UA Part 14: PubSub - 7.2 Message Mappings

Errata exists for this version of the document.

Message mappings specify a specific structure and encoding for NetworkMessages. Such a structure represents the payload for transport protocol mappings like UDP, MQTT or AMQP.

Different mappings are defined for different use cases.

7.2.2 UADP Message Mapping

7.2.2.1 General

The UADP message mapping uses optimized UA Binary encoding and provides message security for OPC UA PubSub. The available protocol mappings are defined in 7.3.

The UADP message mapping defines different optional header fields, variations of field settings and different message types and data encodings.

A Publisher shall support all variations it allows through configuration. The required set of features is defined through profiles in OPC 10000-7.

A Subscriber shall be able to process all possible NetworkMessages and shall be able to skip information the Subscriber is not interested in. The Subscriber may not support all security policies. The capabilities related to processing different DataSet encodings is defined in OPC 10000-7.

7.2.2.2 NetworkMessage

7.2.2.2.1 General

The UADP NetworkMessage header and other parts of the NetworkMessage are shown in Figure 27.

When using security, the payload and the Padding field are encrypted and after that, the whole NetworkMessage is signed if signing and encryption is active. The NetworkMessage shall be signed without being encrypted if only the signing is active.

Figure 27 – UADP NetworkMessage

7.2.2.2.2 NetworkMessage Layout

The encoding of the UADP NetworkMessage is specified in Table 73.

The NetworkMessageContentMask setting of the Publisher controls the flags in the fields UADPFlags and ExtendedFlags1. The SecurityMode setting of the Publisher controls the security enabled flag of the ExtendedFlags1. The setting of the flags shall not change until the configuration of the Publisher is changed.

