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

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a cache cannot determine from the request headers of a subsequentrequest whether this response is the appropriate representation. Seesection 13.6 for use of the Vary header field by caches.Vary = "Vary" ":" ( "*" | 1#field-name )An HTTP/1.1 server SHOULD include a Vary header field with anycacheable response that is subject to server-driven negotiation.Doing so allows a cache to properly interpret future requests on thatresource and informs the user agent about the presence of negotiationFielding, et al. Standards Track [Page 145]RFC 2616 HTTP/1.1 June 1999on that resource. A server MAY include a Vary header field with anon-cacheable response that is subject to server-driven negotiation,since this might provide the user agent with useful information aboutthe dimensions over which the response varies at the time of theresponse.A Vary field value consisting of a list of field-names signals thatthe representation selected for the response is based on a selectionalgorithm which considers ONLY the listed request-header field valuesin selecting the most appropriate representation. A cache MAY assumethat the same selection will be made for future requests with thesame values for the listed field names, for the duration of time forwhich the response is fresh.The field-names given are not limited to the set of standardrequest-header fields defined by this specification. Field names arecase-insensitive.A Vary field value of "*" signals that unspecified parameters notlimited to the request-headers (e.g., the network address of theclient), play a role in the selection of the response representation.The "*" value MUST NOT be generated by a proxy server; it may only begenerated by an origin server.14.45 ViaThe Via general-header field MUST be used by gateways and proxies toindicate the intermediate protocols and recipients between the useragent and the server on requests, and between the origin server andthe client on responses. It is analogous to the "Received" field ofRFC 822 [9] and is intended to be used for tracking message forwards,avoiding request loops, and identifying the protocol capabilities ofall senders along the request/response chain.
Via = "Via" ":" 1#( received-protocol received-by [ comment ] )received-protocol = [ protocol-name "/" ] protocol-versionprotocol-name = tokenprotocol-version = tokenreceived-by = ( host [ ":" port ] ) | pseudonympseudonym = tokenThe received-protocol indicates the protocol version of the messagereceived by the server or client along each segment of therequest/response chain. The received-protocol version is appended tothe Via field value when the message is forwarded so that informationabout the protocol capabilities of upstream applications remainsvisible to all recipients.Fielding, et al. Standards Track [Page 146]RFC 2616 HTTP/1.1 June 1999The protocol-name is optional if and only if it would be "HTTP". Thereceived-by field is normally the host and optional port number of arecipient server or client that subsequently forwarded the message.However, if the real host is considered to be sensitive information,it MAY be replaced by a pseudonym. If the port is not given, it MAYbe assumed to be the default port of the received-protocol.Multiple Via field values represents each proxy or gateway that hasforwarded the message. Each recipient MUST append its informationsuch that the end result is ordered according to the sequence offorwarding applications.Comments MAY be used in the Via header field to identify the softwareof the recipient proxy or gateway, analogous to the User-Agent andServer header fields. However, all comments in the Via field areoptional and MAY be removed by any recipient prior to forwarding themessage.For example, a request message could be sent from an HTTP/1.0 useragent to an internal proxy code-named "fred", which uses HTTP/1.1 toforward the request to a public proxy at nowhere.com, which completesthe request by forwarding it to the origin server at www.ics.uci.edu.The request received by www.ics.uci.edu would then have the followingVia header field:Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)Proxies and gateways used as a portal through a network firewall
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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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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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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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Page 114 and 115: Note: If the request does not inclu
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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 126 and 127: must-revalidateBecause a cache MAY
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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 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
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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
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Page 154 and 155: Content) instead of 200 (OK).- The
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Page 182 and 183: Notes:1) Additional CRLFs may prece
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Page 190 and 191: CREATE had a race that required an
Page 192 and 193: also needed to be a general header,
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