Session Initiation Protocol - Wikipedia
https://en.wikipedia.org/wiki/Session_Initiation_Protocol
The Session Initiation Protocol (SIP) is a signaling protocol used for initiating, maintaining, and terminating real-time sessions that include voice, video and messaging applications.
What is SIP? - YouTube
https://www.youtube.com/watch?v=erICfPV8-Lg
Introduces SIP - the Session Initiation Protocol.