In some situations Clients will not know which hosts have Servers. In these situations the Client will look for a LocalDiscoveryServer with the MulticastExtension on its local host and requests a list of DiscoveryUrls for Servers and DiscoveryServers available on the MulticastSubnet.
The discovery process for this scenario is illustrated in Figure 4.
Figure 4 – The MulticastSubnet Discovery Process
In this scenario the Server uses the RegisterServer2 Service to tell a LocalDiscoveryServer to announce the Server on the MulticastSubnet. The Client will receive the DiscoveryUrl and ServerCapabilityIdentifiers for the Server when it calls FindServersOnNetwork and then connects directly to the Server. When a Client calls FindServers it only receives the Servers running on the same host as the LDS.
Clients running on embedded systems may not have a LDS-ME available on the system, These Clients can 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.