What is the SIP Protocol?
Definition: SIP, or session initiation protocol is a signaling protocol for IP-based telephony applications. A signaling protocol provides the control layer for communications such as the establishment and release of a voice call.

History of SIP
Previous signaling protocol such as SS7 were designed for circuit-switched networks. These networks use dedicated T1 channels for carrying telephony communications and signaling. With dedicated T1 channels, SS7 is able to provide high-quality voice communications, but at high cost due to the requirement of end-to-end dedicated channels. With the advent of IP and packet-based networks, telephony traffic could be routed more efficiently and cheaply. But this required a new packet-based signaling protocol to be developed. SIP was born. Initially designed for voice communications, today it can manage instant messaging, video conferencing, and file transfers.