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Session Initiation Protocol

Last Updated : 22 Sep, 2020
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Session Initiation Protocol was designed by IETF and is described in RFC 3261. It’s the protocol of application layer that describes the way to found out Internet telephone calls, video conferences and other multimedia connections, manage them and terminate them. It’s one module. it’s ready to establish two way party session, multiparty session and multicast session. It’s design independent of underlying transport layer. It’s ready to run on UDP or TCP.

SIP address :
In a regular telephone communication, a phone number identifies the sender and another phone number identifies the receiver. In SIP, the sender and receiver are often identified by any of these.

  1. An Email address.
  2. An IP address.
  3. A Phone number

The representation is represented as URLs using sip scheme :

SIP address schemes

SIP messages :
SIP may be a text based protocol modeled on HTTP. It uses messages in ASCII text. Each message features a header and a body. the varied SIP messages are as follows.

Sr. No. Message Description
1. INVITE It requests for initiation of a session.
2. ACK It confirms that session has initiated.
3. BYE It request for the termination of the session.
4. OPTIONS It query a host about its capabilities.
5. CANCEL It will cancel the pending request.
6. REGISTER It informs a redirection server about the user’s current location.

SIP Session :
A simple session using SIP consists of the following.

  1. Establishing a session.
  2. Communication.
  3. Terminating the session

SIP Session

Establishing a session :
It requires a three-way handshake. The caller will send INVITE message. If the caller is willing to start out, he/she sends a reply message. to verify that a reply code is received, the caller send an ACK message.

Communication :
After establishment of session, the caller and callee communicate using two temporary ports.

Terminating the session :
The session can often terminated by using BYE message send by either caller or callee.


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