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HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2616 (if approved)                                  J. Gettys
Updates: 2817 (if approved)                               Alcatel-Lucent
Intended status: Standards Track                                J. Mogul
Expires: February 5, 2011                                             HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                           Adobe Systems
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                           Y. Lafon, Ed.
                                                                     W3C
                                                         J. Reschke, Ed.
                                                              greenbytes
                                                          August 4, 2010


                  HTTP/1.1, part 2: Message Semantics
                   draft-ietf-httpbis-p2-semantics-11

Abstract

   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.

Editorial Note (To be removed by RFC Editor)

   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 <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
   documents (including fancy diffs) can be found at
   <http://tools.ietf.org/wg/httpbis/>.

   The changes in this draft are summarized in Appendix C.12.

Status of This Memo




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   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 5, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  6
       1.2.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  7



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       1.2.2.  ABNF Rules defined in other Parts of the
               Specification  . . . . . . . . . . . . . . . . . . . .  7
   2.  Method . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.1.  Method Registry  . . . . . . . . . . . . . . . . . . . . .  8
   3.  Request Header Fields  . . . . . . . . . . . . . . . . . . . .  9
   4.  Status Code and Reason Phrase  . . . . . . . . . . . . . . . . 10
     4.1.  Status Code Registry . . . . . . . . . . . . . . . . . . . 12
   5.  Response Header Fields . . . . . . . . . . . . . . . . . . . . 12
   6.  Representation . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Identifying the Resource Associated with a
           Representation . . . . . . . . . . . . . . . . . . . . . . 13
   7.  Method Definitions . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Safe and Idempotent Methods  . . . . . . . . . . . . . . . 14
       7.1.1.  Safe Methods . . . . . . . . . . . . . . . . . . . . . 14
       7.1.2.  Idempotent Methods . . . . . . . . . . . . . . . . . . 14
     7.2.  OPTIONS  . . . . . . . . . . . . . . . . . . . . . . . . . 15
     7.3.  GET  . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.4.  HEAD . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.5.  POST . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.6.  PUT  . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     7.7.  DELETE . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.8.  TRACE  . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.9.  CONNECT  . . . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  Status Code Definitions  . . . . . . . . . . . . . . . . . . . 20
     8.1.  Informational 1xx  . . . . . . . . . . . . . . . . . . . . 20
       8.1.1.  100 Continue . . . . . . . . . . . . . . . . . . . . . 20
       8.1.2.  101 Switching Protocols  . . . . . . . . . . . . . . . 21
     8.2.  Successful 2xx . . . . . . . . . . . . . . . . . . . . . . 21
       8.2.1.  200 OK . . . . . . . . . . . . . . . . . . . . . . . . 21
       8.2.2.  201 Created  . . . . . . . . . . . . . . . . . . . . . 21
       8.2.3.  202 Accepted . . . . . . . . . . . . . . . . . . . . . 22
       8.2.4.  203 Non-Authoritative Information  . . . . . . . . . . 22
       8.2.5.  204 No Content . . . . . . . . . . . . . . . . . . . . 22
       8.2.6.  205 Reset Content  . . . . . . . . . . . . . . . . . . 23
       8.2.7.  206 Partial Content  . . . . . . . . . . . . . . . . . 23
     8.3.  Redirection 3xx  . . . . . . . . . . . . . . . . . . . . . 23
       8.3.1.  300 Multiple Choices . . . . . . . . . . . . . . . . . 24
       8.3.2.  301 Moved Permanently  . . . . . . . . . . . . . . . . 24
       8.3.3.  302 Found  . . . . . . . . . . . . . . . . . . . . . . 25
       8.3.4.  303 See Other  . . . . . . . . . . . . . . . . . . . . 25
       8.3.5.  304 Not Modified . . . . . . . . . . . . . . . . . . . 26
       8.3.6.  305 Use Proxy  . . . . . . . . . . . . . . . . . . . . 26
       8.3.7.  306 (Unused) . . . . . . . . . . . . . . . . . . . . . 26
       8.3.8.  307 Temporary Redirect . . . . . . . . . . . . . . . . 26
     8.4.  Client Error 4xx . . . . . . . . . . . . . . . . . . . . . 27
       8.4.1.  400 Bad Request  . . . . . . . . . . . . . . . . . . . 27
       8.4.2.  401 Unauthorized . . . . . . . . . . . . . . . . . . . 27
       8.4.3.  402 Payment Required . . . . . . . . . . . . . . . . . 27



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       8.4.4.  403 Forbidden  . . . . . . . . . . . . . . . . . . . . 27
       8.4.5.  404 Not Found  . . . . . . . . . . . . . . . . . . . . 28
       8.4.6.  405 Method Not Allowed . . . . . . . . . . . . . . . . 28
       8.4.7.  406 Not Acceptable . . . . . . . . . . . . . . . . . . 28
       8.4.8.  407 Proxy Authentication Required  . . . . . . . . . . 28
       8.4.9.  408 Request Timeout  . . . . . . . . . . . . . . . . . 29
       8.4.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . . 29
       8.4.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . . 29
       8.4.12. 411 Length Required  . . . . . . . . . . . . . . . . . 30
       8.4.13. 412 Precondition Failed  . . . . . . . . . . . . . . . 30
       8.4.14. 413 Request Entity Too Large . . . . . . . . . . . . . 30
       8.4.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . . 30
       8.4.16. 415 Unsupported Media Type . . . . . . . . . . . . . . 30
       8.4.17. 416 Requested Range Not Satisfiable  . . . . . . . . . 30
       8.4.18. 417 Expectation Failed . . . . . . . . . . . . . . . . 31
     8.5.  Server Error 5xx . . . . . . . . . . . . . . . . . . . . . 31
       8.5.1.  500 Internal Server Error  . . . . . . . . . . . . . . 31
       8.5.2.  501 Not Implemented  . . . . . . . . . . . . . . . . . 31
       8.5.3.  502 Bad Gateway  . . . . . . . . . . . . . . . . . . . 31
       8.5.4.  503 Service Unavailable  . . . . . . . . . . . . . . . 31
       8.5.5.  504 Gateway Timeout  . . . . . . . . . . . . . . . . . 32
       8.5.6.  505 HTTP Version Not Supported . . . . . . . . . . . . 32
   9.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 32
     9.1.  Allow  . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     9.2.  Expect . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     9.3.  From . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     9.4.  Location . . . . . . . . . . . . . . . . . . . . . . . . . 34
     9.5.  Max-Forwards . . . . . . . . . . . . . . . . . . . . . . . 35
     9.6.  Referer  . . . . . . . . . . . . . . . . . . . . . . . . . 36
     9.7.  Retry-After  . . . . . . . . . . . . . . . . . . . . . . . 36
     9.8.  Server . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     9.9.  User-Agent . . . . . . . . . . . . . . . . . . . . . . . . 37
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
     10.1. Method Registry  . . . . . . . . . . . . . . . . . . . . . 38
     10.2. Status Code Registry . . . . . . . . . . . . . . . . . . . 38
     10.3. Header Field Registration  . . . . . . . . . . . . . . . . 39
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 40
     11.1. Transfer of Sensitive Information  . . . . . . . . . . . . 40
     11.2. Encoding Sensitive Information in URIs . . . . . . . . . . 41
     11.3. Location Headers and Spoofing  . . . . . . . . . . . . . . 42
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 42
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 42
     13.2. Informative References . . . . . . . . . . . . . . . . . . 43
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 43
   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 44
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 47



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     C.1.  Since RFC2616  . . . . . . . . . . . . . . . . . . . . . . 47
     C.2.  Since draft-ietf-httpbis-p2-semantics-00 . . . . . . . . . 47
     C.3.  Since draft-ietf-httpbis-p2-semantics-01 . . . . . . . . . 47
     C.4.  Since draft-ietf-httpbis-p2-semantics-02 . . . . . . . . . 48
     C.5.  Since draft-ietf-httpbis-p2-semantics-03 . . . . . . . . . 49
     C.6.  Since draft-ietf-httpbis-p2-semantics-04 . . . . . . . . . 49
     C.7.  Since draft-ietf-httpbis-p2-semantics-05 . . . . . . . . . 49
     C.8.  Since draft-ietf-httpbis-p2-semantics-06 . . . . . . . . . 50
     C.9.  Since draft-ietf-httpbis-p2-semantics-07 . . . . . . . . . 50
     C.10. Since draft-ietf-httpbis-p2-semantics-08 . . . . . . . . . 51
     C.11. Since draft-ietf-httpbis-p2-semantics-09 . . . . . . . . . 51
     C.12. Since draft-ietf-httpbis-p2-semantics-10 . . . . . . . . . 51
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52






































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1.  Introduction

   This document defines HTTP/1.1 request and response semantics.  Each
   HTTP message, as defined in [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 to the target resource.

