SIP, like HTTP, is versatile and simple to use. It can set up collaborative multimedia conferencing and voice-enabled e-commerce. SIP is expected to become the norm for VoIP implementations within a couple of years, though today just about all enterprise VoIP vendors would rather keep you locked into their proprietary signaling solutions. The IETF published the first version of SIP, RFC 2543, in 1999 and the most recent version, RFC 3261, last June.
SIP is ideal for VoIP, where a session over the Internet replaces the traditional end-to-end circuit for a voice call in a legacy network. The ITU's H.323 multimedia standard, as well as some vendor-proprietary VoIP phones, also do this. VoIP vendors that built products before SIP emerged have adopted H.323. But SIP is simpler to implement than H.323 and is a lighter-weight protocol with less overhead.
SIP is more than a standards-based replacement for legacy phone connections, though. It makes it easier to implement advanced multimedia services, such as presence, which allows you to determine instantly whether a user can and wants to receive a call on a specific phone, as well as over video and instant messaging sessions. It also lets you ring multiple destinations in a VoIP call.
And SIP is making commercial inroads. Microsoft's WinMessenger IM program, which comes packaged with its XP OS, is based on SIP. WinMessenger also uses SIP to make Internet phone calls. Future 3G wireless WANs, too, will use SIP for setting up and tearing down calls.
Still, there are plenty of misconceptions about what SIP can actually do. SIP does not, for instance, transport digitized voice. That's the job of the RTP (Real-Time Transport Protocol), which transports voice after SIP establishes the call. And before SIP can set up voice, text-messaging or video sessions using various codecs and techniques, you need to determine what features the devices in the session support. That's where the Session Description Protocol comes in: SIP relies on SDP to negotiate the capabilities between two endpoints in a potential conversation.
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