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HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                            J. Reschke, Ed.
Updates: 2817 (if approved)                                   greenbytes
Intended status: Standards Track                         October 4, 2012
Expires: April 7, 2013


     Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
                   draft-ietf-httpbis-p2-semantics-21

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypertext information
   systems.  This document defines the semantics of HTTP/1.1 messages,
   as expressed by request methods, request header fields, response
   status codes, and response header fields, along with the payload of
   messages (metadata and body content) and mechanisms for content
   negotiation.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft takes place on the HTTPBIS working group
   mailing list (ietf-http-wg@w3.org), which is archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

   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 F.41.

Status of This Memo

   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."



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   This Internet-Draft will expire on April 7, 2013.

Copyright Notice

   Copyright (c) 2012 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
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   publication of this document.  Please review these documents
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   7
     1.1.  Conformance and Error Handling  . . . . . . . . . . . . .   7
     1.2.  Syntax Notation . . . . . . . . . . . . . . . . . . . . .   7
   2.  Resource  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Representation  . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Representation Metadata . . . . . . . . . . . . . . . . .   8
       3.1.1.  Data Type . . . . . . . . . . . . . . . . . . . . . .   9
       3.1.2.  Data Encoding . . . . . . . . . . . . . . . . . . . .  12
       3.1.3.  Audience Language . . . . . . . . . . . . . . . . . .  14
       3.1.4.  Identification  . . . . . . . . . . . . . . . . . . .  15
     3.2.  Representation Data . . . . . . . . . . . . . . . . . . .  18
     3.3.  Payload Semantics . . . . . . . . . . . . . . . . . . . .  18
     3.4.  Content Negotiation . . . . . . . . . . . . . . . . . . .  19
       3.4.1.  Proactive Negotiation . . . . . . . . . . . . . . . .  20
       3.4.2.  Reactive Negotiation  . . . . . . . . . . . . . . . .  21
   4.  Product Tokens  . . . . . . . . . . . . . . . . . . . . . . .  22
   5.  Request Methods . . . . . . . . . . . . . . . . . . . . . . .  22



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     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  22
     5.2.  Common Method Properties  . . . . . . . . . . . . . . . .  24
       5.2.1.  Safe Methods  . . . . . . . . . . . . . . . . . . . .  24
       5.2.2.  Idempotent Methods  . . . . . . . . . . . . . . . . .  25
       5.2.3.  Cacheable Methods . . . . . . . . . . . . . . . . . .  25
     5.3.  Method Definitions  . . . . . . . . . . . . . . . . . . .  25
       5.3.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  25
       5.3.2.  HEAD  . . . . . . . . . . . . . . . . . . . . . . . .  26
       5.3.3.  POST  . . . . . . . . . . . . . . . . . . . . . . . .  27
       5.3.4.  PUT . . . . . . . . . . . . . . . . . . . . . . . . .  28
       5.3.5.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  30
       5.3.6.  CONNECT . . . . . . . . . . . . . . . . . . . . . . .  30
       5.3.7.  OPTIONS . . . . . . . . . . . . . . . . . . . . . . .  32
       5.3.8.  TRACE . . . . . . . . . . . . . . . . . . . . . . . .  33
   6.  Request Header Fields . . . . . . . . . . . . . . . . . . . .  33
     6.1.  Controls  . . . . . . . . . . . . . . . . . . . . . . . .  33
       6.1.1.  Max-Forwards  . . . . . . . . . . . . . . . . . . . .  34
       6.1.2.  Expect  . . . . . . . . . . . . . . . . . . . . . . .  34
     6.2.  Conditionals  . . . . . . . . . . . . . . . . . . . . . .  37
     6.3.  Content Negotiation . . . . . . . . . . . . . . . . . . .  38
       6.3.1.  Quality Values  . . . . . . . . . . . . . . . . . . .  38
       6.3.2.  Accept  . . . . . . . . . . . . . . . . . . . . . . .  38
       6.3.3.  Accept-Charset  . . . . . . . . . . . . . . . . . . .  41
       6.3.4.  Accept-Encoding . . . . . . . . . . . . . . . . . . .  41
       6.3.5.  Accept-Language . . . . . . . . . . . . . . . . . . .  42
     6.4.  Authentication Credentials  . . . . . . . . . . . . . . .  44
     6.5.  Context . . . . . . . . . . . . . . . . . . . . . . . . .  44
       6.5.1.  From  . . . . . . . . . . . . . . . . . . . . . . . .  44
       6.5.2.  Referer . . . . . . . . . . . . . . . . . . . . . . .  45
       6.5.3.  User-Agent  . . . . . . . . . . . . . . . . . . . . .  45
   7.  Response Status Codes . . . . . . . . . . . . . . . . . . . .  46
     7.1.  Overview of Status Codes  . . . . . . . . . . . . . . . .  47
     7.2.  Informational 1xx . . . . . . . . . . . . . . . . . . . .  49
       7.2.1.  100 Continue  . . . . . . . . . . . . . . . . . . . .  49
       7.2.2.  101 Switching Protocols . . . . . . . . . . . . . . .  49
     7.3.  Successful 2xx  . . . . . . . . . . . . . . . . . . . . .  50
       7.3.1.  200 OK  . . . . . . . . . . . . . . . . . . . . . . .  50
       7.3.2.  201 Created . . . . . . . . . . . . . . . . . . . . .  50
       7.3.3.  202 Accepted  . . . . . . . . . . . . . . . . . . . .  51
       7.3.4.  203 Non-Authoritative Information . . . . . . . . . .  51
       7.3.5.  204 No Content  . . . . . . . . . . . . . . . . . . .  51
       7.3.6.  205 Reset Content . . . . . . . . . . . . . . . . . .  52
     7.4.  Redirection 3xx . . . . . . . . . . . . . . . . . . . . .  52
       7.4.1.  300 Multiple Choices  . . . . . . . . . . . . . . . .  54
       7.4.2.  301 Moved Permanently . . . . . . . . . . . . . . . .  54
       7.4.3.  302 Found . . . . . . . . . . . . . . . . . . . . . .  55
       7.4.4.  303 See Other . . . . . . . . . . . . . . . . . . . .  55
       7.4.5.  305 Use Proxy . . . . . . . . . . . . . . . . . . . .  56



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       7.4.6.  306 (Unused)  . . . . . . . . . . . . . . . . . . . .  56
       7.4.7.  307 Temporary Redirect  . . . . . . . . . . . . . . .  56
     7.5.  Client Error 4xx  . . . . . . . . . . . . . . . . . . . .  56
       7.5.1.  400 Bad Request . . . . . . . . . . . . . . . . . . .  56
       7.5.2.  402 Payment Required  . . . . . . . . . . . . . . . .  56
       7.5.3.  403 Forbidden . . . . . . . . . . . . . . . . . . . .  57
       7.5.4.  404 Not Found . . . . . . . . . . . . . . . . . . . .  57
       7.5.5.  405 Method Not Allowed  . . . . . . . . . . . . . . .  57
       7.5.6.  406 Not Acceptable  . . . . . . . . . . . . . . . . .  57
       7.5.7.  408 Request Timeout . . . . . . . . . . . . . . . . .  58
       7.5.8.  409 Conflict  . . . . . . . . . . . . . . . . . . . .  58
       7.5.9.  410 Gone  . . . . . . . . . . . . . . . . . . . . . .  58
       7.5.10. 411 Length Required . . . . . . . . . . . . . . . . .  59
       7.5.11. 413 Request Representation Too Large  . . . . . . . .  59
       7.5.12. 414 URI Too Long  . . . . . . . . . . . . . . . . . .  59
       7.5.13. 415 Unsupported Media Type  . . . . . . . . . . . . .  59
       7.5.14. 417 Expectation Failed  . . . . . . . . . . . . . . .  60
       7.5.15. 426 Upgrade Required  . . . . . . . . . . . . . . . .  60
     7.6.  Server Error 5xx  . . . . . . . . . . . . . . . . . . . .  60
       7.6.1.  500 Internal Server Error . . . . . . . . . . . . . .  60
       7.6.2.  501 Not Implemented . . . . . . . . . . . . . . . . .  60
       7.6.3.  502 Bad Gateway . . . . . . . . . . . . . . . . . . .  61
       7.6.4.  503 Service Unavailable . . . . . . . . . . . . . . .  61
       7.6.5.  504 Gateway Timeout . . . . . . . . . . . . . . . . .  61
       7.6.6.  505 HTTP Version Not Supported  . . . . . . . . . . .  61
   8.  Response Header Fields  . . . . . . . . . . . . . . . . . . .  61
     8.1.  Control Data  . . . . . . . . . . . . . . . . . . . . . .  62
       8.1.1.  Origination Date  . . . . . . . . . . . . . . . . . .  62
       8.1.2.  Location  . . . . . . . . . . . . . . . . . . . . . .  65
       8.1.3.  Retry-After . . . . . . . . . . . . . . . . . . . . .  66
     8.2.  Selected Representation Header Fields . . . . . . . . . .  67
       8.2.1.  Vary  . . . . . . . . . . . . . . . . . . . . . . . .  67
     8.3.  Authentication Challenges . . . . . . . . . . . . . . . .  68
     8.4.  Informative . . . . . . . . . . . . . . . . . . . . . . .  68
       8.4.1.  Allow . . . . . . . . . . . . . . . . . . . . . . . .  69
       8.4.2.  Server  . . . . . . . . . . . . . . . . . . . . . . .  69
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  70
     9.1.  Method Registry . . . . . . . . . . . . . . . . . . . . .  70
       9.1.1.  Procedure . . . . . . . . . . . . . . . . . . . . . .  70
       9.1.2.  Considerations for New Methods  . . . . . . . . . . .  70
       9.1.3.  Registrations . . . . . . . . . . . . . . . . . . . .  71
     9.2.  Status Code Registry  . . . . . . . . . . . . . . . . . .  71
       9.2.1.  Procedure . . . . . . . . . . . . . . . . . . . . . .  71
       9.2.2.  Considerations for New Status Codes . . . . . . . . .  71
       9.2.3.  Registrations . . . . . . . . . . . . . . . . . . . .  72
     9.3.  Header Field Registry . . . . . . . . . . . . . . . . . .  73
       9.3.1.  Considerations for New Header Fields  . . . . . . . .  74
       9.3.2.  Registrations . . . . . . . . . . . . . . . . . . . .  75



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     9.4.  Content Coding Registry . . . . . . . . . . . . . . . . .  76
       9.4.1.  Procedure . . . . . . . . . . . . . . . . . . . . . .  76
       9.4.2.  Registrations . . . . . . . . . . . . . . . . . . . .  77
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  77
     10.1. Transfer of Sensitive Information . . . . . . . . . . . .  77
     10.2. Encoding Sensitive Information in URIs  . . . . . . . . .  78
     10.3. Location Header Fields: Spoofing and Information
           Leakage . . . . . . . . . . . . . . . . . . . . . . . . .  79
     10.4. Security Considerations for CONNECT . . . . . . . . . . .  79
     10.5. Privacy Issues Connected to Accept Header Fields  . . . .  79
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  80
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  80
     12.1. Normative References  . . . . . . . . . . . . . . . . . .  80
     12.2. Informative References  . . . . . . . . . . . . . . . . .  81
   Appendix A.  Differences between HTTP and MIME  . . . . . . . . .  83
     A.1.  MIME-Version  . . . . . . . . . . . . . . . . . . . . . .  84
     A.2.  Conversion to Canonical Form  . . . . . . . . . . . . . .  84
     A.3.  Conversion of Date Formats  . . . . . . . . . . . . . . .  84
     A.4.  Introduction of Content-Encoding  . . . . . . . . . . . .  85
     A.5.  No Content-Transfer-Encoding  . . . . . . . . . . . . . .  85
     A.6.  MHTML and Line Length Limitations . . . . . . . . . . . .  85
   Appendix B.  Additional Features  . . . . . . . . . . . . . . . .  85
   Appendix C.  Changes from RFC 2616  . . . . . . . . . . . . . . .  86
   Appendix D.  Imported ABNF  . . . . . . . . . . . . . . . . . . .  88
   Appendix E.  Collected ABNF . . . . . . . . . . . . . . . . . . .  88
   Appendix F.  Change Log (to be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  91
     F.1.  Since RFC 2616  . . . . . . . . . . . . . . . . . . . . .  91
     F.2.  Since draft-ietf-httpbis-p2-semantics-00  . . . . . . . .  91
     F.3.  Since draft-ietf-httpbis-p3-payload-00  . . . . . . . . .  92
     F.4.  Since draft-ietf-httpbis-p2-semantics-01  . . . . . . . .  93
     F.5.  Since draft-ietf-httpbis-p3-payload-01  . . . . . . . . .  93
     F.6.  Since draft-ietf-httpbis-p2-semantics-02  . . . . . . . .  93
     F.7.  Since draft-ietf-httpbis-p3-payload-02  . . . . . . . . .  94
     F.8.  Since draft-ietf-httpbis-p2-semantics-03  . . . . . . . .  95
     F.9.  Since draft-ietf-httpbis-p3-payload-03  . . . . . . . . .  95
     F.10. Since draft-ietf-httpbis-p2-semantics-04  . . . . . . . .  95
     F.11. Since draft-ietf-httpbis-p3-payload-04  . . . . . . . . .  96
     F.12. Since draft-ietf-httpbis-p2-semantics-05  . . . . . . . .  96
     F.13. Since draft-ietf-httpbis-p3-payload-05  . . . . . . . . .  96
     F.14. Since draft-ietf-httpbis-p2-semantics-06  . . . . . . . .  97
     F.15. Since draft-ietf-httpbis-p3-payload-06  . . . . . . . . .  97
     F.16. Since draft-ietf-httpbis-p2-semantics-07  . . . . . . . .  97
     F.17. Since draft-ietf-httpbis-p3-payload-07  . . . . . . . . .  98
     F.18. Since draft-ietf-httpbis-p2-semantics-08  . . . . . . . .  99
     F.19. Since draft-ietf-httpbis-p3-payload-08  . . . . . . . . .  99
     F.20. Since draft-ietf-httpbis-p2-semantics-09  . . . . . . . .  99
     F.21. Since draft-ietf-httpbis-p3-payload-09  . . . . . . . . .  99



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     F.22. Since draft-ietf-httpbis-p2-semantics-10  . . . . . . . . 100
     F.23. Since draft-ietf-httpbis-p3-payload-10  . . . . . . . . . 100
     F.24. Since draft-ietf-httpbis-p2-semantics-11  . . . . . . . . 101
     F.25. Since draft-ietf-httpbis-p3-payload-11  . . . . . . . . . 101
     F.26. Since draft-ietf-httpbis-p2-semantics-12  . . . . . . . . 101
     F.27. Since draft-ietf-httpbis-p3-payload-12  . . . . . . . . . 103
     F.28. Since draft-ietf-httpbis-p2-semantics-13  . . . . . . . . 103
     F.29. Since draft-ietf-httpbis-p3-payload-13  . . . . . . . . . 103
     F.30. Since draft-ietf-httpbis-p2-semantics-14  . . . . . . . . 103
     F.31. Since draft-ietf-httpbis-p3-payload-14  . . . . . . . . . 104
     F.32. Since draft-ietf-httpbis-p2-semantics-15  . . . . . . . . 104
     F.33. Since draft-ietf-httpbis-p3-payload-15  . . . . . . . . . 104
     F.34. Since draft-ietf-httpbis-p2-semantics-16  . . . . . . . . 104
     F.35. Since draft-ietf-httpbis-p3-payload-16  . . . . . . . . . 104
     F.36. Since draft-ietf-httpbis-p2-semantics-17  . . . . . . . . 105
     F.37. Since draft-ietf-httpbis-p3-payload-17  . . . . . . . . . 105
     F.38. Since draft-ietf-httpbis-p2-semantics-18  . . . . . . . . 105
     F.39. Since draft-ietf-httpbis-p3-payload-18  . . . . . . . . . 106
     F.40. Since draft-ietf-httpbis-p2-semantics-19 and
           draft-ietf-httpbis-p3-payload-19  . . . . . . . . . . . . 106
     F.41. Since draft-ietf-httpbis-p2-semantics-20  . . . . . . . . 107
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107





























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1.  Introduction

   Each Hypertext Transfer Protocol (HTTP) message is either a request
   or a response.  A server listens on a connection for a request,
   parses each message received, interprets the message semantics in
   relation to the identified request target, and responds to that
   request with one or more response messages.  A client constructs
   request messages to communicate specific intentions, and examines
   received responses to see if the intentions were carried out and
   determine how to interpret the results.  This document defines
   HTTP/1.1 request and response semantics in terms of the architecture
   defined in [Part1].

   HTTP provides a uniform interface for interacting with a resource
   (Section 2), regardless of its type, nature, or implementation, and
   for transferring content in message payloads in the form of a
   representation (Section 3).

   HTTP semantics include the intentions defined by each request method
   (Section 5), extensions to those semantics that might be described in
   request header fields (Section 6), the meaning of status codes to
   indicate a machine-readable response (Section 7), and the meaning of
   other control data and resource metadata that might be given in
   response header fields (Section 8).

   This document also defines representation metadata that describe how
   a payload is intended to be interpreted by a recipient, the request
   header fields that might influence content selection, and the various
   selection algorithms that are collectively referred to as "content
   negotiation" (Section 3.4).

1.1.  Conformance and Error Handling

   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].

   Conformance criteria and considerations regarding error handling are
   defined in Section 2.5 of [Part1].

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with the list rule extension defined in Section
   1.2 of [Part1].  Appendix D describes rules imported from other
   documents.  Appendix E shows the collected ABNF with the list rule
   expanded.




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2.  Resource

   The target of each HTTP request is called a resource.  HTTP does not
   limit the nature of a resource; it merely defines an interface that
   might be used to interact with resources.  Each resource is
   identified by a Uniform Resource Identifier (URI), as described in
   Section 2.7 of [Part1].

   When a client constructs an HTTP/1.1 request message, it sends the
   "target URI" in one of various forms, as defined in (Section 5.3 of
   [Part1]).  When a request is received, the server reconstructs an
   "effective request URI" for the target resource (Section 5.5 of
   [Part1]).

   One design goal of HTTP is to separate resource identification from
   request semantics, which is made possible by vesting the request
   semantics in the request method (Section 5) and a few request-
   modifying header fields (Section 6).  Resource owners SHOULD NOT
   include request semantics within a URI, such as by specifying an
   action to invoke within the path or query components of the effective
   request URI, unless those semantics are disabled when they are
   inconsistent with the request method.

