Question: What action is triggered at the S-CSCF upon receiving a re-registration request from a UE in response to a network-initiated authentication?
Option 1: De-registration of the Subscriber
Option 2: Immediate Authorization for IP Services
Option 3: A new SIP Digest procedure for user re-authentication
Option 4: Update of UE's Location in the HSS
Explanation: In the context of network-initiated authentications, when a UE responds to an authentication request by initiating a re-registration procedure as instructed by the S-CSCF, the primary goal is to verify the user's credentials again. The S-CSCF achieves this by triggering a new SIP Digest procedure, which is designed for user authentication in IP-based services. This ensures the security and validity of the user's access to the network services.