Subclause 7.3 lists the standard protocols that have been selected for this document and their possible combinations with message mappings.

OPC UA UDP is a simple UDP based protocol that is used to transport UADP NetworkMessages.

A PubSubConnection for UDP shall have a unique Address across all PubSubConnections of an OPC UA Application.

If the Address specifies a domain name then the resolution to an IP address requires access to a domain name resolution service (e.g., the DNS protocol) that maps the domain name onto a usable network address. OPC 10000-7 defines Profiles for different name resolution protocols that Publisher or Subscriber may support.

For OPC UA UDP it is recommended to limit the MaxNetworkMessageSize plus additional headers to a MTU size. The number of frames used for a UADP NetworkMessage influences the probability that UADP NetworkMessages get lost.

Note: The MaxNetworkMessageSize that fits into one MTU is maximum 1472 Byte for IPv4 and 1452 Byte for IPv6. The additional headers have a size of 22 Byte for Ethernet, 20 Byte for IPv4 or 40 Byte for IPv6 and 8 Byte for UDP. This is based on IETF RFC 8200 for IPv6, RFC 791 for IPv4 and RFC 768 for UDP.

For OPC UA UDP the MaxNetworkMessageSize plus additional headers shall be limited to 65535 Byte.

The transport of a UADP NetworkMessage in a UDP packet is defined in Table 170.

Table 170 – UADP message transported over UDP

Name	Description
Frame Header	The frame header.
IP Header	The IP header for the frame contains information like source IP address and destination IP address. IPv4 and IPv6 addresses can be used. The size of the IP header depends on the used version.
UDP Header	The UDP header for the frame contains the source port, destination port, length and checksum. Each field is two byte long. The total size of the UDP header is 8 byte.
UADP NetworkMessage	The UADP NetworkMessage is sent as UDP data.
Frame Footer	The frame footer.

The IANA registered IPv4 multicast address for discovery is 224.0.2.14. It shall only be used for OPC UA discovery purposes. The recommended port for discovery is 4840. Therefore the default DiscoveryAddress has the following form:

opc.udp://224.0.2.14

The default DiscoveryMaxMessageSize for UDP is 4096 bytes.

The transport protocol URL for UDP multicast and broadcast communication is configured on a PubSubConnection for Publisher and Subscriber. The Address parameter for a PubSubConnection is defined in 6.2.7.3.

The Url field in the Address is used as destination address for NetworkMessages sent as UDP datagram. The Address is also used to receive UDP datagrams from the multicast IP address. All DataSetWriters and DataSetReaders that send to and receive from the multicast IP address shall be configured on one PubSubConnection. The Address parameter for WriterGroup datagram TransportSettings shall be null. If an Address is configured on a WriterGroup, the WriterGroup PubSubState shall be Error. The NetworkInterface field in the Address is required if more than one network interface is available.

The syntax of the UDP transport protocol URL used in the Address has the following form:

opc.udp://<address>[:<port>]

The address is either an multicast or broadcast IP address or a registered name like a domain name that can be resolved to a multicast or broadcast IP address.. It is the destination of the UDP datagram.

The IANA registered OPC UA port for UDP communication is 4840. This is the default and recommended port for broadcast and multicast communication. Alternative ports may be used.

It is recommended to use switches with IGMP and MLD support to limit the distribution of multicast traffic to the interested participants.

Note: The Internet Group Management Protocol (IGMP) is a standard protocol used by hosts to report their IP multicast group memberships for IPv4 and needs to be implemented by any host that wishes to receive IP multicast datagrams. IGMP messages are used by multicast routers to learn which multicast groups have members on their attached networks. IGMP messages are also used by switches capable of supporting “IGMP snooping” whereby the switch listens to IGMP messages and only sends the multicast NetworkMessages to ports that have joined the multicast group. The corresponding protocol for IPv6 is the Multicast Listener Discovery (MLD).There are different versions of IGMP and MLD: - IGMP V1 is defined in IETF RFC 1112. - IGMP V2 is defined in IETF RFC 2236. - IGMP V3 is defined in IETF RFC 3376.