3.  Representation

   If we consider that a resource could be anything, and that the
   uniform interface provided by HTTP is similar to a window through
   which one can observe and act upon such a thing only through the
   communication of messages to some independent actor on the other
   side, then we need an abstraction to represent ("take the place of")
   the current or desired state of that thing in our communications.  We
   call that abstraction a "representation" [REST].

   For the purposes of HTTP, a representation is information that
   reflects the current or desired state of a given resource, in a
   format that can be readily communicated via the protocol, consisting
   of a set of representation metadata and a potentially unbounded
   stream of representation data.

3.1.  Representation Metadata

   Representation header fields provide metadata about the
   representation.  When a message includes a payload body, the
   representation header fields describe how to interpret the
   representation data enclosed in the payload body.  In a response to a
   HEAD request, the representation header fields describe the
   representation data that would have been enclosed in the payload body
   if the same request had been a GET.



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   The following header fields are defined to convey representation
   metadata:

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Content-Type      | Section 3.1.1.5        |
   | Content-Encoding  | Section 3.1.2.2        |
   | Content-Language  | Section 3.1.3.2        |
   | Content-Location  | Section 3.1.4.2        |
   | Expires           | Section 7.3 of [Part6] |
   +-------------------+------------------------+

3.1.1.  Data Type

3.1.1.1.  Media Types

   HTTP uses Internet Media Types [RFC2046] in the Content-Type
   (Section 3.1.1.5) and Accept (Section 6.3.2) header fields in order
   to provide open and extensible data typing and type negotiation.

     media-type = type "/" subtype *( OWS ";" OWS parameter )
     type       = token
     subtype    = token

   The type/subtype MAY be followed by parameters in the form of
   attribute/value pairs.

     parameter      = attribute "=" value
     attribute      = token
     value          = word

   The type, subtype, and parameter attribute names are case-
   insensitive.  Parameter values might or might not be case-sensitive,
   depending on the semantics of the parameter name.  The presence or
   absence of a parameter might be significant to the processing of a
   media-type, depending on its definition within the media type
   registry.

   A parameter value that matches the token production can be
   transmitted as either a token or within a quoted-string.  The quoted
   and unquoted values are equivalent.

   Media-type values are registered with the Internet Assigned Number
   Authority (IANA).  The media type registration process is outlined in
   [RFC4288].  Use of non-registered media types is discouraged.





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3.1.1.2.  Character Encodings (charset)

   HTTP uses charset names to indicate the character encoding of a
   textual representation.

   A character encoding is identified by a case-insensitive token.  The
   complete set of tokens is defined by the IANA Character Set registry
   (<http://www.iana.org/assignments/character-sets>).

     charset = token

   Although HTTP allows an arbitrary token to be used as a charset
   value, any token that has a predefined value within the IANA
   Character Set registry MUST represent the character encoding defined
   by that registry.  Applications SHOULD limit their use of character
   encodings to those defined within the IANA registry.

   HTTP uses charset in two contexts: within an Accept-Charset request
   header field (in which the charset value is an unquoted token) and as
   the value of a parameter in a Content-Type header field (within a
   request or response), in which case the parameter value of the
   charset parameter can be quoted.

   Implementers need to be aware of IETF character set requirements
   [RFC3629] [RFC2277].

3.1.1.3.  Canonicalization and Text Defaults

   Internet media types are registered with a canonical form.  A
   representation transferred via HTTP messages MUST be in the
   appropriate canonical form prior to its transmission except for
   "text" types, as defined in the next paragraph.

   When in canonical form, media subtypes of the "text" type use CRLF as
   the text line break.  HTTP relaxes this requirement and allows the
   transport of text media with plain CR or LF alone representing a line
   break when it is done consistently for an entire representation.
   HTTP applications MUST accept CRLF, bare CR, and bare LF as
   indicating a line break in text media received via HTTP.  In
   addition, if the text is in a character encoding that does not use
   octets 13 and 10 for CR and LF respectively, as is the case for some
   multi-byte character encodings, HTTP allows the use of whatever octet
   sequences are defined by that character encoding to represent the
   equivalent of CR and LF for line breaks.  This flexibility regarding
   line breaks applies only to text media in the payload body; a bare CR
   or LF MUST NOT be substituted for CRLF within any of the HTTP control
   structures (such as header fields and multipart boundaries).




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   If a representation is encoded with a content-coding, the underlying
   data MUST be in a form defined above prior to being encoded.

3.1.1.4.  Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more representations within a single message body.  All
   multipart types share a common syntax, as defined in Section 5.1.1 of
   [RFC2046], and include a boundary parameter as part of the media type
   value.  The message body is itself a protocol element; a sender MUST
   generate only CRLF to represent line breaks between body-parts.

   In general, HTTP treats a multipart message body no differently than
   any other media type: strictly as payload.  HTTP does not use the
   multipart boundary as an indicator of message body length.  In all
   other respects, an HTTP user agent SHOULD follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart type.
   The MIME header fields within each body-part of a multipart message
   body do not have any significance to HTTP beyond that defined by
   their MIME semantics.

   A recipient MUST treat an unrecognized multipart subtype as being
   equivalent to "multipart/mixed".

      Note: The "multipart/form-data" type has been specifically defined
      for carrying form data suitable for processing via the POST
      request method, as described in [RFC2388].

3.1.1.5.  Content-Type

   The "Content-Type" header field indicates the media type of the
   representation, which defines both the data format and how that data
   SHOULD be processed by the recipient (within the scope of the request
   method semantics) after any Content-Encoding is decoded.  For
   responses to the HEAD method, the media type is that which would have
   been sent had the request been a GET.

     Content-Type = media-type

   Media types are defined in Section 3.1.1.1.  An example of the field
   is

     Content-Type: text/html; charset=ISO-8859-4

   A sender SHOULD include a Content-Type header field in a message
   containing a payload body, defining the media type of the enclosed
   representation, unless the intended media type is unknown to the
   sender.  If a Content-Type header field is not present, recipients



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   MAY either assume a media type of "application/octet-stream"
   ([RFC2046], Section 4.5.1) or examine the representation data to
   determine its type.

   In practice, resource owners do not always properly configure their
   origin server to provide the correct Content-Type for a given
   representation, with the result that some clients will examine a
   payload's content and override the specified type.  Clients that do
   so risk drawing incorrect conclusions, which might expose additional
   security risks (e.g., "privilege escalation").  Furthermore, it is
   impossible to determine the sender's intent by examining the data
   format: many data formats match multiple media types that differ only
   in processing semantics.  Implementers are encouraged to provide a
   means of disabling such "content sniffing" when it is used.

3.1.2.  Data Encoding

3.1.2.1.  Content Codings

   Content coding values indicate an encoding transformation that has
   been or can be applied to a representation.  Content codings are
   primarily used to allow a representation to be compressed or
   otherwise usefully transformed without losing the identity of its
   underlying media type and without loss of information.  Frequently,
   the representation is stored in coded form, transmitted directly, and
   only decoded by the recipient.

     content-coding   = token

   All content-coding values are case-insensitive and SHOULD be
   registered within the HTTP Content Coding registry, as defined in
   Section 9.4.  They are used in the Accept-Encoding (Section 6.3.4)
   and Content-Encoding (Section 3.1.2.2) header fields.

   The following content-coding values are defined by this
   specification:

      compress (and x-compress): See Section 4.2.1 of [Part1].

      deflate: See Section 4.2.2 of [Part1].

      gzip (and x-gzip): See Section 4.2.3 of [Part1].

3.1.2.2.  Content-Encoding

   The "Content-Encoding" header field indicates what content codings
   have been applied to the representation, beyond those inherent in the
   media type, and thus what decoding mechanisms have to be applied in



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   order to obtain data in the media type referenced by the Content-Type
   header field.  Content-Encoding is primarily used to allow a
   representation's data to be compressed without losing the identity of
   its underlying media type.

     Content-Encoding = 1#content-coding

   An example of its use is

     Content-Encoding: gzip

   If multiple encodings have been applied to a representation, the
   content codings MUST be listed in the order in which they were
   applied.  Additional information about the encoding parameters MAY be
   provided by other header fields not defined by this specification.

   Unlike Transfer-Encoding (Section 3.3.1 of [Part1]), the codings
   listed in Content-Encoding are a characteristic of the
   representation; the representation is defined in terms of the coded
   form, and all other metadata about the representation is about the
   coded form unless otherwise noted in the metadata definition.
   Typically, the representation is only decoded just prior to rendering
   or analogous usage.

   A transforming proxy MAY modify the content coding if the new coding
   is known to be acceptable to the recipient, unless the "no-transform"
   cache-control directive is present in the message.

   If the media type includes an inherent encoding, such as a data
   format that is always compressed, then that encoding would not be
   restated as a Content-Encoding even if it happens to be the same
   algorithm as one of the content codings.  Such a content coding would
   only be listed if, for some bizarre reason, it is applied a second
   time to form the representation.  Likewise, an origin server might
   choose to publish the same payload data as multiple representations
   that differ only in whether the coding is defined as part of Content-
   Type or Content-Encoding, since some user agents will behave
   differently in their handling of each response (e.g., open a "Save as
   ..." dialog instead of automatic decompression and rendering of
   content).

   If the content-coding of a representation in a request message is not
   acceptable to the origin server, the server SHOULD respond with a
   status code of 415 (Unsupported Media Type).







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3.1.3.  Audience Language

3.1.3.1.  Language Tags

   A language tag, as defined in [RFC5646], identifies a natural
   language spoken, written, or otherwise conveyed by human beings for
   communication of information to other human beings.  Computer
   languages are explicitly excluded.  HTTP uses language tags within
   the Accept-Language and Content-Language fields.

   In summary, a language tag is composed of one or more parts: A
   primary language subtag followed by a possibly empty series of
   subtags:

     language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

   White space is not allowed within the tag and all tags are case-
   insensitive.  The name space of language subtags is administered by
   the IANA (see
   <http://www.iana.org/assignments/language-subtag-registry>).

   Example tags include:

     en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN

   See [RFC5646] for further information.

3.1.3.2.  Content-Language

   The "Content-Language" header field describes the natural language(s)
   of the intended audience for the representation.  Note that this
   might not be equivalent to all the languages used within the
   representation.

     Content-Language = 1#language-tag

   Language tags are defined in Section 3.1.3.1.  The primary purpose of
   Content-Language is to allow a user to identify and differentiate
   representations according to the user's own preferred language.
   Thus, if the content is intended only for a Danish-literate audience,
   the appropriate field is

     Content-Language: da

   If no Content-Language is specified, the default is that the content
   is intended for all language audiences.  This might mean that the
   sender does not consider it to be specific to any natural language,
   or that the sender does not know for which language it is intended.



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   Multiple languages MAY be listed for content that is intended for
   multiple audiences.  For example, a rendition of the "Treaty of
   Waitangi", presented simultaneously in the original Maori and English
   versions, would call for

     Content-Language: mi, en

   However, just because multiple languages are present within a
   representation does not mean that it is intended for multiple
   linguistic audiences.  An example would be a beginner's language
   primer, such as "A First Lesson in Latin", which is clearly intended
   to be used by an English-literate audience.  In this case, the
   Content-Language would properly only include "en".

   Content-Language MAY be applied to any media type -- it is not
   limited to textual documents.

3.1.4.  Identification

3.1.4.1.  Identifying a Representation

   When a complete or partial representation is transferred in a message
   payload, it is often desirable for the sender to supply, or the
   recipient to determine, an identifier for a resource corresponding to
   that representation.

   The following rules are used to determine such a URI for the payload
   of a request message:

   o  If the request has a Content-Location header field, then the
      sender asserts that the payload is a representation of the
      resource identified by the Content-Location field-value.  However,
      such an assertion cannot be trusted unless it can be verified by
      other means (not defined by HTTP).  The information might still be
      useful for revision history links.

   o  Otherwise, the payload is unidentified.

   The following rules, to be applied in order until a match is found,
   are used to determine such a URI for the payload of a response
   message:

   1.  If the request is GET or HEAD and the response status code is 200
       (OK), 204 (No Content), 206 (Partial Content), or 304 (Not
       Modified), the payload's identifier is the effective request URI
       (Section 5.5 of [Part1]).





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   2.  If the request is GET or HEAD and the response status code is 203
       (Non-Authoritative Information), the payload is a potentially
       modified representation of the target resource; as such, the
       effective request URI might only act as an identifier for the
       payload's representation when a request is made via the same
       chain of intermediaries.

   3.  If the response has a Content-Location header field and its
       field-value is a reference to the same URI as the effective
       request URI, the payload's identifier is the effective request
       URI.

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

   5.  Otherwise, the payload is unidentified.

3.1.4.2.  Content-Location

   The "Content-Location" header field references a URI that can be used
   as a specific identifier for the representation in this message
   payload.  In other words, if one were to perform a GET on this URI at
   the time of this message's generation, then a 200 (OK) response would
   contain the same representation that is enclosed as payload in this
   message.

     Content-Location = absolute-URI / partial-URI

   The Content-Location value is not a replacement for the effective
   Request URI (Section 5.5 of [Part1]).  It is representation metadata.
   It has the same syntax and semantics as the header field of the same
   name defined for MIME body parts in Section 4 of [RFC2557].  However,
   its appearance in an HTTP message has some special implications for
   HTTP recipients.

   If Content-Location is included in a 2xx (Successful) response
   message and its value refers (after conversion to absolute form) to a
   URI that is the same as the effective request URI, then the response
   payload SHOULD be considered a current representation of that
   resource.  For a GET or HEAD request, this is the same as the default
   semantics when no Content-Location is provided by the server.  For a
   state-changing request like PUT or POST, it implies that the server's
   response contains the new representation of that resource, thereby
   distinguishing it from representations that might only report about



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   the action (e.g., "It worked!").  This allows authoring applications
   to update their local copies without the need for a subsequent GET
   request.

   If Content-Location is included in a 2xx (Successful) response
   message and its field-value refers to a URI that differs from the
   effective request URI, then the origin server claims that the field-
   value is an identifier for the payload's representation.  Such a
   claim can only be trusted if both identifiers share the same resource
   owner, which cannot be programmatically determined via HTTP.

   o  For a response to a GET or HEAD request, this is an indication
      that the effective request URI identifies a resource that is
      subject to content negotiation and the Content-Location field-
      value is a more specific identifier for the selected
      representation.

   o  For a 201 (Created) response to a state-changing method, a
      Content-Location field-value that is identical to the Location
      field-value indicates that this payload is a current
      representation of the newly created resource.

   o  Otherwise, such a Content-Location indicates that this payload is
      a representation reporting on the requested action's status and
      that the same report is available (for future access with GET) at
      the given URI.  For example, a purchase transaction made via a
      POST request might include a receipt document as the payload of
      the 200 (OK) response; the Content-Location field-value provides
      an identifier for retrieving a copy of that same receipt in the
      future.

   If Content-Location is included in a request message, then it MAY be
   interpreted by the origin server as an indication of where the user
   agent originally obtained the content of the enclosed representation
   (prior to any subsequent modification of the content by that user
   agent).  In other words, the user agent is providing the same
   representation metadata that it received with the original
   representation.  However, such interpretation MUST NOT be used to
   alter the semantics of the method requested by the client.  For
   example, if a client makes a PUT request on a negotiated resource and
   the origin server accepts that PUT (without redirection), then the
   new set of values for that resource is expected to be consistent with
   the one representation supplied in that PUT; the Content-Location
   cannot be used as a form of reverse content selection that identifies
   only one of the negotiated representations to be updated.  If the
   user agent had wanted the latter semantics, it would have applied the
   PUT directly to the Content-Location URI.




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   A Content-Location field received in a request message is transitory
   information that SHOULD NOT be saved with other representation
   metadata for use in later responses.  The Content-Location's value
   might be saved for use in other contexts, such as within source links
   or other metadata.

   A cache cannot assume that a representation with a Content-Location
   different from the URI used to retrieve it can be used to respond to
   later requests on that Content-Location URI.

3.2.  Representation Data

   The representation data associated with an HTTP message is either
   provided as the payload body of the message or referred to by the
   message semantics and the effective request URI.  The representation
   data is in a format and encoding defined by the representation
   metadata header fields.

   The data type of the representation data is determined via the header
   fields Content-Type and Content-Encoding.  These define a two-layer,
   ordered encoding model:

     representation-data := Content-Encoding( Content-Type( bits ) )

3.3.  Payload Semantics

   Some HTTP messages transfer a complete or partial representation as
   the message "payload".  In some cases, a payload might only contain
   the associated representation's header fields (e.g., responses to
   HEAD) or only some part(s) of the representation data (e.g., the 206
   (Partial Content) status code).

   The purpose of a payload in a request is defined by the method
   semantics.  In a response, the payload's purpose is defined by both
   the request method and the response status code.

   For example, a representation in the payload of a PUT request
   (Section 5.3.4) represents the desired state of the target resource
   if the request is successfully applied, whereas a representation in
   the payload of a POST request (Section 5.3.3) represents an anonymous
   resource for providing data to be processed, such as the information
   that a user entered within an HTML form.

   Likewise, the payload of a 200 (OK) response to GET (Section 5.3.1)
   contains a representation of the target resource, as observed at the
   time of the message origination date (Section 8.1.1.2), whereas the
   same status code in a response to POST might contain either a
   representation of the processing result or a current representation



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   of the target resource after applying the processing.  Response
   messages with an error status code usually contain a representation
   that describes the error and what next steps are suggested for
   resolving it.

   Header fields that specifically describe the payload, rather than the
   associated representation, are referred to as "payload header
   fields".  Payload header fields are defined in other parts of this
   specification, due to their impact on message parsing.

   +-------------------+--------------------------+
   | Header Field Name | Defined in...            |
   +-------------------+--------------------------+
   | Content-Length    | Section 3.3.2 of [Part1] |
   | Content-Range     | Section 5.2 of [Part5]   |
   | Transfer-Encoding | Section 3.3.1 of [Part1] |
   +-------------------+--------------------------+

3.4.  Content Negotiation

   HTTP responses include a representation which contains information
   for interpretation, whether by a human user or for further
   processing.  Often, the server has different ways of representing the
   same information; for example, in different formats, languages, or
   using different character encodings.

   HTTP clients and their users might have different or variable
   capabilities, characteristics or preferences which would influence
   which representation, among those available from the server, would be
   best for the server to deliver.  For this reason, HTTP provides
   mechanisms for "content negotiation" -- a process of allowing
   selection of a representation of a given resource, when more than one
   is available.

