What is SIP?

SIP, or Session Initiation Protocol, a method that computers use to open an active connection between them. It is used in telephony, instant messaging, and mobile phone calling over LTE.

It describes the way that some software or phones reach out to another computer or phone, and ask if they can open a connection that allows them to be directly linked.

Once that connection is open, it defines the parameters in which the connection will remain open. When the communication is over, it defines the process by which the connection is terminated so that both sides are aware that the connection is no longer active.