- IGMP V3 and MLD V2 are defined in IETF RFC 4604.

IETF RFC 2236 and IETF RFC 3376 discuss host and router requirements for interoperation with older IGMP versions.If OPC UA devices make extensive use of IP multicast for UDP transport, consistent IGMP and MLD usage by OPC UA devices is essential in order to create well-functioning OPC UA Application networks.

OPC UA Applications shall issue an IGMP membership report message (V1, V2 or V3 as appropriate) for IPv4 or a MLD membership report message for IPv6 when enabling a PubSub connection on which they will receive UDP multicast NetworkMessages.

For UDP unicast, the address information for the Subscriber is configured on the PubSubConnection and the address information for the Publisher is configured on the WriterGroup.

The receive port for UDP unicast communication is configured on a PubSubConnection. The Address parameter for a PubSubConnection is defined in 6.2.7.3. All NetworkMessages for one port are received through one PubSubConnection. The filtering and assignment of NetworkMessages for the Subscriber is done based on the PublisherId. The hostname for the Url in the PubSubConnection Address parameter is set to ‘localhost’ since the source address is not used for filtering. The NetworkInterface field in the Address is not required and is only configured if the Subscriber should listen only on the configured interface. If the NetworkInterface is null or empty, the Subscriber should listen on all interfaces.

The syntax of the Url field in the PubSubConnection Address parameter has the following form:

opc.udp://localhost[:<port>]

The destination address is configured on the datagram TransportSettings of a WriterGroup. The Address parameter for a WriterGroup datagram TransportSetting is defined in 6.4.1.3.4. The Address parameter for WriterGroup datagram TransportSettings shall be configured. If no Address is configured on a WriterGroup, the WriterGroup PubSubState shall be Error. The NetworkInterface field in the Address is not required and should be null or empty and shall be ignored.

The syntax of Url field in the WriterGroup datagram TransportSettings Address parameter has the following form:

opc.udp://<host>[:<port>]

The host is either an unicast IP address or a registered name like a hostname or domain name that can be resolved to a unicast IP addresses. The IP address and the port are the destination of the UDP datagram.

The IANA registered OPC UA port for UDP communication is 4840. This is the default and recommended port for unicast communication. Alternative ports may be used.

OPC UA Ethernet is a simple Ethernet based protocol using EtherType 0xB62C that is used to transport UADP NetworkMessages as payload of the Ethernet II frame without IP or UDP headers.

The syntax of the Ethernet transporting protocol URL used in the Address parameter defined in 6.2.7.3 has the following form:

opc.eth://<host>[:<VID>[.PCP]]

The host is a MAC address, an IP address or a registered name like a hostname. The format of a MAC address is six groups of hexadecimal digits, separated by hyphens (e.g. 01-23-45-67-89-ab). A system may also accept hostnames and/or IP addresses if it provides means to resolve it to a MAC address (e.g. DNS and Reverse-ARP).

The VID is the VLAN ID as number.

The PCP is the Priority Code Point as one digit number. This optional parameter is typically configured as part of the QoS settings on the network interface and not of the address.

The transport of a UADP NetworkMessage in an Ethernet II frame is defined in Table 171.

Table 171 – UADP message transported over Ethernet

Name	Description
Frame Header	The frame header with an EtherType of 0xB62C.
UADP NetworkMessage	The UADP NetworkMessage is sent as Ethernet payload.
Frame Footer	The frame footer.

For OPC UA Ethernet the MaxNetworkMessageSize and the DiscoveryMaxMessageSize plus additional headers shall be limited to an Ethernet frame size of 1522 Byte.

