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Versions: (draft-nottingham-httpbis-header-registry) 00 01 02 03 04 05 06 07 08 09 10 11 12

HTTP Working Group                                      R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2818, 7230, 7231, 7232, 7233, 7235,        M. Nottingham, Ed.
           7538, 7615, 7694 (if approved)                         Fastly
Intended status: Standards Track                         J. Reschke, Ed.
Expires: April 5, 2021                                        greenbytes
                                                         October 2, 2020


                             HTTP Semantics
                    draft-ietf-httpbis-semantics-12

Abstract

   The Hypertext Transfer Protocol (HTTP) is a stateless application-
   level protocol for distributed, collaborative, hypertext information
   systems.  This document defines the semantics of HTTP: its
   architecture, terminology, the "http" and "https" Uniform Resource
   Identifier (URI) schemes, core request methods, request header
   fields, response status codes, response header fields, and content
   negotiation.

   This document obsoletes RFC 2818, RFC 7231, RFC 7232, RFC 7233, RFC
   7235, RFC 7538, RFC 7615, RFC 7694, and portions of RFC 7230.

Editorial Note

   This note is to be removed before publishing as an RFC.

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

   Working Group information can be found at <https://httpwg.org/>;
   source code and issues list for this draft can be found at
   <https://github.com/httpwg/http-core>.

   The changes in this draft are summarized in Appendix C.13.

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 https://datatracker.ietf.org/drafts/current/.



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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on April 5, 2021.

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   Copyright (c) 2020 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 (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   9
     1.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . .   9
     1.2.  Evolution . . . . . . . . . . . . . . . . . . . . . . . .   9
     1.3.  Semantics . . . . . . . . . . . . . . . . . . . . . . . .  10
     1.4.  Obsoletes . . . . . . . . . . . . . . . . . . . . . . . .  11
   2.  Conformance . . . . . . . . . . . . . . . . . . . . . . . . .  12
     2.1.  Syntax Notation . . . . . . . . . . . . . . . . . . . . .  12
     2.2.  Requirements Notation . . . . . . . . . . . . . . . . . .  12
     2.3.  Length Requirements . . . . . . . . . . . . . . . . . . .  13
     2.4.  Error Handling  . . . . . . . . . . . . . . . . . . . . .  14
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  14
     3.1.  Resources . . . . . . . . . . . . . . . . . . . . . . . .  14
     3.2.  Connections . . . . . . . . . . . . . . . . . . . . . . .  15



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     3.3.  Messages  . . . . . . . . . . . . . . . . . . . . . . . .  15
     3.4.  User Agent  . . . . . . . . . . . . . . . . . . . . . . .  15
     3.5.  Origin Server . . . . . . . . . . . . . . . . . . . . . .  16
     3.6.  Example Request and Response  . . . . . . . . . . . . . .  16
     3.7.  Intermediaries  . . . . . . . . . . . . . . . . . . . . .  17
     3.8.  Caches  . . . . . . . . . . . . . . . . . . . . . . . . .  19
   4.  Identifiers . . . . . . . . . . . . . . . . . . . . . . . . .  20
     4.1.  URI References  . . . . . . . . . . . . . . . . . . . . .  20
     4.2.  URI Schemes . . . . . . . . . . . . . . . . . . . . . . .  21
       4.2.1.  http URI Scheme . . . . . . . . . . . . . . . . . . .  22
       4.2.2.  https URI Scheme  . . . . . . . . . . . . . . . . . .  22
       4.2.3.  http(s) Normalization and Comparison  . . . . . . . .  23
       4.2.4.  http(s) Deprecated userinfo . . . . . . . . . . . . .  24
       4.2.5.  http(s) References with Fragment Identifiers  . . . .  24
     4.3.  Authoritative Access  . . . . . . . . . . . . . . . . . .  24
       4.3.1.  URI Origin  . . . . . . . . . . . . . . . . . . . . .  24
       4.3.2.  http origins  . . . . . . . . . . . . . . . . . . . .  25
       4.3.3.  https origins . . . . . . . . . . . . . . . . . . . .  26
       4.3.4.  https certificate verification  . . . . . . . . . . .  27
   5.  Message Abstraction . . . . . . . . . . . . . . . . . . . . .  28
     5.1.  Protocol Version  . . . . . . . . . . . . . . . . . . . .  28
     5.2.  Framing . . . . . . . . . . . . . . . . . . . . . . . . .  30
     5.3.  Control Data  . . . . . . . . . . . . . . . . . . . . . .  30
       5.3.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  30
       5.3.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  30
     5.4.  Header Fields . . . . . . . . . . . . . . . . . . . . . .  30
       5.4.1.  Field Ordering and Combination  . . . . . . . . . . .  32
       5.4.2.  Field Limits  . . . . . . . . . . . . . . . . . . . .  33
       5.4.3.  Field Names . . . . . . . . . . . . . . . . . . . . .  33
       5.4.4.  Field Values  . . . . . . . . . . . . . . . . . . . .  33
     5.5.  Payload . . . . . . . . . . . . . . . . . . . . . . . . .  35
       5.5.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . .  35
       5.5.2.  Identification  . . . . . . . . . . . . . . . . . . .  36
       5.5.3.  Payload Metadata  . . . . . . . . . . . . . . . . . .  37
       5.5.4.  Payload Body  . . . . . . . . . . . . . . . . . . . .  37
     5.6.  Trailer Fields  . . . . . . . . . . . . . . . . . . . . .  37
       5.6.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . .  38
       5.6.2.  Limitations . . . . . . . . . . . . . . . . . . . . .  38
       5.6.3.  Processing  . . . . . . . . . . . . . . . . . . . . .  39
     5.7.  Common Rules for Defining Field Values  . . . . . . . . .  39
       5.7.1.  Lists (#rule ABNF Extension)  . . . . . . . . . . . .  39
       5.7.2.  Tokens  . . . . . . . . . . . . . . . . . . . . . . .  41
       5.7.3.  Whitespace  . . . . . . . . . . . . . . . . . . . . .  41
       5.7.4.  Quoted Strings  . . . . . . . . . . . . . . . . . . .  42
       5.7.5.  Comments  . . . . . . . . . . . . . . . . . . . . . .  42
       5.7.6.  Parameters  . . . . . . . . . . . . . . . . . . . . .  43
       5.7.7.  Date/Time Formats . . . . . . . . . . . . . . . . . .  43
   6.  Routing . . . . . . . . . . . . . . . . . . . . . . . . . . .  45



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     6.1.  Target Resource . . . . . . . . . . . . . . . . . . . . .  45
       6.1.1.  Request Target  . . . . . . . . . . . . . . . . . . .  45
       6.1.2.  Host  . . . . . . . . . . . . . . . . . . . . . . . .  46
       6.1.3.  Reconstructing the Target URI . . . . . . . . . . . .  47
     6.2.  Routing Inbound . . . . . . . . . . . . . . . . . . . . .  47
       6.2.1.  To a Cache  . . . . . . . . . . . . . . . . . . . . .  47
       6.2.2.  To a Proxy  . . . . . . . . . . . . . . . . . . . . .  48
       6.2.3.  To the Origin . . . . . . . . . . . . . . . . . . . .  48
     6.3.  Response Correlation  . . . . . . . . . . . . . . . . . .  48
     6.4.  Message Forwarding  . . . . . . . . . . . . . . . . . . .  48
       6.4.1.  Connection  . . . . . . . . . . . . . . . . . . . . .  49
       6.4.2.  Max-Forwards  . . . . . . . . . . . . . . . . . . . .  50
       6.4.3.  Via . . . . . . . . . . . . . . . . . . . . . . . . .  51
     6.5.  Transformations . . . . . . . . . . . . . . . . . . . . .  53
     6.6.  Upgrade . . . . . . . . . . . . . . . . . . . . . . . . .  54
   7.  Representations . . . . . . . . . . . . . . . . . . . . . . .  56
     7.1.  Selected Representation . . . . . . . . . . . . . . . . .  57
     7.2.  Data  . . . . . . . . . . . . . . . . . . . . . . . . . .  57
     7.3.  Metadata  . . . . . . . . . . . . . . . . . . . . . . . .  57
     7.4.  Content-Type  . . . . . . . . . . . . . . . . . . . . . .  58
       7.4.1.  Media Type  . . . . . . . . . . . . . . . . . . . . .  59
       7.4.2.  Charset . . . . . . . . . . . . . . . . . . . . . . .  59
       7.4.3.  Canonicalization and Text Defaults  . . . . . . . . .  60
       7.4.4.  Multipart Types . . . . . . . . . . . . . . . . . . .  61
     7.5.  Content-Encoding  . . . . . . . . . . . . . . . . . . . .  61
       7.5.1.  Content Codings . . . . . . . . . . . . . . . . . . .  62
     7.6.  Content-Language  . . . . . . . . . . . . . . . . . . . .  63
       7.6.1.  Language Tags . . . . . . . . . . . . . . . . . . . .  64
     7.7.  Content-Length  . . . . . . . . . . . . . . . . . . . . .  65
     7.8.  Content-Location  . . . . . . . . . . . . . . . . . . . .  66
     7.9.  Validators  . . . . . . . . . . . . . . . . . . . . . . .  68
       7.9.1.  Weak versus Strong  . . . . . . . . . . . . . . . . .  69
       7.9.2.  Last-Modified . . . . . . . . . . . . . . . . . . . .  71
       7.9.3.  ETag  . . . . . . . . . . . . . . . . . . . . . . . .  73
       7.9.4.  When to Use Entity-Tags and Last-Modified Dates . . .  76
   8.  Methods . . . . . . . . . . . . . . . . . . . . . . . . . . .  77
     8.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  77
     8.2.  Common Method Properties  . . . . . . . . . . . . . . . .  78
       8.2.1.  Safe Methods  . . . . . . . . . . . . . . . . . . . .  79
       8.2.2.  Idempotent Methods  . . . . . . . . . . . . . . . . .  80
       8.2.3.  Methods and Caching . . . . . . . . . . . . . . . . .  81
     8.3.  Method Definitions  . . . . . . . . . . . . . . . . . . .  81
       8.3.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  81
       8.3.2.  HEAD  . . . . . . . . . . . . . . . . . . . . . . . .  82
       8.3.3.  POST  . . . . . . . . . . . . . . . . . . . . . . . .  83
       8.3.4.  PUT . . . . . . . . . . . . . . . . . . . . . . . . .  84
       8.3.5.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  87
       8.3.6.  CONNECT . . . . . . . . . . . . . . . . . . . . . . .  88



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       8.3.7.  OPTIONS . . . . . . . . . . . . . . . . . . . . . . .  89
       8.3.8.  TRACE . . . . . . . . . . . . . . . . . . . . . . . .  90
   9.  Context . . . . . . . . . . . . . . . . . . . . . . . . . . .  91
     9.1.  Request Context . . . . . . . . . . . . . . . . . . . . .  91
       9.1.1.  Expect  . . . . . . . . . . . . . . . . . . . . . . .  92
       9.1.2.  From  . . . . . . . . . . . . . . . . . . . . . . . .  94
       9.1.3.  Referer . . . . . . . . . . . . . . . . . . . . . . .  95
       9.1.4.  TE  . . . . . . . . . . . . . . . . . . . . . . . . .  96
       9.1.5.  Trailer . . . . . . . . . . . . . . . . . . . . . . .  96
       9.1.6.  User-Agent  . . . . . . . . . . . . . . . . . . . . .  97
     9.2.  Response Context  . . . . . . . . . . . . . . . . . . . .  98
       9.2.1.  Allow . . . . . . . . . . . . . . . . . . . . . . . .  98
       9.2.2.  Date  . . . . . . . . . . . . . . . . . . . . . . . .  99
       9.2.3.  Location  . . . . . . . . . . . . . . . . . . . . . . 100
       9.2.4.  Retry-After . . . . . . . . . . . . . . . . . . . . . 101
       9.2.5.  Server  . . . . . . . . . . . . . . . . . . . . . . . 102
   10. Authentication  . . . . . . . . . . . . . . . . . . . . . . . 102
     10.1.  Authentication Scheme  . . . . . . . . . . . . . . . . . 102
     10.2.  Authentication Parameters  . . . . . . . . . . . . . . . 103
     10.3.  Challenge and Response . . . . . . . . . . . . . . . . . 103
     10.4.  Credentials  . . . . . . . . . . . . . . . . . . . . . . 104
     10.5.  Protection Space (Realm) . . . . . . . . . . . . . . . . 105
     10.6.  Authenticating User to Origin Server . . . . . . . . . . 106
       10.6.1.  WWW-Authenticate . . . . . . . . . . . . . . . . . . 106
       10.6.2.  Authorization  . . . . . . . . . . . . . . . . . . . 107
       10.6.3.  Authentication-Info  . . . . . . . . . . . . . . . . 107
     10.7.  Authenticating Client to Proxy . . . . . . . . . . . . . 108
       10.7.1.  Proxy-Authenticate . . . . . . . . . . . . . . . . . 108
       10.7.2.  Proxy-Authorization  . . . . . . . . . . . . . . . . 108
       10.7.3.  Proxy-Authentication-Info  . . . . . . . . . . . . . 109
   11. Content Negotiation . . . . . . . . . . . . . . . . . . . . . 109
     11.1.  Proactive Negotiation  . . . . . . . . . . . . . . . . . 110
       11.1.1.  Shared Negotiation Features  . . . . . . . . . . . . 111
       11.1.2.  Accept . . . . . . . . . . . . . . . . . . . . . . . 113
       11.1.3.  Accept-Charset . . . . . . . . . . . . . . . . . . . 115
       11.1.4.  Accept-Encoding  . . . . . . . . . . . . . . . . . . 116
       11.1.5.  Accept-Language  . . . . . . . . . . . . . . . . . . 117
     11.2.  Reactive Negotiation . . . . . . . . . . . . . . . . . . 119
       11.2.1.  Vary . . . . . . . . . . . . . . . . . . . . . . . . 120
     11.3.  Request Payload Negotiation  . . . . . . . . . . . . . . 121
   12. Conditional Requests  . . . . . . . . . . . . . . . . . . . . 121
     12.1.  Preconditions  . . . . . . . . . . . . . . . . . . . . . 122
       12.1.1.  If-Match . . . . . . . . . . . . . . . . . . . . . . 122
       12.1.2.  If-None-Match  . . . . . . . . . . . . . . . . . . . 124
       12.1.3.  If-Modified-Since  . . . . . . . . . . . . . . . . . 125
       12.1.4.  If-Unmodified-Since  . . . . . . . . . . . . . . . . 127
       12.1.5.  If-Range . . . . . . . . . . . . . . . . . . . . . . 128
     12.2.  Evaluation . . . . . . . . . . . . . . . . . . . . . . . 129



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     12.3.  Precedence . . . . . . . . . . . . . . . . . . . . . . . 130
   13. Range Requests  . . . . . . . . . . . . . . . . . . . . . . . 131
     13.1.  Range Units  . . . . . . . . . . . . . . . . . . . . . . 132
       13.1.1.  Range Specifiers . . . . . . . . . . . . . . . . . . 133
       13.1.2.  Byte Ranges  . . . . . . . . . . . . . . . . . . . . 134
     13.2.  Range  . . . . . . . . . . . . . . . . . . . . . . . . . 135
     13.3.  Accept-Ranges  . . . . . . . . . . . . . . . . . . . . . 137
     13.4.  Content-Range  . . . . . . . . . . . . . . . . . . . . . 137
     13.5.  Media Type multipart/byteranges  . . . . . . . . . . . . 139
   14. Status Codes  . . . . . . . . . . . . . . . . . . . . . . . . 141
     14.1.  Overview of Status Codes . . . . . . . . . . . . . . . . 142
     14.2.  Informational 1xx  . . . . . . . . . . . . . . . . . . . 142
       14.2.1.  100 Continue . . . . . . . . . . . . . . . . . . . . 142
       14.2.2.  101 Switching Protocols  . . . . . . . . . . . . . . 143
     14.3.  Successful 2xx . . . . . . . . . . . . . . . . . . . . . 143
       14.3.1.  200 OK . . . . . . . . . . . . . . . . . . . . . . . 143
       14.3.2.  201 Created  . . . . . . . . . . . . . . . . . . . . 144
       14.3.3.  202 Accepted . . . . . . . . . . . . . . . . . . . . 144
       14.3.4.  203 Non-Authoritative Information  . . . . . . . . . 145
       14.3.5.  204 No Content . . . . . . . . . . . . . . . . . . . 145
       14.3.6.  205 Reset Content  . . . . . . . . . . . . . . . . . 146
       14.3.7.  206 Partial Content  . . . . . . . . . . . . . . . . 146
     14.4.  Redirection 3xx  . . . . . . . . . . . . . . . . . . . . 149
       14.4.1.  300 Multiple Choices . . . . . . . . . . . . . . . . 152
       14.4.2.  301 Moved Permanently  . . . . . . . . . . . . . . . 153
       14.4.3.  302 Found  . . . . . . . . . . . . . . . . . . . . . 153
       14.4.4.  303 See Other  . . . . . . . . . . . . . . . . . . . 154
       14.4.5.  304 Not Modified . . . . . . . . . . . . . . . . . . 154
       14.4.6.  305 Use Proxy  . . . . . . . . . . . . . . . . . . . 155
       14.4.7.  306 (Unused) . . . . . . . . . . . . . . . . . . . . 155
       14.4.8.  307 Temporary Redirect . . . . . . . . . . . . . . . 155
       14.4.9.  308 Permanent Redirect . . . . . . . . . . . . . . . 156
     14.5.  Client Error 4xx . . . . . . . . . . . . . . . . . . . . 156
       14.5.1.  400 Bad Request  . . . . . . . . . . . . . . . . . . 156
       14.5.2.  401 Unauthorized . . . . . . . . . . . . . . . . . . 156
       14.5.3.  402 Payment Required . . . . . . . . . . . . . . . . 157
       14.5.4.  403 Forbidden  . . . . . . . . . . . . . . . . . . . 157
       14.5.5.  404 Not Found  . . . . . . . . . . . . . . . . . . . 157
       14.5.6.  405 Method Not Allowed . . . . . . . . . . . . . . . 158
       14.5.7.  406 Not Acceptable . . . . . . . . . . . . . . . . . 158
       14.5.8.  407 Proxy Authentication Required  . . . . . . . . . 158
       14.5.9.  408 Request Timeout  . . . . . . . . . . . . . . . . 158
       14.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 159
       14.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 159
       14.5.12. 411 Length Required  . . . . . . . . . . . . . . . . 159
       14.5.13. 412 Precondition Failed  . . . . . . . . . . . . . . 160
       14.5.14. 413 Payload Too Large  . . . . . . . . . . . . . . . 160
       14.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 160



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       14.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 160
       14.5.17. 416 Range Not Satisfiable  . . . . . . . . . . . . . 161
       14.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 161
       14.5.19. 418 (Unused) . . . . . . . . . . . . . . . . . . . . 162
       14.5.20. 422 Unprocessable Payload  . . . . . . . . . . . . . 162
       14.5.21. 426 Upgrade Required . . . . . . . . . . . . . . . . 162
     14.6.  Server Error 5xx . . . . . . . . . . . . . . . . . . . . 163
       14.6.1.  500 Internal Server Error  . . . . . . . . . . . . . 163
       14.6.2.  501 Not Implemented  . . . . . . . . . . . . . . . . 163
       14.6.3.  502 Bad Gateway  . . . . . . . . . . . . . . . . . . 163
       14.6.4.  503 Service Unavailable  . . . . . . . . . . . . . . 163
       14.6.5.  504 Gateway Timeout  . . . . . . . . . . . . . . . . 164
       14.6.6.  505 HTTP Version Not Supported . . . . . . . . . . . 164
   15. Extending HTTP  . . . . . . . . . . . . . . . . . . . . . . . 164
     15.1.  Method Extensibility . . . . . . . . . . . . . . . . . . 165
       15.1.1.  Method Registry  . . . . . . . . . . . . . . . . . . 165
       15.1.2.  Considerations for New Methods . . . . . . . . . . . 165
     15.2.  Status Code Extensibility  . . . . . . . . . . . . . . . 166
       15.2.1.  Status Code Registry . . . . . . . . . . . . . . . . 166
       15.2.2.  Considerations for New Status Codes  . . . . . . . . 166
     15.3.  Field Name Extensibility . . . . . . . . . . . . . . . . 167
       15.3.1.  Field Name Registry  . . . . . . . . . . . . . . . . 167
       15.3.2.  Considerations for New Field Names . . . . . . . . . 168
       15.3.3.  Considerations for New Field Values  . . . . . . . . 169
     15.4.  Authentication Scheme Extensibility  . . . . . . . . . . 171
       15.4.1.  Authentication Scheme Registry . . . . . . . . . . . 171
       15.4.2.  Considerations for New Authentication Schemes  . . . 171
     15.5.  Range Unit Extensibility . . . . . . . . . . . . . . . . 172
       15.5.1.  Range Unit Registry  . . . . . . . . . . . . . . . . 172
       15.5.2.  Considerations for New Range Units . . . . . . . . . 173
     15.6.  Content Coding Extensibility . . . . . . . . . . . . . . 173
       15.6.1.  Content Coding Registry  . . . . . . . . . . . . . . 173
       15.6.2.  Considerations for New Content Codings . . . . . . . 173
     15.7.  Upgrade Token Registry . . . . . . . . . . . . . . . . . 174
   16. Security Considerations . . . . . . . . . . . . . . . . . . . 174
     16.1.  Establishing Authority . . . . . . . . . . . . . . . . . 175
     16.2.  Risks of Intermediaries  . . . . . . . . . . . . . . . . 176
     16.3.  Attacks Based on File and Path Names . . . . . . . . . . 176
     16.4.  Attacks Based on Command, Code, or Query Injection . . . 177
     16.5.  Attacks via Protocol Element Length  . . . . . . . . . . 177
     16.6.  Attacks using Shared-dictionary Compression  . . . . . . 178
     16.7.  Disclosure of Personal Information . . . . . . . . . . . 178
     16.8.  Privacy of Server Log Information  . . . . . . . . . . . 179
     16.9.  Disclosure of Sensitive Information in URIs  . . . . . . 179
     16.10. Disclosure of Fragment after Redirects . . . . . . . . . 180
     16.11. Disclosure of Product Information  . . . . . . . . . . . 180
     16.12. Browser Fingerprinting . . . . . . . . . . . . . . . . . 181
     16.13. Validator Retention  . . . . . . . . . . . . . . . . . . 182



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     16.14. Denial-of-Service Attacks Using Range  . . . . . . . . . 182
     16.15. Authentication Considerations  . . . . . . . . . . . . . 183
       16.15.1.  Confidentiality of Credentials  . . . . . . . . . . 183
       16.15.2.  Credentials and Idle Clients  . . . . . . . . . . . 183
       16.15.3.  Protection Spaces . . . . . . . . . . . . . . . . . 184
       16.15.4.  Additional Response Fields  . . . . . . . . . . . . 184
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 184
     17.1.  URI Scheme Registration  . . . . . . . . . . . . . . . . 185
     17.2.  Method Registration  . . . . . . . . . . . . . . . . . . 185
     17.3.  Status Code Registration . . . . . . . . . . . . . . . . 185
     17.4.  HTTP Field Name Registration . . . . . . . . . . . . . . 187
     17.5.  Authentication Scheme Registration . . . . . . . . . . . 189
     17.6.  Content Coding Registration  . . . . . . . . . . . . . . 189
     17.7.  Range Unit Registration  . . . . . . . . . . . . . . . . 189
     17.8.  Media Type Registration  . . . . . . . . . . . . . . . . 189
     17.9.  Port Registration  . . . . . . . . . . . . . . . . . . . 189
     17.10. Upgrade Token Registration . . . . . . . . . . . . . . . 190
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . . 190
     18.1.  Normative References . . . . . . . . . . . . . . . . . . 190
     18.2.  Informative References . . . . . . . . . . . . . . . . . 192
   Appendix A.  Collected ABNF . . . . . . . . . . . . . . . . . . . 198
   Appendix B.  Changes from previous RFCs . . . . . . . . . . . . . 203
     B.1.  Changes from RFC 2818 . . . . . . . . . . . . . . . . . . 203
     B.2.  Changes from RFC 7230 . . . . . . . . . . . . . . . . . . 203
     B.3.  Changes from RFC 7231 . . . . . . . . . . . . . . . . . . 204
     B.4.  Changes from RFC 7232 . . . . . . . . . . . . . . . . . . 205
     B.5.  Changes from RFC 7233 . . . . . . . . . . . . . . . . . . 205
     B.6.  Changes from RFC 7235 . . . . . . . . . . . . . . . . . . 205
     B.7.  Changes from RFC 7538 . . . . . . . . . . . . . . . . . . 205
     B.8.  Changes from RFC 7615 . . . . . . . . . . . . . . . . . . 205
     B.9.  Changes from RFC 7694 . . . . . . . . . . . . . . . . . . 206
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . . 206
     C.1.  Between RFC723x and draft 00  . . . . . . . . . . . . . . 206
     C.2.  Since draft-ietf-httpbis-semantics-00 . . . . . . . . . . 206
     C.3.  Since draft-ietf-httpbis-semantics-01 . . . . . . . . . . 207
     C.4.  Since draft-ietf-httpbis-semantics-02 . . . . . . . . . . 208
     C.5.  Since draft-ietf-httpbis-semantics-03 . . . . . . . . . . 209
     C.6.  Since draft-ietf-httpbis-semantics-04 . . . . . . . . . . 210
     C.7.  Since draft-ietf-httpbis-semantics-05 . . . . . . . . . . 210
     C.8.  Since draft-ietf-httpbis-semantics-06 . . . . . . . . . . 212
     C.9.  Since draft-ietf-httpbis-semantics-07 . . . . . . . . . . 213
     C.10. Since draft-ietf-httpbis-semantics-08 . . . . . . . . . . 214
     C.11. Since draft-ietf-httpbis-semantics-09 . . . . . . . . . . 216
     C.12. Since draft-ietf-httpbis-semantics-10 . . . . . . . . . . 216
     C.13. Since draft-ietf-httpbis-semantics-11 . . . . . . . . . . 217
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 218
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 218




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1.  Introduction

1.1.  Purpose

   The Hypertext Transfer Protocol (HTTP) is a family of stateless,
   application-level, request/response protocols that share a generic
   interface, extensible semantics, and self-descriptive messages to
   enable flexible interaction with network-based hypertext information
   systems.

   HTTP hides the details of how a service is implemented by presenting
   a uniform interface to clients that is independent of the types of
   resources provided.  Likewise, servers do not need to be aware of
   each client's purpose: a request can be considered in isolation
   rather than being associated with a specific type of client or a
   predetermined sequence of application steps.  This allows general-
   purpose implementations to be used effectively in many different
   contexts, reduces interaction complexity, and enables independent
   evolution over time.

   HTTP is also designed for use as an intermediation protocol, wherein
   proxies and gateways can translate non-HTTP information systems into
   a more generic interface.

   One consequence of this flexibility is that the protocol cannot be
   defined in terms of what occurs behind the interface.  Instead, we
   are limited to defining the syntax of communication, the intent of
   received communication, and the expected behavior of recipients.  If
   the communication is considered in isolation, then successful actions
   ought to be reflected in corresponding changes to the observable
   interface provided by servers.  However, since multiple clients might
   act in parallel and perhaps at cross-purposes, we cannot require that
   such changes be observable beyond the scope of a single response.

1.2.  Evolution

   HTTP has been the primary information transfer protocol for the World
   Wide Web since its introduction in 1990.  It began as a trivial
   mechanism for low-latency requests, with a single method (GET) to
   request transfer of a presumed hypertext document identified by a
   given pathname.  This original protocol is now referred to as
   HTTP/0.9.









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   HTTP's version number consists of two decimal digits separated by a
   "." (period or decimal point).  The first digit ("major version")
   indicates the messaging syntax, whereas the second digit ("minor
   version") indicates the highest minor version within that major
   version to which the sender is conformant (able to understand for
   future communication).

   As the Web grew, HTTP was extended to enclose requests and responses
   within messages, transfer arbitrary data formats using MIME-like
   media types, and route requests through intermediaries, eventually
   being defined as HTTP/1.0 [RFC1945].

   HTTP/1.1 was designed to refine the protocol's features while
   retaining compatibility with the existing text-based messaging
   syntax, improving its interoperability, scalability, and robustness
   across the Internet.  This included length-based payload delimiters
   for both fixed and dynamic (chunked) content, a consistent framework
   for content negotiation, opaque validators for conditional requests,
   cache controls for better cache consistency, range requests for
   partial updates, and default persistent connections.  HTTP/1.1 was
   introduced in 1995 and published on the standards track in 1997
   [RFC2068], 1999 [RFC2616], and 2014 ([RFC7230] - [RFC7235]).

   HTTP/2 ([RFC7540]) introduced a multiplexed session layer on top of
   the existing TLS and TCP protocols for exchanging concurrent HTTP
   messages with efficient header field compression and server push.
   HTTP/3 ([HTTP3]) provides greater independence for concurrent
   messages by using QUIC as a secure multiplexed transport over UDP
   instead of TCP.

   All three major versions of HTTP rely on the semantics defined by
   this document.  They have not obsoleted each other because each one
   has specific benefits and limitations depending on the context of
   use.  Implementations are expected to choose the most appropriate
   transport and messaging syntax for their particular context.

   This revision of HTTP separates the definition of semantics (this
   document) and caching ([Caching]) from the current HTTP/1.1 messaging
   syntax ([Messaging]) to allow each major protocol version to progress
   independently while referring to the same core semantics.

1.3.  Semantics

   HTTP provides a uniform interface for interacting with a resource
   (Section 3.1), regardless of its type, nature, or implementation, by
   sending messages that manipulate or transfer representations
   (Section 7).




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   Each message is either a request or a response.  A client constructs
   request messages that communicate its intentions and routes those
   messages toward an identified origin server.  A server listens for
   requests, parses each message received, interprets the message
   semantics in relation to the identified target resource, and responds
   to that request with one or more response messages.  The client
   examines received responses to see if its intentions were carried
   out, determining what to do next based on the received status and
   payloads.

   HTTP semantics include the intentions defined by each request method
   (Section 8), extensions to those semantics that might be described in
   request header fields, status codes that describe the response
   (Section 14), and other control data and resource metadata that might
   be given in response fields.

   Semantics also include representation metadata that describe how a
   payload is intended to be interpreted by a recipient, request header
   fields that might influence content selection, and the various
   selection algorithms that are collectively referred to as "content
   negotiation" (Section 11).

1.4.  Obsoletes

   This document obsoletes the following specifications:

    -------------------------------------------- ----------- ---------
     Title                                        Reference   Changes
    -------------------------------------------- ----------- ---------
     HTTP Over TLS                                [RFC2818]   B.1
     HTTP/1.1 Message Syntax and Routing [*]      [RFC7230]   B.2
     HTTP/1.1 Semantics and Content               [RFC7231]   B.3
     HTTP/1.1 Conditional Requests                [RFC7232]   B.4
     HTTP/1.1 Range Requests                      [RFC7233]   B.5
     HTTP/1.1 Authentication                      [RFC7235]   B.6
     HTTP Status Code 308 (Permanent Redirect)    [RFC7538]   B.7
     HTTP Authentication-Info and Proxy-          [RFC7615]   B.8
     Authentication-Info Response Header Fields
     HTTP Client-Initiated Content-Encoding       [RFC7694]   B.9
    -------------------------------------------- ----------- ---------

                                 Table 1

   [*] This document only obsoletes the portions of RFC 7230 that are
   independent of the HTTP/1.1 messaging syntax and connection
   management; the remaining bits of RFC 7230 are obsoleted by "HTTP/1.1
   Messaging" [Messaging].




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2.  Conformance

2.1.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234], extended with the notation for case-
   sensitivity in strings defined in [RFC7405].

   It also uses a list extension, defined in Section 5.7.1, that allows
   for compact definition of comma-separated lists using a '#' operator
   (similar to how the '*' operator indicates repetition).  Appendix A
   shows the collected grammar with all list operators expanded to
   standard ABNF notation.

   As a convention, ABNF rule names prefixed with "obs-" denote
   "obsolete" grammar rules that appear for historical reasons.

   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).

   Section 5.7 defines some generic syntactic components for field
   values.

   The rule below is defined in [Messaging];

     transfer-coding = <transfer-coding, see [Messaging], Section 7>

   This specification uses the terms "character", "character encoding
   scheme", "charset", and "protocol element" as they are defined in
   [RFC6365].

2.2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This specification targets conformance criteria according to the role
   of a participant in HTTP communication.  Hence, requirements are
   placed on senders, recipients, clients, servers, user agents,
   intermediaries, origin servers, proxies, gateways, or caches,
   depending on what behavior is being constrained by the requirement.



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   Additional (social) requirements are placed on implementations,
   resource owners, and protocol element registrations when they apply
   beyond the scope of a single communication.

   The verb "generate" is used instead of "send" where a requirement
   applies only to implementations that create the protocol element,
   rather than an implementation that forwards a received element
   downstream.

   An implementation is considered conformant if it complies with all of
   the requirements associated with the roles it partakes in HTTP.

   Conformance includes both the syntax and semantics of protocol
   elements.  A sender MUST NOT generate protocol elements that convey a
   meaning that is known by that sender to be false.  A sender MUST NOT
   generate protocol elements that do not match the grammar defined by
   the corresponding ABNF rules.  Within a given message, a sender MUST
   NOT generate protocol elements or syntax alternatives that are only
   allowed to be generated by participants in other roles (i.e., a role
   that the sender does not have for that message).

2.3.  Length Requirements

   When a received protocol element is parsed, the recipient MUST be
   able to parse any value of reasonable length that is applicable to
   the recipient's role and that matches the grammar defined by the
   corresponding ABNF rules.  Note, however, that some received protocol
   elements might not be parsed.  For example, an intermediary
   forwarding a message might parse a field into generic field name and
   field value components, but then forward the field without further
   parsing inside the field value.

   HTTP does not have specific length limitations for many of its
   protocol elements because the lengths that might be appropriate will
   vary widely, depending on the deployment context and purpose of the
   implementation.  Hence, interoperability between senders and
   recipients depends on shared expectations regarding what is a
   reasonable length for each protocol element.  Furthermore, what is
   commonly understood to be a reasonable length for some protocol
   elements has changed over the course of the past two decades of HTTP
   use and is expected to continue changing in the future.

   At a minimum, a recipient MUST be able to parse and process protocol
   element lengths that are at least as long as the values that it
   generates for those same protocol elements in other messages.  For
   example, an origin server that publishes very long URI references to
   its own resources needs to be able to parse and process those same
   references when received as a target URI.



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2.4.  Error Handling

   A recipient MUST interpret a received protocol element according to
   the semantics defined for it by this specification, including
   extensions to this specification, unless the recipient has determined
   (through experience or configuration) that the sender incorrectly
   implements what is implied by those semantics.  For example, an
   origin server might disregard the contents of a received
   Accept-Encoding header field if inspection of the User-Agent header
   field indicates a specific implementation version that is known to
   fail on receipt of certain content codings.

   Unless noted otherwise, a recipient MAY attempt to recover a usable
   protocol element from an invalid construct.  HTTP does not define
   specific error handling mechanisms except when they have a direct
   impact on security, since different applications of the protocol
   require different error handling strategies.  For example, a Web
   browser might wish to transparently recover from a response where the
   Location header field doesn't parse according to the ABNF, whereas a
   systems control client might consider any form of error recovery to
   be dangerous.

   Some requests can be automatically retried by a client in the event
   of an underlying connection failure, as described in Section 8.2.2.

3.  Terminology

   HTTP was created for the World Wide Web (WWW) architecture and has
   evolved over time to support the scalability needs of a worldwide
   hypertext system.  Much of that architecture is reflected in the
   terminology and syntax productions used to define HTTP.

3.1.  Resources

   The target of an 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.  Most resources are
   identified by a Uniform Resource Identifier (URI), as described in
   Section 4.

   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 8) and a few request-
   modifying header fields.  If there is a conflict between the method
   semantics and any semantic implied by the URI itself, as described in
   Section 8.2.1, the method semantics take precedence.





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   HTTP relies upon the Uniform Resource Identifier (URI) standard
   [RFC3986] to indicate the target resource (Section 6.1) and
   relationships between resources.

3.2.  Connections

   HTTP is a client/server protocol that operates over a reliable
   transport- or session-layer "connection".

   An HTTP "client" is a program that establishes a connection to a
   server for the purpose of sending one or more HTTP requests.  An HTTP
   "server" is a program that accepts connections in order to service
   HTTP requests by sending HTTP responses.

   The terms "client" and "server" refer only to the roles that these
   programs perform for a particular connection.  The same program might
   act as a client on some connections and a server on others.

3.3.  Messages

   HTTP is a stateless request/response protocol for exchanging
   "messages" across a connection.  The terms "sender" and "recipient"
   refer to any implementation that sends or receives a given message,
   respectively.

   A client sends requests to a server in the form of a request message
   with a method (Section 8) and request target (Section 6.1.1).  The
   request might also contain header fields (Section 5.4) for request
   modifiers, client information, and representation metadata, a payload
   body (Section 5.5.4) to be processed in accordance with the method,
   and trailer fields (Section 5.6) for metadata collected while sending
   the payload.

   A server responds to a client's request by sending one or more
   response messages, each including a status code (Section 14).  The
   response might also contain header fields for server information,
   resource metadata, and representation metadata, a payload body to be
   interpreted in accordance with the status code, and trailer fields
   for metadata collected while sending the payload.

3.4.  User Agent

   The term "user agent" refers to any of the various client programs
   that initiate a request.

   The most familiar form of user agent is the general-purpose Web
   browser, but that's only a small percentage of implementations.
   Other common user agents include spiders (web-traversing robots),



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   command-line tools, billboard screens, household appliances, scales,
   light bulbs, firmware update scripts, mobile apps, and communication
   devices in a multitude of shapes and sizes.

   Being a user agent does not imply that there is a human user directly
   interacting with the software agent at the time of a request.  In
   many cases, a user agent is installed or configured to run in the
   background and save its results for later inspection (or save only a
   subset of those results that might be interesting or erroneous).
   Spiders, for example, are typically given a start URI and configured
   to follow certain behavior while crawling the Web as a hypertext
   graph.

   Many user agents cannot, or choose not to, make interactive
   suggestions to their user or provide adequate warning for security or
   privacy concerns.  In the few cases where this specification requires
   reporting of errors to the user, it is acceptable for such reporting
   to only be observable in an error console or log file.  Likewise,
   requirements that an automated action be confirmed by the user before
   proceeding might be met via advance configuration choices, run-time
   options, or simple avoidance of the unsafe action; confirmation does
   not imply any specific user interface or interruption of normal
   processing if the user has already made that choice.

3.5.  Origin Server

   The term "origin server" refers to a program that can originate
   authoritative responses for a given target resource.

   The most familiar form of origin server are large public websites.
   However, like user agents being equated with browsers, it is easy to
   be misled into thinking that all origin servers are alike.  Common
   origin servers also include home automation units, configurable
   networking components, office machines, autonomous robots, news
   feeds, traffic cameras, real-time ad selectors, and video-on-demand
   platforms.

3.6.  Example Request and Response

   Most HTTP communication consists of a retrieval request (GET) for a
   representation of some resource identified by a URI.  In the simplest
   case, this might be accomplished via a single bidirectional
   connection (===) between the user agent (UA) and the origin server
   (O).

            request   >
       UA ======================================= O
                                   <   response



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                                  Figure 1

   The following example illustrates a typical message exchange for a
   GET request (Section 8.3.1) on the URI "http://www.example.com/
   hello.txt":

   Client request:

     GET /hello.txt HTTP/1.1
     User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
     Host: www.example.com
     Accept-Language: en, mi


   Server response:

     HTTP/1.1 200 OK
     Date: Mon, 27 Jul 2009 12:28:53 GMT
     Server: Apache
     Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
     ETag: "34aa387-d-1568eb00"
     Accept-Ranges: bytes
     Content-Length: 51
     Vary: Accept-Encoding
     Content-Type: text/plain

     Hello World! My payload includes a trailing CRLF.

3.7.  Intermediaries

   HTTP enables the use of intermediaries to satisfy requests through a
   chain of connections.  There are three common forms of HTTP
   intermediary: proxy, gateway, and tunnel.  In some cases, a single
   intermediary might act as an origin server, proxy, gateway, or
   tunnel, switching behavior based on the nature of each request.

            >             >             >             >
       UA =========== A =========== B =========== C =========== O
                  <             <             <             <

                                  Figure 2

   The figure above shows three intermediaries (A, B, and C) between the
   user agent and origin server.  A request or response message that
   travels the whole chain will pass through four separate connections.
   Some HTTP communication options might apply only to the connection
   with the nearest, non-tunnel neighbor, only to the endpoints of the
   chain, or to all connections along the chain.  Although the diagram



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   is linear, each participant might be engaged in multiple,
   simultaneous communications.  For example, B might be receiving
   requests from many clients other than A, and/or forwarding requests
   to servers other than C, at the same time that it is handling A's
   request.  Likewise, later requests might be sent through a different
   path of connections, often based on dynamic configuration for load
   balancing.

   The terms "upstream" and "downstream" are used to describe
   directional requirements in relation to the message flow: all
   messages flow from upstream to downstream.  The terms "inbound" and
   "outbound" are used to describe directional requirements in relation
   to the request route: "inbound" means toward the origin server and
   "outbound" means toward the user agent.

   A "proxy" is a message-forwarding agent that is chosen by the client,
   usually via local configuration rules, to receive requests for some
   type(s) of absolute URI and attempt to satisfy those requests via
   translation through the HTTP interface.  Some translations are
   minimal, such as for proxy requests for "http" URIs, whereas other
   requests might require translation to and from entirely different
   application-level protocols.  Proxies are often used to group an
   organization's HTTP requests through a common intermediary for the
   sake of security, annotation services, or shared caching.  Some
   proxies are designed to apply transformations to selected messages or
   payloads while they are being forwarded, as described in Section 6.5.

   A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as
   an origin server for the outbound connection but translates received
   requests and forwards them inbound to another server or servers.
   Gateways are often used to encapsulate legacy or untrusted
   information services, to improve server performance through
   "accelerator" caching, and to enable partitioning or load balancing
   of HTTP services across multiple machines.

   All HTTP requirements applicable to an origin server also apply to
   the outbound communication of a gateway.  A gateway communicates with
   inbound servers using any protocol that it desires, including private
   extensions to HTTP that are outside the scope of this specification.
   However, an HTTP-to-HTTP gateway that wishes to interoperate with
   third-party HTTP servers ought to conform to user agent requirements
   on the gateway's inbound connection.

