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

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

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:

http://www.opcfoundation.org/UA/schemas/1.04/ServerCapabilities.csv

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.