   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 [RFC2616].

1.1.  Requirements

   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 [RFC2119].

   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".

1.2.  Syntax Notation

   This specification uses the ABNF syntax defined in Section 1.2 of
   [Part1] (which extends the syntax defined in [RFC5234] with a list
   rule).  Appendix B shows the collected ABNF, with the list rule
   expanded.

   The following core rules are included by reference, as defined in
   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
   HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit



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   sequence of data), SP (space), VCHAR (any visible USASCII character),
   and WSP (whitespace).

1.2.1.  Core Rules

   The core rules below are defined in Section 1.2.2 of [Part1]:

     quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
     token         = <token, defined in [Part1], Section 1.2.2>
     OWS           = <OWS, defined in [Part1], Section 1.2.2>
     RWS           = <RWS, defined in [Part1], Section 1.2.2>
     obs-text      = <obs-text, defined in [Part1], Section 1.2.2>

1.2.2.  ABNF Rules defined in other Parts of the Specification

   The ABNF rules below are defined in other parts:

     absolute-URI  = <absolute-URI, defined in [Part1], Section 2.6>
     comment       = <comment, defined in [Part1], Section 3.2>
     Host          = <Host, defined in [Part1], Section 2.6>
     HTTP-date     = <HTTP-date, defined in [Part1], Section 6.1>
     partial-URI   = <partial-URI, defined in [Part1], Section 2.6>
     product       = <product, defined in [Part1], Section 6.3>
     TE            = <TE, defined in [Part1], Section 9.5>
     URI-reference = <URI-reference, defined in [Part1], Section 2.6>


     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>


     ETag          = <ETag, defined in [Part4], Section 6.1>
     If-Match      = <If-Match, defined in [Part4], Section 6.2>
     If-Modified-Since =
                <If-Modified-Since, defined in [Part4], Section 6.3>
     If-None-Match = <If-None-Match, defined in [Part4], Section 6.4>
     If-Unmodified-Since =
                <If-Unmodified-Since, defined in [Part4], Section 6.5>


     Accept-Ranges = <Accept-Ranges, defined in [Part5], Section 5.1>
     If-Range      = <If-Range, defined in [Part5], Section 5.3>
     Range         = <Range, defined in [Part5], Section 5.4>



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     Age           = <Age, defined in [Part6], Section 3.1>
     Vary          = <Vary, defined in [Part6], Section 3.5>


     Authorization = <Authorization, defined in [Part7], Section 3.1>
     Proxy-Authenticate =
                <Proxy-Authenticate, defined in [Part7], Section 3.2>
     Proxy-Authorization =
                <Proxy-Authorization, defined in [Part7], Section 3.3>
     WWW-Authenticate =
                <WWW-Authenticate, defined in [Part7], Section 3.4>

2.  Method

   The Method token indicates the method to be performed on the target
   resource (Section 4.3 of [Part1]).  The method is case-sensitive.

     Method         = %x4F.50.54.49.4F.4E.53   ; "OPTIONS", Section 7.2
                    / %x47.45.54               ; "GET", Section 7.3
                    / %x48.45.41.44            ; "HEAD", Section 7.4
                    / %x50.4F.53.54            ; "POST", Section 7.5
                    / %x50.55.54               ; "PUT", Section 7.6
                    / %x44.45.4C.45.54.45      ; "DELETE", Section 7.7
                    / %x54.52.41.43.45         ; "TRACE", Section 7.8
                    / %x43.4F.4E.4E.45.43.54   ; "CONNECT", Section 7.9
                    / extension-method
     extension-method = token

   The list of methods allowed by a resource can be specified in an
   Allow header field (Section 9.1).  The status 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 respond with the status code 405 (Method Not
   Allowed) if the method is known by the origin server but not allowed
   for the 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 Section 7.

2.1.  Method Registry

   The HTTP Method Registry defines the name space for the Method token
   in the Request line of an HTTP request.

   Registrations MUST include the following fields:




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   o  Method Name (see Section 2)

   o  Safe ("yes" or "no", see Section 7.1.1)

   o  Pointer to specification text

   Values to be added to this name space are subject to IETF review
   ([RFC5226], Section 4.1).

   The registry itself is maintained at
   <http://www.iana.org/assignments/http-methods>.

3.  Request Header Fields

   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.

     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 3.1
                    / Expect                   ; Section 9.2
                    / From                     ; Section 9.3
                    / Host                     ; [Part1], Section 9.4
                    / If-Match                 ; [Part4], Section 6.2
                    / If-Modified-Since        ; [Part4], Section 6.3
                    / If-None-Match            ; [Part4], Section 6.4
                    / If-Range                 ; [Part5], Section 5.3
                    / If-Unmodified-Since      ; [Part4], Section 6.5
                    / Max-Forwards             ; Section 9.5
                    / Proxy-Authorization      ; [Part7], Section 3.3
                    / Range                    ; [Part5], Section 5.4
                    / Referer                  ; Section 9.6
                    / TE                       ; [Part1], Section 9.5
                    / User-Agent               ; Section 9.9

   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.






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4.  Status Code and Reason Phrase

   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 Section 8, Section 3 of [Part4], Section
   3 of [Part5], and Section 2 of [Part7].

   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.

   The individual values of the numeric status codes defined for
   HTTP/1.1, and an example set of corresponding Reason-Phrase values,
   are presented below.  The reason phrases listed here are only
   recommendations -- they MAY be replaced by local equivalents without
   affecting the protocol.


































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     Status-Code =
          "100"  ; Section 8.1.1: Continue
        / "101"  ; Section 8.1.2: Switching Protocols
        / "200"  ; Section 8.2.1: OK
        / "201"  ; Section 8.2.2: Created
        / "202"  ; Section 8.2.3: Accepted
        / "203"  ; Section 8.2.4: Non-Authoritative Information
        / "204"  ; Section 8.2.5: No Content
        / "205"  ; Section 8.2.6: Reset Content
        / "206"  ; [Part5], Section 3.1: Partial Content
        / "300"  ; Section 8.3.1: Multiple Choices
        / "301"  ; Section 8.3.2: Moved Permanently
        / "302"  ; Section 8.3.3: Found
        / "303"  ; Section 8.3.4: See Other
        / "304"  ; [Part4], Section 3.1: Not Modified
        / "305"  ; Section 8.3.6: Use Proxy
        / "307"  ; Section 8.3.8: Temporary Redirect
        / "400"  ; Section 8.4.1: Bad Request
        / "401"  ; [Part7], Section 2.1: Unauthorized
        / "402"  ; Section 8.4.3: Payment Required
        / "403"  ; Section 8.4.4: Forbidden
        / "404"  ; Section 8.4.5: Not Found
        / "405"  ; Section 8.4.6: Method Not Allowed
        / "406"  ; Section 8.4.7: Not Acceptable
        / "407"  ; [Part7], Section 2.2: Proxy Authentication Required
        / "408"  ; Section 8.4.9: Request Time-out
        / "409"  ; Section 8.4.10: Conflict
        / "410"  ; Section 8.4.11: Gone
        / "411"  ; Section 8.4.12: Length Required
        / "412"  ; [Part4], Section 3.2: Precondition Failed
        / "413"  ; Section 8.4.14: Request Entity Too Large
        / "414"  ; Section 8.4.15: URI Too Long
        / "415"  ; Section 8.4.16: Unsupported Media Type
        / "416"  ; [Part5], Section 3.2: Requested range not satisfiable
        / "417"  ; Section 8.4.18: Expectation Failed
        / "500"  ; Section 8.5.1: Internal Server Error
        / "501"  ; Section 8.5.2: Not Implemented
        / "502"  ; Section 8.5.3: Bad Gateway
        / "503"  ; Section 8.5.4: Service Unavailable
        / "504"  ; Section 8.5.5: Gateway Time-out
        / "505"  ; Section 8.5.6: HTTP Version not supported
        / extension-code

     extension-code = 3DIGIT
     Reason-Phrase  = *( WSP / VCHAR / obs-text )

   HTTP status codes are extensible.  HTTP applications are not required
   to understand the meaning of all registered status codes, though such



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   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 representation
   enclosed with the response, since that representation is likely to
   include human-readable information which will explain the unusual
   status.

