Re: [sr-dev] [SR-Users] Kamailio at Google Summer of Code 2011

2011/3/28 Olle E. Johansson oej@edvina.net:

• In SIP you can send a Subscribe with Expires:0 to "query" the server about the current state of a URI.
• In XMPP, the server doesn't have a state for anything.  It's just a message router.

This is not entirely true. In XMPP basic presence ("online", "away", text message as "I'm at home") is sent as individual messages the server routes to the watchers. But other more modern features (as user avatar/image) use PUB-SUB mechanism, so the watcher must subscribe to a specific user for the watched user.