In some situations Clientswill not know which hosts have Servers. In these situations the Clientwill look for a LocalDiscoveryServerwith the MulticastExtensionon its local host and requests a list of DiscoveryUrlsfor Serversand DiscoveryServersavailable on the MulticastSubnet.

The discovery process for this scenario is illustrated in Figure 4.

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Figure 4– The MulticastSubnet Discovery Process

In this scenario the Serveruses the RegisterServer2 Serviceto tell a LocalDiscoveryServerto announce the Serveron the MulticastSubnet. The Clientwill receive the DiscoveryUrland CapabilityIdentifiersfor the Server when it calls FindServersOnNetworkand then connects directly to the Server. When a Clientcalls FindServersit only receives the Serversrunning on the same host as the LDS.

Clientsrunning on embedded systems may not have a LDS-ME available on the system, These Clientscan support an mDNS Responder which understands how OPC UA concepts are mapped to mDNS messages and maintains the same table of servers as maintained by the LDS-ME. This mapping is described in Annex C.