RFC 3261 - SIP: Session Initiation Protocol
https://tools.ietf.org/html/3261
SIP: Session Initiation Protocol. Status of this Memo. This document specifies an Internet standards track This document describes Session Initiation Protocol (SIP), an application-layer control...
What is SIP? - YouTube
https://www.youtube.com/watch?v=erICfPV8-Lg
Introduces SIP - the Session Initiation Protocol.