4.1.  Status Code Registry

   The HTTP Status Code Registry defines the name space for the Status-
   Code token in the Status-Line of an HTTP response.

   Values to be added to this name space are subject to IETF review
   ([RFC5226], Section 4.1).

   The registry itself is maintained at
   <http://www.iana.org/assignments/http-status-codes>.

5.  Response Header Fields

   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 target resource (Section 4.3 of [Part1]).

     response-header = Accept-Ranges           ; [Part5], Section 5.1
                     / Age                     ; [Part6], Section 3.1
                     / Allow                   ; Section 9.1
                     / ETag                    ; [Part4], Section 6.1
                     / Location                ; Section 9.4
                     / Proxy-Authenticate      ; [Part7], Section 3.2
                     / Retry-After             ; Section 9.7
                     / Server                  ; Section 9.8
                     / Vary                    ; [Part6], Section 3.5
                     / WWW-Authenticate        ; [Part7], Section 3.4

   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.




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6.  Representation

   Request and Response messages MAY transfer a representation if not
   otherwise restricted by the request method or response status code.
   A representation consists of metadata (representation header fields)
   and data (representation body).  When a complete or partial
   representation is enclosed in an HTTP message, it is referred to as
   the payload of the message.  HTTP representations are defined in
   [Part3].

   A representation body is only present in a message when a message-
   body is present, as described in Section 3.3 of [Part1].  The
   representation 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.

6.1.  Identifying the Resource Associated with a Representation

   It is sometimes necessary to determine an identifier for the resource
   associated with a representation.

   An HTTP request representation, when present, is always associated
   with an anonymous (i.e., unidentified) resource.

   In the common case, an HTTP response is a representation of the
   target resource (see Section 4.3 of [Part1]).  However, this is not
   always the case.  To determine the URI of the resource a response is
   associated with, the following rules are used (with the first
   applicable one being selected):

   1.  If the response status code is 200 or 203 and the request method
       was GET, the response payload is a representation of the target
       resource.

   2.  If the response status code is 204, 206, or 304 and the request
       method was GET or HEAD, the response payload is a partial
       representation of the target (see Section 2.8 of [Part6]).

   3.  If the response has a Content-Location header, and that URI is
       the same as the effective request URI, the response payload is a
       representation of the target resource.

   4.  If the response has a Content-Location header, and that URI is
       not the same as the effective request URI, then the response
       asserts that its payload is a representation of the resource
       identified by the Content-Location URI.  However, such an
       assertion cannot be trusted unless it can be verified by other
       means (not defined by HTTP).



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   5.  Otherwise, the response is a representation of an anonymous
       (i.e., unidentified) resource.

   [[TODO-req-uri: The comparison function is going to have to be
   defined somewhere, because we already need to compare URIs for things
   like cache invalidation.]]

7.  Method Definitions

   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.

7.1.  Safe and Idempotent Methods

7.1.1.  Safe Methods

   Implementors need to be aware that the software represents the user
   in their interactions over the Internet, and need to allow the user
   to be aware of any actions they take which might have an unexpected
   significance to themselves or others.

   In particular, the convention has been established that the GET,
   HEAD, OPTIONS, and TRACE 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.

   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.

7.1.2.  Idempotent Methods

   Methods can also have the property of "idempotence" in that, aside
   from error or expiration issues, the intended effect of multiple
   identical requests is the same as for a single request.  The methods
   PUT, DELETE, and all safe methods are idempotent.  It is important to
   note that idempotence refers only to changes requested by the client:
   a server is free to change its state due to multiple requests for the
   purpose of tracking those requests, versioning of results, etc.






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7.2.  OPTIONS

   The OPTIONS method represents a request for information about the
   communication options available on the request/response chain
   identified by the effective 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.

   Responses to this method are not cacheable.

   If the OPTIONS request includes a message-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.

   If the request-target 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).

   If the request-target is not an asterisk, the OPTIONS request applies
   only to the options that are available when communicating with that
   resource.

   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".

   The Max-Forwards request-header field MAY be used to target a
   specific proxy in the request chain (see Section 9.5).  If no Max-
   Forwards field is present in the request, then the forwarded request
   MUST NOT include a Max-Forwards field.






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7.3.  GET

   The GET method means retrieve whatever information (in the form of a
   representation) currently corresponds to the target resource.

   If the target resource is a data-producing process, it is the
   produced data which shall be returned as the representation in the
   response and not the source text of the process, unless that text
   happens to be the output of the process.

   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 representation 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 representations to be refreshed without
   requiring multiple requests or transferring data already held by the
   client.

   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 representation be transferred, as
   described in Section 5.4 of [Part5].  The partial GET method is
   intended to reduce unnecessary network usage by allowing partially-
   retrieved representations to be completed without transferring data
   already held by the client.

   The response to a GET request is cacheable and MAY be used to satisfy
   subsequent GET and HEAD requests (see [Part6]).

   See Section 11.2 for security considerations when used for forms.

7.4.  HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   return a message-body in the response.  The metadata 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 metadata about the representation implied by the
   request without transferring the representation body.  This method is
   often used for testing hypertext links for validity, accessibility,
   and recent modification.

   The response to a HEAD request is cacheable and MAY be used to
   satisfy a subsequent HEAD request; see [Part6].  It also MAY be used
   to update a previously cached representation from that resource; if
   the new field values indicate that the cached representation differs



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   from the current representation (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.

7.5.  POST

   The POST method is used to request that the origin server accept the
   representation enclosed in the request as data to be processed by the
   target resource.  POST is designed to allow a uniform method to cover
   the following functions:

   o  Annotation of existing resources;

   o  Posting a message to a bulletin board, newsgroup, mailing list, or
      similar group of articles;

   o  Providing a block of data, such as the result of submitting a
      form, to a data-handling process;

   o  Extending a database through an append operation.

   The actual function performed by the POST method is determined by the
   server and is usually dependent on the effective request URI.

   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 code,
   depending on whether or not the response includes a representation
   that describes the result.

   If a resource has been created on the origin server, the response
   SHOULD be 201 (Created) and contain a representation which describes
   the status of the request and refers to the new resource, and a
   Location header (see Section 9.4).

   Responses to POST requests are only cacheable when they include
   explicit freshness information (see Section 2.3.1 of [Part6]).  A
   cached POST response with a Content-Location header (see Section 6.7
   of [Part3]) whose value is the effective Request URI MAY be used to
   satisfy subsequent GET and HEAD requests.

   Note that POST caching is not widely implemented.  However, the 303
   (See Other) response can be used to direct the user agent to retrieve
   a cacheable resource.







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7.6.  PUT

   The PUT method requests that the enclosed representation be stored at
   the effective request URI.  If the effective request URI refers to an
   already existing resource, the enclosed representation SHOULD be
   considered a modified version of the one residing on the origin
   server.  Otherwise, if the effective 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 effective 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 target resource could not be created or modified, an
   appropriate error response SHOULD be given that reflects the nature
   of the problem.  The recipient of the representation MUST NOT ignore
   any Content-* headers (headers starting with the prefix "Content-")
   that it does not understand or implement and MUST return a 501 (Not
   Implemented) response in such cases.

