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Call Session Timers

The SIP ALG uses the Session-Expires value to time out a session if a Re-INVITE or UPDATE message is not received. The ALG gets the Session-Expires value, if present, from the 200 OK response to the INVITE and uses this value for signaling timeout. If the ALG receives another INVITE before the session times out, it resets all timeout values to this new INVITE or to default values, and the process is repeated.

As a precautionary measure, the SIP ALG uses hard timeout values to set the maximum amount of time a call can exist. This ensures that the J-series device is protected should one of the following events occur:

End systems crash during a call and a BYE message is not received.
Malicious users never send a BYE in an attempt to attack a SIP ALG.
Poor implementations of SIP proxy fail to process Record-Route and never send a BYE message.
Network failures prevent a BYE message from being received.

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