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Regarding LDS-ME in OPC Discovery
November 24, 2020
11:04, EET
Forum Posts: 30
Member Since:
June 2, 2019
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I am curious to know about the multicast mechanism of discovery in OPC UA.
For the multicast subnet discovery, part 12 says it is useful when clients do not have an idea about the servers present on the network.
In that scenario, the server registers to the LDS-ME using registerServer2 service. Next, when client plugs itself in, the LDS-ME of the client issues a probe message on which the LDS-ME of the server responds with an announcement providing the list of ServerURL’s.

Having read the mDNS specification, I do not seem to get a clarity as to why is it referred as probing here.
Probing ,in mDNS terms is used to have a single authority over the various Resource records of a domain. Could you please tell me how is it probing here in OPC UA?
Also announcment somehow makes sensse, as to making the registered server available for discovery.

Below, i have summarised my understanding. Please correct me if I am wrong..
Looking into the Annexure(part 12), I see that mDNS uses the DNS SRV type query to check for the servers available, so will the probe message will be (Qtype = SRV, Qclass =IN, Qname= example.com), and in response, the server LDS-ME gives an announcement with the host and port information too, example of which is given as “_opcua-tcp._tcp.example.com. 86400 IN SRV 0 5 4840 uaserver.example.com.” in part 12, which ultimately gets mapped to the discoveryURL.

Does the mapping to URL happen in the findServersinNetwork method?
When does the mapping happen?

Extremely curious. Apologies if this is really basic. Please let me know. Would be really greatful.


November 25, 2020
13:19, EET
Bjarne Boström
Forum Posts: 895
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April 3, 2012
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For the time being, I would recommend you to ask this question on the OPC Foundation’s forum: https://opcfoundation.org/forum/.

As far as I’m aware, we basically just call RegisterServer2 on the LDS (Local Discovery Server) on the local machine by default, and it is upto the LDS impls to sync themselves (and we use RegisterServer if the “2” fails). And in practice we have used the LDS provided by the OPC Foundation so far, or that is to say, installing a LDS is outside the scope of our SDKs and Apps, it is something the user of apps would have to install themselves.

FindServersOnNetwork should return urls for which the normal discovery can be done: https://reference.opcfoundation.org/v104/Core/docs/Part4/5.4.3/

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