Question: In IMS-specific confidentiality protection between UE (User Equipment) and P-CSCF (Proxy-Call Session Control Function), what mode of IPsec ESP (Encapsulating Security Payload) is applied for protecting SIP signaling messages at the IP level?
Option 1: Transport Mode
Option 2: Tunnel Mode
Option 3: Hybrid Mode
Option 4: Adaptive Mode
Explanation: In the context of IMS security for confidentiality protection between UE and P-CSCF, IPsec ESP in Transport Mode is specifically utilized. This mode protects the payload of the IP packet (SIP signaling messages) without encrypting or altering the original IP header, maintaining routing information while ensuring message content confidentiality.