SIP timers: retransmissions, timeouts, and session refreshes

SIP timers control how long a transaction waits, when a request is retransmitted, and when an established session should be refreshed. They help the protocol recover from loss but cannot repair a broken route.

Troubleshooting clues

• Repeated requests at increasing intervals can indicate packet loss or a missing response.
• A transaction timeout points to reachability, routing, overload, or a blocked return path.
• Session refresh failures can drop an otherwise established call.
• Excessively aggressive local timers can create false failures on slow paths.

Capture timestamps, request direction, response direction, and transport before changing timer values. Defaults vary by implementation; change them only with a specific failure hypothesis.

Reference: RFC 3261. Related: SIP call flow.