   If the request passes through a cache that has one or more stored
   responses for the effective request URI, those stored responses
   SHOULD be marked as stale if the response to the PUT request has a
   success status code.  Responses to the PUT method are not cacheable.

   The fundamental difference between the POST and PUT requests is
   reflected in the different meaning of the effective request URI.  The
   URI in a POST request identifies the resource that will handle the
   enclosed representation.  That resource might be a data-accepting
   process, a gateway to some other protocol, or a document that accepts
   annotations.  In contrast, the URI in a PUT request identifies the
   resource for which enclosed representation is a new or replacement
   value; 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.

   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.



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   HTTP/1.1 does not define how a PUT method affects the state of an
   origin server.

   Header fields in a PUT request that are recognized as representation
   metadata SHOULD be applied to the resource created or modified by the
   PUT.  Unrecognized header fields SHOULD be ignored.

7.7.  DELETE

   The DELETE method requests that the origin server delete the target
   resource.  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.

   A successful response SHOULD be 200 (OK) if the response includes an
   representation 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 a representation.

   If the request passes through a cache and the effective request URI
   identifies one or more currently cached representations, those
   entries SHOULD be treated as stale.  Responses to the DELETE method
   are not cacheable.

7.8.  TRACE

   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
   message-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 Section 9.5).  A TRACE
   request MUST NOT include a message-body.

   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 9.9 of
   [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.

   If the request is valid, the response SHOULD have a Content-Type of
   "message/http" (see Section 10.3.1 of [Part1]) and contain a message-



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   body that encloses a copy of the entire request message.  Responses
   to the TRACE method are not cacheable.

7.9.  CONNECT

   This specification reserves the method name CONNECT for use with a
   proxy that can dynamically switch to being a tunnel (e.g., SSL
   tunneling [RFC2817]).

8.  Status Code Definitions

   Each Status-Code is described below, including any metadata required
   in the response.

8.1.  Informational 1xx

   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.

   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.

   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).)

8.1.1.  100 Continue

   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 [Part1] for detailed discussion of
   the use and handling of this status code.






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8.1.2.  101 Switching Protocols

   The server understands and is willing to comply with the client's
   request, via the Upgrade message header field (Section 9.8 of
   [Part1]), 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.

   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.

8.2.  Successful 2xx

   This class of status code indicates that the client's request was
   successfully received, understood, and accepted.

8.2.1.  200 OK

   The request has succeeded.  The payload returned with the response is
   dependent on the method used in the request, for example:

   GET  a representation of the target resource is sent in the response;

   HEAD  the same representation as GET, except without the message-
      body;

   POST  a representation describing or containing the result of the
      action;

   TRACE  a representation containing the request message as received by
      the end server.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 200 responses.

8.2.2.  201 Created

   The request has been fulfilled and has resulted in a new resource
   being created.  The newly created resource can be referenced by the
   URI(s) returned in the payload of the response, with the most
   specific URI for the resource given by a Location header field.  The
   response SHOULD include a payload containing a list of resource
   characteristics and location(s) from which the user or user agent can
   choose the one most appropriate.  The payload format is specified by



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   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.

   A 201 response MAY contain an ETag response header field indicating
   the current value of the entity-tag for the representation of the
   resource just created (see Section 6.1 of [Part4]).

8.2.3.  202 Accepted

   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.

   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 representation 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.

8.2.4.  203 Non-Authoritative Information

   The returned metadata in the header fields 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
   metadata 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).

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 203 responses.

8.2.5.  204 No Content

   The server has successfully fulfilled the request, but there is no
   additional content to return in the response payload body.  The
   resource metadata and representation metadata in the response
   message's header fields refer to the target resource and its current
   representation, respectively, after the requested action.  For
   example, if a 204 status code is received in response to a PUT and



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   the response contains an ETag header field, then the value of that
   field is the current entity-tag for the representation that was
   successfully PUT.

   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 metadata SHOULD be applied to the document
   currently in the user agent's active view.

   The 204 response MUST NOT include a message-body, and thus is always
   terminated by the first empty line after the header fields.

8.2.6.  205 Reset Content

   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 a message-body.

8.2.7.  206 Partial Content

   The server has fulfilled the partial GET request for the resource and
   the enclosed payload is a partial representation as defined in
   Section 3.1 of [Part5].

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 206 responses.

8.3.  Redirection 3xx

   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
   known to be "safe", as defined in Section 7.1.1.  A client SHOULD
   detect infinite redirection loops, since such loops generate network
   traffic for each redirection.

      Note: An earlier version of this specification recommended a
      maximum of five redirections ([RFC2068], Section 10.3).  Content
      developers need to be aware that some clients might implement such
      a fixed limitation.





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8.3.1.  300 Multiple Choices

   The target resource more than one representation, each with its own
   specific location, and agent-driven negotiation information (Section
   5 of [Part3]) is being provided so that the user (or user agent) can
   select a preferred representation by redirecting its request to that
   location.

   Unless it was a HEAD request, the response SHOULD include a
   representation containing a list of representation metadata and
   location(s) from which the user or user agent can choose the one most
   appropriate.  The data 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.

   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.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 300 responses.

8.3.2.  301 Moved Permanently

   The target 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 effective request URI to one or more of the
   new references returned by the server, where possible.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 301 responses.

   The new permanent URI SHOULD be given by the Location field in the
   response.  Unless the request method was HEAD, the representation of
   the response SHOULD contain a short hypertext note with a hyperlink
   to the new URI(s).

   If the 301 status code is received in response to a request method
   that is known to be "safe", as defined in Section 7.1.1, 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.




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      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.

8.3.3.  302 Found

   The target resource resides temporarily under a different URI.  Since
   the redirection might be altered on occasion, the client SHOULD
   continue to use the effective request URI for future requests.

   The temporary URI SHOULD be given by the Location field in the
   response.  Unless the request method was HEAD, the representation of
   the response SHOULD contain a short hypertext note with a hyperlink
   to the new URI(s).

   If the 302 status code is received in response to a request method
   that is known to be "safe", as defined in Section 7.1.1, 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.

      Note: HTTP/1.0 ([RFC1945], Section 9.3) and the first version of
      HTTP/1.1 ([RFC2068], Section 10.3.3) 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.  Therefore, a
      previous version of this specification ([RFC2616], Section 10.3.3)
      has added the status codes 303 and 307 for servers that wish to
      make unambiguously clear which kind of reaction is expected of the
      client.

8.3.4.  303 See Other

   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 effective request URI.

   The 303 status code 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



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   original request.

   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 target resource, such that the follow-on
   representation might be useful to recipients without implying that it
   adequately represents the target 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 URI owner(s).

   Except for responses to a HEAD request, the representation of a 303
   response SHOULD contain a short hypertext note with a hyperlink to
   the Location URI.

8.3.5.  304 Not Modified

   The response to the request has not been modified since the
   conditions indicated by the client's conditional GET request, as
   defined in Section 3.1 of [Part4].

8.3.6.  305 Use Proxy

   The 305 status code was defined in a previous version of this
   specification (see Appendix A), and is now deprecated.

8.3.7.  306 (Unused)

   The 306 status code was used in a previous version of the
   specification, is no longer used, and the code is reserved.

8.3.8.  307 Temporary Redirect

   The target resource resides temporarily under a different URI.  Since
   the redirection can change over time, the client SHOULD continue to
   use the effective request URI for future requests.

   The temporary URI SHOULD be given by the Location field in the
   response.  Unless the request method was HEAD, the representation 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 code.  Therefore, the note SHOULD contain
   the information necessary for a user to repeat the original request
   on the new URI.

   If the 307 status code is received in response to a request method
   that is known to be "safe", as defined in Section 7.1.1, then the



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   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.

