source: draft-ietf-httpbis/03/draft-ietf-httpbis-p2-semantics-03.xml @ 912

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<rfc obsoletes="2616" updates="2817" category="std" ipr="full3978" docName="draft-ietf-httpbis-p2-semantics-03">
<front>

  <title abbrev="HTTP/1.1, Part 2">HTTP/1.1, part 2: Message Semantics</title>

  <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
    <organization abbrev="Day Software">Day Software</organization>
    <address>
      <postal>
        <street>23 Corporate Plaza DR, Suite 280</street>
        <city>Newport Beach</city>
        <region>CA</region>
        <code>92660</code>
        <country>USA</country>
      </postal>
      <phone>+1-949-706-5300</phone>
      <facsimile>+1-949-706-5305</facsimile>
      <email>fielding@gbiv.com</email>
      <uri>http://roy.gbiv.com/</uri>
    </address>
  </author>

  <author initials="J." surname="Gettys" fullname="Jim Gettys">
    <organization>One Laptop per Child</organization>
    <address>
      <postal>
        <street>21 Oak Knoll Road</street>
        <city>Carlisle</city>
        <region>MA</region>
        <code>01741</code>
        <country>USA</country>
      </postal>
      <email>jg@laptop.org</email>
      <uri>http://www.laptop.org/</uri>
    </address>
  </author>
  
  <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
    <organization abbrev="HP">Hewlett-Packard Company</organization>
    <address>
      <postal>
        <street>HP Labs, Large Scale Systems Group</street>
        <street>1501 Page Mill Road, MS 1177</street>
        <city>Palo Alto</city>
        <region>CA</region>
        <code>94304</code>
        <country>USA</country>
      </postal>
      <email>JeffMogul@acm.org</email>
    </address>
  </author>

  <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
        <country>USA</country>
      </postal>
      <email>henrikn@microsoft.com</email>
    </address>
  </author>

  <author initials="L." surname="Masinter" fullname="Larry Masinter">
    <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
    <address>
      <postal>
        <street>345 Park Ave</street>
        <city>San Jose</city>
        <region>CA</region>
        <code>95110</code>
        <country>USA</country>
      </postal>
      <email>LMM@acm.org</email>
      <uri>http://larry.masinter.net/</uri>
    </address>
  </author>
  
  <author initials="P." surname="Leach" fullname="Paul J. Leach">
    <organization abbrev="Microsoft">Microsoft Corporation</organization>
    <address>
      <postal>
        <street>1 Microsoft Way</street>
        <city>Redmond</city>
        <region>WA</region>
        <code>98052</code>
      </postal>
      <email>paulle@microsoft.com</email>
    </address>
  </author>
   
  <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
    <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
    <address>
      <postal>
        <street>MIT Computer Science and Artificial Intelligence Laboratory</street>
        <street>The Stata Center, Building 32</street>
        <street>32 Vassar Street</street>
        <city>Cambridge</city>
        <region>MA</region>
        <code>02139</code>
        <country>USA</country>
      </postal>
      <email>timbl@w3.org</email>
      <uri>http://www.w3.org/People/Berners-Lee/</uri>
    </address>
  </author>

  <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
    <organization abbrev="W3C">World Wide Web Consortium</organization>
    <address>
      <postal>
        <street>W3C / ERCIM</street>
        <street>2004, rte des Lucioles</street>
        <city>Sophia-Antipolis</city>
        <region>AM</region>
        <code>06902</code>
        <country>France</country>
      </postal>
      <email>ylafon@w3.org</email>
      <uri>http://www.raubacapeu.net/people/yves/</uri>
    </address>
  </author>

  <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
    <organization abbrev="greenbytes">greenbytes GmbH</organization>
    <address>
      <postal>
        <street>Hafenweg 16</street>
        <city>Muenster</city><region>NW</region><code>48155</code>
        <country>Germany</country>
      </postal>
      <phone>+49 251 2807760</phone>    
      <facsimile>+49 251 2807761</facsimile>    
      <email>julian.reschke@greenbytes.de</email>       
      <uri>http://greenbytes.de/tech/webdav/</uri>      
    </address>
  </author>

  <date month="June" year="2008" day="17"/>

<abstract>
<t>
   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems. HTTP has been in use by the World Wide Web global information
   initiative since 1990. This document is Part 2 of the seven-part specification
   that defines the protocol referred to as "HTTP/1.1" and, taken together,
   obsoletes RFC 2616.  Part 2 defines the semantics of HTTP messages
   as expressed by request methods, request-header fields, response status codes,
   and response-header fields.
</t>
</abstract>

<note title="Editorial Note (To be removed by RFC Editor)">
  <t>
    Discussion of this draft should take place on the HTTPBIS working group
    mailing list (ietf-http-wg@w3.org). The current issues list is
    at <eref target="http://www.tools.ietf.org/wg/httpbis/trac/report/11"/>
    and related documents (including fancy diffs) can be found at
    <eref target="http://www.tools.ietf.org/wg/httpbis/"/>.
  </t>
  <t>
    The changes in this draft are summarized in <xref target="changes.since.02"/>.
  </t>
</note>
</front>
<middle>
<section title="Introduction" anchor="introduction">
<t>
   This document defines HTTP/1.1 request and response semantics.  Each HTTP
   message, as defined in <xref target="Part1"/>, is in the form of either a request or
   a response.  An HTTP server listens on a connection for HTTP requests and
   responds to each request, in the order received on that connection, with
   one or more HTTP response messages.  This document defines the commonly
   agreed upon semantics of the HTTP uniform interface, the intentions defined
   by each request method, and the various response messages that might be
   expected as a result of applying that method for the requested resource.
</t>
<t>
   This document is currently disorganized in order to minimize the changes
   between drafts and enable reviewers to see the smaller errata changes.
   The next draft will reorganize the sections to better reflect the content.
   In particular, the sections will be ordered according to the typical
   processing of an HTTP request message (after message parsing): resource
   mapping, general header fields, methods, request modifiers, response
   status, and resource metadata.  The current mess reflects how widely
   dispersed these topics and associated requirements had become in
   <xref target="RFC2616"/>.
</t>

<section title="Requirements" anchor="intro.requirements">
<t>
   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <xref target="RFC2119"/>.
</t>
<t>
   An implementation is not compliant if it fails to satisfy one or more
   of the MUST or REQUIRED level requirements for the protocols it
   implements. An implementation that satisfies all the MUST or REQUIRED
   level and all the SHOULD level requirements for its protocols is said
   to be "unconditionally compliant"; one that satisfies all the MUST
   level requirements but not all the SHOULD level requirements for its
   protocols is said to be "conditionally compliant."
</t>
</section>
</section>

<section title="Notational Conventions and Generic Grammar" anchor="notation">
  
  
  
  
<t>
  This specification uses the ABNF syntax defined in Section 2.1 of <xref target="Part1"/> and
  the core rules defined in Section 2.2 of <xref target="Part1"/>:
  <cref anchor="abnf.dep">ABNF syntax and basic rules will be adopted from RFC 5234, see
  &lt;http://tools.ietf.org/wg/httpbis/trac/ticket/36&gt;.</cref>
</t>
<figure><artwork type="abnf2616"><![CDATA[
  DIGIT         = <DIGIT, defined in [Part1], Section 2.2>
]]></artwork></figure>
<figure><artwork type="abnf2616"><![CDATA[
  
  comment       = <comment, defined in [Part1], Section 2.2>
  quoted-string = <quoted-string, defined in [Part1], Section 2.2>
  token         = <token, defined in [Part1], Section 2.2>
]]></artwork></figure>
<t anchor="abnf.dependencies">
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  The ABNF rules below are defined in other parts: 
</t>
<figure><!--Part1--><artwork type="abnf2616"><![CDATA[
  absoluteURI   = <absoluteURI, defined in [Part1], Section 3.2.1>
  fragment      = <fragment, defined in [Part1], Section 3.2.1>
  Host          = <Host, defined in [Part1], Section 8.4>
  HTTP-date     = <HTTP-date, defined in [Part1], Section 3.3.1>
  product       = <product, defined in [Part1], Section 3.5>
  relativeURI   = <relativeURI, defined in [Part1], Section 3.2.1>
  TE            = <TE, defined in [Part1], Section 8.8>
]]></artwork></figure>
<figure><!--Part3--><artwork type="abnf2616"><![CDATA[
  Accept        = <Accept, defined in [Part3], Section 6.1>
  Accept-Charset =
             <Accept-Charset, defined in [Part3], Section 6.2>
  Accept-Encoding =
             <Accept-Encoding, defined in [Part3], Section 6.3>
  Accept-Language =
             <Accept-Language, defined in [Part3], Section 6.4>
]]></artwork></figure>
<figure><!--Part4--><artwork type="abnf2616"><![CDATA[
  ETag          = <ETag, defined in [Part4], Section 7.1>
  If-Match      = <If-Match, defined in [Part4], Section 7.2>
  If-Modified-Since =
             <If-Modified-Since, defined in [Part4], Section 7.3>
  If-None-Match = <If-None-Match, defined in [Part4], Section 7.4>
  If-Unmodified-Since =
             <If-Unmodified-Since, defined in [Part4], Section 7.5>
]]></artwork></figure>
<figure><!--Part5--><artwork type="abnf2616"><![CDATA[
  Accept-Ranges = <Accept-Ranges, defined in [Part5], Section 6.1>
  If-Range      = <If-Range, defined in [Part5], Section 6.3>
  Range         = <Range, defined in [Part5], Section 6.4>
]]></artwork></figure>
<figure><!--Part6--><artwork type="abnf2616"><![CDATA[
  Age           = <Age, defined in [Part6], Section 16.1>
  Vary          = <Vary, defined in [Part6], Section 16.5>
]]></artwork><!--Part7--></figure>
<figure><artwork type="abnf2616"><![CDATA[
  Authorization = <Authorization, defined in [Part7], Section 4.1>
  Proxy-Authenticate =
             <Proxy-Authenticate, defined in [Part7], Section 4.2>
  Proxy-Authorization =
             <Proxy-Authorization, defined in [Part7], Section 4.3>
  WWW-Authenticate = 
             <WWW-Authenticate, defined in [Part7], Section 4.4>
]]></artwork></figure>
</section>

<section title="Method" anchor="method">
  
  
<t>
   The Method  token indicates the method to be performed on the
   resource identified by the Request-URI. The method is case-sensitive.
</t>
<figure><iref primary="true" item="Grammar" subitem="Method"/><iref primary="true" item="Grammar" subitem="extension-method"/><artwork type="abnf2616"><![CDATA[
  Method         = %x4F.50.54.49.4F.4E.53   ; "OPTIONS", Section 8.2
                 | %x47.45.54               ; "GET", Section 8.3
                 | %x48.45.41.44            ; "HEAD", Section 8.4
                 | %x50.4F.53.54            ; "POST", Section 8.5
                 | %x50.55.54               ; "PUT", Section 8.6
                 | %x44.45.4C.45.54.45      ; "DELETE", Section 8.7
                 | %x54.52.41.43.45         ; "TRACE", Section 8.8
                 | %x43.4F.4E.4E.45.43.54   ; "CONNECT", Section 8.9
                 | extension-method
  extension-method = token
]]></artwork></figure>
<t>
   The list of methods allowed by a resource can be specified in an
   Allow header field (<xref target="header.allow"/>). The return code of the response
   always notifies the client whether a method is currently allowed on a
   resource, since the set of allowed methods can change dynamically. An
   origin server SHOULD return the status code 405 (Method Not Allowed)
   if the method is known by the origin server but not allowed for the
   requested resource, and 501 (Not Implemented) if the method is
   unrecognized or not implemented by the origin server. The methods GET
   and HEAD MUST be supported by all general-purpose servers. All other
   methods are OPTIONAL; however, if the above methods are implemented,
   they MUST be implemented with the same semantics as those specified
   in <xref target="method.definitions"/>.
</t>
</section>

