Traditional big web services, like a billing system or a service running in the cloud, nearly always have fixed addresses--often HTTP URLs.  Smaller web services, such as those on a PC or a device, often have logical identifiers ("urn:uuid:c3de740f-9227-4706-b3ee-9494243c040d") that let you recognize the service, but which don't actually tell you anything about how to contact the service.

One of the great uses of WS-Discovery is transforming logical addresses into physical addresses (e.g., http://my_pc/service/) so you can actually contact the service.  A service uses the xAddrs field in WS-Discovery messages to express its transport addresses.

But it turns out that these xAddrs are optional.  There are actually a lot of really good cases why you would want to omit them--if they're very large, or if you don't want to generate different Hello messages for every IP address in the system--but the specification doesn't clearly explain when they're often omitted in the real world, and what happens when they are.

So let's go through the four messages a service can send.

• Hello messages sometimes contain xAddrs.  On any given subnet it's likely to encounter Hello messages both with and without xAddrs.  Those without xAddrs will cause a Resolve from any clients interested in the service.  Hello is the only multicast message that can include xAddrs, so it's a little unique--and it differs from ProbeMatches and ResolveMatches in that sense.
• Bye messages never contain xAddrs.  The specification does not allow it (note that this has been changed in WS-Discovery v1.1).
• ResolveMatches messages always contain xAddrs.  It's technically possible for the xAddrs element to be empty but this is unusual and probably not a good way to achieve interoperability.  A service must realistically always produce xAddrs in its ResolveMatches messages.
• ProbeMatches messages usually contain xAddrs.  It's very unusual for services to omit xAddrs in their ProbeMatches messages because it takes no more effort than including them in ResolveMatches--both messages are unicast responses.  Including xAddrs in ProbeMatches lets everyone avoid a Resolve/ResolveMatches exchange.

One interesting consequence of this is that the example sequence on page 11 of WS-Discovery (Probe -> ProbeMatches -> Resolve -> Resolve Matches -> success!) almost never happens.  Services usually include xAddrs in their ProbeMatches messages in the first place.