10.9 If-Modified-Since

The If-Modified-Since request-header field is used with the GET method to make it conditional: if the requested resource has not been modified since the time specified in this field, a copy of the resource will not be returned from the server; instead, a 304 (not modified) response will be returned without any Entity-Body.

       If-Modified-Since = "If-Modified-Since" ":" HTTP-date

An example of the field is:

       If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A conditional GET method requests that the identified resource be transferred only if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes the following cases:

1. If the request would normally result in anything other than a 200 (ok) status, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid.

2. If the resource has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.

3. If the resource has not been modified since a valid If-Modified-Since date, the server shall return a 304 (not modified) response.

The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead.