The link on the letter M is: https://www.w3.org/2005/MWI/BPWG/techs/CachingWithE

Question

The link on the letter M is: https://www.w3.org/2005/MWI/BPWG/techs/CachingWithETag.html

The second link on the letter M is:  http://www.pocketsoap.com/weblog/stories/2002/05/0015.html

4. The HTTP RESPONSE message (Total 60 points) Consider the figure above, where the server is sending a HTTP RESPONSE message back the client Suppose the server-to-client HTTP RESPONSE message is the following HTTP/1.1 200 OK Date: Mon, 25 Feb 2018 18:00:00 +0000 Server: Apache/2.2.3 (CentoS) Last-Modified: Thu, 01 Feb 2018 18:00:00 +0000 ETag: "12345-abc-67890" Content-Length: 2635 Keep-Alive: timeout-23, max-10 Connection: Keep-alive Content-type: text/html

Explanation / Answer

If-None-Match is the HTTP request header. It makes the request conditional. When requesting url generally we have serveral request methods like GET, POST, PUT, DELETE, PATCH. For GET and HEAD request methods, the server will send back the whatever user requested resource, with a Status code of 200(OK), only if that requested resourse doesn't have an ETag matching the given ones. For other methods, the request will be processed only if the eventually existing resource's ETag doesn't match any of the values listed.

When the request condition fails for GET and HEAD request methods for requesting resource, then the server must return HTTP status code 304 (Not Modified). For that request methods apply server-side changes, the status code 412 (Precondition Failed) is used. Note that the server generating a response code of 304 Must generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, ETag, Date, Content-Location, Expires, and Vary.

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