HTTP 304 status code

304 Not Modified

If the client sends a conditional GET request and the request is allowed, but the content of the document (since the last access or according to the conditions of the request) has not changed, The server should return this status code. A 304 response is prohibited from including a message body, so it always ends with the first blank line after the message header.

The response must contain the following header information:

Date, unless the server does not have a clock. If servers without clocks follow these rules, then proxy servers and clients can add the Date field to received response headers themselves (as specified in RFC 2068), and the caching mechanism will work normally.

ETag and/or Content-Location, if the same request should have returned a 200 response.

Expires, Cache-Control, and/or Vary, if its value may be different from the value corresponding to other previous responses to the same variable.

If this response request uses strong cache verification, then this response should not contain other entity headers; otherwise (for example, a conditional GET request uses weak cache verification), this response is prohibited from containing Other entity headers; this avoids inconsistencies between cached entity content and updated entity header information.

If a 304 response indicates that an entity is currently not cached, the caching system must ignore the response and repeat the request without the restriction.

If a 304 response is received that requires an update to a cache entry, the cache system must update the entire entry to reflect the values ​​of all fields that were updated in the response.