The SIP B2BUA acts as refresher for the Session-Timer mechanism as defined in RFC 4028. If the endpoints indicate support for session timers during call-setup, then the SIP B2BUA will use an UPDATE or re-INVITE message if enabled per peer, domain or subscriber via Provisioning to check if the endpoints are still alive and responsive. Both endpoints can renegotiate the timer within a configurable range. All values can be tuned using the Admin Panel or the APIs using Peer-, Domain- and Subscriber-Preferences.
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Keep in mind that the values being used in the signaling are always half the value being configured. So if you want to send a keep-alive every 300 seconds, you need to provision sst_expires to 600. |
If one of the endpoints doesn’t respond to the keep-alive messages or answers with
481 Call/Transaction Does Not Exist, then the call is torn down on both sides. This mechanism
prevents excessive over-billing of calls if one of the endpoints is not reachable anymore or
"forgets" about the call. The BYE message sent by the B2BUA triggers a stop-record for accounting
and also closes the media ports on the Media Relay to stop the call.
Beside the Session-Timer mechanism to prevent calls from being lost or kept open, there is a maximum call length of 21600 seconds per default defined in the B2BUA. This is a security/anti-fraud mechanism to prevent overly long calls causing excessive costs.