ssage sizes but it adds complexity to implementations. A text-based protocol such as SIP can be easily scripted and requires no tools to monitor and interpret mes- sages-a simple packet dump from a LAN provides the ASCII-encoded SIP messages, which can be easily logged, examined, and inspected. Simple applica- tions can be written to test and simulate SIP traffic. The text-based encoding of Comparison to H.323 SIP has made it seem more friendly to Internet and Web developers who have embraced it in developing various applications. Another important difference is that while H.323 is exclusively a signaling protocol, SIP has both presence and instant message capability. The combina- tion of presence and signaling in one protocol using a common universal addressing scheme, URIs, will be a very powerful driver in new applications in the future. This makes SIP an extremely powerful "rendezvous" protocol that allows a user with multiple mobile end points to locate and communicate with another user with multiple capabilities. It is for this reason that the most exciting and innovative services are being developed with SIP rather than H.323. Another important difference is in key vendor support and momentum. SIP has been adopted by some key players in both the PC and telecommunica- tions industries. It is widely regarded in the industry as the signaling protocol session establishment over IP. While it may not immediately replace many established H.323 networks used for basic services such as simple phone calls, it is being adopted or investigated by all major players in the industry, even those who currently have a vested interest in legacy H.323 systems. SIP has also been adopted by mobile operators as the call signaling and instant message protocol for their third generation networks currently under development. This offers the promise of millions of SIP-enabled wireless devices in the next few years. SIP is also being coupled with 802.11 wireless networks for another set of mobile services. Another important differenc