8.4.  Client Error 4xx

   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 a representation 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
   representation to the user.

   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 might erase the client's unacknowledged input
   buffers before they can be read and interpreted by the HTTP
   application.

8.4.1.  400 Bad Request

   The request could not be understood by the server due to malformed
   syntax.  The client SHOULD NOT repeat the request without
   modifications.

8.4.2.  401 Unauthorized

   The request requires user authentication (see Section 2.1 of
   [Part7]).

8.4.3.  402 Payment Required

   This code is reserved for future use.

8.4.4.  403 Forbidden

   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 representation.  If the server does not
   wish to make this information available to the client, the status
   code 404 (Not Found) can be used instead.



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8.4.5.  404 Not Found

   The server has not found anything matching the effective 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.

8.4.6.  405 Method Not Allowed

   The method specified in the Request-Line is not allowed for the
   target resource.  The response MUST include an Allow header
   containing a list of valid methods for the requested resource.

8.4.7.  406 Not Acceptable

   The resource identified by the request is only capable of generating
   response representations which have content characteristics not
   acceptable according to the accept headers sent in the request.

   Unless it was a HEAD request, the response SHOULD include a
   representation containing a list of available representation
   characteristics and location(s) from which the user or user agent can
   choose the one most appropriate.  The data 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.

      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 might 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.

   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.

8.4.8.  407 Proxy Authentication Required

   This code is similar to 401 (Unauthorized), but indicates that the
   client must first authenticate itself with the proxy (see Section 2.2
   of [Part7]).



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8.4.9.  408 Request Timeout

   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.

8.4.10.  409 Conflict

   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 representation would include enough information
   for the user or user agent to fix the problem; however, that might
   not be possible and is not required.

   Conflicts are most likely to occur in response to a PUT request.  For
   example, if versioning were being used and the representation 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 representation would likely contain a list of the
   differences between the two versions in a format defined by the
   response Content-Type.

8.4.11.  410 Gone

   The target 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 effective 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.

   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.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 410 responses.



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8.4.12.  411 Length Required

   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.

8.4.13.  412 Precondition Failed

   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
   Section 3.2 of [Part4].

8.4.14.  413 Request Entity Too Large

   The server is refusing to process a request because the request
   representation 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.

   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.

8.4.15.  414 URI Too Long

   The server is refusing to service the request because the effective
   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 effective request URI.

8.4.16.  415 Unsupported Media Type

   The server is refusing to service the request because the
   representation of the request is in a format not supported by the
   target resource for the requested method.

8.4.17.  416 Requested Range Not Satisfiable

   The request included a Range request-header field (Section 5.4 of
   [Part5]) and none of the range-specifier values in this field overlap
   the current extent of the selected resource.  See Section 3.2 of
   [Part5].



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8.4.18.  417 Expectation Failed

   The expectation given in an Expect request-header field (see
   Section 9.2) 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.

8.5.  Server Error 5xx

   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 a representation containing an explanation
   of the error situation, and whether it is a temporary or permanent
   condition.  User agents SHOULD display any included representation to
   the user.  These response codes are applicable to any request method.

8.5.1.  500 Internal Server Error

   The server encountered an unexpected condition which prevented it
   from fulfilling the request.

8.5.2.  501 Not Implemented

   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.

8.5.3.  502 Bad Gateway

   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.

8.5.4.  503 Service Unavailable

   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.

      Note: The existence of the 503 status code does not imply that a
      server must use it when becoming overloaded.  Some servers might
      wish to simply refuse the connection.




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8.5.5.  504 Gateway Timeout

   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.

      Note to implementors: some deployed proxies are known to return
      400 or 500 when DNS lookups time out.

8.5.6.  505 HTTP Version Not Supported

   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
   2.5 of [Part1], other than with this error message.  The response
   SHOULD contain a representation describing why that version is not
   supported and what other protocols are supported by that server.

9.  Header Field Definitions

   This section defines the syntax and semantics of HTTP/1.1 header
   fields related to request and response semantics.

9.1.  Allow

   The "Allow" response-header field lists the set of methods advertised
   as supported by the target resource.  The purpose of this field is
   strictly to inform the recipient of valid methods associated with the
   resource.

     Allow   = "Allow" ":" OWS Allow-v
     Allow-v = #Method

   Example of use:

     Allow: GET, HEAD, PUT

   The actual set of allowed methods is defined by the origin server at
   the time of each request.

   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.






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9.2.  Expect

   The "Expect" request-header field is used to indicate that particular
   server behaviors are required by the client.

     Expect       = "Expect" ":" OWS Expect-v
     Expect-v     = 1#expectation

     expectation  = "100-continue" / expectation-extension
     expectation-extension = token [ "=" ( token / quoted-string )
                              *expect-params ]
     expect-params = ";" token [ "=" ( token / quoted-string ) ]

   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 code.  The server MUST respond with a
   417 (Expectation Failed) status code if any of the expectations
   cannot be met or, if there are other problems with the request, some
   other 4xx status code.

   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 code.

   Comparison of expectation values is case-insensitive for unquoted
   tokens (including the 100-continue token), and is case-sensitive for
   quoted-string expectation-extensions.

   The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
   return a 417 (Expectation Failed) status code 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.

   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
   Expect header.

   See Section 7.2.3 of [Part1] for the use of the 100 (Continue) status
   code.

9.3.  From

   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 [RFC5322]:




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     From    = "From" ":" OWS From-v
     From-v  = mailbox

     mailbox = <mailbox, defined in [RFC5322], Section 3.4>

   An example is:

     From: webmaster@example.org

   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.

   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.

   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.

9.4.  Location

   The "Location" response-header field is used to identify a newly
   created resource, or to redirect the recipient to a different
   location for completion of the request.

   For 201 (Created) responses, the Location is the URI 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 URI-reference.  When it has the
   form of a relative reference ([RFC3986], Section 4.2), the final
   value is computed by resolving it against the effective request URI
   ([RFC3986], Section 5).

     Location       = "Location" ":" OWS Location-v
     Location-v     = URI-reference




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   Examples are:

     Location: http://www.example.org/pub/WWW/People.html#tim

     Location: /index.html

   There are circumstances in which a fragment identifier in a Location
   URI would not be appropriate:

   o  With a 201 Created response, because in this usage the Location
      header specifies the URI for the entire created resource.

   o  With 305 Use Proxy.

      Note: This specification does not define precedence rules for the
      case where the original URI, as navigated to by the user agent,
      and the Location header field value both contain fragment
      identifiers.

      Note: The Content-Location header field (Section 6.7 of [Part3])
      differs from Location in that the Content-Location identifies the
      most specific resource corresponding to the enclosed
      representation.  It is therefore possible for a response to
      contain header fields for both Location and Content-Location.

9.5.  Max-Forwards

   The "Max-Forwards" request-header field provides a mechanism with the
   TRACE (Section 7.8) and OPTIONS (Section 7.2) methods to limit the
   number of times that the request is forwarded by proxies or gateways.
   This can be useful when the client is attempting to trace a request
   which appears to be failing or looping in mid-chain.

     Max-Forwards   = "Max-Forwards" ":" OWS Max-Forwards-v
     Max-Forwards-v = 1*DIGIT

   The Max-Forwards value is a decimal integer indicating the remaining
   number of times this request message can be forwarded.

   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).

   The Max-Forwards header field MAY be ignored for all other methods



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   defined by this specification and for any extension methods for which
   it is not explicitly referred to as part of that method definition.

9.6.  Referer

   The "Referer" [sic] request-header field allows the client to specify
   the URI of the resource from which the effective request URI was
   obtained (the "referrer", although the header field is misspelled.).

   The Referer header allows servers 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.  Some
   servers use Referer as a means of controlling where they allow links
   from (so-called "deep linking"), but legitimate requests do not
   always contain a Referer header field.

   If the effective request URI was obtained from a source that does not
   have its own URI (e.g., input from the user keyboard), the Referer
   field MUST either be sent with the value "about:blank", or not be
   sent at all.  Note that this requirement does not apply to sources
   with non-HTTP URIs (e.g., FTP).