Table 73 – UADP NetworkMessage

Name	Type	Description
UADPVersion	Bit[0-3]	Bit range 0-3: Version of the UADP NetworkMessage. The UADPVersion for this specification version is 1.
UADPFlags	Bit[4-7]	Bit 4: PublisherId enabled If the PublisherId is enabled, the type of PublisherId is indicated in the ExtendedFlags1 field. Bit 5: GroupHeader enabled Bit 6: PayloadHeader enabled Bit 7: ExtendedFlags1 enabledThe bit shall be false, if ExtendedFlags1 is 0.
ExtendedFlags1	Byte	The ExtendedFlags1 shall be omitted if bit 7 of the UADPFlags is false. If the field is omitted, the Subscriber shall handle the related bits as false. Bit range 0-2: PublisherId Type 000 The PublisherId is of DataType Byte This is the default value if ExtendedFlags1 is omitted 001 The PublisherId is of DataType UInt16 010 The PublisherId is of DataType UInt32 011 The PublisherId is of DataType UInt64 100 The PublisherId is of DataType String 101 Reserved 11x Reserved 111 Reserved Bit 3: DataSetClassId enabled Bit 4: Security enabled If the SecurityMode is SIGN_1 or SIGNANDENCRYPT_2, this flag is set, message security is enabled and the SecurityHeader is contained in the NetworkMessage header. If this flag is not set, the SecurityHeader is omitted. Bit 5: Timestamp enabled Bit 6: PicoSeconds enabled Bit 7: ExtendedFlags2 enabled The bit shall be false, if ExtendedFlags2 is 0.
ExtendedFlags2	Byte	The ExtendedFlags2 shall be omitted if bit 7 of the ExtendedFlags1 is false. If the field is omitted, the Subscriber shall handle the related bits as false. Bit 0: Chunk message defined in in 7.2.2.2.4. Bit 1: PromotedFields enabled Promoted fields can only be sent if the NetworkMessage contains only one DataSetMessage. Bit range 2-4: UADP NetworkMessage type 000 NetworkMessage with DataSetMessage payload defined in 7.2.2.2.4. If the ExtendedFlags2 field is not provided, this is the default NetworkMessage type. 001 NetworkMessage with discovery request payload defined in 7.2.2.3.4. 010 NetworkMessage with discovery response payload defined in 7.2.2.4.2. 011 Reserved 1xxReserved Bit 5: Reserved Bit 6: Reserved Bit 7: Reserved for further extended flag fields
PublisherId	Byte[*]	The PublisherId shall be omitted if bit 4 of the UADPFlags is false. The Id of the Publisher that sent the data. Valid DataTypes are UInteger and String. The DataType is indicated by bits 0-2 of the ExtendedFlags1. A Subscriber can skip NetworkMessages from Publishers it does not expect NetworkMessages from.
DataSetClassId	Guid	The DataSetClassId associated with the DataSets in the NetworkMessage. All DataSetMessages in the NetworkMessage shall have the same DataSetClassId. The DataSetClassId shall be omitted if bit 3 of the ExtendedFlags1 is false.
GroupHeader		The group header shall be omitted if bit 5 of the UADPFlags is false.
GroupFlags	Byte	Bit 0: WriterGroupId enabled Bit 1: GroupVersion enabled Bit 2: NetworkMessageNumber enabled Bit 3: SequenceNumber enabled Bits 4-6: Reserved Bit 7: Reserved for further extended flag fields
WriterGroupId	UInt16	Unique id for the WriterGroup in the Publisher. A Subscriber can skip NetworkMessages from WriterGroups it does not expect NetworkMessages from. This field shall be omitted if bit 0 of the GroupFlags is false.
GroupVersion	VersionTime	Version of the header and payload layout configuration of the NetworkMessages sent for the group. This field shall be omitted if bit 1 of the GroupFlags is false.
NetworkMessage Number	UInt16	Unique number of a NetworkMessage across the combination of PublisherId and WriterGroupId within one PublishingInterval. The number is needed if the DataSetMessages for one group are split into more than one NetworkMessage in a PublishingInterval. The value 0 is invalid. This field shall be omitted if bit 2 of the GroupFlags is false.
SequenceNumber	UInt16	Sequence number for the NetworkMessage. This field shall be omitted if bit 3 of the GroupFlags is false.
PayloadHeader	Byte [*]	The payload header depends on the UADP NetworkMessage Type flags defined in the ExtendedFlags2 bit range 0-3. The default is DataSetMessage if the ExtendedFlags2 field is not enabled. The PayloadHeader shall be omitted if bit 6 of the UADPFlags is false. The PayloadHeader is not contained in the payload but it is contained in the unencrypted NetworkMessage header since it contains information necessary to filter DataSetMessages on the Subscriber side.
Timestamp	DateTime	The time the NetworkMessage was created. The Timestamp shall be omitted if bit 5 of ExtendedFlags1 is false. The PublishingInterval, the SamplingOffset the PublishingOffset and the Timestamp and PicoSeconds in the NetworkMessage header shall use the same time base.
PicoSeconds	UInt16	Specifies the number of 10 picoseconds (1,0 e-11 seconds) intervals which shall be added to the Timestamp. The PicoSeconds shall be omitted if bit 6 of ExtendedFlags1 is false.
PromotedFields		The PromotedFields shall be omitted if bit 4 of the ExtendedFlags2 is false. If the PromotedFields are provided, the number of DataSetMessages in the Network Message shall be one.
Size	UInt16	Total size in Bytes of the Fields contained in the PromotedFields.
Fields	BaseDataType[ ]	Array of promoted fields. The size, order and DataTypes of the fields depend on the settings in the FieldMetaData of the DataSetMetaData associated with the DataSetMessage contained in the NetworkMessage.
SecurityHeader		The security header shall be omitted if bit 4 of the ExtendedFlags1 is false.
SecurityFlags	Byte	Bit 0: NetworkMessage Signed Bit 1: NetworkMessage Encrypted Bit 2: SecurityFooter enabled Bit 3: Force key reset This bit is set if all keys will be made invalid. It is set until the new key is used. The publisher must give subscribers a reasonable time to request new keys. The minimum time is five times the KeepAliveTime configured for the corresponding PubSub group. This flag is typically set if all keys are invalidated to exclude Subscribers, that no longer have access to the keys. Bit range 4-7: Reserved
SecurityTokenId	IntegerId	The ID of the security token that identifies the security key in a SecurityGroup. The relation to the SecurityGroup is done through DataSetWriterIds contained in the NetworkMessage.
NonceLength	Byte	The length of the Nonce used to initialize the encryption algorithm.
MessageNonce	Byte [NonceLength]	A number used exactly once for a given security key. For a given security key a unique nonce shall be generated for every NetworkMessage. The rules for constructing the MessageNonce are defined for the UADP Message Security in 7.2.2.2.3.
SecurityFooterSize	UInt16	The size of the SecurityFooter. The security footer size shall be omitted if bit 2 of the SecurityFlags is false.
Payload	Byte [*]	The payload depends on the UADP NetworkMessage Type flags defined in the ExtendedFlags2 bit range 2-5.
SecurityFooter	Byte [*]	Optional security footer shall be omitted if bit 2 of the SecurityFlags is false. The content of the security footer is defined by the SecurityPolicy.
Signature	Byte [*]	The signature of the NetworkMessage.

