Understanding the tolerance-window setting on 4500

Hi there I wonder if anyone could explain the reference in the user guide for the ‘tolerance widow’.

 

Recently we have noticed that some calls are dropping after 30 mins, after investigation. At the 30min mark, the Softswitch sends a re-invite to the SBC and the SBC send this on to the client, from the wireshark/network traces, it can be seen that the Client does reply with a 200OK.

 

But when I check the SBC sipmsg.log there is no 200OK.

The log show the SBC resending the re-invite a number of times (at 0.5,1,2,4,8 16 second intervals) after ~32 sec, the SBC times out and send a 408 back to the SS.