<section title="Request Header Fields" anchor="request.header.fields">
  
  
<t>
   The request-header fields allow the client to pass additional
   information about the request, and about the client itself, to the
   server. These fields act as request modifiers, with semantics
   equivalent to the parameters on a programming language method
   invocation.
</t>
<figure><iref primary="true" item="Grammar" subitem="request-header"/><artwork type="abnf2616"><![CDATA[
  request-header = Accept                   ; [Part3], Section 6.1
                 | Accept-Charset           ; [Part3], Section 6.2
                 | Accept-Encoding          ; [Part3], Section 6.3
                 | Accept-Language          ; [Part3], Section 6.4
                 | Authorization            ; [Part7], Section 4.1
                 | Expect                   ; Section 10.2
                 | From                     ; Section 10.3
                 | Host                     ; [Part1], Section 8.4
                 | If-Match                 ; [Part4], Section 7.2
                 | If-Modified-Since        ; [Part4], Section 7.3
                 | If-None-Match            ; [Part4], Section 7.4
                 | If-Range                 ; [Part5], Section 6.3
                 | If-Unmodified-Since      ; [Part4], Section 7.5
                 | Max-Forwards             ; Section 10.5
                 | Proxy-Authorization      ; [Part7], Section 4.3
                 | Range                    ; [Part5], Section 6.4
                 | Referer                  ; Section 10.6
                 | TE                       ; [Part1], Section 8.8
                 | User-Agent               ; Section 10.9
]]></artwork></figure>
<t>
   Request-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of request-header
   fields if all parties in the communication recognize them to
   be request-header fields. Unrecognized header fields are treated as
   entity-header fields.
</t>
</section>

<section title="Status Code and Reason Phrase" anchor="status.code.and.reason.phrase">
  
  
<t>
   The Status-Code element is a 3-digit integer result code of the
   attempt to understand and satisfy the request. The status codes listed
   below are defined in <xref target="status.codes"/>.
   The Reason-Phrase is intended to give a short
   textual description of the Status-Code. The Status-Code is intended
   for use by automata and the Reason-Phrase is intended for the human
   user. The client is not required to examine or display the Reason-Phrase.
</t>
<t>  
   The individual values of the numeric status codes defined for
   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
   presented below. The reason phrases listed here are only
   recommendations -- they MAY be replaced by local equivalents without
   affecting the protocol.
</t>
<figure><iref primary="true" item="Grammar" subitem="Status-Code"/><iref primary="true" item="Grammar" subitem="extension-code"/><iref primary="true" item="Grammar" subitem="Reason-Phrase"/><artwork type="abnf2616"><![CDATA[
  Status-Code    =
         "100"  ; Section 9.1.1: Continue
       | "101"  ; Section 9.1.2: Switching Protocols
       | "200"  ; Section 9.2.1: OK
       | "201"  ; Section 9.2.2: Created
       | "202"  ; Section 9.2.3: Accepted
       | "203"  ; Section 9.2.4: Non-Authoritative Information
       | "204"  ; Section 9.2.5: No Content
       | "205"  ; Section 9.2.6: Reset Content
       | "206"  ; Section 9.2.7: Partial Content
       | "300"  ; Section 9.3.1: Multiple Choices
       | "301"  ; Section 9.3.2: Moved Permanently
       | "302"  ; Section 9.3.3: Found
       | "303"  ; Section 9.3.4: See Other
       | "304"  ; Section 9.3.5: Not Modified
       | "305"  ; Section 9.3.6: Use Proxy
       | "307"  ; Section 9.3.8: Temporary Redirect
       | "400"  ; Section 9.4.1: Bad Request
       | "401"  ; Section 9.4.2: Unauthorized
       | "402"  ; Section 9.4.3: Payment Required
       | "403"  ; Section 9.4.4: Forbidden
       | "404"  ; Section 9.4.5: Not Found
       | "405"  ; Section 9.4.6: Method Not Allowed
       | "406"  ; Section 9.4.7: Not Acceptable
       | "407"  ; Section 9.4.8: Proxy Authentication Required
       | "408"  ; Section 9.4.9: Request Time-out
       | "409"  ; Section 9.4.10: Conflict
       | "410"  ; Section 9.4.11: Gone
       | "411"  ; Section 9.4.12: Length Required
       | "412"  ; Section 9.4.13: Precondition Failed
       | "413"  ; Section 9.4.14: Request Entity Too Large
       | "414"  ; Section 9.4.15: Request-URI Too Large
       | "415"  ; Section 9.4.16: Unsupported Media Type
       | "416"  ; Section 9.4.17: Requested range not satisfiable
       | "417"  ; Section 9.4.18: Expectation Failed
       | "500"  ; Section 9.5.1: Internal Server Error
       | "501"  ; Section 9.5.2: Not Implemented
       | "502"  ; Section 9.5.3: Bad Gateway
       | "503"  ; Section 9.5.4: Service Unavailable
       | "504"  ; Section 9.5.5: Gateway Time-out
       | "505"  ; Section 9.5.6: HTTP Version not supported
       | extension-code

  extension-code = 3DIGIT
  Reason-Phrase  = *<TEXT, excluding CR, LF>
]]></artwork></figure>
<t>
   HTTP status codes are extensible. HTTP applications are not required
   to understand the meaning of all registered status codes, though such
   understanding is obviously desirable. However, applications MUST
   understand the class of any status code, as indicated by the first
   digit, and treat any unrecognized response as being equivalent to the
   x00 status code of that class, with the exception that an
   unrecognized response MUST NOT be cached. For example, if an
   unrecognized status code of 431 is received by the client, it can
   safely assume that there was something wrong with its request and
   treat the response as if it had received a 400 status code. In such
   cases, user agents SHOULD present to the user the entity returned
   with the response, since that entity is likely to include human-readable
   information which will explain the unusual status.
</t>

<section title="Status Code Registry" anchor="status.code.registry">
<t>
  The HTTP Status Code Registry defines the name space for the Status-Code
  token in the Status line of an HTTP response.
</t>
<t>
  Values to be added to this name space are subject to IETF review
  (<xref target="RFC5226"/>, Section 4.1).  Any document registering
  new status codes should be traceable through statuses of either 'Obsoletes'
  or 'Updates' to this document.
</t>
<t>
  The registry itself is maintained at <eref target="http://www.iana.org/assignments/http-status-codes"/>.
</t>
</section>

</section>

<section title="Response Header Fields" anchor="response.header.fields">
  
<t>
   The response-header fields allow the server to pass additional
   information about the response which cannot be placed in the Status-Line.
   These header fields give information about the server and about
   further access to the resource identified by the Request-URI.
</t>
<figure><iref primary="true" item="Grammar" subitem="response-header"/><artwork type="abnf2616"><![CDATA[
  response-header = Accept-Ranges           ; [Part5], Section 6.1
                  | Age                     ; [Part6], Section 16.1
                  | Allow                   ; Section 10.1
                  | ETag                    ; [Part4], Section 7.1
                  | Location                ; Section 10.4
                  | Proxy-Authenticate      ; [Part7], Section 4.2
                  | Retry-After             ; Section 10.7
                  | Server                  ; Section 10.8
                  | Vary                    ; [Part6], Section 16.5
                  | WWW-Authenticate        ; [Part7], Section 4.4
]]></artwork></figure>
<t>
   Response-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of response-header
   fields if all parties in the communication recognize them to
   be response-header fields. Unrecognized header fields are treated as
   entity-header fields.
</t>
</section>

<section title="Entity" anchor="entity">
<t>
   Request and Response messages MAY transfer an entity if not otherwise
   restricted by the request method or response status code. An entity
   consists of entity-header fields and an entity-body, although some
   responses will only include the entity-headers. HTTP entity-body and
   entity-header fields are defined in <xref target="Part3"/>.
</t>
<t>
   An entity-body is only present in a message when a message-body is
   present, as described in Section 4.3 of <xref target="Part1"/>. The entity-body is obtained
   from the message-body by decoding any Transfer-Encoding that might
   have been applied to ensure safe and proper transfer of the message.
</t>
</section>


<section title="Method Definitions" anchor="method.definitions">
<t>
   The set of common methods for HTTP/1.1 is defined below. Although
   this set can be expanded, additional methods cannot be assumed to
   share the same semantics for separately extended clients and servers.
</t>

<section title="Safe and Idempotent Methods" anchor="safe.and.idempotent">

<section title="Safe Methods" anchor="safe.methods">
<t>
   Implementors should be aware that the software represents the user in
   their interactions over the Internet, and should be careful to allow
   the user to be aware of any actions they might take which may have an
   unexpected significance to themselves or others.
</t>
<t>
   In particular, the convention has been established that the GET and
   HEAD methods SHOULD NOT  have the significance of taking an action
   other than retrieval. These methods ought to be considered "safe".
   This allows user agents to represent other methods, such as POST, PUT
   and DELETE, in a special way, so that the user is made aware of the
   fact that a possibly unsafe action is being requested.
</t>
<t>
   Naturally, it is not possible to ensure that the server does not
   generate side-effects as a result of performing a GET request; in
   fact, some dynamic resources consider that a feature. The important
   distinction here is that the user did not request the side-effects,
   so therefore cannot be held accountable for them.
</t>
</section>

<section title="Idempotent Methods" anchor="idempotent.methods">
<t>
   Methods can also have the property of "idempotence" in that (aside
   from error or expiration issues) the side-effects of N &gt; 0 identical
   requests is the same as for a single request. The methods GET, HEAD,
   PUT and DELETE share this property. Also, the methods OPTIONS and
   TRACE SHOULD NOT  have side effects, and so are inherently idempotent.
</t>
<t>
   However, it is possible that a sequence of several requests is non-idempotent,
   even if all of the methods executed in that sequence are
   idempotent. (A sequence is idempotent if a single execution of the
   entire sequence always yields a result that is not changed by a
   reexecution of all, or part, of that sequence.) For example, a
   sequence is non-idempotent if its result depends on a value that is
   later modified in the same sequence.
</t>
<t>
   A sequence that never has side effects is idempotent, by definition
   (provided that no concurrent operations are being executed on the
   same set of resources).
</t>
</section>
</section>

<section title="OPTIONS" anchor="OPTIONS">
  <iref primary="true" item="OPTIONS method"/>
  <iref primary="true" item="Methods" subitem="OPTIONS"/>
<t>
   The OPTIONS method represents a request for information about the
   communication options available on the request/response chain
   identified by the Request-URI. This method allows the client to
   determine the options and/or requirements associated with a resource,
   or the capabilities of a server, without implying a resource action
   or initiating a resource retrieval.
</t>
<t>
   Responses to this method are not cacheable.
</t>
<t>
   If the OPTIONS request includes an entity-body (as indicated by the
   presence of Content-Length or Transfer-Encoding), then the media type
   MUST be indicated by a Content-Type field. Although this
   specification does not define any use for such a body, future
   extensions to HTTP might use the OPTIONS body to make more detailed
   queries on the server. A server that does not support such an
   extension MAY discard the request body.
</t>
<t>
   If the Request-URI is an asterisk ("*"), the OPTIONS request is
   intended to apply to the server in general rather than to a specific
   resource. Since a server's communication options typically depend on
   the resource, the "*" request is only useful as a "ping" or "no-op"
   type of method; it does nothing beyond allowing the client to test
   the capabilities of the server. For example, this can be used to test
   a proxy for HTTP/1.1 compliance (or lack thereof).
</t>
<t>
   If the Request-URI is not an asterisk, the OPTIONS request applies
   only to the options that are available when communicating with that
   resource.
</t>
<t>
   A 200 response SHOULD include any header fields that indicate
   optional features implemented by the server and applicable to that
   resource (e.g., Allow), possibly including extensions not defined by
   this specification. The response body, if any, SHOULD also include
   information about the communication options. The format for such a
   body is not defined by this specification, but might be defined by
   future extensions to HTTP. Content negotiation MAY be used to select
   the appropriate response format. If no response body is included, the
   response MUST include a Content-Length field with a field-value of
   "0".
</t>
<t>
   The Max-Forwards request-header field MAY be used to target a
   specific proxy in the request chain. When a proxy receives an OPTIONS
   request on an absoluteURI for which request forwarding is permitted,
   the proxy MUST check for a Max-Forwards field. If the Max-Forwards
   field-value is zero ("0"), the proxy MUST NOT forward the message;
   instead, the proxy SHOULD respond with its own communication options.
   If the Max-Forwards field-value is an integer greater than zero, the
   proxy MUST decrement the field-value when it forwards the request. If
   no Max-Forwards field is present in the request, then the forwarded
   request MUST NOT include a Max-Forwards field.
</t>
</section>

