Heuristic caching without validators

Hi all

p6 and RFC 2616 when talking about heuristic caching, place a should 
level limit on heuristic freshness (10% of time elapsed since L-M), but 
only when there is a Last-Modified header.

However, it has been noted that some caches will store and reuse 
responses that have no validators at all.

Obviously it only makes sense to cache something if it can be re-used, 
and without validators, it can't be re-validated with the O-S, and 
therefore the only way such a resource can be re-used is if the cache 
makes some assumption about freshness.

In previous versions, we had heuristics for minimum effective freshness 
based on content type.  This caused all manner of problems, so we 
dropped it for our current version, however we're finding relatively 
poor cachability of the internet, and so the value of caching is proving 
to be limited.

Since we can't wait forever for web site operators to consider and roll 
out reasonable caching directives, I believe in order to provide some 
real benefit from caching, we need to take a much more aggressive 
stance.  This will require at least heuristic caching, and I'm fairly 
certain also heuristic caching of responses that don't have any 
validators.

Are there any guidelines for this?  Since we're a shared intermediary 
cache, we can't do things like cache for the duration of the browser 
session, so it's going to come down to a table of min freshness per 
content-type (where there are no validators).

Does this issue deserve any discussion in the RFC / p6?  It's very light 
on heuristic freshness - probably for a reason.

Regards

Adrien