   A "tunnel" acts as a blind relay between two connections without
   changing the messages.  Once active, a tunnel is not considered a
   party to the HTTP communication, though the tunnel might have been
   initiated by an HTTP request.  A tunnel ceases to exist when both
   ends of the relayed connection are closed.  Tunnels are used to



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   extend a virtual connection through an intermediary, such as when
   Transport Layer Security (TLS, [RFC8446]) is used to establish
   confidential communication through a shared firewall proxy.

   The above categories for intermediary only consider those acting as
   participants in the HTTP communication.  There are also
   intermediaries that can act on lower layers of the network protocol
   stack, filtering or redirecting HTTP traffic without the knowledge or
   permission of message senders.  Network intermediaries are
   indistinguishable (at a protocol level) from an on-path attacker,
   often introducing security flaws or interoperability problems due to
   mistakenly violating HTTP semantics.

   For example, an "interception proxy" [RFC3040] (also commonly known
   as a "transparent proxy" [RFC1919] or "captive portal") differs from
   an HTTP proxy because it is not chosen by the client.  Instead, an
   interception proxy filters or redirects outgoing TCP port 80 packets
   (and occasionally other common port traffic).  Interception proxies
   are commonly found on public network access points, as a means of
   enforcing account subscription prior to allowing use of non-local
   Internet services, and within corporate firewalls to enforce network
   usage policies.

   HTTP is defined as a stateless protocol, meaning that each request
   message can be understood in isolation.  Many implementations depend
   on HTTP's stateless design in order to reuse proxied connections or
   dynamically load balance requests across multiple servers.  Hence, a
   server MUST NOT assume that two requests on the same connection are
   from the same user agent unless the connection is secured and
   specific to that agent.  Some non-standard HTTP extensions (e.g.,
   [RFC4559]) have been known to violate this requirement, resulting in
   security and interoperability problems.

3.8.  Caches

   A "cache" is a local store of previous response messages and the
   subsystem that controls its message storage, retrieval, and deletion.
   A cache stores cacheable responses in order to reduce the response
   time and network bandwidth consumption on future, equivalent
   requests.  Any client or server MAY employ a cache, though a cache
   cannot be used by a server while it is acting as a tunnel.

   The effect of a cache is that the request/response chain is shortened
   if one of the participants along the chain has a cached response
   applicable to that request.  The following illustrates the resulting
   chain if B has a cached copy of an earlier response from O (via C)
   for a request that has not been cached by UA or A.




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               >             >
          UA =========== A =========== B - - - - - - C - - - - - - O
                     <             <

                                  Figure 3

   A response is "cacheable" if a cache is allowed to store a copy of
   the response message for use in answering subsequent requests.  Even
   when a response is cacheable, there might be additional constraints
   placed by the client or by the origin server on when that cached
   response can be used for a particular request.  HTTP requirements for
   cache behavior and cacheable responses are defined in Section 2 of
   [Caching].

   There is a wide variety of architectures and configurations of caches
   deployed across the World Wide Web and inside large organizations.
   These include national hierarchies of proxy caches to save
   transoceanic bandwidth, collaborative systems that broadcast or
   multicast cache entries, archives of pre-fetched cache entries for
   use in off-line or high-latency environments, and so on.

4.  Identifiers

   Uniform Resource Identifiers (URIs) [RFC3986] are used throughout
   HTTP as the means for identifying resources (Section 3.1).

4.1.  URI References

   URI references are used to target requests, indicate redirects, and
   define relationships.

   The definitions of "URI-reference", "absolute-URI", "relative-part",
   "authority", "port", "host", "path-abempty", "segment", and "query"
   are adopted from the URI generic syntax.  An "absolute-path" rule is
   defined for protocol elements that can contain a non-empty path
   component.  (This rule differs slightly from the path-abempty rule of
   RFC 3986, which allows for an empty path to be used in references,
   and path-absolute rule, which does not allow paths that begin with
   "//".)  A "partial-URI" rule is defined for protocol elements that
   can contain a relative URI but not a fragment component.











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     URI-reference = <URI-reference, see [RFC3986], Section 4.1>
     absolute-URI  = <absolute-URI, see [RFC3986], Section 4.3>
     relative-part = <relative-part, see [RFC3986], Section 4.2>
     authority     = <authority, see [RFC3986], Section 3.2>
     uri-host      = <host, see [RFC3986], Section 3.2.2>
     port          = <port, see [RFC3986], Section 3.2.3>
     path-abempty  = <path-abempty, see [RFC3986], Section 3.3>
     segment       = <segment, see [RFC3986], Section 3.3>
     query         = <query, see [RFC3986], Section 3.4>

     absolute-path = 1*( "/" segment )
     partial-URI   = relative-part [ "?" query ]

   Each protocol element in HTTP that allows a URI reference will
   indicate in its ABNF production whether the element allows any form
   of reference (URI-reference), only a URI in absolute form (absolute-
   URI), only the path and optional query components, or some
   combination of the above.  Unless otherwise indicated, URI references
   are parsed relative to the target URI (Section 6.1).

   It is RECOMMENDED that all senders and recipients support, at a
   minimum, URIs with lengths of 8000 octets in protocol elements.  Note
   that this implies some structures and on-wire representations (for
   example, the request line in HTTP/1.1) will necessarily be larger in
   some cases.

4.2.  URI Schemes

   IANA maintains the registry of URI Schemes [BCP35] at
   <https://www.iana.org/assignments/uri-schemes/>.  Although requests
   might target any URI scheme, the following schemes are inherent to
   HTTP servers:

    ------------ ------------------------------------ -------
     URI Scheme   Description                          Ref.
    ------------ ------------------------------------ -------
     http         Hypertext Transfer Protocol          4.2.1
     https        Hypertext Transfer Protocol Secure   4.2.2
    ------------ ------------------------------------ -------

                             Table 2










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   Note that the presence of an "http" or "https" URI does not imply
   that there is always an HTTP server at the identified origin
   listening for connections.  Anyone can mint a URI, whether or not a
   server exists and whether or not that server currently maps that
   identifier to a resource.  The delegated nature of registered names
   and IP addresses creates a federated namespace whether or not an HTTP
   server is present.

4.2.1.  http URI Scheme

   The "http" URI scheme is hereby defined for minting identifiers
   within the hierarchical namespace governed by a potential HTTP origin
   server listening for TCP ([RFC0793]) connections on a given port.

     http-URI = "http" "://" authority path-abempty [ "?" query ]

   The origin server for an "http" URI is identified by the authority
   component, which includes a host identifier and optional port number
   ([RFC3986], Section 3.2.2).  If the port subcomponent is empty or not
   given, TCP port 80 (the reserved port for WWW services) is the
   default.  The origin determines who has the right to respond
   authoritatively to requests that target the identified resource, as
   defined in Section 4.3.2.

   A sender MUST NOT generate an "http" URI with an empty host
   identifier.  A recipient that processes such a URI reference MUST
   reject it as invalid.

   The hierarchical path component and optional query component identify
   the target resource within that origin server's name space.

4.2.2.  https URI Scheme

   The "https" URI scheme is hereby defined for minting identifiers
   within the hierarchical namespace governed by a potential origin
   server listening for TCP connections on a given port and capable of
   establishing a TLS ([RFC8446]) connection that has been secured for
   HTTP communication.  In this context, "secured" specifically means
   that the server has been authenticated as acting on behalf of the
   identified authority and all HTTP communication with that server has
   been protected for confidentiality and integrity through the use of
   strong encryption.

     https-URI = "https" "://" authority path-abempty [ "?" query ]

   The origin server for an "https" URI is identified by the authority
   component, which includes a host identifier and optional port number
   ([RFC3986], Section 3.2.2).  If the port subcomponent is empty or not



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   given, TCP port 443 (the reserved port for HTTP over TLS) is the
   default.  The origin determines who has the right to respond
   authoritatively to requests that target the identified resource, as
   defined in Section 4.3.3.

   A sender MUST NOT generate an "https" URI with an empty host
   identifier.  A recipient that processes such a URI reference MUST
   reject it as invalid.

   The hierarchical path component and optional query component identify
   the target resource within that origin server's name space.

   A client MUST ensure that its HTTP requests for an "https" resource
   are secured, prior to being communicated, and that it only accepts
   secured responses to those requests.

   Resources made available via the "https" scheme have no shared
   identity with the "http" scheme.  They are distinct origins with
   separate namespaces.  However, an extension to HTTP that is defined
   to apply to all origins with the same host, such as the Cookie
   protocol [RFC6265], can allow information set by one service to
   impact communication with other services within a matching group of
   host domains.

4.2.3.  http(s) Normalization and Comparison

   Since the "http" and "https" schemes conform to the URI generic
   syntax, such URIs are normalized and compared according to the
   algorithm defined in Section 6 of [RFC3986], using the defaults
   described above for each scheme.

   If the port is equal to the default port for a scheme, the normal
   form is to omit the port subcomponent.  When not being used as the
   target of an OPTIONS request, an empty path component is equivalent
   to an absolute path of "/", so the normal form is to provide a path
   of "/" instead.  The scheme and host are case-insensitive and
   normally provided in lowercase; all other components are compared in
   a case-sensitive manner.  Characters other than those in the
   "reserved" set are equivalent to their percent-encoded octets: the
   normal form is to not encode them (see Sections 2.1 and 2.2 of
   [RFC3986]).

   For example, the following three URIs are equivalent:

      http://example.com:80/~smith/home.html
      http://EXAMPLE.com/%7Esmith/home.html
      http://EXAMPLE.com:/%7esmith/home.html




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4.2.4.  http(s) Deprecated userinfo

   The URI generic syntax for authority also includes a userinfo
   subcomponent ([RFC3986], Section 3.2.1) for including user
   authentication information in the URI.  In that subcomponent, the use
   of the format "user:password" is deprecated.

   Some implementations make use of the userinfo component for internal
   configuration of authentication information, such as within command
   invocation options, configuration files, or bookmark lists, even
   though such usage might expose a user identifier or password.

   A sender MUST NOT generate the userinfo subcomponent (and its "@"
   delimiter) when an "http" or "https" URI reference is generated
   within a message as a target URI or field value.

   Before making use of an "http" or "https" URI reference received from
   an untrusted source, a recipient SHOULD parse for userinfo and treat
   its presence as an error; it is likely being used to obscure the
   authority for the sake of phishing attacks.

4.2.5.  http(s) References with Fragment Identifiers

   Fragment identifiers allow for indirect identification of a secondary
   resource, independent of the URI scheme, as defined in Section 3.5 of
   [RFC3986].  Some protocol elements that refer to a URI allow
   inclusion of a fragment, while others do not.  They are distinguished
   by use of the ABNF rule for elements where fragment is allowed;
   otherwise, a specific rule that excludes fragments is used (see
   Section 6.1).

      |  *Note:* the fragment identifier component is not part of the
      |  actual scheme definition for a URI scheme (see Section 4.3 of
      |  [RFC3986]), thus does not appear in the ABNF definitions for
      |  the "http" and "https" URI schemes above.

4.3.  Authoritative Access

   See Section 16.1 for security considerations related to establishing
   authority.

4.3.1.  URI Origin

   The "origin" for a given URI is the triple of scheme, host, and port
   after normalizing the scheme and host to lowercase and normalizing
   the port to remove any leading zeros.  If port is elided from the
   URI, the default port for that scheme is used.  For example, the URI




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      https://Example.Com/happy.js

   would have the origin

      { "https", "example.com", "443" }

   which can also be described as the normalized URI prefix with port
   always present:

      https://example.com:443

   Each origin defines its own namespace and controls how identifiers
   within that namespace are mapped to resources.  In turn, how the
   origin responds to valid requests, consistently over time, determines
   the semantics that users will associate with a URI, and the
   usefulness of those semantics is what ultimately transforms these
   mechanisms into a "resource" for users to reference and access in the
   future.

   Two origins are distinct if they differ in scheme, host, or port.
   Even when it can be verified that the same entity controls two
   distinct origins, the two namespaces under those origins are distinct
   unless explicitly aliased by a server authoritative for that origin.

   Origin is also used within HTML and related Web protocols, beyond the
   scope of this document, as described in [RFC6454].

4.3.2.  http origins

   Although HTTP is independent of the transport protocol, the "http"
   scheme (Section 4.2.1) is specific to associating authority with
   whomever controls the origin server listening for TCP connections on
   the indicated port of whatever host is identified within the
   authority component.  This is a very weak sense of authority because
   it depends on both client-specific name resolution mechanisms and
   communication that might not be secured from an on-path attacker.
   Nevertheless, it is a sufficient minimum for binding "http"
   identifiers to an origin server for consistent resolution within a
   trusted environment.

   If the host identifier is provided as an IP address, the origin
   server is the listener (if any) on the indicated TCP port at that IP
   address.  If host is a registered name, the registered name is an
   indirect identifier for use with a name resolution service, such as
   DNS, to find an address for an appropriate origin server.






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   When an "http" URI is used within a context that calls for access to
   the indicated resource, a client MAY attempt access by resolving the
   host identifier to an IP address, establishing a TCP connection to
   that address on the indicated port, and sending an HTTP request
   message to the server containing the URI's identifying data.

   If the server responds to such a request with a non-interim HTTP
   response message, as described in Section 14, then that response is
   considered an authoritative answer to the client's request.

   Note, however, that the above is not the only means for obtaining an
   authoritative response, nor does it imply that an authoritative
   response is always necessary (see [Caching]).  For example, the Alt-
   Svc header field [RFC7838] allows an origin server to identify other
   services that are also authoritative for that origin.  Access to
   "http" identified resources might also be provided by protocols
   outside the scope of this document.

4.3.3.  https origins

   The "https" scheme (Section 4.2.2) associates authority based on the
   ability of a server to use the private key corresponding to a
   certificate that the client considers to be trustworthy for the
   identified origin server.  The client usually relies upon a chain of
   trust, conveyed from some prearranged or configured trust anchor, to
   deem a certificate trustworthy (Section 4.3.4).

   In HTTP/1.1 and earlier, a client will only attribute authority to a
   server when they are communicating over a successfully established
   and secured connection specifically to that URI origin's host.  The
   connection establishment and certificate verification are used as
   proof of authority.

   In HTTP/2 and HTTP/3, a client will attribute authority to a server
   when they are communicating over a successfully established and
   secured connection if the URI origin's host matches any of the hosts
   present in the server's certificate and the client believes that it
   could open a connection to that host for that URI.  In practice, a
   client will make a DNS query to check that the origin's host contains
   the same server IP address as the established connection.  This
   restriction can be removed by the origin server sending an equivalent
   ORIGIN frame [RFC8336].

   The request target's host and port value are passed within each HTTP
   request, identifying the origin and distinguishing it from other
   namespaces that might be controlled by the same server.  It is the
   origin's responsibility to ensure that any services provided with
   control over its certificate's private key are equally responsible



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   for managing the corresponding "https" namespaces, or at least
   prepared to reject requests that appear to have been misdirected.  A
   server might be unwilling to serve as the origin for some hosts even
   when they have the authority to do so.

   For example, if a network attacker causes connections for port N to
   be received at port Q, checking the target URI is necessary to ensure
   that the attacker can't cause "https://example.com:N/foo" to be
   replaced by "https://example.com:Q/foo" without consent.

   Note that the "https" scheme does not rely on TCP and the connected
   port number for associating authority, since both are outside the
   secured communication and thus cannot be trusted as definitive.
   Hence, the HTTP communication might take place over any channel that
   has been secured, as defined in Section 4.2.2, including protocols
   that don't use TCP.

   When an "https" URI is used within a context that calls for access to
   the indicated resource, a client MAY attempt access by resolving the
   host identifier to an IP address, establishing a TCP connection to
   that address on the indicated port, securing the connection end-to-
   end by successfully initiating TLS over TCP with confidentiality and
   integrity protection, and sending an HTTP request message over that
   connection containing the URI's identifying data.

   If the server responds to such a request with a non-interim HTTP
   response message, as described in Section 14, then that response is
   considered an authoritative answer to the client's request.

   Note, however, that the above is not the only means for obtaining an
   authoritative response, nor does it imply that an authoritative
   response is always necessary (see [Caching]).

4.3.4.  https certificate verification

   To establish a secured connection to dereference a URI, a client MUST
   verify that the service's identity is an acceptable match for the
   URI's origin server.  Certificate verification is used to prevent
   server impersonation by an on-path attacker or by an attacker that
   controls name resolution.  This process requires that a client be
   configured with a set of trust anchors.

   In general, a client MUST verify the service identity using the
   verification process defined in Section 6 of [RFC6125] (for a
   reference identifier of type URI-ID) unless the client has been
   specifically configured to accept some other form of verification.
   For example, a client might be connecting to a server whose address
   and hostname are dynamic, with an expectation that the service will



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   present a specific certificate (or a certificate matching some
   externally defined reference identity) rather than one matching the
   dynamic URI's origin server identifier.

   In special cases, it might be appropriate for a client to simply
   ignore the server's identity, but it must be understood that this
   leaves a connection open to active attack.

   If the certificate is not valid for the URI's origin server, a user
   agent MUST either notify the user (user agents MAY give the user an
   option to continue with the connection in any case) or terminate the
   connection with a bad certificate error.  Automated clients MUST log
   the error to an appropriate audit log (if available) and SHOULD
   terminate the connection (with a bad certificate error).  Automated
   clients MAY provide a configuration setting that disables this check,
   but MUST provide a setting which enables it.

5.  Message Abstraction

   Each major version of HTTP defines its own syntax for the
   communication of messages.  However, they share a common data
   abstraction.

   A message consists of control data to describe and route the message,
   optional header fields that modify or extend the message semantics,
   describe the sender, the payload, or provide additional context, a
   potentially unbounded stream of payload data, and optional trailer
   fields for metadata collected while sending the payload.

   Messages are intended to be self-descriptive.  This means that
   everything a recipient needs to know about the message can be
   determined by looking at the message itself, after decoding or
   reconstituting parts that have been compressed or elided in transit,
   without requiring an understanding of the sender's current
   application state (established via prior messages).

5.1.  Protocol Version

   While HTTP's core semantics don't change between protocol versions,
   the expression of them "on the wire" can change, and so the HTTP
   version number changes when incompatible changes are made to the wire
   format.  Additionally, HTTP allows incremental, backwards-compatible
   changes to be made to the protocol without changing its version
   through the use of defined extension points (Section 15).







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   The protocol version as a whole indicates the sender's conformance
   with the set of requirements laid out in that version's corresponding
   specification of HTTP.  For example, the version "HTTP/1.1" is
   defined by the combined specifications of this document, "HTTP
   Caching" [Caching], and "HTTP/1.1 Messaging" [Messaging].

   HTTP's major version number is incremented when an incompatible
   message syntax is introduced.  The minor number is incremented when
   changes made to the protocol have the effect of adding to the message
   semantics or implying additional capabilities of the sender.

   The minor version advertises the sender's communication capabilities
   even when the sender is only using a backwards-compatible subset of
   the protocol, thereby letting the recipient know that more advanced
   features can be used in response (by servers) or in future requests
   (by clients).

   A client SHOULD send a request version equal to the highest version
   to which the client is conformant and whose major version is no
   higher than the highest version supported by the server, if this is
   known.  A client MUST NOT send a version to which it is not
   conformant.

   A client MAY send a lower request version if it is known that the
   server incorrectly implements the HTTP specification, but only after
   the client has attempted at least one normal request and determined
   from the response status code or header fields (e.g., Server) that
   the server improperly handles higher request versions.

   A server SHOULD send a response version equal to the highest version
   to which the server is conformant that has a major version less than
   or equal to the one received in the request.  A server MUST NOT send
   a version to which it is not conformant.  A server can send a 505
   (HTTP Version Not Supported) response if it wishes, for any reason,
   to refuse service of the client's major protocol version.

   When an HTTP message is received with a major version number that the
   recipient implements, but a higher minor version number than what the
   recipient implements, the recipient SHOULD process the message as if
   it were in the highest minor version within that major version to
   which the recipient is conformant.  A recipient can assume that a
   message with a higher minor version, when sent to a recipient that
   has not yet indicated support for that higher version, is
   sufficiently backwards-compatible to be safely processed by any
   implementation of the same major version.






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   When a major version of HTTP does not define any minor versions, the
   minor version "0" is implied and is used when referring to that
   protocol within a protocol element that requires sending a minor
   version.

5.2.  Framing


   // Message framing defines how each message begins and ends, such
   // that the message can be distinguished from other message (or
   // noise) on the same connection.  Framing is specific to each major
   // version of HTTP.

   One of the functions of message framing is to assure that messages
   are complete.  A message is considered complete when all of the
   octets indicated by its framing are available.  Note that, when no
   explicit framing is used, a response message that is ended by the
   transport connection's close is considered complete even though it
   might be indistinguishable from an incomplete response, unless a
   transport-level error indicates that it is not complete.

5.3.  Control Data

5.3.1.  Request

   HTTP communication is initiated by a user agent for some purpose.
   The purpose is a combination of request semantics and a target
   resource upon which to apply those semantics.

5.3.2.  Response


5.4.  Header Fields

   HTTP messages use key/value pairs to convey data about the message,
   its payload, the target resource, or the connection.  They are called
   "HTTP fields" or just "fields".

   Fields that are sent/received before the message body are referred to
   as "header fields" (or just "headers", colloquially) and are located
   within the "header section" of a message.  We refer to some named
   fields specifically as a "header field" when they are only allowed to
   be sent in the header section.








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   Fields that are sent/received after the header section has ended
   (usually after the message body begins to stream) are referred to as
   "trailer fields" (or just "trailers", colloquially) and located
   within a "trailer section".  One or more trailer sections are only
   possible when supported by the version of HTTP in use and enabled by
   an extensible mechanism for framing message sections.

   Both sections are composed of any number of "field lines", each with
   a "field name" (see Section 5.4.3) identifying the field, and a
   "field line value" that conveys data for the field.

   Each field name present in a section has a corresponding "field
   value" for that section, composed from all field line values with
   that given field name in that section, concatenated together and
   separated with commas.  See Section 5.4.1 for further discussion of
   the semantics of field ordering and combination in messages, and
   Section 5.4.4 for more discussion of field values.

   For example, this section:

      Example-Field: Foo, Bar
      Example-Field: Baz

   contains two field lines, both with the field name "Example-Field".
   The first field line has a field line value of "Foo, Bar", while the
   second field line value is "Baz".  The field value for "Example-
   Field" is a list with three members: "Foo", "Bar", and "Baz".

   The interpretation of a field does not change between minor versions
   of the same major HTTP version, though the default behavior of a
   recipient in the absence of such a field can change.  Unless
   specified otherwise, fields are defined for all versions of HTTP.  In
   particular, the Host and Connection fields ought to be implemented by
   all HTTP/1.x implementations whether or not they advertise
   conformance with HTTP/1.1.

   New fields can be introduced without changing the protocol version if
   their defined semantics allow them to be safely ignored by recipients
   that do not recognize them; see Section 15.3.

   A proxy MUST forward unrecognized header fields unless the field name
   is listed in the Connection header field (Section 6.4.1) or the proxy
   is specifically configured to block, or otherwise transform, such
   fields.  Other recipients SHOULD ignore unrecognized header and
   trailer fields.  These requirements allow HTTP's functionality to be
   enhanced without requiring prior update of deployed intermediaries.





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5.4.1.  Field Ordering and Combination

   The order in which field lines with differing names are received in a
   message is not significant.  However, it is good practice to send
   header fields that contain control data first, such as Host on
   requests and Date on responses, so that implementations can decide
   when not to handle a message as early as possible.  A server MUST NOT
   apply a request to the target resource until the entire request
   header section is received, since later header field lines might
   include conditionals, authentication credentials, or deliberately
   misleading duplicate header fields that would impact request
   processing.

   A recipient MAY combine multiple field lines with the same field name
   into one field line, without changing the semantics of the message,
   by appending each subsequent field line value to the initial field
   line value in order, separated by a comma and OWS (optional
   whitespace).  For consistency, use comma SP.

   The order in which field lines with the same name are received is
   therefore significant to the interpretation of the field value; a
   proxy MUST NOT change the order of these field line values when
   forwarding a message.

   This means that, aside from the well-known exception noted below, a
   sender MUST NOT generate multiple field lines with the same name in a
   message (whether in the headers or trailers), or append a field line
   when a field line of the same name already exists in the message,
   unless that field's definition allows multiple field line values to
   be recombined as a comma-separated list [i.e., at least one
   alternative of the field's definition allows a comma-separated list,
   such as an ABNF rule of #(values) defined in Section 5.7.1].

      |  *Note:* In practice, the "Set-Cookie" header field ([RFC6265])
      |  often appears in a response message across multiple field lines
      |  and does not use the list syntax, violating the above
      |  requirements on multiple field lines with the same field name.
      |  Since it cannot be combined into a single field value,
      |  recipients ought to handle "Set-Cookie" as a special case while
      |  processing fields.  (See Appendix A.2.3 of [Kri2001] for
      |  details.)










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5.4.2.  Field Limits

   HTTP does not place a predefined limit on the length of each field
   line, field value, or on the length of a header or trailer section as
   a whole, as described in Section 2.  Various ad hoc limitations on
   individual lengths are found in practice, often depending on the
   specific field's semantics.

   A server that receives a request header field line, field value, or
   set of fields larger than it wishes to process MUST respond with an
   appropriate 4xx (Client Error) status code.  Ignoring such header
   fields would increase the server's vulnerability to request smuggling
   attacks (Section 11.2 of [Messaging]).

   A client MAY discard or truncate received field lines that are larger
   than the client wishes to process if the field semantics are such
   that the dropped value(s) can be safely ignored without changing the
   message framing or response semantics.

5.4.3.  Field Names

   The field-name token labels the corresponding field value as having
   the semantics defined by that field.  For example, the Date header
   field is defined in Section 9.2.2 as containing the origination
   timestamp for the message in which it appears.

     field-name     = token

   Field names are case-insensitive and ought to be registered within
   the "Hypertext Transfer Protocol (HTTP) Field Name Registry"; see
   Section 15.3.1.

5.4.4.  Field Values

   HTTP field values typically have their syntax defined using ABNF
   ([RFC5234]), using the extension defined in Section 5.7.1 as
   necessary, and are usually constrained to the range of US-ASCII
   characters.  Fields needing a greater range of characters can use an
   encoding such as the one defined in [RFC8187].

     field-value    = *field-content
     field-content  = field-vchar
                      [ 1*( SP / HTAB / field-vchar ) field-vchar ]
     field-vchar    = VCHAR / obs-text

   Historically, HTTP allowed field content with text in the ISO-8859-1
   charset [ISO-8859-1], supporting other charsets only through use of
   [RFC2047] encoding.  In practice, most HTTP field values use only a



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   subset of the US-ASCII charset [USASCII].  Newly defined fields
   SHOULD limit their values to US-ASCII octets.  A recipient SHOULD
   treat other octets in field content (obs-text) as opaque data.

   Field values containing control (CTL) characters such as CR or LF are
   invalid; recipients MUST either reject a field value containing
   control characters, or convert them to SP before processing or
   forwarding the message.

   Leading and trailing whitespace in raw field values is removed upon
   field parsing (e.g., Section 5.1 of [Messaging]).  Field definitions
   where leading or trailing whitespace in values is significant will
   have to use a container syntax such as quoted-string (Section 5.7.4).

   Commas (",") often are used to separate members in field values.
   Fields that allow multiple members are referred to as list-based
   fields.  Fields that only anticipate a single member are referred to
   as singleton fields.

   Because commas are used as a generic delimiter between members, they
   need to be treated with care if they are allowed as data within a
   member.  This is true for both list-based and singleton fields, since
   a singleton field might be sent with multiple members erroneously;
   being able to detect this condition improves interoperability.
   Fields that expect to contain a comma within a member, such as an
   HTTP-date or URI-reference element, ought to be defined with
   delimiters around that element to distinguish any comma within that
   data from potential list separators.

   For example, a textual date and a URI (either of which might contain
   a comma) could be safely carried in list-based 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 fields (such as Content-Type, defined in Section 7.4) use a
   common syntax for parameters that allows both unquoted (token) and
   quoted (quoted-string) syntax for a parameter value (Section 5.7.6).
   Use of common syntax allows recipients to reuse existing parser
   components.  When allowing both forms, the meaning of a parameter
   value ought to be the same whether it was received as a token or a
   quoted string.



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   Historically, HTTP field values could be extended over multiple lines
   by preceding each extra line with at least one space or horizontal
   tab (obs-fold).  This document assumes that any such obsolete line
   folding has been replaced with one or more SP octets prior to
   interpreting the field value, as described in Section 5.2 of
   [Messaging].

      |  *Note:* This specification does not use ABNF rules to define
      |  each "Field Name: Field Value" pair, as was done in earlier
      |  editions (published before [RFC7230]).  Instead, ABNF rules are
      |  named according to each registered field name, wherein the rule
      |  defines the valid grammar for that field's corresponding field
      |  values (i.e., after the field value has been extracted by a
      |  generic field parser).

5.5.  Payload

   Some HTTP messages transfer a complete or partial representation as
   the message "payload".  In some cases, a payload might contain only
   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).

5.5.1.  Purpose

   The purpose of a payload in a request is defined by the method
   semantics.  For example, a representation in the payload of a PUT
   request (Section 8.3.4) 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 8.3.3)
   represents information to be processed by the target resource.

   In a response, the payload's purpose is defined by both the request
   method and the response status code.  For example, the payload of a
   200 (OK) response to GET (Section 8.3.1) represents the current state
   of the target resource, as observed at the time of the message
   origination date (Section 9.2.2), whereas the payload of the same
   status code in a response to POST might represent either the
   processing result or the new state of the target resource after
   applying the processing.  Response messages with an error status code
   usually contain a payload that represents the error condition, such
   that it describes the error state and what next steps are suggested
   for resolving it.








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5.5.2.  Identification

   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.

   For 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 this specification).  The information
      might still be useful for revision history links.

   o  Otherwise, the payload is unidentified.

   For a response message, the following rules are applied in order
   until a match is found:

   1.  If the request method is GET or HEAD and the response status code
       is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not
       Modified), the payload is a representation of the resource
       identified by the target URI (Section 6.1).

   2.  If the request method is GET or HEAD and the response status code
       is 203 (Non-Authoritative Information), the payload is a
       potentially modified or enhanced representation of the target
       resource as provided by an intermediary.

   3.  If the response has a Content-Location header field and its field
       value is a reference to the same URI as the target URI, the
       payload is a representation of the target resource.

   4.  If the response has a Content-Location header field and its field
       value is a reference to a URI different from the target 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 this specification).

   5.  Otherwise, the payload is unidentified.








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5.5.3.  Payload Metadata

   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.

5.5.4.  Payload Body

   The payload body contains the data of a request or response.  This is
   distinct from the message body (e.g., Section 6 of [Messaging]),
   which is how the payload body is transferred "on the wire", and might
   be encoded, depending on the HTTP version in use.

   It is also distinct from a request or response's representation data
   (Section 7.2), which can be inferred from protocol operation, rather
   than necessarily appearing "on the wire."

   The presence of a payload body in a request depends on whether the
   request method used defines semantics for it.

   The presence of a payload body in a response depends on both the
   request method to which it is responding and the response status code
   (Section 14).

   Responses to the HEAD request method (Section 8.3.2) never include a
   payload body because the associated response header fields indicate
   only what their values would have been if the request method had been
   GET (Section 8.3.1).

   2xx (Successful) responses to a CONNECT request method
   (Section 8.3.6) switch the connection to tunnel mode instead of
   having a payload body.

   All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
   responses do not include a payload body.

   All other responses do include a payload body, although that body
   might be of zero length.

5.6.  Trailer Fields










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5.6.1.  Purpose

   In some HTTP versions, additional metadata can be sent after the
   initial header section has been completed (during or after
   transmission of the payload body), such as a message integrity check,
   digital signature, or post-processing status.  For example, the
   chunked coding in HTTP/1.1 allows a trailer section after the payload
   body (Section 7.1.2 of [Messaging]) which can contain trailer fields:
   field names and values that share the same syntax and namespace as
   header fields but that are received after the header section.

   Trailer fields ought to be processed and stored separately from the
   fields in the header section to avoid contradicting message semantics
   known at the time the header section was complete.  The presence or
   absence of certain header fields might impact choices made for the
   routing or processing of the message as a whole before the trailers
   are received; those choices cannot be unmade by the later discovery
   of trailer fields.

5.6.2.  Limitations

   Many fields cannot be processed outside the header section because
   their evaluation is necessary prior to receiving the message body,
   such as those that describe message framing, routing, authentication,
   request modifiers, response controls, or payload format.  A sender
   MUST NOT generate a trailer field unless the sender knows the
   corresponding header field name's definition permits the field to be
   sent in trailers.

   Trailer fields can be difficult to process by intermediaries that
   forward messages from one protocol version to another.  If the entire
   message can be buffered in transit, some intermediaries could merge
   trailer fields into the header section (as appropriate) before it is
   forwarded.  However, in most cases, the trailers are simply
   discarded.  A recipient MUST NOT merge a trailer field into a header
   section unless the recipient understands the corresponding header
   field definition and that definition explicitly permits and defines
   how trailer field values can be safely merged.

   The presence of the keyword "trailers" in the TE header field
   (Section 9.1.4) indicates that the client is willing to accept
   trailer fields, on behalf of itself and any downstream clients.  For
   requests from an intermediary, this implies that all downstream
   clients are willing to accept trailer fields in the forwarded
   response.  Note that the presence of "trailers" does not mean that
   the client(s) will process any particular trailer field in the
   response; only that the trailer section(s) will not be dropped by any
   of the clients.



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   Because of the potential for trailer fields to be discarded in
   transit, a server SHOULD NOT generate trailer fields that it believes
   are necessary for the user agent to receive.

5.6.3.  Processing

   Like header fields, trailer fields with the same name are processed
   in the order received; multiple trailer field lines with the same
   name have the equivalent semantics as appending the multiple values
   as a list of members, even when the field lines are received in
   separate trailer sections.  Trailer fields that might be generated
   more than once during a message MUST be defined as a list value even
   if each member value is only processed once per field line received.

   Trailer fields are expected (but not required) to be processed one
   trailer section at a time.  That is, for each trailer section
   received, a recipient that is looking for trailer fields will parse
   the received section into fields, invoke any associated processing
   for those fields at that point in the message processing, and then
   append those fields to the set of trailer fields received for the
   overall message.

   This behavior allows for iterative processing of trailer fields that
   contain incremental signatures or mid-stream status information, and
   fields that might refer to each other's values within the same
   section.  However, there is no guarantee that trailer sections won't
   shift in relation to the message body stream, or won't be recombined
   (or dropped) in transit, so trailer fields that refer to data outside
   the present trailer section need to use self-descriptive references
   (i.e., refer to the data by name, ordinal position, or an octet
   range) rather than assume it is the data most recently received.

   Likewise, at the end of a message, a recipient MAY treat the entire
   set of received trailer fields as one data structure to be considered
   as the message concludes.  Additional processing expectations, if
   any, can be defined within the field specification for a field
   intended for use in trailers.

5.7.  Common Rules for Defining Field Values

5.7.1.  Lists (#rule ABNF Extension)

   A #rule extension to the ABNF rules of [RFC5234] is used to improve
   readability in the definitions of some list-based field values.







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   A construct "#" is defined, similar to "*", for defining comma-
   delimited lists of elements.  The full form is "<n>#<m>element"
   indicating at least <n> and at most <m> elements, each separated by a
   single comma (",") and optional whitespace (OWS).

5.7.1.1.  Sender Requirements

   In any production that uses the list construct, a sender MUST NOT
   generate empty list elements.  In other words, a sender MUST generate
   lists that satisfy the following syntax:

     1#element => element *( OWS "," OWS element )

   and:

     #element => [ 1#element ]

   and for n >= 1 and m > 1:

     <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )

   Appendix A shows the collected ABNF for senders after the list
   constructs have been expanded.

5.7.1.2.  Recipient Requirements

   Empty elements do not contribute to the count of elements present.  A
   recipient MUST parse and ignore a reasonable number of empty list
   elements: enough to handle common mistakes by senders that merge
   values, but not so much that they could be used as a denial-of-
   service mechanism.  In other words, a recipient MUST accept lists
   that satisfy the following syntax:

     #element => [ element ] *( OWS "," OWS [ element ] )

   Note that because of the potential presence of empty list elements,
   the RFC 5234 ABNF cannot enforce the cardinality of list elements,
   and consequently all cases are mapped as if there was no cardinality
   specified.

   For example, given these ABNF productions:

     example-list      = 1#example-list-elmt
     example-list-elmt = token ; see Section 5.7.2

   Then the following are valid values for example-list (not including
   the double quotes, which are present for delimitation only):




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     "foo,bar"
     "foo ,bar,"
     "foo , ,bar,charlie"

   In contrast, the following values would be invalid, since at least
   one non-empty element is required by the example-list production:

     ""
     ","
     ",   ,"

5.7.2.  Tokens

   Many HTTP field values are defined using common syntax components,
   separated by whitespace or specific delimiting characters.
   Delimiters are chosen from the set of US-ASCII visual characters not
   allowed in a token (DQUOTE and "(),/:;<=>?@[\]{}").

   Tokens are short textual identifiers that do not include whitespace
   or delimiters.

     token          = 1*tchar

     tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                    / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                    / DIGIT / ALPHA
                    ; any VCHAR, except delimiters

5.7.3.  Whitespace

   This specification uses three rules to denote the use of linear
   whitespace: OWS (optional whitespace), RWS (required whitespace), and
   BWS ("bad" whitespace).

   The OWS rule is used where zero or more linear whitespace octets
   might appear.  For protocol elements where optional whitespace is
   preferred to improve readability, a sender SHOULD generate the
   optional whitespace as a single SP; otherwise, a sender SHOULD NOT
   generate optional whitespace except as needed to white out invalid or
   unwanted protocol elements during in-place message filtering.

   The RWS rule is used when at least one linear whitespace octet is
   required to separate field tokens.  A sender SHOULD generate RWS as a
   single SP.

   OWS and RWS have the same semantics as a single SP.  Any content
   known to be defined as OWS or RWS MAY be replaced with a single SP
   before interpreting it or forwarding the message downstream.



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   The BWS rule is used where the grammar allows optional whitespace
   only for historical reasons.  A sender MUST NOT generate BWS in
   messages.  A recipient MUST parse for such bad whitespace and remove
   it before interpreting the protocol element.

   BWS has no semantics.  Any content known to be defined as BWS MAY be
   removed before interpreting it or forwarding the message downstream.

     OWS            = *( SP / HTAB )
                    ; optional whitespace
     RWS            = 1*( SP / HTAB )
                    ; required whitespace
     BWS            = OWS
                    ; "bad" whitespace

5.7.4.  Quoted Strings

   A string of text is parsed as a single value if it is quoted using
   double-quote marks.

     quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
     qdtext         = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
     obs-text       = %x80-FF

   The backslash octet ("\") can be used as a single-octet quoting
   mechanism within quoted-string and comment constructs.  Recipients
   that process the value of a quoted-string MUST handle a quoted-pair
   as if it were replaced by the octet following the backslash.

     quoted-pair    = "\" ( HTAB / SP / VCHAR / obs-text )

   A sender SHOULD NOT generate a quoted-pair in a quoted-string except
   where necessary to quote DQUOTE and backslash octets occurring within
   that string.  A sender SHOULD NOT generate a quoted-pair in a comment
   except where necessary to quote parentheses ["(" and ")"] and
   backslash octets occurring within that comment.

5.7.5.  Comments

   Comments can be included in some HTTP fields by surrounding the
   comment text with parentheses.  Comments are only allowed in fields
   containing "comment" as part of their field value definition.

     comment        = "(" *( ctext / quoted-pair / comment ) ")"
     ctext          = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text






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5.7.6.  Parameters

   Parameters are zero or more instances of a name=value pair; they are
   often used in field values as a common syntax for appending auxiliary
   information to an item.  Each parameter is usually delimited by an
   immediately preceding semicolon.

     parameters      = *( OWS ";" OWS [ parameter ] )
     parameter       = parameter-name "=" parameter-value
     parameter-name  = token
     parameter-value = ( token / quoted-string )

   Parameter names are case-insensitive.  Parameter values might or
   might not be case-sensitive, depending on the semantics of the
   parameter name.  Examples of parameters and some equivalent forms can
   be seen in media types (Section 7.4.1) and the Accept header field
   (Section 11.1.2).

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

      |  *Note:* Parameters do not allow whitespace (not even "bad"
      |  whitespace) around the "=" character.

5.7.7.  Date/Time Formats

   Prior to 1995, there were three different formats commonly used by
   servers to communicate timestamps.  For compatibility with old
   implementations, all three are defined here.  The preferred format is
   a fixed-length and single-zone subset of the date and time
   specification used by the Internet Message Format [RFC5322].

     HTTP-date    = IMF-fixdate / obs-date

   An example of the preferred format is

     Sun, 06 Nov 1994 08:49:37 GMT    ; IMF-fixdate

   Examples of the two obsolete formats are

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

   A recipient that parses a timestamp value in an HTTP field MUST
   accept all three HTTP-date formats.  When a sender generates a field
   that contains one or more timestamps defined as HTTP-date, the sender
   MUST generate those timestamps in the IMF-fixdate format.



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   An HTTP-date value represents time as an instance of Coordinated
   Universal Time (UTC).  The first two formats indicate UTC by the
   three-letter abbreviation for Greenwich Mean Time, "GMT", a
   predecessor of the UTC name; values in the asctime format are assumed
   to be in UTC.  A sender that generates HTTP-date values from a local
   clock ought to use NTP ([RFC5905]) or some similar protocol to
   synchronize its clock to UTC.

   Preferred format:

     IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT
     ; fixed length/zone/capitalization subset of the format
     ; see Section 3.3 of [RFC5322]

     day-name     = %s"Mon" / %s"Tue" / %s"Wed"
                  / %s"Thu" / %s"Fri" / %s"Sat" / %s"Sun"

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

     day          = 2DIGIT
     month        = %s"Jan" / %s"Feb" / %s"Mar" / %s"Apr"
                  / %s"May" / %s"Jun" / %s"Jul" / %s"Aug"
                  / %s"Sep" / %s"Oct" / %s"Nov" / %s"Dec"
     year         = 4DIGIT

     GMT          = %s"GMT"

     time-of-day  = hour ":" minute ":" second
                  ; 00:00:00 - 23:59:60 (leap second)

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

   Obsolete formats:

     obs-date     = rfc850-date / asctime-date

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

     day-name-l   = %s"Monday" / %s"Tuesday" / %s"Wednesday"
                  / %s"Thursday" / %s"Friday" / %s"Saturday"
                  / %s"Sunday"





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

   HTTP-date is case sensitive.  A sender MUST NOT generate additional
   whitespace in an HTTP-date beyond that specifically included as SP in
   the grammar.  The semantics of day-name, day, month, year, and
   time-of-day are the same as those defined for the Internet Message
   Format constructs with the corresponding name ([RFC5322],
   Section 3.3).