7.2.2.2.3 UADP Message Security

7.2.2.2.3.1 General

The algorithm and nonce length used of the UADP NetworkMessage security depend on the selected SecurityPolicy. They are defined by SymmetricPubSubEncryptionAlgorithm and SymmetricPubSubNonceLength.

The keys used to encrypt and sign messages are returned from the GetSecurityKeys method (see 8.4). This Method returns a sequence of random data with a length that depends on the SecurityPolicyUri, which is also returned by the Method. The layout of the random data is defined in Table 74.

Table 74 – Layout of the key data for UADP message security

Name	Type	Description
SigningKey	Byte [SymmetricSignatureAlgorithm Key Length]	Signing key part of the key data returned from GetSecurityKeys. The SymmetricSignatureAlgorithm is defined in the SecurityPolicy.
EncryptingKey	Byte [SymmetricEncryptionAlgorithm KeyLength]	Encryption key part of the key data returned from GetSecurityKeys. The SymmetricEncryptionAlgorithm is defined in the SecurityPolicy.
KeyNonce	Byte [SymmetricPubSubNonceLength]	Nonce part of the key data returned from GetSecurityKeys.

7.2.2.2.3.2 AES-CTR

The layout of the MessageNonce for AES-CTR mode is defined in Table 75.

Table 75 – Layout of the MessageNonce for AES-CTR

Name	Type	Description
Random	Byte [4]	The random part of the MessageNonce. This number does not need to be a cryptographically random number, it can be pseudo-random.
SequenceNumber	UInt32	A strictly monotonically increasing sequence number assigned by the publisher to each NetworkMessage sent for a SecurityTokenId and PublisherId combination. The sequence number is reset to 1 after the key and SecurityTokenId are updated in the Publisher. A receiver should ignore older NetworkMessages than the last sequence processed if it does not handle reordering of NetworkMessages. Receivers need to be aware of sequence numbers roll over (change from 4294967295 to 0). To determine whether a received NetworkMessages is newer than the last processed NetworkMessages the following formula shall be used: (4294967295 + received sequence number – last processed sequence number) modulo 4294967296. Results below 1073741824 indicate that the received NetworkMessages is newer than the last processed NetworkMessages and the received NetworkMessages is processed. Results above 3221225472 indicate that the received message is older (or same) than the last processed NetworkMessages and the received NetworkMessages should be ignored if reordering of NetworkMessages is not necessary. Other results are invalid and the NetworkMessages shall be ignored. The key lifetime should be selected in a way that a new key is used before a rollover for the SequenceNumber happens. Subscribers shall reset the records they keep for sequence numbers if they do not receive messages for two times the keep alive time to deal with Publishers that are out of service and were not able to continue from the last used SequenceNumber.

The message encryption and decryption with AES-CTR mode uses a secret and a counter block. The secret is the EncryptingKey from the key data defined in Table 74. The layout and content of the counter block is defined in Table 76.

Table 76 – Layout of the counter block for UADP message security

Name	Type	Description
KeyNonce	Byte [4]	The KeyNonce portion of the key data returned from GetSecurityKeys.
MessageNonce	Byte [8]	The first 8 bytes of the Nonce in the SecurityHeader of the NetworkMessage. For AES-CTR mode the length of the SecurityHeader Nonce shall be 8 Bytes.
BlockCounter	Byte [4]	The counter for each encrypted block of the NetworkMessage. The counter is a 32-bit big endian integer (the opposite of the normal encoding for UInt32 values in OPC UA. This convention comes from the AES-CTR RFC). The counter starts with 0 at the first block. The counter is incremented by 1 for each block.

AES-CTR mode takes the counter block and encrypts it using the encrypting key. The encrypted key stream is then logically XORed with the data to encrypt or decrypt. The process is repeated for each block in the plain text. No padding is added to the end of the plain text. AES-CTR does not change the size of the plain text data and can be applied directly to a memory buffer containing the message.

The signature is calculated on the entire NetworkMessage including any encrypted data. The signature algorithm is specified by the SecurityPolicyUri in OPC 10000-7.

When a Subscriber receives a NetworkMessage, it shall verify the signature first. If verification fails, it drops the NetworkMessage.

Other SecurityPolicy may specify different key lengths or cryptography algorithms.

7.2.2.2.4 UADP Chunk NetworkMessage

If a NetworkMessage payload like a DataSetMessage or a discovery response message has to be split across multiple NetworkMessages the chunks are sent with the payload header defined in Table 77 and the payload defined in Table 78. A chunk NetworkMessage can only contain chunked payload of one DataSetMessage.

