Session Initiation Protocol

Session Initiation Protocol (SIP) is an IETF-defined “signaling” protocol for creating, modifying, and terminating end-to-end communication sessions over IP networks. It can be used to initiate a broad array of real-time communications, from text-based messaging to videoconferencing to online games.

SIP provides the following services:

• Call set-up and handling.
• Call forwarding.
• Detection of client capabilities. (Is the recipient using a PC with videoconferencing ability, or just an IP phone?)
• Presence detection. (Which device is the recipient using, and is she available to take a call?)
• Dynamic registration and resolution of a user-friendly name (like an e-mail address or phone number) to an IP address.
• Authentication.

Like HyperText Transfer Protocol (HTTP) and the Simple Mail Transfer Protocol (SMTP), SIP is not tied to any particular type of data; rather, it uses the well-established Multi-Purpose Internet Mail Extensions (MIME) standard to describe the types of data being transferred during a session. As a result, an SIP session can be used for almost any form of synchronous communication, including text-based messaging, voice-over-IP telephony, unicast or multicast video conferencing, application sharing, multiplayer gaming, and more.