Question: In IMS (IP Multimedia Subsystem) control plane using SIP over TLS/TCP for firewall traversal, what keep-alive technique MUST be used between the IMS client and the P-CSCF due to the use of TCP as the transport protocol?
Option 1: STUN (Session Traversal Utilities for NAT) for connection-less transports
Option 2: SIP OPTIONS ping
Option 3: CRLF (Carriage Return Line Feed) keep-alive technique for connection-oriented transports
Option 4: HTTP Refresh Header
Explanation: Since TCP is used as the transport protocol between the IMS client and the P-CSCF, which is a connection-oriented transport, the CRLF (Carriage Return Line Feed) keep-alive technique, as defined in RFC 5626 for connection-oriented transports, must be utilized to maintain firewall pinholes. This ensures persistent connectivity, crucial for real-time communication services.