<section title="GET" anchor="GET">
  <iref primary="true" item="GET method"/>
  <iref primary="true" item="Methods" subitem="GET"/>
<t>
   The GET method means retrieve whatever information (in the form of an
   entity) is identified by the Request-URI. If the Request-URI refers
   to a data-producing process, it is the produced data which shall be
   returned as the entity in the response and not the source text of the
   process, unless that text happens to be the output of the process.
</t>
<t>
   The semantics of the GET method change to a "conditional GET" if the
   request message includes an If-Modified-Since, If-Unmodified-Since,
   If-Match, If-None-Match, or If-Range header field. A conditional GET
   method requests that the entity be transferred only under the
   circumstances described by the conditional header field(s). The
   conditional GET method is intended to reduce unnecessary network
   usage by allowing cached entities to be refreshed without requiring
   multiple requests or transferring data already held by the client.
</t>
<t>
   The semantics of the GET method change to a "partial GET" if the
   request message includes a Range header field. A partial GET requests
   that only part of the entity be transferred, as described in Section 6.4 of <xref target="Part5"/>.
   The partial GET method is intended to reduce unnecessary
   network usage by allowing partially-retrieved entities to be
   completed without transferring data already held by the client.
</t>
<t>
   The response to a GET request is cacheable if and only if it meets
   the requirements for HTTP caching described in <xref target="Part6"/>.
</t>
<t>
   See <xref target="encoding.sensitive.information.in.uris"/> for security considerations when used for forms.
</t>
</section>

<section title="HEAD" anchor="HEAD">
  <iref primary="true" item="HEAD method"/>
  <iref primary="true" item="Methods" subitem="HEAD"/>
<t>
   The HEAD method is identical to GET except that the server MUST NOT
   return a message-body in the response. The metainformation contained
   in the HTTP headers in response to a HEAD request SHOULD be identical
   to the information sent in response to a GET request. This method can
   be used for obtaining metainformation about the entity implied by the
   request without transferring the entity-body itself. This method is
   often used for testing hypertext links for validity, accessibility,
   and recent modification.
</t>
<t>
   The response to a HEAD request MAY be cacheable in the sense that the
   information contained in the response MAY be used to update a
   previously cached entity from that resource. If the new field values
   indicate that the cached entity differs from the current entity (as
   would be indicated by a change in Content-Length, Content-MD5, ETag
   or Last-Modified), then the cache MUST treat the cache entry as
   stale.
</t>
</section>

<section title="POST" anchor="POST">
  <iref primary="true" item="POST method"/>
  <iref primary="true" item="Methods" subitem="POST"/>
<t>
   The POST method is used to request that the origin server accept the
   entity enclosed in the request as data to be processed by the resource
   identified by the Request-URI in the Request-Line. POST is designed
   to allow a uniform method to cover the following functions:
  <list style="symbols">
    <t>
      Annotation of existing resources;
    </t>
    <t>
        Posting a message to a bulletin board, newsgroup, mailing list,
        or similar group of articles;
    </t>
    <t>
        Providing a block of data, such as the result of submitting a
        form, to a data-handling process;
    </t>
    <t>
        Extending a database through an append operation.
    </t>
  </list>
</t>
<t>
   The actual function performed by the POST method is determined by the
   server and is usually dependent on the Request-URI.
</t>
<t>
   The action performed by the POST method might not result in a
   resource that can be identified by a URI. In this case, either 200
   (OK) or 204 (No Content) is the appropriate response status,
   depending on whether or not the response includes an entity that
   describes the result.
</t>
<t>
   If a resource has been created on the origin server, the response
   SHOULD be 201 (Created) and contain an entity which describes the
   status of the request and refers to the new resource, and a Location
   header (see <xref target="header.location"/>).
</t>
<t>
   Responses to this method are not cacheable, unless the response
   includes appropriate Cache-Control or Expires header fields. However,
   the 303 (See Other) response can be used to direct the user agent to
   retrieve a cacheable resource.
</t>
</section>

<section title="PUT" anchor="PUT">
  <iref primary="true" item="PUT method"/>
  <iref primary="true" item="Methods" subitem="PUT"/>
<t>
   The PUT method requests that the enclosed entity be stored at the
   supplied Request-URI. If the Request-URI refers to an already
   existing resource, the enclosed entity SHOULD be considered as a
   modified version of the one residing on the origin server. If the
   Request-URI does not point to an existing resource, and that URI is
   capable of being defined as a new resource by the requesting user
   agent, the origin server can create the resource with that URI. If a
   new resource is created at the Request-URI, the origin server MUST
         inform the user agent
   via the 201 (Created) response. If an existing resource is modified,
   either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
   to indicate successful completion of the request. If the resource
   could not be created or modified with the Request-URI, an appropriate
   error response SHOULD be given that reflects the nature of the
   problem. The recipient of the entity MUST NOT ignore any Content-*
   (e.g. Content-Range) headers that it does not understand or implement
   and MUST return a 501 (Not Implemented) response in such cases.
</t>
<t>
   If the request passes through a cache and the Request-URI identifies
   one or more currently cached entities, those entries SHOULD be
   treated as stale. Responses to this method are not cacheable.
</t>
<t>
   The fundamental difference between the POST and PUT requests is
   reflected in the different meaning of the Request-URI. The URI in a
   POST request identifies the resource that will handle the enclosed
   entity. That resource might be a data-accepting process, a gateway to
   some other protocol, or a separate entity that accepts annotations.
   In contrast, the URI in a PUT request identifies the entity enclosed
   with the request -- the user agent knows what URI is intended and the
   server MUST NOT attempt to apply the request to some other resource.
   If the server desires that the request be applied to a different URI,
   it MUST send a 301 (Moved Permanently) response; the user agent MAY
   then make its own decision regarding whether or not to redirect the
   request.
</t>
<t>
   A single resource MAY be identified by many different URIs. For
   example, an article might have a URI for identifying "the current
   version" which is separate from the URI identifying each particular
   version. In this case, a PUT request on a general URI might result in
   several other URIs being defined by the origin server.
</t>
<t>
   HTTP/1.1 does not define how a PUT method affects the state of an
   origin server.
</t>
<t>
   Unless otherwise specified for a particular entity-header, the
   entity-headers in the PUT request SHOULD be applied to the resource
   created or modified by the PUT.
</t>
</section>

<section title="DELETE" anchor="DELETE">
  <iref primary="true" item="DELETE method"/>
  <iref primary="true" item="Methods" subitem="DELETE"/>
<t>
   The DELETE method requests that the origin server delete the resource
   identified by the Request-URI. This method MAY be overridden by human
   intervention (or other means) on the origin server. The client cannot
   be guaranteed that the operation has been carried out, even if the
   status code returned from the origin server indicates that the action
   has been completed successfully. However, the server SHOULD NOT 
   indicate success unless, at the time the response is given, it
   intends to delete the resource or move it to an inaccessible
   location.
</t>
<t>
   A successful response SHOULD be 200 (OK) if the response includes an
   entity describing the status, 202 (Accepted) if the action has not
   yet been enacted, or 204 (No Content) if the action has been enacted
   but the response does not include an entity.
</t>
<t>
   If the request passes through a cache and the Request-URI identifies
   one or more currently cached entities, those entries SHOULD be
   treated as stale. Responses to this method are not cacheable.
</t>
</section>

<section title="TRACE" anchor="TRACE">
  <iref primary="true" item="TRACE method"/>
  <iref primary="true" item="Methods" subitem="TRACE"/>
<t>
   The TRACE method is used to invoke a remote, application-layer loop-back
   of the request message. The final recipient of the request
   SHOULD reflect the message received back to the client as the
   entity-body of a 200 (OK) response. The final recipient is either the
   origin server or the first proxy or gateway to receive a Max-Forwards
   value of zero (0) in the request (see <xref target="header.max-forwards"/>). A TRACE request
   MUST NOT include an entity.
</t>
<t>
   TRACE allows the client to see what is being received at the other
   end of the request chain and use that data for testing or diagnostic
   information. The value of the Via header field (Section 8.9 of <xref target="Part1"/>) is of
   particular interest, since it acts as a trace of the request chain.
   Use of the Max-Forwards header field allows the client to limit the
   length of the request chain, which is useful for testing a chain of
   proxies forwarding messages in an infinite loop.
</t>
<t>
   If the request is valid, the response SHOULD contain the entire
   request message in the entity-body, with a Content-Type of
   "message/http" (see Appendix A.1 of <xref target="Part1"/>). Responses to this method
   MUST NOT be cached.
</t>
</section>

<section title="CONNECT" anchor="CONNECT">
  <iref primary="true" item="CONNECT method"/>
  <iref primary="true" item="Methods" subitem="CONNECT"/>
<t>
   This specification reserves the method name CONNECT for use with a
   proxy that can dynamically switch to being a tunnel (e.g. SSL
   tunneling <xref target="Luo1998"/>).
</t>
</section>
</section>


<section title="Status Code Definitions" anchor="status.codes">
<t>
   Each Status-Code is described below, including a description of which
   method(s) it can follow and any metainformation required in the
   response.
</t>

<section title="Informational 1xx" anchor="status.1xx">
<t>
   This class of status code indicates a provisional response,
   consisting only of the Status-Line and optional headers, and is
   terminated by an empty line. There are no required headers for this
   class of status code. Since HTTP/1.0 did not define any 1xx status
   codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client
   except under experimental conditions.
</t>
<t>
   A client MUST be prepared to accept one or more 1xx status responses
   prior to a regular response, even if the client does not expect a 100
   (Continue) status message. Unexpected 1xx status responses MAY be
   ignored by a user agent.
</t>
<t>
   Proxies MUST forward 1xx responses, unless the connection between the
   proxy and its client has been closed, or unless the proxy itself
   requested the generation of the 1xx response. (For example, if a
   proxy adds a "Expect: 100-continue" field when it forwards a request,
   then it need not forward the corresponding 100 (Continue)
   response(s).)
</t>

<section title="100 Continue" anchor="status.100">
  <iref primary="true" item="100 Continue (status code)"/>
  <iref primary="true" item="Status Codes" subitem="100 Continue"/>
<t>
   The client SHOULD continue with its request. This interim response is
   used to inform the client that the initial part of the request has
   been received and has not yet been rejected by the server. The client
   SHOULD continue by sending the remainder of the request or, if the
   request has already been completed, ignore this response. The server
   MUST send a final response after the request has been completed. See
   Section 7.2.3 of <xref target="Part1"/> for detailed discussion of the use and handling of this
   status code.
</t>
</section>

<section title="101 Switching Protocols" anchor="status.101">
  <iref primary="true" item="101 Switching Protocols (status code)"/>
  <iref primary="true" item="Status Codes" subitem="101 Switching Protocols"/>
<t>
   The server understands and is willing to comply with the client's
   request, via the Upgrade message header field (Section 6.4 of <xref target="Part5"/>), for a
   change in the application protocol being used on this connection. The
   server will switch protocols to those defined by the response's
   Upgrade header field immediately after the empty line which
   terminates the 101 response.
</t>
<t>
   The protocol SHOULD be switched only when it is advantageous to do
   so. For example, switching to a newer version of HTTP is advantageous
   over older versions, and switching to a real-time, synchronous
   protocol might be advantageous when delivering resources that use
   such features.
</t>
</section>
</section>

<section title="Successful 2xx" anchor="status.2xx">
<t>
   This class of status code indicates that the client's request was
   successfully received, understood, and accepted.
</t>

<section title="200 OK" anchor="status.200">
  <iref primary="true" item="200 OK (status code)"/>
  <iref primary="true" item="Status Codes" subitem="200 OK"/>
<t>
   The request has succeeded. The information returned with the response
   is dependent on the method used in the request, for example:
  <list style="hanging">
    <t hangText="GET">
          an entity corresponding to the requested resource is sent in
          the response;
    </t>
    <t hangText="HEAD">
          the entity-header fields corresponding to the requested
          resource are sent in the response without any message-body;
    </t>
    <t hangText="POST">
      an entity describing or containing the result of the action;
    </t>
    <t hangText="TRACE">
      an entity containing the request message as received by the
      end server.
    </t>
  </list>
</t>
</section>

<section title="201 Created" anchor="status.201">
  <iref primary="true" item="201 Created (status code)"/>
  <iref primary="true" item="Status Codes" subitem="201 Created"/>
<t>
   The request has been fulfilled and resulted in a new resource being
   created. The newly created resource can be referenced by the URI(s)
   returned in the entity of the response, with the most specific URI
   for the resource given by a Location header field. The response
   SHOULD include an entity containing a list of resource
   characteristics and location(s) from which the user or user agent can
   choose the one most appropriate. The entity format is specified by
   the media type given in the Content-Type header field. The origin
   server MUST create the resource before returning the 201 status code.
   If the action cannot be carried out immediately, the server SHOULD
   respond with 202 (Accepted) response instead.
</t>
<t>
   A 201 response MAY contain an ETag response header field indicating
   the current value of the entity tag for the requested variant just
   created, see Section 7.1 of <xref target="Part4"/>.
</t>
</section>

