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

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Some origin server implementations might not have a clock available.An origin server without a clock MUST NOT assign Expires or Last-Modified values to a response, unless these values were associatedwith the resource by a system or user with a reliable clock. It MAYassign an Expires value that is known, at or before serverconfiguration time, to be in the past (this allows "pre-expiration"of responses without storing separate Expires values for eachresource).Fielding, et al. Standards Track [Page 125]RFC 2616 HTTP/1.1 June 199914.19 ETagThe ETag response-header field provides the current value of theentity tag for the requested variant. The headers used with entitytags are described in sections 14.24, 14.26 and 14.44. The entity tagMAY be used for comparison with other entities from the same resource(see section 13.3.3).ETag = "ETag" ":" entity-tagExamples:ETag: "xyzzy"ETag: W/"xyzzy"ETag: ""14.20 ExpectThe Expect request-header field is used to indicate that particularserver behaviors are required by the client.Expect= "Expect" ":" 1#expectationexpectation = "100-continue" | expectation-extensionexpectation-extension = token [ "=" ( token | quoted-string )*expect-params ]expect-params = ";" token [ "=" ( token | quoted-string ) ]A server that does not understand or is unable to comply with any ofthe expectation values in the Expect field of a request MUST respond
with appropriate error status. The server MUST respond with a 417(Expectation Failed) status if any of the expectations cannot be metor, if there are other problems with the request, some other 4xxstatus.This header field is defined with extensible syntax to allow forfuture extensions. If a server receives a request containing anExpect field that includes an expectation-extension that it does notsupport, it MUST respond with a 417 (Expectation Failed) status.Comparison of expectation values is case-insensitive for unquotedtokens (including the 100-continue token), and is case-sensitive forquoted-string expectation-extensions.Fielding, et al. Standards Track [Page 126]RFC 2616 HTTP/1.1 June 1999The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUSTreturn a 417 (Expectation Failed) status if it receives a requestwith an expectation that it cannot meet. However, the Expectrequest-header itself is end-to-end; it MUST be forwarded if therequest is forwarded.Many older HTTP/1.0 and HTTP/1.1 applications do not understand theExpect header.See section 8.2.3 for the use of the 100 (continue) status.14.21 ExpiresThe Expires entity-header field gives the date/time after which theresponse is considered stale. A stale cache entry may not normally bereturned by a cache (either a proxy cache or a user agent cache)unless it is first validated with the origin server (or with anintermediate cache that has a fresh copy of the entity). See section13.2 for further discussion of the expiration model.The presence of an Expires field does not imply that the originalresource will change or cease to exist at, before, or after thattime.The format is an absolute date and time as defined by HTTP-date insection 3.3.1; it MUST be in RFC 1123 date format:
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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
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10.4.15 414 Request-URI Too LongThe
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10.5.5 504 Gateway TimeoutThe serve
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Caching would be useless if it did
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Warning (but without removing any e
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If the user has overridden the basi
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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 102 and 103: Fielding, et al. Standards Track [P
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
Page 134 and 135: Note: while the definition of Conte
Page 136 and 137: server SHOULD return them in the or
Page 140 and 141: Expires = "Expires" ":" HTTP-dateAn
Page 142 and 143: A client MUST include a Host header
Page 144 and 145: een modified since the date given b
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 154 and 155: Content) instead of 200 (OK).- The
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Page 164 and 165: this user agent behavior in mind.Re
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Page 172 and 173: The HTTP protocol has evolved consi
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Page 178 and 179: University of California, IrvineIrv
Page 182 and 183: Notes:1) Additional CRLFs may prece
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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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