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

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If the user has overridden the basic caching mechanisms, the useragent SHOULD explicitly indicate to the user whenever this results inthe display of information that might not meet the server'stransparency requirements (in particular, if the displayed entity isknown to be stale). Since the protocol normally allows the user agentto determine if responses are stale or not, this indication need onlybe displayed when this actually happens. The indication need not be adialog box; it could be an icon (for example, a picture of a rottingfish) or some other indicator.If the user has overridden the caching mechanisms in a way that wouldabnormally reduce the effectiveness of caches, the user agent SHOULDcontinually indicate this state to the user (for example, by adisplay of a picture of currency in flames) so that the user does notinadvertently consume excess resources or suffer from excessivelatency.13.1.5 Exceptions to the Rules and WarningsIn some cases, the operator of a cache MAY choose to configure it toreturn stale responses even when not requested by clients. Thisdecision ought not be made lightly, but may be necessary for reasonsof availability or performance, especially when the cache is poorlyconnected to the origin server. Whenever a cache returns a staleresponse, it MUST mark it as such (using a Warning header) enablingthe client software to alert the user that there might be a potentialproblem.Fielding, et al. Standards Track [Page 78]RFC 2616 HTTP/1.1 June 1999It also allows the user agent to take steps to obtain a first-hand orfresh response. For this reason, a cache SHOULD NOT return a staleresponse if the client explicitly requests a first-hand or fresh one,unless it is impossible to comply for technical or policy reasons.13.1.6 Client-controlled BehaviorWhile the origin server (and to a lesser extent, intermediate caches,by their contribution to the age of a response) are the primarysource of expiration information, in some cases the client might needto control a cache's decision about whether to return a cached
esponse without validating it. Clients do this using severaldirectives of the Cache-Control header.A client's request MAY specify the maximum age it is willing toaccept of an unvalidated response; specifying a value of zero forcesthe cache(s) to revalidate all responses. A client MAY also specifythe minimum time remaining before a response expires. Both of theseoptions increase constraints on the behavior of caches, and so cannotfurther relax the cache's approximation of semantic transparency.A client MAY also specify that it will accept stale responses, up tosome maximum amount of staleness. This loosens the constraints on thecaches, and so might violate the origin server's specifiedconstraints on semantic transparency, but might be necessary tosupport disconnected operation, or high availability in the face ofpoor connectivity.13.2 Expiration Model13.2.1 Server-Specified ExpirationHTTP caching works best when caches can entirely avoid makingrequests to the origin server. The primary mechanism for avoidingrequests is for an origin server to provide an explicit expirationtime in the future, indicating that a response MAY be used to satisfysubsequent requests. In other words, a cache can return a freshresponse without first contacting the server.Our expectation is that servers will assign future explicitexpiration times to responses in the belief that the entity is notlikely to change, in a semantically significant way, before theexpiration time is reached. This normally preserves semantictransparency, as long as the server's expiration times are carefullychosen.Fielding, et al. Standards Track [Page 79]RFC 2616 HTTP/1.1 June 1999The expiration mechanism applies only to responses taken from a cacheand not to first-hand responses forwarded immediately to therequesting client.If an origin server wishes to force a semantically transparent cacheto validate every request, it MAY assign an explicit expiration time
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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
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 84 and 85: Warning (but without removing any e
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
Page 130 and 131: new coding is known to be acceptabl
Page 132 and 133: 14.14 Content-LocationThe Content-L
Page 134 and 135: 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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Network Working Group R. Fielding Request for Comments: 2616 ...

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