<section title="202 Accepted" anchor="status.202">
  <iref primary="true" item="202 Accepted (status code)"/>
  <iref primary="true" item="Status Codes" subitem="202 Accepted"/>
<t>
   The request has been accepted for processing, but the processing has
   not been completed.  The request might or might not eventually be
   acted upon, as it might be disallowed when processing actually takes
   place. There is no facility for re-sending a status code from an
   asynchronous operation such as this.
</t>
<t>
   The 202 response is intentionally non-committal. Its purpose is to
   allow a server to accept a request for some other process (perhaps a
   batch-oriented process that is only run once per day) without
   requiring that the user agent's connection to the server persist
   until the process is completed. The entity returned with this
   response SHOULD include an indication of the request's current status
   and either a pointer to a status monitor or some estimate of when the
   user can expect the request to be fulfilled.
</t>
</section>

<section title="203 Non-Authoritative Information" anchor="status.203">
  <iref primary="true" item="203 Non-Authoritative Information (status code)"/>
  <iref primary="true" item="Status Codes" subitem="203 Non-Authoritative Information"/>
<t>
   The returned metainformation in the entity-header is not the
   definitive set as available from the origin server, but is gathered
   from a local or a third-party copy. The set presented MAY be a subset
   or superset of the original version. For example, including local
   annotation information about the resource might result in a superset
   of the metainformation known by the origin server. Use of this
   response code is not required and is only appropriate when the
   response would otherwise be 200 (OK).
</t>
</section>

<section title="204 No Content" anchor="status.204">
  <iref primary="true" item="204 No Content (status code)"/>
  <iref primary="true" item="Status Codes" subitem="204 No Content"/>
<t>
   The server has fulfilled the request but does not need to return an
   entity-body, and might want to return updated metainformation. The
   response MAY include new or updated metainformation in the form of
   entity-headers, which if present SHOULD be associated with the
   requested variant.
</t>
<t>
   If the client is a user agent, it SHOULD NOT  change its document view
   from that which caused the request to be sent. This response is
   primarily intended to allow input for actions to take place without
   causing a change to the user agent's active document view, although
   any new or updated metainformation SHOULD be applied to the document
   currently in the user agent's active view.
</t>
<t>
   The 204 response MUST NOT include a message-body, and thus is always
   terminated by the first empty line after the header fields.
</t>
</section>

<section title="205 Reset Content" anchor="status.205">
  <iref primary="true" item="205 Reset Content (status code)"/>
  <iref primary="true" item="Status Codes" subitem="205 Reset Content"/>
<t>
   The server has fulfilled the request and the user agent SHOULD reset
   the document view which caused the request to be sent. This response
   is primarily intended to allow input for actions to take place via
   user input, followed by a clearing of the form in which the input is
   given so that the user can easily initiate another input action. The
   response MUST NOT include an entity.
</t>
</section>

<section title="206 Partial Content" anchor="status.206">
  <iref primary="true" item="206 Partial Content (status code)"/>
  <iref primary="true" item="Status Codes" subitem="206 Partial Content"/>
<t>
   The server has fulfilled the partial GET request for the resource
   and the enclosed entity is a partial representation as defined in <xref target="Part5"/>.
</t>
</section>
</section>

<section title="Redirection 3xx" anchor="status.3xx">
<t>
   This class of status code indicates that further action needs to be
   taken by the user agent in order to fulfill the request.  The action
   required MAY be carried out by the user agent without interaction
   with the user if and only if the method used in the second request is
   GET or HEAD. A client SHOULD detect infinite redirection loops, since
   such loops generate network traffic for each redirection.
  <list><t>
      Note: previous versions of this specification recommended a
      maximum of five redirections. Content developers should be aware
      that there might be clients that implement such a fixed
      limitation.
  </t></list>
</t>

<section title="300 Multiple Choices" anchor="status.300">
  <iref primary="true" item="300 Multiple Choices (status code)"/>
  <iref primary="true" item="Status Codes" subitem="300 Multiple Choices"/>
<t>
   The requested resource corresponds to any one of a set of
   representations, each with its own specific location, and agent-driven
   negotiation information (Section 5 of <xref target="Part3"/>) is being provided so that
   the user (or user agent) can select a preferred representation and
   redirect its request to that location.
</t>
<t>
   Unless it was a HEAD request, the response SHOULD include an entity
   containing a list of resource characteristics and location(s) from
   which the user or user agent can choose the one most appropriate. The
   entity format is specified by the media type given in the Content-Type
   header field. Depending upon the format and the capabilities of
   the user agent, selection of the most appropriate choice MAY be
   performed automatically. However, this specification does not define
   any standard for such automatic selection.
</t>
<t>
   If the server has a preferred choice of representation, it SHOULD
   include the specific URI for that representation in the Location
   field; user agents MAY use the Location field value for automatic
   redirection. This response is cacheable unless indicated otherwise.
</t>
</section>

<section title="301 Moved Permanently" anchor="status.301">
  <iref primary="true" item="301 Moved Permanently (status code)"/>
  <iref primary="true" item="Status Codes" subitem="301 Moved Permanently"/>
<t>
   The requested resource has been assigned a new permanent URI and any
   future references to this resource SHOULD use one of the returned
   URIs.  Clients with link editing capabilities ought to automatically
   re-link references to the Request-URI to one or more of the new
   references returned by the server, where possible. This response is
   cacheable unless indicated otherwise.
</t>
<t>
   The new permanent URI SHOULD be given by the Location field in the
   response. Unless the request method was HEAD, the entity of the
   response SHOULD contain a short hypertext note with a hyperlink to
   the new URI(s).
</t>
<t>
   If the 301 status code is received in response to a request method
   that is known to be "safe", as defined in <xref target="safe.methods"/>,
   then the request MAY be automatically redirected by the user agent without
   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.
  <list><t>
      Note: When automatically redirecting a POST request after
      receiving a 301 status code, some existing HTTP/1.0 user agents
      will erroneously change it into a GET request.
  </t></list>
</t>
</section>

<section title="302 Found" anchor="status.302">
  <iref primary="true" item="302 Found (status code)"/>
  <iref primary="true" item="Status Codes" subitem="302 Found"/>
<t>
   The requested resource resides temporarily under a different URI.
   Since the redirection might be altered on occasion, the client SHOULD
   continue to use the Request-URI for future requests.  This response
   is only cacheable if indicated by a Cache-Control or Expires header
   field.
</t>
<t>
   The temporary URI SHOULD be given by the Location field in the
   response. Unless the request method was HEAD, the entity of the
   response SHOULD contain a short hypertext note with a hyperlink to
   the new URI(s).
</t>
<t>
   If the 302 status code is received in response to a request method
   that is known to be "safe", as defined in <xref target="safe.methods"/>,
   then the request MAY be automatically redirected by the user agent without
   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.
  <list><t>
      Note: <xref target="RFC1945"/> and <xref target="RFC2068"/> specify that the client is not allowed
      to change the method on the redirected request.  However, most
      existing user agent implementations treat 302 as if it were a 303
      response, performing a GET on the Location field-value regardless
      of the original request method. The status codes 303 and 307 have
      been added for servers that wish to make unambiguously clear which
      kind of reaction is expected of the client.
  </t></list>
</t>
</section>

<section title="303 See Other" anchor="status.303">
  <iref primary="true" item="303 See Other (status code)"/>
  <iref primary="true" item="Status Codes" subitem="303 See Other"/>
<t>
   The server directs the user agent to a different resource, indicated
   by a URI in the Location header field, that provides an indirect
   response to the original request.  The user agent MAY perform a GET
   request on the URI in the Location field in order to obtain a
   representation corresponding to the response, be redirected again,
   or end with an error status.  The Location URI is not a substitute
   reference for the originally requested resource.
</t>
<t>
   The 303 status is generally applicable to any HTTP method.  It is
   primarily used to allow the output of a POST action to redirect
   the user agent to a selected resource, since doing so provides the
   information corresponding to the POST response in a form that
   can be separately identified, bookmarked, and cached independent
   of the original request.
</t>
<t>
   A 303 response to a GET request indicates that the requested
   resource does not have a representation of its own that can be
   transferred by the server over HTTP.  The Location URI indicates a
   resource that is descriptive of the requested resource such that
   the follow-on representation may be useful without implying that
   it adequately represents the previously requested resource.
   Note that answers to the questions of what can be represented, what
   representations are adequate, and what might be a useful description
   are outside the scope of HTTP and thus entirely determined by the
   resource owner(s).
</t>
<t>
   A 303 response SHOULD NOT be cached unless it is indicated as
   cacheable by Cache-Control or Expires header fields.  Except for
   responses to a HEAD request, the entity of a 303 response SHOULD
   contain a short hypertext note with a hyperlink to the Location URI.
</t>
</section>

<section title="304 Not Modified" anchor="status.304">
  <iref primary="true" item="304 Not Modified (status code)"/>
  <iref primary="true" item="Status Codes" subitem="304 Not Modified"/>
<t>
   The response to the request has not been modified since the conditions
   indicated by the client's conditional GET request, as defined in <xref target="Part4"/>.
</t>
</section>

<section title="305 Use Proxy" anchor="status.305">
  <iref primary="true" item="305 Use Proxy (status code)"/>
  <iref primary="true" item="Status Codes" subitem="305 Use Proxy"/>
<t>
   The 305 status was defined in a previous version of this specification
   (see <xref target="changes.from.rfc.2616"/>), and is now deprecated.
</t>
</section>

<section title="306 (Unused)" anchor="status.306">
  <iref primary="true" item="306 (Unused) (status code)"/>
  <iref primary="true" item="Status Codes" subitem="306 (Unused)"/>
<t>
   The 306 status code was used in a previous version of the
   specification, is no longer used, and the code is reserved.
</t>
</section>

<section title="307 Temporary Redirect" anchor="status.307">
  <iref primary="true" item="307 Temporary Redirect (status code)"/>
  <iref primary="true" item="Status Codes" subitem="307 Temporary Redirect"/>
<t>
   The requested resource resides temporarily under a different URI.
   Since the redirection MAY be altered on occasion, the client SHOULD
   continue to use the Request-URI for future requests.  This response
   is only cacheable if indicated by a Cache-Control or Expires header
   field.
</t>
<t>
   The temporary URI SHOULD be given by the Location field in the
   response. Unless the request method was HEAD, the entity of the
   response SHOULD contain a short hypertext note with a hyperlink to
   the new URI(s) , since many pre-HTTP/1.1 user agents do not
   understand the 307 status. Therefore, the note SHOULD contain the
   information necessary for a user to repeat the original request on
   the new URI.
</t>
<t>
   If the 307 status code is received in response to a request method
   that is known to be "safe", as defined in <xref target="safe.methods"/>,
   then the request MAY be automatically redirected by the user agent without
   confirmation.  Otherwise, the user agent MUST NOT automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.
</t>
</section>
</section>

<section title="Client Error 4xx" anchor="status.4xx">
<t>
   The 4xx class of status code is intended for cases in which the
   client seems to have erred. Except when responding to a HEAD request,
   the server SHOULD include an entity containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition. These status codes are applicable to any request method.
   User agents SHOULD display any included entity to the user.
</t>
<t>
   If the client is sending data, a server implementation using TCP
   SHOULD be careful to ensure that the client acknowledges receipt of
   the packet(s) containing the response, before the server closes the
   input connection. If the client continues sending data to the server
   after the close, the server's TCP stack will send a reset packet to
   the client, which may erase the client's unacknowledged input buffers
   before they can be read and interpreted by the HTTP application.
</t>

<section title="400 Bad Request" anchor="status.400">
  <iref primary="true" item="400 Bad Request (status code)"/>
  <iref primary="true" item="Status Codes" subitem="400 Bad Request"/>
<t>
   The request could not be understood by the server due to malformed
   syntax. The client SHOULD NOT  repeat the request without
   modifications.
</t>
</section>

<section title="401 Unauthorized" anchor="status.401">
  <iref primary="true" item="401 Unauthorized (status code)"/>
  <iref primary="true" item="Status Codes" subitem="401 Unauthorized"/>
<t>
   The request requires user authentication (see <xref target="Part7"/>).
</t>
</section>

<section title="402 Payment Required" anchor="status.402">
  <iref primary="true" item="402 Payment Required (status code)"/>
  <iref primary="true" item="Status Codes" subitem="402 Payment Required"/>
<t>
   This code is reserved for future use.
</t>
</section>

