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

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Fielding, et al. Standards Track [Page 143]RFC 2616 HTTP/1.1 June 1999If multiple encodings have been applied to an entity, the transfercodingsMUST be listed in the order in which they were applied.Additional information about the encoding parameters MAY be providedby other entity-header fields not defined by this specification.Many older HTTP/1.0 applications do not understand the Transfer-Encoding header.14.42 UpgradeThe Upgrade general-header allows the client to specify whatadditional communication protocols it supports and would like to useif the server finds it appropriate to switch protocols. The serverMUST use the Upgrade header field within a 101 (Switching Protocols)response to indicate which protocol(s) are being switched.Upgrade= "Upgrade" ":" 1#productFor example,Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11The Upgrade header field is intended to provide a simple mechanismfor transition from HTTP/1.1 to some other, incompatible protocol. Itdoes so by allowing the client to advertise its desire to use anotherprotocol, such as a later version of HTTP with a higher major versionnumber, even though the current request has been made using HTTP/1.1.This eases the difficult transition between incompatible protocols byallowing the client to initiate a request in the more commonlysupported protocol while indicating to the server that it would liketo use a "better" protocol if available (where "better" is determinedby the server, possibly according to the nature of the method and/orresource being requested).The Upgrade header field only applies to switching application-layerprotocols upon the existing transport-layer connection. Upgradecannot be used to insist on a protocol change; its acceptance and useby the server is optional. The capabilities and nature of theapplication-layer communication after the protocol change is entirelydependent upon the new protocol chosen, although the first actionafter changing the protocol MUST be a response to the initial HTTPrequest containing the Upgrade header field.
The Upgrade header field only applies to the immediate connection.Therefore, the upgrade keyword MUST be supplied within a Connectionheader field (section 14.10) whenever Upgrade is present in anHTTP/1.1 message.Fielding, et al. Standards Track [Page 144]RFC 2616 HTTP/1.1 June 1999The Upgrade header field cannot be used to indicate a switch to aprotocol on a different connection. For that purpose, it is moreappropriate to use a 301, 302, 303, or 305 redirection response.This specification only defines the protocol name "HTTP" for use bythe family of Hypertext Transfer Protocols, as defined by the HTTPversion rules of section 3.1 and future updates to thisspecification. Any token can be used as a protocol name; however, itwill only be useful if both the client and server associate the namewith the same protocol.14.43 User-AgentThe User-Agent request-header field contains information about theuser agent originating the request. This is for statistical purposes,the tracing of protocol violations, and automated recognition of useragents for the sake of tailoring responses to avoid particular useragent limitations. User agents SHOULD include this field withrequests. The field can contain multiple product tokens (section 3.8)and comments identifying the agent and any subproducts which form asignificant part of the user agent. By convention, the product tokensare listed in order of their significance for identifying theapplication.User-Agent = "User-Agent" ":" 1*( product | comment )Example:User-Agent: CERN-LineMode/2.15 libwww/2.17b314.44 VaryThe Vary field value indicates the set of request-header fields thatfully determines, while the response is fresh, whether a cache ispermitted to use the response to reply to a subsequent requestwithout revalidation. For uncacheable or stale responses, the Varyfield value advises the user agent about the criteria that were usedto select the representation. A Vary field value of "*" implies that
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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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Page 110 and 111: The Accept request-header field can
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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
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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 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
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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Page 162 and 163: SHOULD NOT, by default, forward the
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Page 168 and 169: strongly correlated to the membersh
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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
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Page 182 and 183: Notes:1) Additional CRLFs may prece
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Page 186 and 187: ead chunk-size and CRLF}read entity
Page 188 and 189: - understand any valid response in
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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