The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants.
These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences." (cit. RFC 3261). It was originally designed by Henning Schulzrinne (Columbia University) and Mark Handley (UCL) starting in 1996. The latest version of the specification is RFC 3261 from the IETF SIP Working Group.
In November 2000, SIP was accepted as a 3GPP signaling protocol and permanent element of the IMS architecture. It is widely used as signaling protocol for Voice over IP, along with H.323 and others. SIP is addressing neutral, with addresses expressed as URL/URIs of various types, such as H.323 address, E.164 telephone numbers or email like addresses.
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The video below is about SIP, a VoIP protocol that allows people and businesses to make phone calls for free or for a modest fee.