Table 77 – Chunked NetworkMessage Payload Header

Name	Type	Description
DataSetWriterId	UInt16	DataSetWriterId contained in the NetworkMessage. The DataSetWriterId identifies the PublishedDataSet and the DataSetWriter responsible for sending Messages for the DataSet. A Subscriber can skip DataSetMessages from DataSetWriters it does not expect DataSetMessages from. The DataSetWriterId shall be set to 0 for discovery response messages.

Table 78 – Chunked NetworkMessage Payload Fields

Name	Type	Description
MessageSequenceNumber	UInt16	Sequence number of the payload as defined for the NetworkMessage type like DataSetMessageSequenceNumber in a DataSetMessage. NetworkMessages may be received out of order. In this case, a chunk for the next payload can be received before the last chunk of the previous payload was received. If the next sequence number is received by a Subscribers that can handle only one payload, the chunks of the previous payload are skipped if they are not completely received yet.
ChunkOffset	UInt32	The byte offset position of the chunk in the complete NetworkMessage payload. The last chunk is received if ChunkOffset plus the size of the current chunk equals TotalSize. The reassembled NetworkMessage payload can be processed after all chunks are received.
TotalSize	UInt32	Total size of the NetworkMessage payload in bytes.
ChunkData	ByteString	The pieces of the original DataSetMessage, are copied into the chunk until the maximum size allowed for a single NetworkMessage is reached minus space for the signature. The data copied into next chunk starts with the byte after the last byte copied into current chunk. A DataSetMessage is completely received when all chunks are received and the DataSetMessage can be processed completely.

7.2.2.3 DataSetMessage

7.2.2.3.1 General

The UADP DataSet payload header and other parts of the NetworkMessage are shown in Figure 28.

Different types of DataSetMessage can be combined in on NetworkMessage.

Figure 28 – UADP DataSet Payload

7.2.2.3.2 DataSet Payload Header

The encoding of the UADP DataSet payload header is specified in Table 79. The payload header is unencrypted. This header shall be omitted if bit 6 of the UADPFlags is false.

Table 79 – UADP DataSet Payload Header

Name	Type	Description
Count	Byte	Number of DataSetMessages contained in the NetworkMessage. The NetworkMessage shall contain at least one DataSetMessages if the NetworkMessage type is DataSetMessage payload.
DataSetWriterIds	UInt16 [Count]	List of DataSetWriterIds contained in the NetworkMessage. The size of the list is defined by the Count. The DataSetWriterId identifies the PublishedDataSet and the DataSetWriter responsible for sending Messages for the DataSet. A Subscriber can skip DataSetMessages from DataSetWriters it does not expect DataSetMessages from.

7.2.2.3.3 DataSet Payload

The DataSet payload is defined in Table 80. The payload is encrypted.

Table 80 – UADP DataSet Payload

Name	Type	Description
Sizes	UInt16 [Count]	List of byte sizes of the DataSetMessages. The size of the list is defined by the Count in the DataSet payload header. If the payload size exceeds 65535, the DataSetMessages shall be allocated to separate NetworkMessages. If a single DataSetMessage exceeds the payload size it shall be split into Chunk NetworkMessages. This field shall be omitted if count is one or if bit 6 of the UADPFlags is false.
DataSetMessages	DataSetMessage [Count]	DataSetMessages contained in the NetworkMessage. The size of the list is defined by the Count in the DataSet payload header. The type of encoding used for the DataSetMessages is defined by the DataSetWriter. The encodings for the DataSetMessage are defined in 7.2.2.3.4.

7.2.2.3.4 DataSetMessage Header

The DataSetMessage header structure and the relation to other parts in a NetworkMessage is shown in Figure 29.

Figure 29 – DataSetMessage Header Structure

The encoding of the DataSetMessage header structure is specified in Table 81.

The DataSetFieldContentMask and the DataSetMessageContentMask settings of the DataSetWriter control the flags in the fields DataSetFlags1 and DataSetFlags2. The setting of the flags shall not change until the configuration of the DataSetWriter is changed.

