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What is SIP

Posted by kathayat on September 25, 2007

….IETF standards track application layer protocol for establishing, modifying and tearing down sessions whose participants are connected directly or via gatewat to a network.

….Key part of the communication system is finding the call participants and contacting them. The problem is made even more interesting if you assume pasticipants may move from place to place, changing their locations and the addressable equipments they are using. Add to this notion that calls need not be restricted to a single voice stream but may involve multiple streams of voice media. Then consider that many – even thousands- of participants might be involved in that call joining and living in a constantly changing topology. Puts all these together and there is obviously a need for some sort of protocol to deal with generalised sessions. SIP fills this rols.

….SIP supports basic four functions

  1. User locations – translating the users name (email or phone) into current network address, keeping track of the users location as it moves to different locations in the network
  2. Feature negotiation – Ensure that all participants in a session agree on the features to be supported among them
  3. Call management – adding, dropping, transfering, on hold
  4. Feature modification – chaning the feature of the session while the session is in progress

…. There are some assumptions SIP follows

  1. SIP should be scalable
  2. SIP should reuse as many existing protocols and protocol design concepts as possible rather than inventing new ones
  3. SIP should maximize the interoperability

…. SIP Messages

  • Like HTTP, SIP is a text-based protocol with request and response messages
  • SIP message consists of  – request/response line followed by header lines and optional body
  • Format of the request/response line is SIP-Method/SIP-Version Req-URI/Status-Code SIP-Version/Reason-Pharse
  • SIP Methods 
    • INVITE, ACK. OPTIONS, BYE, REGISTER, CANCEL
  • SIP Request URI –
    • sip:abcd@someplace.com
    • tel:123456@voidservice.com
  • SIP Headers – keyword:value
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