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

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3.9 Quality ValuesHTTP content negotiation (section 12) uses short "floating point"numbers to indicate the relative importance ("weight") of variousnegotiable parameters. A weight is normalized to a real number inthe range 0 through 1, where 0 is the minimum and 1 the maximumvalue. If a parameter has a quality value of 0, then content withthis parameter is `not acceptable' for the client. HTTP/1.1applications MUST NOT generate more than three digits after thedecimal point. User configuration of these values SHOULD also belimited in this fashion.qvalue = ( "0" [ "." 0*3DIGIT ] )| ( "1" [ "." 0*3("0") ] )"Quality values" is a misnomer, since these values merely representrelative degradation in desired quality.3.10 Language TagsA language tag identifies a natural language spoken, written, orotherwise conveyed by human beings for communication of informationto other human beings. Computer languages are explicitly excluded.HTTP uses language tags within the Accept-Language and Content-Language fields.The syntax and registry of HTTP language tags is the same as thatdefined by RFC 1766 [1]. In summary, a language tag is composed of 1or more parts: A primary language tag and a possibly empty series ofsubtags:language-tag = primary-tag *( "-" subtag )primary-tag = 1*8ALPHAsubtag = 1*8ALPHAWhite space is not allowed within the tag and all tags are caseinsensitive.The name space of language tags is administered by theIANA. Example tags include:en, en-US, en-cockney, i-cherokee, x-pig-latinFielding, et al. Standards Track [Page 29]RFC 2616 HTTP/1.1 June 1999where any two-letter primary-tag is an ISO-639 language abbreviation
and any two-letter initial subtag is an ISO-3166 country code. (Thelast three tags above are not registered tags; all but the last areexamples of tags which could be registered in future.)3.11 Entity TagsEntity tags are used for comparing two or more entities from the samerequested resource. HTTP/1.1 uses entity tags in the ETag (section14.19), If-Match (section 14.24), If-None-Match (section 14.26), andIf-Range (section 14.27) header fields. The definition of how theyare used and compared as cache validators is in section 13.3.3. Anentity tag consists of an opaque quoted string, possibly prefixed bya weakness indicator.entity-tag = [ weak ] opaque-tagweak = "W/"opaque-tag = quoted-stringA "strong entity tag" MAY be shared by two entities of a resourceonly if they are equivalent by octet equality.A "weak entity tag," indicated by the "W/" prefix, MAY be shared bytwo entities of a resource only if the entities are equivalent andcould be substituted for each other with no significant change insemantics. A weak entity tag can only be used for weak comparison.An entity tag MUST be unique across all versions of all entitiesassociated with a particular resource. A given entity tag value MAYbe used for entities obtained by requests on different URIs. The useof the same entity tag value in conjunction with entities obtained byrequests on different URIs does not imply the equivalence of thoseentities.3.12 Range UnitsHTTP/1.1 allows a client to request that only part (a range of) theresponse entity be included within the response. HTTP/1.1 uses rangeunits in the Range (section 14.35) and Content-Range (section 14.16)header fields. An entity can be broken down into subranges accordingto various structural units.range-unit = bytes-unit | other-range-unitbytes-unit = "bytes"other-range-unit = tokenThe only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1implementations MAY ignore ranges specified using other units.Fielding, et al. Standards Track [Page 30]
Page 2: eferred to as "HTTP/1.1", and is an
Page 5 and 6: 12.1 Server-driven Negotiation ....
Page 7 and 8: 14.40 Trailer .....................
Page 9 and 10: 1.2 RequirementsThe key words "MUST
Page 12 and 13: Fielding, et al. Standards Track [P
Page 14 and 15: equest chain ----------------------
Page 16 and 17: Elements separated by a bar ("|") a
Page 18 and 19: HTTP/1.1 header field values can be
Page 20 and 21: An application that sends a request
Page 22 and 23: 3.2.3 URI ComparisonWhen comparing
Page 26 and 27: is not desirable and is discouraged
Page 28 and 29: CRLFchunk = chunk-size [ chunk-exte
Page 30 and 31: When in canonical form, media subty
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 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
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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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validators or strong validators. Cl
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- MAY send a weak entity tag instea
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Unless specifically constrained by
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Fielding, et al. Standards Track [P
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warn-code 1xx, which are deleted ev
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for the associated entity, its enti
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- DELETE- POSTIn order to prevent d
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The Accept request-header field can
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image/jpeg = 0.5text/html;level=2 =
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Note: If the request does not inclu
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Accept-Ranges: bytesbut are not req
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containing an Authorization field,
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- Restrictions on what may be store
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improve privacy in some cases, we c
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must-revalidateBecause a cache MAY
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understand.For example, consider a
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new coding is known to be acceptabl
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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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Network Working Group R. Fielding Request for Comments: 2616 ...

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