[SIPForum-discussion] What is Session timer ?

[Abhisek Acharya]

Hi Khao,

Session timer is one of the attributes used in Sessio-Refresher
mechanism.Session refresher mechanism is used to verify the existance of a
session during a call and the same is achieved by sending various
mid-dialog SESSION-REFREHSER messages in form of UPDATE or RE-INVITE method.

Supported and Require timer is used to denote the support for this
mechanism and the use of these 2 headers are same as mentioned in RFC3261.

Once both the parties are agreed up on a particular time duration say 1800
seconds in Session-Expiry header then UAC or UAS may act as a Refresher who
will send refresher requests in half of the time mentioned and will expect
a 200OK response in reply to that .

The reason we use this mechanism to avoid hang calls in cases if the
release/BYE message is lost in transition from switches across the
network.If BYE is lost the call dueration in intermediate switches shows
 huge duration even if the call gets released much before.

2.In the above process a lot of proxy resource also used as a part of
maintaining the dialog state even if actually the media path has already
stopped.


Pls let me know if you have any problem in the Session-Refresher mechanism.

Regards
Abhisek Acharya
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