Satisfying IMS from HEAD or IMS would help where browser caches

One thing I've noticed with CERN originally and now when testing
squid2/patch2 is that browsers with internal caching tend to
frustrate a corporate cache. The basic problem is that if an
If-Modified-Since request for an object cached in the browser, misses
on the corporate cache, one has to wait for a round trip time (and do
a demand dial, even if the request could be satisfied from the
browser cache) if the corporate cache never had the page (cache was
built after the browser cached the page) or has lost the page
(limited resources, CERN's absolute time limit, or a demand dial
failure, etc., causing the the cache entry to be wiped by an error
response).

What I used to do on CERN, is occasionally trawl the logs for 304s
that had been proxied (or zero lengths, which came from some earlier
browsers) and refetch them at a cheap rate time. However that
involves fetching the whole page, which might prove not to be useful,
and, as a side effect, marking it recently accessed, so that a
potentially large page might be kept longer than the true access
pattern suggested.

I found the following URL which suggests that someone (not sure how
official this is) was considering caching IMS responses in squid:

http://hem.passagen.se/hno/squid/historic/IMS.html

However, it also occurs to me that HEAD ought to contain all the meta
data. However, caching a page in IE4, bypassing squid, then manually
doing a HEAD for the page through squid, then finally restarting IE4,
configured to use squid, results in a TCP-MISS. Searching the code
tends to confirm this, but I wasn't 100% certain at that stage.

Is there anything in HTTP/1.1 or common misimplementations of HTTP
that would make satisfying IMS from HEAD unsafe (assuming reasonable
plausibility checks are done)? Would it be so unsafe as to make it
unreasonable to have a leaf cache option to permit it? If not, doing
TCP-IMS-HIT from HEAD for GET requests, would allow me to fetch just
the HEADs for recent TCP-MISS/304 events.

-- 
David Woolley - Office: David Woolley <djw@bts.co.uk>
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