<section title="403 Forbidden" anchor="status.403">
  <iref primary="true" item="403 Forbidden (status code)"/>
  <iref primary="true" item="Status Codes" subitem="403 Forbidden"/>
<t>
   The server understood the request, but is refusing to fulfill it.
   Authorization will not help and the request SHOULD NOT  be repeated.
   If the request method was not HEAD and the server wishes to make
   public why the request has not been fulfilled, it SHOULD describe the
   reason for the refusal in the entity.  If the server does not wish to
   make this information available to the client, the status code 404
   (Not Found) can be used instead.
</t>
</section>

<section title="404 Not Found" anchor="status.404">
  <iref primary="true" item="404 Not Found (status code)"/>
  <iref primary="true" item="Status Codes" subitem="404 Not Found"/>
<t>
   The server has not found anything matching the Request-URI. No
   indication is given of whether the condition is temporary or
   permanent. The 410 (Gone) status code SHOULD be used if the server
   knows, through some internally configurable mechanism, that an old
   resource is permanently unavailable and has no forwarding address.
   This status code is commonly used when the server does not wish to
   reveal exactly why the request has been refused, or when no other
   response is applicable.
</t>
</section>

<section title="405 Method Not Allowed" anchor="status.405">
  <iref primary="true" item="405 Method Not Allowed (status code)"/>
  <iref primary="true" item="Status Codes" subitem="405 Method Not Allowed"/>
<t>
   The method specified in the Request-Line is not allowed for the
   resource identified by the Request-URI. The response MUST include an
   Allow header containing a list of valid methods for the requested
   resource.
</t>
</section>

<section title="406 Not Acceptable" anchor="status.406">
  <iref primary="true" item="406 Not Acceptable (status code)"/>
  <iref primary="true" item="Status Codes" subitem="406 Not Acceptable"/>
<t>
   The resource identified by the request is only capable of generating
   response entities which have content characteristics not acceptable
   according to the accept headers sent in the request.
</t>
<t>
   Unless it was a HEAD request, the response SHOULD include an entity
   containing a list of available entity characteristics and location(s)
   from which the user or user agent can choose the one most
   appropriate. The entity format is specified by the media type given
   in the Content-Type header field. Depending upon the format and the
   capabilities of the user agent, selection of the most appropriate
   choice MAY be performed automatically. However, this specification
   does not define any standard for such automatic selection.
  <list><t>
      Note: HTTP/1.1 servers are allowed to return responses which are
      not acceptable according to the accept headers sent in the
      request. In some cases, this may even be preferable to sending a
      406 response. User agents are encouraged to inspect the headers of
      an incoming response to determine if it is acceptable.
  </t></list>
</t>
<t>
   If the response could be unacceptable, a user agent SHOULD
   temporarily stop receipt of more data and query the user for a
   decision on further actions.
</t>
</section>

<section title="407 Proxy Authentication Required" anchor="status.407">
  <iref primary="true" item="407 Proxy Authentication Required (status code)"/>
  <iref primary="true" item="Status Codes" subitem="407 Proxy Authentication Required"/>
<t>
   This code is similar to 401 (Unauthorized), but indicates that the
   client must first authenticate itself with the proxy (see <xref target="Part7"/>).
</t>
</section>

<section title="408 Request Timeout" anchor="status.408">
  <iref primary="true" item="408 Request Timeout (status code)"/>
  <iref primary="true" item="Status Codes" subitem="408 Request Timeout"/>
<t>
   The client did not produce a request within the time that the server
   was prepared to wait. The client MAY repeat the request without
   modifications at any later time.
</t>
</section>

<section title="409 Conflict" anchor="status.409">
  <iref primary="true" item="409 Conflict (status code)"/>
  <iref primary="true" item="Status Codes" subitem="409 Conflict"/>
<t>
   The request could not be completed due to a conflict with the current
   state of the resource. This code is only allowed in situations where
   it is expected that the user might be able to resolve the conflict
   and resubmit the request. The response body SHOULD include enough
   information for the user to recognize the source of the conflict.
   Ideally, the response entity would include enough information for the
   user or user agent to fix the problem; however, that might not be
   possible and is not required.
</t>
<t>
   Conflicts are most likely to occur in response to a PUT request. For
   example, if versioning were being used and the entity being PUT
   included changes to a resource which conflict with those made by an
   earlier (third-party) request, the server might use the 409 response
   to indicate that it can't complete the request. In this case, the
   response entity would likely contain a list of the differences
   between the two versions in a format defined by the response
   Content-Type.
</t>
</section>

<section title="410 Gone" anchor="status.410">
  <iref primary="true" item="410 Gone (status code)"/>
  <iref primary="true" item="Status Codes" subitem="410 Gone"/>
<t>
   The requested resource is no longer available at the server and no
   forwarding address is known. This condition is expected to be
   considered permanent. Clients with link editing capabilities SHOULD
   delete references to the Request-URI after user approval. If the
   server does not know, or has no facility to determine, whether or not
   the condition is permanent, the status code 404 (Not Found) SHOULD be
   used instead. This response is cacheable unless indicated otherwise.
</t>
<t>
   The 410 response is primarily intended to assist the task of web
   maintenance by notifying the recipient that the resource is
   intentionally unavailable and that the server owners desire that
   remote links to that resource be removed. Such an event is common for
   limited-time, promotional services and for resources belonging to
   individuals no longer working at the server's site. It is not
   necessary to mark all permanently unavailable resources as "gone" or
   to keep the mark for any length of time -- that is left to the
   discretion of the server owner.
</t>
</section>

<section title="411 Length Required" anchor="status.411">
  <iref primary="true" item="411 Length Required (status code)"/>
  <iref primary="true" item="Status Codes" subitem="411 Length Required"/>
<t>
   The server refuses to accept the request without a defined Content-Length.
   The client MAY repeat the request if it adds a valid
   Content-Length header field containing the length of the message-body
   in the request message.
</t>
</section>

<section title="412 Precondition Failed" anchor="status.412">
  <iref primary="true" item="412 Precondition Failed (status code)"/>
  <iref primary="true" item="Status Codes" subitem="412 Precondition Failed"/>
<t>
   The precondition given in one or more of the request-header fields
   evaluated to false when it was tested on the server, as defined in
   <xref target="Part4"/>.
</t>
</section>

<section title="413 Request Entity Too Large" anchor="status.413">
  <iref primary="true" item="413 Request Entity Too Large (status code)"/>
  <iref primary="true" item="Status Codes" subitem="413 Request Entity Too Large"/>
<t>
   The server is refusing to process a request because the request
   entity is larger than the server is willing or able to process. The
   server MAY close the connection to prevent the client from continuing
   the request.
</t>
<t>
   If the condition is temporary, the server SHOULD include a Retry-After
   header field to indicate that it is temporary and after what
   time the client MAY try again.
</t>
</section>

<section title="414 Request-URI Too Long" anchor="status.414">
  <iref primary="true" item="414 Request-URI Too Long (status code)"/>
  <iref primary="true" item="Status Codes" subitem="414 Request-URI Too Long"/>
<t>
   The server is refusing to service the request because the Request-URI
   is longer than the server is willing to interpret. This rare
   condition is only likely to occur when a client has improperly
   converted a POST request to a GET request with long query
   information, when the client has descended into a URI "black hole" of
   redirection (e.g., a redirected URI prefix that points to a suffix of
   itself), or when the server is under attack by a client attempting to
   exploit security holes present in some servers using fixed-length
   buffers for reading or manipulating the Request-URI.
</t>
</section>

<section title="415 Unsupported Media Type" anchor="status.415">
  <iref primary="true" item="415 Unsupported Media Type (status code)"/>
  <iref primary="true" item="Status Codes" subitem="415 Unsupported Media Type"/>
<t>
   The server is refusing to service the request because the entity of
   the request is in a format not supported by the requested resource
   for the requested method.
</t>
</section>

<section title="416 Requested Range Not Satisfiable" anchor="status.416">
  <iref primary="true" item="416 Requested Range Not Satisfiable (status code)"/>
  <iref primary="true" item="Status Codes" subitem="416 Requested Range Not Satisfiable"/>
<t>
   The request included a Range request-header field (Section 6.4 of <xref target="Part5"/>) and none of
   the range-specifier values in this field overlap the current extent
   of the selected resource.
</t>
</section>

<section title="417 Expectation Failed" anchor="status.417">
  <iref primary="true" item="417 Expectation Failed (status code)"/>
  <iref primary="true" item="Status Codes" subitem="417 Expectation Failed"/>
<t>
   The expectation given in an Expect request-header field (see <xref target="header.expect"/>)
   could not be met by this server, or, if the server is a proxy,
   the server has unambiguous evidence that the request could not be met
   by the next-hop server.
</t>
</section>
</section>

<section title="Server Error 5xx" anchor="status.5xx">
<t>
   Response status codes beginning with the digit "5" indicate cases in
   which the server is aware that it has erred or is incapable of
   performing the request. Except when responding to a HEAD request, the
   server SHOULD include an entity containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition. User agents SHOULD display any included entity to the
   user. These response codes are applicable to any request method.
</t>

<section title="500 Internal Server Error" anchor="status.500">
  <iref primary="true" item="500 Internal Server Error (status code)"/>
  <iref primary="true" item="Status Codes" subitem="500 Internal Server Error"/>
<t>
   The server encountered an unexpected condition which prevented it
   from fulfilling the request.
</t>
</section>

<section title="501 Not Implemented" anchor="status.501">
  <iref primary="true" item="501 Not Implemented (status code)"/>
  <iref primary="true" item="Status Codes" subitem="501 Not Implemented"/>
<t>
   The server does not support the functionality required to fulfill the
   request. This is the appropriate response when the server does not
   recognize the request method and is not capable of supporting it for
   any resource.
</t>
</section>

<section title="502 Bad Gateway" anchor="status.502">
  <iref primary="true" item="502 Bad Gateway (status code)"/>
  <iref primary="true" item="Status Codes" subitem="502 Bad Gateway"/>
<t>
   The server, while acting as a gateway or proxy, received an invalid
   response from the upstream server it accessed in attempting to
   fulfill the request.
</t>
</section>

<section title="503 Service Unavailable" anchor="status.503">
  <iref primary="true" item="503 Service Unavailable (status code)"/>
  <iref primary="true" item="Status Codes" subitem="503 Service Unavailable"/>
<t>
   The server is currently unable to handle the request due to a
   temporary overloading or maintenance of the server. The implication
   is that this is a temporary condition which will be alleviated after
   some delay. If known, the length of the delay MAY be indicated in a
   Retry-After header. If no Retry-After is given, the client SHOULD
   handle the response as it would for a 500 response.
  <list><t>
      Note: The existence of the 503 status code does not imply that a
      server must use it when becoming overloaded. Some servers may wish
      to simply refuse the connection.
  </t></list>
</t>
</section>

<section title="504 Gateway Timeout" anchor="status.504">
  <iref primary="true" item="504 Gateway Timeout (status code)"/>
  <iref primary="true" item="Status Codes" subitem="504 Gateway Timeout"/>
<t>
   The server, while acting as a gateway or proxy, did not receive a
   timely response from the upstream server specified by the URI (e.g.
   HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
   to access in attempting to complete the request.
  <list><t>
      Note: Note to implementors: some deployed proxies are known to
      return 400 or 500 when DNS lookups time out.
  </t></list>
</t>
</section>

<section title="505 HTTP Version Not Supported" anchor="status.505">
  <iref primary="true" item="505 HTTP Version Not Supported (status code)"/>
  <iref primary="true" item="Status Codes" subitem="505 HTTP Version Not Supported"/>
<t>
   The server does not support, or refuses to support, the protocol
   version that was used in the request message. The server is
   indicating that it is unable or unwilling to complete the request
   using the same major version as the client, as described in Section 3.1 of <xref target="Part1"/>,
   other than with this error message. The response SHOULD contain
   an entity describing why that version is not supported and what other
   protocols are supported by that server.
</t>

</section>
</section>
</section>


<section title="Header Field Definitions" anchor="header.fields">
<t>
   This section defines the syntax and semantics of HTTP/1.1 header fields
   related to request and response semantics.
</t>
<t>
   For entity-header fields, both sender and recipient refer to either the
   client or the server, depending on who sends and who receives the entity.
</t>

<section title="Allow" anchor="header.allow">
  <iref primary="true" item="Allow header"/>
  <iref primary="true" item="Headers" subitem="Allow"/>
  
