The clause describes how an application registers itself so it can be discovered. Most Applications will want other applications to discover them. Applications that do not wish to be discovered openly should not register with a DiscoveryServer. In this case such Applications should only publish a DiscoveryUrl via some out-of-band mechanism to be discovered by specific Applications.

Applications register themselves with the LDS on the same host if they wish to be discovered. The registration ensures that the applications is visible for local discovery (see 4.3.4) and MulticastSubnet discovery if the LDS is a LDS-ME (see 4.3.5).

The OPC UA Standard (OPC 10000-4) defines a RegisterServer2 Service which provides additional registration information. All Applications and LocalDiscoveryServer the shall support the RegisterServer2 Service and, for backwards compatibility, the older RegisterServer Service. If an Application encounters an older LDS that returns a Bad_ServiceUnsupported error when calling RegisterServer2 Service it shall try again with RegisterServer Service.

The RegisterServer2 Service allows the Application to specify zero or more ServerCapability Identifiers. ServerCapabilityIdentifiers are short, string identifiers of well-known OPC UA features. Applications can use these identifiers as a filter during discovery.

The set of known ServerCapabilityIdentifiers is specified in Annex D and is limited to features which are considered to be important enough to report before an application makes a connection. For example, support for the GDS information model or the Alarms information model are Server capabilities that have a ServerCapabilityIdentifier defined.

Before an application registers with the LDS it should call the GetEndpoints Service and choose the most secure endpoint supported by the LDS and then call RegisterServer2 or RegisterServer.

Registration with LDS or LDS-ME is illustrated in Figure 1.

Figure 1 – The Registration Process with an LDS

See OPC 10000-4 for more information on the re-registration timer and the IsOnline flag.

Dedicated systems (usually embedded systems) with exactly one Server installed may not have a separate LDS. Such Servers shall become their own LDS or LDS-ME by implementing FindServers and GetEndpoints Services at the well-known address for an LDS. They should also announce themselves on the MulticastSubnet with a basic MulticastExtension. This requires a small subset of an mDNS Responder (see mDNS and Annex C) that announces the Server and responds to mDNS probes. The Server may not provide the caching and address resolution implemented by a full mDNS Responder.