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

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must-revalidateBecause a cache MAY be configured to ignore a server's specifiedexpiration time, and because a client request MAY include a maxstaledirective (which has a similar effect), the protocol alsoincludes a mechanism for the origin server to require revalidationof a cache entry on any subsequent use. When the must-revalidatedirective is present in a response received by a cache, that cacheMUST NOT use the entry after it becomes stale to respond to aFielding, et al. Standards Track [Page 114]RFC 2616 HTTP/1.1 June 1999subsequent request without first revalidating it with the originserver. (I.e., the cache MUST do an end-to-end revalidation everytime, if, based solely on the origin server's Expires or max-agevalue, the cached response is stale.)The must-revalidate directive is necessary to support reliableoperation for certain protocol features. In all circumstances anHTTP/1.1 cache MUST obey the must-revalidate directive; inparticular, if the cache cannot reach the origin server for anyreason, it MUST generate a 504 (Gateway Timeout) response.Servers SHOULD send the must-revalidate directive if and only iffailure to revalidate a request on the entity could result inincorrect operation, such as a silently unexecuted financialtransaction. Recipients MUST NOT take any automated action thatviolates this directive, and MUST NOT automatically provide anunvalidated copy of the entity if revalidation fails.Although this is not recommended, user agents operating undersevere connectivity constraints MAY violate this directive but, ifso, MUST explicitly warn the user that an unvalidated response hasbeen provided. The warning MUST be provided on each unvalidatedaccess, and SHOULD require explicit user confirmation.proxy-revalidateThe proxy-revalidate directive has the same meaning as the mustrevalidatedirective, except that it does not apply to non-shareduser agent caches. It can be used on a response to anauthenticated request to permit the user's cache to store andlater return the response without needing to revalidate it (sinceit has already been authenticated once by that user), while stillrequiring proxies that service many users to revalidate each time(in order to make sure that each user has been authenticated).Note that such authenticated responses also need the public cachecontrol directive in order to allow them to be cached at all.
14.9.5 No-Transform Directiveno-transformImplementors of intermediate caches (proxies) have found it usefulto convert the media type of certain entity bodies. A nontransparentproxy might, for example, convert between imageformats in order to save cache space or to reduce the amount oftraffic on a slow link.Serious operational problems occur, however, when thesetransformations are applied to entity bodies intended for certainkinds of applications. For example, applications for medicalFielding, et al. Standards Track [Page 115]RFC 2616 HTTP/1.1 June 1999imaging, scientific data analysis and those using end-to-endauthentication, all depend on receiving an entity body that is bitfor bit identical to the original entity-body.Therefore, if a message includes the no-transform directive, anintermediate cache or proxy MUST NOT change those headers that arelisted in section 13.5.2 as being subject to the no-transformdirective. This implies that the cache or proxy MUST NOT changeany aspect of the entity-body that is specified by these headers,including the value of the entity-body itself.14.9.6 Cache Control ExtensionsThe Cache-Control header field can be extended through the use of oneor more cache-extension tokens, each with an optional assigned value.Informational extensions (those which do not require a change incache behavior) MAY be added without changing the semantics of otherdirectives. Behavioral extensions are designed to work by acting asmodifiers to the existing base of cache directives. Both the newdirective and the standard directive are supplied, such thatapplications which do not understand the new directive will defaultto the behavior specified by the standard directive, and those thatunderstand the new directive will recognize it as modifying therequirements associated with the standard directive. In this way,extensions to the cache-control directives can be made withoutrequiring changes to the base protocol.This extension mechanism depends on an HTTP cache obeying all of thecache-control directives defined for its native HTTP-version, obeyingcertain extensions, and ignoring all directives that it does not
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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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how the transfer-length of a messag
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connection-token close.If either th
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idempotent (see section 9.1.2). Non
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compatibility with RFC 2068, a serv
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If at any point an error status is
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optional features implemented by th
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- Extending a database through an a
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intervention (or other means) on th
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10.1.2 101 Switching ProtocolsThe s
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esponse MAY include new or updated
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The response MUST include the follo
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If the client is sending data, a se
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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 80 and 81: Fielding, et al. Standards Track [P
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
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
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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
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Page 132 and 133: 14.14 Content-LocationThe Content-L
Page 134 and 135: Note: while the definition of Conte
Page 136 and 137: server SHOULD return them in the or
Page 138 and 139: Some origin server implementations
Page 140 and 141: Expires = "Expires" ":" HTTP-dateAn
Page 142 and 143: A client MUST include a Host header
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Page 146 and 147: RFC 2616 HTTP/1.1 June 1999If none
Page 148 and 149: 14.29 Last-ModifiedThe Last-Modifie
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Page 152 and 153: yte-ranges-specifier = bytes-unit "
Page 154 and 155: Content) instead of 200 (OK).- The
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Page 172 and 173: The HTTP protocol has evolved consi
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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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