The configuration of the DataSetWriters in a WriterGroup can result in a fixed NetworkMessage layout where all DataSets have a static position between NetworkMessages.

In this case the parameters NetworkMessageNumber and DataSetOffset provide information about the static position of the DataSetMessage in a NetworkMessage Subscribers can rely on. If the value of one of the two parameters is 0, the position is not guaranteed to be static.

Note 1: A Publisher can only provide valid values for the parameters NetworkMessageNumber and DataSetOffset if the message mapping allows keeping the value for these Properties constant unless the configuration of the WriterGroup is changed.

The DataSetMessageContentMask defines the flags for the content of the DataSetMessage header. The UADP message mapping specific flags are defined by the UadpDataSetMessageContentMask DataType.

The UadpDataSetMessageContentMask DataType is formally defined in Table 55.

Table 55 – UadpDataSetMessageContentMask Values

Value	Bit No.	Description
Timestamp	0	If this flag is set, a timestamp shall be included in the DataSetMessage header.
PicoSeconds	1	If this flag is set, a PicoSeconds timestamp field shall be included in the DataSetMessage header. This flag is ignored if the HeaderTimestamp flag is not set.
Status	2	If this flag is set, the DataSetMessage status is included in the DataSetMessage header. The rules for creating the DataSetMessage status are defined in Table 16.
MajorVersion	3	If this flag is set, the ConfigurationVersion.MajorVersion is included in the DataSetMessage header.
MinorVersion	4	If this flag is set, the ConfigurationVersion.MinorVersion is included in the DataSetMessage header.
SequenceNumber	5	If this flag is set, the DataSetMessageSequenceNumber is included in the DataSetMessage header.

The UadpDataSetMessageContentMask representation in the AddressSpace is defined in Table 56.

Table 56 – UadpDataSetMessageContentMask Definition

Attributes	Value
BrowseName	UadpDataSetMessageContentMask
IsAbstract	False
References	NodeClass	BrowseName	DataType
Subtype of UInt32 defined in OPC 10000-5.
HasProperty	Variable	OptionSetValues	LocalizedText [ ]

The parameter ConfiguredSize with the DataType UInt16 defines the fixed size in bytes a DataSetMessage uses inside a NetworkMessage. The default value is 0 and it indicates a dynamic length. If a DataSetMessage would be smaller in size (e.g. because of the current values that are encoded) the DataSetMessage is padded with bytes with value zero. In case it would be larger, the Publisher shall set bit 0 of the DataSetFlags1 to false to indicate that the DataSetMessage is not valid.

Note 1 to entry: The parameter ConfiguredSize can be used for different reasons. One reason is the reservation of space inside a NetworkMessage by setting ConfiguredSize to a higher value than the assigned DataSet actually requires. Modifications (e.g. extensions) of the DataSet would then not change the required bandwidth on the network which reduces the risk of side effects. Another reason would be to maintain predictable network behaviour even when using a volatile field DataTypes like String or ByteString.

The parameter NetworkMessageNumber with the DataType UInt16 is a read-only parameter set by the Publisher in the case of a fixed NetworkMessage layout. The default value is 0 and indicates that the position of the DataSetMessage in a NetworkMessage is not fixed.

If the NetworkMessage layout is fixed and all DataSetMessages of a WriterGroup fit into one single NetworkMessage the value of NetworkMessageNumber shall be 1. If the DataSetMessages of a WriterGroup are distributed or chunked over more than one NetworkMessage the first NetworkMessage in a PublishingInterval shall be generated with the value 1, the following NetworkMessages shall be generated with incrementing NetworkMessageNumbers. To avoid a roll-over the number of NetworkMessages generated from one WriterGroup within one PublishingInterval is limited to 65535.

The parameter DataSetOffset with the DataType UInt16 is a read-only parameter set by the Publisher that specifies the offset in bytes inside a NetworkMessage at which the DataSetMessage is located, relative to the beginning of the NetworkMessage. The default value 0 indicates that the position of the DataSetMessage in a NetworkMessage is not fixed.

This Structure DataType is used to represent UADP DataSetMessage mapping specific DataSetWriter parameters. It is a subtype of the DataSetWriterMessageDataType defined in 6.2.3.5.3.

The UadpDataSetWriterMessageDataType is formally defined in Table 57.

Table 57 – UadpDataSetWriterMessageDataType Structure

Name	Type	Description
UadpDataSetWriterMessageDataType	Structure
dataSetMessageContentMask	UadpDataSetMessageContentMask	Defined in 6.3.1.2.2.
configuredSize	UInt16	Defined in 6.3.1.2.3.
networkMessageNumber	UInt16	Defined in 6.3.1.2.4.
dataSetOffset	UInt16	Defined in 6.3.1.2.5.