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

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10.5.5 504 Gateway TimeoutThe server, while acting as a gateway or proxy, did not receive atimely response from the upstream server specified by the URI (e.g.HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it neededto access in attempting to complete the request.Note: Note to implementors: some deployed proxies are known toreturn 400 or 500 when DNS lookups time out.10.5.6 505 HTTP Version Not SupportedThe server does not support, or refuses to support, the HTTP protocolversion that was used in the request message. The server isindicating that it is unable or unwilling to complete the requestusing the same major version as the client, as described in section3.1, other than with this error message. The response SHOULD containan entity describing why that version is not supported and what otherprotocols are supported by that server.11 Access AuthenticationHTTP provides several OPTIONAL challenge-response authenticationmechanisms which can be used by a server to challenge a clientrequest and by a client to provide authentication information. Thegeneral framework for access authentication, and the specification of"basic" and "digest" authentication, are specified in "HTTPAuthentication: Basic and Digest Access Authentication" [43]. Thisspecification adopts the definitions of "challenge" and "credentials"from that specification.12 Content NegotiationMost HTTP responses include an entity which contains information forinterpretation by a human user. Naturally, it is desirable to supplythe user with the "best available" entity corresponding to therequest. Unfortunately for servers and caches, not all users have thesame preferences for what is "best," and not all user agents areequally capable of rendering all entity types. For that reason, HTTPhas provisions for several mechanisms for "content negotiation" --the process of selecting the best representation for a given responsewhen there are multiple representations available.Note: This is not called "format negotiation" because thealternate representations may be of the same media type, but usedifferent capabilities of that type, be in different languages,etc.
Fielding, et al. Standards Track [Page 71]RFC 2616 HTTP/1.1 June 1999Any response containing an entity-body MAY be subject to negotiation,including error responses.There are two kinds of content negotiation which are possible inHTTP: server-driven and agent-driven negotiation. These two kinds ofnegotiation are orthogonal and thus may be used separately or incombination. One method of combination, referred to as transparentnegotiation, occurs when a cache uses the agent-driven negotiationinformation provided by the origin server in order to provideserver-driven negotiation for subsequent requests.12.1 Server-driven NegotiationIf the selection of the best representation for a response is made byan algorithm located at the server, it is called server-drivennegotiation. Selection is based on the available representations ofthe response (the dimensions over which it can vary; e.g. language,content-coding, etc.) and the contents of particular header fields inthe request message or on other information pertaining to the request(such as the network address of the client).Server-driven negotiation is advantageous when the algorithm forselecting from among the available representations is difficult todescribe to the user agent, or when the server desires to send its"best guess" to the client along with the first response (hoping toavoid the round-trip delay of a subsequent request if the "bestguess" is good enough for the user). In order to improve the server'sguess, the user agent MAY include request header fields (Accept,Accept-Language, Accept-Encoding, etc.) which describe itspreferences for such a response.Server-driven negotiation has disadvantages:1. It is impossible for the server to accurately determine whatmight be "best" for any given user, since that would requirecomplete knowledge of both the capabilities of the user agentand the intended use for the response (e.g., does the user wantto view it on screen or print it on paper?).2. Having the user agent describe its capabilities in everyrequest can be both very inefficient (given that only a smallpercentage of responses have multiple representations) and apotential violation of the user's privacy.3. It complicates the implementation of an origin server and thealgorithms for generating responses to a request.
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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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Page 30 and 31: When in canonical form, media subty
Page 32 and 33: 3.9 Quality ValuesHTTP content nego
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
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Page 44 and 45: values for the first digit:- 1xx: I
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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
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Page 64 and 65: 10.1.2 101 Switching ProtocolsThe s
Page 66 and 67: esponse MAY include new or updated
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Page 76 and 77: 10.4.15 414 Request-URI Too LongThe
Page 82 and 83: Caching would be useless if it did
Page 84 and 85: Warning (but without removing any e
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Page 90 and 91: RFC 2616 HTTP/1.1 June 1999the time
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Page 94 and 95: match.In HTTP/1.1, a conditional re
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Page 122 and 123: improve privacy in some cases, we c
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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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