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

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Caching would be useless if it did not significantly improveperformance. The goal of caching in HTTP/1.1 is to eliminate the needto send requests in many cases, and to eliminate the need to sendfull responses in many other cases. The former reduces the number ofnetwork round-trips required for many operations; we use an"expiration" mechanism for this purpose (see section 13.2). Thelatter reduces network bandwidth requirements; we use a "validation"mechanism for this purpose (see section 13.3).Requirements for performance, availability, and disconnectedoperation require us to be able to relax the goal of semantictransparency. The HTTP/1.1 protocol allows origin servers, caches,Fielding, et al. Standards Track [Page 74]RFC 2616 HTTP/1.1 June 1999and clients to explicitly reduce transparency when necessary.However, because non-transparent operation may confuse non-expertusers, and might be incompatible with certain server applications(such as those for ordering merchandise), the protocol requires thattransparency be relaxed- only by an explicit protocol-level request when relaxed byclient or origin server- only with an explicit warning to the end user when relaxed bycache or clientTherefore, the HTTP/1.1 protocol provides these important elements:1. Protocol features that provide full semantic transparency whenthis is required by all parties.2. Protocol features that allow an origin server or user agent toexplicitly request and control non-transparent operation.3. Protocol features that allow a cache to attach warnings toresponses that do not preserve the requested approximation ofsemantic transparency.A basic principle is that it must be possible for the clients todetect any potential relaxation of semantic transparency.Note: The server, cache, or client implementor might be faced withdesign decisions not explicitly discussed in this specification.If a decision might affect semantic transparency, the implementorought to err on the side of maintaining transparency unless a
careful and complete analysis shows significant benefits inbreaking transparency.13.1.1 Cache CorrectnessA correct cache MUST respond to a request with the most up-to-dateresponse held by the cache that is appropriate to the request (seesections 13.2.5, 13.2.6, and 13.12) which meets one of the followingconditions:1. It has been checked for equivalence with what the origin serverwould have returned by revalidating the response with theorigin server (section 13.3);Fielding, et al. Standards Track [Page 75]RFC 2616 HTTP/1.1 June 19992. It is "fresh enough" (see section 13.2). In the default case,this means it meets the least restrictive freshness requirementof the client, origin server, and cache (see section 14.9); ifthe origin server so specifies, it is the freshness requirementof the origin server alone.If a stored response is not "fresh enough" by the mostrestrictive freshness requirement of both the client and theorigin server, in carefully considered circumstances the cacheMAY still return the response with the appropriate Warningheader (see section 13.1.5 and 14.46), unless such a responseis prohibited (e.g., by a "no-store" cache-directive, or by a"no-cache" cache-request-directive; see section 14.9).3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),or error (4xx or 5xx) response message.If the cache can not communicate with the origin server, then acorrect cache SHOULD respond as above if the response can becorrectly served from the cache; if not it MUST return an error orwarning indicating that there was a communication failure.If a cache receives a response (either an entire response, or a 304(Not Modified) response) that it would normally forward to therequesting client, and the received response is no longer fresh, thecache SHOULD forward it to the requesting client without adding a new
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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
Page 32 and 33: 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 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
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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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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