Idle Timeout not working properly

[laragio]

Hi,

we have set up a Captive Portal system with radius authentication. The radius set also the idle timeout to 30 minutes.
Many users have noted that disconnection were applied after only 5-10 minutes of inactivity (let me say it was quite random).

We have read the code on /etc/inc/captiveportal.inc and on the function captiveportal_prune_old() in  periodic RADIUS reauthentication
the timestamp of ipfw are canceled with: exec("/sbin/ipfw zero {$cpdb[$i][1]}");
This timestamp is used to calculate the idle-timeout for every users with the difference between rule-timestamp and current time. If the timestamp is zero is used the login timestamp so, after the first 30 minutes of connection, the users were disconnected only after 5 minutes of inactivity.

Now we have commented out this line and everything is  working fine, and the idle-timeout now is always 30 minutes.

I would like to know why there is this line in the code?  ???

Thanks