   Recipients of a timestamp value in rfc850-date format, which uses a
   two-digit year, MUST interpret a timestamp that appears to be more
   than 50 years in the future as representing the most recent year in
   the past that had the same last two digits.

   Recipients of timestamp values are encouraged to be robust in parsing
   timestamps unless otherwise restricted by the field definition.  For
   example, messages are occasionally forwarded over HTTP from a non-
   HTTP source that might generate any of the date and time
   specifications defined by the Internet Message Format.

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

6.  Routing

   HTTP is used in a wide variety of applications, ranging from general-
   purpose computers to home appliances.  In some cases, communication
   options are hard-coded in a client's configuration.  However, most
   HTTP clients rely on the same resource identification mechanism and
   configuration techniques as general-purpose Web browsers.

   HTTP request message routing is determined by each client based on
   the target resource, the client's proxy configuration, and
   establishment or reuse of an inbound connection.  The corresponding
   response routing follows the same connection chain back to the
   client.

6.1.  Target Resource

6.1.1.  Request Target

   The "request target" is the protocol element that identifies the
   "target resource".




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   Typically, the request target is a URI reference (Section 4) which a
   user agent would resolve to its absolute form in order to obtain the
   "target URI".  The target URI excludes the reference's fragment
   component, if any, since fragment identifiers are reserved for
   client-side processing ([RFC3986], Section 3.5).

   However, there are two special, method-specific forms allowed for the
   request target in specific circumstances:

   o  For CONNECT (Section 8.3.6), the request target is the host name
      and port number of the tunnel destination, separated by a colon.

   o  For OPTIONS (Section 8.3.7), the request target can be a single
      asterisk ("*").

   See the respective method definitions for details.  These forms MUST
   NOT be used with other methods.

6.1.2.  Host

   The "Host" header field in a request provides the host and port
   information from the target URI, enabling the origin server to
   distinguish among resources while servicing requests for multiple
   host names on a single IP address.

     Host = uri-host [ ":" port ] ; Section 4

   Since the Host field value is critical information for handling a
   request, a user agent SHOULD generate Host as the first field in the
   header section.

   For example, a GET request to the origin server for
   <http://www.example.org/pub/WWW/> would begin with:

     GET /pub/WWW/ HTTP/1.1
     Host: www.example.org

   Since the Host header field acts as an application-level routing
   mechanism, it is a frequent target for malware seeking to poison a
   shared cache or redirect a request to an unintended server.  An
   interception proxy is particularly vulnerable if it relies on the
   Host field value for redirecting requests to internal servers, or for
   use as a cache key in a shared cache, without first verifying that
   the intercepted connection is targeting a valid IP address for that
   host.






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6.1.3.  Reconstructing the Target URI

   Once an inbound connection is obtained, the client sends an HTTP
   request message.

   Depending on the nature of the request, the client's target URI might
   be split into components and transmitted (or implied) within various
   parts of a request message.  These parts are recombined by each
   recipient, in accordance with their local configuration and incoming
   connection context, to determine the target URI.  Appendix of
   [Messaging] defines how a server determines the target URI for an
   HTTP/1.1 request.

   Once the target URI has been reconstructed, an origin server needs to
   decide whether or not to provide service for that URI via the
   connection in which the request was received.  For example, the
   request might have been misdirected, deliberately or accidentally,
   such that the information within a received Host header field differs
   from the host or port upon which the connection has been made.  If
   the connection is from a trusted gateway, that inconsistency might be
   expected; otherwise, it might indicate an attempt to bypass security
   filters, trick the server into delivering non-public content, or
   poison a cache.  See Section 16 for security considerations regarding
   message routing.

      |  *Note:* previous specifications defined the recomposed target
      |  URI as a distinct concept, the effective request URI.

6.2.  Routing Inbound

   Once the target URI and its origin are determined, a client decides
   whether a network request is necessary to accomplish the desired
   semantics and, if so, where that request is to be directed.

6.2.1.  To a Cache

   If the client has a cache [Caching] and the request can be satisfied
   by it, then the request is usually directed there first.













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6.2.2.  To a Proxy

   If the request is not satisfied by a cache, then a typical client
   will check its configuration to determine whether a proxy is to be
   used to satisfy the request.  Proxy configuration is implementation-
   dependent, but is often based on URI prefix matching, selective
   authority matching, or both, and the proxy itself is usually
   identified by an "http" or "https" URI.  If a proxy is applicable,
   the client connects inbound by establishing (or reusing) a connection
   to that proxy.

6.2.3.  To the Origin

   If no proxy is applicable, a typical client will invoke a handler
   routine, usually specific to the target URI's scheme, to connect
   directly to an origin for the target resource.  How that is
   accomplished is dependent on the target URI scheme and defined by its
   associated specification.

6.3.  Response Correlation

   A connection might be used for multiple request/response exchanges.
   The mechanism used to correlate between request and response messages
   is version dependent; some versions of HTTP use implicit ordering of
   messages, while others use an explicit identifier.

   Responses (both final and interim) can be sent at any time after a
   request is received, even if it is not yet complete.  However,
   clients (including intermediaries) might abandon a request if the
   response is not forthcoming within a reasonable period of time.

6.4.  Message Forwarding

   As described in Section 3.7, intermediaries can serve a variety of
   roles in the processing of HTTP requests and responses.  Some
   intermediaries are used to improve performance or availability.
   Others are used for access control or to filter content.  Since an
   HTTP stream has characteristics similar to a pipe-and-filter
   architecture, there are no inherent limits to the extent an
   intermediary can enhance (or interfere) with either direction of the
   stream.

   An intermediary not acting as a tunnel MUST implement the Connection
   header field, as specified in Section 6.4.1, and exclude fields from
   being forwarded that are only intended for the incoming connection.






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   An intermediary MUST NOT forward a message to itself unless it is
   protected from an infinite request loop.  In general, an intermediary
   ought to recognize its own server names, including any aliases, local
   variations, or literal IP addresses, and respond to such requests
   directly.

   An HTTP message can be parsed as a stream for incremental processing
   or forwarding downstream.  However, recipients cannot rely on
   incremental delivery of partial messages, since some implementations
   will buffer or delay message forwarding for the sake of network
   efficiency, security checks, or payload transformations.

6.4.1.  Connection

   The "Connection" header field allows the sender to list desired
   control options for the current connection.

   When a field aside from Connection is used to supply control
   information for or about the current connection, the sender MUST list
   the corresponding field name within the Connection header field.
   Note that some versions of HTTP prohibit the use of fields for such
   information, and therefore do not allow the Connection field.

   Intermediaries MUST parse a received Connection header field before a
   message is forwarded and, for each connection-option in this field,
   remove any header or trailer field(s) from the message with the same
   name as the connection-option, and then remove the Connection header
   field itself (or replace it with the intermediary's own connection
   options for the forwarded message).

   Hence, the Connection header field provides a declarative way of
   distinguishing fields that are only intended for the immediate
   recipient ("hop-by-hop") from those fields that are intended for all
   recipients on the chain ("end-to-end"), enabling the message to be
   self-descriptive and allowing future connection-specific extensions
   to be deployed without fear that they will be blindly forwarded by
   older intermediaries.

   Furthermore, intermediaries SHOULD remove or replace field(s) whose
   semantics are known to require removal before forwarding, whether or
   not they appear as a Connection option, after applying those fields'
   semantics.  This includes but is not limited to:

   o  Proxy-Connection (Appendix C.1.2 of [Messaging])

   o  Keep-Alive (Section 19.7.1 of [RFC2068])

   o  TE (Section 9.1.4)



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   o  Trailer (Section 9.1.5)

   o  Transfer-Encoding (Section 6.1 of [Messaging])

   o  Upgrade (Section 6.6)

   The Connection header field's value has the following grammar:

     Connection        = #connection-option
     connection-option = token

   Connection options are case-insensitive.

   A sender MUST NOT send a connection option corresponding to a field
   that is intended for all recipients of the payload.  For example,
   Cache-Control is never appropriate as a connection option
   (Section 5.2 of [Caching]).

   The connection options do not always correspond to a field present in
   the message, since a connection-specific field might not be needed if
   there are no parameters associated with a connection option.  In
   contrast, a connection-specific field that is received without a
   corresponding connection option usually indicates that the field has
   been improperly forwarded by an intermediary and ought to be ignored
   by the recipient.

   When defining new connection options, specification authors ought to
   document it as reserved field name and register that definition in
   the Hypertext Transfer Protocol (HTTP) Field Name Registry
   (Section 15.3.1), to avoid collisions.

6.4.2.  Max-Forwards

   The "Max-Forwards" header field provides a mechanism with the TRACE
   (Section 8.3.8) and OPTIONS (Section 8.3.7) request 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 that
   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 intermediary that receives 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
   intermediary MUST NOT forward the request; instead, the intermediary



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   MUST respond as the final recipient.  If the received Max-Forwards
   value is greater than zero, the intermediary MUST generate an updated
   Max-Forwards field in the forwarded message with a field value that
   is the lesser of a) the received value decremented by one (1) or b)
   the recipient's maximum supported value for Max-Forwards.

   A recipient MAY ignore a Max-Forwards header field received with any
   other request methods.

6.4.3.  Via

   The "Via" header field indicates the presence of intermediate
   protocols and recipients between the user agent and the server (on
   requests) or between the origin server and the client (on responses),
   similar to the "Received" header field in email (Section 3.6.7 of
   [RFC5322]).  Via can be used for tracking message forwards, avoiding
   request loops, and identifying the protocol capabilities of senders
   along the request/response chain.

     Via = #( received-protocol RWS received-by [ RWS comment ] )

     received-protocol = [ protocol-name "/" ] protocol-version
                       ; see Section 6.6
     received-by       = pseudonym [ ":" port ]
     pseudonym         = token

   Each member of the Via field value represents a proxy or gateway that
   has forwarded the message.  Each intermediary appends its own
   information about how the message was received, such that the end
   result is ordered according to the sequence of forwarding recipients.

   A proxy MUST send an appropriate Via header field, as described
   below, in each message that it forwards.  An HTTP-to-HTTP gateway
   MUST send an appropriate Via header field in each inbound request
   message and MAY send a Via header field in forwarded response
   messages.

   For each intermediary, the received-protocol indicates the protocol
   and protocol version used by the upstream sender of the message.
   Hence, the Via field value records the advertised protocol
   capabilities of the request/response chain such that they remain
   visible to downstream recipients; this can be useful for determining
   what backwards-incompatible features might be safe to use in
   response, or within a later request, as described in Section 5.1.
   For brevity, the protocol-name is omitted when the received protocol
   is HTTP.





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   The received-by portion is normally the host and optional port number
   of a recipient server or client that subsequently forwarded the
   message.  However, if the real host is considered to be sensitive
   information, a sender MAY replace it with a pseudonym.  If a port is
   not provided, a recipient MAY interpret that as meaning it was
   received on the default TCP port, if any, for the received-protocol.

   A sender MAY generate comments to identify the software of each
   recipient, analogous to the User-Agent and Server header fields.
   However, comments in Via are optional, and a recipient MAY remove
   them prior to forwarding the message.

   For example, a request message could be sent from an HTTP/1.0 user
   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
   forward the request to a public proxy at p.example.net, which
   completes the request by forwarding it to the origin server at
   www.example.com.  The request received by www.example.com would then
   have the following Via header field:

     Via: 1.0 fred, 1.1 p.example.net

   An intermediary used as a portal through a network firewall SHOULD
   NOT forward the names and ports of hosts within the firewall region
   unless it is explicitly enabled to do so.  If not enabled, such an
   intermediary SHOULD replace each received-by host of any host behind
   the firewall by an appropriate pseudonym for that host.

   An intermediary MAY combine an ordered subsequence of Via header
   field list members into a single member if the entries have identical
   received-protocol values.  For example,

     Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

   could be collapsed to

     Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

   A sender SHOULD NOT combine multiple list members unless they are all
   under the same organizational control and the hosts have already been
   replaced by pseudonyms.  A sender MUST NOT combine members that have
   different received-protocol values.










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6.5.  Transformations

   Some intermediaries include features for transforming messages and
   their payloads.  A proxy might, for example, convert between image
   formats in order to save cache space or to reduce the amount of
   traffic on a slow link.  However, operational problems might occur
   when these transformations are applied to payloads intended for
   critical applications, such as medical imaging or scientific data
   analysis, particularly when integrity checks or digital signatures
   are used to ensure that the payload received is identical to the
   original.

   An HTTP-to-HTTP proxy is called a "transforming proxy" if it is
   designed or configured to modify messages in a semantically
   meaningful way (i.e., modifications, beyond those required by normal
   HTTP processing, that change the message in a way that would be
   significant to the original sender or potentially significant to
   downstream recipients).  For example, a transforming proxy might be
   acting as a shared annotation server (modifying responses to include
   references to a local annotation database), a malware filter, a
   format transcoder, or a privacy filter.  Such transformations are
   presumed to be desired by whichever client (or client organization)
   chose the proxy.

   If a proxy receives a target URI with a host name that is not a fully
   qualified domain name, it MAY add its own domain to the host name it
   received when forwarding the request.  A proxy MUST NOT change the
   host name if the target URI contains a fully qualified domain name.

   A proxy MUST NOT modify the "absolute-path" and "query" parts of the
   received target URI when forwarding it to the next inbound server,
   except as noted above to replace an empty path with "/" or "*".

   A proxy MUST NOT transform the payload (Section 5.5) of a message
   that contains a no-transform cache-control response directive
   (Section 5.2 of [Caching]).  Note that this does not include changes
   to the message body that do not affect the payload, such as transfer
   codings (Section 7 of [Messaging]).

   A proxy MAY transform the payload of a message that does not contain
   a no-transform cache-control directive.  A proxy that transforms the
   payload of a 200 (OK) response can inform downstream recipients that
   a transformation has been applied by changing the response status
   code to 203 (Non-Authoritative Information) (Section 14.3.4).







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   A proxy SHOULD NOT modify header fields that provide information
   about the endpoints of the communication chain, the resource state,
   or the selected representation (other than the payload) unless the
   field's definition specifically allows such modification or the
   modification is deemed necessary for privacy or security.

6.6.  Upgrade

   The "Upgrade" header field is intended to provide a simple mechanism
   for transitioning from HTTP/1.1 to some other protocol on the same
   connection.

   A client MAY send a list of protocol names in the Upgrade header
   field of a request to invite the server to switch to one or more of
   the named protocols, in order of descending preference, before
   sending the final response.  A server MAY ignore a received Upgrade
   header field if it wishes to continue using the current protocol on
   that connection.  Upgrade cannot be used to insist on a protocol
   change.

     Upgrade          = #protocol

     protocol         = protocol-name ["/" protocol-version]
     protocol-name    = token
     protocol-version = token

   Although protocol names are registered with a preferred case,
   recipients SHOULD use case-insensitive comparison when matching each
   protocol-name to supported protocols.

   A server that sends a 101 (Switching Protocols) response MUST send an
   Upgrade header field to indicate the new protocol(s) to which the
   connection is being switched; if multiple protocol layers are being
   switched, the sender MUST list the protocols in layer-ascending
   order.  A server MUST NOT switch to a protocol that was not indicated
   by the client in the corresponding request's Upgrade header field.  A
   server MAY choose to ignore the order of preference indicated by the
   client and select the new protocol(s) based on other factors, such as
   the nature of the request or the current load on the server.

   A server that sends a 426 (Upgrade Required) response MUST send an
   Upgrade header field to indicate the acceptable protocols, in order
   of descending preference.

   A server MAY send an Upgrade header field in any other response to
   advertise that it implements support for upgrading to the listed
   protocols, in order of descending preference, when appropriate for a
   future request.



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   The following is a hypothetical example sent by a client:

     GET /hello HTTP/1.1
     Host: www.example.com
     Connection: upgrade
     Upgrade: websocket, IRC/6.9, RTA/x11


   The capabilities and nature of the application-level communication
   after the protocol change is entirely dependent upon the new
   protocol(s) chosen.  However, immediately after sending the 101
   (Switching Protocols) response, the server is expected to continue
   responding to the original request as if it had received its
   equivalent within the new protocol (i.e., the server still has an
   outstanding request to satisfy after the protocol has been changed,
   and is expected to do so without requiring the request to be
   repeated).

   For example, if the Upgrade header field is received in a GET request
   and the server decides to switch protocols, it first responds with a
   101 (Switching Protocols) message in HTTP/1.1 and then immediately
   follows that with the new protocol's equivalent of a response to a
   GET on the target resource.  This allows a connection to be upgraded
   to protocols with the same semantics as HTTP without the latency cost
   of an additional round trip.  A server MUST NOT switch protocols
   unless the received message semantics can be honored by the new
   protocol; an OPTIONS request can be honored by any protocol.

   The following is an example response to the above hypothetical
   request:

     HTTP/1.1 101 Switching Protocols
     Connection: upgrade
     Upgrade: websocket

     [... data stream switches to websocket with an appropriate response
     (as defined by new protocol) to the "GET /hello" request ...]

   When Upgrade is sent, the sender MUST also send a Connection header
   field (Section 6.4.1) that contains an "upgrade" connection option,
   in order to prevent Upgrade from being accidentally forwarded by
   intermediaries that might not implement the listed protocols.  A
   server MUST ignore an Upgrade header field that is received in an
   HTTP/1.0 request.

   A client cannot begin using an upgraded protocol on the connection
   until it has completely sent the request message (i.e., the client
   can't change the protocol it is sending in the middle of a message).



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   If a server receives both an Upgrade and an Expect header field with
   the "100-continue" expectation (Section 9.1.1), the server MUST send
   a 100 (Continue) response before sending a 101 (Switching Protocols)
   response.

   The Upgrade header field only applies to switching protocols on top
   of the existing connection; it cannot be used to switch the
   underlying connection (transport) protocol, nor to switch the
   existing communication to a different connection.  For those
   purposes, it is more appropriate to use a 3xx (Redirection) response
   (Section 14.4).

   This specification only defines the protocol name "HTTP" for use by
   the family of Hypertext Transfer Protocols, as defined by the HTTP
   version rules of Section 5.1 and future updates to this
   specification.  Additional protocol names ought to be registered
   using the registration procedure defined in Section 15.7.

7.  Representations

   A "representation" is information that is intended to reflect a past,
   current, or desired state of a given resource, in a format that can
   be readily communicated via the protocol.  A representation consists
   of a set of representation metadata and a potentially unbounded
   stream of representation data.

   HTTP allows "information hiding" behind its uniform interface by
   phrasing communication with respect to a transferable representation
   of the resource state, rather than transferring the resource itself.
   This allows the resource identified by a URI to be anything,
   including temporal functions like "the current weather in Laguna
   Beach", while potentially providing information that represents that
   resource at the time a message is generated [REST].

   The uniform interface is similar to a window through which one can
   observe and act upon a thing only through the communication of
   messages to an independent actor on the other side.  A shared
   abstraction is needed to represent ("take the place of") the current
   or desired state of that thing in our communications.  When a
   representation is hypertext, it can provide both a representation of
   the resource state and processing instructions that help guide the
   recipient's future interactions.









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7.1.  Selected Representation

   An origin server might be provided with, or be capable of generating,
   multiple representations that are each intended to reflect the
   current state of a target resource.  In such cases, some algorithm is
   used by the origin server to select one of those representations as
   most applicable to a given request, usually based on content
   negotiation.  This "selected representation" is used to provide the
   data and metadata for evaluating conditional requests (Section 12.1)
   and constructing the payload for 200 (OK), 206 (Partial Content), and
   304 (Not Modified) responses to GET (Section 8.3.1).

7.2.  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 target 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 ) )

7.3.  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.

   The following header fields convey representation metadata:














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    ------------------ ------
     Field Name         Ref.
    ------------------ ------
     Content-Type       7.4
     Content-Encoding   7.5
     Content-Language   7.6
     Content-Length     7.7
     Content-Location   7.8
    ------------------ ------

             Table 3

7.4.  Content-Type

   The "Content-Type" header field indicates the media type of the
   associated representation: either the representation enclosed in the
   message payload or the selected representation, as determined by the
   message semantics.  The indicated media type defines both the data
   format and how that data is intended to be processed by a recipient,
   within the scope of the received message semantics, after any content
   codings indicated by Content-Encoding are decoded.

     Content-Type = media-type

   Media types are defined in Section 7.4.1.  An example of the field is

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

   A sender that generates a message containing a payload body SHOULD
   generate a Content-Type header field in that message unless the
   intended media type of the enclosed representation is unknown to the
   sender.  If a Content-Type header field is not present, the recipient
   MAY either assume a media type of "application/octet-stream"
   ([RFC2046], Section 4.5.1) or examine the 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.  Some user agents examine a payload's content and, in
   certain cases, override the received type (for example, see
   [Sniffing]).  This "MIME sniffing" risks drawing incorrect
   conclusions about the data, which might expose the user to additional
   security risks (e.g., "privilege escalation").  Furthermore, it is
   impossible to determine the sender's intended processing model 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 to disable such sniffing.





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   Furthermore, although Content-Type is defined as a singleton field,
   it is sometimes incorrectly generated multiple times, resulting in a
   combined field value that appears to be a list.  Recipients often
   attempt to handle this error by using the last syntactically valid
   member of the list, but note that some implementations might have
   different error handling behaviors, leading to interoperability and/
   or security issues.

7.4.1.  Media Type

   HTTP uses media types [RFC2046] in the Content-Type (Section 7.4) and
   Accept (Section 11.1.2) header fields in order to provide open and
   extensible data typing and type negotiation.  Media types define both
   a data format and various processing models: how to process that data
   in accordance with each context in which it is received.

     media-type = type "/" subtype parameters
     type       = token
     subtype    = token

   The type and subtype tokens are case-insensitive.

   The type/subtype MAY be followed by semicolon-delimited parameters
   (Section 5.7.6) in the form of name=value pairs.  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.  Parameter values might or might not be case-sensitive,
   depending on the semantics of the parameter name.

   For example, the following media types are equivalent in describing
   HTML text data encoded in the UTF-8 character encoding scheme, but
   the first is preferred for consistency (the "charset" parameter value
   is defined as being case-insensitive in [RFC2046], Section 4.1.2):

     text/html;charset=utf-8
     Text/HTML;Charset="utf-8"
     text/html; charset="utf-8"
     text/html;charset=UTF-8

   Media types ought to be registered with IANA according to the
   procedures defined in [BCP13].

7.4.2.  Charset

   HTTP uses charset names to indicate or negotiate the character
   encoding scheme of a textual representation [RFC6365].  A charset is
   identified by a case-insensitive token.




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     charset = token

   Charset names ought to be registered in the IANA "Character Sets"
   registry (<https://www.iana.org/assignments/character-sets>)
   according to the procedures defined in Section 2 of [RFC2978].

      |  *Note:* In theory, charset names are defined by the "mime-
      |  charset" ABNF rule defined in Section 2.3 of [RFC2978] (as
      |  corrected in [Err1912]).  That rule allows two characters that
      |  are not included in "token" ("{" and "}"), but no charset name
      |  registered at the time of this writing includes braces (see
      |  [Err5433]).

7.4.3.  Canonicalization and Text Defaults

   Media types are registered with a canonical form in order to be
   interoperable among systems with varying native encoding formats.
   Representations selected or transferred via HTTP ought to be in
   canonical form, for many of the same reasons described by the
   Multipurpose Internet Mail Extensions (MIME) [RFC2045].  However, the
   performance characteristics of email deployments (i.e., store and
   forward messages to peers) are significantly different from those
   common to HTTP and the Web (server-based information services).
   Furthermore, MIME's constraints for the sake of compatibility with
   older mail transfer protocols do not apply to HTTP (see Appendix B of
   [Messaging]).

   MIME's canonical form requires that media subtypes of the "text" type
   use CRLF as the text line break.  HTTP allows the transfer of text
   media with plain CR or LF alone representing a line break, when such
   line breaks are consistent for an entire representation.  An HTTP
   sender MAY generate, and a recipient MUST be able to parse, line
   breaks in text media that consist of CRLF, bare CR, or bare LF.  In
   addition, text media in HTTP is not limited to charsets that use
   octets 13 and 10 for CR and LF, respectively.  This flexibility
   regarding line breaks applies only to text within a representation
   that has been assigned a "text" media type; it does not apply to
   "multipart" types or HTTP elements outside the payload body (e.g.,
   header fields).

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









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7.4.4.  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.

   HTTP message framing does not use the multipart boundary as an
   indicator of message body length, though it might be used by
   implementations that generate or process the payload.  For example,
   the "multipart/form-data" type is often used for carrying form data
   in a request, as described in [RFC7578], and the "multipart/
   byteranges" type is defined by this specification for use in some 206
   (Partial Content) responses (see Section 14.3.7).

7.5.  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
   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 = #content-coding

   An example of its use is

     Content-Encoding: gzip

   If one or more encodings have been applied to a representation, the
   sender that applied the encodings MUST generate a Content-Encoding
   header field that lists the content codings in the order in which
   they were applied.  Note that the coding named "identity" is reserved
   for its special role in Accept-Encoding, and thus SHOULD NOT be
   included.

   Additional information about the encoding parameters can be provided
   by other header fields not defined by this specification.









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   Unlike Transfer-Encoding (Section 6.1 of [Messaging]), 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.

   If the media type includes an inherent encoding, such as a data
   format that is always compressed, then that encoding would not be
   restated in 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 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).

   An origin server MAY respond with a status code of 415 (Unsupported
   Media Type) if a representation in the request message has a content
   coding that is not acceptable.

7.5.1.  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 final recipient.

     content-coding   = token

   All content codings are case-insensitive and ought to be registered
   within the "HTTP Content Coding Registry", as described in
   Section 15.6

   Content-coding values are used in the Accept-Encoding
   (Section 11.1.4) and Content-Encoding (Section 7.5) header fields.

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






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    ------------ ------------------------------------------- ---------
     Name         Description                                 Ref.
    ------------ ------------------------------------------- ---------
     compress     UNIX "compress" data format [Welch]         7.5.1.1
     deflate      "deflate" compressed data ([RFC1951])       7.5.1.2
                  inside the "zlib" data format ([RFC1950])
     gzip         GZIP file format [RFC1952]                  7.5.1.3
     identity     Reserved                                    11.1.4
     x-compress   Deprecated (alias for compress)             7.5.1.1
     x-gzip       Deprecated (alias for gzip)                 7.5.1.3
    ------------ ------------------------------------------- ---------

                                 Table 4

7.5.1.1.  Compress Coding

   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
   [Welch] that is commonly produced by the UNIX file compression
   program "compress".  A recipient SHOULD consider "x-compress" to be
   equivalent to "compress".

7.5.1.2.  Deflate Coding

   The "deflate" coding is a "zlib" data format [RFC1950] containing a
   "deflate" compressed data stream [RFC1951] that uses a combination of
   the Lempel-Ziv (LZ77) compression algorithm and Huffman coding.

      |  *Note:* Some non-conformant implementations send the "deflate"
      |  compressed data without the zlib wrapper.

7.5.1.3.  Gzip Coding

   The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy
   Check (CRC) that is commonly produced by the gzip file compression
   program [RFC1952].  A recipient SHOULD consider "x-gzip" to be
   equivalent to "gzip".

7.6.  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 = #language-tag






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   Language tags are defined in Section 7.6.1.  The primary purpose of
   Content-Language is to allow a user to identify and differentiate
   representations according to the users' 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.

   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.

7.6.1.  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 header fields.  Accept-Language uses the broader
   language-range production defined in Section 11.1.5, whereas
   Content-Language uses the language-tag production defined below.

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

   A language tag is a sequence of one or more case-insensitive subtags,
   each separated by a hyphen character ("-", %x2D).  In most cases, a
   language tag consists of a primary language subtag that identifies a
   broad family of related languages (e.g., "en" = English), which is



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   optionally followed by a series of subtags that refine or narrow that
   language's range (e.g., "en-CA" = the variety of English as
   communicated in Canada).  Whitespace is not allowed within a language
   tag.  Example tags include:

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

   See [RFC5646] for further information.

7.7.  Content-Length

   The "Content-Length" header field indicates the associated
   representation's data length as a decimal non-negative integer number
   of octets.  When transferring a representation in a message, Content-
   Length refers specifically to the amount of data enclosed so that it
   can be used to delimit framing of the message body (e.g., Section 6.2
   of [Messaging]).  In other cases, Content-Length indicates the
   selected representation's current length, which can be used by
   recipients to estimate transfer time or compare to previously stored
   representations.

     Content-Length = 1*DIGIT

   An example is

     Content-Length: 3495

   A sender MUST NOT send a Content-Length header field in any message
   that contains a Transfer-Encoding header field.

   A user agent SHOULD send a Content-Length in a request message when
   no Transfer-Encoding is sent and the request method defines a meaning
   for an enclosed payload body.  For example, a Content-Length header
   field is normally sent in a POST request even when the value is 0
   (indicating an empty payload body).  A user agent SHOULD NOT send a
   Content-Length header field when the request message does not contain
   a payload body and the method semantics do not anticipate such a
   body.

   A server MAY send a Content-Length header field in a response to a
   HEAD request (Section 8.3.2); a server MUST NOT send Content-Length
   in such a response unless its field value equals the decimal number
   of octets that would have been sent in the payload body of a response
   if the same request had used the GET method.







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   A server MAY send a Content-Length header field in a 304 (Not
   Modified) response to a conditional GET request (Section 14.4.5); a
   server MUST NOT send Content-Length in such a response unless its
   field value equals the decimal number of octets that would have been
   sent in the payload body of a 200 (OK) response to the same request.

   A server MUST NOT send a Content-Length header field in any response
   with a status code of 1xx (Informational) or 204 (No Content).  A
   server MUST NOT send a Content-Length header field in any 2xx
   (Successful) response to a CONNECT request (Section 8.3.6).

   Aside from the cases defined above, in the absence of Transfer-
   Encoding, an origin server SHOULD send a Content-Length header field
   when the payload body size is known prior to sending the complete
   header section.  This will allow downstream recipients to measure
   transfer progress, know when a received message is complete, and
   potentially reuse the connection for additional requests.

   Any Content-Length field value greater than or equal to zero is
   valid.  Since there is no predefined limit to the length of a
   payload, a recipient MUST anticipate potentially large decimal
   numerals and prevent parsing errors due to integer conversion
   overflows (Section 16.5).

   If a message is received that has a Content-Length header field value
   consisting of the same decimal value as a comma-separated list
   (Section 5.7.1) - for example, "Content-Length: 42, 42" - indicating
   that duplicate Content-Length header fields have been generated or
   combined by an upstream message processor, then the recipient MUST
   either reject the message as invalid or replace the duplicated field
   values with a single valid Content-Length field containing that
   decimal value prior to determining the message body length or
   forwarding the message.

7.8.  Content-Location

   The "Content-Location" header field references a URI that can be used
   as an identifier for a specific resource corresponding to the
   representation in this message's payload.  In other words, if one
   were to perform a GET request 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 field value is either an absolute-URI or a partial-URI.  In the
   latter case (Section 4), the referenced URI is relative to the target
   URI ([RFC3986], Section 5).



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   The Content-Location value is not a replacement for the target URI
   (Section 6.1).  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 target URI, then the recipient MAY
   consider the payload to be a current representation of that resource
   at the time indicated by the message origination date.  For a GET
   (Section 8.3.1) or HEAD (Section 8.3.2) 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 (Section 8.3.4) or
   POST (Section 8.3.3), it implies that the server's response contains
   the new representation of that resource, thereby distinguishing it
   from representations that might only report about 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
   target URI, then the origin server claims that the URI is an
   identifier for a different resource corresponding to the enclosed
   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 target URI refers to 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.





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   A user agent that sends Content-Location in a request message is
   stating that its value refers to where the user agent originally
   obtained the content of the enclosed representation (prior to any
   modifications made by that user agent).  In other words, the user
   agent is providing a back link to the source of the original
   representation.

   An origin server that receives a Content-Location field in a request
   message MUST treat the information as transitory request context
   rather than as metadata to be saved verbatim as part of the
   representation.  An origin server MAY use that context to guide in
   processing the request or to save it for other uses, such as within
   source links or versioning metadata.  However, an origin server MUST
   NOT use such context information to alter the request semantics.

   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 state of 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 identifier to update
   only one of the negotiated representations.  If the user agent had
   wanted the latter semantics, it would have applied the PUT directly
   to the Content-Location URI.

7.9.  Validators

   Validator header fields convey metadata about the selected
   representation (Section 7).  In responses to safe requests, validator
   fields describe the selected representation chosen by the origin
   server while handling the response.  Note that, depending on the
   status code semantics, the selected representation for a given
   response is not necessarily the same as the representation enclosed
   as response payload.

   In a successful response to a state-changing request, validator
   fields describe the new representation that has replaced the prior
   selected representation as a result of processing the request.

   For example, an ETag field in a 201 (Created) response communicates
   the entity-tag of the newly created resource's representation, so
   that it can be used in later conditional requests to prevent the
   "lost update" problem Section 12.1.









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    --------------- -------
     Field Name      Ref.
    --------------- -------
     ETag            7.9.3
     Last-Modified   7.9.2
    --------------- -------

            Table 5

   This specification defines two forms of metadata that are commonly
   used to observe resource state and test for preconditions:
   modification dates (Section 7.9.2) and opaque entity tags
   (Section 7.9.3).  Additional metadata that reflects resource state
   has been defined by various extensions of HTTP, such as Web
   Distributed Authoring and Versioning (WebDAV, [RFC4918]), that are
   beyond the scope of this specification.  A resource metadata value is
   referred to as a "validator" when it is used within a precondition.

7.9.1.  Weak versus Strong

   Validators come in two flavors: strong or weak.  Weak validators are
   easy to generate but are far less useful for comparisons.  Strong
   validators are ideal for comparisons but can be very difficult (and
   occasionally impossible) to generate efficiently.  Rather than impose
   that all forms of resource adhere to the same strength of validator,
   HTTP exposes the type of validator in use and imposes restrictions on
   when weak validators can be used as preconditions.

   A "strong validator" is representation metadata that changes value
   whenever a change occurs to the representation data that would be
   observable in the payload body of a 200 (OK) response to GET.

   A strong validator might change for reasons other than a change to
   the representation data, such as when a semantically significant part
   of the representation metadata is changed (e.g., Content-Type), but
   it is in the best interests of the origin server to only change the
   value when it is necessary to invalidate the stored responses held by
   remote caches and authoring tools.

   Cache entries might persist for arbitrarily long periods, regardless
   of expiration times.  Thus, a cache might attempt to validate an
   entry using a validator that it obtained in the distant past.  A
   strong validator is unique across all versions of all representations
   associated with a particular resource over time.  However, there is
   no implication of uniqueness across representations of different
   resources (i.e., the same strong validator might be in use for
   representations of multiple resources at the same time and does not
   imply that those representations are equivalent).



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   There are a variety of strong validators used in practice.  The best
   are based on strict revision control, wherein each change to a
   representation always results in a unique node name and revision
   identifier being assigned before the representation is made
   accessible to GET.  A collision-resistant hash function applied to
   the representation data is also sufficient if the data is available
   prior to the response header fields being sent and the digest does
   not need to be recalculated every time a validation request is
   received.  However, if a resource has distinct representations that
   differ only in their metadata, such as might occur with content
   negotiation over media types that happen to share the same data
   format, then the origin server needs to incorporate additional
   information in the validator to distinguish those representations.

   In contrast, a "weak validator" is representation metadata that might
   not change for every change to the representation data.  This
   weakness might be due to limitations in how the value is calculated,
   such as clock resolution, an inability to ensure uniqueness for all
   possible representations of the resource, or a desire of the resource
   owner to group representations by some self-determined set of
   equivalency rather than unique sequences of data.  An origin server
   SHOULD change a weak entity-tag whenever it considers prior
   representations to be unacceptable as a substitute for the current
   representation.  In other words, a weak entity-tag ought to change
   whenever the origin server wants caches to invalidate old responses.

   For example, the representation of a weather report that changes in
   content every second, based on dynamic measurements, might be grouped
   into sets of equivalent representations (from the origin server's
   perspective) with the same weak validator in order to allow cached
   representations to be valid for a reasonable period of time (perhaps
   adjusted dynamically based on server load or weather quality).
   Likewise, a representation's modification time, if defined with only
   one-second resolution, might be a weak validator if it is possible
   for the representation to be modified twice during a single second
   and retrieved between those modifications.

   Likewise, a validator is weak if it is shared by two or more
   representations of a given resource at the same time, unless those
   representations have identical representation data.  For example, if
   the origin server sends the same validator for a representation with
   a gzip content coding applied as it does for a representation with no
   content coding, then that validator is weak.  However, two
   simultaneous representations might share the same strong validator if
   they differ only in the representation metadata, such as when two
   different media types are available for the same representation data.





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   Strong validators are usable for all conditional requests, including
   cache validation, partial content ranges, and "lost update"
   avoidance.  Weak validators are only usable when the client does not
   require exact equality with previously obtained representation data,
   such as when validating a cache entry or limiting a web traversal to
   recent changes.

7.9.2.  Last-Modified

   The "Last-Modified" header field in a response provides a timestamp
   indicating the date and time at which the origin server believes the
   selected representation was last modified, as determined at the
   conclusion of handling the request.

     Last-Modified = HTTP-date

   An example of its use is

     Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

7.9.2.1.  Generation

   An origin server SHOULD send Last-Modified for any selected
   representation for which a last modification date can be reasonably
   and consistently determined, since its use in conditional requests
   and evaluating cache freshness ([Caching]) results in a substantial
   reduction of HTTP traffic on the Internet and can be a significant
   factor in improving service scalability and reliability.

   A representation is typically the sum of many parts behind the
   resource interface.  The last-modified time would usually be the most
   recent time that any of those parts were changed.  How that value is
   determined for any given resource is an implementation detail beyond
   the scope of this specification.  What matters to HTTP is how
   recipients of the Last-Modified header field can use its value to
   make conditional requests and test the validity of locally cached
   responses.

   An origin server SHOULD obtain the Last-Modified value of the
   representation as close as possible to the time that it generates the
   Date field value for its response.  This allows a recipient to make
   an accurate assessment of the representation's modification time,
   especially if the representation changes near the time that the
   response is generated.

   An origin server with a clock MUST NOT send a Last-Modified date that
   is later than the server's time of message origination (Date).  If
   the last modification time is derived from implementation-specific



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   metadata that evaluates to some time in the future, according to the
   origin server's clock, then the origin server MUST replace that value
   with the message origination date.  This prevents a future
   modification date from having an adverse impact on cache validation.

   An origin server without a clock MUST NOT assign Last-Modified values
   to a response unless these values were associated with the resource
   by some other system or user with a reliable clock.

7.9.2.2.  Comparison

   A Last-Modified time, when used as a validator in a request, is
   implicitly weak unless it is possible to deduce that it is strong,
   using the following rules:

   o  The validator is being compared by an origin server to the actual
      current validator for the representation and,

   o  That origin server reliably knows that the associated
      representation did not change twice during the second covered by
      the presented validator.

   or

   o  The validator is about to be used by a client in an
      If-Modified-Since, If-Unmodified-Since, or If-Range header field,
      because the client has a cache entry for the associated
      representation, and

   o  That cache entry includes a Date value, which gives the time when
      the origin server sent the original response, and

   o  The presented Last-Modified time is at least 60 seconds before the
      Date value.

   or

   o  The validator is being compared by an intermediate cache to the
      validator stored in its cache entry for the representation, and

   o  That cache entry includes a Date value, which gives the time when
      the origin server sent the original response, and

   o  The presented Last-Modified time is at least 60 seconds before the
      Date value.






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   This method relies on the fact that if two different responses were
   sent by the origin server during the same second, but both had the
   same Last-Modified time, then at least one of those responses would
   have a Date value equal to its Last-Modified time.  The arbitrary
   60-second limit guards against the possibility that the Date and
   Last-Modified values are generated from different clocks or at
   somewhat different times during the preparation of the response.  An
   implementation MAY use a value larger than 60 seconds, if it is
   believed that 60 seconds is too short.

7.9.3.  ETag

   The "ETag" field in a response provides the current entity-tag for
   the selected representation, as determined at the conclusion of
   handling the request.  An entity-tag is an opaque validator for
   differentiating between multiple representations of the same
   resource, regardless of whether those multiple representations are
   due to resource state changes over time, content negotiation
   resulting in multiple representations being valid at the same time,
   or both.  An entity-tag consists of an opaque quoted string, possibly
   prefixed by a weakness indicator.

     ETag       = entity-tag

     entity-tag = [ weak ] opaque-tag
     weak       = %s"W/"
     opaque-tag = DQUOTE *etagc DQUOTE
     etagc      = %x21 / %x23-7E / obs-text
                ; VCHAR except double quotes, plus obs-text

      |  *Note:* Previously, opaque-tag was defined to be a quoted-
      |  string ([RFC2616], Section 3.11); thus, some recipients might
      |  perform backslash unescaping.  Servers therefore ought to avoid
      |  backslash characters in entity tags.

   An entity-tag can be more reliable for validation than a modification
   date in situations where it is inconvenient to store modification
   dates, where the one-second resolution of HTTP date values is not
   sufficient, or where modification dates are not consistently
   maintained.

   Examples:

     ETag: "xyzzy"
     ETag: W/"xyzzy"
     ETag: ""





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   An entity-tag can be either a weak or strong validator, with strong
   being the default.  If an origin server provides an entity-tag for a
   representation and the generation of that entity-tag does not satisfy
   all of the characteristics of a strong validator (Section 7.9.1),
   then the origin server MUST mark the entity-tag as weak by prefixing
   its opaque value with "W/" (case-sensitive).

   A sender MAY send the Etag field in a trailer section (see
   Section 5.6).  However, since trailers are often ignored, it is
   preferable to send Etag as a header field unless the entity-tag is
   generated while sending the message body.

7.9.3.1.  Generation

   The principle behind entity-tags is that only the service author
   knows the implementation of a resource well enough to select the most
   accurate and efficient validation mechanism for that resource, and
   that any such mechanism can be mapped to a simple sequence of octets
   for easy comparison.  Since the value is opaque, there is no need for
   the client to be aware of how each entity-tag is constructed.

