SIP is the abbreviation for Session Initiation Protocol. This is a signaling
protocol for IP telephony that is used to establish, modify and terminate
VoIP telephone calls. SIP was developed and standardized in RFC 3261 by IETF
(Internet Engineering Task Force).
SIP makes it possible to establish a phone call by describing the
communication that is needed for this purpose. You can find more information
about this process in the What is SDP article.
Figure 1 - The communication channel of a SIP system
In the world of VoIP, the SIP made revolutional changes. This protocol is
similar to the HTTP protocol: it is text based, and very opened and flexible.
Due to this, SIP protocol has largely replaced the H323 standard.