phranque - 1:41 pm on Feb 1, 2011 (gmt 0)

the actual "mechanism" is:
- the server sends a HTTP response header with the requested resource like:

Etag: "nnnnnnnn"

- if the browser caches that resource it would also maintain the Etag for that resource.
- the next time that browser requests that resource it also sends a HTTP request header like:

If-None-Match: "nnnnnnnn"

- if the server can match Etags with the current version of that resource, instead of responding with that resource it will send a HTTP 304 Not Modified status.
- then the browser will use the cached resource instead of having to wait for another copy.