     Referer        = "Referer" ":" OWS Referer-v
     Referer-v      = absolute-URI / partial-URI

   Example:

     Referer: http://www.example.org/hypertext/Overview.html

   If the field value is a relative URI, it SHOULD be interpreted
   relative to the effective request URI.  The URI MUST NOT include a
   fragment.  See Section 11.2 for security considerations.

9.7.  Retry-After

   The response-header "Retry-After" 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.

     Retry-After   = "Retry-After" ":" OWS Retry-After-v
     Retry-After-v = HTTP-date / delta-seconds




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   Time spans are non-negative decimal integers, representing time in
   seconds.

     delta-seconds  = 1*DIGIT

   Two examples of its use are

     Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
     Retry-After: 120

   In the latter example, the delay is 2 minutes.

9.8.  Server

   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 6.3 of
   [Part1]) and comments (Section 3.2 of [Part1]) identifying the server
   and any significant subproducts.  The product tokens are listed in
   order of their significance for identifying the application.

     Server         = "Server" ":" OWS Server-v
     Server-v       = product
                      *( RWS ( product / comment ) )

   Example:

     Server: CERN/3.0 libwww/2.17

   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 9.9 of [Part1]).

      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.

9.9.  User-Agent

   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.




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   User agents SHOULD include this field with requests.  The field can
   contain multiple product tokens (Section 6.3 of [Part1]) and comments
   (Section 3.2 of [Part1]) 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.

     User-Agent     = "User-Agent" ":" OWS User-Agent-v
     User-Agent-v   = product
                      *( RWS ( product / comment ) )

   Example:

     User-Agent: CERN-LineMode/2.15 libwww/2.17b3

10.  IANA Considerations

10.1.  Method Registry

   The registration procedure for HTTP Methods is defined by Section 2.1
   of this document.

   The HTTP Method Registry shall be created at
   <http://www.iana.org/assignments/http-methods> and be populated with
   the registrations below:

   +---------+------+-------------+
   | Method  | Safe | Reference   |
   +---------+------+-------------+
   | CONNECT | no   | Section 7.9 |
   | DELETE  | no   | Section 7.7 |
   | GET     | yes  | Section 7.3 |
   | HEAD    | yes  | Section 7.4 |
   | OPTIONS | yes  | Section 7.2 |
   | POST    | no   | Section 7.5 |
   | PUT     | no   | Section 7.6 |
   | TRACE   | yes  | Section 7.8 |
   +---------+------+-------------+

10.2.  Status Code Registry

   The registration procedure for HTTP Status Codes -- previously
   defined in Section 7.1 of [RFC2817] -- is now defined by Section 4.1
   of this document.

   The HTTP Status Code Registry located at
   <http://www.iana.org/assignments/http-status-codes> shall be updated
   with the registrations below:



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   +-------+-------------------------------+----------------+
   | Value | Description                   | Reference      |
   +-------+-------------------------------+----------------+
   | 100   | Continue                      | Section 8.1.1  |
   | 101   | Switching Protocols           | Section 8.1.2  |
   | 200   | OK                            | Section 8.2.1  |
   | 201   | Created                       | Section 8.2.2  |
   | 202   | Accepted                      | Section 8.2.3  |
   | 203   | Non-Authoritative Information | Section 8.2.4  |
   | 204   | No Content                    | Section 8.2.5  |
   | 205   | Reset Content                 | Section 8.2.6  |
   | 300   | Multiple Choices              | Section 8.3.1  |
   | 301   | Moved Permanently             | Section 8.3.2  |
   | 302   | Found                         | Section 8.3.3  |
   | 303   | See Other                     | Section 8.3.4  |
   | 305   | Use Proxy                     | Section 8.3.6  |
   | 306   | (Unused)                      | Section 8.3.7  |
   | 307   | Temporary Redirect            | Section 8.3.8  |
   | 400   | Bad Request                   | Section 8.4.1  |
   | 402   | Payment Required              | Section 8.4.3  |
   | 403   | Forbidden                     | Section 8.4.4  |
   | 404   | Not Found                     | Section 8.4.5  |
   | 405   | Method Not Allowed            | Section 8.4.6  |
   | 406   | Not Acceptable                | Section 8.4.7  |
   | 407   | Proxy Authentication Required | Section 8.4.8  |
   | 408   | Request Timeout               | Section 8.4.9  |
   | 409   | Conflict                      | Section 8.4.10 |
   | 410   | Gone                          | Section 8.4.11 |
   | 411   | Length Required               | Section 8.4.12 |
   | 413   | Request Entity Too Large      | Section 8.4.14 |
   | 414   | URI Too Long                  | Section 8.4.15 |
   | 415   | Unsupported Media Type        | Section 8.4.16 |
   | 417   | Expectation Failed            | Section 8.4.18 |
   | 500   | Internal Server Error         | Section 8.5.1  |
   | 501   | Not Implemented               | Section 8.5.2  |
   | 502   | Bad Gateway                   | Section 8.5.3  |
   | 503   | Service Unavailable           | Section 8.5.4  |
   | 504   | Gateway Timeout               | Section 8.5.5  |
   | 505   | HTTP Version Not Supported    | Section 8.5.6  |
   +-------+-------------------------------+----------------+

10.3.  Header Field Registration

   The Message Header Field Registry located at <http://www.iana.org/
   assignments/message-headers/message-header-index.html> shall be
   updated with the permanent registrations below (see [RFC3864]):





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   +-------------------+----------+----------+-------------+
   | Header Field Name | Protocol | Status   | Reference   |
   +-------------------+----------+----------+-------------+
   | Allow             | http     | standard | Section 9.1 |
   | Expect            | http     | standard | Section 9.2 |
   | From              | http     | standard | Section 9.3 |
   | Location          | http     | standard | Section 9.4 |
   | Max-Forwards      | http     | standard | Section 9.5 |
   | Referer           | http     | standard | Section 9.6 |
   | Retry-After       | http     | standard | Section 9.7 |
   | Server            | http     | standard | Section 9.8 |
   | User-Agent        | http     | standard | Section 9.9 |
   +-------------------+----------+----------+-------------+

   The change controller is: "IETF (iesg@ietf.org) - Internet
   Engineering Task Force".

11.  Security Considerations

   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.

11.1.  Transfer of Sensitive Information

   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.

   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.

   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.

   The Referer header allows reading patterns to be studied and reverse
   links drawn.  Although it can be very useful, its power can be abused



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   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.

   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.

   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.

   The User-Agent (Section 9.9) or Server (Section 9.8) 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.

   Some methods, like TRACE (Section 7.8), expose information that was
   sent in request headers within the body of their response.  Clients
   SHOULD be careful with sensitive information, like Cookies,
   Authorization credentials and other headers that might be used to
   collect data from the client.

11.2.  Encoding Sensitive Information in URIs

   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.

   Clients SHOULD NOT include a Referer header field in a (non-secure)
   HTTP request if the referring page was transferred with a secure
   protocol.

   Authors of services SHOULD NOT use GET-based forms for the submission
   of sensitive data because that data will be placed in the request-
   target.  Many existing servers, proxies, and user agents log or
   display the request-target in places where it might be visible to
   third parties.  Such services can use POST-based form submission
   instead.



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11.3.  Location Headers and Spoofing

   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.

12.  Acknowledgments

13.  References

13.1.  Normative References

   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
              and Message Parsing", draft-ietf-httpbis-p1-messaging-11
              (work in progress), August 2010.

   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
              and Content Negotiation", draft-ietf-httpbis-p3-payload-11
              (work in progress), August 2010.

   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
              Requests", draft-ietf-httpbis-p4-conditional-11 (work in
              progress), August 2010.

   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
              Partial Responses", draft-ietf-httpbis-p5-range-11 (work
              in progress), August 2010.

   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
              6: Caching", draft-ietf-httpbis-p6-cache-11 (work in
              progress), August 2010.