   This specification defines two patterns of content negotiation;
   "proactive", where the server selects the representation based upon
   the client's stated preferences, and "reactive" negotiation, where
   the server provides a list of representations for the client to
   choose from, based upon their metadata.  In addition, there are other
   patterns: some applications use an "active content" pattern, where
   the server returns active content which runs on the client and, based
   on client available parameters, selects additional resources to
   invoke.  "Transparent Content Negotiation" ([RFC2295]) has also been
   proposed.

   These patterns are all widely used, and have trade-offs in
   applicability and practicality.  In particular, when the number of
   preferences or capabilities to be expressed by a client are large



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   (such as when many different formats are supported by a user-agent),
   proactive negotiation becomes unwieldy, and might not be appropriate.
   Conversely, when the number of representations to choose from is very
   large, reactive negotiation might not be appropriate.

   Note that, in all cases, the supplier of representations has the
   responsibility for determining which representations might be
   considered to be the "same information".

3.4.1.  Proactive Negotiation

   If the selection of the best representation for a response is made by
   an algorithm located at the server, it is called proactive
   negotiation.  Selection is based on the available representations of
   the response (the dimensions over which it can vary; e.g., language,
   content-coding, etc.) and the contents of particular header fields in
   the request message or on other information pertaining to the request
   (such as the network address of the client).

   Proactive negotiation is advantageous when the algorithm for
   selecting from among the available representations is difficult to
   describe to the user agent, or when the server desires to send its
   "best guess" to the client along with the first response (hoping to
   avoid the round-trip delay of a subsequent request if the "best
   guess" is good enough for the user).  In order to improve the
   server's guess, the user agent MAY include request header fields
   (Accept, Accept-Language, Accept-Encoding, etc.) which describe its
   preferences for such a response.

   Proactive negotiation has disadvantages:

   1.  It is impossible for the server to accurately determine what
       might be "best" for any given user, since that would require
       complete knowledge of both the capabilities of the user agent and
       the intended use for the response (e.g., does the user want to
       view it on screen or print it on paper?).

   2.  Having the user agent describe its capabilities in every request
       can be both very inefficient (given that only a small percentage
       of responses have multiple representations) and a potential
       violation of the user's privacy.

   3.  It complicates the implementation of an origin server and the
       algorithms for generating responses to a request.

   4.  It might limit a public cache's ability to use the same response
       for multiple user's requests.




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   Proactive negotiation allows the user agent to specify its
   preferences, but it cannot expect responses to always honor them.
   For example, the origin server might not implement proactive
   negotiation, or it might decide that sending a response that doesn't
   conform to them is better than sending a 406 (Not Acceptable)
   response.

   HTTP/1.1 includes the following header fields for enabling proactive
   negotiation through description of user agent capabilities and user
   preferences: Accept (Section 6.3.2), Accept-Charset (Section 6.3.3),
   Accept-Encoding (Section 6.3.4), Accept-Language (Section 6.3.5), and
   User-Agent (Section 6.5.3).  However, an origin server is not limited
   to these dimensions and MAY vary the response based on any aspect of
   the request, including aspects of the connection (e.g., IP address)
   or information within extension header fields not defined by this
   specification.

      Note: In practice, User-Agent based negotiation is fragile,
      because new clients might not be recognized.

   The Vary header field (Section 8.2.1) can be used to express the
   parameters the server uses to select a representation that is subject
   to proactive negotiation.

3.4.2.  Reactive Negotiation

   With reactive negotiation, selection of the best representation for a
   response is performed by the user agent after receiving an initial
   response from the origin server.  Selection is based on a list of the
   available representations of the response included within the header
   fields or body of the initial response, with each representation
   identified by its own URI.  Selection from among the representations
   can be performed automatically (if the user agent is capable of doing
   so) or manually by the user selecting from a generated (possibly
   hypertext) menu.

   Reactive negotiation is advantageous when the response would vary
   over commonly-used dimensions (such as type, language, or encoding),
   when the origin server is unable to determine a user agent's
   capabilities from examining the request, and generally when public
   caches are used to distribute server load and reduce network usage.

   Reactive negotiation suffers from the disadvantage of needing a
   second request to obtain the best alternate representation.  This
   second request is only efficient when caching is used.  In addition,
   this specification does not define any mechanism for supporting
   automatic selection, though it also does not prevent any such
   mechanism from being developed as an extension and used within



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   HTTP/1.1.

   This specification defines the 300 (Multiple Choices) and 406 (Not
   Acceptable) status codes for enabling reactive negotiation when the
   server is unwilling or unable to provide a varying response using
   proactive negotiation.

4.  Product Tokens

   Product tokens are used to allow communicating applications to
   identify themselves by software name and version.  Most fields using
   product tokens also allow sub-products which form a significant part
   of the application to be listed, separated by whitespace.  By
   convention, the products are listed in order of their significance
   for identifying the application.

     product         = token ["/" product-version]
     product-version = token

   Examples:

     User-Agent: CERN-LineMode/2.15 libwww/2.17b3
     Server: Apache/0.8.4

   Product tokens SHOULD be short and to the point.  They MUST NOT be
   used for advertising or other non-essential information.  Although
   any token octet MAY appear in a product-version, this token SHOULD
   only be used for a version identifier (i.e., successive versions of
   the same product SHOULD only differ in the product-version portion of
   the product value).

5.  Request Methods

5.1.  Overview

   The request method token is the primary source of request semantics;
   it indicates the purpose for which the client has made this request
   and what is expected by the client as a successful result.  The
   request semantics MAY be further specialized by the semantics of some
   header fields when present in a request (Section 6) if those
   additional semantics do not conflict with the method.

     method = token

   HTTP was originally designed to be usable as an interface to
   distributed object systems.  The request method was envisioned as
   applying semantics to a target resource in much the same way as
   invoking a defined method on an identified object would apply



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   semantics.  The method token is case-sensitive because it might be
   used as a gateway to object-based systems with case-sensitive method
   names.

   Unlike distributed objects, the standardized request methods in HTTP
   are not resource-specific, since uniform interfaces provide for
   better visibility and reuse in network-based systems [REST].  Once
   defined, a standardized method MUST have the same semantics when
   applied to any resource, though each resource determines for itself
   whether those semantics are implemented or allowed.

   This specification defines a number of standardized methods that are
   commonly used in HTTP, as outlined by the following table.  By
   convention, standardized methods are defined in all-uppercase ASCII
   letters.

   +---------+-------------------------------------------------+-------+
   | Method  | Description                                     | Sec.  |
   +---------+-------------------------------------------------+-------+
   | GET     | Transfer a current representation of the target | 5.3.1 |
   |         | resource.                                       |       |
   | HEAD    | Same as GET, but do not include a message body  | 5.3.2 |
   |         | in the response.                                |       |
   | POST    | Perform resource-specific processing on the     | 5.3.3 |
   |         | request payload.                                |       |
   | PUT     | Replace all current representations of the      | 5.3.4 |
   |         | target resource with the request payload.       |       |
   | DELETE  | Remove all current representations of the       | 5.3.5 |
   |         | target resource.                                |       |
   | CONNECT | Establish a tunnel to the server identified by  | 5.3.6 |
   |         | the target resource.                            |       |
   | OPTIONS | Describe the communication options for the      | 5.3.7 |
   |         | target resource.                                |       |
   | TRACE   | Perform a message loop-back test along the path | 5.3.8 |
   |         | to the target resource.                         |       |
   +---------+-------------------------------------------------+-------+

   The methods GET and HEAD MUST be supported by all general-purpose
   servers.  All other methods are OPTIONAL.  When implemented, a server
   MUST implement the above methods according to the semantics defined
   for them in Section 5.3.

   Additional methods MAY be used in HTTP; many have already been
   standardized outside the scope of this specification and registered
   within the HTTP Method Registry maintained by IANA, as defined in
   Section 9.1.

   The set of methods allowed by a target resource can be listed in an



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   Allow header field (Section 8.4.1).  However, the set of allowed
   methods can change dynamically.  When a request message is received
   that is unrecognized or not implemented by an origin server, the
   origin server SHOULD respond with the 501 (Not Implemented) status
   code.  When a request message is received that is known by an origin
   server but not allowed for the target resource, the origin server
   SHOULD respond with the 405 (Method Not Allowed) status code.

5.2.  Common Method Properties

5.2.1.  Safe Methods

   Request methods are considered "safe" if their defined semantics are
   essentially read-only; i.e., the client does not request, and does
   not expect, any state change on the origin server as a result of
   applying a safe method to a target resource.  Likewise, reasonable
   use of a safe method is not expected to cause any harm, loss of
   property, or unusual burden on the origin server.

   This definition of safe methods does not prevent an implementation
   from including behavior that is potentially harmful, not entirely
   read-only, or which causes side-effects while invoking a safe method.
   What is important, however, is that the client did not request that
   additional behavior and cannot be held accountable for it.  For
   example, most servers append request information to access log files
   at the completion of every response, regardless of the method, and
   that is considered safe even though the log storage might become full
   and crash the server.  Likewise, a safe request initiated by
   selecting an advertisement on the Web will often have the side-effect
   of charging an advertising account.

   The GET, HEAD, OPTIONS, and TRACE request methods are defined to be
   safe.

   The purpose of distinguishing between safe and unsafe methods is to
   allow automated retrieval processes (spiders) and cache performance
   optimization (pre-fetching) to work without fear of causing harm.  In
   addition, it allows a user agent to apply appropriate constraints on
   the automated use of unsafe methods when processing potentially
   untrusted content.

   A user agent SHOULD distinguish between safe and unsafe methods when
   presenting potential actions to a user, such that the user can be
   made aware of an unsafe action before it is requested.

   When a resource is constructed such that parameters within the
   effective request URI have the effect of selecting an action, it is
   the resource owner's responsibility to ensure that the action is



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   consistent with the request method semantics.  For example, it is
   common for Web-based content editing software to use actions within
   query parameters, such as "page?do=delete".  If the purpose of such a
   resource is to perform an unsafe action, then the resource MUST
   disable or disallow that action when it is accessed using a safe
   request method.  Failure to do so will result in unfortunate side-
   effects when automated processes perform a GET on every URI reference
   for the sake of link maintenance, pre-fetching, building a search
   index, etc.

5.2.2.  Idempotent Methods

   Request methods are considered "idempotent" if the intended effect of
   multiple identical requests is the same as for a single request.
   PUT, DELETE, and all safe request methods are idempotent.

   Like the definition of safe, the idempotent property only applies to
   what has been requested by the user; a server is free to log each
   request separately, retain a revision control history, or implement
   other non-idempotent side-effects for each idempotent request.

   Idempotent methods are distinguished because the request can be
   repeated automatically if a communication failure occurs before the
   client is able to read the server's response.  For example, if a
   client sends a PUT request and the underlying connection is closed
   before any response is received, then it can establish a new
   connection and retry the idempotent request because it knows that
   repeating the request will have the same effect even if the original
   request succeeded.  Note, however, that repeated failures would
   indicate a problem within the server.

5.2.3.  Cacheable Methods

   Request methods are considered "cacheable" if it is possible and
   useful to answer a current client request with a stored response from
   a prior request.  GET and HEAD are defined to be cacheable.  In
   general, safe methods that do not depend on a current or
   authoritative response are cacheable, though the overwhelming
   majority of caches only support GET and HEAD.  HTTP requirements for
   cache behavior and cacheable responses are defined in [Part6].

5.3.  Method Definitions

5.3.1.  GET

   The GET method requests transfer of a current representation of the
   target resource.




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   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 ([Part4]).  A
   conditional GET requests that the representation be transferred only
   under the circumstances described by the conditional header field(s).
   The conditional GET request 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 ([Part5]).  A partial
   GET requests that only part of the representation be transferred, as
   described in Section 5.4 of [Part5].  The partial GET request is
   intended to reduce unnecessary network usage by allowing partially-
   retrieved representations to be completed without transferring data
   already held by the client.

   A payload within a GET request message has no defined semantics;
   sending a payload body on a GET request might cause some existing
   implementations to reject the request.

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

   See Section 10.2 for security considerations when used for forms.

5.3.2.  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 header fields 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 data.  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.  It also has potential side
   effects on previously stored responses to GET; see Section 5 of
   [Part6].




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   A payload within a HEAD request message has no defined semantics;
   sending a payload body on a HEAD request might cause some existing
   implementations to reject the request.

5.3.3.  POST

   The POST method requests 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 field (see Section 8.1.2).

   Responses to POST requests are only cacheable when they include
   explicit freshness information (see Section 4.1.1 of [Part6]).  A
   cached POST response with a Content-Location header field (see
   Section 3.1.4.2) whose value is the effective Request URI MAY be used
   to satisfy subsequent GET and HEAD (not POST) 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 representation of the resource.







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5.3.4.  PUT

   The PUT method requests that the state of the target resource be
   created or replaced with the state defined by the representation
   enclosed in the request message payload.  A successful PUT of a given
   representation would suggest that a subsequent GET on that same
   target resource will result in an equivalent representation being
   returned in a 200 (OK) response.  However, there is no guarantee that
   such a state change will be observable, since the target resource
   might be acted upon by other user agents in parallel, or might be
   subject to dynamic processing by the origin server, before any
   subsequent GET is received.  A successful response only implies that
   the user agent's intent was achieved at the time of its processing by
   the origin server.

   If the target resource does not have a current representation and the
   PUT successfully creates one, then the origin server MUST inform the
   user agent by sending a 201 (Created) response.  If the target
   resource does have a current representation and that representation
   is successfully modified in accordance with the state of the enclosed
   representation, then either a 200 (OK) or 204 (No Content) response
   SHOULD be sent to indicate successful completion of the request.

   Unrecognized header fields SHOULD be ignored (i.e., not saved as part
   of the resource state).

   An origin server SHOULD verify that the PUT representation is
   consistent with any constraints which the server has for the target
   resource that cannot or will not be changed by the PUT.  This is
   particularly important when the origin server uses internal
   configuration information related to the URI in order to set the
   values for representation metadata on GET responses.  When a PUT
   representation is inconsistent with the target resource, the origin
   server SHOULD either make them consistent, by transforming the
   representation or changing the resource configuration, or respond
   with an appropriate error message containing sufficient information
   to explain why the representation is unsuitable.  The 409 (Conflict)
   or 415 (Unsupported Media Type) status codes are suggested, with the
   latter being specific to constraints on Content-Type values.

   For example, if the target resource is configured to always have a
   Content-Type of "text/html" and the representation being PUT has a
   Content-Type of "image/jpeg", then the origin server SHOULD do one
   of:

   a.  reconfigure the target resource to reflect the new media type;





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   b.  transform the PUT representation to a format consistent with that
       of the resource before saving it as the new resource state; or,

   c.  reject the request with a 415 (Unsupported Media Type) response
       indicating that the target resource is limited to "text/html",
       perhaps including a link to a different resource that would be a
       suitable target for the new representation.

   HTTP does not define exactly how a PUT method affects the state of an
   origin server beyond what can be expressed by the intent of the user
   agent request and the semantics of the origin server response.  It
   does not define what a resource might be, in any sense of that word,
   beyond the interface provided via HTTP.  It does not define how
   resource state is "stored", nor how such storage might change as a
   result of a change in resource state, nor how the origin server
   translates resource state into representations.  Generally speaking,
   all implementation details behind the resource interface are
   intentionally hidden by the server.

   The fundamental difference between the POST and PUT methods is
   highlighted by the different intent for the target resource.  The
   target resource in a POST request is intended to handle the enclosed
   representation as a data-accepting process, such as for a gateway to
   some other protocol or a document that accepts annotations.  In
   contrast, the target resource in a PUT request is intended to take
   the enclosed representation as a new or replacement value.  Hence,
   the intent of PUT is idempotent and visible to intermediaries, even
   though the exact effect is only known by the origin server.

   Proper interpretation of a PUT request presumes that the user agent
   knows what target resource is desired.  A service that is intended to
   select a proper URI on behalf of the client, after receiving a state-
   changing request, SHOULD be implemented using the POST method rather
   than PUT.  If the origin server will not make the requested PUT state
   change to the target resource and instead wishes to have it applied
   to a different resource, such as when the resource has been moved to
   a different URI, then the origin server 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 PUT request applied to the target resource MAY have side-effects on
   other resources.  For example, an article might have a URI for
   identifying "the current version" (a resource) which is separate from
   the URIs identifying each particular version (different resources
   that at one point shared the same state as the current version
   resource).  A successful PUT request on "the current version" URI
   might therefore create a new version resource in addition to changing
   the state of the target resource, and might also cause links to be



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   added between the related resources.

   An origin server SHOULD reject any PUT request that contains a
   Content-Range header field (Section 5.2 of [Part5]), since it might
   be misinterpreted as partial content (or might be partial content
   that is being mistakenly PUT as a full representation).  Partial
   content updates are possible by targeting a separately identified
   resource with state that overlaps a portion of the larger resource,
   or by using a different method that has been specifically defined for
   partial updates (for example, the PATCH method defined in [RFC5789]).

   Responses to the PUT method are not cacheable.  If a PUT request
   passes through a cache that has one or more stored responses for the
   effective request URI, those stored responses will be invalidated
   (see Section 6 of [Part6]).

5.3.5.  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 a
   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.

   A payload within a DELETE request message has no defined semantics;
   sending a payload body on a DELETE request might cause some existing
   implementations to reject the request.

   Responses to the DELETE method are not cacheable.  If a DELETE
   request passes through a cache that has one or more stored responses
   for the effective request URI, those stored responses will be
   invalidated (see Section 6 of [Part6]).

5.3.6.  CONNECT

   The CONNECT method requests that the proxy establish a tunnel to the
   request-target and, if successful, thereafter restrict its behavior
   to blind forwarding of packets until the connection is closed.

   When using CONNECT, the request-target MUST use the authority form



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   (Section 5.3 of [Part1]); i.e., the request-target consists of only
   the host name and port number of the tunnel destination, separated by
   a colon.  For example,

     CONNECT server.example.com:80 HTTP/1.1
     Host: server.example.com:80


   Any 2xx (Successful) response to a CONNECT request indicates that the
   proxy has established a connection to the requested host and port,
   and has switched to tunneling the current connection to that server
   connection.  The tunneled data from the server begins immediately
   after the blank line that concludes the successful response's header
   block.

   A server SHOULD NOT send any Transfer-Encoding or Content-Length
   header fields in a successful response.  A client MUST ignore any
   Content-Length or Transfer-Encoding header fields received in a
   successful response.