   For example, a resource that has implementation-specific versioning
   applied to all changes might use an internal revision number, perhaps
   combined with a variance identifier for content negotiation, to
   accurately differentiate between representations.  Other
   implementations might use a collision-resistant hash of
   representation content, a combination of various file attributes, or
   a modification timestamp that has sub-second resolution.

   An origin server SHOULD send an ETag for any selected representation
   for which detection of changes can be reasonably and consistently
   determined, since the entity-tag's use in conditional requests and
   evaluating cache freshness ([Caching]) can result in a substantial
   reduction of HTTP network traffic and can be a significant factor in
   improving service scalability and reliability.

7.9.3.2.  Comparison

   There are two entity-tag comparison functions, depending on whether
   or not the comparison context allows the use of weak validators:

   o  Strong comparison: two entity-tags are equivalent if both are not
      weak and their opaque-tags match character-by-character.

   o  Weak comparison: two entity-tags are equivalent if their opaque-
      tags match character-by-character, regardless of either or both
      being tagged as "weak".




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   The example below shows the results for a set of entity-tag pairs and
   both the weak and strong comparison function results:

    -------- -------- ------------------- -----------------
     ETag 1   ETag 2   Strong Comparison   Weak Comparison
    -------- -------- ------------------- -----------------
     W/"1"    W/"1"    no match            match
     W/"1"    W/"2"    no match            no match
     W/"1"    "1"      no match            match
     "1"      "1"      match               match
    -------- -------- ------------------- -----------------

                            Table 6

7.9.3.3.  Example: Entity-Tags Varying on Content-Negotiated Resources

   Consider a resource that is subject to content negotiation
   (Section 11), and where the representations sent in response to a GET
   request vary based on the Accept-Encoding request header field
   (Section 11.1.4):

   >> Request:

     GET /index HTTP/1.1
     Host: www.example.com
     Accept-Encoding: gzip


   In this case, the response might or might not use the gzip content
   coding.  If it does not, the response might look like:

   >> Response:

     HTTP/1.1 200 OK
     Date: Fri, 26 Mar 2010 00:05:00 GMT
     ETag: "123-a"
     Content-Length: 70
     Vary: Accept-Encoding
     Content-Type: text/plain

     Hello World!
     Hello World!
     Hello World!
     Hello World!
     Hello World!

   An alternative representation that does use gzip content coding would
   be:



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   >> Response:

     HTTP/1.1 200 OK
     Date: Fri, 26 Mar 2010 00:05:00 GMT
     ETag: "123-b"
     Content-Length: 43
     Vary: Accept-Encoding
     Content-Type: text/plain
     Content-Encoding: gzip

     ...binary data...

      |  *Note:* Content codings are a property of the representation
      |  data, so a strong entity-tag for a content-encoded
      |  representation has to be distinct from the entity tag of an
      |  unencoded representation to prevent potential conflicts during
      |  cache updates and range requests.  In contrast, transfer
      |  codings (Section 7 of [Messaging]) apply only during message
      |  transfer and do not result in distinct entity-tags.

7.9.4.  When to Use Entity-Tags and Last-Modified Dates

   In 200 (OK) responses to GET or HEAD, an origin server:

   o  SHOULD send an entity-tag validator unless it is not feasible to
      generate one.

   o  MAY send a weak entity-tag instead of a strong entity-tag, if
      performance considerations support the use of weak entity-tags, or
      if it is unfeasible to send a strong entity-tag.

   o  SHOULD send a Last-Modified value if it is feasible to send one.

   In other words, the preferred behavior for an origin server is to
   send both a strong entity-tag and a Last-Modified value in successful
   responses to a retrieval request.

   A client:

   o  MUST send that entity-tag in any cache validation request (using
      If-Match or If-None-Match) if an entity-tag has been provided by
      the origin server.

   o  SHOULD send the Last-Modified value in non-subrange cache
      validation requests (using If-Modified-Since) if only a Last-
      Modified value has been provided by the origin server.





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   o  MAY send the Last-Modified value in subrange cache validation
      requests (using If-Unmodified-Since) if only a Last-Modified value
      has been provided by an HTTP/1.0 origin server.  The user agent
      SHOULD provide a way to disable this, in case of difficulty.

   o  SHOULD send both validators in cache validation requests if both
      an entity-tag and a Last-Modified value have been provided by the
      origin server.  This allows both HTTP/1.0 and HTTP/1.1 caches to
      respond appropriately.

8.  Methods

8.1.  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 method's semantics might be further specialized by the
   semantics of some header fields when present in a request if those
   additional semantics do not conflict with the method.  For example, a
   client can send conditional request header fields (Section 12.1) to
   make the requested action conditional on the current state of the
   target resource.

     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
   semantics.

   The method token is case-sensitive because it might be used as a
   gateway to object-based systems with case-sensitive method names.  By
   convention, standardized methods are defined in all-uppercase US-
   ASCII letters.

   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 ought to 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.




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    --------- -------------------------------------------- -------
     Method    Description                                  Ref.
    --------- -------------------------------------------- -------
     GET       Transfer a current representation of the     8.3.1
               target resource.
     HEAD      Same as GET, but do not transfer the         8.3.2
               response body.
     POST      Perform resource-specific processing on      8.3.3
               the request payload.
     PUT       Replace all current representations of the   8.3.4
               target resource with the request payload.
     DELETE    Remove all current representations of the    8.3.5
               target resource.
     CONNECT   Establish a tunnel to the server             8.3.6
               identified by the target resource.
     OPTIONS   Describe the communication options for the   8.3.7
               target resource.
     TRACE     Perform a message loop-back test along the   8.3.8
               path to the target resource.
    --------- -------------------------------------------- -------

                               Table 7

   All general-purpose servers MUST support the methods GET and HEAD.
   All other methods are OPTIONAL.

   The set of methods allowed by a target resource can be listed in an
   Allow header field (Section 9.2.1).  However, the set of allowed
   methods can change dynamically.  When a request method 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 method 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.

   Additional methods, outside the scope of this specification, have
   been specified for use in HTTP.  All such methods ought to be
   registered within the "Hypertext Transfer Protocol (HTTP) Method
   Registry", as described in Section 15.1.

8.2.  Common Method Properties










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8.2.1.  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, that is not
   entirely read-only, or that 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.

   Of the request methods defined by this specification, the GET, HEAD,
   OPTIONS, and TRACE 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 target
   URI have the effect of selecting an action, it is the resource
   owner's responsibility to ensure that the action is 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 owner 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.




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8.2.2.  Idempotent Methods

   A request method is considered "idempotent" if the intended effect on
   the server of multiple identical requests with that method is the
   same as the effect for a single such request.  Of the request methods
   defined by this specification, PUT, DELETE, and 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 the client can establish a new
   connection and retry the idempotent request.  It knows that repeating
   the request will have the same intended effect, even if the original
   request succeeded, though the response might differ.

   A client SHOULD NOT automatically retry a request with a non-
   idempotent method unless it has some means to know that the request
   semantics are actually idempotent, regardless of the method, or some
   means to detect that the original request was never applied.

   For example, a user agent that knows (through design or
   configuration) that a POST request to a given resource is safe can
   repeat that request automatically.  Likewise, a user agent designed
   specifically to operate on a version control repository might be able
   to recover from partial failure conditions by checking the target
   resource revision(s) after a failed connection, reverting or fixing
   any changes that were partially applied, and then automatically
   retrying the requests that failed.

   Some clients use weaker signals to initiate automatic retries.  For
   example, when a POST request is sent, but the underlying transport
   connection is closed before any part of the response is received.
   Although this is commonly implemented, it is not recommended.

   A proxy MUST NOT automatically retry non-idempotent requests.  A
   client SHOULD NOT automatically retry a failed automatic retry.








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8.2.3.  Methods and Caching

   For a cache to store and use a response, the associated method needs
   to explicitly allow caching, and detail under what conditions a
   response can be used to satisfy subsequent requests; a method
   definition which does not do so cannot be cached.  For additional
   requirements see [Caching].

   This specification defines caching semantics for GET, HEAD, and POST,
   although the overwhelming majority of cache implementations only
   support GET and HEAD.

8.3.  Method Definitions

8.3.1.  GET

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

   GET is the primary mechanism of information retrieval and the focus
   of almost all performance optimizations.  Hence, when people speak of
   retrieving some identifiable information via HTTP, they are generally
   referring to making a GET request.  A successful response reflects
   the quality of "sameness" identified by the target URI.  In turn,
   constructing applications such that they produce a URI for each
   important resource results in more resources being available for
   other applications, producing a network effect that promotes further
   expansion of the Web.

   It is tempting to think of resource identifiers as remote file system
   pathnames and of representations as being a copy of the contents of
   such files.  In fact, that is how many resources are implemented (see
   Section 16.3 for related security considerations).  However, there
   are no such limitations in practice.

   The HTTP interface for a resource is just as likely to be implemented
   as a tree of content objects, a programmatic view on various database
   records, or a gateway to other information systems.  Even when the
   URI mapping mechanism is tied to a file system, an origin server
   might be configured to execute the files with the request as input
   and send the output as the representation rather than transfer the
   files directly.  Regardless, only the origin server needs to know how
   each of its resource identifiers corresponds to an implementation and
   how each implementation manages to select and send a current
   representation of the target resource in a response to GET.






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   A client can alter the semantics of GET to be a "range request",
   requesting transfer of only some part(s) of the selected
   representation, by sending a Range header field in the request
   (Section 13.2).

   A client SHOULD NOT generate a body in a GET request.  A payload
   received in a GET request has no defined semantics, cannot alter the
   meaning or target of the request, and might lead some implementations
   to reject the request and close the connection because of its
   potential as a request smuggling attack (Section 11.2 of
   [Messaging]).

   The response to a GET request is cacheable; a cache MAY use it to
   satisfy subsequent GET and HEAD requests unless otherwise indicated
   by the Cache-Control header field (Section 5.2 of [Caching]).  A
   cache that receives a payload in a GET request is likely to ignore
   that payload and cache regardless of the payload contents.

   When information retrieval is performed with a mechanism that
   constructs a target URI from user-provided information, such as the
   query fields of a form using GET, potentially sensitive data might be
   provided that would not be appropriate for disclosure within a URI
   (see Section 16.9).  In some cases, the data can be filtered or
   transformed such that it would not reveal such information.  In
   others, particularly when there is no benefit from caching a
   response, using the POST method (Section 8.3.3) instead of GET will
   usually transmit such information in the request body rather than
   construct a new URI.

8.3.2.  HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   send a message body in the response (i.e., the response terminates at
   the end of the header section).  The server SHOULD send the same
   header fields in response to a HEAD request as it would have sent if
   the request had been a GET, except that the payload header fields
   (Section 5.5) MAY be omitted.  This method can be used for obtaining
   metadata about the selected representation without transferring the
   representation data and is often used for testing hypertext links for
   validity, accessibility, and recent modification.

   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.







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   The response to a HEAD request is cacheable; a cache MAY use it to
   satisfy subsequent HEAD requests unless otherwise indicated by the
   Cache-Control header field (Section 5.2 of [Caching]).  A HEAD
   response might also have an effect on previously cached responses to
   GET; see Section 4.3.5 of [Caching].

8.3.3.  POST

   The POST method requests that the target resource process the
   representation enclosed in the request according to the resource's
   own specific semantics.  For example, POST is used for the following
   functions (among others):

   o  Providing a block of data, such as the fields entered into an HTML
      form, to a data-handling process;

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

   o  Creating a new resource that has yet to be identified by the
      origin server; and

   o  Appending data to a resource's existing representation(s).

   An origin server indicates response semantics by choosing an
   appropriate status code depending on the result of processing the
   POST request; almost all of the status codes defined by this
   specification could be received in a response to POST (the exceptions
   being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not
   Satisfiable)).

   If one or more resources has been created on the origin server as a
   result of successfully processing a POST request, the origin server
   SHOULD send a 201 (Created) response containing a Location header
   field that provides an identifier for the primary resource created
   (Section 9.2.3) and a representation that describes the status of the
   request while referring to the new resource(s).

   Responses to POST requests are only cacheable when they include
   explicit freshness information (see Section 4.2.1 of [Caching]) and a
   Content-Location header field that has the same value as the POST's
   target URI (Section 7.8).  A cached POST response can be reused to
   satisfy a later GET or HEAD request, but not a POST request, since
   POST is required to be written through to the origin server, because
   it is unsafe; see Section 4 of [Caching].






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   If the result of processing a POST would be equivalent to a
   representation of an existing resource, an origin server MAY redirect
   the user agent to that resource by sending a 303 (See Other) response
   with the existing resource's identifier in the Location field.  This
   has the benefits of providing the user agent a resource identifier
   and transferring the representation via a method more amenable to
   shared caching, though at the cost of an extra request if the user
   agent does not already have the representation cached.

8.3.4.  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
   sent 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 the origin server MUST send either a 200 (OK) or
   a 204 (No Content) response to indicate successful completion of the
   request.

   An origin server SHOULD ignore unrecognized header and trailer fields
   received in a PUT request (i.e., do not save them as part of the
   resource state).















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   An origin server SHOULD verify that the PUT representation is
   consistent with any constraints 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", the origin server ought to do one of:

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

   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.

   An origin server MUST NOT send a validator header field
   (Section 7.9), such as an ETag or Last-Modified field, in a
   successful response to PUT unless the request's representation data
   was saved without any transformation applied to the body (i.e., the
   resource's new representation data is identical to the representation
   data received in the PUT request) and the validator field value
   reflects the new representation.  This requirement allows a user
   agent to know when the representation body it has in memory remains
   current as a result of the PUT, thus not in need of being retrieved



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   again from the origin server, and that the new validator(s) received
   in the response can be used for future conditional requests in order
   to prevent accidental overwrites (Section 12.1).

   The fundamental difference between the POST and PUT methods is
   highlighted by the different intent for the enclosed representation.
   The target resource in a POST request is intended to handle the
   enclosed representation according to the resource's own semantics,
   whereas the enclosed representation in a PUT request is defined as
   replacing the state of the target resource.  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 which target resource is desired.  A service that selects 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 an appropriate 3xx
   (Redirection) 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 can have side effects on
   other resources.  For example, an article might have a URI for
   identifying "the current version" (a resource) that 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
   added between the related resources.

   An origin server that allows PUT on a given target resource MUST send
   a 400 (Bad Request) response to a PUT request that contains a
   Content-Range header field (Section 13.4), since the payload is
   likely to be partial content that has been 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 successful PUT
   request passes through a cache that has one or more stored responses
   for the target URI, those stored responses will be invalidated (see
   Section 4.4 of [Caching]).



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8.3.5.  DELETE

   The DELETE method requests that the origin server remove the
   association between the target resource and its current
   functionality.  In effect, this method is similar to the rm command
   in UNIX: it expresses a deletion operation on the URI mapping of the
   origin server rather than an expectation that the previously
   associated information be deleted.

   If the target resource has one or more current representations, they
   might or might not be destroyed by the origin server, and the
   associated storage might or might not be reclaimed, depending
   entirely on the nature of the resource and its implementation by the
   origin server (which are beyond the scope of this specification).
   Likewise, other implementation aspects of a resource might need to be
   deactivated or archived as a result of a DELETE, such as database or
   gateway connections.  In general, it is assumed that the origin
   server will only allow DELETE on resources for which it has a
   prescribed mechanism for accomplishing the deletion.

   Relatively few resources allow the DELETE method - its primary use is
   for remote authoring environments, where the user has some direction
   regarding its effect.  For example, a resource that was previously
   created using a PUT request, or identified via the Location header
   field after a 201 (Created) response to a POST request, might allow a
   corresponding DELETE request to undo those actions.  Similarly,
   custom user agent implementations that implement an authoring
   function, such as revision control clients using HTTP for remote
   operations, might use DELETE based on an assumption that the server's
   URI space has been crafted to correspond to a version repository.

   If a DELETE method is successfully applied, the origin server SHOULD
   send

   o  a 202 (Accepted) status code if the action will likely succeed but
      has not yet been enacted,

   o  a 204 (No Content) status code if the action has been enacted and
      no further information is to be supplied, or

   o  a 200 (OK) status code if the action has been enacted and the
      response message includes a representation describing the status.

   A client SHOULD NOT generate a body in a DELETE request.  A payload
   received in a DELETE request has no defined semantics, cannot alter
   the meaning or target of the request, and might lead some
   implementations to reject the request.




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   Responses to the DELETE method are not cacheable.  If a successful
   DELETE request passes through a cache that has one or more stored
   responses for the target URI, those stored responses will be
   invalidated (see Section 4.4 of [Caching]).

8.3.6.  CONNECT

   The CONNECT method requests that the recipient establish a tunnel to
   the destination origin server identified by the request target and,
   if successful, thereafter restrict its behavior to blind forwarding
   of data, in both directions, until the tunnel is closed.  Tunnels are
   commonly used to create an end-to-end virtual connection, through one
   or more proxies, which can then be secured using TLS (Transport Layer
   Security, [RFC8446]).

   Because CONNECT changes the request/response nature of an HTTP
   connection, specific HTTP versions might have different ways of
   mapping its semantics into the protocol's wire format.

   CONNECT is intended only for use in requests to a proxy.  An origin
   server that receives a CONNECT request for itself MAY respond with a
   2xx (Successful) status code to indicate that a connection is
   established.  However, most origin servers do not implement CONNECT.

   A client sending a CONNECT request MUST send the authority component
   (described in Section 3.2 of [RFC3986]) as the request target; 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


   The recipient proxy can establish a tunnel either by directly
   connecting to the request target or, if configured to use another
   proxy, by forwarding the CONNECT request to the next inbound proxy.
   Any 2xx (Successful) response indicates that the sender (and all
   inbound proxies) will switch to tunnel mode immediately after the
   blank line that concludes the successful response's header section;
   data received after that blank line is from the server identified by
   the request target.  Any response other than a successful response
   indicates that the tunnel has not yet been formed and that the
   connection remains governed by HTTP.








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   A tunnel is closed when a tunnel intermediary detects that either
   side has closed its connection: the intermediary MUST attempt to send
   any outstanding data that came from the closed side to the other
   side, close both connections, and then discard any remaining data
   left undelivered.

   Proxy authentication might be used to establish the authority to
   create a tunnel.  For example,

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


   There are significant risks in establishing a tunnel to arbitrary
   servers, particularly when the destination is a well-known or
   reserved TCP port that is not intended for Web traffic.  For example,
   a CONNECT to "example.com:25" would suggest that the proxy connect to
   the reserved port for SMTP traffic; if allowed, that could trick the
   proxy into relaying spam email.  Proxies that support CONNECT SHOULD
   restrict its use to a limited set of known ports or a configurable
   whitelist of safe request targets.

   A server MUST NOT send any Transfer-Encoding or Content-Length header
   fields in a 2xx (Successful) response to CONNECT.  A client MUST
   ignore any Content-Length or Transfer-Encoding header fields received
   in a successful response to CONNECT.

   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.

   Responses to the CONNECT method are not cacheable.

8.3.7.  OPTIONS

   The OPTIONS method requests information about the communication
   options available for the target resource, at either the origin
   server or an intervening intermediary.  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.









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   An OPTIONS request with an asterisk ("*") as the request target
   (Section 6.1) applies 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
   to the options that are available when communicating with the target
   resource.

   A server generating a successful response to OPTIONS SHOULD send any
   header that might indicate optional features implemented by the
   server and applicable to the target resource (e.g., Allow), including
   potential extensions not defined by this specification.  The response
   payload, if any, might also describe the communication options in a
   machine or human-readable representation.  A standard format for such
   a representation is not defined by this specification, but might be
   defined by future extensions to HTTP.

   A client MAY send a Max-Forwards header field in an OPTIONS request
   to target a specific recipient in the request chain (see
   Section 6.4.2).  A proxy MUST NOT generate a Max-Forwards header
   field while forwarding a request unless that request was received
   with a Max-Forwards field.

   A client that generates an OPTIONS request containing a payload body
   MUST send a valid Content-Type header field describing the
   representation media type.  Note that this specification does not
   define any use for such a payload.

   Responses to the OPTIONS method are not cacheable.

8.3.8.  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, excluding some fields described below,
   back to the client as the message body of a 200 (OK) response with a
   Content-Type of "message/http" (Section 10.1 of [Messaging]).  The
   final recipient is either the origin server or the first server to
   receive a Max-Forwards value of zero (0) in the request
   (Section 6.4.2).

   A client MUST NOT generate fields in a TRACE request containing
   sensitive data that might be disclosed by the response.  For example,
   it would be foolish for a user agent to send stored user credentials



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   Section 10 or cookies [RFC6265] in a TRACE request.  The final
   recipient of the request SHOULD exclude any request fields that are
   likely to contain sensitive data when that recipient generates the
   response 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 6.4.3) 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.

   A client MUST NOT send a message body in a TRACE request.

   Responses to the TRACE method are not cacheable.

9.  Context

9.1.  Request Context

   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.

   The following request header fields provide additional information
   about the request context, including information about the user, user
   agent, and resource behind the request.

    ------------ -------
     Field Name   Ref.
    ------------ -------
     Expect       9.1.1
     From         9.1.2
     Referer      9.1.3
     TE           9.1.4
     Trailer      9.1.5
     User-Agent   9.1.6
    ------------ -------

          Table 8







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9.1.1.  Expect

   The "Expect" header field in a request indicates a certain set of
   behaviors (expectations) that need to be supported by the server in
   order to properly handle this request.

     Expect =      #expectation
     expectation = token [ "=" ( token / quoted-string ) parameters ]

   The Expect field value is case-insensitive.

   The only expectation defined by this specification is "100-continue"
   (with no defined parameters).

   A server that receives an Expect field value containing a member
   other than 100-continue MAY respond with a 417 (Expectation Failed)
   status code to indicate that the unexpected expectation cannot be
   met.

   A 100-continue expectation informs recipients that the client is
   about to send a (presumably large) message body in this request and
   wishes to receive a 100 (Continue) interim response if the method,
   target URI, and header fields are not sufficient to cause an
   immediate success, redirect, or error response.  This allows the
   client to wait for an indication that it is worthwhile to send the
   message body before actually doing so, which can improve efficiency
   when the message body is huge or when the client anticipates that an
   error is likely (e.g., when sending a state-changing method, for the
   first time, without previously verified authentication credentials).

   For example, a request that begins with

     PUT /somewhere/fun HTTP/1.1
     Host: origin.example.com
     Content-Type: video/h264
     Content-Length: 1234567890987
     Expect: 100-continue


   allows the origin server to immediately respond with an error
   message, such as 401 (Unauthorized) or 405 (Method Not Allowed),
   before the client starts filling the pipes with an unnecessary data
   transfer.

   Requirements for clients:

   o  A client MUST NOT generate a 100-continue expectation in a request
      that does not include a message body.



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   o  A client that will wait for a 100 (Continue) response before
      sending the request message body MUST send an Expect header field
      containing a 100-continue expectation.

   o  A client that sends a 100-continue expectation is not required to
      wait for any specific length of time; such a client MAY proceed to
      send the message body even if it has not yet received a response.
      Furthermore, since 100 (Continue) responses cannot be sent through
      an HTTP/1.0 intermediary, such a client SHOULD NOT wait for an
      indefinite period before sending the message body.

   o  A client that receives a 417 (Expectation Failed) status code in
      response to a request containing a 100-continue expectation SHOULD
      repeat that request without a 100-continue expectation, since the
      417 response merely indicates that the response chain does not
      support expectations (e.g., it passes through an HTTP/1.0 server).

   Requirements for servers:

   o  A server that receives a 100-continue expectation in an HTTP/1.0
      request MUST ignore that expectation.

   o  A server MAY omit sending a 100 (Continue) response if it has
      already received some or all of the message body for the
      corresponding request, or if the framing indicates that there is
      no message body.

   o  A server that sends a 100 (Continue) response MUST ultimately send
      a final status code, once the message body is received and
      processed, unless the connection is closed prematurely.

   o  A server that responds with a final status code before reading the
      entire request payload body SHOULD indicate whether it intends to
      close the connection (e.g., see Section 9.6 of [Messaging]) or
      continue reading the payload body.

   An origin server MUST, upon receiving an HTTP/1.1 (or later) request
   that has a method, target URI, and complete header section that
   contains a 100-continue expectation and indicates a request message
   body will follow, either send an immediate response with a final
   status code, if that status can be determined by examining just the
   method, target URI, and header fields, or send an immediate 100
   (Continue) response to encourage the client to send the request's
   message body.  The origin server MUST NOT wait for the message body
   before sending the 100 (Continue) response.






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   A proxy MUST, upon receiving an HTTP/1.1 (or later) request that has
   a method, target URI, and complete header section that contains a
   100-continue expectation and indicates a request message body will
   follow, either send an immediate response with a final status code,
   if that status can be determined by examining just the method, target
   URI, and header fields, or begin forwarding the request toward the
   origin server by sending a corresponding request-line and header
   section to the next inbound server.  If the proxy believes (from
   configuration or past interaction) that the next inbound server only
   supports HTTP/1.0, the proxy MAY generate an immediate 100 (Continue)
   response to encourage the client to begin sending the message body.

      |  *Note:* The Expect header field was added after the original
      |  publication of HTTP/1.1 [RFC2068] as both the means to request
      |  an interim 100 (Continue) response and the general mechanism
      |  for indicating must-understand extensions.  However, the
      |  extension mechanism has not been used by clients and the must-
      |  understand requirements have not been implemented by many
      |  servers, rendering the extension mechanism useless.  This
      |  specification has removed the extension mechanism in order to
      |  simplify the definition and processing of 100-continue.

9.1.2.  From

   The "From" header field contains an Internet email address for a
   human user who controls the requesting user agent.  The address ought
   to be machine-usable, as defined by "mailbox" in Section 3.4 of
   [RFC5322]:

     From    = mailbox

     mailbox = <mailbox, see [RFC5322], Section 3.4>

   An example is:

     From: webmaster@example.org

   The From header field is rarely sent by non-robotic user agents.  A
   user agent SHOULD NOT send a From header field without explicit
   configuration by the user, since that might conflict with the user's
   privacy interests or their site's security policy.

   A robotic user agent SHOULD send a valid From header field so that
   the person responsible for running the robot can be contacted if
   problems occur on servers, such as if the robot is sending excessive,
   unwanted, or invalid requests.





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   A server SHOULD NOT use the From header field for access control or
   authentication, since most recipients will assume that the field
   value is public information.

9.1.3.  Referer

   The "Referer" [sic] header field allows the user agent to specify a
   URI reference for the resource from which the target URI was obtained
   (i.e., the "referrer", though the field name is misspelled).  A user
   agent MUST NOT include the fragment and userinfo components of the
   URI reference [RFC3986], if any, when generating the Referer field
   value.

     Referer = absolute-URI / partial-URI

   The field value is either an absolute-URI or a partial-URI.  In the
   latter case (Section 4), the referenced URI is relative to the target
   URI ([RFC3986], Section 5).

   The Referer header field allows servers to generate back-links to
   other resources for simple analytics, logging, optimized caching,
   etc.  It also allows obsolete or mistyped links to be found for
   maintenance.  Some servers use the Referer header field as a means of
   denying links from other sites (so-called "deep linking") or
   restricting cross-site request forgery (CSRF), but not all requests
   contain it.

   Example:

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

   If the target URI was obtained from a source that does not have its
   own URI (e.g., input from the user keyboard, or an entry within the
   user's bookmarks/favorites), the user agent MUST either exclude the
   Referer field or send it with a value of "about:blank".

   The Referer field has the potential to reveal information about the
   request context or browsing history of the user, which is a privacy
   concern if the referring resource's identifier reveals personal
   information (such as an account name) or a resource that is supposed
   to be confidential (such as behind a firewall or internal to a
   secured service).  Most general-purpose user agents do not send the
   Referer header field when the referring resource is a local "file" or
   "data" URI.  A user agent MUST NOT send a Referer header field in an
   unsecured HTTP request if the referring page was received with a
   secure protocol.  See Section 16.9 for additional security
   considerations.




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   Some intermediaries have been known to indiscriminately remove
   Referer header fields from outgoing requests.  This has the
   unfortunate side effect of interfering with protection against CSRF
   attacks, which can be far more harmful to their users.
   Intermediaries and user agent extensions that wish to limit
   information disclosure in Referer ought to restrict their changes to
   specific edits, such as replacing internal domain names with
   pseudonyms or truncating the query and/or path components.  An
   intermediary SHOULD NOT modify or delete the Referer header field
   when the field value shares the same scheme and host as the target
   URI.

9.1.4.  TE

   The "TE" header field in a request can be used to indicate that the
   sender will not discard trailer fields when it contains a "trailers"
   member, as described in Section 5.6.

   Additionally, specific HTTP versions can use it to indicate the
   transfer codings the client is willing to accept in the response.

   The TE field-value consists of a list of tokens, each allowing for
   optional parameters (as described in Section 5.7.6).

     TE        = #t-codings
     t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
     t-ranking = OWS ";" OWS "q=" rank
     rank      = ( "0" [ "." 0*3DIGIT ] )
               / ( "1" [ "." 0*3("0") ] )

9.1.5.  Trailer

   The "Trailer" header field provides a list of field names that the
   sender anticipates sending as trailer fields within that message.
   This allows a recipient to prepare for receipt of the indicated
   metadata before it starts processing the body.

     Trailer = #field-name

   For example, a sender might indicate that a message integrity check
   will be computed as the payload is being streamed and provide the
   final signature as a trailer field.  This allows a recipient to
   perform the same check on the fly as the payload data is received.

   A sender that intends to generate one or more trailer fields in a
   message SHOULD generate a Trailer header field in the header section
   of that message to indicate which fields might be present in the
   trailers.



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9.1.6.  User-Agent

   The "User-Agent" header field contains information about the user
   agent originating the request, which is often used by servers to help
   identify the scope of reported interoperability problems, to work
   around or tailor responses to avoid particular user agent
   limitations, and for analytics regarding browser or operating system
   use.  A user agent SHOULD send a User-Agent field in each request
   unless specifically configured not to do so.

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

   The User-Agent field value consists of one or more product
   identifiers, each followed by zero or more comments (Section 5.7.5),
   which together identify the user agent software and its significant
   subproducts.  By convention, the product identifiers are listed in
   decreasing order of their significance for identifying the user agent
   software.  Each product identifier consists of a name and optional
   version.

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

   A sender SHOULD limit generated product identifiers to what is
   necessary to identify the product; a sender MUST NOT generate
   advertising or other nonessential information within the product
   identifier.  A sender SHOULD NOT generate information in
   product-version that is not a version identifier (i.e., successive
   versions of the same product name ought to differ only in the
   product-version portion of the product identifier).

   Example:

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

   A user agent SHOULD NOT generate a User-Agent field containing
   needlessly fine-grained detail and SHOULD limit the addition of
   subproducts by third parties.  Overly long and detailed User-Agent
   field values increase request latency and the risk of a user being
   identified against their wishes ("fingerprinting").

   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.  If a user
   agent masquerades as a different user agent, recipients can assume
   that the user intentionally desires to see responses tailored for
   that identified user agent, even if they might not work as well for
   the actual user agent being used.



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9.2.  Response Context

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

   The response header fields allow the server to pass additional
   information about the response beyond the status code.  These header
   fields give information about the server, about further access to the
   target resource, or about related resources.

   Although each response header field has a defined meaning, in
   general, the precise semantics might be further refined by the
   semantics of the request method and/or response status code.

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

    ------------- -------
     Field Name    Ref.
    ------------- -------
     Allow         9.2.1
     Date          9.2.2
     Location      9.2.3
     Retry-After   9.2.4
     Server        9.2.5
    ------------- -------

           Table 9

9.2.1.  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









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   The actual set of allowed methods is defined by the origin server at
   the time of each request.  An origin server MUST generate an Allow
   field in a 405 (Method Not Allowed) response and MAY do so in any
   other response.  An empty Allow field value indicates that the
   resource allows no methods, which might occur in a 405 response if
   the resource has been temporarily disabled by configuration.

   A proxy MUST NOT modify the Allow header field - it does not need to
   understand all of the indicated methods in order to handle them
   according to the generic message handling rules.

9.2.2.  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 5.7.7.

     Date = HTTP-date

   An example is

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

   When a Date header field is generated, the sender SHOULD generate its
   field value as the best available approximation of the date and time
   of message generation.  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 message origination.

   An origin server MUST NOT send a Date header field if it does not
   have a clock capable of providing a reasonable approximation of the
   current instance in Coordinated Universal Time.  An origin server MAY
   send a Date header field if the response is in the 1xx
   (Informational) or 5xx (Server Error) class of status codes.  An
   origin server MUST send a Date header field in all other cases.

   A recipient with a clock that receives a response message without a
   Date header field MUST record the time it was received and append a
   corresponding Date header field to the message's header section if it
   is cached or forwarded downstream.

   A user agent MAY send a Date header field in a request, though
   generally will not do so unless it is believed to convey useful
   information to the server.  For example, custom applications of HTTP
   might convey a Date if the server is expected to adjust its
   interpretation of the user's request based on differences between the
   user agent and server clocks.



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9.2.3.  Location

   The "Location" header field is used in some responses to refer to a
   specific resource in relation to the response.  The type of
   relationship is defined by the combination of request method and
   status code semantics.

     Location = URI-reference

   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 target URI ([RFC3986],
   Section 5).

   For 201 (Created) responses, the Location value refers to the primary
   resource created by the request.  For 3xx (Redirection) responses,
   the Location value refers to the preferred target resource for
   automatically redirecting the request.

   If the Location value provided in a 3xx (Redirection) response does
   not have a fragment component, a user agent MUST process the
   redirection as if the value inherits the fragment component of the
   URI reference used to generate the target URI (i.e., the redirection
   inherits the original reference's fragment, if any).

   For example, a GET request generated for the URI reference
   "http://www.example.org/~tim" might result in a 303 (See Other)
   response containing the header field:

     Location: /People.html#tim

   which suggests that the user agent redirect to
   "http://www.example.org/People.html#tim"

   Likewise, a GET request generated for the URI reference
   "http://www.example.org/index.html#larry" might result in a 301
   (Moved Permanently) response containing the header field:

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

   which suggests that the user agent redirect to
   "http://www.example.net/index.html#larry", preserving the original
   fragment identifier.

   There are circumstances in which a fragment identifier in a Location
   value would not be appropriate.  For example, the Location header
   field in a 201 (Created) response is supposed to provide a URI that
   is specific to the created resource.



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      |  *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 for the
      |  sake of robustness.  A Location field value cannot allow a list
      |  of members because the comma list separator is a valid data
      |  character within a URI-reference.  If an invalid message is
      |  sent with multiple Location field instances, a recipient along
      |  the path might combine the field instances into one value.
      |  Recovery of a valid Location field value from that situation is
      |  difficult and not interoperable across implementations.

      |  *Note:* The Content-Location header field (Section 7.8) differs
      |  from Location in that the Content-Location refers to the most
      |  specific resource corresponding to the enclosed representation.
      |  It is therefore possible for a response to contain both the
      |  Location and Content-Location header fields.

9.2.4.  Retry-After

   Servers send the "Retry-After" header field to indicate how long the
   user agent ought to wait before making a follow-up request.  When
   sent with a 503 (Service Unavailable) response, Retry-After indicates
   how long the service is expected to be unavailable to the client.
   When sent with any 3xx (Redirection) response, Retry-After indicates
   the minimum time that the user agent is asked to wait before issuing
   the redirected request.

   The value of this field can be either an HTTP-date or a number of
   seconds to delay after the response is received.

     Retry-After = HTTP-date / delay-seconds

   A delay-seconds value is a non-negative decimal integer, representing
   time in seconds.

     delay-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.








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9.2.5.  Server

   The "Server" header field contains information about the software
   used by the origin server to handle the request, which is often used
   by clients to help identify the scope of reported interoperability
   problems, to work around or tailor requests to avoid particular
   server limitations, and for analytics regarding server or operating
   system use.  An origin server MAY generate a Server field in its
   responses.

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

   The Server field value consists of one or more product identifiers,
   each followed by zero or more comments (Section 5.7.5), which
   together identify the origin server software and its significant
   subproducts.  By convention, the product identifiers are listed in
   decreasing order of their significance for identifying the origin
   server software.  Each product identifier consists of a name and
   optional version, as defined in Section 9.1.6.

   Example:

     Server: CERN/3.0 libwww/2.17

   An origin server SHOULD NOT generate a Server field containing
   needlessly fine-grained detail and SHOULD limit the addition of
   subproducts by third parties.  Overly long and detailed Server field
   values increase response latency and potentially reveal internal
   implementation details that might make it (slightly) easier for
   attackers to find and exploit known security holes.

10.  Authentication

10.1.  Authentication Scheme

   HTTP provides a general framework for access control and
   authentication, via an extensible set of challenge-response
   authentication schemes, which can be used by a server to challenge a
   client request and by a client to provide authentication information.
   It uses a case-insensitive token to identify the authentication
   scheme

     auth-scheme    = token








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   Aside from the general framework, this document does not specify any
   authentication schemes.  New and existing authentication schemes are
   specified independently and ought to be registered within the
   "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".
   For example, the "basic" and "digest" authentication schemes are
   defined by RFC 7617 and RFC 7616, respectively.

10.2.  Authentication Parameters

   The authentication scheme is followed by additional information
   necessary for achieving authentication via that scheme as either a
   comma-separated list of parameters or a single sequence of characters
   capable of holding base64-encoded information.

     token68        = 1*( ALPHA / DIGIT /
                          "-" / "." / "_" / "~" / "+" / "/" ) *"="

   The token68 syntax allows the 66 unreserved URI characters
   ([RFC3986]), plus a few others, so that it can hold a base64,
   base64url (URL and filename safe alphabet), base32, or base16 (hex)
   encoding, with or without padding, but excluding whitespace
   ([RFC4648]).

   Authentication parameters are name=value pairs, where the name token
   is matched case-insensitively and each parameter name MUST only occur
   once per challenge.

     auth-param     = token BWS "=" BWS ( token / quoted-string )

   Parameter values can be expressed either as "token" or as "quoted-
   string" (Section 5.7).  Authentication scheme definitions need to
   accept both notations, both for senders and recipients, to allow
   recipients to use generic parsing components regardless of the
   authentication scheme.

   For backwards compatibility, authentication scheme definitions can
   restrict the format for senders to one of the two variants.  This can
   be important when it is known that deployed implementations will fail
   when encountering one of the two formats.

10.3.  Challenge and Response

   A 401 (Unauthorized) response message is used by an origin server to
   challenge the authorization of a user agent, including a
   WWW-Authenticate header field containing at least one challenge
   applicable to the requested resource.





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   A 407 (Proxy Authentication Required) response message is used by a
   proxy to challenge the authorization of a client, including a
   Proxy-Authenticate header field containing at least one challenge
   applicable to the proxy for the requested resource.

     challenge   = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

      |  *Note:* Many clients fail to parse a challenge that contains an
      |  unknown scheme.  A workaround for this problem is to list well-
      |  supported schemes (such as "basic") first.

   A user agent that wishes to authenticate itself with an origin server
   - usually, but not necessarily, after receiving a 401 (Unauthorized)
   - can do so by including an Authorization header field with the
   request.

   A client that wishes to authenticate itself with a proxy - usually,
   but not necessarily, after receiving a 407 (Proxy Authentication
   Required) - can do so by including a Proxy-Authorization header field
   with the request.

10.4.  Credentials

   Both the Authorization field value and the Proxy-Authorization field
   value contain the client's credentials for the realm of the resource
   being requested, based upon a challenge received in a response
   (possibly at some point in the past).  When creating their values,
   the user agent ought to do so by selecting the challenge with what it
   considers to be the most secure auth-scheme that it understands,
   obtaining credentials from the user as appropriate.  Transmission of
   credentials within header field values implies significant security
   considerations regarding the confidentiality of the underlying
   connection, as described in Section 16.15.1.

     credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]

   Upon receipt of a request for a protected resource that omits
   credentials, contains invalid credentials (e.g., a bad password) or
   partial credentials (e.g., when the authentication scheme requires
   more than one round trip), an origin server SHOULD send a 401
   (Unauthorized) response that contains a WWW-Authenticate header field
   with at least one (possibly new) challenge applicable to the
   requested resource.








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   Likewise, upon receipt of a request that omits proxy credentials or
   contains invalid or partial proxy credentials, a proxy that requires
   authentication SHOULD generate a 407 (Proxy Authentication Required)
   response that contains a Proxy-Authenticate header field with at
   least one (possibly new) challenge applicable to the proxy.

   A server that receives valid credentials that are not adequate to
   gain access ought to respond with the 403 (Forbidden) status code
   (Section 14.5.4).

   HTTP does not restrict applications to this simple challenge-response
   framework for access authentication.  Additional mechanisms can be
   used, such as authentication at the transport level or via message
   encapsulation, and with additional header fields specifying
   authentication information.  However, such additional mechanisms are
   not defined by this specification.

   Note that various custom mechanisms for user authentication use the
   Set-Cookie and Cookie header fields, defined in [RFC6265], for
   passing tokens related to authentication.

10.5.  Protection Space (Realm)

   The "realm" authentication parameter is reserved for use by
   authentication schemes that wish to indicate a scope of protection.

   A protection space is defined by the canonical root URI (the scheme
   and authority components of the target URI; see Section 6.1) of the
   server being accessed, in combination with the realm value if
   present.  These realms allow the protected resources on a server to
   be partitioned into a set of protection spaces, each with its own
   authentication scheme and/or authorization database.  The realm value
   is a string, generally assigned by the origin server, that can have
   additional semantics specific to the authentication scheme.  Note
   that a response can have multiple challenges with the same auth-
   scheme but with different realms.

   The protection space determines the domain over which credentials can
   be automatically applied.  If a prior request has been authorized,
   the user agent MAY reuse the same credentials for all other requests
   within that protection space for a period of time determined by the
   authentication scheme, parameters, and/or user preferences (such as a
   configurable inactivity timeout).  Unless specifically allowed by the
   authentication scheme, a single protection space cannot extend
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   For historical reasons, a sender MUST only generate the quoted-string
   syntax.  Recipients might have to support both token and quoted-
   string syntax for maximum interoperability with existing clients that
   have been accepting both notations for a long time.

10.6.  Authenticating User to Origin Server

10.6.1.  WWW-Authenticate

   The "WWW-Authenticate" header field indicates the authentication
   scheme(s) and parameters applicable to the target resource.

     WWW-Authenticate = #challenge

   A server generating a 401 (Unauthorized) response MUST send a WWW-
   Authenticate header field containing at least one challenge.  A
   server MAY generate a WWW-Authenticate header field in other response
   messages to indicate that supplying credentials (or different
   credentials) might affect the response.

   A proxy forwarding a response MUST NOT modify any WWW-Authenticate
   fields in that response.

