Thanks you all guys.

So, the XMPP protocol only defines the C<-->S communications. Developers can add more features based on this protocol, distribute services, for instance. They need to implement this by them self, am I right?

You know that I plan to write an IM, and originally, I intend to use Windows IOCP, but it may take me so much time on it. A college introduce Jabber to me, it' really popular. But, my concern is the efficiency, I don't know whether the server works well if there are so many connections at a time.