   Any response other than a successful response indicates that the
   tunnel has not yet been formed and that the connection remains
   governed by HTTP.

   Proxy authentication might be used to establish the authority to
   create a tunnel:

     CONNECT server.example.com:80 HTTP/1.1
     Host: server.example.com:80
     Proxy-Authorization: basic aGVsbG86d29ybGQ=


   A payload within a CONNECT request message has no defined semantics;
   sending a payload body on a CONNECT request might cause some existing
   implementations to reject the request.

   Similar to a pipelined HTTP/1.1 request, data to be tunneled from
   client to server MAY be sent immediately after the request (before a
   response is received).  The usual caveats also apply: data can be
   discarded if the eventual response is negative, and the connection
   can be reset with no response if more than one TCP segment is
   outstanding.

   It might be the case that the proxy itself can only reach the
   requested origin server through another proxy.  In this case, the
   first proxy SHOULD make a CONNECT request of that next proxy,
   requesting a tunnel to the authority.  A proxy MUST NOT respond with
   any 2xx status code unless it has either a direct or tunnel



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   connection established to the authority.

   If at any point either one of the peers gets disconnected, any
   outstanding data that came from that peer will be passed to the other
   one, and after that also the other connection will be terminated by
   the proxy.  If there is outstanding data to that peer undelivered,
   that data will be discarded.

   An origin server which receives a CONNECT request for itself MAY
   respond with a 2xx status code to indicate that a connection is
   established.  However, most origin servers do not implement CONNECT.

5.3.7.  OPTIONS

   The OPTIONS method requests information about the communication
   options available on the request/response chain identified by the
   effective request URI.  This method allows a 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 the OPTIONS method are not cacheable.

   If the OPTIONS request includes a payload, 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 (Section 5.3 of [Part1]) 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 conformance
   (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 (OK) 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 payload, if any, SHOULD also
   include information about the communication options.  The format for
   such a payload is not defined by this specification, but might be



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   defined by future extensions to HTTP.  Content negotiation MAY be
   used to select the appropriate representation.  If no payload body is
   included, the response MUST include a Content-Length field with a
   field-value of "0".

   The Max-Forwards header field MAY be used to target a specific proxy
   in the request chain (see Section 6.1.1).  If no Max-Forwards field
   is present in the request, then the forwarded request MUST NOT
   include a Max-Forwards field.

5.3.8.  TRACE

   The TRACE method requests a remote, application-level 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 to receive a Max-Forwards value of zero (0)
   in the request (see Section 6.1.1).  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 5.7 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 7.3.1 of [Part1]) and contain a message
   body that encloses a copy of the entire request message.  Responses
   to the TRACE method are not cacheable.

6.  Request Header Fields

   A client sends request header fields to provide more information
   about the request context, make the request conditional based on the
   target resource state, suggest preferred formats for the response,
   supply authentication credentials, or modify the expected request
   processing.  These fields act as request modifiers, similar to the
   parameters on a programming language method invocation.

6.1.  Controls

   Controls are request header fields that direct specific handling of
   the request.





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   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Host              | Section 5.4 of [Part1] |
   | Max-Forwards      | Section 6.1.1          |
   | Expect            | Section 6.1.2          |
   | Range             | Section 5.4 of [Part5] |
   +-------------------+------------------------+

6.1.1.  Max-Forwards

   The "Max-Forwards" header field provides a mechanism with the TRACE
   (Section 5.3.8) and OPTIONS (Section 5.3.7) methods to limit the
   number of times that the request is forwarded by proxies.  This can
   be useful when the client is attempting to trace a request which
   appears to be failing or looping mid-chain.

     Max-Forwards = 1*DIGIT

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

   Each 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 request
   methods.

6.1.2.  Expect

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

     Expect       = 1#expectation

     expectation  = expect-name [ BWS "=" BWS expect-value ]
                                *( OWS ";" [ OWS expect-param ] )
     expect-param = expect-name [ BWS "=" BWS expect-value ]

     expect-name  = token
     expect-value = token / quoted-string

   If all received Expect header field(s) are syntactically valid but



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   contain an expectation that the recipient does not understand or
   cannot comply with, the recipient MUST respond with a 417
   (Expectation Failed) status code.  A recipient of a syntactically
   invalid Expectation header field MUST respond with a 4xx status code
   other than 417.

   The only expectation defined by this specification is:

   100-continue

      The "100-continue" expectation is defined below.  It does not
      support any expect-params.

   Comparison is case-insensitive for names (expect-name), and case-
   sensitive for values (expect-value).

   The Expect mechanism is hop-by-hop: the above requirements apply to
   any server, including proxies.  However, the Expect header field
   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 field.

6.1.2.1.  Use of the 100 (Continue) Status

   The purpose of the 100 (Continue) status code (Section 7.2.1) is to
   allow a client that is sending a request message with a payload to
   determine if the origin server is willing to accept the request
   (based on the request header fields) before the client sends the
   payload body.  In some cases, it might either be inappropriate or
   highly inefficient for the client to send the payload body if the
   server will reject the message without looking at the body.

   Requirements for HTTP/1.1 clients:

   o  If a client will wait for a 100 (Continue) response before sending
      the payload body, it MUST send an Expect header field with the
      "100-continue" expectation.

   o  A client MUST NOT send an Expect header field with the "100-
      continue" expectation if it does not intend to send a payload
      body.

   Because of the presence of older implementations, the protocol allows
   ambiguous situations in which a client might send "Expect: 100-
   continue" without receiving either a 417 (Expectation Failed) or a
   100 (Continue) status code.  Therefore, when a client sends this



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   header field to an origin server (possibly via a proxy) from which it
   has never seen a 100 (Continue) status code, the client SHOULD NOT
   wait for an indefinite period before sending the payload body.

   Requirements for HTTP/1.1 origin servers:

   o  Upon receiving a request which includes an Expect header field
      with the "100-continue" expectation, an origin server MUST either
      respond with 100 (Continue) status code and continue to read from
      the input stream, or respond with a final status code.  The origin
      server MUST NOT wait for the payload body before sending the 100
      (Continue) response.  If it responds with a final status code, it
      MAY close the transport connection or it MAY continue to read and
      discard the rest of the request.  It MUST NOT perform the request
      method if it returns a final status code.

   o  An origin server SHOULD NOT send a 100 (Continue) response if the
      request message does not include an Expect header field with the
      "100-continue" expectation, and MUST NOT send a 100 (Continue)
      response if such a request comes from an HTTP/1.0 (or earlier)
      client.  There is an exception to this rule: for compatibility
      with [RFC2068], a server MAY send a 100 (Continue) status code in
      response to an HTTP/1.1 PUT or POST request that does not include
      an Expect header field with the "100-continue" expectation.  This
      exception, the purpose of which is to minimize any client
      processing delays associated with an undeclared wait for 100
      (Continue) status code, applies only to HTTP/1.1 requests, and not
      to requests with any other HTTP-version value.

   o  An origin server MAY omit a 100 (Continue) response if it has
      already received some or all of the payload body for the
      corresponding request.

   o  An origin server that sends a 100 (Continue) response MUST
      ultimately send a final status code, once the payload body is
      received and processed, unless it terminates the transport
      connection prematurely.

   o  If an origin server receives a request that does not include an
      Expect header field with the "100-continue" expectation, the
      request includes a payload body, and the server responds with a
      final status code before reading the entire payload body from the
      transport connection, then the server SHOULD NOT close the
      transport connection until it has read the entire request, or
      until the client closes the connection.  Otherwise, the client
      might not reliably receive the response message.  However, this
      requirement ought not be construed as preventing a server from
      defending itself against denial-of-service attacks, or from badly



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      broken client implementations.

   Requirements for HTTP/1.1 proxies:

   o  If a proxy receives a request that includes an Expect header field
      with the "100-continue" expectation, and the proxy either knows
      that the next-hop server complies with HTTP/1.1 or higher, or does
      not know the HTTP version of the next-hop server, it MUST forward
      the request, including the Expect header field.

   o  If the proxy knows that the version of the next-hop server is
      HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
      respond with a 417 (Expectation Failed) status code.

   o  Proxies SHOULD maintain a record of the HTTP version numbers
      received from recently-referenced next-hop servers.

   o  A proxy MUST NOT forward a 100 (Continue) response if the request
      message was received from an HTTP/1.0 (or earlier) client and did
      not include an Expect header field with the "100-continue"
      expectation.  This requirement overrides the general rule for
      forwarding of 1xx responses (see Section 7.2.1).

6.2.  Conditionals

   Conditionals are request header fields that indicate a precondition
   to be tested before applying the method semantics to the target
   resource.  Each precondition is based on metadata that is expected to
   change if the selected representation of the target resource is
   changed.  The HTTP/1.1 conditional request mechanisms are defined in
   [Part4].

   +---------------------+------------------------+
   | Header Field Name   | Defined in...          |
   +---------------------+------------------------+
   | If-Match            | Section 3.1 of [Part4] |
   | If-None-Match       | Section 3.2 of [Part4] |
   | If-Modified-Since   | Section 3.3 of [Part4] |
   | If-Unmodified-Since | Section 3.4 of [Part4] |
   | If-Range            | Section 5.3 of [Part5] |
   +---------------------+------------------------+










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6.3.  Content Negotiation

   +-------------------+---------------+
   | Header Field Name | Defined in... |
   +-------------------+---------------+
   | Accept            | Section 6.3.2 |
   | Accept-Charset    | Section 6.3.3 |
   | Accept-Encoding   | Section 6.3.4 |
   | Accept-Language   | Section 6.3.5 |
   +-------------------+---------------+

6.3.1.  Quality Values

   Many of the request header fields for proactive content negotiation
   use a common parameter, named "q" (case-insensitive), to assign a
   relative "weight" to the preference for that associated kind of
   content.  This weight is referred to as a "quality value" (or
   "qvalue") because the same parameter name is often used within server
   configurations to assign a weight to the relative quality of the
   various representations that can be selected for a resource.

   The weight is normalized to a real number in the range 0 through 1,
   where 0.001 is the least preferred and 1 is the most preferred; a
   value of 0 means "not acceptable".  If no "q" parameter is present,
   the default weight is 1.

     weight = OWS ";" OWS "q=" qvalue
     qvalue = ( "0" [ "." 0*3DIGIT ] )
            / ( "1" [ "." 0*3("0") ] )

   A sender of qvalue MUST NOT generate more than three digits after the
   decimal point.  User configuration of these values ought to be
   limited in the same fashion.

6.3.2.  Accept

   The "Accept" header field can be used by user agents to specify
   response media types that are acceptable.  Accept header fields can
   be used to indicate that the request is specifically limited to a
   small set of desired types, as in the case of a request for an in-
   line image.










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     Accept = #( media-range [ accept-params ] )

     media-range    = ( "*/*"
                      / ( type "/" "*" )
                      / ( type "/" subtype )
                      ) *( OWS ";" OWS parameter )
     accept-params  = weight *( accept-ext )
     accept-ext     = OWS ";" OWS token [ "=" word ]

   The asterisk "*" character is used to group media types into ranges,
   with "*/*" indicating all media types and "type/*" indicating all
   subtypes of that type.  The media-range MAY include media type
   parameters that are applicable to that range.

   Each media-range MAY be followed by one or more accept-params,
   beginning with the "q" parameter for indicating a relative weight, as
   defined in Section 6.3.1.  The first "q" parameter (if any) separates
   the media-range parameter(s) from the accept-params.

      Note: Use of the "q" parameter name to separate media type
      parameters from Accept extension parameters is due to historical
      practice.  Although this prevents any media type parameter named
      "q" from being used with a media range, such an event is believed
      to be unlikely given the lack of any "q" parameters in the IANA
      media type registry and the rare usage of any media type
      parameters in Accept.  Future media types are discouraged from
      registering any parameter named "q".

   The example

     Accept: audio/*; q=0.2, audio/basic

   SHOULD be interpreted as "I prefer audio/basic, but send me any audio
   type if it is the best available after an 80% mark-down in quality".

   A request without any Accept header field implies that the user agent
   will accept any media type in response.  If an Accept header field is
   present in a request and none of the available representations for
   the response have a media type that is listed as acceptable, the
   origin server MAY either honor the Accept header field by sending a
   406 (Not Acceptable) response or disregard the Accept header field by
   treating the response as if it is not subject to content negotiation.

   A more elaborate example is

     Accept: text/plain; q=0.5, text/html,
             text/x-dvi; q=0.8, text/x-c




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   Verbally, this would be interpreted as "text/html and text/x-c are
   the preferred media types, but if they do not exist, then send the
   text/x-dvi representation, and if that does not exist, send the text/
   plain representation".

   Media ranges can be overridden by more specific media ranges or
   specific media types.  If more than one media range applies to a
   given type, the most specific reference has precedence.  For example,

     Accept: text/*, text/plain, text/plain;format=flowed, */*

   have the following precedence:

   1.  text/plain;format=flowed

   2.  text/plain

   3.  text/*

   4.  */*

   The media type quality factor associated with a given type is
   determined by finding the media range with the highest precedence
   which matches that type.  For example,

     Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
             text/html;level=2;q=0.4, */*;q=0.5

   would cause the following values to be associated:

   +-------------------+---------------+
   | Media Type        | Quality Value |
   +-------------------+---------------+
   | text/html;level=1 | 1             |
   | text/html         | 0.7           |
   | text/plain        | 0.3           |
   | image/jpeg        | 0.5           |
   | text/html;level=2 | 0.4           |
   | text/html;level=3 | 0.7           |
   +-------------------+---------------+

   Note: A user agent might be provided with a default set of quality
   values for certain media ranges.  However, unless the user agent is a
   closed system which cannot interact with other rendering agents, this
   default set ought to be configurable by the user.






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6.3.3.  Accept-Charset

   The "Accept-Charset" header field can be used by user agents to
   indicate what character encodings are acceptable in a response
   payload.  This field allows clients capable of understanding more
   comprehensive or special-purpose character encodings to signal that
   capability to a server which is capable of representing documents in
   those character encodings.

     Accept-Charset = 1#( ( charset / "*" ) [ weight ] )

   Character encoding values (a.k.a., charsets) are described in
   Section 3.1.1.2.  Each charset MAY be given an associated quality
   value which represents the user's preference for that charset, as
   defined in Section 6.3.1.  An example is

     Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

   The special value "*", if present in the Accept-Charset field,
   matches every character encoding which is not mentioned elsewhere in
   the Accept-Charset field.  If no "*" is present in an Accept-Charset
   field, then any character encodings not explicitly mentioned in the
   field are considered "not acceptable" to the client.

   A request without any Accept-Charset header field implies that the
   user agent will accept any character encoding in response.

   If an Accept-Charset header field is present in a request and none of
   the available representations for the response have a character
   encoding that is listed as acceptable, the origin server MAY either
   honor the Accept-Charset header field by sending a 406 (Not
   Acceptable) response or disregard the Accept-Charset header field by
   treating the response as if it is not subject to content negotiation.

6.3.4.  Accept-Encoding

   The "Accept-Encoding" header field can be used by user agents to
   indicate what response content-codings (Section 3.1.2.1) are
   acceptable in the response.  An "identity" token is used as a synonym
   for "no encoding" in order to communicate when no encoding is
   preferred.

     Accept-Encoding  = #( codings [ weight ] )
     codings          = content-coding / "identity" / "*"

   Each codings value MAY be given an associated quality value which
   represents the preference for that encoding, as defined in
   Section 6.3.1.



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   For example,

     Accept-Encoding: compress, gzip
     Accept-Encoding:
     Accept-Encoding: *
     Accept-Encoding: compress;q=0.5, gzip;q=1.0
     Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

   A server tests whether a content-coding for a given representation is
   acceptable, according to an Accept-Encoding field, using these rules:

   1.  The special "*" symbol in an Accept-Encoding field matches any
       available content-coding not explicitly listed in the header
       field.

   2.  If the representation has no content-coding, then it is
       acceptable by default unless specifically excluded by the Accept-
       Encoding field stating either "identity;q=0" or "*;q=0" without a
       more specific entry for "identity".

   3.  If the representation's content-coding is one of the content-
       codings listed in the Accept-Encoding field, then it is
       acceptable unless it is accompanied by a qvalue of 0.  (As
       defined in Section 6.3.1, a qvalue of 0 means "not acceptable".)

   4.  If multiple content-codings are acceptable, then the acceptable
       content-coding with the highest non-zero qvalue is preferred.

   An Accept-Encoding header field with a combined field-value that is
   empty implies that the user agent does not want any content-coding in
   response.  If an Accept-Encoding header field is present in a request
   and none of the available representations for the response have a
   content-coding that is listed as acceptable, the origin server SHOULD
   send a response without any content-coding.

   A request without an Accept-Encoding header field implies that the
   user agent will accept any content-coding in response.

      Note: Most HTTP/1.0 applications do not recognize or obey qvalues
      associated with content-codings.  This means that qvalues will not
      work and are not permitted with x-gzip or x-compress.

6.3.5.  Accept-Language

   The "Accept-Language" header field can be used by user agents to
   indicate the set of natural languages that are preferred in the
   response.  Language tags are defined in Section 3.1.3.1.




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     Accept-Language = 1#( language-range [ weight ] )
     language-range  =
               <language-range, defined in [RFC4647], Section 2.1>

   Each language-range can be given an associated quality value which
   represents an estimate of the user's preference for the languages
   specified by that range, as defined in Section 6.3.1.  For example,

     Accept-Language: da, en-gb;q=0.8, en;q=0.7

   would mean: "I prefer Danish, but will accept British English and
   other types of English". (see also Section 2.3 of [RFC4647])

   For matching, Section 3 of [RFC4647] defines several matching
   schemes.  Implementations can offer the most appropriate matching
   scheme for their requirements.

      Note: The "Basic Filtering" scheme ([RFC4647], Section 3.3.1) is
      identical to the matching scheme that was previously defined in
      Section 14.4 of [RFC2616].

   It might be contrary to the privacy expectations of the user to send
   an Accept-Language header field with the complete linguistic
   preferences of the user in every request.  For a discussion of this
   issue, see Section 10.5.

   As intelligibility is highly dependent on the individual user, it is
   recommended that client applications make the choice of linguistic
   preference available to the user.  If the choice is not made
   available, then the Accept-Language header field MUST NOT be given in
   the request.

      Note: When making the choice of linguistic preference available to
      the user, we remind implementers of the fact that users are not
      familiar with the details of language matching as described above,
      and ought to be provided appropriate guidance.  As an example,
      users might assume that on selecting "en-gb", they will be served
      any kind of English document if British English is not available.
      A user agent might suggest in such a case to add "en" to get the
      best matching behavior.