   User agents are advised to take special care in parsing the field
   value, as it might contain more than one challenge, and each
   challenge can contain a comma-separated list of authentication
   parameters.  Furthermore, the header field itself can occur multiple
   times.

   For instance:

     WWW-Authenticate: Newauth realm="apps", type=1,
                       title="Login to \"apps\"", Basic realm="simple"

   This header field contains two challenges; one for the "Newauth"
   scheme with a realm value of "apps", and two additional parameters
   "type" and "title", and another one for the "Basic" scheme with a
   realm value of "simple".

   Some user agents do not recognise this form, however.  As a result,
   sending a WWW-Authenticate field value with more than one member on
   the same field line might not be interoperable.









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      |  *Note:* The challenge grammar production uses the list syntax
      |  as well.  Therefore, a sequence of comma, whitespace, and comma
      |  can be considered either as applying to the preceding
      |  challenge, or to be an empty entry in the list of challenges.
      |  In practice, this ambiguity does not affect the semantics of
      |  the header field value and thus is harmless.

10.6.2.  Authorization

   The "Authorization" header field allows a user agent to authenticate
   itself with an origin server - usually, but not necessarily, after
   receiving a 401 (Unauthorized) response.  Its value consists of
   credentials containing the authentication information of the user
   agent for the realm of the resource being requested.

     Authorization = credentials

   If a request is authenticated and a realm specified, the same
   credentials are presumed to be valid for all other requests within
   this realm (assuming that the authentication scheme itself does not
   require otherwise, such as credentials that vary according to a
   challenge value or using synchronized clocks).

   A proxy forwarding a request MUST NOT modify any Authorization fields
   in that request.  See Section 3.3 of [Caching] for details of and
   requirements pertaining to handling of the Authorization field by
   HTTP caches.

10.6.3.  Authentication-Info

   HTTP authentication schemes can use the Authentication-Info response
   header field to communicate information after the client's
   authentication credentials have been accepted.  This information can
   include a finalization message from the server (e.g., it can contain
   the server authentication).

   The field value is a list of parameters (name/value pairs), using the
   "auth-param" syntax defined in Section 10.3.  This specification only
   describes the generic format; authentication schemes using
   Authentication-Info will define the individual parameters.  The
   "Digest" Authentication Scheme, for instance, defines multiple
   parameters in Section 3.5 of [RFC7616].

     Authentication-Info = #auth-param







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   The Authentication-Info header field can be used in any HTTP
   response, independently of request method and status code.  Its
   semantics are defined by the authentication scheme indicated by the
   Authorization header field (Section 10.6.2) of the corresponding
   request.

   A proxy forwarding a response is not allowed to modify the field
   value in any way.

   Authentication-Info can be sent as a trailer field (Section 5.6) when
   the authentication scheme explicitly allows this.

10.7.  Authenticating Client to Proxy

10.7.1.  Proxy-Authenticate

   The "Proxy-Authenticate" header field consists of at least one
   challenge that indicates the authentication scheme(s) and parameters
   applicable to the proxy for this request.  A proxy MUST send at least
   one Proxy-Authenticate header field in each 407 (Proxy Authentication
   Required) response that it generates.

     Proxy-Authenticate = #challenge

   Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
   only to the next outbound client on the response chain.  This is
   because only the client that chose a given proxy is likely to have
   the credentials necessary for authentication.  However, when multiple
   proxies are used within the same administrative domain, such as
   office and regional caching proxies within a large corporate network,
   it is common for credentials to be generated by the user agent and
   passed through the hierarchy until consumed.  Hence, in such a
   configuration, it will appear as if Proxy-Authenticate is being
   forwarded because each proxy will send the same challenge set.

   Note that the parsing considerations for WWW-Authenticate apply to
   this header field as well; see Section 10.6.1 for details.

10.7.2.  Proxy-Authorization

   The "Proxy-Authorization" header field allows the client to identify
   itself (or its user) to a proxy that requires authentication.  Its
   value consists of credentials containing the authentication
   information of the client for the proxy and/or realm of the resource
   being requested.

     Proxy-Authorization = credentials




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   Unlike Authorization, the Proxy-Authorization header field applies
   only to the next inbound proxy that demanded authentication using the
   Proxy-Authenticate field.  When multiple proxies are used in a chain,
   the Proxy-Authorization header field is consumed by the first inbound
   proxy that was expecting to receive credentials.  A proxy MAY relay
   the credentials from the client request to the next proxy if that is
   the mechanism by which the proxies cooperatively authenticate a given
   request.

10.7.3.  Proxy-Authentication-Info

   The Proxy-Authentication-Info response header field is equivalent to
   Authentication-Info, except that it applies to proxy authentication
   (Section 10.3) and its semantics are defined by the authentication
   scheme indicated by the Proxy-Authorization header field
   (Section 10.7.2) of the corresponding request:

     Proxy-Authentication-Info = #auth-param

   However, unlike Authentication-Info, the Proxy-Authentication-Info
   header field applies only to the next outbound client on the response
   chain.  This is because only the client that chose a given proxy is
   likely to have the credentials necessary for authentication.
   However, when multiple proxies are used within the same
   administrative domain, such as office and regional caching proxies
   within a large corporate network, it is common for credentials to be
   generated by the user agent and passed through the hierarchy until
   consumed.  Hence, in such a configuration, it will appear as if
   Proxy-Authentication-Info is being forwarded because each proxy will
   send the same field value.

11.  Content Negotiation

   When responses convey payload information, whether indicating a
   success or an error, the origin server often has different ways of
   representing that information; for example, in different formats,
   languages, or encodings.  Likewise, different users or user agents
   might have differing capabilities, characteristics, or preferences
   that could influence which representation, among those available,
   would be best to deliver.  For this reason, HTTP provides mechanisms
   for content negotiation.

   This specification defines three patterns of content negotiation that
   can be made visible within the protocol: "proactive" negotiation,
   where the server selects the representation based upon the user
   agent's stated preferences, "reactive" negotiation, where the server
   provides a list of representations for the user agent to choose from,
   and "request payload" negotiation, where the user agent selects the



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   representation for a future request based upon the server's stated
   preferences in past responses.  Other patterns of content negotiation
   include "conditional content", where the representation consists of
   multiple parts that are selectively rendered based on user agent
   parameters, "active content", where the representation contains a
   script that makes additional (more specific) requests based on the
   user agent characteristics, and "Transparent Content Negotiation"
   ([RFC2295]), where content selection is performed by an intermediary.
   These patterns are not mutually exclusive, and each has trade-offs in
   applicability and practicality.

   Note that, in all cases, HTTP is not aware of the resource semantics.
   The consistency with which an origin server responds to requests,
   over time and over the varying dimensions of content negotiation, and
   thus the "sameness" of a resource's observed representations over
   time, is determined entirely by whatever entity or algorithm selects
   or generates those responses.

11.1.  Proactive Negotiation

   When content negotiation preferences are sent by the user agent in a
   request to encourage an algorithm located at the server to select the
   preferred representation, it is called proactive negotiation (a.k.a.,
   server-driven negotiation).  Selection is based on the available
   representations for a response (the dimensions over which it might
   vary, such as language, content-coding, etc.) compared to various
   information supplied in the request, including both the explicit
   negotiation fields below and implicit characteristics, such as the
   client's network address or parts of the User-Agent field.

   Proactive negotiation is advantageous when the algorithm for
   selecting from among the available representations is difficult to
   describe to a user agent, or when the server desires to send its
   "best guess" to the user agent 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, a user agent MAY send request header fields that
   describe its preferences.

   Proactive negotiation has serious disadvantages:

   o  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?);





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   o  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 risk
      to the user's privacy;

   o  It complicates the implementation of an origin server and the
      algorithms for generating responses to a request; and,

   o  It limits the reusability of responses for shared caching.

   A user agent cannot rely on proactive negotiation preferences being
   consistently honored, since the origin server might not implement
   proactive negotiation for the requested resource or might decide that
   sending a response that doesn't conform to the user agent's
   preferences is better than sending a 406 (Not Acceptable) response.

   A Vary header field (Section 11.2.1) is often sent in a response
   subject to proactive negotiation to indicate what parts of the
   request information were used in the selection algorithm.

   The following request header fields can be sent by a user agent to
   engage in proactive negotiation of the response content, as defined
   in Section 11.1.  The preferences sent in these fields apply to any
   content in the response, including representations of the target
   resource, representations of error or processing status, and
   potentially even the miscellaneous text strings that might appear
   within the protocol.

    ----------------- --------
     Field Name        Ref.
    ----------------- --------
     Accept            11.1.2
     Accept-Charset    11.1.3
     Accept-Encoding   11.1.4
     Accept-Language   11.1.5
    ----------------- --------

             Table 10

11.1.1.  Shared Negotiation Features











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11.1.1.1.  Absence

   For each of these header fields, a request that does not contain the
   field implies that the user agent has no preference on that axis of
   negotiation.  If the header field is present in a request and none of
   the available representations for the response can be considered
   acceptable according to it, the origin server can either honor the
   header field by sending a 406 (Not Acceptable) response or disregard
   the header field by treating the response as if it is not subject to
   content negotiation for that request header field.  This does not
   imply, however, that the client will be able to use the
   representation.

   *Note:* Sending these header fields makes it easier for a server to
   identify an individual by virtue of the user agent's request
   characteristics (Section 16.12).

11.1.1.2.  Quality Values

   The content negotiation fields defined by this specification 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.

11.1.1.3.  Wildcard Values

   Most of these header fields, where indicated, define a wildcard value
   ("*") to select unspecified values.  If no wildcard is present, all
   values not explicitly mentioned in the field are considered "not
   acceptable" to the client.





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   *Note:* In practice, using wildcards in content negotiation has
   limited practical value, because it is seldom useful to say, for
   example, "I prefer image/* more or less than (some other specific
   value)".  Clients can explicitly request a 406 (Not Acceptable)
   response if a more preferred format is not available by sending
   Accept: */*;q=0, but they still need to be able to handle a different
   response, since the server is allowed to ignore their preference.

11.1.2.  Accept

   The "Accept" header field can be used by user agents to specify their
   preferences regarding response media types.  For example, 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.

   When sent by a server in a response, Accept provides information
   about what content types are preferred in the payload of a subsequent
   request to the same resource.

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

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

   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 can include media type
   parameters that are applicable to that range.

   Each media-range might be followed by zero or more applicable media
   type parameters (e.g., charset), an optional "q" parameter for
   indicating a relative weight (Section 11.1.1.2), and then zero or
   more extension parameters.  The "q" parameter is necessary if any
   extensions (accept-ext) are present, since it acts as a separator
   between the two parameter sets.











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      |  *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

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

   A more elaborate example is

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

   Verbally, this would be interpreted as "text/html and text/x-c are
   the equally 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
   that matches the type.  For example,

     Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,
             text/plain;format=fixed;q=0.4, */*;q=0.5



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   would cause the following values to be associated:

    -------------------------- ---------------
     Media Type                 Quality Value
    -------------------------- ---------------
     text/plain;format=flowed   1
     text/plain                 0.7
     text/html                  0.3
     image/jpeg                 0.5
     text/plain;format=fixed    0.4
     text/html;level=3          0.7
    -------------------------- ---------------

                     Table 11

   *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 that cannot interact with other rendering agents, this
   default set ought to be configurable by the user.

11.1.3.  Accept-Charset

   The "Accept-Charset" header field can be sent by a user agent to
   indicate its preferences for charsets in textual response content.
   For example, this field allows user agents capable of understanding
   more comprehensive or special-purpose charsets to signal that
   capability to an origin server that is capable of representing
   information in those charsets.

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

   Charset names are defined in Section 7.4.2.  A user agent MAY
   associate a quality value with each charset to indicate the user's
   relative preference for that charset, as defined in Section 11.1.1.2.
   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 charset that is not mentioned elsewhere in the Accept-
   Charset field.

   *Note:* Accept-Charset is deprecated because UTF-8 has become nearly
   ubiquitous and sending a detailed list of user-preferred charsets
   wastes bandwidth, increases latency, and makes passive fingerprinting
   far too easy (Section 16.12).  Most general-purpose user agents do
   not send Accept-Charset, unless specifically configured to do so.




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11.1.4.  Accept-Encoding

   The "Accept-Encoding" header field can be used to indicate
   preferences regarding the use of content codings (Section 7.5.1).

   When sent by a user agent in a request, Accept-Encoding indicates the
   content codings acceptable in a response.

   When sent by a server in a response, Accept-Encoding provides
   information about what content codings are preferred in the payload
   of a subsequent request to the same resource.

   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
   representing the preference for that encoding, as defined in
   Section 11.1.1.2.  The asterisk "*" symbol in an Accept-Encoding
   field matches any available content-coding not explicitly listed in
   the header field.

   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 using these rules:

   1.  If no Accept-Encoding field is in the request, any content-coding
       is considered acceptable by the user agent.

   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 value, then it is
       acceptable unless it is accompanied by a qvalue of 0.  (As
       defined in Section 11.1.1.2, a qvalue of 0 means "not
       acceptable".)



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   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 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.

   When the Accept-Encoding header field is present in a response, it
   indicates what content codings the resource was willing to accept in
   the associated request.  The field value is evaluated the same way as
   in a request.

   Note that this information is specific to the associated request; the
   set of supported encodings might be different for other resources on
   the same server and could change over time or depend on other aspects
   of the request (such as the request method).

   Servers that fail a request due to an unsupported content coding
   ought to respond with a 415 (Unsupported Media Type) status and
   include an Accept-Encoding header field in that response, allowing
   clients to distinguish between issues related to content codings and
   media types.  In order to avoid confusion with issues related to
   media types, servers that fail a request with a 415 status for
   reasons unrelated to content codings MUST NOT include the Accept-
   Encoding header field.

   The most common use of Accept-Encoding is in responses with a 415
   (Unsupported Media Type) status code, in response to optimistic use
   of a content coding by clients.  However, the header field can also
   be used to indicate to clients that content codings are supported, to
   optimize future interactions.  For example, a resource might include
   it in a 2xx (Successful) response when the request payload was big
   enough to justify use of a compression coding but the client failed
   do so.

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

11.1.5.  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 7.6.1.



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

   Each language-range can be given an associated quality value
   representing an estimate of the user's preference for the languages
   specified by that range, as defined in Section 11.1.1.2.  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".

   Note that some recipients treat the order in which language tags are
   listed as an indication of descending priority, particularly for tags
   that are assigned equal quality values (no value is the same as q=1).
   However, this behavior cannot be relied upon.  For consistency and to
   maximize interoperability, many user agents assign each language tag
   a unique quality value while also listing them in order of decreasing
   quality.  Additional discussion of language priority lists can be
   found in 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.  The "Basic Filtering" scheme
   ([RFC4647], Section 3.3.1) is identical to the matching scheme that
   was previously defined for HTTP 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 (Section 16.12).

   Since intelligibility is highly dependent on the individual user,
   user agents need to allow user control over the linguistic preference
   (either through configuration of the user agent itself or by
   defaulting to a user controllable system setting).  A user agent that
   does not provide such control to the user MUST NOT send an Accept-
   Language header field.

      |  *Note:* User agents ought to provide guidance to users when
      |  setting a preference, since users are rarely familiar with the
      |  details of language matching as described above.  For 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 the list for better matching behavior.




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11.2.  Reactive Negotiation

   With reactive negotiation (a.k.a., agent-driven negotiation),
   selection of the best response representation (regardless of the
   status code) is performed by the user agent after receiving an
   initial response from the origin server that contains a list of
   resources for alternative representations.  If the user agent is not
   satisfied by the initial response representation, it can perform a
   GET request on one or more of the alternative resources, selected
   based on metadata included in the list, to obtain a different form of
   representation for that response.  Selection of alternatives might be
   performed automatically by the user agent or manually by the user
   selecting from a generated (possibly hypertext) menu.

   Note that the above refers to representations of the response, in
   general, not representations of the resource.  The alternative
   representations are only considered representations of the target
   resource if the response in which those alternatives are provided has
   the semantics of being a representation of the target resource (e.g.,
   a 200 (OK) response to a GET request) or has the semantics of
   providing links to alternative representations for the target
   resource (e.g., a 300 (Multiple Choices) response to a GET request).

   A server might choose not to send an initial representation, other
   than the list of alternatives, and thereby indicate that reactive
   negotiation by the user agent is preferred.  For example, the
   alternatives listed in responses with the 300 (Multiple Choices) and
   406 (Not Acceptable) status codes include information about the
   available representations so that the user or user agent can react by
   making a selection.

   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 disadvantages of transmitting a
   list of alternatives to the user agent, which degrades user-perceived
   latency if transmitted in the header section, and needing a second
   request to obtain an alternate representation.  Furthermore, this
   specification does not define a mechanism for supporting automatic
   selection, though it does not prevent such a mechanism from being
   developed as an extension.







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11.2.1.  Vary

   The "Vary" header field in a response describes what parts of a
   request message, aside from the method and target URI, might
   influence the origin server's process for selecting and representing
   this response.

     Vary = #( "*" / field-name )

   A Vary field value is a list of request field names, known as the
   selecting header fields, that might have a role in selecting the
   representation for this response.  Potential selecting header fields
   are not limited to those defined by this specification.

   If the list contains "*", it signals that other aspects of the
   request might play a role in selecting the response representation,
   possibly including elements outside the message syntax (e.g., the
   client's network address).  A recipient will not be able to determine
   whether this response is appropriate for a later request without
   forwarding the request to the origin server.  A proxy MUST NOT
   generate "*" in a Vary field value.

   For example, a response that contains

     Vary: accept-encoding, accept-language

   indicates that the origin server might have used the request's
   Accept-Encoding and Accept-Language fields (or lack thereof) as
   determining factors while choosing the content for this response.

   An origin server might send Vary with a list of fields for two
   purposes:

   1.  To inform cache recipients that they MUST NOT use this response
       to satisfy a later request unless the later request has the same
       values for the listed fields as the original request (Section 4.1
       of [Caching]).  In other words, Vary expands the cache key
       required to match a new request to the stored cache entry.

   2.  To inform user agent recipients that this response is subject to
       content negotiation (Section 11) and that a different
       representation might be sent in a subsequent request if
       additional parameters are provided in the listed header fields
       (proactive negotiation).

   An origin server SHOULD send a Vary header field when its algorithm
   for selecting a representation varies based on aspects of the request
   message other than the method and target URI, unless the variance



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   cannot be crossed or the origin server has been deliberately
   configured to prevent cache transparency.  For example, there is no
   need to send the Authorization field name in Vary because reuse
   across users is constrained by the field definition (Section 10.6.2).
   Likewise, an origin server might use Cache-Control response
   directives (Section 5.2 of [Caching]) to supplant Vary if it
   considers the variance less significant than the performance cost of
   Vary's impact on caching.

11.3.  Request Payload Negotiation

   When content negotiation preferences are sent in a server's response,
   the listed preferences are called request payload negotiation because
   they intend to influence selection of an appropriate payload for
   subsequent requests to that resource.  For example, the
   Accept-Encoding field (Section 11.1.4) can be sent in a response to
   indicate preferred content codings for subsequent requests to that
   resource [RFC7694].

      |  Similarly, Section 3.1 of [RFC5789] defines the "Accept-Patch"
      |  response header field which allows discovery of which content
      |  types are accepted in PATCH requests.

12.  Conditional Requests

   A conditional request is an HTTP request with one or more request
   header fields that indicate a precondition to be tested before
   applying the request method to the target resource.  Section 12.2
   defines when preconditions are applied.  Section 12.3 defines the
   order of evaluation when more than one precondition is present.

   Conditional GET requests are the most efficient mechanism for HTTP
   cache updates [Caching].  Conditionals can also be applied to state-
   changing methods, such as PUT and DELETE, to prevent the "lost
   update" problem: one client accidentally overwriting the work of
   another client that has been acting in parallel.

   Conditional request preconditions are based on the state of the
   target resource as a whole (its current value set) or the state as
   observed in a previously obtained representation (one value in that
   set).  A resource might have multiple current representations, each
   with its own observable state.  The conditional request mechanisms
   assume that the mapping of requests to a selected representation
   (Section 7) will be consistent over time if the server intends to
   take advantage of conditionals.  Regardless, if the mapping is
   inconsistent and the server is unable to select the appropriate
   representation, then no harm will result when the precondition
   evaluates to false.



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12.1.  Preconditions

   The following request header fields allow a client to place a
   precondition on the state of the target resource, so that the action
   corresponding to the method semantics will not be applied if the
   precondition evaluates to false.  Each precondition defined by this
   specification consists of a comparison between a set of validators
   obtained from prior representations of the target resource to the
   current state of validators for the selected representation
   (Section 7.9).  Hence, these preconditions evaluate whether the state
   of the target resource has changed since a given state known by the
   client.  The effect of such an evaluation depends on the method
   semantics and choice of conditional, as defined in Section 12.2.

    --------------------- --------
     Field Name            Ref.
    --------------------- --------
     If-Match              12.1.1
     If-None-Match         12.1.2
     If-Modified-Since     12.1.3
     If-Unmodified-Since   12.1.4
     If-Range              12.1.5
    --------------------- --------

               Table 12

12.1.1.  If-Match

   The "If-Match" header field makes the request method conditional on
   the recipient origin server either having at least one current
   representation of the target resource, when the field value is "*",
   or having a current representation of the target resource that has an
   entity-tag matching a member of the list of entity-tags provided in
   the field value.

   An origin server MUST use the strong comparison function when
   comparing entity-tags for If-Match (Section 7.9.3.2), since the
   client intends this precondition to prevent the method from being
   applied if there have been any changes to the representation data.

     If-Match = "*" / #entity-tag

   Examples:

     If-Match: "xyzzy"
     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
     If-Match: *




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   If-Match is most often used with state-changing methods (e.g., POST,
   PUT, DELETE) to prevent accidental overwrites when multiple user
   agents might be acting in parallel on the same resource (i.e., to
   prevent the "lost update" problem).  It can also be used with any
   method to abort a request if the selected representation does not
   match one that the client has already stored (or partially stored)
   from a prior request.

   An origin server that receives an If-Match header field MUST evaluate
   the condition as per Section 12.2 prior to performing the method.

   To evaluate a received If-Match header field:

   1.  If the field value is "*", the condition is true if the origin
       server has a current representation for the target resource.

   2.  If the field value is a list of entity-tags, the condition is
       true if any of the listed tags match the entity-tag of the
       selected representation.

   3.  Otherwise, the condition is false.

   An origin server MUST NOT perform the requested method if a received
   If-Match condition evaluates to false.  Instead, the origin server
   MAY indicate that the conditional request failed by responding with a
   412 (Precondition Failed) status code.  Alternatively, if the request
   is a state-changing operation that appears to have already been
   applied to the selected representation, the origin server MAY respond
   with a 2xx (Successful) status code (i.e., the change requested by
   the user agent has already succeeded, but the user agent might not be
   aware of it, perhaps because the prior response was lost or an
   equivalent change was made by some other user agent).

   Allowing an origin server to send a success response when a change
   request appears to have already been applied is more efficient for
   many authoring use cases, but comes with some risk if multiple user
   agents are making change requests that are very similar but not
   cooperative.  For example, multiple user agents writing to a common
   resource as a semaphore (e.g., a non-atomic increment) are likely to
   collide and potentially lose important state transitions.  For those
   kinds of resources, an origin server is better off being stringent in
   sending 412 for every failed precondition on an unsafe method.  In
   other cases, excluding the ETag field from a success response might
   encourage the user agent to perform a GET as its next request to
   eliminate confusion about the resource's current state.

   The If-Match header field can be ignored by caches and intermediaries
   because it is not applicable to a stored response.



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   Note that an If-Match header field with a list value containing "*"
   and other values (including other instances of "*") is unlikely to be
   interoperable.

12.1.2.  If-None-Match

   The "If-None-Match" header field makes the request method conditional
   on a recipient cache or origin server either not having any current
   representation of the target resource, when the field value is "*",
   or having a selected representation with an entity-tag that does not
   match any of those listed in the field value.

   A recipient MUST use the weak comparison function when comparing
   entity-tags for If-None-Match (Section 7.9.3.2), since weak entity-
   tags can be used for cache validation even if there have been changes
   to the representation data.

     If-None-Match = "*" / #entity-tag

   Examples:

     If-None-Match: "xyzzy"
     If-None-Match: W/"xyzzy"
     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
     If-None-Match: *

   If-None-Match is primarily used in conditional GET requests to enable
   efficient updates of cached information with a minimum amount of
   transaction overhead.  When a client desires to update one or more
   stored responses that have entity-tags, the client SHOULD generate an
   If-None-Match header field containing a list of those entity-tags
   when making a GET request; this allows recipient servers to send a
   304 (Not Modified) response to indicate when one of those stored
   responses matches the selected representation.

   If-None-Match can also be used with a value of "*" to prevent an
   unsafe request method (e.g., PUT) from inadvertently modifying an
   existing representation of the target resource when the client
   believes that the resource does not have a current representation
   (Section 8.2.1).  This is a variation on the "lost update" problem
   that might arise if more than one client attempts to create an
   initial representation for the target resource.

   An origin server that receives an If-None-Match header field MUST
   evaluate the condition as per Section 12.2 prior to performing the
   method.




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   To evaluate a received If-None-Match header field:

   1.  If the field value is "*", the condition is false if the origin
       server has a current representation for the target resource.

   2.  If the field value is a list of entity-tags, the condition is
       false if one of the listed tags matches the entity-tag of the
       selected representation.

   3.  Otherwise, the condition is true.

   An origin server MUST NOT perform the requested method if the
   condition evaluates to false; instead, the origin server MUST respond
   with either a) the 304 (Not Modified) status code if the request
   method is GET or HEAD or b) the 412 (Precondition Failed) status code
   for all other request methods.

   Requirements on cache handling of a received If-None-Match header
   field are defined in Section 4.3.2 of [Caching].

   Note that an If-None-Match header field with a list value containing
   "*" and other values (including other instances of "*") is unlikely
   to be interoperable.

12.1.3.  If-Modified-Since

   The "If-Modified-Since" header field makes a GET or HEAD request
   method conditional on the selected representation's modification date
   being more recent than the date provided in the field value.
   Transfer of the selected representation's data is avoided if that
   data has not changed.

     If-Modified-Since = HTTP-date

   An example of the field is:

     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

   A recipient MUST ignore If-Modified-Since if the request contains an
   If-None-Match header field; the condition in If-None-Match is
   considered to be a more accurate replacement for the condition in If-
   Modified-Since, and the two are only combined for the sake of
   interoperating with older intermediaries that might not implement
   If-None-Match.







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   A recipient MUST ignore the If-Modified-Since header field if the
   received field value is not a valid HTTP-date, the field value has
   more than one member, or if the request method is neither GET nor
   HEAD.

   A recipient MUST interpret an If-Modified-Since field value's
   timestamp in terms of the origin server's clock.

   If-Modified-Since is typically used for two distinct purposes: 1) to
   allow efficient updates of a cached representation that does not have
   an entity-tag and 2) to limit the scope of a web traversal to
   resources that have recently changed.

   When used for cache updates, a cache will typically use the value of
   the cached message's Last-Modified field to generate the field value
   of If-Modified-Since.  This behavior is most interoperable for cases
   where clocks are poorly synchronized or when the server has chosen to
   only honor exact timestamp matches (due to a problem with Last-
   Modified dates that appear to go "back in time" when the origin
   server's clock is corrected or a representation is restored from an
   archived backup).  However, caches occasionally generate the field
   value based on other data, such as the Date header field of the
   cached message or the local clock time that the message was received,
   particularly when the cached message does not contain a Last-Modified
   field.

   When used for limiting the scope of retrieval to a recent time
   window, a user agent will generate an If-Modified-Since field value
   based on either its own local clock or a Date header field received
   from the server in a prior response.  Origin servers that choose an
   exact timestamp match based on the selected representation's
   Last-Modified field will not be able to help the user agent limit its
   data transfers to only those changed during the specified window.

   An origin server that receives an If-Modified-Since header field
   SHOULD evaluate the condition as per Section 12.2 prior to performing
   the method.  The origin server SHOULD NOT perform the requested
   method if the selected representation's last modification date is
   earlier than or equal to the date provided in the field value;
   instead, the origin server SHOULD generate a 304 (Not Modified)
   response, including only those metadata that are useful for
   identifying or updating a previously cached response.

   Requirements on cache handling of a received If-Modified-Since header
   field are defined in Section 4.3.2 of [Caching].






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12.1.4.  If-Unmodified-Since

   The "If-Unmodified-Since" header field makes the request method
   conditional on the selected representation's last modification date
   being earlier than or equal to the date provided in the field value.
   This field accomplishes the same purpose as If-Match for cases where
   the user agent does not have an entity-tag for the representation.

     If-Unmodified-Since = HTTP-date

   An example of the field is:

     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

   A recipient MUST ignore If-Unmodified-Since if the request contains
   an If-Match header field; the condition in If-Match is considered to
   be a more accurate replacement for the condition in If-Unmodified-
   Since, and the two are only combined for the sake of interoperating
   with older intermediaries that might not implement If-Match.

   A recipient MUST ignore the If-Unmodified-Since header field if the
   received field value is not a valid HTTP-date (including when the
   field value appears to be a list of dates).

   A recipient MUST interpret an If-Unmodified-Since field value's
   timestamp in terms of the origin server's clock.

   If-Unmodified-Since is most often used with state-changing methods
   (e.g., POST, PUT, DELETE) to prevent accidental overwrites when
   multiple user agents might be acting in parallel on a resource that
   does not supply entity-tags with its representations (i.e., to
   prevent the "lost update" problem).  It can also be used with any
   method to abort a request if the selected representation does not
   match one that the client already stored (or partially stored) from a
   prior request.

   An origin server that receives an If-Unmodified-Since header field
   MUST evaluate the condition as per Section 12.2 prior to performing
   the method.

   If the selected representation has a last modification date, the
   origin server MUST NOT perform the requested method if that date is
   more recent than the date provided in the field value.  Instead, the
   origin server MAY indicate that the conditional request failed by
   responding with a 412 (Precondition Failed) status code.
   Alternatively, if the request is a state-changing operation that
   appears to have already been applied to the selected representation,
   the origin server MAY respond with a 2xx (Successful) status code



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   (i.e., the change requested by the user agent has already succeeded,
   but the user agent might not be aware of it, perhaps because the
   prior response was lost or an equivalent change was made by some
   other user agent).

   Allowing an origin server to send a success response when a change
   request appears to have already been applied is more efficient for
   many authoring use cases, but comes with some risk if multiple user
   agents are making change requests that are very similar but not
   cooperative.  In those cases, an origin server is better off being
   stringent in sending 412 for every failed precondition on an unsafe
   method.

   The If-Unmodified-Since header field can be ignored by caches and
   intermediaries because it is not applicable to a stored response.

12.1.5.  If-Range

   The "If-Range" header field provides a special conditional request
   mechanism that is similar to the If-Match and If-Unmodified-Since
   header fields but that instructs the recipient to ignore the Range
   header field if the validator doesn't match, resulting in transfer of
   the new selected representation instead of a 412 (Precondition
   Failed) response.

   If a client has a partial copy of a representation and wishes to have
   an up-to-date copy of the entire representation, it could use the
   Range header field with a conditional GET (using either or both of
   If-Unmodified-Since and If-Match.)  However, if the precondition
   fails because the representation has been modified, the client would
   then have to make a second request to obtain the entire current
   representation.

   The "If-Range" header field allows a client to "short-circuit" the
   second request.  Informally, its meaning is as follows: if the
   representation is unchanged, send me the part(s) that I am requesting
   in Range; otherwise, send me the entire representation.

     If-Range = entity-tag / HTTP-date

   A client MUST NOT generate an If-Range header field in a request that
   does not contain a Range header field.  A server MUST ignore an If-
   Range header field received in a request that does not contain a
   Range header field.  An origin server MUST ignore an If-Range header
   field received in a request for a target resource that does not
   support Range requests.





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   A client MUST NOT generate an If-Range header field containing an
   entity-tag that is marked as weak.  A client MUST NOT generate an If-
   Range header field containing an HTTP-date unless the client has no
   entity-tag for the corresponding representation and the date is a
   strong validator in the sense defined by Section 7.9.2.2.

   A server that evaluates an If-Range precondition MUST use the strong
   comparison function when comparing entity-tags (Section 7.9.3.2) and
   MUST evaluate the condition as false if an HTTP-date validator is
   provided that is not a strong validator in the sense defined by
   Section 7.9.2.2.  A valid entity-tag can be distinguished from a
   valid HTTP-date by examining the first two characters for a DQUOTE.

   If the validator given in the If-Range header field matches the
   current validator for the selected representation of the target
   resource, then the server SHOULD process the Range header field as
   requested.  If the validator does not match, the server MUST ignore
   the Range header field.  Note that this comparison by exact match,
   including when the validator is an HTTP-date, differs from the
   "earlier than or equal to" comparison used when evaluating an
   If-Unmodified-Since conditional.

12.2.  Evaluation

   Except when excluded below, a recipient cache or origin server MUST
   evaluate received request preconditions after it has successfully
   performed its normal request checks and just before it would process
   the request body (if any) or perform the action associated with the
   request method.  A server MUST ignore all received preconditions if
   its response to the same request without those conditions, prior to
   processing the request body, would have been a status code other than
   a 2xx (Successful) or 412 (Precondition Failed).  In other words,
   redirects and failures that can be detected before significant
   processing occurs take precedence over the evaluation of
   preconditions.

   A server that is not the origin server for the target resource and
   cannot act as a cache for requests on the target resource MUST NOT
   evaluate the conditional request header fields defined by this
   specification, and it MUST forward them if the request is forwarded,
   since the generating client intends that they be evaluated by a
   server that can provide a current representation.  Likewise, a server
   MUST ignore the conditional request header fields defined by this
   specification when received with a request method that does not
   involve the selection or modification of a selected representation,
   such as CONNECT, OPTIONS, or TRACE.





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   Note that protocol extensions can modify the conditions under which
   revalidation is triggered.  For example, the "immutable" cache
   directive (defined by [RFC8246]) instructs caches to forgo
   revalidation of fresh responses even when requested by the client.

   Conditional request header fields that are defined by extensions to
   HTTP might place conditions on all recipients, on the state of the
   target resource in general, or on a group of resources.  For
   instance, the "If" header field in WebDAV can make a request
   conditional on various aspects of multiple resources, such as locks,
   if the recipient understands and implements that field ([RFC4918],
   Section 10.4).

   Although conditional request header fields are defined as being
   usable with the HEAD method (to keep HEAD's semantics consistent with
   those of GET), there is no point in sending a conditional HEAD
   because a successful response is around the same size as a 304 (Not
   Modified) response and more useful than a 412 (Precondition Failed)
   response.

12.3.  Precedence

   When more than one conditional request header field is present in a
   request, the order in which the fields are evaluated becomes
   important.  In practice, the fields defined in this document are
   consistently implemented in a single, logical order, since "lost
   update" preconditions have more strict requirements than cache
   validation, a validated cache is more efficient than a partial
   response, and entity tags are presumed to be more accurate than date
   validators.

   A recipient cache or origin server MUST evaluate the request
   preconditions defined by this specification in the following order:

   1.  When recipient is the origin server and If-Match is present,
       evaluate the If-Match precondition:

       o  if true, continue to step 3

       o  if false, respond 412 (Precondition Failed) unless it can be
          determined that the state-changing request has already
          succeeded (see Section 12.1.1)

   2.  When recipient is the origin server, If-Match is not present, and
       If-Unmodified-Since is present, evaluate the If-Unmodified-Since
       precondition:

       o  if true, continue to step 3



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       o  if false, respond 412 (Precondition Failed) unless it can be
          determined that the state-changing request has already
          succeeded (see Section 12.1.4)

   3.  When If-None-Match is present, evaluate the If-None-Match
       precondition:

       o  if true, continue to step 5

       o  if false for GET/HEAD, respond 304 (Not Modified)

       o  if false for other methods, respond 412 (Precondition Failed)

   4.  When the method is GET or HEAD, If-None-Match is not present, and
       If-Modified-Since is present, evaluate the If-Modified-Since
       precondition:

       o  if true, continue to step 5

       o  if false, respond 304 (Not Modified)

   5.  When the method is GET and both Range and If-Range are present,
       evaluate the If-Range precondition:

       o  if the validator matches and the Range specification is
          applicable to the selected representation, respond 206
          (Partial Content)

   6.  Otherwise,

       o  all conditions are met, so perform the requested action and
          respond according to its success or failure.

   Any extension to HTTP that defines additional conditional request
   header fields ought to define its own expectations regarding the
   order for evaluating such fields in relation to those defined in this
   document and other conditionals that might be found in practice.

13.  Range Requests

   Clients often encounter interrupted data transfers as a result of
   canceled requests or dropped connections.  When a client has stored a
   partial representation, it is desirable to request the remainder of
   that representation in a subsequent request rather than transfer the
   entire representation.  Likewise, devices with limited local storage
   might benefit from being able to request only a subset of a larger
   representation, such as a single page of a very large document, or
   the dimensions of an embedded image.



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   Range requests are an OPTIONAL feature of HTTP, designed so that
   recipients not implementing this feature (or not supporting it for
   the target resource) can respond as if it is a normal GET request
   without impacting interoperability.  Partial responses are indicated
   by a distinct status code to not be mistaken for full responses by
   caches that might not implement the feature.

13.1.  Range Units

   Representation data can be partitioned into subranges when there are
   addressable structural units inherent to that data's content coding
   or media type.  For example, octet (a.k.a., byte) boundaries are a
   structural unit common to all representation data, allowing
   partitions of the data to be identified as a range of bytes at some
   offset from the start or end of that data.

   This general notion of a "range unit" is used in the Accept-Ranges
   (Section 13.3) response header field to advertise support for range
   requests, the Range (Section 13.2) request header field to delineate
   the parts of a representation that are requested, and the
   Content-Range (Section 13.4) payload header field to describe which
   part of a representation is being transferred.

     range-unit       = token

   All range unit names are case-insensitive and ought to be registered
   within the "HTTP Range Unit Registry", as defined in Section 15.5.1

   Range units are intended to be extensible, as described in
   Section 15.5.  The following range unit names are defined by this
   document:

    ----------------- ---------------------------------- --------
     Range Unit Name   Description                        Ref.
    ----------------- ---------------------------------- --------
     bytes             a range of octets                  13.1.2
     none              reserved as keyword to indicate    13.3
                       range requests are not supported
    ----------------- ---------------------------------- --------

                               Table 13










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13.1.1.  Range Specifiers

   Ranges are expressed in terms of a range unit paired with a set of
   range specifiers.  The range unit name determines what kinds of
   range-spec are applicable to its own specifiers.  Hence, the
   following gramar is generic: each range unit is expected to specify
   requirements on when int-range, suffix-range, and other-range are
   allowed.

   A range request can specify a single range or a set of ranges within
   a single representation.

     ranges-specifier = range-unit "=" range-set
     range-set        = 1#range-spec
     range-spec       = int-range
                      / suffix-range
                      / other-range

   An int-range is a range expressed as two non-negative integers or as
   one non-negative integer through to the end of the representation
   data.  The range unit specifies what the integers mean (e.g., they
   might indicate unit offsets from the beginning, inclusive numbered
   parts, etc.).

     int-range     = first-pos "-" [ last-pos ]
     first-pos     = 1*DIGIT
     last-pos      = 1*DIGIT

   An int-range is invalid if the last-pos value is present and less
   than the first-pos.

   A suffix-range is a range expressed as a suffix of the representation
   data with the provided non-negative integer maximum length (in range
   units).  In other words, the last N units of the representation data.

     suffix-range  = "-" suffix-length
     suffix-length = 1*DIGIT

   To provide for extensibility, the other-range rule is a mostly
   unconstrained grammar that allows application-specific or future
   range units to define additional range specifiers.

     other-range   = 1*( %x21-2B / %x2D-7E )
                   ; 1*(VCHAR excluding comma)







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13.1.2.  Byte Ranges

   The "bytes" range unit is used to express subranges of a
   representation data's octet sequence.  Each byte range is expressed
   as an integer range at some offset, relative to either the beginning
   (int-range) or end (suffix-range) of the representation data.  Byte
   ranges do not use the other-range specifier.

   The first-pos value in a bytes int-range gives the offset of the
   first byte in a range.  The last-pos value gives the offset of the
   last byte in the range; that is, the byte positions specified are
   inclusive.  Byte offsets start at zero.

   If the representation data has a content coding applied, each byte
   range is calculated with respect to the encoded sequence of bytes,
   not the sequence of underlying bytes that would be obtained after
   decoding.

   Examples of bytes range specifiers:

   o  The first 500 bytes (byte offsets 0-499, inclusive):

           bytes=0-499

   o  The second 500 bytes (byte offsets 500-999, inclusive):

           bytes=500-999

   A client can limit the number of bytes requested without knowing the
   size of the selected representation.  If the last-pos value is
   absent, or if the value is greater than or equal to the current
   length of the representation data, the byte range is interpreted as
   the remainder of the representation (i.e., the server replaces the
   value of last-pos with a value that is one less than the current
   length of the selected representation).

   A client can request the last N bytes (N > 0) of the selected
   representation using a suffix-range.  If the selected representation
   is shorter than the specified suffix-length, the entire
   representation is used.

   Additional examples, assuming a representation of length 10000:

   o  The final 500 bytes (byte offsets 9500-9999, inclusive):

           bytes=-500

      Or:



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           bytes=9500-

   o  The first and last bytes only (bytes 0 and 9999):

           bytes=0-0,-1

   o  The first, middle, and last 1000 bytes:

           bytes= 0-999, 4500-5499, -1000

   o  Other valid (but not canonical) specifications of the second 500
      bytes (byte offsets 500-999, inclusive):

           bytes=500-600,601-999
           bytes=500-700,601-999

   If a valid bytes range-set includes at least one range-spec with a
   first-pos that is less than the current length of the representation,
   or at least one suffix-range with a non-zero suffix-length, then the
   bytes range-set is satisfiable.  Otherwise, the bytes range-set is
   unsatisfiable.

   If the selected representation has zero length, the only satisfiable
   form of range-spec is a suffix-range with a non-zero suffix-length.

   In the byte-range syntax, first-pos, last-pos, and suffix-length are
   expressed as decimal number of octets.  Since there is no predefined
   limit to the length of a payload, recipients MUST anticipate
   potentially large decimal numerals and prevent parsing errors due to
   integer conversion overflows.

13.2.  Range

   The "Range" header field on a GET request modifies the method
   semantics to request transfer of only one or more subranges of the
   selected representation data (Section 7.2), rather than the entire
   selected representation.

