Subclause 7.3lists 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 PubSubConnectionfor UDP shall have a unique Addressacross all PubSubConnectionsof an OPC UA Application.

For OPC UA UDP it is recommended to limit the MaxNetworkMessageSizeplus additional headers to a MTU size. The number of frames used for a UADP NetworkMessageinfluences the probability that UADP NetworkMessagesget 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 MaxNetworkMessageSizeplus additional headers shall be limited to 65535 Byte.

The transport of a UADP NetworkMessagein a UDP packet is defined in Table 161.

Table 161– 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 DiscoveryAddresshas the following form:

opc.udp://224.0.2.14

The default DiscoveryMaxMessageSizefor UDP is 4096 bytes.

The transport protocol URL for UDP multicast and broadcast communication is configured on a PubSubConnectionfor Publisherand Subscriber. The Addressparameter for a PubSubConnectionis defined in 6.2.7.3.

The Urlfield in the Addressis used as destination address for NetworkMessagessent as UDP datagram. The Addressis also used to receive UDP datagrams from the multicast IP address. All DataSetWritersand DataSetReadersthat send to and receive from the multicast IP address shall be configured on one PubSubConnection. The Addressparameter forWriterGroupdatagram TransportSettingsshall be null. If an Addressis configured on a WriterGroup, the WriterGroup PubSubStateshall be Error. The NetworkInterfacefield in the Addressis required if more than one network interface is available.

The syntax of the UDP transport protocol URL used in the Addresshas 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 NetworkMessagesto 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 Applicationnetworks.

OPC UA Applicationsshall 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 Subscriberis configured on the PubSubConnectionand the address information for the Publisheris configured on the WriterGroup.

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

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

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

The destination address is configured on the datagram TransportSettingsof a WriterGroup. The Addressparameter for a WriterGroupdatagram TransportSettingis defined in 6.4.1.3.4. The Addressparameter forWriterGroupdatagram TransportSettingsshall be configured. If no Addressis configured on a WriterGroup, the WriterGroup PubSubStateshall be Error. The NetworkInterfacefield in the Addressis not required and should be null or empty and shall be ignored.

The syntax of Url field in the WriterGroupdatagram TransportSettings Addressparameter 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 NetworkMessagesas payload of the Ethernet II frame without IP or UDP headers.

The syntax of the Ethernet transporting protocol URL used in the Addressparameter defined in 6.2.7.3has 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 NetworkMessagein an Ethernet II frame is defined in Table 162.

Table 162– 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 MaxNetworkMessageSizeand the DiscoveryMaxMessageSizeplus additional headers shall be limited to an Ethernet frame size of 1522 Byte.

Note: The MaxNetworkMessageSizeis 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 Brokerthat relays messages between applications that cannot communicate directly.

Publisherssend AMQP messages to AMQP endpoints. Subscribers listen to AMQP endpoints for incoming messages. If a Brokeris involved it may persist messages so they can be delivered even if the subscriber is not online. Brokersmay 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 7.2.2and a JSON message mapping defined in 7.2.2.6.4.7. AMQP Brokers have an upper limit on message size. The limit is defined by the AMQP field max-message-size. The mechanism for handling NetworkMessagesthat exceed the Brokerlimits depends on the MessageMapping. For MessageMappingsthat support chunking, the NetworkMessageshall be broken into multiple chunks. The chunk size plus the AMQP header should not exceed the AMQP max-message-size. For MessageMappingsthat do not support chunking, the NetworkMessagesexceeding the maximum size mut be skipped. Diagnostic information for such error scenarios are provided through the Eventsof the type PubSubTransportLimitsExceedEventTypedefined in 9.1.13.2and through the FailedTransmissionscounter of the PubSubDiagnosticsWriterGroupTypedefined in 9.1.11.9.

Security with AMQP is primary provided by a TLS connection between the Publisheror Subscriberand the AMQP Broker, however, this requires that the AMQP Brokerbe 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 NetworkMessagesand binary message encoding defined in 7.2.2.3. 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 Addressparameter defined in 6.2.7.3has 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 AuthenticationProfileUriof the PubSubConnection, DataSetWriterGroup, DataSetWriteror DataSetReaderentities.

If no authentication information is provided in the form of ResourceUriand 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 WriterGroupand DataSetWriter.

The properties are defined through the KeyValuePairarray in the ConnectionProperties WriterGroupPropertiesand DataSetWriterProperties. The NamespaceIndexof the QualifiedNamein the KeyValuePairshall be 0 for AMQP standard properties. The Nameof the QualifiedNameis 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 Valueof the KeyValuePairis converted to an AMQP data type using the rules defined in Table 165. 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 BrokerTransportQualityOfServicevalues as follows:

If the KeepAliveTimeis 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 KeepAliveTimeis specified, the implementation should set a reasonable default value.

When setting the maximum message sizes for the Link, the MaxNetworkMessageSizeof the PubSubGroupshall 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 Publisheror Subscriberis 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 163describes how these fields shall be populated when an AMQP message is constructed.

Table 163– 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 7.3.4.8.1and 7.3.4.8.3.

