if-modified-since vs if-none-match

Question

What could be the difference between if-modified-since and if-none-match? I have a feeling that if-none-match is used for files whereas if-modified-since is used for pages?<

Answer 1

Unless stated as weak by the server, an ETag is considered a strong validator, and can thus be used to satify a conditional ranged request. However, most automatically generated ETags exhibit difficulties in server farm situations, since they often use inode information and / or a unique persistent counter. In practice, I have found the Last Modified header to be sufficient for fairly static content, e.g. serving up protected static content, since the write time of the file makes a reasonably good validator.

The ETag is by far the most flexible. Conforming clients are required to send the ETag in a conditional request, whereas they SHOULD send both if available.