     Range = ranges-specifier

   A server MAY ignore the Range header field.  However, origin servers
   and intermediate caches ought to support byte ranges when possible,
   since they support efficient recovery from partially failed transfers
   and partial retrieval of large representations.  A server MUST ignore
   a Range header field received with a request method other than GET.






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   An origin server MUST ignore a Range header field that contains a
   range unit it does not understand.  A proxy MAY discard a Range
   header field that contains a range unit it does not understand.

   A server that supports range requests MAY ignore or reject a Range
   header field that consists of more than two overlapping ranges, or a
   set of many small ranges that are not listed in ascending order,
   since both are indications of either a broken client or a deliberate
   denial-of-service attack (Section 16.14).  A client SHOULD NOT
   request multiple ranges that are inherently less efficient to process
   and transfer than a single range that encompasses the same data.

   A server that supports range requests MAY ignore a Range header field
   when the selected representation has no body (i.e., the selected
   representation data is of zero length).

   A client that is requesting multiple ranges SHOULD list those ranges
   in ascending order (the order in which they would typically be
   received in a complete representation) unless there is a specific
   need to request a later part earlier.  For example, a user agent
   processing a large representation with an internal catalog of parts
   might need to request later parts first, particularly if the
   representation consists of pages stored in reverse order and the user
   agent wishes to transfer one page at a time.

   The Range header field is evaluated after evaluating the precondition
   header fields defined in Section 12.1, and only if the result in
   absence of the Range header field would be a 200 (OK) response.  In
   other words, Range is ignored when a conditional GET would result in
   a 304 (Not Modified) response.

   The If-Range header field (Section 12.1.5) can be used as a
   precondition to applying the Range header field.

   If all of the preconditions are true, the server supports the Range
   header field for the target resource, and the specified range(s) are
   valid and satisfiable (as defined in Section 13.1.2), the server
   SHOULD send a 206 (Partial Content) response with a payload
   containing one or more partial representations that correspond to the
   satisfiable ranges requested.

   If all of the preconditions are true, the server supports the Range
   header field for the target resource, and the specified range(s) are
   invalid or unsatisfiable, the server SHOULD send a 416 (Range Not
   Satisfiable) response.






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13.3.  Accept-Ranges

   The "Accept-Ranges" header field allows a server to indicate that it
   supports range requests for the target resource.

     Accept-Ranges     = acceptable-ranges
     acceptable-ranges = 1#range-unit / "none"

   An origin server that supports byte-range requests for a given target
   resource MAY send

     Accept-Ranges: bytes

   to indicate what range units are supported.  A client MAY generate
   range requests without having received this header field for the
   resource involved.  Range units are defined in Section 13.1.

   A server that does not support any kind of range request for the
   target resource MAY send

     Accept-Ranges: none

   to advise the client not to attempt a range request.

13.4.  Content-Range

   The "Content-Range" header field is sent in a single part 206
   (Partial Content) response to indicate the partial range of the
   selected representation enclosed as the message payload, sent in each
   part of a multipart 206 response to indicate the range enclosed
   within each body part, and sent in 416 (Range Not Satisfiable)
   responses to provide information about the selected representation.

     Content-Range       = range-unit SP
                           ( range-resp / unsatisfied-range )

     range-resp          = incl-range "/" ( complete-length / "*" )
     incl-range          = first-pos "-" last-pos
     unsatisfied-range   = "*/" complete-length

     complete-length     = 1*DIGIT

   If a 206 (Partial Content) response contains a Content-Range header
   field with a range unit (Section 13.1) that the recipient does not
   understand, the recipient MUST NOT attempt to recombine it with a
   stored representation.  A proxy that receives such a message SHOULD
   forward it downstream.




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   For byte ranges, a sender SHOULD indicate the complete length of the
   representation from which the range has been extracted, unless the
   complete length is unknown or difficult to determine.  An asterisk
   character ("*") in place of the complete-length indicates that the
   representation length was unknown when the header field was
   generated.

   The following example illustrates when the complete length of the
   selected representation is known by the sender to be 1234 bytes:

     Content-Range: bytes 42-1233/1234

   and this second example illustrates when the complete length is
   unknown:

     Content-Range: bytes 42-1233/*

   A Content-Range field value is invalid if it contains a range-resp
   that has a last-pos value less than its first-pos value, or a
   complete-length value less than or equal to its last-pos value.  The
   recipient of an invalid Content-Range MUST NOT attempt to recombine
   the received content with a stored representation.

   A server generating a 416 (Range Not Satisfiable) response to a byte-
   range request SHOULD send a Content-Range header field with an
   unsatisfied-range value, as in the following example:

     Content-Range: bytes */1234

   The complete-length in a 416 response indicates the current length of
   the selected representation.

   The Content-Range header field has no meaning for status codes that
   do not explicitly describe its semantic.  For this specification,
   only the 206 (Partial Content) and 416 (Range Not Satisfiable) status
   codes describe a meaning for Content-Range.

   The following are examples of Content-Range values in which the
   selected representation contains a total of 1234 bytes:

   o  The first 500 bytes:

           Content-Range: bytes 0-499/1234

   o  The second 500 bytes:

           Content-Range: bytes 500-999/1234




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   o  All except for the first 500 bytes:

           Content-Range: bytes 500-1233/1234

   o  The last 500 bytes:

           Content-Range: bytes 734-1233/1234

13.5.  Media Type multipart/byteranges

   When a 206 (Partial Content) response message includes the content of
   multiple ranges, they are transmitted as body parts in a multipart
   message body ([RFC2046], Section 5.1) with the media type of
   "multipart/byteranges".

   The multipart/byteranges media type includes one or more body parts,
   each with its own Content-Type and Content-Range fields.  The
   required boundary parameter specifies the boundary string used to
   separate each body part.

   Implementation Notes:

   1.  Additional CRLFs might precede the first boundary string in the
       body.

   2.  Although [RFC2046] permits the boundary string to be quoted, some
       existing implementations handle a quoted boundary string
       incorrectly.

   3.  A number of clients and servers were coded to an early draft of
       the byteranges specification that used a media type of multipart/
       x-byteranges , which is almost (but not quite) compatible with
       this type.

   Despite the name, the "multipart/byteranges" media type is not
   limited to byte ranges.  The following example uses an "exampleunit"
   range unit:














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     HTTP/1.1 206 Partial Content
     Date: Tue, 14 Nov 1995 06:25:24 GMT
     Last-Modified: Tue, 14 July 04:58:08 GMT
     Content-Length: 2331785
     Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

     --THIS_STRING_SEPARATES
     Content-Type: video/example
     Content-Range: exampleunit 1.2-4.3/25

     ...the first range...
     --THIS_STRING_SEPARATES
     Content-Type: video/example
     Content-Range: exampleunit 11.2-14.3/25

     ...the second range
     --THIS_STRING_SEPARATES--

   The following information serves as the registration form for the
   multipart/byteranges media type.

   Type name:  multipart

   Subtype name:  byteranges

   Required parameters:  boundary

   Optional parameters:  N/A

   Encoding considerations:  only "7bit", "8bit", or "binary" are
      permitted

   Security considerations:  see Section 16

   Interoperability considerations:  N/A

   Published specification:  This specification (see Section 13.5).

   Applications that use this media type:  HTTP components supporting
      multiple ranges in a single request.

   Fragment identifier considerations:  N/A

   Additional information:  Deprecated alias names for this type:  N/A

                            Magic number(s):  N/A

                            File extension(s):  N/A



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                            Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  See Aut
      hors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section.

   Change controller:  IESG

14.  Status Codes

   The (response) status code is a three-digit integer code giving the
   result of the attempt to understand and satisfy the request.

   HTTP status codes are extensible.  HTTP clients are not required to
   understand the meaning of all registered status codes, though such
   understanding is obviously desirable.  However, a client MUST
   understand the class of any status code, as indicated by the first
   digit, and treat an unrecognized status code as being equivalent to
   the x00 status code of that class.

   For example, if an unrecognized status code of 471 is received by a
   client, the client can assume that there was something wrong with its
   request and treat the response as if it had received a 400 (Bad
   Request) status code.  The response message will usually contain a
   representation that explains the 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
   five values for the first digit:

   o  1xx (Informational): The request was received, continuing process

   o  2xx (Successful): The request 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



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   A single request can have multiple associated responses: zero or more
   interim (non-final) responses with status codes in the
   "informational" (1xx) range, followed by exactly one final response
   with a status code in one of the other ranges.

14.1.  Overview of Status Codes

   The status codes listed below are defined in this specification.  The
   reason phrases listed here are only recommendations - they can be
   replaced by local equivalents without affecting the protocol.

   Responses with status codes that are defined as heuristically
   cacheable (e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410,
   414, and 501 in this specification) can be reused by a cache with
   heuristic expiration unless otherwise indicated by the method
   definition or explicit cache controls [Caching]; all other status
   codes are not heuristically cacheable.

   Additional status codes, outside the scope of this specification,
   have been specified for use in HTTP.  All such status codes ought to
   be registered within the "Hypertext Transfer Protocol (HTTP) Status
   Code Registry", as described in Section 15.2.

14.2.  Informational 1xx

   The 1xx (Informational) class of status code indicates an interim
   response for communicating connection status or request progress
   prior to completing the requested action and sending a final
   response.  1xx responses are terminated by the end of the header
   section.  Since HTTP/1.0 did not define any 1xx status codes, a
   server MUST NOT send a 1xx response to an HTTP/1.0 client.

   A client MUST be able to parse one or more 1xx responses received
   prior to a final response, even if the client does not expect one.  A
   user agent MAY ignore unexpected 1xx responses.

   A proxy MUST forward 1xx responses 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).

14.2.1.  100 Continue

   The 100 (Continue) status code indicates that the initial part of a
   request has been received and has not yet been rejected by the
   server.  The server intends to send a final response after the
   request has been fully received and acted upon.




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   When the request contains an Expect header field that includes a
   100-continue expectation, the 100 response indicates that the server
   wishes to receive the request payload body, as described in
   Section 9.1.1.  The client ought to continue sending the request and
   discard the 100 response.

   If the request did not contain an Expect header field containing the
   100-continue expectation, the client can simply discard this interim
   response.

14.2.2.  101 Switching Protocols

   The 101 (Switching Protocols) status code indicates that the server
   understands and is willing to comply with the client's request, via
   the Upgrade header field (Section 6.6), for a change in the
   application protocol being used on this connection.  The server MUST
   generate an Upgrade header field in the response that indicates which
   protocol(s) will be switched to immediately after the empty line that
   terminates the 101 response.

   It is assumed that the server will only agree to switch protocols
   when it is advantageous to do so.  For example, switching to a newer
   version of HTTP might be advantageous over older versions, and
   switching to a real-time, synchronous protocol might be advantageous
   when delivering resources that use such features.

14.3.  Successful 2xx

   The 2xx (Successful) class of status code indicates that the client's
   request was successfully received, understood, and accepted.

14.3.1.  200 OK

   The 200 (OK) status code indicates that the request has succeeded.
   The payload sent in a 200 response depends on the request method.
   For the methods defined by this specification, the intended meaning
   of the payload can be summarized as:

   GET  a representation of the target resource;

   HEAD  the same representation as GET, but without the representation
      data;

   POST  a representation of the status of, or results obtained from,
      the action;

   PUT, DELETE  a representation of the status of the action;




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   OPTIONS  a representation of the communications options;

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

   Aside from responses to CONNECT, a 200 response always has a payload,
   though an origin server MAY generate a payload body of zero length.
   If no payload is desired, an origin server ought to send 204 (No
   Content) instead.  For CONNECT, no payload is allowed because the
   successful result is a tunnel, which begins immediately after the 200
   response header section.

   A 200 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

14.3.2.  201 Created

   The 201 (Created) status code indicates that the request has been
   fulfilled and has resulted in one or more new resources being
   created.  The primary resource created by the request is identified
   by either a Location header field in the response or, if no Location
   field is received, by the target URI.

   The 201 response payload typically describes and links to the
   resource(s) created.  See Section 7.9 for a discussion of the meaning
   and purpose of validator header fields, such as ETag and
   Last-Modified, in a 201 response.

14.3.3.  202 Accepted

   The 202 (Accepted) status code indicates that 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 in HTTP for re-sending a status code from an asynchronous
   operation.

   The 202 response is intentionally noncommittal.  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 sent with this
   response ought to describe the request's current status and point to
   (or embed) a status monitor that can provide the user with an
   estimate of when the request will be fulfilled.





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14.3.4.  203 Non-Authoritative Information

   The 203 (Non-Authoritative Information) status code indicates that
   the request was successful but the enclosed payload has been modified
   from that of the origin server's 200 (OK) response by a transforming
   proxy (Section 6.5).  This status code allows the proxy to notify
   recipients when a transformation has been applied, since that
   knowledge might impact later decisions regarding the content.  For
   example, future cache validation requests for the content might only
   be applicable along the same request path (through the same proxies).

   The 203 response is similar to the Warning code of 214 Transformation
   Applied (Section 5.5 of [Caching]), which has the advantage of being
   applicable to responses with any status code.

   A 203 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

14.3.5.  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 send in the response payload body.  Metadata in the
   response header fields refer to the target resource and its selected
   representation after the requested action was applied.

   For example, if a 204 status code is received in response to a PUT
   request and the response contains an ETag field, then the PUT was
   successful and the ETag field value contains the entity-tag for 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 does not need to 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
   its 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.

   A 204 response is terminated by the first empty line after the header
   fields because it cannot contain a message body.



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   A 204 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

14.3.6.  205 Reset Content

   The 205 (Reset Content) status code indicates that the server has
   fulfilled the request and desires that the user agent reset the
   "document view", which caused the request to be sent, to its original
   state as received from the origin server.

   This response is intended to support a common data entry use case
   where the user receives content that supports data entry (a form,
   notepad, canvas, etc.), enters or manipulates data in that space,
   causes the entered data to be submitted in a request, and then the
   data entry mechanism is reset for the next entry so that the user can
   easily initiate another input action.

   Since the 205 status code implies that no additional content will be
   provided, a server MUST NOT generate a payload in a 205 response.

14.3.7.  206 Partial Content

   The 206 (Partial Content) status code indicates that the server is
   successfully fulfilling a range request for the target resource by
   transferring one or more parts of the selected representation.

   When a 206 response is generated, the server MUST generate the
   following header fields, in addition to those required in the
   subsections below, if the field would have been sent in a 200 (OK)
   response to the same request: Date, Cache-Control, ETag, Expires,
   Content-Location, and Vary.

   A Content-Length field present in a 206 response indicates the number
   of octets in the body of this message, which is usually not the
   complete length of the selected representation.  Each Content-Range
   field includes information about the selected representation's
   complete length.

   If a 206 is generated in response to a request with an If-Range
   header field, the sender SHOULD NOT generate other representation
   header fields beyond those required, because the client is understood
   to already have a prior response containing those header fields.
   Otherwise, the sender MUST generate all of the representation header
   fields that would have been sent in a 200 (OK) response to the same
   request.





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   A 206 response is heuristically cacheable; i.e., unless otherwise
   indicated by explicit cache controls (see Section 4.2.2 of
   [Caching]).

14.3.7.1.  Single Part

   If a single part is being transferred, the server generating the 206
   response MUST generate a Content-Range header field, describing what
   range of the selected representation is enclosed, and a payload
   consisting of the range.  For example:

     HTTP/1.1 206 Partial Content
     Date: Wed, 15 Nov 1995 06:25:24 GMT
     Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
     Content-Range: bytes 21010-47021/47022
     Content-Length: 26012
     Content-Type: image/gif

     ... 26012 bytes of partial image data ...

14.3.7.2.  Multiple Parts

   If multiple parts are being transferred, the server generating the
   206 response MUST generate a "multipart/byteranges" payload, as
   defined in Section 13.5, and a Content-Type header field containing
   the multipart/byteranges media type and its required boundary
   parameter.  To avoid confusion with single-part responses, a server
   MUST NOT generate a Content-Range header field in the HTTP header
   section of a multiple part response (this field will be sent in each
   part instead).

   Within the header area of each body part in the multipart payload,
   the server MUST generate a Content-Range header field corresponding
   to the range being enclosed in that body part.  If the selected
   representation would have had a Content-Type header field in a 200
   (OK) response, the server SHOULD generate that same Content-Type
   field in the header area of each body part.  For example:














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     HTTP/1.1 206 Partial Content
     Date: Wed, 15 Nov 1995 06:25:24 GMT
     Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
     Content-Length: 1741
     Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

     --THIS_STRING_SEPARATES
     Content-Type: application/pdf
     Content-Range: bytes 500-999/8000

     ...the first range...
     --THIS_STRING_SEPARATES
     Content-Type: application/pdf
     Content-Range: bytes 7000-7999/8000

     ...the second range
     --THIS_STRING_SEPARATES--

   When multiple ranges are requested, a server MAY coalesce any of the
   ranges that overlap, or that are separated by a gap that is smaller
   than the overhead of sending multiple parts, regardless of the order
   in which the corresponding range-spec appeared in the received Range
   header field.  Since the typical overhead between parts of a
   multipart/byteranges payload is around 80 bytes, depending on the
   selected representation's media type and the chosen boundary
   parameter length, it can be less efficient to transfer many small
   disjoint parts than it is to transfer the entire selected
   representation.

   A server MUST NOT generate a multipart response to a request for a
   single range, since a client that does not request multiple parts
   might not support multipart responses.  However, a server MAY
   generate a multipart/byteranges payload with only a single body part
   if multiple ranges were requested and only one range was found to be
   satisfiable or only one range remained after coalescing.  A client
   that cannot process a multipart/byteranges response MUST NOT generate
   a request that asks for multiple ranges.

   When a multipart response payload is generated, the server SHOULD
   send the parts in the same order that the corresponding range-spec
   appeared in the received Range header field, excluding those ranges
   that were deemed unsatisfiable or that were coalesced into other
   ranges.  A client that receives a multipart response MUST inspect the
   Content-Range header field present in each body part in order to
   determine which range is contained in that body part; a client cannot
   rely on receiving the same ranges that it requested, nor the same
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14.3.7.3.  Combining Parts

   A response might transfer only a subrange of a representation if the
   connection closed prematurely or if the request used one or more
   Range specifications.  After several such transfers, a client might
   have received several ranges of the same representation.  These
   ranges can only be safely combined if they all have in common the
   same strong validator (Section 7.9.1).

   A client that has received multiple partial responses to GET requests
   on a target resource MAY combine those responses into a larger
   continuous range if they share the same strong validator.

   If the most recent response is an incomplete 200 (OK) response, then
   the header fields of that response are used for any combined response
   and replace those of the matching stored responses.

   If the most recent response is a 206 (Partial Content) response and
   at least one of the matching stored responses is a 200 (OK), then the
   combined response header fields consist of the most recent 200
   response's header fields.  If all of the matching stored responses
   are 206 responses, then the stored response with the most recent
   header fields is used as the source of header fields for the combined
   response, except that the client MUST use other header fields
   provided in the new response, aside from Content-Range, to replace
   all instances of the corresponding header fields in the stored
   response.

   The combined response message body consists of the union of partial
   content ranges in the new response and each of the selected
   responses.  If the union consists of the entire range of the
   representation, then the client MUST process the combined response as
   if it were a complete 200 (OK) response, including a Content-Length
   header field that reflects the complete length.  Otherwise, the
   client MUST process the set of continuous ranges as one of the
   following: an incomplete 200 (OK) response if the combined response
   is a prefix of the representation, a single 206 (Partial Content)
   response containing a multipart/byteranges body, or multiple 206
   (Partial Content) responses, each with one continuous range that is
   indicated by a Content-Range header field.

14.4.  Redirection 3xx

   The 3xx (Redirection) class of status code indicates that further
   action needs to be taken by the user agent in order to fulfill the
   request.  There are several types of redirects:





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   1.  Redirects that indicate this resource might be available at a
       different URI, as provided by the Location field, as in the
       status codes 301 (Moved Permanently), 302 (Found), 307 (Temporary
       Redirect), and 308 (Permanent Redirect).

   2.  Redirection that offers a choice among matching resources capable
       of representing this resource, as in the 300 (Multiple Choices)
       status code.

   3.  Redirection to a different resource, identified by the Location
       field, that can represent an indirect response to the request, as
       in the 303 (See Other) status code.

   4.  Redirection to a previously stored result, as in the 304 (Not
       Modified) status code.

   If a Location header field (Section 9.2.3) is provided, the user
   agent MAY automatically redirect its request to the URI referenced by
   the Location field value, even if the specific status code is not
   understood.  Automatic redirection needs to be done with care for
   methods not known to be safe, as defined in Section 8.2.1, since the
   user might not wish to redirect an unsafe request.

   When automatically following a redirected request, the user agent
   SHOULD resend the original request message with the following
   modifications:

   1.  Replace the target URI with the URI referenced by the redirection
       response's Location header field value after resolving it
       relative to the original request's target URI.

   2.  Remove header fields that were automatically generated by the
       implementation, replacing them with updated values as appropriate
       to the new request.  This includes:

       1.  Connection-specific header fields (see Section 6.4.1),

       2.  Header fields specific to the client's proxy configuration,
           including (but not limited to) Proxy-Authorization,

       3.  Origin-specific header fields (if any), including (but not
           limited to) Host,

       4.  Validating header fields that were added by the
           implementation's cache (e.g., If-None-Match,
           If-Modified-Since),





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       5.  Resource-specific header fields, including (but not limited
           to) Referer, Origin, Authorization, and Cookie.

   3.  Consider removing header fields that were not automatically
       generated by the implementation (i.e., those present in the
       request because they were added by the calling context) where
       there are security implications; this includes but is not limited
       to Authorization and Cookie.

   4.  Change the request method according to the redirecting status
       code's semantics, if applicable.

   5.  If the request method has been changed to GET or HEAD, remove
       content-specific header fields, including (but not limited to)
       Content-Encoding, Content-Language, Content-Location,
       Content-Type, Content-Length, Digest, ETag, Last-Modified.

      |  *Note:* In HTTP/1.0, the status codes 301 (Moved Permanently)
      |  and 302 (Found) were defined for the first type of redirect
      |  ([RFC1945], Section 9.3).  Early user agents split on whether
      |  the method applied to the redirect target would be the same as
      |  the original request or would be rewritten as GET.  Although
      |  HTTP originally defined the former semantics for 301 and 302
      |  (to match its original implementation at CERN), and defined 303
      |  (See Other) to match the latter semantics, prevailing practice
      |  gradually converged on the latter semantics for 301 and 302 as
      |  well.  The first revision of HTTP/1.1 added 307 (Temporary
      |  Redirect) to indicate the former semantics of 302 without being
      |  impacted by divergent practice.  For the same reason, 308
      |  (Permanent Redirect) was later on added in [RFC7538] to match
      |  301.  Over 10 years later, most user agents still do method
      |  rewriting for 301 and 302; therefore, [RFC7231] made that
      |  behavior conformant when the original request is POST.

   A client 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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14.4.1.  300 Multiple Choices

   The 300 (Multiple Choices) status code indicates that the target
   resource has more than one representation, each with its own more
   specific identifier, and information about the alternatives is being
   provided so that the user (or user agent) can select a preferred
   representation by redirecting its request to one or more of those
   identifiers.  In other words, the server desires that the user agent
   engage in reactive negotiation to select the most appropriate
   representation(s) for its needs (Section 11).

   If the server has a preferred choice, the server SHOULD generate a
   Location header field containing a preferred choice's URI reference.
   The user agent MAY use the Location field value for automatic
   redirection.

   For request methods other than HEAD, the server SHOULD generate a
   payload in the 300 response containing a list of representation
   metadata and URI reference(s) from which the user or user agent can
   choose the one most preferred.  The user agent MAY make a selection
   from that list automatically if it understands the provided media
   type.  A specific format for automatic selection is not defined by
   this specification because HTTP tries to remain orthogonal to the
   definition of its payloads.  In practice, the representation is
   provided in some easily parsed format believed to be acceptable to
   the user agent, as determined by shared design or content
   negotiation, or in some commonly accepted hypertext format.

   A 300 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

      |  *Note:* The original proposal for the 300 status code defined
      |  the URI header field as providing a list of alternative
      |  representations, such that it would be usable for 200, 300, and
      |  406 responses and be transferred in responses to the HEAD
      |  method.  However, lack of deployment and disagreement over
      |  syntax led to both URI and Alternates (a subsequent proposal)
      |  being dropped from this specification.  It is possible to
      |  communicate the list as a Link header field value [RFC8288]
      |  whose members have a relationship of "alternate", though
      |  deployment is a chicken-and-egg problem.









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14.4.2.  301 Moved Permanently

   The 301 (Moved Permanently) status code indicates that the target
   resource has been assigned a new permanent URI and any future
   references to this resource ought to use one of the enclosed URIs.
   Clients with link-editing capabilities ought to automatically re-link
   references to the target URI to one or more of the new references
   sent by the server, where possible.

   The server SHOULD generate a Location header field in the response
   containing a preferred URI reference for the new permanent URI.  The
   user agent MAY use the Location field value for automatic
   redirection.  The server's response payload usually contains a short
   hypertext note with a hyperlink to the new URI(s).

      |  *Note:* For historical reasons, a user agent MAY change the
      |  request method from POST to GET for the subsequent request.  If
      |  this behavior is undesired, the 308 (Permanent Redirect) status
      |  code can be used instead.

   A 301 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

14.4.3.  302 Found

   The 302 (Found) status code indicates that the target resource
   resides temporarily under a different URI.  Since the redirection
   might be altered on occasion, the client ought to continue to use the
   target URI for future requests.

   The server SHOULD generate a Location header field in the response
   containing a URI reference for the different URI.  The user agent MAY
   use the Location field value for automatic redirection.  The server's
   response payload usually contains a short hypertext note with a
   hyperlink to the different URI(s).

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










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14.4.4.  303 See Other

   The 303 (See Other) 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, which is intended to provide an
   indirect response to the original request.  A user agent can perform
   a retrieval request targeting that URI (a GET or HEAD request if
   using HTTP), which might also be redirected, 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
   target URI.

   This status code is 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 origin server does
   not have a representation of the target resource that can be
   transferred by the server over HTTP.  However, the Location field
   value refers to a resource that is descriptive of the target
   resource, such that making a retrieval request on that other resource
   might result in a representation that is useful to recipients without
   implying that it represents the original 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.

   Except for responses to a HEAD request, the representation of a 303
   response ought to contain a short hypertext note with a hyperlink to
   the same URI reference provided in the Location header field.

14.4.5.  304 Not Modified

   The 304 (Not Modified) status code indicates that a conditional GET
   or HEAD request has been received and would have resulted in a 200
   (OK) response if it were not for the fact that the condition
   evaluated to false.  In other words, there is no need for the server
   to transfer a representation of the target resource because the
   request indicates that the client, which made the request
   conditional, already has a valid representation; the server is
   therefore redirecting the client to make use of that stored
   representation as if it were the payload of a 200 (OK) response.






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   The server generating a 304 response MUST generate any of the
   following header fields that would have been sent in a 200 (OK)
   response to the same request: Cache-Control, Content-Location, Date,
   ETag, Expires, and Vary.

   Since the goal of a 304 response is to minimize information transfer
   when the recipient already has one or more cached representations, a
   sender SHOULD NOT generate representation metadata other than the
   above listed fields unless said metadata exists for the purpose of
   guiding cache updates (e.g., Last-Modified might be useful if the
   response does not have an ETag field).

   Requirements on a cache that receives a 304 response are defined in
   Section 4.3.4 of [Caching].  If the conditional request originated
   with an outbound client, such as a user agent with its own cache
   sending a conditional GET to a shared proxy, then the proxy SHOULD
   forward the 304 response to that client.

   A 304 response cannot contain a message-body; it is always terminated
   by the first empty line after the header fields.

14.4.6.  305 Use Proxy

   The 305 (Use Proxy) status code was defined in a previous version of
   this specification and is now deprecated (Appendix B of [RFC7231]).

14.4.7.  306 (Unused)

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

14.4.8.  307 Temporary Redirect

   The 307 (Temporary Redirect) status code indicates that the target
   resource resides temporarily under a different URI and the user agent
   MUST NOT change the request method if it performs an automatic
   redirection to that URI.  Since the redirection can change over time,
   the client ought to continue using the original target URI for future
   requests.

   The server SHOULD generate a Location header field in the response
   containing a URI reference for the different URI.  The user agent MAY
   use the Location field value for automatic redirection.  The server's
   response payload usually contains a short hypertext note with a
   hyperlink to the different URI(s).






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14.4.9.  308 Permanent Redirect

   The 308 (Permanent Redirect) status code indicates that the target
   resource has been assigned a new permanent URI and any future
   references to this resource ought to use one of the enclosed URIs.
   Clients with link editing capabilities ought to automatically re-link
   references to the target URI to one or more of the new references
   sent by the server, where possible.

   The server SHOULD generate a Location header field in the response
   containing a preferred URI reference for the new permanent URI.  The
   user agent MAY use the Location field value for automatic
   redirection.  The server's response payload usually contains a short
   hypertext note with a hyperlink to the new URI(s).

   A 308 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

      |  *Note:* This status code is much younger (June 2014) than its
      |  sibling codes, and thus might not be recognized everywhere.
      |  See Section 4 of [RFC7538] for deployment considerations.

14.5.  Client Error 4xx

   The 4xx (Client Error) class of status code indicates that the client
   seems to have erred.  Except when responding to a HEAD request, the
   server SHOULD send 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.

14.5.1.  400 Bad Request

   The 400 (Bad Request) status code indicates that the server cannot or
   will not process the request due to something that is perceived to be
   a client error (e.g., malformed request syntax, invalid request
   message framing, or deceptive request routing).

14.5.2.  401 Unauthorized

   The 401 (Unauthorized) status code indicates that the request has not
   been applied because it lacks valid authentication credentials for
   the target resource.  The server generating a 401 response MUST send
   a WWW-Authenticate header field (Section 10.6.1) containing at least
   one challenge applicable to the target resource.





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   If the request included authentication credentials, then the 401
   response indicates that authorization has been refused for those
   credentials.  The user agent MAY repeat the request with a new or
   replaced Authorization header field (Section 10.6.2).  If the 401
   response contains the same challenge as the prior response, and the
   user agent has already attempted authentication at least once, then
   the user agent SHOULD present the enclosed representation to the
   user, since it usually contains relevant diagnostic information.

14.5.3.  402 Payment Required

   The 402 (Payment Required) status code is reserved for future use.

14.5.4.  403 Forbidden

   The 403 (Forbidden) status code indicates that the server understood
   the request but refuses to fulfill it.  A server that wishes to make
   public why the request has been forbidden can describe that reason in
   the response payload (if any).

   If authentication credentials were provided in the request, the
   server considers them insufficient to grant access.  The client
   SHOULD NOT automatically repeat the request with the same
   credentials.  The client MAY repeat the request with new or different
   credentials.  However, a request might be forbidden for reasons
   unrelated to the credentials.

   An origin server that wishes to "hide" the current existence of a
   forbidden target resource MAY instead respond with a status code of
   404 (Not Found).

14.5.5.  404 Not Found

   The 404 (Not Found) status code indicates that the origin server did
   not find a current representation for the target resource or is not
   willing to disclose that one exists.  A 404 status code does not
   indicate whether this lack of representation is temporary or
   permanent; the 410 (Gone) status code is preferred over 404 if the
   origin server knows, presumably through some configurable means, that
   the condition is likely to be permanent.

   A 404 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).







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14.5.6.  405 Method Not Allowed

   The 405 (Method Not Allowed) status code indicates that the method
   received in the request-line is known by the origin server but not
   supported by the target resource.  The origin server MUST generate an
   Allow header field in a 405 response containing a list of the target
   resource's currently supported methods.

   A 405 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

14.5.7.  406 Not Acceptable

   The 406 (Not Acceptable) status code indicates that the target
   resource does not have a current representation that would be
   acceptable to the user agent, according to the proactive negotiation
   header fields received in the request (Section 11.1), and the server
   is unwilling to supply a default representation.

   The server SHOULD generate a payload containing a list of available
   representation characteristics and corresponding resource identifiers
   from which the user or user agent can choose the one most
   appropriate.  A user agent MAY automatically select the most
   appropriate choice from that list.  However, this specification does
   not define any standard for such automatic selection, as described in
   Section 14.4.1.

14.5.8.  407 Proxy Authentication Required

   The 407 (Proxy Authentication Required) status code is similar to 401
   (Unauthorized), but it indicates that the client needs to
   authenticate itself in order to use a proxy for this request.  The
   proxy MUST send a Proxy-Authenticate header field (Section 10.7.1)
   containing a challenge applicable to that proxy for the request.  The
   client MAY repeat the request with a new or replaced
   Proxy-Authorization header field (Section 10.7.2).

14.5.9.  408 Request Timeout

   The 408 (Request Timeout) status code indicates that the server did
   not receive a complete request message within the time that it was
   prepared to wait.  If the client has an outstanding request in
   transit, the client MAY repeat that request on a new connection.







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14.5.10.  409 Conflict

   The 409 (Conflict) status code indicates that the request could not
   be completed due to a conflict with the current state of the target
   resource.  This code is used in situations where the user might be
   able to resolve the conflict and resubmit the request.  The server
   SHOULD generate a payload that includes enough information for a user
   to recognize the source of the conflict.

   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 that conflict with those made by
   an earlier (third-party) request, the origin server might use a 409
   response to indicate that it can't complete the request.  In this
   case, the response representation would likely contain information
   useful for merging the differences based on the revision history.

14.5.11.  410 Gone

   The 410 (Gone) status code indicates that access to the target
   resource is no longer available at the origin server and that this
   condition is likely to be permanent.  If the origin server does not
   know, or has no facility to determine, whether or not the condition
   is permanent, the status code 404 (Not Found) ought to be used
   instead.

   The 410 response is primarily intended to assist the task of web
   maintenance by notifying the recipient that the resource is
   intentionally unavailable and that the server owners desire that
   remote links to that resource be removed.  Such an event is common
   for limited-time, promotional services and for resources belonging to
   individuals no longer associated with the origin 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.

   A 410 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

14.5.12.  411 Length Required

   The 411 (Length Required) status code indicates that the server
   refuses to accept the request without a defined Content-Length
   (Section 7.7).  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.




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14.5.13.  412 Precondition Failed

   The 412 (Precondition Failed) status code indicates that one or more
   conditions given in the request header fields evaluated to false when
   tested on the server.  This response status code allows the client to
   place preconditions on the current resource state (its current
   representations and metadata) and, thus, prevent the request method
   from being applied if the target resource is in an unexpected state.

14.5.14.  413 Payload Too Large

   The 413 (Payload Too Large) status code indicates that the server is
   refusing to process a request because the request payload is larger
   than the server is willing or able to process.  The server MAY
   terminate the request, if the protocol version in use allows it;
   otherwise, the server MAY close the connection.

   If the condition is temporary, the server SHOULD generate a
   Retry-After header field to indicate that it is temporary and after
   what time the client MAY try again.

14.5.15.  414 URI Too Long

   The 414 (URI Too Long) status code indicates that the server is
   refusing to service the request because the target 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 "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 potential security holes.

   A 414 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

14.5.16.  415 Unsupported Media Type

   The 415 (Unsupported Media Type) status code indicates that the
   origin server is refusing to service the request because the payload
   is in a format not supported by this method on the target resource.

   The format problem might be due to the request's indicated
   Content-Type or Content-Encoding, or as a result of inspecting the
   data directly.






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   If the problem was caused by an unsupported content coding, the
   Accept-Encoding response header field (Section 11.1.4) ought to be
   used to indicate what (if any) content codings would have been
   accepted in the request.

   On the other hand, if the cause was an unsupported media type, the
   Accept response header field (Section 11.1.2) can be used to indicate
   what media types would have been accepted in the request.

14.5.17.  416 Range Not Satisfiable

   The 416 (Range Not Satisfiable) status code indicates that the set of
   ranges in the request's Range header field (Section 13.2) has been
   rejected either because none of the requested ranges are satisfiable
   or because the client has requested an excessive number of small or
   overlapping ranges (a potential denial of service attack).

   Each range unit defines what is required for its own range sets to be
   satisfiable.  For example, Section 13.1.2 defines what makes a bytes
   range set satisfiable.

   When this status code is generated in response to a byte-range
   request, the sender SHOULD generate a Content-Range header field
   specifying the current length of the selected representation
   (Section 13.4).

   For example:

     HTTP/1.1 416 Range Not Satisfiable
     Date: Fri, 20 Jan 2012 15:41:54 GMT
     Content-Range: bytes */47022

      |  *Note:* Because servers are free to ignore Range, many
      |  implementations will respond with the entire selected
      |  representation in a 200 (OK) response.  That is partly because
      |  most clients are prepared to receive a 200 (OK) to complete the
      |  task (albeit less efficiently) and partly because clients might
      |  not stop making an invalid partial request until they have
      |  received a complete representation.  Thus, clients cannot
      |  depend on receiving a 416 (Range Not Satisfiable) response even
      |  when it is most appropriate.

14.5.18.  417 Expectation Failed

   The 417 (Expectation Failed) status code indicates that the
   expectation given in the request's Expect header field
   (Section 9.1.1) could not be met by at least one of the inbound
   servers.



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14.5.19.  418 (Unused)

   [RFC2324] was an April 1 RFC that lampooned the various ways HTTP was
   abused; one such abuse was the definition of an application-specific
   418 status code.  In the intervening years, this status code has been
   widely implemented as an "Easter Egg", and therefore is effectively
   consumed by this use.

   Therefore, the 418 status code is reserved in the IANA HTTP Status
   Code Registry.  This indicates that the status code cannot be
   assigned to other applications currently.  If future circumstances
   require its use (e.g., exhaustion of 4NN status codes), it can be re-
   assigned to another use.

14.5.20.  422 Unprocessable Payload

   The 422 (Unprocessable Payload) status code indicates that the server
   understands the content type of the request payload (hence a 415
   (Unsupported Media Type) status code is inappropriate), and the
   syntax of the request payload is correct, but was unable to process
   the contained instructions.  For example, this status code can be
   sent if an XML request payload contains well-formed (i.e.,
   syntactically correct), but semantically erroneous XML instructions.

14.5.21.  426 Upgrade Required

   The 426 (Upgrade Required) status code indicates that the server
   refuses to perform the request using the current protocol but might
   be willing to do so after the client upgrades to a different
   protocol.  The server MUST send an Upgrade header field in a 426
   response to indicate the required protocol(s) (Section 6.6).

   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.










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14.6.  Server Error 5xx

   The 5xx (Server Error) class of status code indicates that the server
   is aware that it has erred or is incapable of performing the
   requested method.  Except when responding to a HEAD request, the
   server SHOULD send a representation containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition.  A user agent SHOULD display any included representation
   to the user.  These response codes are applicable to any request
   method.

14.6.1.  500 Internal Server Error

   The 500 (Internal Server Error) status code indicates that the server
   encountered an unexpected condition that prevented it from fulfilling
   the request.

14.6.2.  501 Not Implemented

   The 501 (Not Implemented) status code indicates that the server does
   not support the functionality required to fulfill the request.  This
   is the appropriate response when the server does not recognize the
   request method and is not capable of supporting it for any resource.

   A 501 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

14.6.3.  502 Bad Gateway

   The 502 (Bad Gateway) status code indicates that the server, while
   acting as a gateway or proxy, received an invalid response from an
   inbound server it accessed while attempting to fulfill the request.

14.6.4.  503 Service Unavailable

   The 503 (Service Unavailable) status code indicates that the server
   is currently unable to handle the request due to a temporary overload
   or scheduled maintenance, which will likely be alleviated after some
   delay.  The server MAY send a Retry-After header field
   (Section 9.2.4) to suggest an appropriate amount of time for the
   client to wait before retrying the request.

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





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14.6.5.  504 Gateway Timeout

   The 504 (Gateway Timeout) status code indicates that the server,
   while acting as a gateway or proxy, did not receive a timely response
   from an upstream server it needed to access in order to complete the
   request.

14.6.6.  505 HTTP Version Not Supported

   The 505 (HTTP Version Not Supported) status code indicates that the
   server does not support, or refuses to support, the major version of
   HTTP 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 5.1, other than
   with this error message.  The server SHOULD generate a representation
   for the 505 response that describes why that version is not supported
   and what other protocols are supported by that server.

15.  Extending HTTP

   HTTP defines a number of generic extension points that can be used to
   introduce capabilities to the protocol without introducing a new
   version, including methods, status codes, field names, and further
   extensibility points within defined fields, such as authentication
   schemes and cache-directives (see Cache-Control in Section 5.2.3 of
   [Caching]).  Because the semantics of HTTP are not versioned, these
   extension points are persistent; the version of the protocol in use
   does not affect their semantics.

   Version-independent extensions are discouraged from depending on or
   interacting with the specific version of the protocol in use.  When
   this is unavoidable, careful consideration needs to be given to how
   the extension can interoperate across versions.

   Additionally, specific versions of HTTP might have their own
   extensibility points, such as transfer-codings in HTTP/1.1
   (Section 6.1 of [Messaging]) and HTTP/2 ([RFC7540]) SETTINGS or frame
   types.  These extension points are specific to the version of the
   protocol they occur within.

   Version-specific extensions cannot override or modify the semantics
   of a version-independent mechanism or extension point (like a method
   or header field) without explicitly being allowed by that protocol
   element.  For example, the CONNECT method (Section 8.3.6) allows
   this.






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   These guidelines assure that the protocol operates correctly and
   predictably, even when parts of the path implement different versions
   of HTTP.

15.1.  Method Extensibility

15.1.1.  Method Registry

   The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained
   by IANA at <https://www.iana.org/assignments/http-methods>, registers
   method names.

   HTTP method registrations MUST include the following fields:

   o  Method Name (see Section 8)

   o  Safe ("yes" or "no", see Section 8.2.1)

   o  Idempotent ("yes" or "no", see Section 8.2.2)

   o  Pointer to specification text

   Values to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

15.1.2.  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 6 of [Messaging]) 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".

   A new method definition needs to indicate whether it is safe
   (Section 8.2.1), idempotent (Section 8.2.2), cacheable
   (Section 8.2.3), what semantics are to be associated with the payload
   body if any is present in the request and what refinements the method
   makes to header field or status code semantics.  If the new method is
   cacheable, its definition ought to describe how, and under what



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   conditions, a cache can store a response and use it to satisfy a
   subsequent request.  The new method ought to describe whether it can
   be made conditional (Section 12.1) and, if so, how a server responds
   when the condition is false.  Likewise, if the new method might have
   some use for partial response semantics (Section 13.2), it ought to
   document this, too.

      |  *Note:* Avoid defining a method name that starts with "M-",
      |  since that prefix might be misinterpreted as having the
      |  semantics assigned to it by [RFC2774].

