What is SIP?

The Session Initiation Protocol (SIP) is a signaling protocol that is used by technology products for creating session-oriented connections between two or more endpoints in an IP network

These endpoints could be IP telephones, instant messaging clients, or a collaborative multimedia conference application.

The core protocol itself is defined within the IETF SIP working group. Additional work is done in the IETF SIPPING working group, which is chartered to document the use of SIP for several applications related to telephony and multimedia, and to develop requirements for extensions to SIP needed for those applications.

read more here at the SIP Forum