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New Protocol of IP Phone
  Mention voice over IP and most people have at last heard of the H.323 protocol. Well. If H .323 can be considered vanilla, then Session Initialization Protocol (SIP) could be strawberry. Its not any better or worse , just different..

SIP is part of an Internet Engineering Task Force(IETF)proposal to replace parts of H.323. Just as h.323 is a collection of protocols, SIP is one of several protocols that will work together to complete calls.

SIP is an application layer, control/signaling protocol for creating, modifying and terminating sessions with one or more participants, distance learning ,internet telephone calls and multimedia

distribution .SIP can invite persons and robots, such as media storage services, to participate in a call.

The protocol may be used to initiate sessions, invite members to sessions advertised by other means or initiate multiparty calls using a multipoint control unit. SIP transparently support name mapping and redirection services, allowing the implementation of ISDN and intelligent network telephony subscriber services such as personal mobility.

The protocol does not offer conference control services, nor does it prescribe how a conference is to be managed. SIP does not allocate multicast addresses and it does not reserve, but it can convey to the invited system the information necessary to do this. 

SIP supports five facets of establishing and terminating multimedia communications: user location, capabilities and availability, call setup and call handing.

Callers and call receivers are identified by SIP addresses. A caller first locates the appropriate server, then sends a SIP request(probably an invite).In a perfect world, the request arrives at its destination, where the client accepts the call by returning a SIP response code 200,As with other TCP/IP reply codes, a response code that starts with a 2 is good, then the originating caller sends an acknowledgement back to the recipient, which is a bit unusual because the station that initiates the call also sends the acknowledgement.

SIP can communicate via multicast, a mesh of unicast relationships, or a combination of multicast and unicast.

A SIP transaction consist of a SIP request and the appropriate response. To facilitate pairing a request with its response, there are several fields that contain identical values on the request and the response. These fields include the call ID field, the command sequence number, the to field, the from field, and the tag(if present),thats correct-to and from fields are identical in both directions, this aids in problem resolution when youre using your protocol analyzer to troubleshoot network anomalies,

To oversimplify, the IETF created SIP and its brethren protocols because of a belief that H .323 would not scale well. Theres no question that H.232 is way in front of SIP at this point in the race .It will be interesting to see what it looks like at the finish line.

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