them. They are used here to specify PubSub properties not directly supported by the MQTT protocol.

Table 207 describes how these properties shall be populated when a MQTT version 5.0 message is constructed.

Table 207 – OPC UA MQTT message properties

MQTT property name	MQTT property type	MQTT property value
UAMessageType	User Property	Valid values are “ua-<MqttMessageType>” where the MqttMessageTypes are defined in 7.3.5.7.2.
Content Type	Standard	The MIME type for the message body. The MIME types are specified in the message body subsections 7.3.5.9.1 and 7.3.5.9.2.

The MQTT message header shall include additional fields defined on the PubSubConnection, WriterGroup or DataSetWriter through the KeyValuePair array in the WriterGroupProperties and DataSetWriterProperties. The NamespaceIndex of the QualifiedName in the KeyValuePair shall be 0. The Name of the QualifiedName is constructed from a message prefix and the MQTT property name with the following syntax.

Name = <MqttMessageType>-<MQTT property name>

The Name of the key in the KeyValuePair shall have a prefix “message” followed by a hyphen and the MQTT property name.

Table 208 defines the MQTT standard message properties.

Table 208 – OPC UA MQTT standard message property configuration

MQTT property name	OPC UA DataTypes	MQTT data types	Description
RETAIN	Boolean	RETAIN bit in the header	RETAIN configuration for DataSetMessages.
Message Expiry Interval	UInt32	Four Byte Integer	Not available as message property for MQTT 3.1.1.

Any name not in the Table 208 is assumed to be a MQTT User Property.

When a field is added to the header as a MQTT User Property the value is encoded as UTF-8 encoded String. If the value is not a String, then it is encoded using the VerboseEncoding OPC UA JSON Data Encoding rules in OPC 10000-6. Promoted fields can only be sent for fields which are assumed to be a MQTT User Property and if the NetworkMessage contains only one DataSetMessage. The MQTT message header shall include additional promoted fields of the DataSet as a list of MQTT User Property name-value pairs. DataSet fields with the PromotedField flag set in the FieldMetaData fieldFlags are copied into the MQTT header. The FieldMetaData Structure is defined in 6.2.3.2.4. For a UADP message mapping the promoted fields are also included in the UADP NetworkMessage. Promoted fields shall always be included in the header even if the DataSetMessage body is a delta frame and the DataSet field is not included in the delta frame. In this case the last known value is sent in the header.