xAddrs optionality in WS-Discovery messages

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…

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