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6.4.  Authentication Credentials

   +---------------------+------------------------+
   | Header Field Name   | Defined in...          |
   +---------------------+------------------------+
   | Authorization       | Section 4.1 of [Part7] |
   | Proxy-Authorization | Section 4.3 of [Part7] |
   +---------------------+------------------------+

6.5.  Context

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | From              | Section 6.5.1          |
   | Referer           | Section 6.5.2          |
   | TE                | Section 4.3 of [Part1] |
   | User-Agent        | Section 6.5.3          |
   +-------------------+------------------------+

6.5.1.  From

   The "From" 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]:

     From    = 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 field 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.



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   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.

6.5.2.  Referer

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

   The Referer header field 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 target 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 = 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 10.2 for security considerations.

6.5.3.  User-Agent

   The "User-Agent" header field contains information about the user
   agent originating the request.  User agents SHOULD include this field
   with requests.

   Typically, it is used for statistical purposes, the tracing of
   protocol violations, and tailoring responses to avoid particular user
   agent limitations.

   The field can contain multiple product tokens (Section 4) and
   comments (Section 3.2 of [Part1]) identifying the agent and its
   significant subproducts.  By convention, the product tokens are



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   listed in order of their significance for identifying the
   application.

   Because this field is usually sent on every request a user agent
   makes, implementations are encouraged not to include needlessly fine-
   grained detail, and to limit (or even prohibit) the addition of
   subproducts by third parties.  Overly long and detailed User-Agent
   field values make requests larger and can also be used to identify
   ("fingerprint") the user against their wishes.

   Likewise, implementations are encouraged not to use the product
   tokens of other implementations in order to declare compatibility
   with them, as this circumvents the purpose of the field.  Finally,
   they are encouraged not to use comments to identify products; doing
   so makes the field value more difficult to parse.

     User-Agent = product *( RWS ( product / comment ) )

   Example:

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

7.  Response Status Codes

   The status-code element is a 3-digit integer result code of the
   attempt to understand and satisfy the request.

   HTTP status codes are extensible.  HTTP applications are not required
   to understand the meaning of all registered status codes, though such
   understanding is obviously desirable.  However, applications MUST
   understand the class of any status code, as indicated by the first
   digit, and treat any unrecognized response as being equivalent to the
   x00 status code of that class, with the exception that an
   unrecognized response MUST NOT be cached.  For example, if an
   unrecognized status code of 431 is received by the client, it can
   safely assume that there was something wrong with its request and
   treat the response as if it had received a 400 status code.  In such
   cases, user agents SHOULD present to the user the representation
   enclosed with the response, since that representation is likely to
   include human-readable information which will explain the unusual
   status.

   The first digit of the status-code defines the class of response.
   The last two digits do not have any categorization role.  There are 5
   values for the first digit:

   o  1xx (Informational): Request received, continuing process




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   o  2xx (Successful): The action was successfully received,
      understood, and accepted

   o  3xx (Redirection): Further action needs to be taken in order to
      complete the request

   o  4xx (Client Error): The request contains bad syntax or cannot be
      fulfilled

   o  5xx (Server Error): The server failed to fulfill an apparently
      valid request

   For most status codes the response can carry a payload, in which case
   a Content-Type header field indicates the payload's media type
   (Section 3.1.1.5).

7.1.  Overview of Status Codes

   The status codes listed below are defined in this specification,
   Section 4 of [Part4], Section 3 of [Part5], and Section 3 of [Part7].
   The reason phrases listed here are only recommendations -- they can
   be replaced by local equivalents without affecting the protocol.





























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   +-------------+------------------------------+----------------------+
   | status-code | reason-phrase                | Defined in...        |
   +-------------+------------------------------+----------------------+
   | 100         | Continue                     | Section 7.2.1        |
   | 101         | Switching Protocols          | Section 7.2.2        |
   | 200         | OK                           | Section 7.3.1        |
   | 201         | Created                      | Section 7.3.2        |
   | 202         | Accepted                     | Section 7.3.3        |
   | 203         | Non-Authoritative            | Section 7.3.4        |
   |             | Information                  |                      |
   | 204         | No Content                   | Section 7.3.5        |
   | 205         | Reset Content                | Section 7.3.6        |
   | 206         | Partial Content              | Section 3.1 of       |
   |             |                              | [Part5]              |
   | 300         | Multiple Choices             | Section 7.4.1        |
   | 301         | Moved Permanently            | Section 7.4.2        |
   | 302         | Found                        | Section 7.4.3        |
   | 303         | See Other                    | Section 7.4.4        |
   | 304         | Not Modified                 | Section 4.1 of       |
   |             |                              | [Part4]              |
   | 305         | Use Proxy                    | Section 7.4.5        |
   | 307         | Temporary Redirect           | Section 7.4.7        |
   | 400         | Bad Request                  | Section 7.5.1        |
   | 401         | Unauthorized                 | Section 3.1 of       |
   |             |                              | [Part7]              |
   | 402         | Payment Required             | Section 7.5.2        |
   | 403         | Forbidden                    | Section 7.5.3        |
   | 404         | Not Found                    | Section 7.5.4        |
   | 405         | Method Not Allowed           | Section 7.5.5        |
   | 406         | Not Acceptable               | Section 7.5.6        |
   | 407         | Proxy Authentication         | Section 3.2 of       |
   |             | Required                     | [Part7]              |
   | 408         | Request Time-out             | Section 7.5.7        |
   | 409         | Conflict                     | Section 7.5.8        |
   | 410         | Gone                         | Section 7.5.9        |
   | 411         | Length Required              | Section 7.5.10       |
   | 412         | Precondition Failed          | Section 4.2 of       |
   |             |                              | [Part4]              |
   | 413         | Request Representation Too   | Section 7.5.11       |
   |             | Large                        |                      |
   | 414         | URI Too Long                 | Section 7.5.12       |
   | 415         | Unsupported Media Type       | Section 7.5.13       |
   | 416         | Requested range not          | Section 3.2 of       |
   |             | satisfiable                  | [Part5]              |
   | 417         | Expectation Failed           | Section 7.5.14       |
   | 426         | Upgrade Required             | Section 7.5.15       |
   | 500         | Internal Server Error        | Section 7.6.1        |
   | 501         | Not Implemented              | Section 7.6.2        |



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   | 502         | Bad Gateway                  | Section 7.6.3        |
   | 503         | Service Unavailable          | Section 7.6.4        |
   | 504         | Gateway Time-out             | Section 7.6.5        |
   | 505         | HTTP Version not supported   | Section 7.6.6        |
   +-------------+------------------------------+----------------------+

   Note that this list is not exhaustive -- it does not include
   extension status codes defined in other specifications.

7.2.  Informational 1xx

   This class of status code indicates a provisional response,
   consisting only of the status-line and optional header fields, and is
   terminated by an empty line.  There are no required header fields 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 an "Expect: 100-continue" field when it forwards a
   request, then it need not forward the corresponding 100 (Continue)
   response(s).)

7.2.1.  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 6.1.2.1 for detailed discussion of the use
   and handling of this status code.

7.2.2.  101 Switching Protocols

   The server understands and is willing to comply with the client's
   request, via the Upgrade message header field (Section 6.3 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



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   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.

7.3.  Successful 2xx

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

7.3.1.  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 4.1.2 of [Part6]) to
   determine freshness for 200 responses.

7.3.2.  201 Created

   The request has been fulfilled and has resulted in one or more new
   resources being created.

   Newly created resources are typically linked to from the response
   payload, with the most relevant URI also being carried in the
   Location header field.  If the newly created resource's URI is the
   same as the Effective Request URI, this information can be omitted
   (e.g., in the case of a response to a PUT request).

   The origin server MUST create the resource(s) 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



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   the current value of the entity-tag for the representation of the
   resource identified by the Location header field or, in case the
   Location header field was omitted, by the Effective Request URI (see
   Section 2.3 of [Part4]).

7.3.3.  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.

7.3.4.  203 Non-Authoritative Information

   The representation in the response has been transformed or otherwise
   modified by a transforming proxy (Section 2.3 of [Part1]).  Note that
   the behavior of transforming intermediaries is controlled by the no-
   transform Cache-Control directive (Section 7.2 of [Part6]).

   This status code is only appropriate when the response status code
   would have been 200 (OK) otherwise.  When the status code before
   transformation would have been different, the 214 Transformation
   Applied warn-code (Section 7.5 of [Part6]) is appropriate.

   Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to
   determine freshness for 203 responses.

7.3.5.  204 No Content

   The 204 (No Content) status code indicates that the server has
   successfully fulfilled the request and that there is no additional
   content to return in the response payload body.  Metadata in the
   response header fields refer to the target resource and its current
   representation after the requested action.

   For example, if a 204 status code is received in response to a PUT
   request and the response contains an ETag header field, then the PUT
   was successful and the ETag field-value contains the entity-tag for



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   the new representation of that target resource.

   The 204 response allows a server to indicate that the action has been
   successfully applied to the target resource while implying that the
   user agent SHOULD NOT traverse away from its current "document view"
   (if any).  The server assumes that the user agent will provide some
   indication of the success to its user, in accord with its own
   interface, and apply any new or updated metadata in the response to
   the active representation.

   For example, a 204 status code is commonly used with document editing
   interfaces corresponding to a "save" action, such that the document
   being saved remains available to the user for editing.  It is also
   frequently used with interfaces that expect automated data transfers
   to be prevalent, such as within distributed version control systems.

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

7.3.6.  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 message body included with the response MUST be empty.  Note that
   receivers still need to parse the response according to the algorithm
   defined in Section 3.3 of [Part1].

7.4.  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.  If the
   required action involves a subsequent HTTP request, it 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 5.2.1.

   There are several types of redirects:

   1.  Redirects of the request to another URI, either temporarily or
       permanently.  The new URI is specified in the Location header
       field.  In this specification, the status codes 301 (Moved
       Permanently), 302 (Found), and 307 (Temporary Redirect) fall
       under this category.




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   2.  Redirection to a new location that represents an indirect
       response to the request, such as the result of a POST operation
       to be retrieved with a subsequent GET request.  This is status
       code 303 (See Other).

   3.  Redirection offering a choice of matching resources for use by
       reactive content negotiation (Section 3.4.2).  This is status
       code 300 (Multiple Choices).

   4.  Other kinds of redirection, such as to a cached result (status
       code 304 (Not Modified), see Section 4.1 of [Part4]).

      Note: In HTTP/1.0, only the status codes 301 (Moved Permanently)
      and 302 (Found) were defined for the first type of redirect, and
      the second type did not exist at all ([RFC1945], Section 9.3).
      However it turned out that web forms using POST expected redirects
      to change the operation for the subsequent request to retrieval
      (GET).  To address this use case, HTTP/1.1 introduced the second
      type of redirect with the status code 303 (See Other) ([RFC2068],
      Section 10.3.4).  As user agents did not change their behavior to
      maintain backwards compatibility, the first revision of HTTP/1.1
      added yet another status code, 307 (Temporary Redirect), for which
      the backwards compatibility problems did not apply ([RFC2616],
      Section 10.3.8).  Over 10 years later, most user agents still do
      method rewriting for status codes 301 and 302, therefore this
      specification makes that behavior conformant in case the original
      request was POST.

   A Location header field on a 3xx response indicates that a client MAY
   automatically redirect to the URI provided; see Section 8.1.2.

   Note that for methods not known to be "safe", as defined in
   Section 5.2.1, automatic redirection needs to done with care, since
   the redirect might change the conditions under which the request was
   issued.

   Clients SHOULD detect and intervene in cyclical redirections (i.e.,
   "infinite" redirection loops).

      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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7.4.1.  300 Multiple Choices

   The target resource has more than one representation, each with its
   own specific location, and reactive negotiation information
   (Section 3.4) 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.  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 4.1.2 of [Part6]) to
   determine freshness for 300 responses.

7.4.2.  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 4.1.2 of [Part6]) to
   determine freshness for 301 responses.

   The new permanent URI SHOULD be given by the Location field in the
   response.  A response payload can contain a short hypertext note with
   a hyperlink to the new URI(s).

      Note: For historic reasons, user agents MAY change the request
      method from POST to GET for the subsequent request.  If this
      behavior is undesired, status code 307 (Temporary Redirect) can be
      used instead.








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7.4.3.  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.  A response payload can contain a short hypertext note with
   a hyperlink to the new URI(s).

      Note: For historic reasons, user agents MAY change the request
      method from POST to GET for the subsequent request.  If this
      behavior is undesired, status code 307 (Temporary Redirect) can be
      used instead.

7.4.4.  303 See Other

   The 303 status code indicates that the server is redirecting the user
   agent to a different resource, as indicated by a URI in the Location
   header field, that is intended to provide an indirect response to the
   original request.  In order to satisfy the original request, a user
   agent SHOULD perform a retrieval request using the Location URI (a
   GET or HEAD request if using HTTP), which can itself be redirected
   further, and present the eventual result as an answer to the original
   request.  Note that the new URI in the Location header field is not
   considered equivalent to the effective request URI.

   This 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
   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.




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7.4.5.  305 Use Proxy

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

7.4.6.  306 (Unused)

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

7.4.7.  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.  A response payload can contain a short hypertext note with
   a hyperlink to the new URI(s).

      Note: This status code is similar to 302 (Found), except that it
      does not allow rewriting the request method from POST to GET.
      This specification defines no equivalent counterpart for 301
      (Moved Permanently) ([status-308], however, defines the status
      code 308 (Permanent Redirect) for this purpose).

7.5.  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.

7.5.1.  400 Bad Request

   The server cannot or will not process the request, due to a client
   error (e.g., malformed syntax).

7.5.2.  402 Payment Required

   This code is reserved for future use.







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7.5.3.  403 Forbidden

   The server understood the request, but refuses to authorize it.
   Providing different user authentication credentials might be
   successful, but any credentials that were provided in the request are
   insufficient.  The request SHOULD NOT be repeated with the same
   credentials.

   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) MAY be used instead.

7.5.4.  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.

7.5.5.  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 field
   containing a list of valid methods for the requested resource.

7.5.6.  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 and Accept-* header fields 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.  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.






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      Note: HTTP/1.1 servers are allowed to return responses which are
      not acceptable according to the accept header fields sent in the
      request.  In some cases, this might even be preferable to sending
      a 406 response.  User agents are encouraged to inspect the header
      fields 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.

7.5.7.  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.

7.5.8.  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 payload 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.

7.5.9.  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



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   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 4.1.2 of [Part6]) to
   determine freshness for 410 responses.

7.5.10.  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.

7.5.11.  413 Request Representation 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.

7.5.12.  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 request-target.

7.5.13.  415 Unsupported Media Type

   The server is refusing to service the request because the request
   payload is in a format not supported by this request method on the
   target resource.






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7.5.14.  417 Expectation Failed

   The expectation given in an Expect header field (see Section 6.1.2)
   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.

7.5.15.  426 Upgrade Required

   The request can not be completed without a prior protocol upgrade.
   This response MUST include an Upgrade header field (Section 6.3 of
   [Part1]) specifying the required protocols.

   Example:

     HTTP/1.1 426 Upgrade Required
     Upgrade: HTTP/3.0
     Connection: Upgrade
     Content-Length: 53
     Content-Type: text/plain

     This service requires use of the HTTP/3.0 protocol.

   The server SHOULD include a message body in the 426 response which
   indicates in human readable form the reason for the error and
   describes any alternative courses which might be available to the
   user.

7.6.  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.

7.6.1.  500 Internal Server Error

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

7.6.2.  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



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   any resource.

7.6.3.  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.

7.6.4.  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 field (Section 8.1.3).  If no
   Retry-After is given, the client SHOULD handle the response as it
   would for a 500 (Internal Server Error) response.

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

7.6.5.  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 implementers: some deployed proxies are known to return
      400 (Bad Request) or 500 (Internal Server Error) when DNS lookups
      time out.

7.6.6.  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
   2.6 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.

8.  Response Header Fields

   The response header fields allow the server to pass additional
   information about the response which cannot be placed in the status-



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   line.  These header fields give information about the server and
   about further access to the target resource (Section 5.5 of [Part1]).

8.1.  Control Data

   Response header fields can supply control data that supplements the
   status code or instructs the client where to go next.

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Age               | Section 7.1 of [Part6] |
   | Date              | Section 8.1.1.2        |
   | Location          | Section 8.1.2          |
   | Retry-After       | Section 8.1.3          |
   +-------------------+------------------------+

8.1.1.  Origination Date

8.1.1.1.  Date/Time Formats

   HTTP applications have historically allowed three different formats
   for date/time stamps.  However, the preferred format is a fixed-
   length subset of that defined by [RFC1123]:

     Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 1123

   The other formats are described here only for compatibility with
   obsolete implementations.

     Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
     Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

   HTTP/1.1 clients and servers that parse a date value MUST accept all
   three formats (for compatibility with HTTP/1.0), though they MUST
   only generate the RFC 1123 format for representing HTTP-date values
   in header fields.

   All HTTP date/time stamps MUST be represented in Greenwich Mean Time
   (GMT), without exception.  For the purposes of HTTP, GMT is exactly
   equal to UTC (Coordinated Universal Time).  This is indicated in the
   first two formats by the inclusion of "GMT" as the three-letter
   abbreviation for time zone, and MUST be assumed when reading the
   asctime format.  HTTP-date is case sensitive and MUST NOT include
   additional whitespace beyond that specifically included as SP in the
   grammar.

     HTTP-date    = rfc1123-date / obs-date



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   Preferred format:

     rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
     ; fixed length subset of the format defined in
     ; Section 5.2.14 of [RFC1123]

     day-name     = %x4D.6F.6E ; "Mon", case-sensitive
                  / %x54.75.65 ; "Tue", case-sensitive
                  / %x57.65.64 ; "Wed", case-sensitive
                  / %x54.68.75 ; "Thu", case-sensitive
                  / %x46.72.69 ; "Fri", case-sensitive
                  / %x53.61.74 ; "Sat", case-sensitive
                  / %x53.75.6E ; "Sun", case-sensitive

     date1        = day SP month SP year
                  ; e.g., 02 Jun 1982

     day          = 2DIGIT
     month        = %x4A.61.6E ; "Jan", case-sensitive
                  / %x46.65.62 ; "Feb", case-sensitive
                  / %x4D.61.72 ; "Mar", case-sensitive
                  / %x41.70.72 ; "Apr", case-sensitive
                  / %x4D.61.79 ; "May", case-sensitive
                  / %x4A.75.6E ; "Jun", case-sensitive
                  / %x4A.75.6C ; "Jul", case-sensitive
                  / %x41.75.67 ; "Aug", case-sensitive
                  / %x53.65.70 ; "Sep", case-sensitive
                  / %x4F.63.74 ; "Oct", case-sensitive
                  / %x4E.6F.76 ; "Nov", case-sensitive
                  / %x44.65.63 ; "Dec", case-sensitive
     year         = 4DIGIT

     GMT   = %x47.4D.54 ; "GMT", case-sensitive

     time-of-day  = hour ":" minute ":" second
                    ; 00:00:00 - 23:59:59

     hour         = 2DIGIT
     minute       = 2DIGIT
     second       = 2DIGIT

   The semantics of day-name, day, month, year, and time-of-day are the
   same as those defined for the RFC 5322 constructs with the
   corresponding name ([RFC5322], Section 3.3).

