SIP overview

The Session Initiation Protocol (SIP) is an internet protocol that enables two or more users to establish a communication path and exchange data. This exchange of data is called a session. The data can be audio, video, or text. SIP enables users to identify each other, establish a connection, agree on a data format, exchange information, and end the communication.

Protocol layers

SIP is a layered protocol as defined in RFC 3261. Each layer provides a processing stage with defined functionality. The following table describes these layers:

Layer

Description

Application layer

Provides an interface to an application that enables the application to communicate with other applications.

Transport layer

Splits data into packets for transmission and reassembles them when the packets are received. Hides the details of transport from the upper layers.

Network layer

Packages the data with the correct address information.

Datalink layer and physical layer

The datalink layer creates an error free transmission channel.

The physical layer moves the actual data from one point to another.

The following illustration shows the SIP layers:

layers.gif

The functionality of each layer includes the services and functions of all of the layers below it.