<t>
      The Allow response-header field lists the set of methods advertised as
      supported by the resource identified by the Request-URI. The purpose of
      this field is strictly to inform the recipient of valid methods
      associated with the resource. An Allow header field MUST be
      present in a 405 (Method Not Allowed) response.
</t>
<figure><iref primary="true" item="Grammar" subitem="Allow"/><artwork type="abnf2616"><![CDATA[
  Allow   = "Allow" ":" #Method
]]></artwork></figure>
<t>
      Example of use:
</t>
<figure><artwork type="example"><![CDATA[
       Allow: GET, HEAD, PUT
]]></artwork></figure>
<t>
      The actual set of allowed methods is defined
      by the origin server at the time of each request.
</t>
<t>
      A proxy MUST NOT modify the Allow header field even if it does not
      understand all the methods specified, since the user agent might
      have other means of communicating with the origin server.
</t>
</section>

<section title="Expect" anchor="header.expect">
  <iref primary="true" item="Expect header"/>
  <iref primary="true" item="Headers" subitem="Expect"/>
  
  
  
  
<t>
   The Expect request-header field is used to indicate that particular
   server behaviors are required by the client.
</t>
<figure><iref primary="true" item="Grammar" subitem="Expect"/><iref primary="true" item="Grammar" subitem="expectation"/><iref primary="true" item="Grammar" subitem="expectation-extension"/><iref primary="true" item="Grammar" subitem="expect-params"/><artwork type="abnf2616"><![CDATA[
  Expect       =  "Expect" ":" 1#expectation
  
  expectation  =  "100-continue" | expectation-extension
  expectation-extension =  token [ "=" ( token | quoted-string )
                           *expect-params ]
  expect-params =  ";" token [ "=" ( token | quoted-string ) ]
]]></artwork></figure>
<t>
   A server that does not understand or is unable to comply with any of
   the expectation values in the Expect field of a request MUST respond
   with appropriate error status. The server MUST respond with a 417
   (Expectation Failed) status if any of the expectations cannot be met
   or, if there are other problems with the request, some other 4xx
   status.
</t>
<t>
   This header field is defined with extensible syntax to allow for
   future extensions. If a server receives a request containing an
   Expect field that includes an expectation-extension that it does not
   support, it MUST respond with a 417 (Expectation Failed) status.
</t>
<t>
   Comparison of expectation values is case-insensitive for unquoted
   tokens (including the 100-continue token), and is case-sensitive for
   quoted-string expectation-extensions.
</t>
<t>
   The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
   return a 417 (Expectation Failed) status if it receives a request
   with an expectation that it cannot meet. However, the Expect
   request-header itself is end-to-end; it MUST be forwarded if the
   request is forwarded.
</t>
<t>
   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
   Expect header.
</t>
<t>
   See Section 7.2.3 of <xref target="Part1"/> for the use of the 100 (Continue) status.
</t>
</section>

<section title="From" anchor="header.from">
  <iref primary="true" item="From header"/>
  <iref primary="true" item="Headers" subitem="From"/>
  
  
<t>
   The From request-header field, if given, SHOULD contain an Internet
   e-mail address for the human user who controls the requesting user
   agent. The address SHOULD be machine-usable, as defined by "mailbox"
   in Section 3.4 of <xref target="RFC2822"/>:
</t>
<figure><iref primary="true" item="Grammar" subitem="From"/><artwork type="abnf2616"><![CDATA[
  From    = "From" ":" mailbox
  
  mailbox = <mailbox, defined in [RFC2822], Section 3.4>
]]></artwork></figure>
<t>
   An example is:
</t>
<figure><artwork type="example"><![CDATA[
    From: webmaster@example.org
]]></artwork></figure>
<t>
   This header field MAY be used for logging purposes and as a means for
   identifying the source of invalid or unwanted requests. It SHOULD NOT 
   be used as an insecure form of access protection. The interpretation
   of this field is that the request is being performed on behalf of the
   person given, who accepts responsibility for the method performed. In
   particular, robot agents SHOULD include this header so that the
   person responsible for running the robot can be contacted if problems
   occur on the receiving end.
</t>
<t>
   The Internet e-mail address in this field MAY be separate from the
   Internet host which issued the request. For example, when a request
   is passed through a proxy the original issuer's address SHOULD be
   used.
</t>
<t>
   The client SHOULD NOT  send the From header field without the user's
   approval, as it might conflict with the user's privacy interests or
   their site's security policy. It is strongly recommended that the
   user be able to disable, enable, and modify the value of this field
   at any time prior to a request.
</t>
</section>

<section title="Location" anchor="header.location">
  <iref primary="true" item="Location header"/>
  <iref primary="true" item="Headers" subitem="Location"/>
  
<t>
   The Location response-header field is used for the identification of a 
   new resource or to redirect the recipient to a location other than the 
   Request-URI for completion of the request.  For 201 (Created)
   responses, the Location is that of the new resource which was created
   by the request. For 3xx responses, the location SHOULD indicate the
   server's preferred URI for automatic redirection to the resource. The
   field value consists of a single absolute URI.
</t>
<figure><iref primary="true" item="Grammar" subitem="Location"/><artwork type="abnf2616"><![CDATA[
  Location       = "Location" ":" absoluteURI [ "#" fragment ]
]]></artwork></figure>
<t>
   An example is:
</t>
<figure><artwork type="example"><![CDATA[
    Location: http://www.example.org/pub/WWW/People.html
]]></artwork></figure>
<t>
  <list><t>
      Note: The Content-Location header field (Section 6.7 of <xref target="Part3"/>) differs
      from Location in that the Content-Location identifies the original
      location of the entity enclosed in the request. It is therefore
      possible for a response to contain header fields for both Location
      and Content-Location.
  </t></list>
</t>
<t>
   There are circumstances in which a fragment identifier in a Location URL would not be appropriate:
   <list style="symbols">
      <t>With a 201 Created response, because in this usage the Location header specifies the URL for the entire created resource.</t>
      <t>With a 300 Multiple Choices, since the choice decision is intended to be made on resource characteristics and not fragment characteristics.</t>
      <t>With 305 Use Proxy.</t>
   </list>
</t>
</section>

<section title="Max-Forwards" anchor="header.max-forwards">
  <iref primary="true" item="Max-Forwards header"/>
  <iref primary="true" item="Headers" subitem="Max-Forwards"/>
  
<t>
   The Max-Forwards request-header field provides a mechanism with the
   TRACE (<xref target="TRACE"/>) and OPTIONS (<xref target="OPTIONS"/>) methods to limit the
   number of proxies or gateways that can forward the request to the
   next inbound server. This can be useful when the client is attempting
   to trace a request chain which appears to be failing or looping in
   mid-chain.
</t>
<figure><iref primary="true" item="Grammar" subitem="Max-Forwards"/><artwork type="abnf2616"><![CDATA[
  Max-Forwards   = "Max-Forwards" ":" 1*DIGIT
]]></artwork></figure>
<t>
   The Max-Forwards value is a decimal integer indicating the remaining
   number of times this request message may be forwarded.
</t>
<t>
   Each proxy or gateway recipient of a TRACE or OPTIONS request
   containing a Max-Forwards header field MUST check and update its
   value prior to forwarding the request. If the received value is zero
   (0), the recipient MUST NOT forward the request; instead, it MUST
   respond as the final recipient. If the received Max-Forwards value is
   greater than zero, then the forwarded message MUST contain an updated
   Max-Forwards field with a value decremented by one (1).
</t>
<t>
   The Max-Forwards header field MAY be ignored for all other methods
   defined by this specification and for any extension methods for which
   it is not explicitly referred to as part of that method definition.
</t>
</section>

<section title="Referer" anchor="header.referer">
  <iref primary="true" item="Referer header"/>
  <iref primary="true" item="Headers" subitem="Referer"/>
  
<t>
   The Referer[sic] request-header field allows the client to specify,
   for the server's benefit, the address (URI) of the resource from
   which the Request-URI was obtained (the "referrer", although the
   header field is misspelled.) The Referer request-header allows a
   server to generate lists of back-links to resources for interest,
   logging, optimized caching, etc. It also allows obsolete or mistyped
   links to be traced for maintenance. The Referer field MUST NOT be
   sent if the Request-URI was obtained from a source that does not have
   its own URI, such as input from the user keyboard.
</t>
<figure><iref primary="true" item="Grammar" subitem="Referer"/><artwork type="abnf2616"><![CDATA[
  Referer        = "Referer" ":" ( absoluteURI | relativeURI )
]]></artwork></figure>
<t>
   Example:
</t>
<figure><artwork type="example"><![CDATA[
    Referer: http://www.example.org/hypertext/Overview.html
]]></artwork></figure>
<t>
   If the field value is a relative URI, it SHOULD be interpreted
   relative to the Request-URI. The URI MUST NOT include a fragment. See
   <xref target="encoding.sensitive.information.in.uris"/> for security considerations.
</t>
</section>

<section title="Retry-After" anchor="header.retry-after">
  <iref primary="true" item="Retry-After header"/>
  <iref primary="true" item="Headers" subitem="Retry-After"/>
  
<t>
   The Retry-After response-header field can be used with a 503 (Service
   Unavailable) response to indicate how long the service is expected to
   be unavailable to the requesting client. This field MAY also be used
   with any 3xx (Redirection) response to indicate the minimum time the
   user-agent is asked wait before issuing the redirected request. The
   value of this field can be either an HTTP-date or an integer number
   of seconds (in decimal) after the time of the response.
</t>
<figure><iref primary="true" item="Grammar" subitem="Retry-After"/><artwork type="abnf2616"><![CDATA[
  Retry-After   = "Retry-After" ":" ( HTTP-date | delta-seconds )
]]></artwork></figure>
<t anchor="rule.delta-seconds">
  
   Time spans are non-negative decimal integers, representing time in
   seconds.
</t>
<figure><iref primary="true" item="Grammar" subitem="delta-seconds"/><artwork type="abnf2616"><![CDATA[
  delta-seconds  = 1*DIGIT
]]></artwork></figure>
<t>
   Two examples of its use are
</t>
<figure><artwork type="example"><![CDATA[
    Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
    Retry-After: 120
]]></artwork></figure>
<t>
   In the latter example, the delay is 2 minutes.
</t>
</section>

<section title="Server" anchor="header.server">
  <iref primary="true" item="Server header"/>
  <iref primary="true" item="Headers" subitem="Server"/>
  
<t>
   The Server response-header field contains information about the
   software used by the origin server to handle the request. The field
   can contain multiple product tokens (Section 3.5 of <xref target="Part1"/>) and comments
   identifying the server and any significant subproducts. The product
   tokens are listed in order of their significance for identifying the
   application.
</t>
<figure><iref primary="true" item="Grammar" subitem="Server"/><artwork type="abnf2616"><![CDATA[
  Server         = "Server" ":" 1*( product | comment )
]]></artwork></figure>
<t>
   Example:
</t>
<figure><artwork type="example"><![CDATA[
    Server: CERN/3.0 libwww/2.17
]]></artwork></figure>
<t>
   If the response is being forwarded through a proxy, the proxy
   application MUST NOT modify the Server response-header. Instead, it
   MUST include a Via field (as described in Section 8.9 of <xref target="Part1"/>).
  <list><t>
      Note: Revealing the specific software version of the server might
      allow the server machine to become more vulnerable to attacks
      against software that is known to contain security holes. Server
      implementors are encouraged to make this field a configurable
      option.
  </t></list>
</t>
</section>

<section title="User-Agent" anchor="header.user-agent">
  <iref primary="true" item="User-Agent header"/>
  <iref primary="true" item="Headers" subitem="User-Agent"/>
  
<t>
   The User-Agent request-header field contains information about the
   user agent originating the request. This is for statistical purposes,
   the tracing of protocol violations, and automated recognition of user
   agents for the sake of tailoring responses to avoid particular user
   agent limitations. User agents SHOULD include this field with
   requests. The field can contain multiple product tokens (Section 3.5 of <xref target="Part1"/>)
   and comments identifying the agent and any subproducts which form a
   significant part of the user agent. By convention, the product tokens
   are listed in order of their significance for identifying the
   application.
</t>
<figure><iref primary="true" item="Grammar" subitem="User-Agent"/><artwork type="abnf2616"><![CDATA[
  User-Agent     = "User-Agent" ":" 1*( product | comment )
]]></artwork></figure>
<t>
   Example:
</t>
<figure><artwork type="example"><![CDATA[
    User-Agent: CERN-LineMode/2.15 libwww/2.17b3
]]></artwork></figure>
</section>

