Question: During IMS registration, what happens after the UE receives message SM6 and TLS was selected by the P-CSCF?
Option 1: The UE sends SM7 without any security headers over the existing connection.
Option 2: The UE establishes a new TCP connection before initiating the TLS handshake with the P-CSCF.
Option 3: The UE performs a TLS handshake with the P-CSCF and then sends SM7 over this newly established TLS connection, including sip-sec-agree negotiation headers.
Option 4: The UE skips the TLS handshake and directly sends SM7 to the S-CSCF for authentication.
Explanation: This is correct because, after receiving SM6 with TLS selected, the UE initiates a TLS handshake with the P-CSCF. Following successful establishment of the TLS connection, the UE sends message SM7 over this secure connection, including necessary sip-sec-agree negotiation headers to complete security setup for the session, ensuring subsequent signaling communications are integrity protected.