Re: Improving If-Modified-Since

> It doesn't solve the problem of two dates being exactly the
> same but the file has been modified, or the problem of a file getting
> modified and having it's date set into the past.  Date's alone
> are not a strong enough versioning system.  And don't try and tell me 
> that these don't happen.  They are as likely to happen as a file with
> a date far forward, and these kinds of problems are happening all
> the time.

These problems happen, but they often happen in ways that preserve
length. If you want something stronger than date, then use a checksum;
if you're going to use a checksum, you might as well use MD5 or SHA.

Checksums can be pre-computed by the server and periodically
recomputed by them as well, sent along as content-md5 headers and just
sent back in HTTP/1.1 get-if-not-content-MD5 request.