SIP- What is the Session Initiation Protocol (SIP)?

• Session Initiation Protocol (SIP) was designed by IETF and is described in RFC 3261.

Features of SIP

• It is an application layer protocol that describes how to set up Internet telephone calls, video conferences and other multimedia connections, manage them and terminate them.

• Unlike 4.323, which is a complete protocol suite, SIP is a single module.

• SIP can establish two way party session (ordinary telephone calls), multiparty session (where everyone can hear and speak), multicast session (one sender, many receivers)

• The SIP just handles setup, management and termination of sessions. Other protocols such as RTPIR TCP are used for data transport.

• SIP is designed independent of the underlying transport layer; it can run on UDP or TCP.

SIP Addresses

• In a regular telephone communication, a telephone number identifies the sender and another telephone number identifies the receiver.

• In SIP, the sender and receiver can be identified by any of these:

1. An Email address

2. An IP address

3. A telephone number

• All these addresses are represented as URLs using the sip scheme or format.

                             SIP: [email protected]     E-mail address

                             SIP: dineshthakur@ 201.23.45.78                  IP address

                             SIP: dineshthakur@+91-9815618378           Phone Number

SIP Messages

• SIP is a text based protocol modeled on HTTP.

• Like HTTP, SIP uses messages in ASCII text.

• Each message has a header and a body.

• The header consists of several lines that describe the structure of the message, caller’s capability, and media types and so on.

• The various SIP messages are listed in the table below:

SIP Session

A simple session using SIP consists of:

1. Establishing a session

2. Communication

3. Terminating the session

1. Establishing a Session

• Establishing a session requires a three-way handshake.

• The caller sends an INVITE message, using TCP or UDP to begin the communication.

• If the caller is willing to start, he/she sends a reply message.

• To confirm that a reply code has been received the caller sends an ACK message.

2. Communication

• After the session has been established the caller and caller can communicate by using two temporary ports.

3. Terminating the Session

• The session can be terminated with a BYE message sent by either party.

Comparison between H.323 and SIP