SIP Session Timer Call Flows

IMG 2020 is the UAC and does the refresh

The IMG 2020 transmits an INVITE message and requests a Session Timer by including a Session-Expires header. The Remote Gateway then transmits a 422 Session Interval Too Small response with Min-SE header. The IMG 2020 sends out new INVITE with Min-SE and an updated Session-Expires value. The Remote Gateway transmits a 200 OK that sets the Session Timer to 1800 seconds and the IMG 2020 as the refresher. The IMG 2020 then starts a 900 second session refresh timer. After refresh timer expires, a refresh request with Session-Expires value set to current value 1800 seconds and refresher to uac is transmitted from IMG 2020. The IMG 2020 then receives a 200 OK and resets the timer. Once again the IMG 2020 sends out refresh request after session refresh timer expires, but the remote gateway crashed so a 408 Request Timeout is received. A BYE is then transmitted and the call is terminated.

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IMG 2020 is UAC and Remote Gateway does the Refresh

The IMG 2020 transmits an INVITE that has Supported header with option tag ‘timer’ and Session-Expires to request session timer. The remote gateway accepts it. The session interval is set to 1800 seconds and refresher=uac is set to the remote gateway. The IMG 2020 starts a 1768 second Session End timer. The IMG 2020 receives a session refresh request before the session end timer expires. The IMG 2020 sends 200 OK back and set refresher to UAC so that the role of refresher doesn’t change. The IMG 2020 restarts 1768 seconds session end timer. Now the remote gateway crashes and no refresh request is sent. 1768 seconds later the session end timer expires, and IMG 2020 sends out BYE and terminates the call.

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IMG 2020 is UAS and Remote Gateway does the Refresh

IMG 2020 receives INVITE that has Supported header with option tag ‘timer’. IMG 2020 sends 200 OK and requests session timer by including Require header with tag ‘timer’ and Session-Expires header. The session interval is set to 1800 seconds and refresher is the remote gateway. IMG 2020 starts 1768 seconds session end timer. IMG 2020 receives session refresh request before the session end timer expires. IMG 2020 sends 200 OK back and set refresher to UAC so that the role of refresher doesn’t change. IMG 2020 restarts 1768 seconds session end timer. Now the remote gateway crashes and no refresh request sent. 1768 seconds later the session end timer expires, IMG 2020 sends out BYE and terminates the call.

IMG 2020 is UAS and does the Refresh

The IMG 2020 receives an INVITE message that has a Supported header with option tag ‘timer’ and Session-Expires header with a 90 second value. Since the 2020 SBC’s minimum session timer is configured to 1800 seconds, the IMG 2020 sends back a 422 response with Min-SE header set to 1800. The remote gateway sends new INVITE message with an updated session timer. The IMG 2020 accepts it. The session interval is set to 1800 seconds and the refresher=uac is the IMG 2020. The IMG 2020 starts a 900 second session refresh timer. After refresh timer expires, the IMG 2020 sends out refresh request with Session-Expires value set to current value of 1800 seconds, refresher is set to the uac to ensure the IMG 2020 will always perform a refresh. The IMG 2020 receives 200 OK and resets the timer. Once again the IMG 2020 sends out refresh request after session refresh timer expires, but now the remote gateway crashed so that the IMG 2020 receives 408 Request Timeout. The IMG 2020 sends out BYE and the call is terminated.

IMG 2020 is UAC and Requests Session Timer

The IMG 2020 requests a Session Timer by including Session-Expires header on the INVITE. The IMG 2020 receives 200 OK without Session-Expires header that indicates the remote gateway does not support a session timer. Since the IMG 2020 is configured to enforce a session timer, the IMG 2020 sets session interval to 1800 seconds and refresher to uac. The IMG 2020 starts a 900 second session refresh timer. After refresh timer expires, the IMG 2020 sends out refresh request with Session-Expires value set to current value 1800 seconds and refresher to uac. The IMG 2020 receives 200 OK and resets the timer. Once again the IMG 2020 sends out refresh request after session refresh timer expires, but now the remote gateway crashed so that the IMG 2020 receives a 408 Request Timeout. The IMG 2020 sends out BYE and the call is terminated.

IMG 2020 is UAS and configured to enforce session timer on INVITE

The IMG 2020 receives an INVITE message that the Supported header does not contain tag ‘timer’. The IMG 2020 sends back 200 OK. Since the IMG 2020 is configured to enforce session timer, the IMG 2020 sets session interval to 1800 seconds and the refresher=uas. The IMG 2020 starts 900 second session refresh timer. After refresh timer expires, the IMG 2020 sends out a refresh request with a Session-Expires value set to the current value of 1800 seconds. The refresher is set to uac to ensure the IMG 2020 will always perform a refresh. The IMG 2020 then receives 200 OK and resets the timer. Once again the IMG 2020 sends out a refresh request after session refresh timer expires, but the remote gateway has crashed and the IMG 2020 receives a 408 Request Timeout. The IMG 2020 sends out BYE and the call is terminated.