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

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The Accept request-header field can be used to specify certain mediatypes which are acceptable for the response. Accept headers can beused to indicate that the request is specifically limited to a smallset of desired types, as in the case of a request for an in-lineimage.Accept = "Accept" ":"#( media-range [ accept-params ] )media-range = ( "*/*"| ( type "/" "*" )| ( type "/" subtype )) *( ";" parameter )accept-params = ";" "q" "=" qvalue *( accept-extension )accept-extension = ";" token [ "=" ( token | quoted-string ) ]The asterisk "*" character is used to group media types into ranges,with "*/*" indicating all media types and "type/*" indicating allsubtypes of that type. The media-range MAY include media typeparameters that are applicable to that range.Each media-range MAY be followed by one or more accept-params,beginning with the "q" parameter for indicating a relative qualityfactor. The first "q" parameter (if any) separates the media-rangeparameter(s) from the accept-params. Quality factors allow the useror user agent to indicate the relative degree of preference for thatmedia-range, using the qvalue scale from 0 to 1 (section 3.9). Thedefault value is q=1.Note: Use of the "q" parameter name to separate media typeparameters from Accept extension parameters is due to historicalpractice. Although this prevents any media type parameter named"q" from being used with a media range, such an event is believedto be unlikely given the lack of any "q" parameters in the IANAmedia type registry and the rare usage of any media typeparameters in Accept. Future media types are discouraged fromregistering any parameter named "q".Fielding, et al. Standards Track [Page 100]RFC 2616 HTTP/1.1 June 1999The exampleAccept: audio/*; q=0.2, audio/basic
SHOULD be interpreted as "I prefer audio/basic, but send me any audiotype if it is the best available after an 80% mark-down in quality."If no Accept header field is present, then it is assumed that theclient accepts all media types. If an Accept header field is present,and if the server cannot send a response which is acceptableaccording to the combined Accept field value, then the server SHOULDsend a 406 (not acceptable) response.A more elaborate example isAccept: text/plain; q=0.5, text/html,text/x-dvi; q=0.8, text/x-cVerbally, this would be interpreted as "text/html and text/x-c arethe preferred media types, but if they do not exist, then send thetext/x-dvi entity, and if that does not exist, send the text/plainentity."Media ranges can be overridden by more specific media ranges orspecific media types. If more than one media range applies to a giventype, the most specific reference has precedence. For example,Accept: text/*, text/html, text/html;level=1, */*have the following precedence:1) text/html;level=12) text/html3) text/*4) */*The media type quality factor associated with a given type isdetermined by finding the media range with the highest precedencewhich matches that type. For example,Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,text/html;level=2;q=0.4, */*;q=0.5would cause the following values to be associated:text/html;level=1 = 1text/html = 0.7text/plain = 0.3Fielding, et al. Standards Track [Page 101]RFC 2616 HTTP/1.1 June 1999
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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
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 68 and 69: Fielding, et al. Standards Track [P
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 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 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
Page 136 and 137: server SHOULD return them in the or
Page 138 and 139: Some origin server implementations
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
Page 150 and 151: MAY require that behavior be consis
Page 152 and 153: yte-ranges-specifier = bytes-unit "
Page 154 and 155: Content) instead of 200 (OK).- The
Page 156 and 157: 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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