Clients benefit if they have more information about a Server before they connect, however, providing this information imposes a burden on the mechanisms used to discover Servers. The challenge is to find the right balance between the two objectives.
ServerCapabilityIdentifiers are the way this specification achieves the balance. These identifiers are short and map onto a subset of OPC UA features which are likely to be useful during the discovery process. The identifiers are short because of length restrictions for fields used in the mDNS specification. Table 71 is a non-normative list of possible identifiers.
Table 71 – Examples of ServerCapabilityIdentifiers
|NA||No capability information is available. Cannot be used in combination with any other capability.|
|DA||Provides current data.|
|HD||Provides historical data.|
|AC||Provides alarms and conditions that may require operator interaction.|
|HE||Provides historical alarms and events.|
|GDS||Supports the Global Discovery Server information model.|
|LDS||Only supports the Discovery Services. Cannot be used in combination with any other capability.|
|DI||Supports the Device Integration (DI) information model (see DI).|
|ADI||Supports the Analyser Device Integration (ADI) information model (see ADI).|
|FDI||Supports the Field Device Integration (FDI) information model (see FDI).|
|FDIC||Supports the Field Device Integration (FDI) Communication Server information model (see FDI).|
|PLC||Supports the PLCopen information model (see PLCopen).|
|S95||Supports the ISA95 information model (see ISA-95).|
|RCP||Supports the reverse connect capabilities defined in OPC 10000-6.|
|PUB||Supports the Publisher capabilities defined in OPC 10000-14.|
|SUB||Supports the Subscriber capabilities defined in OPC 10000-14.|
The normative set of ServerCapabilityIdentifiers can be found here:
Application developers shall always use the linked CSV.
Client applications that support the PUB or SUB capability can send or receive PubSub Messages but do not support the PubSub information model.
Client applications that support the RCP capability allow Servers to connect, however, they do not support GetEndpoints Service.