Note: The MaxNetworkMessageSize is typically 1500 Byte since the additional headers have a size of 22 Byte and it consists of 6 Byte destination address, 6 Byte source address, 2 Byte EtherType, 4 Byte frame check sequence and optionally 4 Byte VLAN tag. This is based on Q-tagged frames defined in IEEE Std 802.3-2018.

The IANA registered OPC UA EtherType for UADP communication is 0xB62C.

The Advanced Message Queuing Protocol (AMQP) is an open standard application layer protocol for Message Oriented Middleware. AMQP is often used with a Broker that relays messages between applications that cannot communicate directly.

Publishers send AMQP messages to AMQP endpoints. Subscribers listen to AMQP endpoints for incoming messages. If a Broker is involved it 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 AMQP protocol defines a binary encoding for all messages with a header and a body. The header allows applications to insert additional information as name-value pairs that are serialized using the AMQP binary encoding. The body is an opaque binary blob that can contain any data serialized using an encoding chosen by the application.

This document defines two possible message mappings for the AMQP message body: the UADP message mapping defined in 7.2.3 and a JSON message mapping defined in 7.2.4.6.9. AMQP Brokers have an upper limit on message size. The limit is defined by the AMQP field max-message-size. The mechanism for handling NetworkMessages that exceed the Broker limits depends on the MessageMapping. For MessageMappings that support chunking, the NetworkMessage shall be broken into multiple chunks. The chunk size plus the AMQP header should not exceed the AMQP max-message-size. For MessageMappings that do not support chunking, the NetworkMessages exceeding the maximum size mut be skipped. Diagnostic information for such error scenarios are provided through the Events of the type PubSubTransportLimitsExceedEventType defined in 9.1.13.2 and through the FailedTransmissions counter of the PubSubDiagnosticsWriterGroupType defined in 9.1.11.9.

Security with AMQP is primary provided by a TLS connection between the Publisher or Subscriber and the AMQP Broker, however, this requires that the AMQP Broker be trusted. For that reason, it may be necessary to provide end-to-end security. Applications that require end-to-end security with AMQP need to use the UADP NetworkMessages and binary message encoding defined in 7.2.4.4. JSON encoded message bodies rely on the security mechanisms provided by AMQP and the AMQP Broker.

The syntax of the AMQP transporting protocol URL used in the Address parameter defined in 6.2.7.3 has the following form:

amqps://<domain name>[:<port>][/<path>]

The default port is 5671. The protocol prefix above provides transport security.

amqp://<domain name>[:<port>][/<path>]

The default port is 5672.

The syntax for an AMQP URL over Web Sockets has the following form:

wss://<domain name>[:<port>][/<path>]

The default port is 443.

Authentication shall be performed according to the configured AuthenticationProfileUri of the PubSubConnection, DataSetWriterGroup, DataSetWriter or DataSetReader entities.

If no authentication information is provided in the form of ResourceUri and AuthenticationProfileUri, SASL Anonymous is implied.

If the authentication profile specifies SASL PLAIN authentication, a separate connection for each new Authentication setting is required.

AMQP allows sending properties as part of opening the connection, session establishment and link attach.

The connection properties apply to any connection, session or link created as part of the PubSubConnection, or subordinate configuration entities, such as WriterGroup and DataSetWriter.

The properties are defined through the KeyValuePair array in the ConnectionProperties WriterGroupProperties and DataSetWriterProperties. The NamespaceIndex of the QualifiedName in the KeyValuePair shall be 0 for AMQP standard properties. The Name of the QualifiedName is constructed from a prefix and the AMQP property name with the following syntax.

Name = <target prefix>-<AMQP property name>

The target prefix can have the following values:

• Connection;
• session;
• link.

The Value of the KeyValuePair is converted to an AMQP data type using the rules defined in Table 174. If there is no rule defined for a data type, the property shall not be included.

The connection properties are intended to be used sparingly to optimize interoperability with existing broker endpoints.

