Changeset 183 for draft-ietf-httpbis/latest/p4-conditional.html
- Timestamp:
- 02/02/08 01:04:56 (14 years ago)
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draft-ietf-httpbis/latest/p4-conditional.html
r182 r183 745 745 ETag: W/"xyzzy" 746 746 ETag: "" 747 </pre><div id="rfc.iref.i.1"></div> 747 </pre><p id="rfc.section.6.1.p.4">The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable 748 validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date 749 values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification 750 dates. 751 </p> 752 <p id="rfc.section.6.1.p.5">The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an 753 appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality 754 would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0) 755 are never used for purposes of validating a cache entry. 756 </p> 757 <div id="rfc.iref.i.1"></div> 748 758 <div id="rfc.iref.h.2"></div> 749 759 <h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a> <a id="header.if-match" href="#header.if-match">If-Match</a></h2> … … 899 909 </p> 900 910 <p id="rfc.section.6.6.p.8">HTTP/1.1 servers <em class="bcp14">SHOULD</em> send Last-Modified whenever feasible. 911 </p> 912 <p id="rfc.section.6.6.p.9">The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered 913 to be valid if the entity has not been modified since the Last-Modified value. 901 914 </p> 902 915 <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a> <a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
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