Alternative cache expire method?

Hello,

as far as I can see, squid unlinks all files in cache that is expired.
Is it possible to contact the source to check if it is really expired
before unlinking? Has it already been done? Were there any reasons not
to do so?

The reason why I am asking is that I encountered big files that were
unchanged since two years and getting those files in my cache was a
quite bandwidth intensive exercise. I know that the ttl can be tuned in
squid.conf to try to find an optimum between cache consistency and
conserving bandwidth, but I think only the origin of the file can give
an authoritative answer whether the file is out of date or is still
valid.

If verifying and replenishing the ttl or unlinking the file gets too
expensive in terms of bandwidth, there could be a threshold based
heuristics when to switch from one method to another method.

Any comments?
Thank you for any pointers.

-- 
Kang-Jin Lee                             lee@arco.met.fu-berlin.de
                                         lee@tengu.in-berlin.de
                                         2:2410/810