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

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idempotent (see section 9.1.2). Non-idempotent methods or sequencesMUST NOT be automatically retried, although user agents MAY offer ahuman operator the choice of retrying the request(s). Confirmation byuser-agent software with semantic understanding of the applicationMAY substitute for user confirmation. The automatic retry SHOULD NOTbe repeated if the second sequence of requests fails.Servers SHOULD always respond to at least one request per connection,if at all possible. Servers SHOULD NOT close a connection in themiddle of transmitting a response, unless a network or client failureis suspected.Clients that use persistent connections SHOULD limit the number ofsimultaneous connections that they maintain to a given server. Asingle-user client SHOULD NOT maintain more than 2 connections withany server or proxy. A proxy SHOULD use up to 2*N connections toanother server or proxy, where N is the number of simultaneouslyactive users. These guidelines are intended to improve HTTP responsetimes and avoid congestion.8.2 Message Transmission Requirements8.2.1 Persistent Connections and Flow ControlHTTP/1.1 servers SHOULD maintain persistent connections and use TCP'sflow control mechanisms to resolve temporary overloads, rather thanterminating connections with the expectation that clients will retry.The latter technique can exacerbate network congestion.Fielding, et al. Standards Track [Page 47]RFC 2616 HTTP/1.1 June 19998.2.2 Monitoring Connections for Error Status MessagesAn HTTP/1.1 (or later) client sending a message-body SHOULD monitorthe network connection for an error status while it is transmittingthe request. If the client sees an error status, it SHOULDimmediately cease transmitting the body. If the body is being sentusing a "chunked" encoding (section 3.6), a zero length chunk andempty trailer MAY be used to prematurely mark the end of the message.If the body was preceded by a Content-Length header, the client MUSTclose the connection.8.2.3 Use of the 100 (Continue) Status
The purpose of the 100 (Continue) status (see section 10.1.1) is toallow a client that is sending a request message with a request bodyto determine if the origin server is willing to accept the request(based on the request headers) before the client sends the requestbody. In some cases, it might either be inappropriate or highlyinefficient for the client to send the body if the server will rejectthe message without looking at the body.Requirements for HTTP/1.1 clients:- If a client will wait for a 100 (Continue) response beforesending the request body, it MUST send an Expect request-headerfield (section 14.20) with the "100-continue" expectation.- A client MUST NOT send an Expect request-header field (section14.20) with the "100-continue" expectation if it does not intendto send a request body.Because of the presence of older implementations, the protocol allowsambiguous situations in which a client may send "Expect: 100-continue" without receiving either a 417 (Expectation Failed) statusor a 100 (Continue) status. Therefore, when a client sends thisheader field to an origin server (possibly via a proxy) from which ithas never seen a 100 (Continue) status, the client SHOULD NOT waitfor an indefinite period before sending the request body.Requirements for HTTP/1.1 origin servers:- Upon receiving a request which includes an Expect request-headerfield with the "100-continue" expectation, an origin server MUSTeither respond with 100 (Continue) status and continue to readfrom the input stream, or respond with a final status code. Theorigin server MUST NOT wait for the request body before sendingthe 100 (Continue) response. If it responds with a final statuscode, it MAY close the transport connection or it MAY continueFielding, et al. Standards Track [Page 48]RFC 2616 HTTP/1.1 June 1999to read and discard the rest of the request. It MUST NOTperform the requested method if it returns a final status code.- An origin server SHOULD NOT send a 100 (Continue) response ifthe request message does not include an Expect request-headerfield with the "100-continue" expectation, and MUST NOT send a100 (Continue) response if such a request comes from an HTTP/1.0(or earlier) client. There is an exception to this rule: for
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 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
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 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
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
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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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