A writer negotiates the delivery guarantees for its link using the snd-settle-mode settlement policy (settled, unsettled, mixed) it will use, and the desired rcv-settle-mode (first, second) of the broker.

Vice versa, the reader negotiates delivery guarantees using its rcv-settle-mode (first, second) and the desired snd-settle-mode (settled, unsettled) of the broker.

This matches to the BrokerTransportQualityOfService values as follows:

• AtMostOnce or BestEffort – messages are pre-settled at the sender endpoint and not sent again. Messages may be lost in transit. This is the default setting.
• AtLeastOnce – messages are received and settled at the receiver without waiting for the sender to settle.
• ExactlyOnce – messages are received, the sender settles and then the receiver settles.

If the KeepAliveTime is set on a WriterGroup, a value slightly higher than the configured value of the group should be used as AMQP idle time-out of the AMQP connection ensuring that the connection is disconnected if the keep alive message was not sent by any writer. Otherwise, if no KeepAliveTime is specified, the implementation should set a reasonable default value.

When setting the maximum message sizes for the Link, the MaxNetworkMessageSize of the PubSubGroup shall be used. If this value is 0, the implementation chooses a reasonable maximum.

Other limits are up to the implementation and depend on the capabilities of the OS or on the capabilities of the device the Publisher or Subscriber is running on, and can be made configurable through configuration model extensions or by other means.

The AMQP message header has a number of standard fields which are called properties in the AMQP specification. Table 172 describes how these fields shall be populated when an AMQP message is constructed.

Table 172 – AMQP standard header fields

Field Name	Source
message-id	A globally unique value per message.
Subject	Valid values are ua-data or ua-metadata.
Content-type	The MIME type for the message body. The MIME types are specified in the message body subclauses 7.3.4.8.1 and 7.3.4.8.3.

The subject defines the type of the message contained in the AMQP body. A value of “ua-data” specifies the body contains a UADP or JSON NetworkMessage. A value of “ua-metadata” specifies a body that contains a UA Binary or JSON encoded DataSetMetaData Message. The content-type specifies the whether the message is binary or JSON data.

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

Name = message-<AMQP property name>

Table 173 defines the AMQP standard message properties.

Table 173 – OPC UA AMQP standard header QualifiedName Name mappings

AMQP standard property name	OPC UA DataType	AMQP data type	Note
to	String	*
user-id	ByteString	binary
reply-to	String	string
correlation-id	ByteString	*
absolute-expiry-time	Duration	timestamp	The absolute-expiry-time is calculated by adding the message-absolute-expiry-time (Duration) from the DataSetWriterProperties to the current time of the DataSetMessage creation.
Group-id	String	string
reply-to-group-id	String	string
creation-time	Boolean	timestamp	The creation-time is set to the current time of the DataSetMessage creation if the message-creation-time (Boolean) in the DataSetWriterProperties is True, or else if the value is False or if the property is not configured, the AMQP property is not set.
Content-encoding	String	symbol

Any name not in the table is assumed to be an application property. In this case the namespace provided as part of the QualifiedName shall be the ApplicationUri.

The AMQP message header shall include additional promoted fields of the DataSet as a list of name-value pairs. DataSet fields with the PromotedField flag set in the FieldMetaData fieldFlags are copied into the AMQP header. The FieldMetaData Structure is defined in 6.2.3.2.4. 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.

When a field is added to the header it is converted to an AMQP data type using the rules defined in Table 174. If there is no rule defined for the data type, the field shall not be included.