</section>

<section title="IANA Considerations" anchor="IANA.considerations">
<section title="Status Code Registry" anchor="status.code.registration">
<t>
  The registration procedure for HTTP Status Codes -- previously defined
  in Section 7.1 of <xref target="RFC2817"/> -- is now defined
  by <xref target="status.code.registry"/> of this document.
</t>
<!--AUTOGENERATED FROM extract-status-code-defs.xslt, do not edit manually-->
<!--(START)-->
<t>
    The HTTP Status Code Registry located at <eref target="http://www.iana.org/assignments/http-status-codes"/>
    should be updated with the registrations below:
  </t>
<texttable>
   <ttcol>Value</ttcol>
   <ttcol>Description</ttcol>
   <ttcol>Reference</ttcol>

   <c>100</c>
   <c>Continue</c>
   <c>
      <xref target="status.100"/>
   </c>

   <c>101</c>
   <c>Switching Protocols</c>
   <c>
      <xref target="status.101"/>
   </c>

   <c>200</c>
   <c>OK</c>
   <c>
      <xref target="status.200"/>
   </c>

   <c>201</c>
   <c>Created</c>
   <c>
      <xref target="status.201"/>
   </c>

   <c>202</c>
   <c>Accepted</c>
   <c>
      <xref target="status.202"/>
   </c>

   <c>203</c>
   <c>Non-Authoritative Information</c>
   <c>
      <xref target="status.203"/>
   </c>

   <c>204</c>
   <c>No Content</c>
   <c>
      <xref target="status.204"/>
   </c>

   <c>205</c>
   <c>Reset Content</c>
   <c>
      <xref target="status.205"/>
   </c>

   <c>206</c>
   <c>Partial Content</c>
   <c>
      <xref target="status.206"/>
   </c>

   <c>300</c>
   <c>Multiple Choices</c>
   <c>
      <xref target="status.300"/>
   </c>

   <c>301</c>
   <c>Moved Permanently</c>
   <c>
      <xref target="status.301"/>
   </c>

   <c>302</c>
   <c>Found</c>
   <c>
      <xref target="status.302"/>
   </c>

   <c>303</c>
   <c>See Other</c>
   <c>
      <xref target="status.303"/>
   </c>

   <c>304</c>
   <c>Not Modified</c>
   <c>
      <xref target="status.304"/>
   </c>

   <c>305</c>
   <c>Use Proxy</c>
   <c>
      <xref target="status.305"/>
   </c>

   <c>306</c>
   <c>(Unused)</c>
   <c>
      <xref target="status.306"/>
   </c>

   <c>307</c>
   <c>Temporary Redirect</c>
   <c>
      <xref target="status.307"/>
   </c>

   <c>400</c>
   <c>Bad Request</c>
   <c>
      <xref target="status.400"/>
   </c>

   <c>401</c>
   <c>Unauthorized</c>
   <c>
      <xref target="status.401"/>
   </c>

   <c>402</c>
   <c>Payment Required</c>
   <c>
      <xref target="status.402"/>
   </c>

   <c>403</c>
   <c>Forbidden</c>
   <c>
      <xref target="status.403"/>
   </c>

   <c>404</c>
   <c>Not Found</c>
   <c>
      <xref target="status.404"/>
   </c>

   <c>405</c>
   <c>Method Not Allowed</c>
   <c>
      <xref target="status.405"/>
   </c>

   <c>406</c>
   <c>Not Acceptable</c>
   <c>
      <xref target="status.406"/>
   </c>

   <c>407</c>
   <c>Proxy Authentication Required</c>
   <c>
      <xref target="status.407"/>
   </c>

   <c>408</c>
   <c>Request Timeout</c>
   <c>
      <xref target="status.408"/>
   </c>

   <c>409</c>
   <c>Conflict</c>
   <c>
      <xref target="status.409"/>
   </c>

   <c>410</c>
   <c>Gone</c>
   <c>
      <xref target="status.410"/>
   </c>

   <c>411</c>
   <c>Length Required</c>
   <c>
      <xref target="status.411"/>
   </c>

   <c>412</c>
   <c>Precondition Failed</c>
   <c>
      <xref target="status.412"/>
   </c>

   <c>413</c>
   <c>Request Entity Too Large</c>
   <c>
      <xref target="status.413"/>
   </c>

   <c>414</c>
   <c>Request-URI Too Long</c>
   <c>
      <xref target="status.414"/>
   </c>

   <c>415</c>
   <c>Unsupported Media Type</c>
   <c>
      <xref target="status.415"/>
   </c>

   <c>416</c>
   <c>Requested Range Not Satisfiable</c>
   <c>
      <xref target="status.416"/>
   </c>

   <c>417</c>
   <c>Expectation Failed</c>
   <c>
      <xref target="status.417"/>
   </c>

   <c>500</c>
   <c>Internal Server Error</c>
   <c>
      <xref target="status.500"/>
   </c>

   <c>501</c>
   <c>Not Implemented</c>
   <c>
      <xref target="status.501"/>
   </c>

   <c>502</c>
   <c>Bad Gateway</c>
   <c>
      <xref target="status.502"/>
   </c>

   <c>503</c>
   <c>Service Unavailable</c>
   <c>
      <xref target="status.503"/>
   </c>

   <c>504</c>
   <c>Gateway Timeout</c>
   <c>
      <xref target="status.504"/>
   </c>

   <c>505</c>
   <c>HTTP Version Not Supported</c>
   <c>
      <xref target="status.505"/>
   </c>
</texttable>
<!--(END)-->
</section>
<section title="Message Header Registration" anchor="message.header.registration">
<!--AUTOGENERATED FROM extract-header-defs.xslt, do not edit manually-->
<!--(START)-->
<t>
    The Message Header Registry located at <eref target="http://www.iana.org/assignments/message-headers/message-header-index.html"/> should be updated
    with the permanent registrations below (see <xref target="RFC3864"/>):
  </t>
<texttable>
   <ttcol>Header Field Name</ttcol>
   <ttcol>Protocol</ttcol>
   <ttcol>Status</ttcol>
   <ttcol>Reference</ttcol>

   <c>Allow</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.allow"/>
   </c>

   <c>Expect</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.expect"/>
   </c>

   <c>From</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.from"/>
   </c>

   <c>Location</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.location"/>
   </c>

   <c>Max-Forwards</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.max-forwards"/>
   </c>

   <c>Referer</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.referer"/>
   </c>

   <c>Retry-After</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.retry-after"/>
   </c>

   <c>Server</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.server"/>
   </c>

   <c>User-Agent</c>
   <c>http</c>
   <c>standard</c>
   <c>
      <xref target="header.user-agent"/>
   </c>
</texttable>
<t>
    The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
</t>
<!--(END)-->
</section>
</section>

<section title="Security Considerations" anchor="security.considerations">
<t>
   This section is meant to inform application developers, information
   providers, and users of the security limitations in HTTP/1.1 as
   described by this document. The discussion does not include
   definitive solutions to the problems revealed, though it does make
   some suggestions for reducing security risks.
</t>

<section title="Transfer of Sensitive Information" anchor="security.sensitive">
<t>
   Like any generic data transfer protocol, HTTP cannot regulate the
   content of the data that is transferred, nor is there any a priori
   method of determining the sensitivity of any particular piece of
   information within the context of any given request. Therefore,
   applications SHOULD supply as much control over this information as
   possible to the provider of that information. Four header fields are
   worth special mention in this context: Server, Via, Referer and From.
</t>
<t>
   Revealing the specific software version of the server might allow the
   server machine to become more vulnerable to attacks against software
   that is known to contain security holes. Implementors SHOULD make the
   Server header field a configurable option.
</t>
<t>
   Proxies which serve as a portal through a network firewall SHOULD
   take special precautions regarding the transfer of header information
   that identifies the hosts behind the firewall. In particular, they
   SHOULD remove, or replace with sanitized versions, any Via fields
   generated behind the firewall.
</t>
<t>
   The Referer header allows reading patterns to be studied and reverse
   links drawn. Although it can be very useful, its power can be abused
   if user details are not separated from the information contained in
   the Referer. Even when the personal information has been removed, the
   Referer header might indicate a private document's URI whose
   publication would be inappropriate.
</t>
<t>
   The information sent in the From field might conflict with the user's
   privacy interests or their site's security policy, and hence it
   SHOULD NOT  be transmitted without the user being able to disable,
   enable, and modify the contents of the field. The user MUST be able
   to set the contents of this field within a user preference or
   application defaults configuration.
</t>
<t>
   We suggest, though do not require, that a convenient toggle interface
   be provided for the user to enable or disable the sending of From and
   Referer information.
</t>
<t>
   The User-Agent (<xref target="header.user-agent"/>) or Server (<xref target="header.server"/>) header
   fields can sometimes be used to determine that a specific client or
   server have a particular security hole which might be exploited.
   Unfortunately, this same information is often used for other valuable
   purposes for which HTTP currently has no better mechanism.
</t>
</section>

<section title="Encoding Sensitive Information in URIs" anchor="encoding.sensitive.information.in.uris">
<t>
   Because the source of a link might be private information or might
   reveal an otherwise private information source, it is strongly
   recommended that the user be able to select whether or not the
   Referer field is sent. For example, a browser client could have a
   toggle switch for browsing openly/anonymously, which would
   respectively enable/disable the sending of Referer and From
   information.
</t>
<t>
   Clients SHOULD NOT include a Referer header field in a (non-secure)
   HTTP request if the referring page was transferred with a secure
   protocol.
</t>
<t>
   Authors of services should not use
   GET-based forms for the submission of sensitive data because that
   data will be encoded in the Request-URI. Many existing
   servers, proxies, and user agents log or display the Request-URI in
   places where it might be visible to third parties. Such services can
   use POST-based form submission instead.
</t>
</section>

<section title="Location Headers and Spoofing" anchor="location.spoofing">
<t>
   If a single server supports multiple organizations that do not trust
   one another, then it MUST check the values of Location and Content-Location
   headers in responses that are generated under control of
   said organizations to make sure that they do not attempt to
   invalidate resources over which they have no authority.
</t>
</section>

</section>

<section title="Acknowledgments" anchor="ack">
</section>
</middle>
<back>

<references title="Normative References">

<reference anchor="Part1">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Day Software">Day Software</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>One Laptop per Child</organization>
      <address><email>jg@laptop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="June" year="2008"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-03"/>
  
</reference>

<reference anchor="Part3">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 3: Message Payload and Content Negotiation</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Day Software">Day Software</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>One Laptop per Child</organization>
      <address><email>jg@laptop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="June" year="2008"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-03"/>
  
</reference>

<reference anchor="Part4">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 4: Conditional Requests</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Day Software">Day Software</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>One Laptop per Child</organization>
      <address><email>jg@laptop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="June" year="2008"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-03"/>
  
</reference>

<reference anchor="Part5">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 5: Range Requests and Partial Responses</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Day Software">Day Software</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>One Laptop per Child</organization>
      <address><email>jg@laptop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="June" year="2008"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-03"/>
  
</reference>

<reference anchor="Part6">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 6: Caching</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Day Software">Day Software</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>One Laptop per Child</organization>
      <address><email>jg@laptop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="June" year="2008"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p6-cache-03"/>
  
</reference>

<reference anchor="Part7">
  <front>
    <title abbrev="HTTP/1.1">HTTP/1.1, part 7: Authentication</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding" role="editor">
      <organization abbrev="Day Software">Day Software</organization>
      <address><email>fielding@gbiv.com</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>One Laptop per Child</organization>
      <address><email>jg@laptop.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization abbrev="HP">Hewlett-Packard Company</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="Henrik Frystyk Nielsen">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>henrikn@microsoft.com</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="Larry Masinter">
      <organization abbrev="Adobe Systems">Adobe Systems, Incorporated</organization>
      <address><email>LMM@acm.org</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="Paul J. Leach">
      <organization abbrev="Microsoft">Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization abbrev="W3C/MIT">World Wide Web Consortium</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="Y." surname="Lafon" fullname="Yves Lafon" role="editor">
      <organization abbrev="W3C">World Wide Web Consortium</organization>
      <address><email>ylafon@w3.org</email></address>
    </author>
    <author initials="J. F." surname="Reschke" fullname="Julian F. Reschke" role="editor">
      <organization abbrev="greenbytes">greenbytes GmbH</organization>
      <address><email>julian.reschke@greenbytes.de</email></address>
    </author>
    <date month="June" year="2008"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-03"/>
  
