SIP is an IETF application layer protocol for establishing, manipulating, and tearing down sessions. SIP’s main purpose is to help session originators deliver invitations to potential session participants wherever they may be. In a nut shell, that is SIP’s role.
So SIP is not the panacea – because it was never built to be that way. Let’s review two of the fundamental assumptions behind SIP’s design:
Reusing Existing Protocols – SIP was designed to specifically reuse as many existing protocols and protocol design concepts. For example, SIP was modeled after HTTP, using URLs for addressing and SDP to convey session information.
Maximizing Interoperability – SIP was also designed so that it would be easy to bind SIP functions to existing protocols and applications, such as e-mail and Web browsers. SIP does this by limiting itself to a modular philosophy – just like many other Internet protocols – and focusing on a specific set of functions.
It’s actually good news that SIP does not try to solve everything single-handedly. We can examine this statement more closely with a quick look at the H.323 approach to IP telephony. H.323 is not a single protocol but rather an entire suite of protocols that cover everything from soup to nuts – codecs, call control, conferencing, and many other functions in one vertically integrated stack.
The advantage to this approach is that by strictly controlling so many aspects of the implementation it is easier to ensure that H.323 based systems function well together. On the down side, H.323 becomes heavy and cumbersome. Flexibility is sacrificed as one is tied to a single family of technologies.
For a mature technology this may not be a problem, since the best solutions are likely to have been discovered and incorporated into standards. However for a field as young and fast changing as IP telephony, where many problems and solutions are still under debate, flexibility is more important. SIP is part of this flexible approach, as it uses a wide variety of protocols, each addressing a different aspect of the problem space. The advantage is the ability to choose from among many competing technologies and move to newer and better ones as they emerge. This has always been the philosophy behind SIP and this is the approach of the IETF to IP telephony in general.
SIP is an important piece of this modular approach to IP telephony protocols. SIP addresses the need for a protocol to deal with generalized sessions. This involves finding potential call participants and contacting them as they move from place to place, changing their location and the even equipment they are using. Calls may require the use of multiple streams of various media, and very large numbers of participants might be involved in a call – and even joining and leaving in a constantly changing topology. This is what SIP does!
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