Table 174 – OPC UA AMQP header field conversion rules

OPC UA DataType	Conversion Rules to AMQP data types.
Boolean	AMQP ‘boolean’ type.
Sbyte	AMQP ‘byte’ type.
Byte	AMQP ‘ubyte’ type.
Int16	AMQP ‘short’ type.
UInt16	AMQP ‘ushort’ type.
Int32	AMQP ‘int’ type.
UInt32	AMQP ‘uint’ type.
Int64	AMQP ‘long’ type.
UInt64	AMQP ‘ulong’ type.
Float	AMQP ‘float’ type.
Double	AMQP ‘double’ type.
String	AMQP ‘string’ type.
ByteString	AMQP ‘binary’ type.
DateTime	AMQP ‘timestamp’ type. This conversion may result in loss of precision on some platforms. The rules for dealing with the loss of precision are described in OPC 10000-6.
Guid	AMQP ‘uuid’ type.
QualifiedName	The QualifiedName is encoded as an AMQP ‘string’ type with the format <NamespaceUri>’#’<Name>.
LocalizedText	Not supported and the related field is discarded.
NodeId	If the NamespaceIndex = 0 the value is encoded as an AMQP ‘string’ type using the format for a NodeId defined in OPC 10000-6. If the NamespaceIndex > 0 the value is converted to an ExpandedNodeId with a NamespaceUri and is encoded as an AMQP ‘string’ type using the format for an ExpandedNodeId defined in OPC 10000-6.
ExpandedNodeId	If the NamespaceUri is not provided the rules for the NodeId are used. If the NamespaceUri is provided the value is encoded as an AMQP ‘string’ type using the format for an ExpandedNodeId defined in OPC 10000-6.
StatusCode	AMQP ‘uint’ type.
Variant	If the value has a supported datatype it uses that conversion; otherwise it is not supported and the related field is discarded.
Structure	Not supported and the related field is discarded.
Structure with option fields	Not supported and the related field is discarded.
Array	Not supported and the related field is discarded.
Union	Not supported and the related field is discarded.

The message body is encoded in the AMQP bare-message application-data section as an AMQP ‘binary’ value.

A JSON body is encoded as defined for the JSON message mapping defined in 7.2.4.6.9.

The corresponding MIME type is application/json.

A UADP body is encoded as defined for the UADP message mapping defined in 7.2.3.

The corresponding MIME type is application/opcua+uadp.

If the encoded AMQP message size exceeds the Broker limits it shall be broken into multiple chunks as described in 7.2.4.4.4.

It is recommended to create the MetaDataQueueName as described in 6.4.2.5.5 by appending the text “$Metadata” to the related QueueName.

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 7.2.4.6.9.

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.

The syntax of the MQTT transporting protocol URL used in the Address parameter defined in 6.2.7.3 has the following form:

mqtts://<domain name>[:<port>][/<path>]

The protocol prefix mqtts provides transport security. The default port is 8883.

mqtt://<domain name>[:<port>][/<path>]

The protocol prefix without transport security is mqtt. The default port is 1883.

wss://<domain name>[:<port>][/<path>]

The protocol prefix for MQTT over secure Web Sockets is wss. The default port is 443.

MQTT supports the use of Username/Password authentication in the initial CONNECT packet. Aside from password credentials, implementations can use this mechanism to pass any form of secret, such as an authentication token. However, if CONNECT authentication is used, the connection should be secured.

MQTT version 5.0 also supports enhanced authentication, whereby clients can specify the desired SASL authentication method during initial CONNECT and finish the secret exchange with the broker using subsequent AUTH packets, or reauthenticate on an existing connection.

Authentication shall be performed according to the configured AuthenticationProfileUri of the PubSubConnection, DataSetWriterGroup, DataSetWriter or DataSetReader entities.

If no authentication information is provided in the form of ResourceUri and AuthenticationProfileUri, SASL Anonymous is implied.

If the authentication profile specifies SASL PLAIN authentication, a separate connection for each authentication setting is required.

The MQTT transport mapping for version 3.1.1 only supports the connection property ClientID using a KeyValuePair. Any other configured setting in the connection properties shall be silently discarded.

If the ClientID is not configured, the PublisherId is used as ClientID. If the PublisherId has a UInteger DataType, the UInteger value is converted to a String for the ClientID.

MQTT version 5.0 allows Publishers and Subscribers to provide MQTT connection properties as part of opening the connection.

