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

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RFC 2616 HTTP/1.1 June 1999If none of the entity tags match, then the server MAY perform therequested method as if the If-None-Match header field did not exist,but MUST also ignore any If-Modified-Since header field(s) in therequest. That is, if no entity tags match, then the server MUST NOTreturn a 304 (Not Modified) response.If the request would, without the If-None-Match header field, resultin anything other than a 2xx or 304 status, then the If-None-Matchheader MUST be ignored. (See section 13.3.4 for a discussion ofserver behavior when both If-Modified-Since and If-None-Match appearin the same request.)The meaning of "If-None-Match: *" is that the method MUST NOT beperformed if the representation selected by the origin server (or bya cache, possibly using the Vary mechanism, see section 14.44)exists, and SHOULD be performed if the representation does not exist.This feature is intended to be useful in preventing races between PUToperations.Examples:If-None-Match: "xyzzy"If-None-Match: W/"xyzzy"If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"If-None-Match: *The result of a request having both an If-None-Match header field andeither an If-Match or an If-Unmodified-Since header fields isundefined by this specification.14.27 If-RangeIf a client has a partial copy of an entity in its cache, and wishesto have an up-to-date copy of the entire entity in its cache, itcould use the Range request-header with a conditional GET (usingeither or both of If-Unmodified-Since and If-Match.) However, if thecondition fails because the entity has been modified, the clientwould then have to make a second request to obtain the entire currententity-body.The If-Range header allows a client to "short-circuit" the secondrequest. Informally, its meaning is `if the entity is unchanged, sendme the part(s) that I am missing; otherwise, send me the entire newentity'.If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
Fielding, et al. Standards Track [Page 133]RFC 2616 HTTP/1.1 June 1999If the client has no entity tag for an entity, but does have a Last-Modified date, it MAY use that date in an If-Range header. (Theserver can distinguish between a valid HTTP-date and any form ofentity-tag by examining no more than two characters.) The If-Rangeheader SHOULD only be used together with a Range header, and MUST beignored if the request does not include a Range header, or if theserver does not support the sub-range operation.If the entity tag given in the If-Range header matches the currententity tag for the entity, then the server SHOULD provide thespecified sub-range of the entity using a 206 (Partial content)response. If the entity tag does not match, then the server SHOULDreturn the entire entity using a 200 (OK) response.14.28 If-Unmodified-SinceThe If-Unmodified-Since request-header field is used with a method tomake it conditional. If the requested resource has not been modifiedsince the time specified in this field, the server SHOULD perform therequested operation as if the If-Unmodified-Since header were notpresent.If the requested variant has been modified since the specified time,the server MUST NOT perform the requested operation, and MUST returna 412 (Precondition Failed).If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-dateAn example of the field is:If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMTIf the request normally (i.e., without the If-Unmodified-Sinceheader) would result in anything other than a 2xx or 412 status, theIf-Unmodified-Since header SHOULD be ignored.If the specified date is invalid, the header is ignored.The result of a request having both an If-Unmodified-Since headerfield and either an If-None-Match or an If-Modified-Since headerfields is undefined by this specification.
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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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in the past. This means that the re
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RFC 2616 HTTP/1.1 June 1999the time
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simple:response_is_fresh = (freshne
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match.In HTTP/1.1, a conditional re
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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
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Page 148 and 149: 14.29 Last-ModifiedThe Last-Modifie
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Network Working Group R. Fielding Request for Comments: 2616 ...

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