   Obsolete formats:

     obs-date     = rfc850-date / asctime-date



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     rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
     date2        = day "-" month "-" 2DIGIT
                    ; day-month-year (e.g., 02-Jun-82)

     day-name-l   = %x4D.6F.6E.64.61.79 ; "Monday", case-sensitive
            / %x54.75.65.73.64.61.79 ; "Tuesday", case-sensitive
            / %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive
            / %x54.68.75.72.73.64.61.79 ; "Thursday", case-sensitive
            / %x46.72.69.64.61.79 ; "Friday", case-sensitive
            / %x53.61.74.75.72.64.61.79 ; "Saturday", case-sensitive
            / %x53.75.6E.64.61.79 ; "Sunday", case-sensitive


     asctime-date = day-name SP date3 SP time-of-day SP year
     date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))
                    ; month day (e.g., Jun  2)

      Note: Recipients of date values are encouraged to be robust in
      accepting date values that might have been sent by non-HTTP
      applications, as is sometimes the case when retrieving or posting
      messages via proxies/gateways to SMTP or NNTP.

      Note: HTTP requirements for the date/time stamp format apply only
      to their usage within the protocol stream.  Clients and servers
      are not required to use these formats for user presentation,
      request logging, etc.

8.1.1.2.  Date

   The "Date" header field represents the date and time at which the
   message was originated, having the same semantics as the Origination
   Date Field (orig-date) defined in Section 3.6.1 of [RFC5322].  The
   field value is an HTTP-date, as defined in Section 8.1.1.1; it MUST
   be sent in rfc1123-date format.

     Date = HTTP-date

   An example is

     Date: Tue, 15 Nov 1994 08:12:31 GMT

   Origin servers MUST include a Date header field in all responses,
   except in these cases:

   1.  If the response status code is 100 (Continue) or 101 (Switching
       Protocols), the response MAY include a Date header field, at the
       server's option.




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   2.  If the response status code conveys a server error, e.g., 500
       (Internal Server Error) or 503 (Service Unavailable), and it is
       inconvenient or impossible to generate a valid Date.

   3.  If the server does not have a clock that can provide a reasonable
       approximation of the current time, its responses MUST NOT include
       a Date header field.

   A received message that does not have a Date header field MUST be
   assigned one by the recipient if the message will be cached by that
   recipient.

   Clients can use the Date header field as well; in order to keep
   request messages small, they are advised not to include it when it
   doesn't convey any useful information (as is usually the case for
   requests that do not contain a payload).

   The HTTP-date sent in a Date header field SHOULD NOT represent a date
   and time subsequent to the generation of the message.  It SHOULD
   represent the best available approximation of the date and time of
   message generation, unless the implementation has no means of
   generating a reasonably accurate date and time.  In theory, the date
   ought to represent the moment just before the payload is generated.
   In practice, the date can be generated at any time during the message
   origination without affecting its semantic value.

8.1.2.  Location

   The "Location" header field MAY be sent in responses to refer to a
   specific resource in accordance with the semantics of the status
   code.

     Location = URI-reference

   For 201 (Created) responses, the Location is the URI of the new
   resource which was created by the request.  For 3xx (Redirection)
   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).  If the original URI, as navigated to by the
   user agent, did contain a fragment identifier, and the final value
   does not, then the original URI's fragment identifier is added to the
   final value.





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   For example, the original URI "http://www.example.org/~tim", combined
   with a field value given as:

     Location: /pub/WWW/People.html#tim

   would result in a final value of
   "http://www.example.org/pub/WWW/People.html#tim"

   An original URI "http://www.example.org/index.html#larry", combined
   with a field value given as:

     Location: http://www.example.net/index.html

   would result in a final value of
   "http://www.example.net/index.html#larry", preserving the original
   fragment identifier.

      Note: Some recipients attempt to recover from Location fields that
      are not valid URI references.  This specification does not mandate
      or define such processing, but does allow it.

   There are circumstances in which a fragment identifier in a Location
   URI would not be appropriate.  For instance, when it appears in a 201
   (Created) response, where the Location header field specifies the URI
   for the entire created resource.

      Note: The Content-Location header field (Section 3.1.4.2) 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.

8.1.3.  Retry-After

   The 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 to 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 = HTTP-date / delta-seconds

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




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     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.

8.2.  Selected Representation Header Fields

   We use the term "selected representation" to refer to the the current
   representation of a target resource that would have been selected in
   a successful response if the same request had used the method GET and
   excluded any conditional request header fields.

   Additional header fields define metadata about the selected
   representation, which might differ from the representation included
   in the message for responses to some state-changing methods.  The
   following header fields are defined as selected representation
   metadata:

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | ETag              | Section 2.3 of [Part4] |
   | Last-Modified     | Section 2.2 of [Part4] |
   | Vary              | Section 8.2.1          |
   +-------------------+------------------------+

8.2.1.  Vary

   The "Vary" header field conveys the set of header fields that were
   used to select the representation.

   Caches use this information as part of determining whether a stored
   response can be used to satisfy a given request (Section 4.3 of
   [Part6]).

   In uncacheable or stale responses, the Vary field value advises the
   user agent about the criteria that were used to select the
   representation.

     Vary = "*" / 1#field-name

   The set of header fields named by the Vary field value is known as
   the selecting header fields.




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   A server SHOULD include a Vary header field with any cacheable
   response that is subject to proactive negotiation.  Doing so allows a
   cache to properly interpret future requests on that resource and
   informs the user agent about the presence of negotiation on that
   resource.  A server MAY include a Vary header field with a non-
   cacheable response that is subject to proactive negotiation, since
   this might provide the user agent with useful information about the
   dimensions over which the response varies at the time of the
   response.

   A Vary field value of "*" signals that unspecified parameters not
   limited to the header fields (e.g., the network address of the
   client), play a role in the selection of the response representation;
   therefore, a cache cannot determine whether this response is
   appropriate.  A proxy MUST NOT generate the "*" value.

   The field-names given are not limited to the set of standard header
   fields defined by this specification.  Field names are case-
   insensitive.

8.3.  Authentication Challenges

   Authentication challenges indicate what mechanisms are available for
   the client to provide authentication credentials in future requests.

   +--------------------+------------------------+
   | Header Field Name  | Defined in...          |
   +--------------------+------------------------+
   | WWW-Authenticate   | Section 4.4 of [Part7] |
   | Proxy-Authenticate | Section 4.2 of [Part7] |
   +--------------------+------------------------+

8.4.  Informative

   The remaining response header fields provide more information about
   the target resource for potential use in later requests.

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Accept-Ranges     | Section 5.1 of [Part5] |
   | Allow             | Section 8.4.1          |
   | Server            | Section 8.4.2          |
   +-------------------+------------------------+







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8.4.1.  Allow

   The "Allow" 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 request methods associated
   with the resource.

     Allow = #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 -- it does not need to
   understand all the methods specified in order to handle them
   according to the generic message handling rules.

8.4.2.  Server

   The "Server" header field contains information about the software
   used by the origin server to handle the request.

   The field can contain multiple product tokens (Section 4) 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 = 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 header field.  Instead, it
   MUST include a Via field (as described in Section 5.7 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
      implementers are encouraged to make this field a configurable
      option.






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9.  IANA Considerations

9.1.  Method Registry

   The HTTP Method Registry defines the name space for the request
   method token (Section 5).  The method registry is maintained at
   <http://www.iana.org/assignments/http-methods>.

9.1.1.  Procedure

   HTTP method registrations MUST include the following fields:

   o  Method Name (see Section 5)

   o  Safe ("yes" or "no", see Section 5.2.1)

   o  Idempotent ("yes" or "no", see Section 5.2.2)

   o  Pointer to specification text

   Values to be added to this name space require IETF Review (see
   [RFC5226], Section 4.1).

9.1.2.  Considerations for New Methods

   Standardized methods are generic; that is, they are potentially
   applicable to any resource, not just one particular media type, kind
   of resource, or application.  As such, it is preferred that new
   methods be registered in a document that isn't specific to a single
   application or data format, since orthogonal technologies deserve
   orthogonal specification.

   Since message parsing (Section 3.3 of [Part1]) needs to be
   independent of method semantics (aside from responses to HEAD),
   definitions of new methods cannot change the parsing algorithm or
   prohibit the presence of a message body on either the request or the
   response message.  Definitions of new methods can specify that only a
   zero-length message body is allowed by requiring a Content-Length
   header field with a value of "0".

   New method definitions need to indicate whether they are safe
   (Section 5.2.1), idempotent (Section 5.2.2), cacheable
   (Section 5.2.3), and what semantics are to be associated with the
   payload body if any is present in the request.  If a method is
   cacheable, the method definition ought to describe how, and under
   what conditions, a cache can store a response and use it to satisfy a
   subsequent request.




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9.1.3.  Registrations

   The HTTP Method Registry shall be populated with the registrations
   below:

   +---------+------+------------+---------------+
   | Method  | Safe | Idempotent | Reference     |
   +---------+------+------------+---------------+
   | CONNECT | no   | no         | Section 5.3.6 |
   | DELETE  | no   | yes        | Section 5.3.5 |
   | GET     | yes  | yes        | Section 5.3.1 |
   | HEAD    | yes  | yes        | Section 5.3.2 |
   | OPTIONS | yes  | yes        | Section 5.3.7 |
   | POST    | no   | no         | Section 5.3.3 |
   | PUT     | no   | yes        | Section 5.3.4 |
   | TRACE   | yes  | yes        | Section 5.3.8 |
   +---------+------+------------+---------------+

9.2.  Status Code Registry

   The HTTP Status Code Registry defines the name space for the response
   status-code token (Section 7).  The status code registry is
   maintained at <http://www.iana.org/assignments/http-status-codes>.

   This section replaces the registration procedure for HTTP Status
   Codes previously defined in Section 7.1 of [RFC2817].

9.2.1.  Procedure

   Values to be added to the HTTP status code name space require IETF
   Review (see [RFC5226], Section 4.1).

9.2.2.  Considerations for New Status Codes

   When it is necessary to express semantics for a response that are not
   defined by current status codes, a new status code can be registered.
   HTTP status codes are generic; they are potentially applicable to any
   resource, not just one particular media type, "type" of resource, or
   application.  As such, it is preferred that new status codes be
   registered in a document that isn't specific to a single application.

   New status codes are required to fall under one of the categories
   defined in Section 7.  To allow existing parsers to properly handle
   them, new status codes cannot disallow a payload, although they can
   mandate a zero-length payload body.

   A definition for a new status code ought to explain the request
   conditions that produce a response containing that status code (e.g.,



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   combinations of request header fields and/or method(s)) along with
   any dependencies on response header fields (e.g., what fields are
   required and what fields can modify the semantics).  A response that
   can transfer a payload ought to specify expected cache behavior
   (e.g., cacheability and freshness criteria, as described in [Part6])
   and whether the payload has any implied association with an
   identified resource (Section 3.1.4.1).

9.2.3.  Registrations

   The HTTP Status Code Registry shall be updated with the registrations
   below:







































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   +-------+----------------------------------+----------------+
   | Value | Description                      | Reference      |
   +-------+----------------------------------+----------------+
   | 100   | Continue                         | Section 7.2.1  |
   | 101   | Switching Protocols              | Section 7.2.2  |
   | 200   | OK                               | Section 7.3.1  |
   | 201   | Created                          | Section 7.3.2  |
   | 202   | Accepted                         | Section 7.3.3  |
   | 203   | Non-Authoritative Information    | Section 7.3.4  |
   | 204   | No Content                       | Section 7.3.5  |
   | 205   | Reset Content                    | Section 7.3.6  |
   | 300   | Multiple Choices                 | Section 7.4.1  |
   | 301   | Moved Permanently                | Section 7.4.2  |
   | 302   | Found                            | Section 7.4.3  |
   | 303   | See Other                        | Section 7.4.4  |
   | 305   | Use Proxy                        | Section 7.4.5  |
   | 306   | (Unused)                         | Section 7.4.6  |
   | 307   | Temporary Redirect               | Section 7.4.7  |
   | 400   | Bad Request                      | Section 7.5.1  |
   | 402   | Payment Required                 | Section 7.5.2  |
   | 403   | Forbidden                        | Section 7.5.3  |
   | 404   | Not Found                        | Section 7.5.4  |
   | 405   | Method Not Allowed               | Section 7.5.5  |
   | 406   | Not Acceptable                   | Section 7.5.6  |
   | 408   | Request Timeout                  | Section 7.5.7  |
   | 409   | Conflict                         | Section 7.5.8  |
   | 410   | Gone                             | Section 7.5.9  |
   | 411   | Length Required                  | Section 7.5.10 |
   | 413   | Request Representation Too Large | Section 7.5.11 |
   | 414   | URI Too Long                     | Section 7.5.12 |
   | 415   | Unsupported Media Type           | Section 7.5.13 |
   | 417   | Expectation Failed               | Section 7.5.14 |
   | 426   | Upgrade Required                 | Section 7.5.15 |
   | 500   | Internal Server Error            | Section 7.6.1  |
   | 501   | Not Implemented                  | Section 7.6.2  |
   | 502   | Bad Gateway                      | Section 7.6.3  |
   | 503   | Service Unavailable              | Section 7.6.4  |
   | 504   | Gateway Timeout                  | Section 7.6.5  |
   | 505   | HTTP Version Not Supported       | Section 7.6.6  |
   +-------+----------------------------------+----------------+

9.3.  Header Field Registry

   HTTP header fields are registered within the Message Header Field
   Registry located at <http://www.iana.org/assignments/message-headers/
   message-header-index.html>, as defined by [RFC3864].





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9.3.1.  Considerations for New Header Fields

   Header fields are key:value pairs that can be used to communicate
   data about the message, its payload, the target resource, or the
   connection (i.e., control data).  See Section 3.2 of [Part1] for a
   general definition of header field syntax in HTTP messages.

   The requirements for header field names are defined in Section 4.1 of
   [RFC3864].  Authors of specifications defining new fields are advised
   to keep the name as short as practical, and not to prefix them with
   "X-" if they are to be registered (either immediately or in the
   future).

   New header field values typically have their syntax defined using
   ABNF ([RFC5234]), using the extension defined in Appendix B of
   [Part1] as necessary, and are usually constrained to the range of
   ASCII characters.  Header fields needing a greater range of
   characters can use an encoding such as the one defined in [RFC5987].

   Because commas (",") are used as a generic delimiter between field-
   values, they need to be treated with care if they are allowed in the
   field-value's payload.  Typically, components that might contain a
   comma are protected with double-quotes using the quoted-string ABNF
   production (Section 3.2.4 of [Part1]).

   For example, a textual date and a URI (either of which might contain
   a comma) could be safely carried in field-values like these:

     Example-URI-Field: "http://example.com/a.html,foo",
                        "http://without-a-comma.example.com/"
     Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"

   Note that double-quote delimiters almost always are used with the
   quoted-string production; using a different syntax inside double-
   quotes will likely cause unnecessary confusion.

   Many header fields use a format including (case-insensitively) named
   parameters (for instance, Content-Type, defined in Section 3.1.1.5).
   Allowing both unquoted (token) and quoted (quoted-string) syntax for
   the parameter value enables recipients to use existing parser
   components.  When allowing both forms, the meaning of a parameter
   value ought to be independent of the syntax used for it (for an
   example, see the notes on parameter handling for media types in
   Section 3.1.1.1).

   Authors of specifications defining new header fields are advised to
   consider documenting:




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   o  Whether the field is a single value, or whether it can be a list
      (delimited by commas; see Section 3.2 of [Part1]).

      If it does not use the list syntax, document how to treat messages
      where the header field occurs multiple times (a sensible default
      would be to ignore the header field, but this might not always be
      the right choice).

      Note that intermediaries and software libraries might combine
      multiple header field instances into a single one, despite the
      header field not allowing this.  A robust format enables
      recipients to discover these situations (good example: "Content-
      Type", as the comma can only appear inside quoted strings; bad
      example: "Location", as a comma can occur inside a URI).

   o  Under what conditions the header field can be used; e.g., only in
      responses or requests, in all messages, only on responses to a
      particular request method.

   o  Whether it is appropriate to list the field-name in the Connection
      header field (i.e., if the header field is to be hop-by-hop, see
      Section 6.1 of [Part1]).

   o  Under what conditions intermediaries are allowed to modify the
      header field's value, insert or delete it.

   o  How the header field might interact with caching (see [Part6]).

   o  Whether the header field is useful or allowable in trailers (see
      Section 4.1 of [Part1]).

   o  Whether the header field ought to be preserved across redirects.

9.3.2.  Registrations

   The Message Header Field Registry shall be updated with the following
   permanent registrations:














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   +-------------------+----------+----------+-----------------+
   | Header Field Name | Protocol | Status   | Reference       |
   +-------------------+----------+----------+-----------------+
   | Accept            | http     | standard | Section 6.3.2   |
   | Accept-Charset    | http     | standard | Section 6.3.3   |
   | Accept-Encoding   | http     | standard | Section 6.3.4   |
   | Accept-Language   | http     | standard | Section 6.3.5   |
   | Allow             | http     | standard | Section 8.4.1   |
   | Content-Encoding  | http     | standard | Section 3.1.2.2 |
   | Content-Language  | http     | standard | Section 3.1.3.2 |
   | Content-Location  | http     | standard | Section 3.1.4.2 |
   | Content-Type      | http     | standard | Section 3.1.1.5 |
   | Date              | http     | standard | Section 8.1.1.2 |
   | Expect            | http     | standard | Section 6.1.2   |
   | From              | http     | standard | Section 6.5.1   |
   | Location          | http     | standard | Section 8.1.2   |
   | MIME-Version      | http     | standard | Appendix A.1    |
   | Max-Forwards      | http     | standard | Section 6.1.1   |
   | Referer           | http     | standard | Section 6.5.2   |
   | Retry-After       | http     | standard | Section 8.1.3   |
   | Server            | http     | standard | Section 8.4.2   |
   | User-Agent        | http     | standard | Section 6.5.3   |
   | Vary              | http     | standard | Section 8.2.1   |
   +-------------------+----------+----------+-----------------+

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

9.4.  Content Coding Registry

   The HTTP Content Coding Registry defines the name space for content
   coding names (Section 4.2 of [Part1]).  The content coding registry
   is maintained at <http://www.iana.org/assignments/http-parameters>.