The connection properties apply to any connection created as part of the PubSubConnection, or subordinate configuration entities, such as the WriterGroup and the DataSetWriter.

The properties are defined through the KeyValuePair array in the ConnectionProperties. The NamespaceIndex of the QualifiedName in the KeyValuePair shall be 0.

Table 175 formally defines the ConnectionProperties used for MQTT connection configuration.

Table 175 – MQTT ConnectionProperties

Key	DataType	Description
0:MqttVersion	String	Defines the MQTT version to use for the MQTT connection. Possible values are “3.1.1”, “5.0” and “BestAvailable”. The default value is BestAvailable.
0:MqttTopicPrefix	String	The <Prefix> part of the Topic convention defined in 7.3.5.7.1. The default value is “opcua”

For MQTT properties, the Name of the QualifiedName is constructed from a prefix “connection” followed by a hyphen and the MQTT property name with the following syntax.

Name = connection-<MQTT property name>

Table 176 defines the MQTT standard connection properties.

Table 176 – OPC UA MQTT standard connection property configuration

MQTT property name	OPC UA DataTypes	MQTT data types
ClientID	String	UTF-8 Encoded String
Receive Maximum	UInt16	Two Byte Integer
Maximum Packet Size	UInt32	Four Byte Integer
Session Expiry Interval	UInt32	Four Byte Integer
Topic Alias Maximum	UInt16	Two Byte Integer
Request Response Information	Boolean	Byte
Request Problem Information	Boolean	Byte

Any name not in the Table 176 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 non-reversible OPC UA JSON Data Encoding rules in OPC 10000-6.

The BrokerTransportQualityOfService values map to MQTT publish and subscribe QoS settings as follows:

• AtMostOnce and BestEffort is mapped to MQTT QoS 0.
• AtLeastOnce is mapped to MQTT QoS 1.
• ExactlyOnce is mapped to MQTT QoS 2.

If the KeepAliveTime is set on a WriterGroup, a value slightly higher than the configured value of the group in seconds should be set as MQTT Keep Alive ensuring that the connection is disconnected if the keep alive message was not sent by any writer in the specified time. If multiple WriterGroups are configured, the group with the highest KeepAliveTime setting is used for the calculation.

The implementation chooses packet and message size limits depending on the capabilities of the OS or of the capabilities of the device the application is running on. They can be made configurable through configuration model extensions or by other means.

MQTT messages are sent to Topics which provide context and other information about the message. Topics are hierarchical paths that allow Subscribers to use wildcards to select multiple Topics. Therefore, Topics are most useful when they follow a predictable pattern. This clause defines the Topic conventions for use with the MQTT mapping. All Publishers shall be able to support these conventions.

A Topic has the following general pattern:

<Prefix>/<Encoding>/<MqttMessageType>/<PublisherId>/[<WriterGroup>/<DataSetWriter>]

Where Topic levels are:

• <Prefix> is a system defined prefix that provides a scope for the PublisherIds;The default value is ‘opcua’.
• <Encoding> specifies the encoding of messages sent to the Topic (‘json’, ‘uadp’, etc);
• <MqttMessageType> specifies the content of the messages published to the Topic as defined in 7.3.5.7.2;
• <PublisherId> uniquely identifies a Publisher within the scope of the prefix;
• <WriterGroup> is the name of a WriterGroup within the Publisher;
• <DataSetWriter> is the name of a DataSetWriter within the WriterGroup;

The <Prefix> should be one Topic level, however, system designers may break <Prefix> into multiple Topic levels. Note that using multiple Topic levels prevents Subscribers from automatically discovering Publishers in the system unless they are preconfigured with the <Prefix> used for the system.

The possible values for the <Encoding> are ‘json’ or ‘uadp’ based on the message mappings defined in 7.2.3 and 7.2.4.6.9.

When Publishers are configured to publish to an MQTT Broker they shall have PublisherId assigned that can be used as Topic level.

