SIP主要信令与扩展功能

SIP allows two end points to establish media sessions with each other. The main signaling functions of the protocol are as follows:

• Location of an end point;

• Contacting an end point to determine willingness to establish a session;

• Exchange of media information to allow session to be established;

• Modification of existing media sessions;

• Tear-down of existing media sessions.

SIP has also been extended to request and deliver presence information (on-line/off-line status and location information such as that contained in a "buddy" list) as well as instant message sessions. These functions include:

• Publishing and uploading of presence information;

• Requesting delivery of presence information;

• Presence and other event notification;

• Transporting of instant messages.