Send a 304 Not Modified response status from the server send in response to a client request that contains headers
If-None-Match, if the request resource hasn’t changed.
For example if a client request for a web page includes the header
If-Modified-Since: Fri, 22 Jul 2016 14:34:40 GMT and the page wasn’t modified since then, respond with the status line
HTTP/1.1 304 Not Modified.