The Topic levels that appear after the <PublisherId> depend on the <MqttMessageType>.

When these Topic conventions are used the wildcards in Table 177 may be used:

Table 177 – Examples of MQTT Wildcards

Wildcard	Notes
opcua/json/status/#	Subscribes to the status of all Publishers in the “opcua” scope. This allows the Subscriber to detect when Publishers come online or disappear.
opcua/json/metadata/#	Subscribes to the metadata of all Publishers in the “opcua” scope. This allows the Subscriber to detect changes to metadata.
opcua/json/data/device-one/#	Subscribes to all data produced by Publisher “device-one” in the “opcua” scope.
opcua/json/data/+/+/diagnostic	Subscribes to all Publishers that offer a “diagnostic” writer.

The MQTT Topic syntax places restrictions on what characters may be used in a Topic level. Specifically, Topic levels are any UTF8 string that:

• Does not start with a $
• Does not include /, + or #
• Does not include non-printable characters or whitespace other than the space character (U+0020).

<MqttMessageType> Topic levels exist for each MessageType defined in 7.2.2. Additional information about each Topic level is provided in Table 178.

Table 178 – MQTT Topic level MessageType mapping

MessageType	MqttMessageType	RETAIN	Required	Specification Reference
DataSetMessage	data	False	Yes	Defined in 7.3.5.7.3. A system specific Topic may be used instead The RETAIN false is the default setting.
DataSetMetaData	metadata	True	Yes	Defined in 7.3.5.7.4. A system specific Topic may be used instead.
ApplicationDescription	application	True	No	Defined in 7.3.5.7.5.
ServerEndpoints	endpoints	True	No	Defined in 7.3.5.7.6.
Status	status	True	Yes	Defined in 7.3.5.7.7.
PubSubConnection	connection	True	Yes	Defined in 7.3.5.7.8.

The data Topic has the form:

<Prefix>/<Encoding>/data/<PublisherId>/<WriterGroup>[/<DataSetWriter>]

The <PublisherId> Topic level is the PublisherId for the application sending the messages.

The <WriterGroup> Topic level is the name of a WriterGroup within the Publisher.

The <DataSetWriter> Topic level is the name of a DataSetWriter within the WriterGroup. If no QueueName is specified at the DataSetWriter level then the QueueName in WriterGroup TransportSettings is used and the DataSetWriter name is not part of the Topic.

The data Topic level is the default if the system owner does not have their own Topic tree. The Topic actually used is specified in the Connection Message as QueueName in the DataSetWriter or WriterGroup TransportSettings.

The messages are instances of the DataSetMessage MessageType (see 7.2.2).

The corresponding PubSubConnection Messag e is sent to the Topic with the same <Prefix>, <Encoding> and <PublisherId>. The PubSubConnection specifies the WriterGroups and DataSetWriters for a Publisher.

The metadata Topic has the form:

<Prefix>/<Encoding>/metadata/<PublisherId>/<WriterGroup>/<DataSetWriter>

The <PublisherId> Topic level is the PublisherId for the application sending the messages.

The <WriterGroup> Topic level is the name of a WriterGroup within the Publisher.

The <DataSetWriter> Topic level the name of a DataSetWriter within the WriterGroup.

The metadata Topic is the default if the system owner does not have their own Topic tree. The Topic actually used is specified in the Connection Message as MetaDataQueueName in the DataSetWriter TransportSettings.

The messages are instances of the DataSetMetaData MessageType (see 7.2.2).

The corresponding PubSubConnection Messag e is sent to the Topic with the same <Prefix>, <Encoding> and <PublisherId>. The PubSubConnection specifies the WriterGroups and DataSetWriters for a Publisher.

The application Topic has the form:

<Prefix>/<Encoding>/application/<PublisherId>

The <PublisherId> Topic level is the PublisherId for the application sending the messages.

