Question: In a SIP-based IMS network, when a non-Initial REGISTER message is protected by Transport Layer Security (TLS) and forwarded by the Proxy-CSCF to the Serving-CSCF, what action does the P-CSCF take regarding an integrity protection indicator?
Option 1: The P-CSCF includes a generic security indicator without specifying the protection method.
Option 2: The P-CSCF never attaches any indicators for non-Initial REGISTER messages.
Option 3: The P-CSCF attaches an appropriate TLS integrity protection indicator to distinguish it from IPsec protection.
Option 4: The P-CSCF only attaches the indicator if the S-CSCF explicitly requests it.
Explanation: This is correct because, according to the specified behavior for non-Initial REGISTER messages protected by TLS, the P-CSCF must attach a specific indicator that allows the S-CSCF to differentiate between protection methods (TLS vs. IPsec), ensuring the S-CSCF can appropriately handle the security context of the message.