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

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Warning (but without removing any existing Warning headers). A cacheSHOULD NOT attempt to revalidate a response simply because thatresponse became stale in transit; this might lead to an infiniteloop. A user agent that receives a stale response without a WarningMAY display a warning indication to the user.13.1.2 WarningsWhenever a cache returns a response that is neither first-hand nor"fresh enough" (in the sense of condition 2 in section 13.1.1), itMUST attach a warning to that effect, using a Warning general-header.The Warning header and the currently defined warnings are describedin section 14.46. The warning allows clients to take appropriateaction.Warnings MAY be used for other purposes, both cache-related andotherwise. The use of a warning, rather than an error status code,distinguish these responses from true failures.Warnings are assigned three digit warn-codes. The first digitindicates whether the Warning MUST or MUST NOT be deleted from astored cache entry after a successful revalidation:Fielding, et al. Standards Track [Page 76]RFC 2616 HTTP/1.1 June 19991xx Warnings that describe the freshness or revalidation status ofthe response, and so MUST be deleted after a successfulrevalidation. 1XX warn-codes MAY be generated by a cache only whenvalidating a cached entry. It MUST NOT be generated by clients.2xx Warnings that describe some aspect of the entity body or entityheaders that is not rectified by a revalidation (for example, alossy compression of the entity bodies) and which MUST NOT bedeleted after a successful revalidation.See section 14.46 for the definitions of the codes themselves.HTTP/1.0 caches will cache all Warnings in responses, withoutdeleting the ones in the first category. Warnings in responses thatare passed to HTTP/1.0 caches carry an extra warning-date field,which prevents a future HTTP/1.1 recipient from believing anerroneously cached Warning.Warnings also carry a warning text. The text MAY be in anyappropriate natural language (perhaps based on the client's Acceptheaders), and include an OPTIONAL indication of what character set is
used.Multiple warnings MAY be attached to a response (either by the originserver or by a cache), including multiple warnings with the same codenumber. For example, a server might provide the same warning withtexts in both English and Basque.When multiple warnings are attached to a response, it might not bepractical or reasonable to display all of them to the user. Thisversion of HTTP does not specify strict priority rules for decidingwhich warnings to display and in what order, but does suggest someheuristics.13.1.3 Cache-control MechanismsThe basic cache mechanisms in HTTP/1.1 (server-specified expirationtimes and validators) are implicit directives to caches. In somecases, a server or client might need to provide explicit directivesto the HTTP caches. We use the Cache-Control header for this purpose.The Cache-Control header allows a client or server to transmit avariety of directives in either requests or responses. Thesedirectives typically override the default caching algorithms. As ageneral rule, if there is any apparent conflict between headervalues, the most restrictive interpretation is applied (that is, theone that is most likely to preserve semantic transparency). However,Fielding, et al. Standards Track [Page 77]RFC 2616 HTTP/1.1 June 1999in some cases, cache-control directives are explicitly specified asweakening the approximation of semantic transparency (for example,"max-stale" or "public").The cache-control directives are described in detail in section 14.9.13.1.4 Explicit User Agent WarningsMany user agents make it possible for users to override the basiccaching mechanisms. For example, the user agent might allow the userto specify that cached entities (even explicitly stale ones) arenever validated. Or the user agent might habitually add "Cache-Control: max-stale=3600" to every request. The user agent SHOULD NOTdefault to either non-transparent behavior, or behavior that resultsin abnormally ineffective caching, but MAY be explicitly configuredto do so by an explicit action of the user.
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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
Page 34 and 35: RFC 2616 HTTP/1.1 June 1999HTTP/1.1
Page 36 and 37: message-body = entity-body| Transfe
Page 38 and 39: in advance.Messages MUST NOT includ
Page 40 and 41: equested resource, and 501 (Not Imp
Page 42 and 43: names) MUST use the following rules
Page 44 and 45: values for the first digit:- 1xx: I
Page 46 and 47: Fielding, et al. Standards Track [P
Page 48 and 49: how the transfer-length of a messag
Page 50 and 51: connection-token close.If either th
Page 52 and 53: idempotent (see section 9.1.2). Non
Page 54 and 55: compatibility with RFC 2068, a serv
Page 56 and 57: If at any point an error status is
Page 58 and 59: optional features implemented by th
Page 60 and 61: - Extending a database through an a
Page 62 and 63: intervention (or other means) on th
Page 64 and 65: 10.1.2 101 Switching ProtocolsThe s
Page 66 and 67: esponse MAY include new or updated
Page 70 and 71: The response MUST include the follo
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 86 and 87: If the user has overridden the basi
Page 88 and 89: in the past. This means that the re
Page 90 and 91: RFC 2616 HTTP/1.1 June 1999the time
Page 92 and 93: simple:response_is_fresh = (freshne
Page 94 and 95: match.In HTTP/1.1, a conditional re
Page 96 and 97: validators or strong validators. Cl
Page 98 and 99: - MAY send a weak entity tag instea
Page 100 and 101: Unless specifically constrained by
Page 104 and 105: warn-code 1xx, which are deleted ev
Page 106 and 107: for the associated entity, its enti
Page 108 and 109: - DELETE- POSTIn order to prevent d
Page 110 and 111: The Accept request-header field can
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
Page 118 and 119: containing an Authorization field,
Page 120 and 121: - Restrictions on what may be store
Page 122 and 123: improve privacy in some cases, we c
Page 126 and 127: must-revalidateBecause a cache MAY
Page 128 and 129: understand.For example, consider a
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Page 132 and 133: 14.14 Content-LocationThe Content-L
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Note: while the definition of Conte
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server SHOULD return them in the or
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Some origin server implementations
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Expires = "Expires" ":" HTTP-dateAn
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A client MUST include a Host header
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een modified since the date given b
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RFC 2616 HTTP/1.1 June 1999If none
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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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