The messages are instances of the ApplicationDescription MessageType (see 7.2.2).

The endpoints Topic has the form:

<Prefix>/<Encoding>/endpoints/<PublisherId>

The <PublisherId> Topic level is the PublisherId for the application sending the messages.

The messages are instances of the ServerEndpoints MessageType (see 7.2.2).

The status Topic has the form:

<Prefix>/<Encoding>/status/<PublisherId>

The <PublisherId> Topic level is the PublisherId for the application sending the messages.

The messages are instances of the Status MessageType (see 7.2.2).

A Publisher that is exclusively using a PublisherId shall register a Status message as an MQTT Will message when it creates the connection to the MQTT Broker. This message is sent automatically if the Publisher loses its connection the MQTT Broker. The IsCyclic shall be FALSE in this case. The PubSubState value of the Will message shall be Error.

The connection Topic has the form:

<Prefix>/<Encoding>/connection/<PublisherId>

The <PublisherId> Topic level is the PublisherId for the application sending the messages. This value shall be the same as the PublisherId in PubSubConnection provided in the message.

The PublisherId in the PubSubConnection uniquely identifies the Publisher within the scope defined by the <Prefix>.

The TransportProfileUri in the PubSubConnection specifies the <Encoding> used for all messages for the combination of <Encoding>/<PublisherId>. Table 179 specifies the mapping between a TransportProfileUri and the encoding.

Table 179 – TransportProfileUri encodings

URI	Encoding
http://opcfoundation.org/UA-Profile/Transport/pubsub-mqtt-json	json
http://opcfoundation.org/UA-Profile/Transport/pubsub-mqtt-uadp	uadp

The messages are instances of the PubSubConnection MessageType (see 7.2.2).

The default setting for the MQTT RETAIN flag are defined in Table 178. Table 181 defines an option to change the RETAIN flag setting for DataSetMessages.

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

Table 180 – OPC UA MQTT message properties

MQTT property name	MQTT property type	MQTT property value
UAMessageType	User Property	Valid values are ua-data or ua-metadata.
Content Type	Standard	The MIME type for the message body. The MIME types are specified in the message body subsections 7.3.5.9.2 and 7.3.5.9.3.

The MQTT message header shall include additional fields defined on the 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 = message-<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 181 defines the MQTT standard message properties.

Table 181 – 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.
Response Topic	String	UTF-8 Encoded String	Not available as message property for MQTT 3.1.1.
Correlation Data	ByteString	Binary Data	Not available as message property for MQTT 3.1.1.
Message Expiry Interval	UInt32	Four Byte Integer	Not available as message property for MQTT 3.1.1.
Content Type	String	UTF-8 Encoded String	Configuration option for the MIME.type of the message body for MQTT 3.1.1 and 5.0. Not available as message property for MQTT 3.1.1.

Any name not in the Table 181 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 non-reversible 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.

When sending a MQTT Version 5.0 message the UAMessageType property shall be set to uadata for data messages or ua-metadata for metadata messages.

A JSON body is encoded as defined for the JSON message mapping defined in 7.2.4.6.9.

When sending a MQTT Version 5.0 message the MQTT ContentType property shall be set to application/json when sending uncompressed JSON messages.

JSON messages can become quite large. In order to save bandwidth and to reduce message size, on MQTT Version 5.0 the MQTT ContentType property allows to select a compression type as encoding for a JSON message.

When sending a gzip (RFC 1952) compressed JSON message on MQTT Version 5.0 the MQTT ContentType property shall be set to application/json+gzip.

A UADP body is encoded as defined for the UADP message mapping defined in 7.2.3.

It is expected that the software used to receive UADP NetworkMessage can process the body without needing to know how it was transported.

If the encoded MQTT message size exceeds the Broker limits, it is broken into multiple chunks as described in 7.2.4.4.4.

When sending such message over MQTT Version 5.0 the ContentType property shall be set to application/opcua+uadp.