Network Working Group R. Fielding Request for Comments: 2616 ...

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- MAY send a weak entity tag instead of a strong entity tag, ifperformance considerations support the use of weak entity tags,or if it is unfeasible to send a strong entity tag.- SHOULD send a Last-Modified value if it is feasible to send one,unless the risk of a breakdown in semantic transparency thatcould result from using this date in an If-Modified-Since headerwould lead to serious problems.In other words, the preferred behavior for an HTTP/1.1 origin serveris to send both a strong entity tag and a Last-Modified value.In order to be legal, a strong entity tag MUST change whenever theassociated entity value changes in any way. A weak entity tag SHOULDchange whenever the associated entity changes in a semanticallysignificant way.Note: in order to provide semantically transparent caching, anorigin server must avoid reusing a specific strong entity tagvalue for two different entities, or reusing a specific weakentity tag value for two semantically different entities. Cacheentries might persist for arbitrarily long periods, regardless ofexpiration times, so it might be inappropriate to expect that acache will never again attempt to validate an entry using avalidator that it obtained at some point in the past.HTTP/1.1 clients:- If an entity tag has been provided by the origin server, MUSTuse that entity tag in any cache-conditional request (using If-Match or If-None-Match).- If only a Last-Modified value has been provided by the originserver, SHOULD use that value in non-subrange cache-conditionalrequests (using If-Modified-Since).Fielding, et al. Standards Track [Page 89]RFC 2616 HTTP/1.1 June 1999- If only a Last-Modified value has been provided by an HTTP/1.0origin server, MAY use that value in subrange cache-conditionalrequests (using If-Unmodified-Since:). The user agent SHOULDprovide a way to disable this, in case of difficulty.- If both an entity tag and a Last-Modified value have beenprovided by the origin server, SHOULD use both validators in
cache-conditional requests. This allows both HTTP/1.0 andHTTP/1.1 caches to respond appropriately.An HTTP/1.1 origin server, upon receiving a conditional request thatincludes both a Last-Modified date (e.g., in an If-Modified-Since orIf-Unmodified-Since header field) and one or more entity tags (e.g.,in an If-Match, If-None-Match, or If-Range header field) as cachevalidators, MUST NOT return a response status of 304 (Not Modified)unless doing so is consistent with all of the conditional headerfields in the request.An HTTP/1.1 caching proxy, upon receiving a conditional request thatincludes both a Last-Modified date and one or more entity tags ascache validators, MUST NOT return a locally cached response to theclient unless that cached response is consistent with all of theconditional header fields in the request.Note: The general principle behind these rules is that HTTP/1.1servers and clients should transmit as much non-redundantinformation as is available in their responses and requests.HTTP/1.1 systems receiving this information will make the mostconservative assumptions about the validators they receive.HTTP/1.0 clients and caches will ignore entity tags. Generally,last-modified values received or used by these systems willsupport transparent and efficient caching, and so HTTP/1.1 originservers should provide Last-Modified values. In those rare caseswhere the use of a Last-Modified value as a validator by anHTTP/1.0 system could result in a serious problem, then HTTP/1.1origin servers should not provide one.13.3.5 Non-validating ConditionalsThe principle behind entity tags is that only the service authorknows the semantics of a resource well enough to select anappropriate cache validation mechanism, and the specification of anyvalidator comparison function more complex than byte-equality wouldopen up a can of worms. Thus, comparisons of any other headers(except Last-Modified, for compatibility with HTTP/1.0) are neverused for purposes of validating a cache entry.Fielding, et al. Standards Track [Page 90]RFC 2616 HTTP/1.1 June 199913.4 Response Cacheability
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eferred to as "HTTP/1.1", and is an
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12.1 Server-driven Negotiation ....
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14.40 Trailer .....................
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1.2 RequirementsThe key words "MUST
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Fielding, et al. Standards Track [P
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equest chain ----------------------
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Elements separated by a bar ("|") a
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HTTP/1.1 header field values can be
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An application that sends a request
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3.2.3 URI ComparisonWhen comparing
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is not desirable and is discouraged
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CRLFchunk = chunk-size [ chunk-exte
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When in canonical form, media subty
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3.9 Quality ValuesHTTP content nego
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RFC 2616 HTTP/1.1 June 1999HTTP/1.1
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message-body = entity-body| Transfe
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in advance.Messages MUST NOT includ
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equested resource, and 501 (Not Imp
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names) MUST use the following rules
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values for the first digit:- 1xx: I
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Page 48 and 49: how the transfer-length of a messag
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Page 64 and 65: 10.1.2 101 Switching ProtocolsThe s
Page 66 and 67: esponse MAY include new or updated
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Page 72 and 73: If the client is sending data, a se
Page 74 and 75: If the response could be unacceptab
Page 76 and 77: 10.4.15 414 Request-URI Too LongThe
Page 78 and 79: 10.5.5 504 Gateway TimeoutThe serve
Page 82 and 83: Caching would be useless if it did
Page 84 and 85: Warning (but without removing any e
Page 86 and 87: If the user has overridden the basi
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Page 90 and 91: RFC 2616 HTTP/1.1 June 1999the time
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Page 94 and 95: match.In HTTP/1.1, a conditional re
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Page 112 and 113: image/jpeg = 0.5text/html;level=2 =
Page 114 and 115: Note: If the request does not inclu
Page 116 and 117: Accept-Ranges: bytesbut are not req
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Page 120 and 121: - Restrictions on what may be store
Page 122 and 123: improve privacy in some cases, we c
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Page 132 and 133: 14.14 Content-LocationThe Content-L
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14.29 Last-ModifiedThe Last-Modifie
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MAY require that behavior be consis
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yte-ranges-specifier = bytes-unit "
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Content) instead of 200 (OK).- The
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The TE request-header field indicat
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a cache cannot determine from the r
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SHOULD NOT, by default, forward the
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this user agent behavior in mind.Re
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designers and implementors be parti
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strongly correlated to the membersh
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serious security considerations. Co
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The HTTP protocol has evolved consi
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Protocol -- HTTP/1.0", RFC 1945, Ma
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Implementation and Analysis", RFC 1
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University of California, IrvineIrv
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Notes:1) Additional CRLFs may prece
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full compliance with the MIME proto
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ead chunk-size and CRLF}read entity
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- understand any valid response in
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CREATE had a race that required an
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also needed to be a general header,
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Network Working Group R. Fielding Request for Comments: 2616 ...

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