Imagine a communications environment where a central directory server not only knows how to reach an individuals work phone, cell phone, and pager, but also her instant messaging (IM) program, e-mail, and PDA. Not only that, but also imagine that the central directory server also knows a party's communication preferences and capabilities, and can intelligently alert a called party when someone is trying to reach her. Finally, imagine that phone calls to an unavailable person can be intelligently rerouted to another person or group depending upon a number of interrelated factors such as time of day, whether the called person is scheduled to be in a meeting, or whether one or more of her modes of communication is unreachable.

These capabilities aren't some dream of a far-off utopian future, but are available today thanks to a remarkable advance in communications: Session Initiation Protocol (SIP). SIP is the glue and the intelligence that makes these advanced communications capabilities possible.