Last modified--expires preliminary explanation

Last-modified and expires are for browsers, and the ETag is client-independent, so it fits in the REST architecture. Both are applied on the browser side of the difference is: expires date before arrival, the browser will not make new requests, unless the user by the browser refresh, so last-modified and expires basically is to reduce the number of browser requests to the server, The ETag is more focused on the client-server connection.

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Http://www.cnblogs.com/sunxucool/p/3342758.html

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Just like
expired on so dont worry about it, or it expires at a impossible time point
Last modifed at some time, so it s a fact.

Last modified--expires preliminary explanation