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Functions Of SIP

As the name implies, the protocol is used to setup, modify and terminate sessions. Its main signaling functions are: 

1. Location of an endpoint - to determine the end system(user) to be used for communication. 
2. User Availability - contact the called party and determine its willingness to engage in communication. 
3. Endpoint Capabilities - negotiation of media and media parameters for a successful session setup. 
4. Session Modification - SIP allows changing features of a session while it is still in progress. 
5. Call Management
 - involves adding, dropping or transferring participants, termination of sessions and                                            invoking services. 

    SIP invitations carry session descriptions that allow the caller and the callee to agree/negotiate on a set of compatible media types. This agreement is done by means of negotiation based on the medium (text, voice, other) the transport (usually RTP) and the encoding codec. The session description is written in SDP (session description protocol) carried along with a SIP message, which means that SIP uses  this protocol for describing multimedia sessions.

This means that SIP could be used with other protocols to depict a complete multimedia architecture. For example, RTP (Real-Time transport protocol) for transferring real-time data and Quality Of Service feedback, RTSP (Real-Time streaming protocol) for controlling delivery of streaming media etc. 

    SIP does not provide services, rather it is used in conjunction with other protocols to ensure that certain services are provided like conference control, resource reservation, session description, caller-ID etc. However, functionality of SIP and its operations does not depend on any of these protocols.                        

    SIP sounds very simple, and it is. This simplicity is very much required to make the protocol stable, but this did not confine the protocol to a limited boundary be it in the sense of usefulness or in the  area of applications. SIP is highly extensible and dozens of extensions already exists. 
SIP has also been extended to provide features/functions such as Registration, Presence and Instant Messaging, Video-conferencing, Online Gaming etc.

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