Table 81 – DataSetMessage Header Structure

Name	Type	Description
DataSetFlags1	Byte	Bit 0: DataSetMessage is valid. If this bit is set to false, the rest of this DataSetMessage is considered invalid, and shall not be processed by the Subscriber. Bit range 1-2: Field Encoding 00 The DataSet fields are encoded as Variant The Variant can contain a StatusCode instead of the expected DataType if the status of the field is Bad. The Variant can contain a DataValue with the value and the statusCode if the status of the field is Uncertain. 01 RawData Field Encoding The DataSet fields are encoded in the DataTypes specified in the DataSetMetaData for the DataSet. The encoding is handled like a Structure DataType where the DataSet fields are handled like Structure fields and fields with Structure DataType are handled like nested structures. All restrictions for the encoding of Structure DataTypes also apply to the RawData Field Encoding. 10 DataValue Field Encoding The DataSet fields are encoded as DataValue. This option is set if the DataSet is configured to send more than the Value. 11 Reserved Bit 3: DataSetMessageSequenceNumber enabled Bit 4: Status enabled Bit 5: ConfigurationVersionMajorVersion enabled Bit 6: ConfigurationVersionMinorVersion enabled Bit 7: DataSetFlags2 enabled The bit shall be false, if DataSetFlags2 is 0.
DataSetFlags2	Byte	The DataSetFlags2 shall be omitted if bit 7 of the DataSetFlags1 is false. If the field is omitted, the Subscriber shall handle the related bits as false. Bit range 0-3: UADP DataSetMessage type 0000Data Key Frame (see 7.2.2.3.5) If the DataSetFlags2 field is not provided, this is the default DataSetMessage type. 0001Data Delta Frame (see 7.2.2.3.6) 0010Event (see 7.2.2.3.7) 0011Keep Alive (see 7.2.2.3.8) 01xxReserved 1xxxReserved Bit 4: Timestamp enabled Bit 5: PicoSeconds included in the DataSetMessage header Bit 6: Reserved Bit 7: Reserved for further extended flag fields
DataSetMessage‌SequenceNumber	UInt16	A strictly monotonically increasing sequence number assigned by the publisher to each DataSetMessage sent. A receiver should ignore older DataSetMessage than the last sequence processed if it does not handle reordering of DataSetMessages. Receivers need to be aware of sequence numbers roll over (change from 65535 to 0). To determine whether a received DataSetMessage is newer than the last processed DataSetMessage the following formula shall be used: (65535 + received sequence number – last processed sequence number) modulo 65536 Results below 16384 indicate that the received DataSetMessage is newer than the last processed DataSetMessage and the received DataSetMessage is processed. Results above 49162 indicate that the received message is older (or same) than the last processed DataSetMessage and the received DataSetMessage should be ignored if reordering of DataSetMessages is not necessary. Other results are invalid and the DataSetMessage shall be ignored. The field shall be omitted if Bit 2 of DataSetFlags1 is false.
Timestamp	UtcTime	The time the Data was collected. The Timestamp shall be omitted if Bit 3 of DataSetFlags1 is false.
PicoSeconds	UInt16	Specifies the number of 10 picoseconds (1,0 e-11 seconds) intervals which shall be added to the Timestamp. The field shall be omitted if Bit 4 of DataSetFlags2 is false.
Status	UInt16	The overall status of the DataSet. This is the high order 16 bits of the StatusCode DataType representing the numeric value of the Severity and SubCode of the StatusCode DataType. The field shall be omitted if Bit 4 of DataSetFlags1 is false.
ConfigurationVersion MajorVersion	VersionTime	The major version of the configuration version of the DataSet used as consistency check with the DataSetMetaData available on the Subscriber side. The field shall be omitted if Bit 5 of DataSetFlags1 is false.
ConfigurationVersion MinorVersion	VersionTime	The minor version of the configuration version of the DataSet used as consistency check with the DataSetMetaData available on the Subscriber side. The field shall be omitted if Bit 6 of DataSetFlags1 is false.

7.2.2.3.5 Data Key Frame DataSetMessage

The data key frame DataSetMessage data and related headers are shown in Figure 30.

Figure 30 – Data Key Frame DataSetMessage Data

The encoding of the data key DataSetMessage structure is specified in Table 82.

Table 82 – Data Key Frame DataSetMessage Structure

Name	Type	Description
FieldCount	UInt16	Number of fields of the DataSet contained in the DataSetMessage. The FieldCount shall be omitted if RawData field encoding is set in the EncodingFlags defined in 7.2.2.3.4.
DataSetFields	BaseDataType[]	The field values of the DataSet. The field encoding depends on the EncodingFlags of the DataSetMessage Header defined in 7.2.2.3.4. The default encoding is Variant if bit 0 and 1 are not set.

7.2.2.3.6 Data Delta Frame DataSetMessage

The data delta frame DataSetMessage data and the related headers are shown in Figure 31.

Figure 31 – Data Delta Frame DataSetMessage

The information for a single value in delta frame messages is larger because of the additional index necessary for sending just changed data. The Publisher shall send a key frame message if the delta frame message is larger than a key frame message.

The encoding of the data delta frame DataSetMessage structure is specified in Table 83.