9.4.1.  Procedure

   Content Coding registrations MUST include the following fields:

   o  Name

   o  Description

   o  Pointer to specification text

   Names of content codings MUST NOT overlap with names of transfer
   codings (Section 4 of [Part1]), unless the encoding transformation is
   identical (as is the case for the compression codings defined in
   Section 4.2 of [Part1]).



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   Values to be added to this name space require IETF Review (see
   Section 4.1 of [RFC5226]), and MUST conform to the purpose of content
   coding defined in this section.

9.4.2.  Registrations

   The HTTP Content Codings Registry shall be updated with the
   registrations below:

   +----------+----------------------------------------+---------------+
   | Name     | Description                            | Reference     |
   +----------+----------------------------------------+---------------+
   | compress | UNIX "compress" program method         | Section 4.2.1 |
   |          |                                        | of [Part1]    |
   | deflate  | "deflate" compression mechanism        | Section 4.2.2 |
   |          | ([RFC1951]) used inside the "zlib"     | of [Part1]    |
   |          | data format ([RFC1950])                |               |
   | gzip     | Same as GNU zip [RFC1952]              | Section 4.2.3 |
   |          |                                        | of [Part1]    |
   | identity | reserved (synonym for "no encoding" in | Section 6.3.4 |
   |          | Accept-Encoding header field)          |               |
   +----------+----------------------------------------+---------------+

10.  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.

10.1.  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.  Implementers 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



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   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 field allows reading patterns to be studied and
   reverse links drawn.  Although it can be very useful, its power can
   be abused if user details are not separated from the information
   contained in the Referer.  Even when the personal information has
   been removed, the Referer header field 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 6.5.3) or Server (Section 8.4.2) header
   fields can sometimes be used to determine that a specific client or
   server has 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.

   Furthermore, the User-Agent header field might contain enough entropy
   to be used, possibly in conjunction with other material, to uniquely
   identify the user.

   Some request methods, like TRACE (Section 5.3.8), expose information
   that was sent in request header fields within the body of their
   response.  Clients SHOULD be careful with sensitive information, like
   Cookies, Authorization credentials, and other header fields that
   might be used to collect data from the client.

10.2.  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.




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   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.

10.3.  Location Header Fields: Spoofing and Information Leakage

   If a single server supports multiple organizations that do not trust
   one another, then it MUST check the values of Location and Content-
   Location header fields 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.

   Furthermore, appending the fragment identifier from one URI to
   another one obtained from a Location header field might leak
   confidential information to the target server -- although the
   fragment identifier is not transmitted in the final request, it might
   be visible to the user agent through other means, such as scripting.

10.4.  Security Considerations for CONNECT

   Since tunneled data is opaque to the proxy, there are additional
   risks to tunneling to other well-known or reserved ports.  A HTTP
   client CONNECTing to port 25 could relay spam via SMTP, for example.
   As such, proxies SHOULD restrict CONNECT access to a small number of
   known ports.

10.5.  Privacy Issues Connected to Accept Header Fields

   Accept header fields can reveal information about the user to all
   servers which are accessed.  The Accept-Language header field in
   particular can reveal information the user would consider to be of a
   private nature, because the understanding of particular languages is
   often strongly correlated to the membership of a particular ethnic
   group.  User agents which offer the option to configure the contents
   of an Accept-Language header field to be sent in every request are
   strongly encouraged to let the configuration process include a
   message which makes the user aware of the loss of privacy involved.

   An approach that limits the loss of privacy would be for a user agent
   to omit the sending of Accept-Language header fields by default, and
   to ask the user whether or not to start sending Accept-Language



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   header fields to a server if it detects, by looking for any Vary
   header fields generated by the server, that such sending could
   improve the quality of service.

   Elaborate user-customized accept header fields sent in every request,
   in particular if these include quality values, can be used by servers
   as relatively reliable and long-lived user identifiers.  Such user
   identifiers would allow content providers to do click-trail tracking,
   and would allow collaborating content providers to match cross-server
   click-trails or form submissions of individual users.  Note that for
   many users not behind a proxy, the network address of the host
   running the user agent will also serve as a long-lived user
   identifier.  In environments where proxies are used to enhance
   privacy, user agents ought to be conservative in offering accept
   header field configuration options to end users.  As an extreme
   privacy measure, proxies could filter the accept header fields in
   relayed requests.  General purpose user agents which provide a high
   degree of header field configurability SHOULD warn users about the
   loss of privacy which can be involved.

11.  Acknowledgments

   See Section 9 of [Part1].

12.  References

12.1.  Normative References

   [Part1]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Message Syntax and
                 Routing", draft-ietf-httpbis-p1-messaging-21 (work in
                 progress), October 2012.

   [Part4]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Conditional Requests",
                 draft-ietf-httpbis-p4-conditional-21 (work in
                 progress), October 2012.

   [Part5]       Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
                 "Hypertext Transfer Protocol (HTTP/1.1): Range
                 Requests", draft-ietf-httpbis-p5-range-21 (work in
                 progress), October 2012.

   [Part6]       Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
                 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                 draft-ietf-httpbis-p6-cache-21 (work in progress),
                 October 2012.




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   [Part7]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Authentication",
                 draft-ietf-httpbis-p7-auth-21 (work in progress),
                 October 2012.

   [RFC1950]     Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data
                 Format Specification version 3.3", RFC 1950, May 1996.

   [RFC1951]     Deutsch, P., "DEFLATE Compressed Data Format
                 Specification version 1.3", RFC 1951, May 1996.

   [RFC1952]     Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and
                 G. Randers-Pehrson, "GZIP file format specification
                 version 4.3", RFC 1952, May 1996.

   [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of Internet
                 Message Bodies", RFC 2045, November 1996.

   [RFC2046]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Two: Media Types",
                 RFC 2046, November 1996.

   [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",
                 STD 66, RFC 3986, January 2005.

   [RFC4647]     Phillips, A., Ed. and M. Davis, Ed., "Matching of
                 Language Tags", BCP 47, RFC 4647, September 2006.

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

   [RFC5646]     Phillips, A., Ed. and M. Davis, Ed., "Tags for
                 Identifying Languages", BCP 47, RFC 5646,
                 September 2009.

12.2.  Informative References

   [REST]        Fielding, R., "Architectural Styles and the Design of
                 Network-based Software Architectures", Doctoral
                 Dissertation, University of California, Irvine ,
                 September 2000,
                 <http://roy.gbiv.com/pubs/dissertation/top.htm>.



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   [RFC1123]     Braden, R., "Requirements for Internet Hosts -
                 Application and Support", STD 3, RFC 1123,
                 October 1989.

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

   [RFC2049]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Five: Conformance Criteria
                 and Examples", RFC 2049, November 1996.

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

   [RFC2076]     Palme, J., "Common Internet Message Headers", RFC 2076,
                 February 1997.

   [RFC2277]     Alvestrand, H., "IETF Policy on Character Sets and
                 Languages", BCP 18, RFC 2277, January 1998.

   [RFC2295]     Holtman, K. and A. Mutz, "Transparent Content
                 Negotiation in HTTP", RFC 2295, March 1998.

   [RFC2388]     Masinter, L., "Returning Values from Forms:  multipart/
                 form-data", RFC 2388, August 1998.

   [RFC2557]     Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
                 "MIME Encapsulation of Aggregate Documents, such as
                 HTML (MHTML)", RFC 2557, March 1999.

   [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.

   [RFC3629]     Yergeau, F., "UTF-8, a transformation format of ISO
                 10646", STD 63, RFC 3629, November 2003.

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

   [RFC4288]     Freed, N. and J. Klensin, "Media Type Specifications
                 and Registration Procedures", BCP 13, RFC 4288,



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                 December 2005.

   [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.

   [RFC5789]     Dusseault, L. and J. Snell, "PATCH Method for HTTP",
                 RFC 5789, March 2010.

   [RFC5987]     Reschke, J., "Character Set and Language Encoding for
                 Hypertext Transfer Protocol (HTTP) Header Field
                 Parameters", RFC 5987, August 2010.

   [RFC6151]     Turner, S. and L. Chen, "Updated Security
                 Considerations for the MD5 Message-Digest and the HMAC-
                 MD5 Algorithms", RFC 6151, March 2011.

   [RFC6266]     Reschke, J., "Use of the Content-Disposition Header
                 Field in the Hypertext Transfer Protocol (HTTP)",
                 RFC 6266, June 2011.

   [status-308]  Reschke, J., "The Hypertext Transfer Protocol (HTTP)
                 Status Code 308 (Permanent Redirect)",
                 draft-reschke-http-status-308-07 (work in progress),
                 March 2012.

Appendix A.  Differences between HTTP and MIME

   HTTP/1.1 uses many of the constructs defined for Internet Mail
   ([RFC5322]) and the Multipurpose Internet Mail Extensions (MIME
   [RFC2045]) to allow a message body to be transmitted in an open
   variety of representations and with extensible mechanisms.  However,
   RFC 2045 discusses mail, and HTTP has a few features that are
   different from those described in MIME.  These differences were
   carefully chosen to optimize performance over binary connections, to
   allow greater freedom in the use of new media types, to make date
   comparisons easier, and to acknowledge the practice of some early
   HTTP servers and clients.

   This appendix describes specific areas where HTTP differs from MIME.
   Proxies and gateways to strict MIME environments SHOULD be aware of
   these differences and provide the appropriate conversions where
   necessary.  Proxies and gateways from MIME environments to HTTP also
   need to be aware of the differences because some conversions might be
   required.



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A.1.  MIME-Version

   HTTP is not a MIME-compliant protocol.  However, HTTP/1.1 messages
   MAY include a single MIME-Version header field to indicate what
   version of the MIME protocol was used to construct the message.  Use
   of the MIME-Version header field indicates that the message is in
   full conformance with the MIME protocol (as defined in [RFC2045]).
   Proxies/gateways are responsible for ensuring full conformance (where
   possible) when exporting HTTP messages to strict MIME environments.

     MIME-Version = 1*DIGIT "." 1*DIGIT

   MIME version "1.0" is the default for use in HTTP/1.1.  However,
   HTTP/1.1 message parsing and semantics are defined by this document
   and not the MIME specification.

A.2.  Conversion to Canonical Form

   MIME requires that an Internet mail body-part be converted to
   canonical form prior to being transferred, as described in Section 4
   of [RFC2049].  Section 3.1.1.3 of this document describes the forms
   allowed for subtypes of the "text" media type when transmitted over
   HTTP.  [RFC2046] requires that content with a type of "text"
   represent line breaks as CRLF and forbids the use of CR or LF outside
   of line break sequences.  HTTP allows CRLF, bare CR, and bare LF to
   indicate a line break within text content when a message is
   transmitted over HTTP.

   Where it is possible, a proxy or gateway from HTTP to a strict MIME
   environment SHOULD translate all line breaks within the text media
   types described in Section 3.1.1.3 of this document to the RFC 2049
   canonical form of CRLF.  Note, however, that this might be
   complicated by the presence of a Content-Encoding and by the fact
   that HTTP allows the use of some character encodings which do not use
   octets 13 and 10 to represent CR and LF, respectively, as is the case
   for some multi-byte character encodings.

   Conversion will break any cryptographic checksums applied to the
   original content unless the original content is already in canonical
   form.  Therefore, the canonical form is recommended for any content
   that uses such checksums in HTTP.

A.3.  Conversion of Date Formats

   HTTP/1.1 uses a restricted set of date formats (Section 8.1.1.1) to
   simplify the process of date comparison.  Proxies and gateways from
   other protocols SHOULD ensure that any Date header field present in a
   message conforms to one of the HTTP/1.1 formats and rewrite the date



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   if necessary.

A.4.  Introduction of Content-Encoding

   MIME does not include any concept equivalent to HTTP/1.1's Content-
   Encoding header field.  Since this acts as a modifier on the media
   type, proxies and gateways from HTTP to MIME-compliant protocols MUST
   either change the value of the Content-Type header field or decode
   the representation before forwarding the message.  (Some experimental
   applications of Content-Type for Internet mail have used a media-type
   parameter of ";conversions=<content-coding>" to perform a function
   equivalent to Content-Encoding.  However, this parameter is not part
   of the MIME standards).

A.5.  No Content-Transfer-Encoding

   HTTP does not use the Content-Transfer-Encoding field of MIME.
   Proxies and gateways from MIME-compliant protocols to HTTP MUST
   remove any Content-Transfer-Encoding prior to delivering the response
   message to an HTTP client.

   Proxies and gateways from HTTP to MIME-compliant protocols are
   responsible for ensuring that the message is in the correct format
   and encoding for safe transport on that protocol, where "safe
   transport" is defined by the limitations of the protocol being used.
   Such a proxy or gateway SHOULD label the data with an appropriate
   Content-Transfer-Encoding if doing so will improve the likelihood of
   safe transport over the destination protocol.

A.6.  MHTML and Line Length Limitations

   HTTP implementations which share code with MHTML [RFC2557]
   implementations need to be aware of MIME line length limitations.
   Since HTTP does not have this limitation, HTTP does not fold long
   lines.  MHTML messages being transported by HTTP follow all
   conventions of MHTML, including line length limitations and folding,
   canonicalization, etc., since HTTP transports all message-bodies as
   payload (see Section 3.1.1.4) and does not interpret the content or
   any MIME header lines that might be contained therein.

Appendix B.  Additional Features

   [RFC1945] and [RFC2068] document protocol elements used by some
   existing HTTP implementations, but not consistently and correctly
   across most HTTP/1.1 applications.  Implementers are advised to be
   aware of these features, but cannot rely upon their presence in, or
   interoperability with, other HTTP/1.1 applications.  Some of these
   describe proposed experimental features, and some describe features



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   that experimental deployment found lacking that are now addressed in
   the base HTTP/1.1 specification.

   A number of other header fields, such as Content-Disposition and
   Title, from SMTP and MIME are also often implemented (see [RFC6266]
   and [RFC2076]).

Appendix C.  Changes from RFC 2616

   Remove base URI setting semantics for "Content-Location" due to poor
   implementation support, which was caused by too many broken servers
   emitting bogus Content-Location header fields, and also the
   potentially undesirable effect of potentially breaking relative links
   in content-negotiated resources.  (Section 3.1.4.2)

   Clarify definition of POST.  (Section 5.3.3)

   Remove requirement to handle all Content-* header fields; ban use of
   Content-Range with PUT.  (Section 5.3.4)

   Take over definition of CONNECT method from [RFC2817].
   (Section 5.3.6)

   Restrict "Max-Forwards" header field to OPTIONS and TRACE
   (previously, extension methods could have used it as well).
   (Section 6.1.1)

   The ABNF for the "Expect" header field has been both fixed (allowing
   parameters for value-less expectations as well) and simplified
   (allowing trailing semicolons after "100-continue" when they were
   invalid before).  (Section 6.1.2)

   Remove ISO-8859-1 special-casing in Accept-Charset.  (Section 6.3.3)

   Allow "Referer" field value of "about:blank" as alternative to not
   specifying it.  (Section 6.5.2)

   Broadened the definition of 203 (Non-Authoritative Information) to
   include cases of payload transformations as well.  (Section 7.3.4)

   Status codes 301, 302, and 307: removed the normative requirements on
   both response payloads and user interaction.  (Section 7.4)

   Failed to consider that there are many other request methods that are
   safe to automatically redirect, and further that the user agent is
   able to make that determination based on the request method
   semantics.  Furthermore, allow user agents to rewrite the method from
   POST to GET for status codes 301 and 302.  (Sections 7.4.2, 7.4.3 and



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   7.4.7)

   Deprecate 305 (Use Proxy) status code, because user agents did not
   implement it.  It used to indicate that the target resource needs to
   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 7.4.5)

   Define status 426 (Upgrade Required) (this was incorporated from
   [RFC2817]).  (Section 7.5.15)

   Correct syntax of "Location" header field 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 8.1.2)

   Reclassify "Allow" as response header field, removing the option to
   specify it in a PUT request.  Relax the server requirement on the
   contents of the Allow header field and remove requirement on clients
   to always trust the header field value.  (Section 8.4.1)

   In the description of the "Server" header field, the "Via" field was
   described as a SHOULD.  The requirement was and is stated correctly
   in the description of the Via header field in Section 5.7 of [Part1].
   (Section 8.4.2)

   Clarify contexts that charset is used in.  (Section 3.1.1.2)

   Remove the default character encoding of "ISO-8859-1" for text media
   types; the default now is whatever the media type definition says.
   (Section 3.1.1.3)

   Registration of Content Codings now requires IETF Review
   (Section 9.4)

   Remove definition of "Content-MD5 header" field because it was
   inconsistently implemented with respect to partial responses, and
   also because of known deficiencies in the hash algorithm itself (see
   [RFC6151] for details).

   Introduce Method Registry.  (Section 9.1)

   Take over the Status Code Registry, previously defined in Section 7.1
   of [RFC2817].  (Section 9.2)

   Remove reference to non-existant identity transfer-coding value
   tokens.  (Appendix A.5)



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   Remove discussion of Content-Disposition header field, it is now
   defined by [RFC6266].  (Appendix B)

Appendix D.  Imported ABNF

   The following core rules are included by reference, as defined in
   Appendix B.1 of [RFC5234]: 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), HTAB (horizontal tab), LF
   (line feed), OCTET (any 8-bit sequence of data), SP (space), and
   VCHAR (any visible US-ASCII character).

