Question: In a 3GPP IP-based service scenario, if a User Equipment's (UE) authentication response is incorrect, what typically happens to the message at the P-CSCF before it reaches the S-CSCF for verification?
Option 1: The P-CSCF forwards the message to the S-CSCF with an error indicator.
Option 2: The IPsec layer at the P-CSCF modifies the message to alert the S-CSCF of potential authentication issues.
Option 3: The IPsec layer at the P-CSCF discards the message due to an integrity check failure caused by the incorrect Integrity Key.
Option 4: The P-CSCF buffers the message, awaiting a retransmission request from the S-CSCF before proceeding.
Explanation: When a UE's authentication response is incorrect, it typically leads to an incorrect Integrity Key being used. This causes the integrity check at the P-CSCF to fail. As a result, the IPsec layer discards the message to prevent potentially compromised data from proceeding, ensuring security and integrity of communications.