Request Timeout response status code means that the server would like to shut down this unused connection. It is sent on an idle connection by some servers, even without any previous request by the client.
A server should send the "close"
Connection header field in the response, since
408 implies that the server has decided to close the connection rather than continue waiting.
This response is used much more since some browsers, like Chrome, Firefox 27+, or IE9, use HTTP pre-connection mechanisms to speed up surfing. Also note that some servers merely shut down the connection without sending this message.
400 Request Timeout
|RFC 7231, section 6.5.7: 408 Request Timeout||Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content|