input: [Code] 304 (Not modified) [Description] 
output: The requested page hasn't been modified since the last request. When the server returns this response, it doesn't return the contents of the page. You should configure your server to return this response (called the If-Modified-Since HTTP header) when a page hasn't changed since the last time the requestor asked for it. This saves you bandwidth and overhead because your server can tell Googlebot that a page hasn't changed since the last time it was crawled.