15.2.  Status Code Extensibility

15.2.1.  Status Code Registry

   The "Hypertext Transfer Protocol (HTTP) Status Code Registry",
   maintained by IANA at <https://www.iana.org/assignments/http-status-
   codes>, registers status code numbers.

   A registration MUST include the following fields:

   o  Status Code (3 digits)

   o  Short Description

   o  Pointer to specification text

   Values to be added to the HTTP status code namespace require IETF
   Review (see [RFC8126], Section 4.8).

15.2.2.  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.
   Status codes are generic; they are potentially applicable to any
   resource, not just one particular media type, kind of resource, or
   application of HTTP.  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 14.  To allow existing parsers to process the
   response message, new status codes cannot disallow a payload,
   although they can mandate a zero-length payload body.

   Proposals for new status codes that are not yet widely deployed ought
   to avoid allocating a specific number for the code until there is
   clear consensus that it will be registered; instead, early drafts can



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   use a notation such as "4NN", or "3N0" .. "3N9", to indicate the
   class of the proposed status code(s) without consuming a number
   prematurely.

   The definition of a new status code ought to explain the request
   conditions that would cause a response containing that status code
   (e.g., combinations of request header fields and/or method(s)) along
   with any dependencies on response header fields (e.g., what fields
   are required, what fields can modify the semantics, and what field
   semantics are further refined when used with the new status code).

   By default, a status code applies only to the request corresponding
   to the response it occurs within.  If a status code applies to a
   larger scope of applicability - for example, all requests to the
   resource in question, or all requests to a server - this must be
   explicitly specified.  When doing so, it should be noted that not all
   clients can be expected to consistently apply a larger scope, because
   they might not understand the new status code.

   The definition of a new status code ought to specify whether or not
   it is cacheable.  Note that all status codes can be cached if the
   response they occur in has explicit freshness information; however,
   status codes that are defined as being cacheable are allowed to be
   cached without explicit freshness information.  Likewise, the
   definition of a status code can place constraints upon cache
   behavior.  See [Caching] for more information.

   Finally, the definition of a new status code ought to indicate
   whether the payload has any implied association with an identified
   resource (Section 5.5.2).

15.3.  Field Name Extensibility

15.3.1.  Field Name Registry

   The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines
   the namespace for HTTP field names.

   Any party can request registration of a HTTP field.  See
   Section 15.3.3 for considerations to take into account when creating
   a new HTTP field.

   The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is
   located at <https://www.iana.org/assignments/http-fields/>.
   Registration requests can be made by following the instructions
   located there or by sending an email to the "ietf-http-wg@ietf.org"
   mailing list.




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   Field names are registered on the advice of a Designated Expert
   (appointed by the IESG or their delegate).  Fields with the status
   'permanent' are Specification Required ([RFC8126], Section 4.6).

   Registration requests consist of at least the following information:

   Field name:
      The requested field name.  It MUST conform to the field-name
      syntax defined in Section 5.4.3, and SHOULD be restricted to just
      letters, digits, hyphen ('-') and underscore ('_') characters,
      with the first character being a letter.

   Status:
      "permanent" or "provisional".

   Specification document(s):
      Reference to the document that specifies the field, preferably
      including a URI that can be used to retrieve a copy of the
      document.  An indication of the relevant section(s) can also be
      included, but is not required.

   And, optionally:

   Comments:  Additional information, such as about reserved entries.

   The Expert(s) can define additional fields to be collected in the
   registry, in consultation with the community.

   Standards-defined names have a status of "permanent".  Other names
   can also be registered as permanent, if the Expert(s) find that they
   are in use, in consultation with the community.  Other names should
   be registered as "provisional".

   Provisional entries can be removed by the Expert(s) if - in
   consultation with the community - the Expert(s) find that they are
   not in use.  The Experts can change a provisional entry's status to
   permanent at any time.

   Note that names can be registered by third parties (including the
   Expert(s)), if the Expert(s) determines that an unregistered name is
   widely deployed and not likely to be registered in a timely manner
   otherwise.

15.3.2.  Considerations for New Field Names

   There is no limit on the introduction of new field names, each
   presumably defining new semantics.




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   New fields can be defined such that, when they are understood by a
   recipient, they might override or enhance the interpretation of
   previously defined fields, define preconditions on request
   evaluation, or refine the meaning of responses.

   Authors of specifications defining new fields are advised to choose a
   short but descriptive field name.  Short names avoid needless data
   transmission; descriptive names avoid confusion and "squatting" on
   names that might have broader uses.

   To that end, limited-use fields (such as a header confined to a
   single application or use case) are encouraged to use a name that
   includes its name (or an abbreviation) as a prefix; for example, if
   the Foo Application needs a Description field, it might use "Foo-
   Desc"; "Description" is too generic, and "Foo-Description" is
   needlessly long.

   While the field-name syntax is defined to allow any token character,
   in practice some implementations place limits on the characters they
   accept in field-names.  To be interoperable, new field names SHOULD
   constrain themselves to alphanumeric characters, "-", and ".", and
   SHOULD begin with an alphanumeric character.

   Field names ought not be prefixed with "X-"; see [BCP178] for further
   information.

   Other prefixes are sometimes used in HTTP field names; for example,
   "Accept-" is used in many content negotiation headers.  These
   prefixes are only an aid to recognizing the purpose of a field, and
   do not trigger automatic processing.

15.3.3.  Considerations for New Field Values

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

   o  Whether the field has a singleton or list-based value (see
      Section 5.4.4).

      If it is a singleton field, document how to treat messages where
      the multiple members are present (a sensible default would be to
      ignore the field, but this might not always be the right choice).









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      Note that intermediaries and software libraries might combine
      multiple field instances into a single one, despite the field
      being defined as a singleton.  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 field can be used; e.g., only in
      responses or requests, in all messages, only on responses to a
      particular request method, etc.

   o  What the scope of applicability for the information conveyed in
      the field is.  By default, fields apply only to the message they
      are associated with, but some response fields are designed to
      apply to all representations of a resource, the resource itself,
      or an even broader scope.  Specifications that expand the scope of
      a response field will need to carefully consider issues such as
      content negotiation, the time period of applicability, and (in
      some cases) multi-tenant server deployments.

   o  Whether the field should be stored by origin servers that
      understand it upon a PUT request.

   o  Whether the field semantics are further refined by the context,
      such as by existing request methods or status codes.

   o  Whether it is appropriate to list the field name in the Connection
      header field (i.e., if the field is to be hop-by-hop; see
      Section 6.4.1).

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

   o  Whether it is appropriate to list the field name in a Vary
      response header field (e.g., when the request header field is used
      by an origin server's content selection algorithm; see
      Section 11.2.1).

   o  Whether the field is allowable in trailers (see Section 5.6).

   o  Whether the field ought to be preserved across redirects.

   o  Whether it introduces any additional security considerations, such
      as disclosure of privacy-related data.







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15.4.  Authentication Scheme Extensibility

15.4.1.  Authentication Scheme Registry

   The "Hypertext Transfer Protocol (HTTP) Authentication Scheme
   Registry" defines the namespace for the authentication schemes in
   challenges and credentials.  It is maintained at
   <https://www.iana.org/assignments/http-authschemes>.

   Registrations MUST include the following fields:

   o  Authentication Scheme Name

   o  Pointer to specification text

   o  Notes (optional)

   Values to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

15.4.2.  Considerations for New Authentication Schemes

   There are certain aspects of the HTTP Authentication framework that
   put constraints on how new authentication schemes can work:

   o  HTTP authentication is presumed to be stateless: all of the
      information necessary to authenticate a request MUST be provided
      in the request, rather than be dependent on the server remembering
      prior requests.  Authentication based on, or bound to, the
      underlying connection is outside the scope of this specification
      and inherently flawed unless steps are taken to ensure that the
      connection cannot be used by any party other than the
      authenticated user (see Section 3.7).

   o  The authentication parameter "realm" is reserved for defining
      protection spaces as described in Section 10.5.  New schemes MUST
      NOT use it in a way incompatible with that definition.

   o  The "token68" notation was introduced for compatibility with
      existing authentication schemes and can only be used once per
      challenge or credential.  Thus, new schemes ought to use the auth-
      param syntax instead, because otherwise future extensions will be
      impossible.

   o  The parsing of challenges and credentials is defined by this
      specification and cannot be modified by new authentication
      schemes.  When the auth-param syntax is used, all parameters ought
      to support both token and quoted-string syntax, and syntactical



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      constraints ought to be defined on the field value after parsing
      (i.e., quoted-string processing).  This is necessary so that
      recipients can use a generic parser that applies to all
      authentication schemes.

      *Note:* The fact that the value syntax for the "realm" parameter
      is restricted to quoted-string was a bad design choice not to be
      repeated for new parameters.

   o  Definitions of new schemes ought to define the treatment of
      unknown extension parameters.  In general, a "must-ignore" rule is
      preferable to a "must-understand" rule, because otherwise it will
      be hard to introduce new parameters in the presence of legacy
      recipients.  Furthermore, it's good to describe the policy for
      defining new parameters (such as "update the specification" or
      "use this registry").

   o  Authentication schemes need to document whether they are usable in
      origin-server authentication (i.e., using WWW-Authenticate), and/
      or proxy authentication (i.e., using Proxy-Authenticate).

   o  The credentials carried in an Authorization header field are
      specific to the user agent and, therefore, have the same effect on
      HTTP caches as the "private" Cache-Control response directive
      (Section 5.2.2.7 of [Caching]), within the scope of the request in
      which they appear.

      Therefore, new authentication schemes that choose not to carry
      credentials in the Authorization header field (e.g., using a newly
      defined header field) will need to explicitly disallow caching, by
      mandating the use of Cache-Control response directives (e.g.,
      "private").

   o  Schemes using Authentication-Info, Proxy-Authentication-Info, or
      any other authentication related response header field need to
      consider and document the related security considerations (see
      Section 16.15.4).

15.5.  Range Unit Extensibility

15.5.1.  Range Unit Registry

   The "HTTP Range Unit Registry" defines the namespace for the range
   unit names and refers to their corresponding specifications.  It is
   maintained at <https://www.iana.org/assignments/http-parameters>.

   Registration of an HTTP Range Unit MUST include the following fields:




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   o  Name

   o  Description

   o  Pointer to specification text

   Values to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

15.5.2.  Considerations for New Range Units

   Other range units, such as format-specific boundaries like pages,
   sections, records, rows, or time, are potentially usable in HTTP for
   application-specific purposes, but are not commonly used in practice.
   Implementors of alternative range units ought to consider how they
   would work with content codings and general-purpose intermediaries.

15.6.  Content Coding Extensibility

15.6.1.  Content Coding Registry

   The "HTTP Content Coding Registry", maintained by IANA at
   <https://www.iana.org/assignments/http-parameters/>, registers
   content-coding names.

   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 7 of [Messaging]), unless the encoding
   transformation is identical (as is the case for the compression
   codings defined in Section 7.5.1).

   Values to be added to this namespace require IETF Review (see
   Section 4.8 of [RFC8126]) and MUST conform to the purpose of content
   coding defined in Section 7.5.1.

15.6.2.  Considerations for New Content Codings

   New content codings ought to be self-descriptive whenever possible,
   with optional parameters discoverable within the coding format
   itself, rather than rely on external metadata that might be lost
   during transit.



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15.7.  Upgrade Token Registry

   The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry"
   defines the namespace for protocol-name tokens used to identify
   protocols in the Upgrade header field.  The registry is maintained at
   <https://www.iana.org/assignments/http-upgrade-tokens>.

   Each registered protocol name is associated with contact information
   and an optional set of specifications that details how the connection
   will be processed after it has been upgraded.

   Registrations happen on a "First Come First Served" basis (see
   Section 4.4 of [RFC8126]) and are subject to the following rules:

   1.  A protocol-name token, once registered, stays registered forever.

   2.  A protocol-name token is case-insensitive and registered with the
       preferred case to be generated by senders.

   3.  The registration MUST name a responsible party for the
       registration.

   4.  The registration MUST name a point of contact.

   5.  The registration MAY name a set of specifications associated with
       that token.  Such specifications need not be publicly available.

   6.  The registration SHOULD name a set of expected "protocol-version"
       tokens associated with that token at the time of registration.

   7.  The responsible party MAY change the registration at any time.
       The IANA will keep a record of all such changes, and make them
       available upon request.

   8.  The IESG MAY reassign responsibility for a protocol token.  This
       will normally only be used in the case when a responsible party
       cannot be contacted.

16.  Security Considerations

   This section is meant to inform developers, information providers,
   and users of known security concerns relevant to HTTP semantics and
   its use for transferring information over the Internet.
   Considerations related to message syntax, parsing, and routing are
   discussed in Section 11 of [Messaging].






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   The list of considerations below is not exhaustive.  Most security
   concerns related to HTTP semantics are about securing server-side
   applications (code behind the HTTP interface), securing user agent
   processing of payloads received via HTTP, or secure use of the
   Internet in general, rather than security of the protocol.  Various
   organizations maintain topical information and links to current
   research on Web application security (e.g., [OWASP]).

16.1.  Establishing Authority

   HTTP relies on the notion of an authoritative response: a response
   that has been determined by (or at the direction of) the origin
   server identified within the target URI to be the most appropriate
   response for that request given the state of the target resource at
   the time of response message origination.

   When a registered name is used in the authority component, the "http"
   URI scheme (Section 4.2.1) relies on the user's local name resolution
   service to determine where it can find authoritative responses.  This
   means that any attack on a user's network host table, cached names,
   or name resolution libraries becomes an avenue for attack on
   establishing authority for "http" URIs.  Likewise, the user's choice
   of server for Domain Name Service (DNS), and the hierarchy of servers
   from which it obtains resolution results, could impact the
   authenticity of address mappings; DNS Security Extensions (DNSSEC,
   [RFC4033]) are one way to improve authenticity.

   Furthermore, after an IP address is obtained, establishing authority
   for an "http" URI is vulnerable to attacks on Internet Protocol
   routing.

   The "https" scheme (Section 4.2.2) is intended to prevent (or at
   least reveal) many of these potential attacks on establishing
   authority, provided that the negotiated connection is secured and the
   client properly verifies that the communicating server's identity
   matches the target URI's authority component (Section 4.3.4).
   Correctly implementing such verification can be difficult (see
   [Georgiev]).

   Authority for a given origin server can be delegated through protocol
   extensions; for example, [RFC7838].  Likewise, the set of servers
   that a connection is considered authoritative for can be changed with
   a protocol extension like [RFC8336].

   Providing a response from a non-authoritative source, such as a
   shared proxy cache, is often useful to improve performance and
   availability, but only to the extent that the source can be trusted
   or the distrusted response can be safely used.



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   Unfortunately, communicating authority to users can be difficult.
   For example, phishing is an attack on the user's perception of
   authority, where that perception can be misled by presenting similar
   branding in hypertext, possibly aided by userinfo obfuscating the
   authority component (see Section 4.2.1).  User agents can reduce the
   impact of phishing attacks by enabling users to easily inspect a
   target URI prior to making an action, by prominently distinguishing
   (or rejecting) userinfo when present, and by not sending stored
   credentials and cookies when the referring document is from an
   unknown or untrusted source.

16.2.  Risks of Intermediaries

   HTTP intermediaries are inherently situated for on-path attacks.
   Compromise of the systems on which the intermediaries run can result
   in serious security and privacy problems.  Intermediaries might have
   access to security-related information, personal information about
   individual users and organizations, and proprietary information
   belonging to users and content providers.  A compromised
   intermediary, or an intermediary implemented or configured without
   regard to security and privacy considerations, might be used in the
   commission of a wide range of potential attacks.

   Intermediaries that contain a shared cache are especially vulnerable
   to cache poisoning attacks, as described in Section 7 of [Caching].

   Implementers need to consider the privacy and security implications
   of their design and coding decisions, and of the configuration
   options they provide to operators (especially the default
   configuration).

   Users need to be aware that intermediaries are no more trustworthy
   than the people who run them; HTTP itself cannot solve this problem.

16.3.  Attacks Based on File and Path Names

   Origin servers frequently make use of their local file system to
   manage the mapping from target URI to resource representations.  Most
   file systems are not designed to protect against malicious file or
   path names.  Therefore, an origin server needs to avoid accessing
   names that have a special significance to the system when mapping the
   target resource to files, folders, or directories.

   For example, UNIX, Microsoft Windows, and other operating systems use
   ".." as a path component to indicate a directory level above the
   current one, and they use specially named paths or file names to send
   data to system devices.  Similar naming conventions might exist
   within other types of storage systems.  Likewise, local storage



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   systems have an annoying tendency to prefer user-friendliness over
   security when handling invalid or unexpected characters,
   recomposition of decomposed characters, and case-normalization of
   case-insensitive names.

   Attacks based on such special names tend to focus on either denial-
   of-service (e.g., telling the server to read from a COM port) or
   disclosure of configuration and source files that are not meant to be
   served.

16.4.  Attacks Based on Command, Code, or Query Injection

   Origin servers often use parameters within the URI as a means of
   identifying system services, selecting database entries, or choosing
   a data source.  However, data received in a request cannot be
   trusted.  An attacker could construct any of the request data
   elements (method, target URI, header fields, or body) to contain data
   that might be misinterpreted as a command, code, or query when passed
   through a command invocation, language interpreter, or database
   interface.

   For example, SQL injection is a common attack wherein additional
   query language is inserted within some part of the target URI or
   header fields (e.g., Host, Referer, etc.).  If the received data is
   used directly within a SELECT statement, the query language might be
   interpreted as a database command instead of a simple string value.
   This type of implementation vulnerability is extremely common, in
   spite of being easy to prevent.

   In general, resource implementations ought to avoid use of request
   data in contexts that are processed or interpreted as instructions.
   Parameters ought to be compared to fixed strings and acted upon as a
   result of that comparison, rather than passed through an interface
   that is not prepared for untrusted data.  Received data that isn't
   based on fixed parameters ought to be carefully filtered or encoded
   to avoid being misinterpreted.

   Similar considerations apply to request data when it is stored and
   later processed, such as within log files, monitoring tools, or when
   included within a data format that allows embedded scripts.

16.5.  Attacks via Protocol Element Length

   Because HTTP uses mostly textual, character-delimited fields, parsers
   are often vulnerable to attacks based on sending very long (or very
   slow) streams of data, particularly where an implementation is
   expecting a protocol element with no predefined length (Section 2.3).




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   To promote interoperability, specific recommendations are made for
   minimum size limits on fields (Section 5.4.2).  These are minimum
   recommendations, chosen to be supportable even by implementations
   with limited resources; it is expected that most implementations will
   choose substantially higher limits.

   A server can reject a message that has a target URI that is too long
   (Section 14.5.15) or a request payload that is too large
   (Section 14.5.14).  Additional status codes related to capacity
   limits have been defined by extensions to HTTP [RFC6585].

   Recipients ought to carefully limit the extent to which they process
   other protocol elements, including (but not limited to) request
   methods, response status phrases, field names, numeric values, and
   body chunks.  Failure to limit such processing can result in buffer
   overflows, arithmetic overflows, or increased vulnerability to
   denial-of-service attacks.

16.6.  Attacks using Shared-dictionary Compression

   Some attacks on encrypted protocols use the differences in size
   created by dynamic compression to reveal confidential information;
   for example, [BREACH].  These attacks rely on creating a redundancy
   between attacker-controlled content and the confidential information,
   such that a dynamic compression algorithm using the same dictionary
   for both content will compress more efficiently when the attacker-
   controlled content matches parts of the confidential content.

   HTTP messages can be compressed in a number of ways, including using
   TLS compression, content-codings, transfer-codings, and other
   extension or version-specific mechanisms.

   The most effective mitigation for this risk is to disable compression
   on sensitive data, or to strictly separate sensitive data from
   attacker-controlled data so that they cannot share the same
   compression dictionary.  With careful design, a compression scheme
   can be designed in a way that is not considered exploitable in
   limited use cases, such as HPACK ([RFC7541]).

16.7.  Disclosure of Personal Information

   Clients are often privy to large amounts of personal information,
   including both information provided by the user to interact with
   resources (e.g., the user's name, location, mail address, passwords,
   encryption keys, etc.) and information about the user's browsing
   activity over time (e.g., history, bookmarks, etc.).  Implementations
   need to prevent unintentional disclosure of personal information.




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16.8.  Privacy of Server Log Information

   A server is in the position to save personal data about a user's
   requests over time, which might identify their reading patterns or
   subjects of interest.  In particular, log information gathered at an
   intermediary often contains a history of user agent interaction,
   across a multitude of sites, that can be traced to individual users.

   HTTP log information is confidential in nature; its handling is often
   constrained by laws and regulations.  Log information needs to be
   securely stored and appropriate guidelines followed for its analysis.
   Anonymization of personal information within individual entries
   helps, but it is generally not sufficient to prevent real log traces
   from being re-identified based on correlation with other access
   characteristics.  As such, access traces that are keyed to a specific
   client are unsafe to publish even if the key is pseudonymous.

   To minimize the risk of theft or accidental publication, log
   information ought to be purged of personally identifiable
   information, including user identifiers, IP addresses, and user-
   provided query parameters, as soon as that information is no longer
   necessary to support operational needs for security, auditing, or
   fraud control.

16.9.  Disclosure of Sensitive Information in URIs

   URIs are intended to be shared, not secured, even when they identify
   secure resources.  URIs are often shown on displays, added to
   templates when a page is printed, and stored in a variety of
   unprotected bookmark lists.  Many servers, proxies, and user agents
   log or display the target URI in places where it might be visible to
   third parties.  It is therefore unwise to include information within
   a URI that is sensitive, personally identifiable, or a risk to
   disclose.

















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   When an application uses client-side mechanisms to construct a target
   URI out of user-provided information, such as the query fields of a
   form using GET, potentially sensitive data might be provided that
   would not be appropriate for disclosure within a URI.  POST is often
   preferred in such cases because it usually doesn't construct a URI;
   instead, POST of a form transmits the potentially sensitive data in
   the request body.  However, this hinders caching and uses an unsafe
   method for what would otherwise be a safe request.  Alternative
   workarounds include transforming the user-provided data prior to
   constructing the URI, or filtering the data to only include common
   values that are not sensitive.  Likewise, redirecting the result of a
   query to a different (server-generated) URI can remove potentially
   sensitive data from later links and provide a cacheable response for
   later reuse.

   Since the Referer header field tells a target site about the context
   that resulted in a request, it has the potential to reveal
   information about the user's immediate browsing history and any
   personal information that might be found in the referring resource's
   URI.  Limitations on the Referer header field are described in
   Section 9.1.3 to address some of its security considerations.

16.10.  Disclosure of Fragment after Redirects

   Although fragment identifiers used within URI references are not sent
   in requests, implementers ought to be aware that they will be visible
   to the user agent and any extensions or scripts running as a result
   of the response.  In particular, when a redirect occurs and the
   original request's fragment identifier is inherited by the new
   reference in Location (Section 9.2.3), this might have the effect of
   disclosing one site's fragment to another site.  If the first site
   uses personal information in fragments, it ought to ensure that
   redirects to other sites include a (possibly empty) fragment
   component in order to block that inheritance.

16.11.  Disclosure of Product Information

   The User-Agent (Section 9.1.6), Via (Section 6.4.3), and Server
   (Section 9.2.5) header fields often reveal information about the
   respective sender's software systems.  In theory, this can make it
   easier for an attacker to exploit known security holes; in practice,
   attackers tend to try all potential holes regardless of the apparent
   software versions being used.








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   Proxies that serve as a portal through a network firewall ought to
   take special precautions regarding the transfer of header information
   that might identify hosts behind the firewall.  The Via header field
   allows intermediaries to replace sensitive machine names with
   pseudonyms.

16.12.  Browser Fingerprinting

   Browser fingerprinting is a set of techniques for identifying a
   specific user agent over time through its unique set of
   characteristics.  These characteristics might include information
   related to its TCP behavior, feature capabilities, and scripting
   environment, though of particular interest here is the set of unique
   characteristics that might be communicated via HTTP.  Fingerprinting
   is considered a privacy concern because it enables tracking of a user
   agent's behavior over time ([Bujlow]) without the corresponding
   controls that the user might have over other forms of data collection
   (e.g., cookies).  Many general-purpose user agents (i.e., Web
   browsers) have taken steps to reduce their fingerprints.

   There are a number of request header fields that might reveal
   information to servers that is sufficiently unique to enable
   fingerprinting.  The From header field is the most obvious, though it
   is expected that From will only be sent when self-identification is
   desired by the user.  Likewise, Cookie header fields are deliberately
   designed to enable re-identification, so fingerprinting concerns only
   apply to situations where cookies are disabled or restricted by the
   user agent's configuration.

   The User-Agent header field might contain enough information to
   uniquely identify a specific device, usually when combined with other
   characteristics, particularly if the user agent sends excessive
   details about the user's system or extensions.  However, the source
   of unique information that is least expected by users is proactive
   negotiation (Section 11.1), including the Accept, Accept-Charset,
   Accept-Encoding, and Accept-Language header fields.

   In addition to the fingerprinting concern, detailed use of the
   Accept-Language header field can reveal information the user might
   consider to be of a private nature.  For example, understanding a
   given language set might be strongly correlated to membership in a
   particular ethnic group.  An approach that limits such loss of
   privacy would be for a user agent to omit the sending of Accept-
   Language except for sites that have been whitelisted, perhaps via
   interaction after detecting a Vary header field that indicates
   language negotiation might be useful.





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   In environments where proxies are used to enhance privacy, user
   agents ought to be conservative in sending proactive negotiation
   header fields.  General-purpose user agents that provide a high
   degree of header field configurability ought to inform users about
   the loss of privacy that might result if too much detail is provided.
   As an extreme privacy measure, proxies could filter the proactive
   negotiation header fields in relayed requests.

16.13.  Validator Retention

   The validators defined by this specification are not intended to
   ensure the validity of a representation, guard against malicious
   changes, or detect on-path attacks.  At best, they enable more
   efficient cache updates and optimistic concurrent writes when all
   participants are behaving nicely.  At worst, the conditions will fail
   and the client will receive a response that is no more harmful than
   an HTTP exchange without conditional requests.

   An entity-tag can be abused in ways that create privacy risks.  For
   example, a site might deliberately construct a semantically invalid
   entity-tag that is unique to the user or user agent, send it in a
   cacheable response with a long freshness time, and then read that
   entity-tag in later conditional requests as a means of re-identifying
   that user or user agent.  Such an identifying tag would become a
   persistent identifier for as long as the user agent retained the
   original cache entry.  User agents that cache representations ought
   to ensure that the cache is cleared or replaced whenever the user
   performs privacy-maintaining actions, such as clearing stored cookies
   or changing to a private browsing mode.

16.14.  Denial-of-Service Attacks Using Range

   Unconstrained multiple range requests are susceptible to denial-of-
   service attacks because the effort required to request many
   overlapping ranges of the same data is tiny compared to the time,
   memory, and bandwidth consumed by attempting to serve the requested
   data in many parts.  Servers ought to ignore, coalesce, or reject
   egregious range requests, such as requests for more than two
   overlapping ranges or for many small ranges in a single set,
   particularly when the ranges are requested out of order for no
   apparent reason.  Multipart range requests are not designed to
   support random access.









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16.15.  Authentication Considerations

   Everything about the topic of HTTP authentication is a security
   consideration, so the list of considerations below is not exhaustive.
   Furthermore, it is limited to security considerations regarding the
   authentication framework, in general, rather than discussing all of
   the potential considerations for specific authentication schemes
   (which ought to be documented in the specifications that define those
   schemes).  Various organizations maintain topical information and
   links to current research on Web application security (e.g.,
   [OWASP]), including common pitfalls for implementing and using the
   authentication schemes found in practice.

16.15.1.  Confidentiality of Credentials

   The HTTP authentication framework does not define a single mechanism
   for maintaining the confidentiality of credentials; instead, each
   authentication scheme defines how the credentials are encoded prior
   to transmission.  While this provides flexibility for the development
   of future authentication schemes, it is inadequate for the protection
   of existing schemes that provide no confidentiality on their own, or
   that do not sufficiently protect against replay attacks.
   Furthermore, if the server expects credentials that are specific to
   each individual user, the exchange of those credentials will have the
   effect of identifying that user even if the content within
   credentials remains confidential.

   HTTP depends on the security properties of the underlying transport-
   or session-level connection to provide confidential transmission of
   fields.  In other words, if a server limits access to authenticated
   users using this framework, the server needs to ensure that the
   connection is properly secured in accordance with the nature of the
   authentication scheme used.  For example, services that depend on
   individual user authentication often require a connection to be
   secured with TLS ("Transport Layer Security", [RFC8446]) prior to
   exchanging any credentials.

16.15.2.  Credentials and Idle Clients

   Existing HTTP clients and user agents typically retain authentication
   information indefinitely.  HTTP does not provide a mechanism for the
   origin server to direct clients to discard these cached credentials,
   since the protocol has no awareness of how credentials are obtained
   or managed by the user agent.  The mechanisms for expiring or
   revoking credentials can be specified as part of an authentication
   scheme definition.





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   Circumstances under which credential caching can interfere with the
   application's security model include but are not limited to:

   o  Clients that have been idle for an extended period, following
      which the server might wish to cause the client to re-prompt the
      user for credentials.

   o  Applications that include a session termination indication (such
      as a "logout" or "commit" button on a page) after which the server
      side of the application "knows" that there is no further reason
      for the client to retain the credentials.

   User agents that cache credentials are encouraged to provide a
   readily accessible mechanism for discarding cached credentials under
   user control.

16.15.3.  Protection Spaces

   Authentication schemes that solely rely on the "realm" mechanism for
   establishing a protection space will expose credentials to all
   resources on an origin server.  Clients that have successfully made
   authenticated requests with a resource can use the same
   authentication credentials for other resources on the same origin
   server.  This makes it possible for a different resource to harvest
   authentication credentials for other resources.

   This is of particular concern when an origin server hosts resources
   for multiple parties under the same canonical root URI
   (Section 10.5).  Possible mitigation strategies include restricting
   direct access to authentication credentials (i.e., not making the
   content of the Authorization request header field available), and
   separating protection spaces by using a different host name (or port
   number) for each party.

16.15.4.  Additional Response Fields

   Adding information to responses that are sent over an unencrypted
   channel can affect security and privacy.  The presence of the
   Authentication-Info and Proxy-Authentication-Info header fields alone
   indicates that HTTP authentication is in use.  Additional information
   could be exposed by the contents of the authentication-scheme
   specific parameters; this will have to be considered in the
   definitions of these schemes.

17.  IANA Considerations

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



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17.1.  URI Scheme Registration

   Please update the registry of URI Schemes [BCP35] at
   <https://www.iana.org/assignments/uri-schemes/> with the permanent
   schemes listed in the first table of Section 3.1.

17.2.  Method Registration

   Please update the "Hypertext Transfer Protocol (HTTP) Method
   Registry" at <https://www.iana.org/assignments/http-methods> with the
   registration procedure of Section 15.1.1 and the method names
   summarized in the following table.

                   --------- ------ ------------ -------
                    Method    Safe   Idempotent   Ref.
                   --------- ------ ------------ -------
                    *         no     no           17.2
                    CONNECT   no     no           8.3.6
                    DELETE    no     yes          8.3.5
                    GET       yes    yes          8.3.1
                    HEAD      yes    yes          8.3.2
                    OPTIONS   yes    yes          8.3.7
                    POST      no     no           8.3.3
                    PUT       no     yes          8.3.4
                    TRACE     yes    yes          8.3.8
                   --------- ------ ------------ -------

                                  Table 14

   The method name "*" is reserved, since using that name as HTTP method
   name might conflict with special semantics in fields such as "Access-
   Control-Request-Method".

17.3.  Status Code Registration

   Please update the "Hypertext Transfer Protocol (HTTP) Status Code
   Registry" at <https://www.iana.org/assignments/http-status-codes>
   with the registration procedure of Section 15.2.1 and the status code
   values summarized in the following table.

             ------- ------------------------------- ---------
              Value   Description                     Ref.
             ------- ------------------------------- ---------
              100     Continue                        14.2.1
              101     Switching Protocols             14.2.2
              200     OK                              14.3.1
              201     Created                         14.3.2
              202     Accepted                        14.3.3



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              203     Non-Authoritative Information   14.3.4
              204     No Content                      14.3.5
              205     Reset Content                   14.3.6
              206     Partial Content                 14.3.7
              300     Multiple Choices                14.4.1
              301     Moved Permanently               14.4.2
              302     Found                           14.4.3
              303     See Other                       14.4.4
              304     Not Modified                    14.4.5
              305     Use Proxy                       14.4.6
              306     (Unused)                        14.4.7
              307     Temporary Redirect              14.4.8
              308     Permanent Redirect              14.4.9
              400     Bad Request                     14.5.1
              401     Unauthorized                    14.5.2
              402     Payment Required                14.5.3
              403     Forbidden                       14.5.4
              404     Not Found                       14.5.5
              405     Method Not Allowed              14.5.6
              406     Not Acceptable                  14.5.7
              407     Proxy Authentication Required   14.5.8
              408     Request Timeout                 14.5.9
              409     Conflict                        14.5.10
              410     Gone                            14.5.11
              411     Length Required                 14.5.12
              412     Precondition Failed             14.5.13
              413     Payload Too Large               14.5.14
              414     URI Too Long                    14.5.15
              415     Unsupported Media Type          14.5.16
              416     Range Not Satisfiable           14.5.17
              417     Expectation Failed              14.5.18
              418     (Unused)                        14.5.19
              422     Unprocessable Payload           14.5.20
              426     Upgrade Required                14.5.21
              500     Internal Server Error           14.6.1
              501     Not Implemented                 14.6.2
              502     Bad Gateway                     14.6.3
              503     Service Unavailable             14.6.4
              504     Gateway Timeout                 14.6.5
              505     HTTP Version Not Supported      14.6.6
             ------- ------------------------------- ---------

                                  Table 15

   Additionally, please update the following entry in the Hypertext
   Transfer Protocol (HTTP) Status Code Registry:

   Value:  418



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   Description:  (Unused)

   Reference  Section 14.5.19

17.4.  HTTP Field Name Registration

   Please create a new registry as outlined in Section 15.3.1.

   After creating the registry, all entries in the Permanent and
   Provisional Message Header Registries with the protocol 'http' are to
   be moved to it, with the following changes applied:

   1.  The 'Applicable Protocol' field is to be omitted.

   2.  Entries with a status of 'standard', 'experimental', 'reserved',
       or 'informational' are to have a status of 'permanent'.

   3.  Provisional entries without a status are to have a status of
       'provisional'.

   4.  Permanent entries without a status (after confirmation that the
       registration document did not define one) will have a status of
       'provisional'.  The Expert(s) can choose to update their status
       if there is evidence that another is more appropriate.

   Please annotate the Permanent and Provisional Message Header
   registries to indicate that HTTP field name registrations have moved,
   with an appropriate link.

   After that is complete, please update the new registry with the field
   names listed in the following table.

    --------------------------- ------------ --------
     Field Name                  Status       Ref.
    --------------------------- ------------ --------
     Accept                      standard     11.1.2
     Accept-Charset              deprecated   11.1.3
     Accept-Encoding             standard     11.1.4
     Accept-Language             standard     11.1.5
     Accept-Ranges               standard     13.3
     Allow                       standard     9.2.1
     Authentication-Info         standard     10.6.3
     Authorization               standard     10.6.2
     Connection                  standard     6.4.1
     Content-Encoding            standard     7.5
     Content-Language            standard     7.6
     Content-Length              standard     7.7
     Content-Location            standard     7.8



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     Content-Range               standard     13.4
     Content-Type                standard     7.4
     Date                        standard     9.2.2
     ETag                        standard     7.9.3
     Expect                      standard     9.1.1
     From                        standard     9.1.2
     Host                        standard     6.1.2
     If-Match                    standard     12.1.1
     If-Modified-Since           standard     12.1.3
     If-None-Match               standard     12.1.2
     If-Range                    standard     12.1.5
     If-Unmodified-Since         standard     12.1.4
     Last-Modified               standard     7.9.2
     Location                    standard     9.2.3
     Max-Forwards                standard     6.4.2
     Proxy-Authenticate          standard     10.7.1
     Proxy-Authentication-Info   standard     10.7.3
     Proxy-Authorization         standard     10.7.2
     Range                       standard     13.2
     Referer                     standard     9.1.3
     Retry-After                 standard     9.2.4
     Server                      standard     9.2.5
     TE                          standard     9.1.4
     Trailer                     standard     9.1.5
     Upgrade                     standard     6.6
     User-Agent                  standard     9.1.6
     Vary                        standard     11.2.1
     Via                         standard     6.4.3
     WWW-Authenticate            standard     10.6.1
    --------------------------- ------------ --------

                         Table 16

   Furthermore, the field name "*" is reserved, since using that name as
   an HTTP header field might conflict with its special semantics in the
   Vary header field (Section 11.2.1).

    ------------ ---------- -------- ------------
     Field Name   Status     Ref.     Comments
    ------------ ---------- -------- ------------
     *            standard   11.2.1   (reserved)
    ------------ ---------- -------- ------------

                       Table 17







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   Finally, please update the "Content-MD5" entry in the new registry to
   have a status of 'obsoleted' with references to Section 14.15 of
   [RFC2616] (for the definition of the header field) and Appendix B of
   [RFC7231] (which removed the field definition from the updated
   specification).

17.5.  Authentication Scheme Registration

   Please update the "Hypertext Transfer Protocol (HTTP) Authentication
   Scheme Registry" at <https://www.iana.org/assignments/http-
   authschemes> with the registration procedure of Section 15.4.1.  No
   authentication schemes are defined in this document.

17.6.  Content Coding Registration

   Please update the "HTTP Content Coding Registry" at
   <https://www.iana.org/assignments/http-parameters/> with the
   registration procedure of Section 15.6.1 and the content coding names
   summarized in the table of Section 7.5.1.

17.7.  Range Unit Registration

   Please update the "HTTP Range Unit Registry" at
   <https://www.iana.org/assignments/http-parameters/> with the
   registration procedure of Section 15.5.1 and the range unit names
   summarized in the table of Section 13.1.

17.8.  Media Type Registration

   Please update the "Media Types" registry at
   <https://www.iana.org/assignments/media-types> with the registration
   information in Section 13.5 for the media type "multipart/
   byteranges".

17.9.  Port Registration

   Please update the "Service Name and Transport Protocol Port Number"
   registry at <https://www.iana.org/assignments/service-names-port-
   numbers/> for the services on ports 80 and 443 that use UDP or TCP
   to:

   1.  use this document as "Reference", and

   2.  when currently unspecified, set "Assignee" to "IESG" and
       "Contact" to "IETF_Chair".






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17.10.  Upgrade Token Registration

   Please update the "Hypertext Transfer Protocol (HTTP) Upgrade Token
   Registry" at <https://www.iana.org/assignments/http-upgrade-tokens>
   with the registration procedure of Section 15.7 and the upgrade token
   names summarized in the following table.

    ------ ------------------- ------------------------- ------
     Name   Description         Expected Version Tokens   Ref.
    ------ ------------------- ------------------------- ------
     HTTP   Hypertext           any DIGIT.DIGIT (e.g,     5.1
            Transfer Protocol   "2.0")
    ------ ------------------- ------------------------- ------

                              Table 18

18.  References

18.1.  Normative References

   [Caching]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", Work in Progress, Internet-Draft,
              draft-ietf-httpbis-cache-12, October 2, 2020,
              <https://tools.ietf.org/html/draft-ietf-httpbis-cache-12>.

   [Messaging]
              Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1 Messaging", Work in Progress, Internet-
              Draft, draft-ietf-httpbis-messaging-12, October 2, 2020,
              <https://tools.ietf.org/html/draft-ietf-httpbis-messaging-
              12>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1950]  Deutsch, L.P. and J-L. Gailly, "ZLIB Compressed Data
              Format Specification version 3.3", RFC 1950,
              DOI 10.17487/RFC1950, May 1996,
              <https://www.rfc-editor.org/info/rfc1950>.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
              <https://www.rfc-editor.org/info/rfc1951>.







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   [RFC1952]  Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L.P., and
              G. Randers-Pehrson, "GZIP file format specification
              version 4.3", RFC 1952, DOI 10.17487/RFC1952, May 1996,
              <https://www.rfc-editor.org/info/rfc1952>.

   [RFC2045]  Freed, N. and N.S. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4647]  Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
              Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September
              2006, <https://www.rfc-editor.org/info/rfc4647>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.



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   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              DOI 10.17487/RFC6365, September 2011,
              <https://www.rfc-editor.org/info/rfc6365>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

   [Welch]    Welch, T. A., "A Technique for High-Performance Data
              Compression", IEEE Computer 17(6),
              DOI 10.1109/MC.1984.1659158, June 1984,
              <https://ieeexplore.ieee.org/document/1659158/>.

18.2.  Informative References

   [BCP13]    Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, January 2013,
              <https://www.rfc-editor.org/info/bcp13>.

   [BCP178]   Saint-Andre, P., Crocker, D., and M. Nottingham,
              "Deprecating the "X-" Prefix and Similar Constructs in
              Application Protocols", BCP 178, RFC 6648, June 2012,
              <https://www.rfc-editor.org/info/bcp178>.

   [BCP35]    Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, June 2015,
              <https://www.rfc-editor.org/info/bcp35>.

   [BREACH]   Gluck, Y., Harris, N., and A. Prado, "BREACH: Reviving the
              CRIME Attack", July 2013,
              <http://breachattack.com/resources/
              BREACH%20-%20SSL,%20gone%20in%2030%20seconds.pdf>.








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   [Bujlow]   Bujlow, T., Carela-Espanol, V., Sole-Pareta, J., and P.
              Barlet-Ros, "A Survey on Web Tracking: Mechanisms,
              Implications, and Defenses",
              DOI 10.1109/JPROC.2016.2637878, Proceedings of the
              IEEE 105(8), August 2017,
              <https://doi.org/10.1109/JPROC.2016.2637878>.

   [Err1912]  RFC Errata, Erratum ID 1912, RFC 2978,
              <https://www.rfc-editor.org/errata/eid1912>.

   [Err5433]  RFC Errata, Erratum ID 5433, RFC 2978,
              <https://www.rfc-editor.org/errata/eid5433>.

   [Georgiev] Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
              D., and V. Shmatikov, "The Most Dangerous Code in the
              World: Validating SSL Certificates in Non-browser
              Software", DOI 10.1145/2382196.2382204, In Proceedings of
              the 2012 ACM Conference on Computer and Communications
              Security (CCS '12), pp. 38-49, October 2012,
              <https://doi.org/10.1145/2382196.2382204>.

