Session Initiation Protocol. Session Initiation Protocol (SIP) is a communications protocol for signalling and controlling multimedia communication sessions. It aims to provide call functions and features similar to normal telephone network. The features that permit familiar telephone-like operations (i.e. dialling a number, causing a phone to ring, hearing ring back tones or a busy signal) are performed by what are called in the SIP world, proxy servers and user agents (basically, the phone).
Sometimes, a device called a Session Border Controller is used to provide extra features. Often used at the border between service providers. May provide all sorts of facilities, including security, NAT, QoS and priority.
SIP is commonly used by Internet (IP) telephone systems but is increasing being used to set up audio and video over IP circuits for broadcast use.
