Question: Upon receiving a REGISTER request, what is the sequence of authentication methods checked by the Serving-Call Session Control Function (S-CSCF) before requesting the Home Subscriber Server (HSS) to distinguish between SIP Digest and Network Access Authentication (NBA)?
Option 1: IMS AKA -> TNA -> GIBA -> HSS for SIP Digest/NBA
Option 2: TNA -> IMS AKA -> GIBA -> HSS for SIP Digest/NBA
Option 3: GIBA -> TNA -> IMS AKA -> HSS for SIP Digest/NBA
Option 4: IMS AKA -> TNA -> GIBA, with SIP Digest/NBA distinction always handled by the HSS as a final step
Explanation: The S-CSCF checks for IMS AKA first, then TNA if not IMS AKA, followed by GIBA for non-IMS-AKA requests. If none apply, it requests the HSS to distinguish between SIP Digest and NBA as a final step, ensuring backward compatibility with earlier HSS releases that may only support specific authentication schemes.