Table 83 – Data Delta Frame DataSetMessage Structure

Name	Type	Description
FieldCount	UInt16	Number of fields of the DataSet contained in the DataSetMessage.
DeltaFrameFields	Structure[]	The subset of field values of the DataSet contained in the delta frame.
FieldIndex	UInt16	The index of the Field in the DataSet. The index is based on the field position in the DataSetMetaData with the configuration version defined in the ConfigurationVersion field.
FieldValue	BaseDataType	The field values of the DataSet. The field encoding depends on the EncodingFlags of the DataSetMessage Header defined in 7.2.2.3.4. The default encoding is Variant if bit 2 and 3 are not set.

7.2.2.3.7 Event DataSetMessage

The Event DataSetMessage data and the related headers are shown in Figure 32.

Figure 32 – Event DataSetMessage

The encoding of the Event DataSetMessage structure is specified in Table 84.

Table 84 – Event DataSetMessage Structure

Name	Type	Description
FieldCount	UInt16	Number of fields of the DataSet contained in the DataSetMessage.
DataSetFields	BaseDataType[]	The field values of the DataSet. The fields of Event DataSetMessages shall be encoded as Variant. The Field Encoding DataSetFlags1 of the DataSetMessage header (bit 1 and 2) defined in 7.2.2.3.4 shall be set to false.

7.2.2.3.8 KeepAlive Message

The keep alive message does not add any additional fields. The message and the related headers are shown in Figure 33.

Figure 33 – KeepAlive Message

The sequence number contains the next expected sequence number for the DataSetWriter.

7.2.2.4 Discovery Messages

7.2.2.4.1 UADP Discovery Request NetworkMessage

7.2.2.4.1.1 General

The NetworkMessage flags used with the discovery request messages shall use the following bit values.

UADPFlags bits 5 and 6 shall be false, bits 4 and 7 shall be true
ExtendedFlags1 bits 3, 5 and 6 shall be false, bits 4 and 7 shall be true
ExtendedFlags2 bit 2 shall be true, all other bits shall be false

The setting of the flags ensures a known value for the first five fields in the NetworkMessage on the Publisher as receiver. The actual security settings for the NetworkMessage are indicated by the SecurityHeader.

7.2.2.4.1.2 Traffic Reduction

A variety of rules are used to reduce the amount of traffic on the network in the case of multicast or broadcast communication.

A Subscriber should cache configuration information for PublisherId and DataSetWriterIds of interest.

If a Subscriber requires information from Publishers after a startup or version change detection, discovery requests shall be randomly delayed in the range of 100-500 ms. The request shall be skipped if the information is already received during this time or another Subscriber sent already a request and the response to this request is used.

Discovery requests for different DataSetWriters in one WriterGroup shall be aggregated into one discovery response.

A Publisher shall delay subsequent responses for a combination of request type and identifier like the DataSetWriterId for at least 500 ms. Duplicate requests, that have not yet been responded to, shall be discarded by the Publisher.

A Subscriber shall wait for a response at least 500 ms. As long as not all responses are received, the Subscriber requests the missing information. It shall double the time period between follwing requests until all needed response are received or denied.

7.2.2.4.1.3 Discovery Request Header

The encoding of the discovery request header structure is specified in Table 85.

Table 85 – Discovery Request Header Structure

Name	Type	Description
RequestType	Byte	The following types of discovery request messages are defined. 0Reserved 1 Publisher information request message (see 7.2.2.4.1.4)

7.2.2.4.1.4 Publisher Information Request Message

The encoding of the Publisher information request message structure is specified in Table 86.

Table 86 – Publisher Information Request Message Structure

Name	Type	Description
InformationType	Byte	The following types of Publisher information requests are defined. 0Reserved 1 Publisher Server Endpoints 2 DataSetMetaData 3 DataSetWriter configuration
DataSetWriterIds	UInt16[]	List of DataSetWriterIds the information is requested for. If the request is not related to DataSet, the array shall be null.

7.2.2.4.2 UADP Discovery Response NetworkMessage

7.2.2.4.2.1 General

The NetworkMessage flags used with the discovery response messages shall use the following bit values.

UADPFlags bits 5 and 6 shall be false, bits 4 and 7 shall be true
ExtendedFlags1 bits 3, 5 and 6 shall be false, bit 7 shall be true
ExtendedFlags2 bit 1 shall be false and the NetworkMessage type shall be discovery response

The setting of the flags ensures a known value for the first five fields in the NetworkMessage for Publishers expected by the Subscriber. The actual security settings for the NetworkMessage are indicated by the SecurityHeader.

7.2.2.4.2.2 Discovery Response Header

The encoding of the discovery response header structure is specified in Table 87.

