Session Initiation Protocol (SIP) provides signaling and call setup for IP-based communication sessions and is used to set up and terminate voice or video calls.
SIP is an application-layer control protocol that can establish, modify, and terminate multimedia sessions (conferences) such as Internet telephony calls. SIP can also invite participants to already existing sessions, such as multicast conferences. Media can be added to (and removed from) an existing session. SIP transparently supports name mapping and redirection services, which supports personal mobility  - users can maintain a single externally visible identifier regardless of their network location.