   The rules below are defined in [Part1]:

     BWS           = <BWS, defined in [Part1], Section 3.2.1>
     OWS           = <OWS, defined in [Part1], Section 3.2.1>
     RWS           = <RWS, defined in [Part1], Section 3.2.1>
     URI-reference = <URI-reference, defined in [Part1], Section 2.7>
     absolute-URI  = <absolute-URI, defined in [Part1], Section 2.7>
     comment       = <comment, defined in [Part1], Section 3.2.4>
     field-name    = <comment, defined in [Part1], Section 3.2>
     partial-URI   = <partial-URI, defined in [Part1], Section 2.7>
     quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
     token         = <token, defined in [Part1], Section 3.2.4>
     word          = <word, defined in [Part1], Section 3.2.4>

Appendix E.  Collected ABNF

   Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
    OWS ( media-range [ accept-params ] ) ] ) ]
   Accept-Charset = *( "," OWS ) ( ( charset / "*" ) [ weight ] ) *( OWS
    "," [ OWS ( ( charset / "*" ) [ weight ] ) ] )
   Accept-Encoding = [ ( "," / ( codings [ weight ] ) ) *( OWS "," [ OWS
    ( codings [ weight ] ) ] ) ]
   Accept-Language = *( "," OWS ) ( language-range [ weight ] ) *( OWS
    "," [ OWS ( language-range [ weight ] ) ] )
   Allow = [ ( "," / method ) *( OWS "," [ OWS method ] ) ]

   BWS = <BWS, defined in [Part1], Section 3.2.1>

   Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
    content-coding ] )
   Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
    language-tag ] )
   Content-Location = absolute-URI / partial-URI
   Content-Type = media-type

   Date = HTTP-date



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   Expect = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )

   From = mailbox

   GMT = %x47.4D.54 ; GMT

   HTTP-date = rfc1123-date / obs-date

   Location = URI-reference

   MIME-Version = 1*DIGIT "." 1*DIGIT
   Max-Forwards = 1*DIGIT

   OWS = <OWS, defined in [Part1], Section 3.2.1>

   RWS = <RWS, defined in [Part1], Section 3.2.1>
   Referer = absolute-URI / partial-URI
   Retry-After = HTTP-date / delta-seconds

   Server = product *( RWS ( product / comment ) )

   URI-reference = <URI-reference, defined in [Part1], Section 2.7>
   User-Agent = product *( RWS ( product / comment ) )

   Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
    ) )

   absolute-URI = <absolute-URI, defined in [Part1], Section 2.7>
   accept-ext = OWS ";" OWS token [ "=" word ]
   accept-params = weight *accept-ext
   asctime-date = day-name SP date3 SP time-of-day SP year
   attribute = token

   charset = token
   codings = content-coding / "identity" / "*"
   comment = <comment, defined in [Part1], Section 3.2.4>
   content-coding = token

   date1 = day SP month SP year
   date2 = day "-" month "-" 2DIGIT
   date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
   day = 2DIGIT









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   day-name = %x4D.6F.6E ; Mon
    / %x54.75.65 ; Tue
    / %x57.65.64 ; Wed
    / %x54.68.75 ; Thu
    / %x46.72.69 ; Fri
    / %x53.61.74 ; Sat
    / %x53.75.6E ; Sun
   day-name-l = %x4D.6F.6E.64.61.79 ; Monday
    / %x54.75.65.73.64.61.79 ; Tuesday
    / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
    / %x54.68.75.72.73.64.61.79 ; Thursday
    / %x46.72.69.64.61.79 ; Friday
    / %x53.61.74.75.72.64.61.79 ; Saturday
    / %x53.75.6E.64.61.79 ; Sunday
   delta-seconds = 1*DIGIT

   expect-name = token
   expect-param = expect-name [ BWS "=" BWS expect-value ]
   expect-value = token / quoted-string
   expectation = expect-name [ BWS "=" BWS expect-value ] *( OWS ";" [
    OWS expect-param ] )

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

   hour = 2DIGIT

   language-range = <language-range, defined in [RFC4647], Section 2.1>
   language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

   mailbox = <mailbox, defined in [RFC5322], Section 3.4>
   media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
    ";" OWS parameter )
   media-type = type "/" subtype *( OWS ";" OWS parameter )
   method = token
   minute = 2DIGIT
   month = %x4A.61.6E ; Jan
    / %x46.65.62 ; Feb
    / %x4D.61.72 ; Mar
    / %x41.70.72 ; Apr
    / %x4D.61.79 ; May
    / %x4A.75.6E ; Jun
    / %x4A.75.6C ; Jul
    / %x41.75.67 ; Aug
    / %x53.65.70 ; Sep
    / %x4F.63.74 ; Oct
    / %x4E.6F.76 ; Nov
    / %x44.65.63 ; Dec




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   obs-date = rfc850-date / asctime-date

   parameter = attribute "=" value
   partial-URI = <partial-URI, defined in [Part1], Section 2.7>
   product = token [ "/" product-version ]
   product-version = token

   quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
   qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )

   rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
   rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT

   second = 2DIGIT
   subtype = token

   time-of-day = hour ":" minute ":" second
   token = <token, defined in [Part1], Section 3.2.4>
   type = token

   value = word

   weight = OWS ";" OWS "q=" qvalue
   word = <word, defined in [Part1], Section 3.2.4>

   year = 4DIGIT

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

F.1.  Since RFC 2616

   Extracted relevant partitions from [RFC2616].

F.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>)





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   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]

F.3.  Since draft-ietf-httpbis-p3-payload-00

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/8>: "Media Type
      Registrations" (<http://purl.org/NET/http-errata#media-reg>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/14>: "Clarification
      regarding quoting of charset values"
      (<http://purl.org/NET/http-errata#charactersets>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/16>: "Remove
      'identity' token references"
      (<http://purl.org/NET/http-errata#identity>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/25>: "Accept-
      Encoding BNF"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/46>: "RFC1700
      references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/55>: "Updating to
      RFC4288"





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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
      Reference"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/68>: "Encoding
      References Normative"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
      to-date references"

F.4.  Since draft-ietf-httpbis-p2-semantics-01

   Closed issues:

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

   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 field.

   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.

F.5.  Since draft-ietf-httpbis-p3-payload-01

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

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

F.6.  Since draft-ietf-httpbis-p2-semantics-02

   Closed issues:

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



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   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 field"

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

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

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

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

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

F.7.  Since draft-ietf-httpbis-p3-payload-02

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/67>: "Quoting
      Charsets"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/115>: "missing
      default for qvalue in description of Accept-Encoding"

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

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




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F.8.  Since draft-ietf-httpbis-p2-semantics-03

   Closed issues:

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

F.9.  Since draft-ietf-httpbis-p3-payload-03

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/67>: "Quoting
      Charsets"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/113>: "language tag
      matching (Accept-Language) vs RFC4647"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/121>: "RFC 1806 has
      been replaced by RFC2183"

   Other changes:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/68>: "Encoding
      References Normative" -- rephrase the annotation and reference
      BCP97.

F.10.  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"




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   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
      field value format definitions.

F.11.  Since draft-ietf-httpbis-p3-payload-04

   Closed issues:

   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
      field value format definitions.

F.12.  Since draft-ietf-httpbis-p2-semantics-05

   Closed issues:

   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.

F.13.  Since draft-ietf-httpbis-p3-payload-05

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/118>: "Join
      "Differences Between HTTP Entities and RFC 2045 Entities"?"



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   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.

   Other changes:

   o  Move definition of quality values into Part 1.

F.14.  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"

F.15.  Since draft-ietf-httpbis-p3-payload-06

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/80>: "Content-
      Location isn't special"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content
      Sniffing"

F.16.  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"



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   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 field ABNF should use 'URI'"

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

   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?"

F.17.  Since draft-ietf-httpbis-p3-payload-07

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/13>: "Updated
      reference for language tags"

   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/154>: "Content-
      Location base-setting problems"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content
      Sniffing"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/188>: "pick IANA
      policy (RFC5226) for Transfer Coding / Content Coding"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/189>: "move
      definitions of gzip/deflate/compress to part 1"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/148>: "update IANA
      requirements wrt Transfer-Coding values" (add the IANA
      Considerations subsection)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/149>: "update IANA
      requirements wrt Content-Coding values" (add the IANA
      Considerations subsection)



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F.18.  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)

F.19.  Since draft-ietf-httpbis-p3-payload-08

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/81>: "Content
      Negotiation for media types"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/181>: "Accept-
      Language: which RFC4647 filtering?"

F.20.  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 field payload handling"

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

F.21.  Since draft-ietf-httpbis-p3-payload-09

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/122>: "MIME-Version
      not listed in P1, general header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/143>: "IANA registry
      for content/transfer encodings"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content
      Sniffing"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/200>: "use of term
      "word" when talking about header field structure"



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   Partly resolved issues:

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

F.22.  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"

   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"

F.23.  Since draft-ietf-httpbis-p3-payload-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/80>: "Content-
      Location isn't special"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/90>: "Delimiting
      messages with multipart/byteranges"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/136>: "confusing
      req. language for Content-Location"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-
      Location on 304 responses"



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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/183>: "'requested
      resource' in content-encoding definition"

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

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/178>: "Content-MD5
      and partial responses"

F.24.  Since draft-ietf-httpbis-p2-semantics-11

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/229>:
      "Considerations for new status codes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/230>:
      "Considerations for new methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/232>: "User-Agent
      guidelines" (relating to the 'User-Agent' header field)

F.25.  Since draft-ietf-httpbis-p3-payload-11

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/123>: "Factor out
      Content-Disposition"

F.26.  Since draft-ietf-httpbis-p2-semantics-12

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment
      combination / precedence during redirects" (added warning about
      having a fragid on the redirect might cause inconvenience in some
      cases)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/79>: "Content-* vs.
      PUT"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/88>: "205 Bodies"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/102>: "Understanding
      Content-* on non-PUT requests"




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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/104>: "Header field
      type defaulting"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/137>: "duplicate
      ABNF for reason-phrase"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/180>: "Note special
      status of Content-* prefix in header field registration
      procedures"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/203>: "Max-Forwards
      vs extension methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/213>: "What is the
      value space of HTTP status codes?" (actually fixed in
      draft-ietf-httpbis-p2-semantics-11)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header Field
      Classification"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/225>: "PUT side
      effect: invalidation or just stale?"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/226>: "proxies not
      supporting certain methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/239>: "Migrate
      CONNECT from RFC2817 to p2"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/240>: "Migrate
      Upgrade details from RFC2817"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/267>: "clarify PUT
      semantics'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/275>: "duplicate
      ABNF for 'Method'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
      ABNFs for header fields"






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F.27.  Since draft-ietf-httpbis-p3-payload-12

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header Field
      Classification"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
      ABNFs for header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/277>: "potentially
      misleading MAY in media-type def"

F.28.  Since draft-ietf-httpbis-p2-semantics-13

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
      ABNFs for header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/251>: "message body
      in CONNECT request"

F.29.  Since draft-ietf-httpbis-p3-payload-13

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/20>: "Default
      charsets for text media types"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/178>: "Content-MD5
      and partial responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
      ABNFs for header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/281>: "confusing
      undefined parameter in media range example"

F.30.  Since draft-ietf-httpbis-p2-semantics-14

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/255>: "Clarify
      status code for rate limiting"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/294>: "clarify 403
      forbidden"



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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/296>: "Clarify 203
      Non-Authoritative Information"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/298>: "update
      default reason phrase for 413"

F.31.  Since draft-ietf-httpbis-p3-payload-14

   None.

F.32.  Since draft-ietf-httpbis-p2-semantics-15

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/285>: "Strength of
      requirements on Accept re: 406"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/303>: "400 response
      isn't generic"

F.33.  Since draft-ietf-httpbis-p3-payload-15

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/285>: "Strength of
      requirements on Accept re: 406"

F.34.  Since draft-ietf-httpbis-p2-semantics-16

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/160>: "Redirects and
      non-GET methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document
      HTTP's error-handling philosophy"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/231>:
      "Considerations for new header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/310>: "clarify 303
      redirect on HEAD"

F.35.  Since draft-ietf-httpbis-p3-payload-16

   Closed issues:





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document
      HTTP's error-handling philosophy"

F.36.  Since draft-ietf-httpbis-p2-semantics-17

   Closed issues:

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/255>: "Clarify
      status code for rate limiting" (change backed out because a new
      status code is being defined for this purpose)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/312>: "should there
      be a permanent variant of 307"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/325>: "When are
      Location's semantics triggered?"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/327>: "'expect'
      grammar missing OWS"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/329>: "header field
      considerations: quoted-string vs use of double quotes"

F.37.  Since draft-ietf-httpbis-p3-payload-17

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/323>: "intended
      maturity level vs normative references"

F.38.  Since draft-ietf-httpbis-p2-semantics-18

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/227>: "Combining
      HEAD responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/238>: "Requirements
      for user intervention during redirects"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/250>: "message-body
      in CONNECT response"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/295>: "Applying
      original fragment to 'plain' redirected URI"



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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/302>: "Misplaced
      text on connection handling in p2"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/331>: "clarify that
      201 doesn't require Location header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/332>: "relax
      requirements on hypertext in 3/4/5xx error responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/333>: "example for
      426 response should have a payload"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/336>: "drop
      indirection entries for status codes"

F.39.  Since draft-ietf-httpbis-p3-payload-18

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/330>: "is ETag a
      representation header field?"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/338>: "Content-
      Location doesn't constrain the cardinality of representations"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/346>: "make IANA
      policy definitions consistent"

F.40.  Since draft-ietf-httpbis-p2-semantics-19 and
       draft-ietf-httpbis-p3-payload-19

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/312>: "should there
      be a permanent variant of 307"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/347>: "clarify that
      201 can imply *multiple* resources were created"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/351>: "merge P2 and
      P3"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF
      requirements for recipients"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/364>: "Capturing
      more information in the method registry"




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/368>: "note
      introduction of new IANA registries as normative changes"

F.41.  Since draft-ietf-httpbis-p2-semantics-20

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/378>: "is 'q=' case-
      sensitive?"

   Other changes:

   o  Conformance criteria and considerations regarding error handling
      are now defined in Part 1.

   o  Properly explain what HTTP semantics are and why.  Rewrite
      introductory description of methods.  Rewrite definition of "safe"
      to be more operable and weaken the original same-origin
      restrictions to be more consistent with modern UAs.  Rewrite
      definition of "idempotent", add definition of "cacheable".

   o  Conneg terminology change: "server-driven" => "proactive" (UA
      sends Accept* fields), "agent-driven" => "reactive" (UA waits for
      300/Alternatives)

   o  Move description of "100-continue" from Part 1 over here.

   o  Move definition of "Vary" header field from Part 6 over here.

   o  Rewrite definition of "representation".

Index

   1
      1xx Informational (status code class)  49

   2
      2xx Successful (status code class)  50

   3
      3xx Redirection (status code class)  52

   4
      4xx Client Error (status code class)  56

   5
      5xx Server Error (status code class)  60




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   1
      100 Continue (status code)  49
      100-continue (expect value)  35
      101 Switching Protocols (status code)  49

   2
      200 OK (status code)  50
      201 Created (status code)  50
      202 Accepted (status code)  51
      203 Non-Authoritative Information (status code)  51
      204 No Content (status code)  51
      205 Reset Content (status code)  52

   3
      300 Multiple Choices (status code)  54
      301 Moved Permanently (status code)  54
      302 Found (status code)  55
      303 See Other (status code)  55
      305 Use Proxy (status code)  56
      306 (Unused) (status code)  56
      307 Temporary Redirect (status code)  56

   4
      400 Bad Request (status code)  56
      402 Payment Required (status code)  56
      403 Forbidden (status code)  57
      404 Not Found (status code)  57
      405 Method Not Allowed (status code)  57
      406 Not Acceptable (status code)  57
      408 Request Timeout (status code)  58
      409 Conflict (status code)  58
      410 Gone (status code)  58
      411 Length Required (status code)  59
      413 Request Representation Too Large (status code)  59
      414 URI Too Long (status code)  59
      415 Unsupported Media Type (status code)  59
      417 Expectation Failed (status code)  60
      426 Upgrade Required (status code)  60

   5
      500 Internal Server Error (status code)  60
      501 Not Implemented (status code)  60
      502 Bad Gateway (status code)  61
      503 Service Unavailable (status code)  61
      504 Gateway Timeout (status code)  61
      505 HTTP Version Not Supported (status code)  61

   A



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      Accept header field  38
      Accept-Charset header field  41
      Accept-Encoding header field  41
      Accept-Language header field  42
      Allow header field  69

   C
      cacheable  25
      compress (content coding)  12
      CONNECT method  30
      content coding  12
      content negotiation  7
      Content-Encoding header field  12
      Content-Language header field  14
      Content-Location header field  16
      Content-Transfer-Encoding header field  85
      Content-Type header field  11

   D
      Date header field  64
      deflate (content coding)  12
      DELETE method  30

   E
      Expect header field  34
      Expect Values
         100-continue  35

   F
      From header field  44

   G
      GET method  25
      Grammar
         Accept  39
         Accept-Charset  41
         Accept-Encoding  41
         accept-ext  39
         Accept-Language  43
         accept-params  39
         Allow  69
         asctime-date  64
         attribute  9
         charset  10
         codings  41
         content-coding  12
         Content-Encoding  13
         Content-Language  14



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         Content-Location  16
         Content-Type  11
         Date  64
         date1  63
         day  63
         day-name  63
         day-name-l  63
         delta-seconds  66
         Expect  34
         expect-name  34
         expect-param  34
         expect-value  34
         expectation  34
         From  44
         GMT  63
         hour  63
         HTTP-date  62
         language-range  43
         language-tag  14
         Location  65
         Max-Forwards  34
         media-range  39
         media-type  9
         method  22
         MIME-Version  84
         minute  63
         month  63
         obs-date  63
         parameter  9
         product  22
         product-version  22
         qvalue  38
         Referer  45
         Retry-After  66
         rfc850-date  64
         rfc1123-date  63
         second  63
         Server  69
         subtype  9
         time-of-day  63
         type  9
         User-Agent  46
         value  9
         Vary  67
         weight  38
         year  63
      gzip (content coding)  12




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   H
      HEAD method  26

   I
      idempotent  25

   L
      Location header field  65

   M
      Max-Forwards header field  34
      MIME-Version header field  84

   O
      OPTIONS method  32

   P
      payload  18
      POST method  27
      PUT method  28

   R
      Referer header field  45
      representation  8
      Retry-After header field  66

   S
      safe  24
      selected representation  67
      Server header field  69
      Status Codes Classes
         1xx Informational  49
         2xx Successful  50
         3xx Redirection  52
         4xx Client Error  56
         5xx Server Error  60

   T
      TRACE method  33

   U
      User-Agent header field  45

   V
      Vary header field  67

   X
      x-compress (content coding)  12



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      x-gzip (content coding)  12

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


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

   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/





























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