Question: In IPsec ESP for SIP signaling between UE and P-CSCF, what determines the choice between transport mode and UDP encapsulated tunnel mode?
Option 1: Type of SIP signaling messages being protected
Option 2: Presence or absence of Network Address Translation (NAT)
Option 3: Selected integrity algorithm for ESP
Option 4: Key expansion function used for deriving IKESP
Explanation: The choice between transport mode and UDP encapsulated tunnel mode for IPsec ESP in protecting SIP signaling between UE and P-CSCF is contingent upon whether NAT (Network Address Translation) traversal support is required. If no NAT is present, transport mode can be used; otherwise, UDP encapsulated tunnel mode is employed to facilitate NAT traversal, ensuring connectivity across networks that utilize NAT.