The HTTP If-Modified-Since request header makes a request Conditional.
The server sends back the requested resource, with a 200 status, only if it has been modified after the date in the If-Modified-Since header.
If the resource has not been modified since, the response is a 304 without any body, and the Last-Modified response header of the previous request contains the date of the last modification....request header makes a request conditional . The server sends back the...If-None-Match , If-Unmodified-Since conditional request headers 304 Not Modified...