Question: In an IMS (IP Multimedia Subsystem) registration process, if an S-CSCF (Serving-Call Session Control Function) encounters a REGISTER request with an Authorization header containing an "integrity-protected" flag set to "tls-connected" and an "algorithm" parameter valued as "AKAv2-SHA-256", what security mechanism is indicated for the registration process?
Option 1: IMS AKA (Authentication and Key Agreement) without TLS
Option 2: SIP Digest with TLS, but not specifically AKAv2
Option 3: IMS AKA with HTTP Digest AKAv2 over a TLS session set up prior to registration, as defined for WebRTC over IMS
Option 4: Generic Bootstrapping Architecture (GIBA) with TLS
Explanation: The specific combination of the "integrity-protected" flag set to "tls-connected" and the "algorithm" parameter valued as "AKAv2-SHA-256" within the Authorization header of a REGISTER request indicates IMS AKA with HTTP Digest AKAv2, where the security context is established over a TLS session prior to registration, particularly defined for WebRTC services over IMS, ensuring secure authentication and key agreement in real-time communication scenarios.