Table 87 – Discovery Response Header Structure

Name	Type	Description
ResponseType	Byte	The following types of discovery response messages are defined. 0Reserved 1Publisher Endpoint message (see 7.2.2.4.2.3) 2DataSet Metadata message (see 7.2.2.4.2.4) 3DataSetWriter configuration message (see 7.2.2.4.2.5)
SequenceNumber	UInt16	A strictly monotonically increasing sequence number assigned to each discovery response sent in the scope of a PublisherId.

7.2.2.4.2.3 Publisher Endpoints Message

The encoding of the available Endpoints of a Publisher is specified in Table 88.

Table 88 – Publisher Endpoints Message Structure

Name	Type	Description
Endpoints	EndpointDescription[]	The OPC UA Server Endpoints of the Publisher. The EndpointDescription is defined in OPC 10000-4.
statusCode	StatusCode	Status code indicating the capability of the Publisher to provide Endpoints.

7.2.2.4.2.4 DataSetMetaData Message

The encoding of the DataSet metadata message structure is specified in Table 89. It contains the current layout and DataSetMetaData for the DataSet.

The ConfigurationVersion in the DataSetMessage header shall match the ConfigurationVersion in the DataSetMetaData.

The Publisher shall send this message without a corresponding discovery request if the DataSetMetaData changed for the DataSet.

Table 89 – DataSetMetaData Message Structure

Name	Type	Description
DataSetWriterId	UInt16	DataSetWriterId of the DataSet described with the MetaData.
MetaData	DataSetMetaDataType	The current DataSet metadata for the DataSet related to the DataSetWriterId. The DataSetMetaDataType is defined in 6.2.2.1.2.
statusCode	StatusCode	Status code indicating the capability of the Publisher to provide MetaData for the DataSetWriterId.

7.2.2.4.2.5 DataSetWriter Configuration Message

The encoding of the DataSetWriter configuration data message structure is specified in Table 90. It contains the current configuration of the WriterGroup and the DataSetWriter for the DataSet.

The Publisher shall send this message without a corresponding discovery request if the configuration of the WriterGroup changed.

Table 90 – DataSetWriter Configuration Message Structure

Name	Type	Description
DataSetWriterIds	UInt16[]	DataSetWriterIds contained in the configuration information.
DataSetWriterConfig	WriterGroupDataType	The current WriterGroup and DataSetWriter settings for the DataSet related to the DataSetWriterId. The WriterGroupDataType is defined in 6.2.5.6. The field DataSetWriters of the WriterGroupDataType shall contain only the entry for the requested or changed DataSetWriters in the WriterGroup.
statusCodes	StatusCode[]	Status codes indicating the capability of the Publisher to provide configuration information for the DataSetWriterIds. The size of the array shall match the size of the DataSetWriterIds array.

7.2.3 JSON Message Mapping

7.2.3.1 General

JSON is a format that uses human readable text. It is defined in RFC 7159.

The JSON based message mapping allows OPC UA Applications to interoperate with web and enterprise software that use this format.

7.2.3.2 NetworkMessage

Each JSON NetworkMessage contains one or more JSON DataSetMessages. The JSON NetworkMessage is a JSON object with the fields defined in Table 91.

Table 91 – JSON NetworkMessage Definition

Name	Type	Description
MessageId	String	A globally unique identifier for the message. This value is mandatory.
MessageType	String	This value shall be “ua-data”. This value is mandatory.
PublisherId	String	A unique identifier for the Publisher. It identifies the source of the message. This value is optional. The presence of the value depends on the setting in the JsonNetworkMessageContentMask. The source is the PublisherId on a PubSubConnection (see 6.2.6.1).
DataSetClassId	String	The DataSetClassId associated with the DataSets in the NetworkMessage. This value is optional. The presence of the value depends on the setting in the JsonNetworkMessageContentMask. If specified, all DataSetMessages in the NetworkMessage shall have the same DataSetClassId. The source is the DataSetClassId on the PublishedDataSet (see 6.2.2.2) associated with the DataSetWriters that produced the DataSetMessages.
Messages	*	A JSON array of JSON DataSetMessages (see 7.2.3.3). This value is mandatory.

All fields with a concrete DataType defined are encoded using reversible OPC UA JSON Data Encoding defined in OPC 10000-6.

The fields in the JSON NetworkMessage are controlled by the NetworkMessageContentMask of the JSON NetworkMessage mapping(see 6.3.2.1.1).

If the NetworkMessageHeader bit of the NetworkMessageContentMask is not set, the NetworkMessage is the contents of the Messages field (e.g. a JSON array of DataSetMessages).