</reference>

<reference anchor="RFC2119">
  <front>
    <title>Key words for use in RFCs to Indicate Requirement Levels</title>
    <author initials="S." surname="Bradner" fullname="Scott Bradner">
      <organization>Harvard University</organization>
      <address><email>sob@harvard.edu</email></address>
    </author>
    <date month="March" year="1997"/>
  </front>
  <seriesInfo name="BCP" value="14"/>
  <seriesInfo name="RFC" value="2119"/>
</reference>

</references>

<references title="Informative References">

<reference anchor="Luo1998">
  <front>
    <title>Tunneling TCP based protocols through Web proxy servers</title>
    <author initials="A." surname="Luotonen" fullname="A. Luotonen">
      <organization/>
    </author>
    <date year="1998" month="August"/>
  </front>
  <seriesInfo name="Internet-Draft" value="draft-luotonen-web-proxy-tunneling-01"/>
</reference>

<reference anchor="RFC1945">
  <front>
    <title abbrev="HTTP/1.0">Hypertext Transfer Protocol -- HTTP/1.0</title>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization>MIT, Laboratory for Computer Science</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <author initials="R.T." surname="Fielding" fullname="Roy T. Fielding">
      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="H.F." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
      <organization>W3 Consortium, MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <date month="May" year="1996"/>
  </front>
  <seriesInfo name="RFC" value="1945"/>
</reference>

<reference anchor="RFC2068">
  <front>
    <title abbrev="HTTP/1.1">Hypertext Transfer Protocol -- HTTP/1.1</title>
    <author initials="R." surname="Fielding" fullname="Roy T. Fielding">
      <organization>University of California, Irvine, Department of Information and Computer Science</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="Jim Gettys">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>jg@w3.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="Jeffrey C. Mogul">
      <organization>Digital Equipment Corporation, Western Research Laboratory</organization>
      <address><email>mogul@wrl.dec.com</email></address>
    </author>
    <author initials="H." surname="Nielsen" fullname="Henrik Frystyk Nielsen">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="Tim Berners-Lee">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <date month="January" year="1997"/>
  </front>
  <seriesInfo name="RFC" value="2068"/>
</reference>

<reference anchor="RFC2616">
  <front>
    <title>Hypertext Transfer Protocol -- HTTP/1.1</title>
    <author initials="R." surname="Fielding" fullname="R. Fielding">
      <organization>University of California, Irvine</organization>
      <address><email>fielding@ics.uci.edu</email></address>
    </author>
    <author initials="J." surname="Gettys" fullname="J. Gettys">
      <organization>W3C</organization>
      <address><email>jg@w3.org</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="J. Mogul">
      <organization>Compaq Computer Corporation</organization>
      <address><email>mogul@wrl.dec.com</email></address>
    </author>
    <author initials="H." surname="Frystyk" fullname="H. Frystyk">
      <organization>MIT Laboratory for Computer Science</organization>
      <address><email>frystyk@w3.org</email></address>
    </author>
    <author initials="L." surname="Masinter" fullname="L. Masinter">
      <organization>Xerox Corporation</organization>
      <address><email>masinter@parc.xerox.com</email></address>
    </author>
    <author initials="P." surname="Leach" fullname="P. Leach">
      <organization>Microsoft Corporation</organization>
      <address><email>paulle@microsoft.com</email></address>
    </author>
    <author initials="T." surname="Berners-Lee" fullname="T. Berners-Lee">
      <organization>W3C</organization>
      <address><email>timbl@w3.org</email></address>
    </author>
    <date month="June" year="1999"/>
  </front>
  <seriesInfo name="RFC" value="2616"/>
</reference>

<reference anchor="RFC2817">
  <front>
    <title>Upgrading to TLS Within HTTP/1.1</title>
    <author initials="R." surname="Khare" fullname="R. Khare">
      <organization>4K Associates / UC Irvine</organization>
      <address><email>rohit@4K-associates.com</email></address>
    </author>
    <author initials="S." surname="Lawrence" fullname="S. Lawrence">
      <organization>Agranat Systems, Inc.</organization>
      <address><email>lawrence@agranat.com</email></address>
    </author>
    <date year="2000" month="May"/>
  </front>
  <seriesInfo name="RFC" value="2817"/>
</reference>

<reference anchor="RFC2822">
  <front>
    <title>Internet Message Format</title>
    <author initials="P." surname="Resnick" fullname="P. Resnick">
      <organization>QUALCOMM Incorporated</organization>
    </author>
    <date year="2001" month="April"/>
  </front> 
  <seriesInfo name="RFC" value="2822"/>
</reference>

<reference anchor="RFC3864">
  <front>
    <title>Registration Procedures for Message Header Fields</title>
    <author initials="G." surname="Klyne" fullname="G. Klyne">
      <organization>Nine by Nine</organization>
      <address><email>GK-IETF@ninebynine.org</email></address>
    </author>
    <author initials="M." surname="Nottingham" fullname="M. Nottingham">
      <organization>BEA Systems</organization>
      <address><email>mnot@pobox.com</email></address>
    </author>
    <author initials="J." surname="Mogul" fullname="J. Mogul">
      <organization>HP Labs</organization>
      <address><email>JeffMogul@acm.org</email></address>
    </author>
    <date year="2004" month="September"/>
  </front>
  <seriesInfo name="BCP" value="90"/>
  <seriesInfo name="RFC" value="3864"/>
</reference>

<reference anchor="RFC5226">
  <front>
    <title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
    <author initials="T." surname="Narten" fullname="T. Narten">
      <organization>IBM</organization>
      <address><email>narten@us.ibm.com</email></address>
    </author>
    <author initials="H." surname="Alvestrand" fullname="H. Alvestrand">
      <organization>Google</organization>
      <address><email>Harald@Alvestrand.no</email></address>
    </author>
    <date year="2008" month="May"/>
  </front>
  <seriesInfo name="BCP" value="26"/>
  <seriesInfo name="RFC" value="5226"/>
</reference>

</references>

<section title="Compatibility with Previous Versions" anchor="compatibility">
<section title="Changes from RFC 2068" anchor="changes.from.rfc.2068">
<t>
   Clarified which error code should be used for inbound server failures
   (e.g. DNS failures). (<xref target="status.504"/>).
</t>
<t>
   201 (Created) had a race that required an Etag be sent when a resource is
   first created. (<xref target="status.201"/>).
</t>
<t>
   Rewrite of message transmission requirements to make it much harder
   for implementors to get it wrong, as the consequences of errors here
   can have significant impact on the Internet, and to deal with the
   following problems:
  <list style="numbers">
      <t>Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
         this was incorrectly placing a requirement on the behavior of
         an implementation of a future version of HTTP/1.x</t>

      <t>Made it clear that user-agents should retry requests, not
         "clients" in general.</t>

      <t>Converted requirements for clients to ignore unexpected 100
         (Continue) responses, and for proxies to forward 100 responses,
         into a general requirement for 1xx responses.</t>

      <t>Modified some TCP-specific language, to make it clearer that
         non-TCP transports are possible for HTTP.</t>

      <t>Require that the origin server MUST NOT wait for the request
         body before it sends a required 100 (Continue) response.</t>

      <t>Allow, rather than require, a server to omit 100 (Continue) if
         it has already seen some of the request body.</t>

      <t>Allow servers to defend against denial-of-service attacks and
         broken clients.</t>
  </list>
</t>
<t>
   This change adds the Expect header and 417 status code.
</t>
<t>
   Clean up confusion between 403 and 404 responses. (Section <xref target="status.403" format="counter"/>,
   <xref target="status.404" format="counter"/>, and <xref target="status.410" format="counter"/>)
</t>
<t>
   The PATCH<iref item="PATCH method" primary="true"/><iref item="Methods" subitem="PATCH" primary="true"/>, LINK<iref item="LINK method" primary="true"/><iref item="Methods" subitem="LINK" primary="true"/>, UNLINK<iref item="UNLINK method" primary="true"/><iref item="Methods" subitem="UNLINK" primary="true"/> methods were defined but not commonly
   implemented in previous versions of this specification. See Section 19.6.1 of <xref target="RFC2068"/>.
</t>
</section>

<section title="Changes from RFC 2616" anchor="changes.from.rfc.2616">
<t>
  This document takes over the Status Code Registry, previously defined
  in Section 7.1 of <xref target="RFC2817"/>.
  (<xref target="status.code.registry"/>)
</t>
<t>
  Clarify definition of POST.
  (<xref target="POST"/>)
</t>
<t>
  Failed to consider that there are
  many other request methods that are safe to automatically redirect,
  and further that the user agent is able to make that determination
  based on the request method semantics.
  (Sections <xref format="counter" target="status.301"/>,
  <xref format="counter" target="status.302"/> and
  <xref format="counter" target="status.307"/>)
</t>
<t>
  Deprecate 305 Use Proxy status code, because user agents did not implement it.
  It used to indicate that the requested resource must be accessed through the
  proxy given by the Location field. The Location field gave the URI of the
  proxy. The recipient was expected to repeat this single request via the proxy.
  (<xref target="status.305"/>)
</t>
<t>
  Reclassify Allow header as response header, removing the option to
  specify it in a PUT request.
  Relax the server requirement on the contents of the Allow header and
  remove requirement on clients to always trust the header value.
  (<xref target="header.allow"/>)
</t>
<t>
  Correct syntax of Location header to allow fragment,
  as referred symbol wasn't what was expected, and add some
  clarifications as to when it would not be appropriate.
  (<xref target="header.location"/>)
</t>
<t>
  In the description of the Server header, the Via field
  was described as a SHOULD. The requirement was and is stated
  correctly in the description of the Via header in Section 8.9 of <xref target="Part1"/>.
  (<xref target="header.server"/>)
</t>
</section>

</section>

<section title="Change Log (to be removed by RFC Editor before publication)" anchor="change.log">

<section title="Since RFC2616">
<t>
  Extracted relevant partitions from <xref target="RFC2616"/>.
</t>
</section>

<section title="Since draft-ietf-httpbis-p2-semantics-00">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/5"/>:
      "Via is a MUST"
      (<eref target="http://purl.org/NET/http-errata#via-must"/>)
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/6"/>:
      "Fragments allowed in Location"
      (<eref target="http://purl.org/NET/http-errata#location-fragments"/>)
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/10"/>:
      "Safe Methods vs Redirection"
      (<eref target="http://purl.org/NET/http-errata#saferedirect"/>)
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/17"/>:
      "Revise description of the POST method"
      (<eref target="http://purl.org/NET/http-errata#post"/>)
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35"/>:
      "Normative and Informative references"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/42"/>:
      "RFC2606 Compliance"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/65"/>:
      "Informative references"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/84"/>:
      "Redundant cross-references"
    </t>
  </list>
</t>
<t>
  Other changes:
  <list style="symbols"> 
    <t>
      Move definitions of 304 and 412 condition codes to <xref target="Part4"/>
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p2-semantics-01">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/21"/>:
      "PUT side effects"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/91"/>:
      "Duplicate Host header requirements"
    </t>
  </list>
</t>
<t>
  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Move "Product Tokens" section (back) into Part 1, as "token" is used
      in the definition of the Upgrade header.
    </t>
    <t>
      Add explicit references to BNF syntax and rules imported from other parts of the specification.
    </t>
    <t>
      Copy definition of delta-seconds from Part6 instead of referencing it.
    </t>
  </list>
</t>
</section>

<section title="Since draft-ietf-httpbis-p2-semantics-02" anchor="changes.since.02">
<t>
  Closed issues:
  <list style="symbols"> 
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/24"/>:
      "Requiring Allow in 405 responses"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/59"/>:
      "Status Code Registry"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/61"/>:
      "Redirection vs. Location"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/70"/>:
      "Cacheability of 303 response"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/76"/>:
      "305 Use Proxy"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/105"/>:
      "Classification for Allow header"
    </t>
    <t>
      <eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/112"/>:
      "PUT - 'store under' vs 'store at'"
    </t>
  </list>
</t>
<t>
  Ongoing work on IANA Message Header Registration (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/40"/>):
  <list style="symbols"> 
    <t>
      Reference RFC 3984, and update header registrations for headers defined
      in this document.
    </t>
  </list>
</t>
<t>
  Ongoing work on ABNF conversion (<eref target="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36"/>):
  <list style="symbols"> 
    <t>
      Replace string literals when the string really is case-sensitive (method).
    </t>
  </list>
</t>
</section>

</section>

</back>
</rfc>