Answer by Rakesh for Stop browser to make HTTP requests for images that should stay cached - mod_expires

Cache Validation and the 304 response

There are a number of situations in which Internet Explorer needs to check whether a cached entry is valid:

• The cached entry has no expiration date and the content is being accessed for the first time in a browser session

• The cached entry has an expiration date but it has expired

• The user has requested a page update by clicking the Refresh button or pressing F5

If the cached entry has a last modification date, IE sends it in the If-Modified-Since header of a GET request message:

GET /images/logo.gif HTTP/1.1Accept: */*Referer: http://www.google.com/Accept-Encoding: gzip, deflateIf-Modified-Since: Thu, 23 Sep 2004 17:42:04 GMTUser-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;)Host: www.google.com

The server checks the If-Modified-Since header and responds accordingly. If the content has not been changed since the date/time specified, it replies with a status code of 304 and a response message that just contains headers:

HTTP/1.1 304 Not ModifiedContent-Type: text/htmlServer: GWS/2.1Content-Length: 0Date: Thu, 04 Oct 2004 12:00:00 GMT

The response can be quickly downloaded because it contains no content and causes IE to read the data it requires from the cache. In effect, it is like a redirection to the local browser cache.

If the requested object has actually changed since the date/time in the If-Modified-Since header, the server responses with a status code of 200 and supplies the modified version of the resource.