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SIP Timers Overview
The BSG uses SIP timers to clean up calls that are initiated but never established and established calls that are inactive.
SIP Timers for Calls in Initiation Stage
For calls in the initiation stage, the BSG uses Timer C as defined in RFC 3261 SIP: Session Initiation Protocol. For every INVITE request sent, an expiration time is calculated as the minimum time between the incoming INVITE request’s Expire header and the value configured for Timer C. Timer C works as follows:
- When the INVITE request is sent, an expiration timer is set.
- When a non-100 provisional request arrives, the timer is reset to its original value.
- When a final response to the INVITE request arrives, the timer is canceled.
If the final response to the invite request is not received by the time Timer C expires, the invite is cancelled by sending a CANCEL message to the call recipient and a 408 error message (request timeout) is sent to the caller.
SIP Timers for Established Calls
For calls that are already established, an inactivity timer works as follows:
- When the call is established, an inactivity timer is set on the INVITE usage.
- Each new SIP request on that dialog resets the timer to its original value.
If the call is inactive (no signaling on the dialog) for the length of the inactivity time, a BYE message is sent for both user agents on the call.