If the DataSetMessageHeader bit of the NetworkMessageContentMask is not set, the content of the Messages field is an array of content from the Payload field for each DataSetMessage (see 7.2.3.3).

If the SingleDataSetMessage bit of the NetworkMessageContentMask is set, the content of the Messages field is a JSON object containing a single DataSetMessage.

If the NetworkMessageHeader and the DataSetMessageHeader bits are not set and SingleDataSetMessage bit is set, the NetworkMessage is a JSON object containing the set of name/value pairs defined for a single DataSet.

If the JSON encoded NetworkMessage size exceeds the Broker limits the message is dropped and a PubSubTransportLimitsExceeded Event is reported.

7.2.3.3 DataSetMessage

A DataSetMessage is produced by a DataSetWriter and contains list of name/value pairs which are specified by the PublishedDataSet associated with the DataSetWriter. The contents of the DataSetMessage are formally described by a DataSetMetData Objects. A DataSetMessage is a JSON object with the fields defined in Table 92.

DataSetWriters may periodically provide keep-alive messages which are DataSetMessages without any Payload field.

Table 92 – JSON DataSetMessage Definition

Name	Type	Description
DataSetWriterId	String	An identifier for DataSetWriter which created the DataSetMessage. This value is mandatory. It is unique within the scope of a Publisher.
SequenceNumber	UInt32	A strictly monotonically increasing sequence number assigned to the DataSetMessage by the DataSetWriter. This value is optional. The presence of the value depends on the setting in the JsonDataSetMessageContentMask.
MetaDataVersion	ConfigurationVersionDataType	The version of the DataSetMetaData which describes the contents of the Payload. This value is optional. The presence of the value depends on the setting in the JsonDataSetMessageContentMask.
Timestamp	DateTime	A timestamp which applies to all values contained in the DataSetMessage. This value is optional. The presence of the value depends on the setting in the JsonDataSetMessageContentMask.
Status	StatusCode	A status code which applies to all values contained in the DataSetMessage. This value is optional. The presence of the value depends on the setting in the JsonDataSetMessageContentMask.
Payload	Object	A JSON object containing the name-value pairs specified by the PublishedDataSet. The format of the value depends on the DataType of the field and the flags specified by the DataSetMessageContentMask.

All fields with a concrete DataType are encoded using reversible OPC UA JSON Data Encoding defined in OPC 10000-6.

The fields in the DataSetMessage are specified by the DataSetFieldContentMask in the DataSetWriter parameters.

DataSetFieldContentMask specifies the format of the field values in the Payload according to the following rules:

If the DataSetFieldContentMask results in a RawData representation, the field value is a Variant encoded using the non-reversible OPC UA JSON Data Encoding defined in OPC 10000-6.
If the DataSetFieldContentMask results in a DataValue representation, the field value is a DataValue encoded using the non-reversible OPC UA JSON Data Encoding defined in OPC 10000-6.
If the DataSetFieldContentMask results in a Variant representation, the field value is encoded as a Variant encoded using the reversible OPC UA JSON Data Encoding defined in OPC 10000-6.

7.2.3.4 Discovery Messages

7.2.3.4.1 General

The JSON message mapping defines only one optional discovery message for the exchange of the DataSetMetaData. The main purpose is the exchange of additional information not contained in the DataSetMessages like Properties for the DataSet fields.

7.2.3.4.2 DataSetMetaData

DataSetMetaData describe the content a DataSet published by a DataSetWriter. More specifically, it specifies the names and data types of the values that shall appear in the Payload of a DataSetMessage.

When the DataSetMetaData of a DataSet changes the, DataSetWriter may be configured to publish the updated value through the mechanism defined by the transport protocol mapping.

The DataSetWriterId and Version fields in a DataSetMessage are used to correlate a DataSetMessage with a DataSetMetaData.

A DataSetMetaData is a JSON object with the fields defined in Table 93.

Table 93 – JSON DataSetMetaData Definition

Name	Type	Description
MessageId	String	A globally unique identifier for the message. This value is mandatory.
MessageType	String	This value shall be “ua-metadata”. This value is mandatory.
PublisherId	String	A unique identifier for the Publisher. It identifies the source of the message. This value is mandatory.
DataSetWriterId	UInt16	An identifier for DataSetWriter which published the DataSetMetaData. This value is mandatory. It is unique within the scope of a Publisher.
MetaData	DataSetMetaDataType	The metadata as defined in 6.2.2.1.2. This value is mandatory.

All fields with a concrete DataType are encoded using reversible OPC UA JSON Data Encoding defined in OPC 10000-6.