presence server
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... This section summarizes the specific aspects of the Extensible
Messaging and Presence Protocol that MUST be supported by instant
messaging and presence servers and clients in order to be considered
compliant implementations. All such applications MUST comply with
...
... requirements for instant messaging
and presence servers and clients; note well that the requirements
...
... In addition to core server compliance requirements, an instant
messaging and presence server MUST additionally support the following
protocols:
...