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SIP"The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences." (RFC 3261). It was originally designed by Henning Schulzrinne (Columbia University) and Mark Handley (UCL) starting in 1996. The latest version of the specification is RFC 3261 from the IETF SIP Working Group. In November 2000, SIP was accepted as a 3GPP signaling protocol and permanent element of the IMS architecture. It is widely used as a signaling protocol for Voice over IP, along with H.323 and others. SIP is addressing neutral, with addresses expressed as URL/URIs of various types, such as H.323 address, E.164 telephone numbers or email like addresses. SIP is a lightweight, transport-independent, text-based protocol. SIP has the following features: Lightweight, in that SIP has only six methods, reducing complexity Transport-independent, because SIP can be used with UDP, TCP, ATM & so on. Text-based, allowing for low overhead
SIP (Session Initiation Protocol) is a signaling protocol used for locating remote users and establishing interactive communications. SIP has several noteworthy design qualities. Two important design elements of SIP are: separation of session establishment and session description cooperation with other protocols. Separating session establishment from session description means that SIP itself, at the core, doesn't know or need to know, what kind of session is being established, voice, video, chat, etc. Protocol cooperation simply means that SIP doesn't try to redefine everything. It works with other protocols and standards, without trying to replace things that fundamentally work. Together, these design elements mean that SIP can support many kinds of communications beyond voice-over-IP and conferencing, including things we haven't even thought of yet. SIP advocates point immediately to SIPs roots in the IETF and therefore its natural fit with the Internet to demonstrate SIP's superiority over ITU and other signaling standards for migrating telecommunications to the Internet. While there is some truth to this, it is probably not SIP's inherit technical merit that makes it stand out. A side effect of SIP's IETF and Internet nature is that it is attractive to developers and designers because of its familiarity; it looks like an Internet protocol so developers can leverage previous experience and knowledge. This has created a great deal of momentum around SIP. In the end, SIP is probably not that remarkable on pure technical merit. And that's fine. HTTP is not a particularly impressive protocol on technical merit alone, yet it resulted in the Internet that had existed for years before HTTP becoming known as 'the web' after HTTP. SIP is an adequate protocol, technically; it can do the job. And much like HTTP, SIP will rely on a lot of other protocols and applications to actually do anything useful. HTTP arrived at a time on the Internet when there were a number of independent applications in place that worked fine for their part, such as file transfer, and electronic documents. HTTP provided the glue to make these existing independent technologies more effective. SIP is the glue for a new set of existing applications, beyond just the obvious 'IP telephone' to include multimedia, mobility, IM and presence, e-commerce, web services, and many others. There have already been movements to employ SIP in such diverse areas as control of home appliances, for hearing and speech disabled persons, and even in cooperation with routing protocols like MPLS. It is probably mostly a matter of SIP being in the right place at the right time, and, as such, it has tremendous potential to be a very disruptive technology. |
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