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.
CapabilityIdentifiers 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 115 is a non-normative list of possible identifiers.
Table 115 – Examples of CapabilityIdentifiers
|
Identifier |
Description |
|
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 |
The ApplicationType is DiscoveryServer. Only used by a standalone LDS implementation. |
|
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 |
Accepts reverse connect requests as defined in OPC 10000-6. |
|
PUB |
Supports the Publisher capabilities defined in OPC 10000-14. |
|
PSC |
Supports the PubSub Configuration model defined in OPC 10000-14. |
|
ALIAS |
Supports the Alias Names capabilities defined in OPC 10000-17. |
|
SKS |
Supports the Security Key Server (SKS) capabilities defined in OPC 10000-14. |
The normative set of CapabilityIdentifiers can be found here:
http://www.opcfoundation.org/UA/schemas/ServerCapabilities.csv
This CSV will be changed to meet the needs of companion specifications and will not trigger an update to this document. Application developers shall always use the linked CSV.
Applications that support the PUB capability can send PubSub Messages but may not support the PubSub information model.
Client applications that support the RCP capability allow Servers to connect, however, they do not support GetEndpoints Service.