And are you sure NOOP every 15 seconds will be suffisant ? your NAT seems to expire quickly.
Catch me using NAT ;)
I do, however, know that certain NAT-like boxes expire much more quickly than 29 minutes, at least when there are many idle connections. I haven't checked, but suspect that either linux 2.2 or 2.4 running as router may have some feature whereby it'll throw out some old connections if there's more than n connections being NAT'd. That's just a guess, really. I know one site where it reliably happens, but the sysadmins there are too clueless to know why.
--Arnt