   [HTTP3]    Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-31, September 25, 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-http-31>.

   [ISO-8859-1]
              International Organization for Standardization,
              "Information technology -- 8-bit single-byte coded graphic
              character sets -- Part 1: Latin alphabet No. 1", ISO/
              IEC 8859-1:1998, 1998.

   [Kri2001]  Kristol, D., "HTTP Cookies: Standards, Privacy, and
              Politics", ACM Transactions on Internet Technology 1(2),
              November 2001, <http://arxiv.org/abs/cs.SE/0105018>.

   [OWASP]    van der Stock, A., Ed., "A Guide to Building Secure Web
              Applications and Web Services", The Open Web Application
              Security Project (OWASP) 2.0.1, July 27, 2005,
              <https://www.owasp.org/>.

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





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   [RFC1919]  Chatel, M., "Classical versus Transparent IP Proxies",
              RFC 1919, DOI 10.17487/RFC1919, March 1996,
              <https://www.rfc-editor.org/info/rfc1919>.

   [RFC1945]  Berners-Lee, T., Fielding, R.T., and H.F. Nielsen,
              "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945,
              DOI 10.17487/RFC1945, May 1996,
              <https://www.rfc-editor.org/info/rfc1945>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/info/rfc2047>.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, DOI 10.17487/RFC2068, January 1997,
              <https://www.rfc-editor.org/info/rfc2068>.

   [RFC2145]  Mogul, J.C., Fielding, R.T., Gettys, J., and H.F. Nielsen,
              "Use and Interpretation of HTTP Version Numbers",
              RFC 2145, DOI 10.17487/RFC2145, May 1997,
              <https://www.rfc-editor.org/info/rfc2145>.

   [RFC2295]  Holtman, K. and A.H. Mutz, "Transparent Content
              Negotiation in HTTP", RFC 2295, DOI 10.17487/RFC2295,
              March 1998, <https://www.rfc-editor.org/info/rfc2295>.

   [RFC2324]  Masinter, L., "Hyper Text Coffee Pot Control Protocol
              (HTCPCP/1.0)", RFC 2324, DOI 10.17487/RFC2324, April 1,
              1998, <https://www.rfc-editor.org/info/rfc2324>.

   [RFC2557]  Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
              "MIME Encapsulation of Aggregate Documents, such as HTML
              (MHTML)", RFC 2557, DOI 10.17487/RFC2557, March 1999,
              <https://www.rfc-editor.org/info/rfc2557>.

   [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,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [RFC2617]  Franks, J., Hallam-Baker, P.M., Hostetler, J.L., Lawrence,
              S.D., Leach, P.J., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, DOI 10.17487/RFC2617, June 1999,
              <https://www.rfc-editor.org/info/rfc2617>.



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   [RFC2774]  Frystyk, H., Leach, P., and S. Lawrence, "An HTTP
              Extension Framework", RFC 2774, DOI 10.17487/RFC2774,
              February 2000, <https://www.rfc-editor.org/info/rfc2774>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <https://www.rfc-editor.org/info/rfc2978>.

   [RFC3040]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
              Replication and Caching Taxonomy", RFC 3040,
              DOI 10.17487/RFC3040, January 2001,
              <https://www.rfc-editor.org/info/rfc3040>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4559]  Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
              Kerberos and NTLM HTTP Authentication in Microsoft
              Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
              <https://www.rfc-editor.org/info/rfc4559>.

   [RFC4918]  Dusseault, L.M., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918,
              DOI 10.17487/RFC4918, June 2007,
              <https://www.rfc-editor.org/info/rfc4918>.

   [RFC5322]  Resnick, P., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, DOI 10.17487/RFC5789, March 2010,
              <https://www.rfc-editor.org/info/rfc5789>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.



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   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
              <https://www.rfc-editor.org/info/rfc6585>.

   [RFC7230]  Fielding, R., Ed. and J. F. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. F. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Semantics and Content",
              RFC 7231, DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7232]  Fielding, R., Ed. and J. F. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Conditional Requests",
              RFC 7232, DOI 10.17487/RFC7232, June 2014,
              <https://www.rfc-editor.org/info/rfc7232>.

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. F. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, DOI 10.17487/RFC7233, June 2014,
              <https://www.rfc-editor.org/info/rfc7233>.

   [RFC7235]  Fielding, R., Ed. and J. F. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC7538]  Reschke, J. F., "The Hypertext Transfer Protocol Status
              Code 308 (Permanent Redirect)", RFC 7538,
              DOI 10.17487/RFC7538, April 2015,
              <https://www.rfc-editor.org/info/rfc7538>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
              <https://www.rfc-editor.org/info/rfc7541>.





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   [RFC7578]  Masinter, L., "Returning Values from Forms: multipart/
              form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015,
              <https://www.rfc-editor.org/info/rfc7578>.

   [RFC7615]  Reschke, J. F., "HTTP Authentication-Info and Proxy-
              Authentication-Info Response Header Fields", RFC 7615,
              DOI 10.17487/RFC7615, September 2015,
              <https://www.rfc-editor.org/info/rfc7615>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <https://www.rfc-editor.org/info/rfc7616>.

   [RFC7617]  Reschke, J. F., "The 'Basic' HTTP Authentication Scheme",
              RFC 7617, DOI 10.17487/RFC7617, September 2015,
              <https://www.rfc-editor.org/info/rfc7617>.

   [RFC7694]  Reschke, J. F., "Hypertext Transfer Protocol (HTTP)
              Client-Initiated Content-Encoding", RFC 7694,
              DOI 10.17487/RFC7694, November 2015,
              <https://www.rfc-editor.org/info/rfc7694>.

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/info/rfc7838>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8187]  Reschke, J. F., "Indicating Character Encoding and
              Language for HTTP Header Field Parameters", RFC 8187,
              DOI 10.17487/RFC8187, September 2017,
              <https://www.rfc-editor.org/info/rfc8187>.

   [RFC8246]  McManus, P., "HTTP Immutable Responses", RFC 8246,
              DOI 10.17487/RFC8246, September 2017,
              <https://www.rfc-editor.org/info/rfc8246>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC8336]  Nottingham, M. and E. Nygren, "The ORIGIN HTTP/2 Frame",
              RFC 8336, DOI 10.17487/RFC8336, March 2018,
              <https://www.rfc-editor.org/info/rfc8336>.



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   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [Sniffing] WHATWG, "MIME Sniffing",
              <https://mimesniff.spec.whatwg.org>.

Appendix A.  Collected ABNF

   In the collected ABNF below, list rules are expanded as per
   Section 5.7.1.1.

   Accept = [ ( media-range [ accept-params ] ) *( OWS "," OWS (
    media-range [ accept-params ] ) ) ]
   Accept-Charset = [ ( ( charset / "*" ) [ weight ] ) *( OWS "," OWS (
    ( charset / "*" ) [ weight ] ) ) ]
   Accept-Encoding = [ ( codings [ weight ] ) *( OWS "," OWS ( codings [
    weight ] ) ) ]
   Accept-Language = [ ( language-range [ weight ] ) *( OWS "," OWS (
    language-range [ weight ] ) ) ]
   Accept-Ranges = acceptable-ranges
   Allow = [ method *( OWS "," OWS method ) ]
   Authentication-Info = [ auth-param *( OWS "," OWS auth-param ) ]
   Authorization = credentials

   BWS = OWS

   Connection = [ connection-option *( OWS "," OWS connection-option )
    ]
   Content-Encoding = [ content-coding *( OWS "," OWS content-coding )
    ]
   Content-Language = [ language-tag *( OWS "," OWS language-tag ) ]
   Content-Length = 1*DIGIT
   Content-Location = absolute-URI / partial-URI
   Content-Range = range-unit SP ( range-resp / unsatisfied-range )
   Content-Type = media-type

   Date = HTTP-date

   ETag = entity-tag
   Expect = [ expectation *( OWS "," OWS expectation ) ]

   From = mailbox

   GMT = %x47.4D.54 ; GMT

   HTTP-date = IMF-fixdate / obs-date
   Host = uri-host [ ":" port ]



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   IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
   If-Match = "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]
   If-Modified-Since = HTTP-date
   If-None-Match = "*" / [ entity-tag *( OWS "," OWS entity-tag ) ]
   If-Range = entity-tag / HTTP-date
   If-Unmodified-Since = HTTP-date

   Last-Modified = HTTP-date
   Location = URI-reference

   Max-Forwards = 1*DIGIT

   OWS = *( SP / HTAB )

   Proxy-Authenticate = [ challenge *( OWS "," OWS challenge ) ]
   Proxy-Authentication-Info = [ auth-param *( OWS "," OWS auth-param )
    ]
   Proxy-Authorization = credentials

   RWS = 1*( SP / HTAB )
   Range = ranges-specifier
   Referer = absolute-URI / partial-URI
   Retry-After = HTTP-date / delay-seconds

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

   TE = [ t-codings *( OWS "," OWS t-codings ) ]
   Trailer = [ field-name *( OWS "," OWS field-name ) ]

   URI-reference = <URI-reference, see [RFC3986], Section 4.1>
   Upgrade = [ protocol *( OWS "," OWS protocol ) ]
   User-Agent = product *( RWS ( product / comment ) )

   Vary = [ ( "*" / field-name ) *( OWS "," OWS ( "*" / field-name ) )
    ]
   Via = [ ( received-protocol RWS received-by [ RWS comment ] ) *( OWS
    "," OWS ( received-protocol RWS received-by [ RWS comment ] ) ) ]

   WWW-Authenticate = [ challenge *( OWS "," OWS challenge ) ]

   absolute-URI = <absolute-URI, see [RFC3986], Section 4.3>
   absolute-path = 1*( "/" segment )
   accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
   accept-params = weight *accept-ext
   acceptable-ranges = ( range-unit *( OWS "," OWS range-unit ) ) /
    "none"
   asctime-date = day-name SP date3 SP time-of-day SP year
   auth-param = token BWS "=" BWS ( token / quoted-string )



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   auth-scheme = token
   authority = <authority, see [RFC3986], Section 3.2>

   challenge = auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","
    OWS auth-param ) ] ) ]
   charset = token
   codings = content-coding / "identity" / "*"
   comment = "(" *( ctext / quoted-pair / comment ) ")"
   complete-length = 1*DIGIT
   connection-option = token
   content-coding = token
   credentials = auth-scheme [ 1*SP ( token68 / [ auth-param *( OWS ","
    OWS auth-param ) ] ) ]
   ctext = HTAB / SP / %x21-27 ; '!'-'''
    / %x2A-5B ; '*'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text

   date1 = day SP month SP year
   date2 = day "-" month "-" 2DIGIT
   date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
   day = 2DIGIT
   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
   delay-seconds = 1*DIGIT

   entity-tag = [ weak ] opaque-tag
   etagc = "!" / %x23-7E ; '#'-'~'
    / obs-text
   expectation = token [ "=" ( token / quoted-string ) parameters ]

   field-content = field-vchar [ 1*( SP / HTAB / field-vchar )
    field-vchar ]
   field-name = token
   field-value = *field-content
   field-vchar = VCHAR / obs-text



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   first-pos = 1*DIGIT

   hour = 2DIGIT
   http-URI = "http://" authority path-abempty [ "?" query ]
   https-URI = "https://" authority path-abempty [ "?" query ]

   incl-range = first-pos "-" last-pos
   int-range = first-pos "-" [ last-pos ]

   language-range = <language-range, see [RFC4647], Section 2.1>
   language-tag = <Language-Tag, see [RFC5646], Section 2.1>
   last-pos = 1*DIGIT

   mailbox = <mailbox, see [RFC5322], Section 3.4>
   media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) )
    parameters
   media-type = type "/" subtype parameters
   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

   obs-date = rfc850-date / asctime-date
   obs-text = %x80-FF
   opaque-tag = DQUOTE *etagc DQUOTE
   other-range = 1*( %x21-2B ; '!'-'+'
    / %x2D-7E ; '-'-'~'
    )

   parameter = parameter-name "=" parameter-value
   parameter-name = token
   parameter-value = ( token / quoted-string )
   parameters = *( OWS ";" OWS [ parameter ] )
   partial-URI = relative-part [ "?" query ]
   path-abempty = <path-abempty, see [RFC3986], Section 3.3>
   port = <port, see [RFC3986], Section 3.2.3>
   product = token [ "/" product-version ]
   product-version = token



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   protocol = protocol-name [ "/" protocol-version ]
   protocol-name = token
   protocol-version = token
   pseudonym = token

   qdtext = HTAB / SP / "!" / %x23-5B ; '#'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text
   query = <query, see [RFC3986], Section 3.4>
   quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
   quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
   qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )

   range-resp = incl-range "/" ( complete-length / "*" )
   range-set = range-spec *( OWS "," OWS range-spec )
   range-spec = int-range / suffix-range / other-range
   range-unit = token
   ranges-specifier = range-unit "=" range-set
   rank = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
   received-by = pseudonym [ ":" port ]
   received-protocol = [ protocol-name "/" ] protocol-version
   relative-part = <relative-part, see [RFC3986], Section 4.2>
   rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT

   second = 2DIGIT
   segment = <segment, see [RFC3986], Section 3.3>
   subtype = token
   suffix-length = 1*DIGIT
   suffix-range = "-" suffix-length

   t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
   t-ranking = OWS ";" OWS "q=" rank
   tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
    "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
   time-of-day = hour ":" minute ":" second
   token = 1*tchar
   token68 = 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" )
    *"="
   transfer-coding = <transfer-coding, see [Messaging], Section 7>
   type = token

   unsatisfied-range = "*/" complete-length
   uri-host = <host, see [RFC3986], Section 3.2.2>

   weak = %x57.2F ; W/
   weight = OWS ";" OWS "q=" qvalue

   year = 4DIGIT



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Appendix B.  Changes from previous RFCs

B.1.  Changes from RFC 2818

   None yet.

B.2.  Changes from RFC 7230

   The sections introducing HTTP's design goals, history, architecture,
   conformance criteria, protocol versioning, URIs, message routing, and
   header fields have been moved here (without substantive change).

   The description of an origin and authoritative access to origin
   servers has been extended for both "http" and "https" URIs to account
   for alternative services and secured connections that are not
   necessarily based on TCP.  (Section 4.2.1, Section 4.2.2,
   Section 4.3.1, Section 6.2.3)

   "Field value" now refers to the value after multiple instances are
   combined with commas - by far the most common use.  To refer to a
   single header line's value, use "field line value".  (Section 5.4)

   Parameters in media type, media range, and expectation can be empty
   via one or more trailing semicolons.  (Section 5.7.6)

   Trailer field semantics now transcend the specifics of chunked
   encoding.  Use of trailer fields has been further limited to only
   allow generation as a trailer field when the sender knows the field
   defines that usage and to only allow merging into the header section
   if the recipient knows the corresponding field definition permits and
   defines how to merge.  In all other cases, implementations are
   encouraged to either store the trailer fields separately or discard
   them instead of merging.  (Section 5.6.2)

   Trailer fields can now potentially appear as multiple trailer
   sections, if allowed by the HTTP version and framing in use, with
   processing described as being iterative as each section is received.
   (Section 5.6.3)

   Made the priority of the absolute form of the request URI over the
   Host header by origin servers explicit, to align with proxy handling.
   (Section 6.1.2)









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   The grammar definition for the Via field's "received-by" was expanded
   in 7230 due to changes in the URI grammar for host [RFC3986] that are
   not desirable for Via. For simplicity, we have removed uri-host from
   the received-by production because it can be encompassed by the
   existing grammar for pseudonym.  In particular, this change removed
   comma from the allowed set of charaters for a host name in received-
   by.  (Section 6.4.3)

   Added status code 308 (previously defined in [RFC7538]) so that it's
   defined closer to status codes 301, 302, and 307.  (Section 14.4.9)

   Added status code 422 (previously defined in Section 11.2 of
   [RFC4918]) because of its general applicability.  (Section 14.5.20)

   The description of an origin and authoritative access to origin
   servers has been extended for both "http" and "https" URIs to account
   for alternative services and secured connections that are not
   necessarily based on TCP.  (Section 4.2.1, Section 4.2.2,
   Section 4.3.1, Section 6.2.3)

B.3.  Changes from RFC 7231

   Minimum URI lengths to be supported by implementations are now
   recommended.  (Section 3.1)

   Clarify that control characters in field values are to be rejected or
   mapped to SP.  (Section 5.4.4)

   Parameters in media type, media range, and expectation can be empty
   via one or more trailing semicolons.  (Section 5.7.6)

   The term "effective request URI" has been replaced with "target URI".
   (Section 6.1)

   Range units are compared in a case insensitive fashion.
   (Section 13.1)

   Restrictions on client retries have been loosened, to reflect
   implementation behavior.  (Section 8.2.2)

   Clarified that request bodies on GET and DELETE are not
   interoperable.  (Section 8.3.1, Section 8.3.5)

   Removed a superfluous requirement about setting Content-Length from
   the description of the OPTIONS method.  (Section 8.3.7)

   Restore list-based grammar for Expect for compatibility with RFC
   2616.  (Section 9.1.1)



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   Allow Accept and Accept-Encoding in response messages; the latter was
   introduced by [RFC7694].  (Section 11.3)

   The process of creating a redirected request has been clarified.
   (Section 14.4)

   The semantics of "*" in the Vary header field when other values are
   present was clarified.  (Section 11.2.1)

B.4.  Changes from RFC 7232

   Preconditions can now be evaluated before the request body is
   processed rather than waiting until the response would otherwise be
   successful.  (Section 12.2)

   Removed edge case requirement on If-Match and If-Unmodified-Since
   that a validator not be sent in a 2xx response when validation fails
   and the server decides that the same change request has already been
   applied.  (Section 12.1.1 and Section 12.1.4)

   Clarified that If-Unmodified-Since doesn't apply to a resource
   without a concept of modification time.  (Section 12.1.4)

B.5.  Changes from RFC 7233

   Refactored the range-unit and ranges-specifier grammars to simplify
   and reduce artificial distinctions between bytes and other
   (extension) range units, removing the overlapping grammar of other-
   range-unit by defining range units generically as a token and placing
   extensions within the scope of a range-spec (other-range).  This
   disambiguates the role of list syntax (commas) in all range sets,
   including extension range units, for indicating a range-set of more
   than one range.  Moving the extension grammar into range specifiers
   also allows protocol specific to byte ranges to be specified
   separately.

B.6.  Changes from RFC 7235

   None yet.

B.7.  Changes from RFC 7538

   None yet.

B.8.  Changes from RFC 7615

   None yet.




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B.9.  Changes from RFC 7694

   This specification includes the extension defined in [RFC7694], but
   leaves out examples and deployment considerations.

Appendix C.  Change Log

   This section is to be removed before publishing as an RFC.

C.1.  Between RFC723x and draft 00

   The changes were purely editorial:

   o  Change boilerplate and abstract to indicate the "draft" status,
      and update references to ancestor specifications.

   o  Remove version "1.1" from document title, indicating that this
      specification applies to all HTTP versions.

   o  Adjust historical notes.

   o  Update links to sibling specifications.

   o  Replace sections listing changes from RFC 2616 by new empty
      sections referring to RFC 723x.

   o  Remove acknowledgements specific to RFC 723x.

   o  Move "Acknowledgements" to the very end and make them unnumbered.

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

   The changes in this draft are editorial, with respect to HTTP as a
   whole, to merge core HTTP semantics into this document:

   o  Merged introduction, architecture, conformance, and ABNF
      extensions from RFC 7230 (Messaging).

   o  Rearranged architecture to extract conformance, http(s) schemes,
      and protocol versioning into a separate major section.

   o  Moved discussion of MIME differences to [Messaging] since that is
      primarily concerned with transforming 1.1 messages.

   o  Merged entire content of RFC 7232 (Conditional Requests).

   o  Merged entire content of RFC 7233 (Range Requests).




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   o  Merged entire content of RFC 7235 (Auth Framework).

   o  Moved all extensibility tips, registration procedures, and
      registry tables from the IANA considerations to normative
      sections, reducing the IANA considerations to just instructions
      that will be removed prior to publication as an RFC.

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

   o  Improve [Welch] citation (<https://github.com/httpwg/http-core/
      issues/63>)

   o  Remove HTTP/1.1-ism about Range Requests
      (<https://github.com/httpwg/http-core/issues/71>)

   o  Cite RFC 8126 instead of RFC 5226 (<https://github.com/httpwg/
      http-core/issues/75>)

   o  Cite RFC 7538 instead of RFC 7238 (<https://github.com/httpwg/
      http-core/issues/76>)

   o  Cite RFC 8288 instead of RFC 5988 (<https://github.com/httpwg/
      http-core/issues/77>)

   o  Cite RFC 8187 instead of RFC 5987 (<https://github.com/httpwg/
      http-core/issues/78>)

   o  Cite RFC 7578 instead of RFC 2388 (<https://github.com/httpwg/
      http-core/issues/79>)

   o  Cite RFC 7595 instead of RFC 4395 (<https://github.com/httpwg/
      http-core/issues/80>)

   o  improve ABNF readability for qdtext (<https://github.com/httpwg/
      http-core/issues/81>, <https://www.rfc-editor.org/errata/eid4891>)

   o  Clarify "resource" vs "representation" in definition of status
      code 416 (<https://github.com/httpwg/http-core/issues/83>,
      <https://www.rfc-editor.org/errata/eid4664>)

   o  Resolved erratum 4072, no change needed here
      (<https://github.com/httpwg/http-core/issues/84>,
      <https://www.rfc-editor.org/errata/eid4072>)

   o  Clarify DELETE status code suggestions
      (<https://github.com/httpwg/http-core/issues/85>,
      <https://www.rfc-editor.org/errata/eid4436>)




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   o  In Section 13.4, fix ABNF for "other-range-resp" to use VCHAR
      instead of CHAR (<https://github.com/httpwg/http-core/issues/86>,
      <https://www.rfc-editor.org/errata/eid4707>)

   o  Resolved erratum 5162, no change needed here
      (<https://github.com/httpwg/http-core/issues/89>,
      <https://www.rfc-editor.org/errata/eid5162>)

   o  Replace "response code" with "response status code" and "status-
      code" (the ABNF production name from the HTTP/1.1 message format)
      by "status code" (<https://github.com/httpwg/http-core/issues/94>,
      <https://www.rfc-editor.org/errata/eid4050>)

   o  Added a missing word in Section 14.4 (<https://github.com/httpwg/
      http-core/issues/98>, <https://www.rfc-editor.org/errata/eid4452>)

   o  In Section 5.7.1, fixed an example that had trailing whitespace
      where it shouldn't (<https://github.com/httpwg/http-core/
      issues/104>, <https://www.rfc-editor.org/errata/eid4169>)

   o  In Section 14.3.7, remove words that were potentially misleading
      with respect to the relation to the requested ranges
      (<https://github.com/httpwg/http-core/issues/102>,
      <https://www.rfc-editor.org/errata/eid4358>)

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

   o  Included (Proxy-)Auth-Info header field definition from RFC 7615
      (<https://github.com/httpwg/http-core/issues/9>)

   o  In Section 8.3.3, clarify POST caching
      (<https://github.com/httpwg/http-core/issues/17>)

   o  Add Section 14.5.19 to reserve the 418 status code
      (<https://github.com/httpwg/http-core/issues/43>)

   o  In Section 3.3 and Section 9.1.1, clarified when a response can be
      sent (<https://github.com/httpwg/http-core/issues/82>)

   o  In Section 7.4.2, explain the difference between the "token"
      production, the RFC 2978 ABNF for charset names, and the actual
      registration practice (<https://github.com/httpwg/http-core/
      issues/100>, <https://www.rfc-editor.org/errata/eid4689>)








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   o  In Section 3.1, removed the fragment component in the URI scheme
      definitions as per Section 4.3 of [RFC3986], furthermore moved
      fragment discussion into a separate section
      (<https://github.com/httpwg/http-core/issues/103>,
      <https://www.rfc-editor.org/errata/eid4251>, <https://www.rfc-
      editor.org/errata/eid4252>)

   o  In Section 5.1, add language about minor HTTP version number
      defaulting (<https://github.com/httpwg/http-core/issues/115>)

   o  Added Section 14.5.20 for status code 422, previously defined in
      Section 11.2 of [RFC4918] (<https://github.com/httpwg/http-core/
      issues/123>)

   o  In Section 14.5.17, fixed prose about byte range comparison
      (<https://github.com/httpwg/http-core/issues/135>,
      <https://www.rfc-editor.org/errata/eid5474>)

   o  In Section 3.3, explain that request/response correlation is
      version specific (<https://github.com/httpwg/http-core/
      issues/145>)

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

   o  In Section 14.4.9, include status code 308 from RFC 7538
      (<https://github.com/httpwg/http-core/issues/3>)

   o  In Section 7.4.1, clarify that the charset parameter value is
      case-insensitive due to the definition in RFC 2046
      (<https://github.com/httpwg/http-core/issues/13>)

   o  Define a separate registry for HTTP header field names
      (<https://github.com/httpwg/http-core/issues/42>)

   o  In Section 11.1, refactor and clarify description of wildcard
      ("*") handling (<https://github.com/httpwg/http-core/issues/46>)

   o  Deprecate Accept-Charset (<https://github.com/httpwg/http-core/
      issues/61>)

   o  In Section 12.2, mention Cache-Control: immutable
      (<https://github.com/httpwg/http-core/issues/69>)

   o  In Section 5.4.1, clarify when header field combination is allowed
      (<https://github.com/httpwg/http-core/issues/74>)

   o  In Section 17.4, instruct IANA to mark Content-MD5 as obsolete
      (<https://github.com/httpwg/http-core/issues/93>)



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   o  Use RFC 7405 ABNF notation for case-sensitive string constants
      (<https://github.com/httpwg/http-core/issues/133>)

   o  Rework Section 3.3 to be more version-independent
      (<https://github.com/httpwg/http-core/issues/142>)

   o  In Section 8.3.5, clarify that DELETE needs to be successful to
      invalidate cache (<https://github.com/httpwg/http-core/
      issues/167>, <https://www.rfc-editor.org/errata/eid5541>)

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

   o  In Section 5.4.4, fix field-content ABNF
      (<https://github.com/httpwg/http-core/issues/19>,
      <https://www.rfc-editor.org/errata/eid4189>)

   o  Move Section 5.7.6 into its own section
      (<https://github.com/httpwg/http-core/issues/45>)

   o  In Section 7.4, reference MIME Sniffing
      (<https://github.com/httpwg/http-core/issues/51>)

   o  In Section 5.7.1, simplify the #rule mapping for recipients
      (<https://github.com/httpwg/http-core/issues/164>,
      <https://www.rfc-editor.org/errata/eid5257>)

   o  In Section 8.3.7, remove misleading text about "extension" of HTTP
      is needed to define method payloads (<https://github.com/httpwg/
      http-core/issues/204>)

   o  Fix editorial issue in Section 7 (<https://github.com/httpwg/http-
      core/issues/223>)

   o  In Section 14.5.20, rephrase language not to use "entity" anymore,
      and also avoid lowercase "may" (<https://github.com/httpwg/http-
      core/issues/224>)

   o  Move discussion of retries from [Messaging] into Section 8.2.2
      (<https://github.com/httpwg/http-core/issues/230>)

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

   o  Moved transport-independent part of the description of trailers
      into Section 5.6 (<https://github.com/httpwg/http-core/issues/16>)

   o  Loosen requirements on retries based upon implementation behavior
      (<https://github.com/httpwg/http-core/issues/27>)




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   o  In Section 17.9, update IANA port registry for TCP/UDP on ports 80
      and 443 (<https://github.com/httpwg/http-core/issues/36>)

   o  In Section 15.3.3, revise guidelines for new header field names
      (<https://github.com/httpwg/http-core/issues/47>)

   o  In Section 8.2.3, remove concept of "cacheable methods" in favor
      of prose (<https://github.com/httpwg/http-core/issues/54>,
      <https://www.rfc-editor.org/errata/eid5300>)

   o  In Section 16.1, mention that the concept of authority can be
      modified by protocol extensions (<https://github.com/httpwg/http-
      core/issues/143>)

   o  Create new subsection on payload body in Section 5.5.4, taken from
      portions of message body (<https://github.com/httpwg/http-core/
      issues/159>)

   o  Moved definition of "Whitespace" into new container "Generic
      Syntax" (<https://github.com/httpwg/http-core/issues/162>)

   o  In Section 3.1, recommend minimum URI size support for
      implementations (<https://github.com/httpwg/http-core/issues/169>)

   o  In Section 13.1, refactored the range-unit and ranges-specifier
      grammars (<https://github.com/httpwg/http-core/issues/196>,
      <https://www.rfc-editor.org/errata/eid5620>)

   o  In Section 8.3.1, caution against a request body more strongly
      (<https://github.com/httpwg/http-core/issues/202>)

   o  Reorganized text in Section 15.3.3 (<https://github.com/httpwg/
      http-core/issues/214>)

   o  In Section 14.5.4, replace "authorize" with "fulfill"
      (<https://github.com/httpwg/http-core/issues/218>)

   o  In Section 8.3.7, removed a misleading statement about Content-
      Length (<https://github.com/httpwg/http-core/issues/235>,
      <https://www.rfc-editor.org/errata/eid5806>)

   o  In Section 16.1, add text from RFC 2818
      (<https://github.com/httpwg/http-core/issues/236>)

   o  Changed "cacheable by default" to "heuristically cacheable"
      throughout (<https://github.com/httpwg/http-core/issues/242>)





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

   o  In Section 6.4.3, simplify received-by grammar (and disallow comma
      character) (<https://github.com/httpwg/http-core/issues/24>)

   o  In Section 5.4.3, give guidance on interoperable field names
      (<https://github.com/httpwg/http-core/issues/30>)

   o  In Section 5.7.3, define the semantics and possible replacement of
      whitespace when it is known to occur (<https://github.com/httpwg/
      http-core/issues/53>, <https://www.rfc-editor.org/errata/eid5163>)

   o  In Section 5.4, introduce field terminology and distinguish
      between field line values and field values; use terminology
      consistently throughout (<https://github.com/httpwg/http-core/
      issues/111>)

   o  Moved #rule definition into Section 5.4.4 and whitespace into
      Section 2.1 (<https://github.com/httpwg/http-core/issues/162>)

   o  In Section 13.1, explicitly call out range unit names as case-
      insensitive, and encourage registration
      (<https://github.com/httpwg/http-core/issues/179>)

   o  In Section 7.5.1, explicitly call out content codings as case-
      insensitive, and encourage registration
      (<https://github.com/httpwg/http-core/issues/179>)

   o  In Section 5.4.3, explicitly call out field names as case-
      insensitive (<https://github.com/httpwg/http-core/issues/179>)

   o  In Section 16.12, cite [Bujlow] (<https://github.com/httpwg/http-
      core/issues/185>)

   o  In Section 14, formally define "final" and "interim" status codes
      (<https://github.com/httpwg/http-core/issues/245>)

   o  In Section 8.3.5, caution against a request body more strongly
      (<https://github.com/httpwg/http-core/issues/258>)

   o  In Section 7.9.3, note that Etag can be used in trailers
      (<https://github.com/httpwg/http-core/issues/262>)

   o  In Section 17.4, consider reserved fields as well
      (<https://github.com/httpwg/http-core/issues/273>)






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   o  In Section 4.2.4, be more correct about what was deprecated by RFC
      3986 (<https://github.com/httpwg/http-core/issues/278>,
      <https://www.rfc-editor.org/errata/eid5964>)

   o  In Section 5.4.1, recommend comma SP when combining field lines
      (<https://github.com/httpwg/http-core/issues/148>)

   o  In Section 6.1.2, make explicit requirements on origin server to
      use authority from absolute-form when available
      (<https://github.com/httpwg/http-core/issues/191>)

   o  In Section 4.2.1, Section 4.2.2, Section 4.3.1, and Section 6.2.3,
      refactored schemes to define origin and authoritative access to an
      origin server for both "http" and "https" URIs to account for
      alternative services and secured connections that are not
      necessarily based on TCP (<https://github.com/httpwg/http-core/
      issues/237>)

   o  In Section 2.2, reference RFC 8174 as well
      (<https://github.com/httpwg/http-core/issues/303>)

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

   o  In Section 13.2, explicitly reference the definition of
      representation data as including any content codings
      (<https://github.com/httpwg/http-core/issues/11>)

   o  Move TE: trailers from [Messaging] into Section 5.6.2
      (<https://github.com/httpwg/http-core/issues/18>)

   o  In Section 7.7, adjust requirements for handling multiple content-
      length values (<https://github.com/httpwg/http-core/issues/59>)

   o  In Section 12.1.1 and Section 12.1.2, clarified condition
      evaluation (<https://github.com/httpwg/http-core/issues/72>)

   o  In Section 5.4.4, remove concept of obs-fold, as that is
      HTTP/1-specific (<https://github.com/httpwg/http-core/issues/116>)

   o  In Section 11, introduce the concept of request payload
      negotiation (Section 11.3) and define for Accept-Encoding
      (<https://github.com/httpwg/http-core/issues/119>)

   o  In Section 14.3.6, Section 14.5.9, and Section 14.5.14, remove
      HTTP/1-specific, connection-related requirements
      (<https://github.com/httpwg/http-core/issues/144>)





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   o  In Section 8.3.6, correct language about what is forwarded
      (<https://github.com/httpwg/http-core/issues/170>)

   o  Throughout, replace "effective request URI", "request-target" and
      similar with "target URI" (<https://github.com/httpwg/http-core/
      issues/259>)

   o  In Section 15.3.3 and Section 15.2.2, describe how extensions
      should consider scope of applicability
      (<https://github.com/httpwg/http-core/issues/265>)

   o  In Section 3.3, don't rely on the HTTP/1.1 Messaging specification
      to define "message" (<https://github.com/httpwg/http-core/
      issues/311>)

   o  In Section 7.8 and Section 9.1.3, note that URL resolution is
      necessary (<https://github.com/httpwg/http-core/issues/321>)

   o  In Section 7, explicitly reference 206 as one of the status codes
      that provide representation data (<https://github.com/httpwg/http-
      core/issues/325>)

   o  In Section 12.1.4, refine requirements so that they don't apply to
      resources without a concept of modification time
      (<https://github.com/httpwg/http-core/issues/326>)

   o  In Section 10.7.1, specify the scope as a request, not a target
      resource (<https://github.com/httpwg/http-core/issues/331>)

   o  In Section 3.3, introduce concept of "complete" messages
      (<https://github.com/httpwg/http-core/issues/334>)

   o  In Section 6.1, Section 8.3.6, and Section 8.3.7, refine use of
      "request target" (<https://github.com/httpwg/http-core/
      issues/340>)

   o  Throughout, remove "status-line" and "request-line", as these are
      HTTP/1.1-specific (<https://github.com/httpwg/http-core/
      issues/361>)

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

   o  In Section 14.5.17, remove duplicate definition of what makes a
      range satisfiable and refer instead to each range unit's
      definition (<https://github.com/httpwg/http-core/issues/12>)






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   o  In Section 13.1.2 and Section 13.2, clarify that a selected
      representation of zero length can only be satisfiable as a suffix
      range and that a server can still ignore Range for that case
      (<https://github.com/httpwg/http-core/issues/12>)

   o  In Section 11.1.2 and Section 14.5.16, allow "Accept" as response
      field (<https://github.com/httpwg/http-core/issues/48>)

   o  Appendix A now uses the sender variant of the "#" list expansion
      (<https://github.com/httpwg/http-core/issues/192>)

   o  In Section 11.2.1, make the field list-based even when "*" is
      present (<https://github.com/httpwg/http-core/issues/272>)

   o  In Section 15.3.1, add optional "Comments" entry
      (<https://github.com/httpwg/http-core/issues/273>)

   o  In Section 17.4, reserve "*" as field name
      (<https://github.com/httpwg/http-core/issues/274>)

   o  In Section 17.2, reserve "*" as method name
      (<https://github.com/httpwg/http-core/issues/274>)

   o  In Section 12.1.1 and Section 12.1.2, state that multiple "*" is
      unlikely to be interoperable (<https://github.com/httpwg/http-
      core/issues/305>)

   o  In Section 11.1.2, avoid use of obsolete media type parameter on
      text/html (<https://github.com/httpwg/http-core/issues/375>,
      <https://www.rfc-editor.org/errata/eid6149>)

   o  Rephrase prose in Section 3.3 to become version-agnostic
      (<https://github.com/httpwg/http-core/issues/372>)

   o  In Section 5.4.4, instruct recipients how to deal with control
      characters in field values (<https://github.com/httpwg/http-core/
      issues/377>)

   o  In Section 5.4.4, update note about field ABNF
      (<https://github.com/httpwg/http-core/issues/380>)

   o  Add Section 15 about Extending and Versioning HTTP
      (<https://github.com/httpwg/http-core/issues/384>)

   o  In Section 14.1, include status 308 in list of heuristically
      cacheable status codes (<https://github.com/httpwg/http-core/
      issues/385>)




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   o  In Section 7.5, make it clearer that "identity" is not to be
      included (<https://github.com/httpwg/http-core/issues/388>)

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

   o  Switch to xml2rfc v3 mode for draft generation
      (<https://github.com/httpwg/http-core/issues/394>)

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

   o  In Section 16.6, mention compression attacks
      (<https://github.com/httpwg/http-core/issues/6>)

   o  In Section 15.6.1, advise to make new content codings self-
      descriptive (<https://github.com/httpwg/http-core/issues/21>)

   o  In Section 5.7.6, introduced the "parameters" ABNF rule, allowing
      empty parameters and trailing semicolons within media type, media
      range, and expectation (<https://github.com/httpwg/http-core/
      issues/33>)

   o  In Section 14.4, explain how to create a redirected request
      (<https://github.com/httpwg/http-core/issues/38>)

   o  In Section 7.4, defined error handling for multiple members
      (<https://github.com/httpwg/http-core/issues/39>)

   o  In Section 1, revise the introduction and introduce HTTP/2 and
      HTTP/3 (<https://github.com/httpwg/http-core/issues/64>)

   o  In Section 7.7, added a definition for Content-Length that
      encompasses its various roles in describing message payload or
      selected representation length; in Section 14.3.7, noted that
      Content-Length counts only the message body (not the selected
      representation) and that the complete length is in each
      Content-Range (<https://github.com/httpwg/http-core/issues/118>)

   o  Noted that "WWW-Authenticate" with more than one value on a line
      is sometimes not interoperable [Messaging]
      (<https://github.com/httpwg/http-core/issues/136>)

   o  In Section 12.1.1 and Section 12.1.4, removed requirement that a
      validator not be sent in a 2xx response when validation fails and
      the server decides that the same change request has already been
      applied (<https://github.com/httpwg/http-core/issues/166>)

   o  Moved requirements specific to HTTP/1.1 from Section 6.1.2 to
      [Messaging] (<https://github.com/httpwg/http-core/issues/182>)



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   o  In Section 5.4.4, introduce the terms "singleton field" and "list-
      based field" (also - in various places - discuss what to do when a
      singleton field is received as a list)
      (<https://github.com/httpwg/http-core/issues/193>)

   o  In Section 9.1.1, change the ABNF back to be a list of
      expectations, as defined in RFC 2616 (<https://github.com/httpwg/
      http-core/issues/203>)

   o  In Section 9.1.5 (Trailer), Section 6.4.3 (Via), Section 6.6
      (Upgrade), Section 6.4.1 (Connection), Section 7.5
      (Content-Encoding), Section 7.6 (Content-Language), Section 9.1.1
      (Expect), Section 12.1.1 (If-Match), Section 12.1.2
      (If-None-Match), Section 11.1.3 (Accept-Charset), Section 11.1.5
      (Accept-Language), Section 11.2.1 (Vary), Section 10.6.1
      (WWW-Authenticate), and Section 10.7.1 (Proxy-Authenticate),
      adjust ABNF to allow empty lists (<https://github.com/httpwg/http-
      core/issues/210>)

   o  In Section 8.3.1 and Section 16.9, provide a more nuanced
      explanation of sensitive data in GET-based forms and describe
      workarounds (<https://github.com/httpwg/http-core/issues/277>)

   o  In Section 12.2, allow preconditions to be evaluated before the
      request body (if any) is processed (<https://github.com/httpwg/
      http-core/issues/261>)

   o  In Section 5.4 and Section 5.6.3, allow for trailer fields in
      multiple trailer sections, depending on the HTTP version and
      framing in use, with processing being iterative as each section is
      received (<https://github.com/httpwg/http-core/issues/313>)

   o  Moved definitions of "TE" and "Upgrade" from [Messaging]
      (<https://github.com/httpwg/http-core/issues/392>)

   o  Moved 1.1-specific discussion of TLS to Messaging and rewrote
      Section 4.3.4 to refer to RFC6125 (<https://github.com/httpwg/
      http-core/issues/404>)

   o  Moved definition of "Connection" from [Messaging]
      (<https://github.com/httpwg/http-core/issues/407>)

C.13.  Since draft-ietf-httpbis-semantics-11

   o  The entire document has been reorganized, with no changes to
      content except editorial for the reorganization
      (<https://github.com/httpwg/http-core/issues/368>)




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   o  Move IANA Upgrade Token Registry instructions from [Messaging]
      (<https://github.com/httpwg/http-core/issues/450>)

Acknowledgments

   This edition of the HTTP specification builds on the many
   contributions that went into RFC 1945, RFC 2068, RFC 2145, RFC 2616,
   and RFC 2818, including substantial contributions made by the
   previous authors, editors, and Working Group Chairs: Tim Berners-Lee,
   Jean-François Groff, Ari Luotonen, Roy T.  Fielding, Henrik Frystyk
   Nielsen, Jim Gettys, Jeffrey C.  Mogul, Larry Masinter, Paul J.
   Leach, Eric Rescorla, and Yves Lafon.

   See Section 10 of [RFC7230] for further acknowledgements from prior
   revisions.

   In addition, this document has reincorporated the HTTP Authentication
   Framework, previously defined in RFC 7235 and RFC 2617.  We thank
   John Franks, Phillip M.  Hallam-Baker, Jeffery L.  Hostetler, Scott
   D.  Lawrence, Paul J.  Leach, Ari Luotonen, and Lawrence C.  Stewart
   for their work on that specification.  See Section 6 of [RFC2617] for
   further acknowledgements.


   // New acks to be added here.

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe
   345 Park Ave
   San Jose, CA 95110
   United States of America

   Email: fielding@gbiv.com
   URI:   https://roy.gbiv.com/


   Mark Nottingham (editor)
   Fastly
   Prahran VIC
   Australia

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/






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   Julian Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   48155 Münster
   Germany

   Email: julian.reschke@greenbytes.de
   URI:   https://greenbytes.de/tech/webdav/











































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