The subjectdefines 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 WriterGroupor DataSetWriterthrough the KeyValuePairarray in the WriterGroupPropertiesand DataSetWriterProperties. The NamespaceIndexof the QualifiedNamein the KeyValuePairshall be 0 for AMQP standard message properties. The Nameof the QualifiedNameis constructed from a message prefix and the AMQP property name with the following syntax.

Name = message-<AMQP property name>

Table 164defines the AMQP standard message properties.

Table 164– 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 DataSetWriterPropertiesto the current time of the DataSetMessagecreation.

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 DataSetMessagecreation if the message-creation-time (Boolean) in the DataSetWriterPropertiesis 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 QualifiedNameshall be the ApplicationUri.

The AMQP message header shall include additional promoted fields of the DataSetas a list of name-value pairs. DataSetfields with the PromotedFieldflag set in the FieldMetaData fieldFlagsare copied into the AMQP header. The FieldMetaData Structureis defined in 6.2.3.2.3. Promoted fields shall always be included in the header even if the DataSetMessagebody is a delta frame and the DataSetfield 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 165. If there is no rule defined for the data type, the field shall not be included.

Table 165– 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 7.2.2.6.4.7.

The corresponding MIME type is application/json.

A UADP body is encoded as defined for the UADP message mapping defined in 7.2.2.

The corresponding MIME type is application/opcua+uadp.

If the encoded AMQP message size exceeds the Brokerlimits it shall be broken into multiple chunks as described in 7.2.2.4.4.

It is recommended to create the MetaDataQueueNameas described in 6.4.2.5.5by 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 Brokerthat relays messages between applications that cannot communicate directly.

Publisherssend MQTT messages to MQTT brokers. Subscriberssubscribe to MQTT brokers for messages. A Brokermay persist messages so they can be delivered even if the Subscriberis not online. Brokersmay 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 DataSetMessagedefined in 7.2.2and a JSON encoded DataSetMessagedefined in 7.2.2.6.4.7.

MQTT version 3.1.1 does not provide a mechanism for specifying the encoding of the MQTT message which means the Subscribersneed to be configured in advance with knowledge of the expected encoding. As a consequence, Publishersshould only publish NetworkMessagesusing 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 Publisheror Subscriberand 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 NetworkMessagesand binary message encoding defined in 7.2.2. 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 Addressparameter defined in 6.2.7.3has 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 AuthenticationProfileUriof the PubSubConnection, DataSetWriterGroup, DataSetWriteror DataSetReaderentities.

If no authentication information is provided in the form of ResourceUriand 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 does only support the connection property ClientID using a KeyValuePair. Any other configured setting in the connection properties shall be silently discarded.

MQTT version 5.0 allows Publishersand Subscribersto 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 WriterGroupand the DataSetWriter.

The properties are defined through the KeyValuePairarray in the ConnectionProperties. The NamespaceIndexof the QualifiedNamein the KeyValuePairshall be 0. The Nameof the QualifiedNameis constructed from a prefix “connection” followed by a hyphen and the MQTT property name with the following syntax.

Name = connection-<MQTT property name>

Table 166defines the MQTT standard connection properties.

Table 166– 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 166is 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 Encodingrules in OPC 10000-6.

The BrokerTransportQualityOfServicevalues 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 KeepAliveTimeis 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 WriterGroupsare configured, the group with the highest KeepAliveTimesetting 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.

The default setting for the MQTT RETAIN flag is false, except for metadata messages published to the MetaDataQueueNameas described in 6.4.2.5.6.

The MQTT version 3.1.1 protocol does not support message headers. Any promoted field or additional fields defined on the WriterGroupor DataSetWriterother 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 PubSubproperties not directly supported by the MQTT protocol.

Table 167describes how these properties shall be populated when a MQTT version 5.0 message is constructed.

Table 167– 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 7.3.5.8.2and 7.3.5.8.3.

The MQTT message header shall include additional fields defined on the WriterGroupor DataSetWriter through theKeyValuePair array in theWriterGroupProperties andDataSetWriterProperties. TheNamespaceIndex of theQualifiedName in theKeyValuePair shall be 0. The Nameof the QualifiedNameis 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 168defines the MQTT standard message properties.

Table 168– OPC UA MQTT standard message property configuration

MQTT property name

OPC UA DataTypes

MQTT data types

Description

RETAIN

Boolean

RETAIN bit in the header

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 168is 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 Encodingrules 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 DataSetas a list of MQTT User Property name-value pairs. DataSetfields with the PromotedFieldflag set in the FieldMetaData fieldFlags are copied into the MQTT header. TheFieldMetaData Structure is defined in 6.2.3.2.3. 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 theDataSetMessage body is a delta frame and the DataSetfield 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 UAMessageTypeproperty 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 7.2.2.6.4.7.

When sending a MQTT Version 5.0 message the MQTT ContentTypeproperty 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 7.2.2.

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

If the encoded MQTT message size exceeds the Brokerlimits, it is broken into multiple chunks as described in 7.2.2.4.4.

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

It is recommended that the MetaDataQueueNameas described in 6.4.2.5.5is configured as a sub-topic of the related QueueNamewith the name “$Metadata”. The MQTT RETAIN flag shall be set for metadata messages.