   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
              draft-ietf-httpbis-p7-auth-11 (work in progress),



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              August 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", RFC 3986,
              STD 66, January 2005.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

13.2.  Informative References

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, January 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
              HTTP/1.1", RFC 2817, May 2000.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5322]  Resnick, P., "Internet Message Format", RFC 5322,
              October 2008.

Appendix A.  Changes from RFC 2616

   This document takes over the Status Code Registry, previously defined
   in Section 7.1 of [RFC2817].  (Section 4.1)

   Clarify definition of POST.  (Section 7.5)

   Failed to consider that there are many other request methods that are
   safe to automatically redirect, and further that the user agent is



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   able to make that determination based on the request method
   semantics.  (Sections 8.3.2, 8.3.3 and 8.3.8)

   Deprecate 305 Use Proxy status code, because user agents did not
   implement it.  It used to indicate that the target 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.  (Section 8.3.6)

   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.  (Section 9.1)

   Correct syntax of Location header to allow URI references (including
   relative references and fragments), as referred symbol "absoluteURI"
   wasn't what was expected, and add some clarifications as to when use
   of fragments would not be appropriate.  (Section 9.4)

   Allow Referer value of "about:blank" as alternative to not specifying
   it.  (Section 9.6)

   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 9.9 of [Part1].
   (Section 9.8)

Appendix B.  Collected ABNF

   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>
   Accept-Ranges = <Accept-Ranges, defined in [Part5], Section 5.1>
   Age = <Age, defined in [Part6], Section 3.1>
   Allow = "Allow:" OWS Allow-v
   Allow-v = [ ( "," / Method ) *( OWS "," [ OWS Method ] ) ]
   Authorization = <Authorization, defined in [Part7], Section 3.1>

   ETag = <ETag, defined in [Part4], Section 6.1>
   Expect = "Expect:" OWS Expect-v
   Expect-v = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )

   From = "From:" OWS From-v
   From-v = mailbox

   HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
   Host = <Host, defined in [Part1], Section 2.6>



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   If-Match = <If-Match, defined in [Part4], Section 6.2>
   If-Modified-Since =
    <If-Modified-Since, defined in [Part4], Section 6.3>
   If-None-Match = <If-None-Match, defined in [Part4], Section 6.4>
   If-Range = <If-Range, defined in [Part5], Section 5.3>
   If-Unmodified-Since =
    <If-Unmodified-Since, defined in [Part4], Section 6.5>

   Location = "Location:" OWS Location-v
   Location-v = URI-reference

   Max-Forwards = "Max-Forwards:" OWS Max-Forwards-v
   Max-Forwards-v = 1*DIGIT
   Method = %x4F.50.54.49.4F.4E.53 ; OPTIONS
    / %x47.45.54 ; GET
    / %x48.45.41.44 ; HEAD
    / %x50.4F.53.54 ; POST
    / %x50.55.54 ; PUT
    / %x44.45.4C.45.54.45 ; DELETE
    / %x54.52.41.43.45 ; TRACE
    / %x43.4F.4E.4E.45.43.54 ; CONNECT
    / extension-method

   OWS = <OWS, defined in [Part1], Section 1.2.2>

   Proxy-Authenticate =
    <Proxy-Authenticate, defined in [Part7], Section 3.2>
   Proxy-Authorization =
    <Proxy-Authorization, defined in [Part7], Section 3.3>

   RWS = <RWS, defined in [Part1], Section 1.2.2>
   Range = <Range, defined in [Part5], Section 5.4>
   Reason-Phrase = *( WSP / VCHAR / obs-text )
   Referer = "Referer:" OWS Referer-v
   Referer-v = absolute-URI / partial-URI
   Retry-After = "Retry-After:" OWS Retry-After-v
   Retry-After-v = HTTP-date / delta-seconds

   Server = "Server:" OWS Server-v
   Server-v = product *( RWS ( product / comment ) )
   Status-Code = "100" / "101" / "200" / "201" / "202" / "203" / "204" /
    "205" / "206" / "300" / "301" / "302" / "303" / "304" / "305" /
    "307" / "400" / "401" / "402" / "403" / "404" / "405" / "406" /
    "407" / "408" / "409" / "410" / "411" / "412" / "413" / "414" /
    "415" / "416" / "417" / "500" / "501" / "502" / "503" / "504" /
    "505" / extension-code

   TE = <TE, defined in [Part1], Section 9.5>



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   URI-reference = <URI-reference, defined in [Part1], Section 2.6>
   User-Agent = "User-Agent:" OWS User-Agent-v
   User-Agent-v = product *( RWS ( product / comment ) )

   Vary = <Vary, defined in [Part6], Section 3.5>

   WWW-Authenticate =
    <WWW-Authenticate, defined in [Part7], Section 3.4>

   absolute-URI = <absolute-URI, defined in [Part1], Section 2.6>

   comment = <comment, defined in [Part1], Section 3.2>

   delta-seconds = 1*DIGIT

   expect-params = ";" token [ "=" ( token / quoted-string ) ]
   expectation = "100-continue" / expectation-extension
   expectation-extension = token [ "=" ( token / quoted-string )
    *expect-params ]
   extension-code = 3DIGIT
   extension-method = token

   mailbox = <mailbox, defined in [RFC5322], Section 3.4>

   obs-text = <obs-text, defined in [Part1], Section 1.2.2>

   partial-URI = <partial-URI, defined in [Part1], Section 2.6>
   product = <product, defined in [Part1], Section 6.3>

   quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>

   request-header = Accept / Accept-Charset / Accept-Encoding /
    Accept-Language / Authorization / Expect / From / Host / If-Match /
    If-Modified-Since / If-None-Match / If-Range / If-Unmodified-Since /
    Max-Forwards / Proxy-Authorization / Range / Referer / TE /
    User-Agent
   response-header = Accept-Ranges / Age / Allow / ETag / Location /
    Proxy-Authenticate / Retry-After / Server / Vary / WWW-Authenticate

   token = <token, defined in [Part1], Section 1.2.2>

   ABNF diagnostics:

   ; Reason-Phrase defined but not used
   ; Status-Code defined but not used
   ; request-header defined but not used
   ; response-header defined but not used




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Appendix C.  Change Log (to be removed by RFC Editor before publication)

C.1.  Since RFC2616

   Extracted relevant partitions from [RFC2616].

C.2.  Since draft-ietf-httpbis-p2-semantics-00

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/5>: "Via is a MUST"
      (<http://purl.org/NET/http-errata#via-must>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/6>: "Fragments
      allowed in Location"
      (<http://purl.org/NET/http-errata#location-fragments>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods
      vs Redirection" (<http://purl.org/NET/http-errata#saferedirect>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/17>: "Revise
      description of the POST method"
      (<http://purl.org/NET/http-errata#post>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
      Informative references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/42>: "RFC2606
      Compliance"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
      references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/84>: "Redundant
      cross-references"

   Other changes:

   o  Move definitions of 304 and 412 condition codes to [Part4]

C.3.  Since draft-ietf-httpbis-p2-semantics-01

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/21>: "PUT side
      effects"





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/91>: "Duplicate Host
      header requirements"

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Move "Product Tokens" section (back) into Part 1, as "token" is
      used in the definition of the Upgrade header.

   o  Add explicit references to BNF syntax and rules imported from
      other parts of the specification.

   o  Copy definition of delta-seconds from Part6 instead of referencing
      it.

C.4.  Since draft-ietf-httpbis-p2-semantics-02

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/24>: "Requiring
      Allow in 405 responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/59>: "Status Code
      Registry"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/61>: "Redirection
      vs. Location"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/70>: "Cacheability
      of 303 response"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/76>: "305 Use Proxy"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/105>:
      "Classification for Allow header"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/112>: "PUT - 'store
      under' vs 'store at'"

   Ongoing work on IANA Message Header Registration
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):

   o  Reference RFC 3984, and update header registrations for headers
      defined in this document.

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):




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   o  Replace string literals when the string really is case-sensitive
      (method).

