The key purpose of the Session Initiation Protocol $($SIP$)$ is to$?$

Question $15$ options:

define the protocol required to establish and tear down communications, including voice and video calls flowing over a packet network.

define the signaling required to establish and tear down communications, including voice and video calls flowing over a PSTN$.$

define the protocol required to establish and tear down communications, including voice and video calls flowing over a circuit$-$switched network.

define the signaling required to establish and tear down communications, including voice and video calls flowing over a packet network.