SIP refers to "Session Initiation Protocol". It is a fundamental protocol in the IMS network, that is responsible for the establishment, modification, and termination of multimedia sessions, such as voice and video calls.

In addition, SIP is responsible for the registration of user endpoints with the IMS network. It handles the process of associating a user's public identity (e.g., phone number, SIP URI) with their current IP address or contact information.

Also SIP provides the mechanisms for locating and routing signalling messages to the appropriate IMS network elements, such as the P-CSCF and the S-CSCF.