C.5.  Since draft-ietf-httpbis-p2-semantics-03

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/98>: "OPTIONS
      request bodies"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/119>: "Description
      of CONNECT should refer to RFC2817"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/125>: "Location
      Content-Location reference request/response mixup"

   Ongoing work on Method Registry
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/72>):

   o  Added initial proposal for registration process, plus initial
      content (non-HTTP/1.1 methods to be added by a separate
      specification).

C.6.  Since draft-ietf-httpbis-p2-semantics-04

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/103>: "Content-*"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/132>: "RFC 2822 is
      updated by RFC 5322"

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Use "/" instead of "|" for alternatives.

   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").

   o  Rewrite ABNFs to spell out whitespace rules, factor out header
      value format definitions.

C.7.  Since draft-ietf-httpbis-p2-semantics-05

   Closed issues:





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/94>: "Reason-Phrase
      BNF"

   Final work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Add appendix containing collected and expanded ABNF, reorganize
      ABNF introduction.

C.8.  Since draft-ietf-httpbis-p2-semantics-06

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/144>: "Clarify when
      Referer is sent"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/164>: "status codes
      vs methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/170>: "Do not
      require "updates" relation for specs that register status codes or
      method names"

C.9.  Since draft-ietf-httpbis-p2-semantics-07

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/27>: "Idempotency"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/33>: "TRACE security
      considerations"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/110>: "Clarify rules
      for determining what entities a response carries"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/140>: "update note
      citing RFC 1945 and 2068"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/182>: "update note
      about redirect limit"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/191>: "Location
      header ABNF should use 'URI'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/192>: "fragments in
      Location vs status 303"





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
      registrations for optional status codes"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/171>: "Are OPTIONS
      and TRACE safe?"

C.10.  Since draft-ietf-httpbis-p2-semantics-08

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods
      vs Redirection" (we missed the introduction to the 3xx status
      codes when fixing this previously)

C.11.  Since draft-ietf-httpbis-p2-semantics-09

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment
      combination / precedence during redirects"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/185>: "Location
      header payload handling"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
      requested resource's URI"

C.12.  Since draft-ietf-httpbis-p2-semantics-10

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
      'Requested Variant'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
      entity / representation / variant terminology"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
      Caching"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/190>: "OPTIONS vs
      Max-Forwards"





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: "Status codes
      and caching"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
      removing the 'changes from 2068' sections"

Index

   1
      100 Continue (status code)  20
      101 Switching Protocols (status code)  21

   2
      200 OK (status code)  21
      201 Created (status code)  21
      202 Accepted (status code)  22
      203 Non-Authoritative Information (status code)  22
      204 No Content (status code)  22
      205 Reset Content (status code)  23
      206 Partial Content (status code)  23

   3
      300 Multiple Choices (status code)  24
      301 Moved Permanently (status code)  24
      302 Found (status code)  25
      303 See Other (status code)  25
      304 Not Modified (status code)  26
      305 Use Proxy (status code)  26
      306 (Unused) (status code)  26
      307 Temporary Redirect (status code)  26

   4
      400 Bad Request (status code)  27
      401 Unauthorized (status code)  27
      402 Payment Required (status code)  27
      403 Forbidden (status code)  27
      404 Not Found (status code)  28
      405 Method Not Allowed (status code)  28
      406 Not Acceptable (status code)  28
      407 Proxy Authentication Required (status code)  28
      408 Request Timeout (status code)  29
      409 Conflict (status code)  29
      410 Gone (status code)  29
      411 Length Required (status code)  30
      412 Precondition Failed (status code)  30
      413 Request Entity Too Large (status code)  30
      414 URI Too Long (status code)  30
      415 Unsupported Media Type (status code)  30



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      416 Requested Range Not Satisfiable (status code)  30
      417 Expectation Failed (status code)  31

   5
      500 Internal Server Error (status code)  31
      501 Not Implemented (status code)  31
      502 Bad Gateway (status code)  31
      503 Service Unavailable (status code)  31
      504 Gateway Timeout (status code)  32
      505 HTTP Version Not Supported (status code)  32

   A
      Allow header  32

   C
      CONNECT method  20

   D
      DELETE method  19

   E
      Expect header  33

   F
      From header  33

   G
      GET method  16
      Grammar
         Allow  32
         Allow-v  32
         delta-seconds  37
         Expect  33
         expect-params  33
         Expect-v  33
         expectation  33
         expectation-extension  33
         extension-code  11
         extension-method  8
         From  34
         From-v  34
         Location  34
         Location-v  34
         Max-Forwards  35
         Max-Forwards-v  35
         Method  8
         Reason-Phrase  11
         Referer  36



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         Referer-v  36
         request-header  9
         response-header  12
         Retry-After  36
         Retry-After-v  36
         Server  37
         Server-v  37
         Status-Code  11
         User-Agent  38
         User-Agent-v  38

   H
      HEAD method  16
      Headers
         Allow  32
         Expect  33
         From  33
         Location  34
         Max-Forwards  35
         Referer  36
         Retry-After  36
         Server  37
         User-Agent  37

   I
      Idempotent Methods  14

   L
      Location header  34

   M
      Max-Forwards header  35
      Methods
         CONNECT  20
         DELETE  19
         GET  16
         HEAD  16
         OPTIONS  15
         POST  17
         PUT  18
         TRACE  19

   O
      OPTIONS method  15

   P
      POST method  17
      PUT method  18



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   R
      Referer header  36
      Retry-After header  36

   S
      Safe Methods  14
      Server header  37
      Status Codes
         100 Continue  20
         101 Switching Protocols  21
         200 OK  21
         201 Created  21
         202 Accepted  22
         203 Non-Authoritative Information  22
         204 No Content  22
         205 Reset Content  23
         206 Partial Content  23
         300 Multiple Choices  24
         301 Moved Permanently  24
         302 Found  25
         303 See Other  25
         304 Not Modified  26
         305 Use Proxy  26
         306 (Unused)  26
         307 Temporary Redirect  26
         400 Bad Request  27
         401 Unauthorized  27
         402 Payment Required  27
         403 Forbidden  27
         404 Not Found  28
         405 Method Not Allowed  28
         406 Not Acceptable  28
         407 Proxy Authentication Required  28
         408 Request Timeout  29
         409 Conflict  29
         410 Gone  29
         411 Length Required  30
         412 Precondition Failed  30
         413 Request Entity Too Large  30
         414 URI Too Long  30
         415 Unsupported Media Type  30
         416 Requested Range Not Satisfiable  30
         417 Expectation Failed  31
         500 Internal Server Error  31
         501 Not Implemented  31
         502 Bad Gateway  31
         503 Service Unavailable  31
         504 Gateway Timeout  32



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         505 HTTP Version Not Supported  32

   T
      TRACE method  19

   U
      User-Agent header  37

Authors' Addresses

   Roy T. Fielding (editor)
   Day Software
   23 Corporate Plaza DR, Suite 280
   Newport Beach, CA  92660
   USA

   Phone: +1-949-706-5300
   Fax:   +1-949-706-5305
   EMail: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


   Jim Gettys
   Alcatel-Lucent Bell Labs
   21 Oak Knoll Road
   Carlisle, MA  01741
   USA

   EMail: jg@freedesktop.org
   URI:   http://gettys.wordpress.com/


   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304
   USA

   EMail: JeffMogul@acm.org











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   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052
   USA

   EMail: henrikn@microsoft.com


   Larry Masinter
   Adobe Systems, Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: LMM@acm.org
   URI:   http://larry.masinter.net/


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052

   EMail: paulle@microsoft.com


   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139
   USA

   EMail: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/














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   Yves Lafon (editor)
   World Wide Web Consortium
   W3C / ERCIM
   2004, rte des Lucioles
   Sophia-Antipolis, AM  06902
   France

   EMail: ylafon@w3.org
   URI:   http://www.raubacapeu.net/people/yves/


   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Phone: +49 251 2807760
   Fax:   +49 251 2807761
   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/






























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