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. F. Reschke, Ed.
Expires: February 28, April 5, 2021                                        greenbytes
                                                         August 27,
                                                         October 2, 2020

                             HTTP Semantics
                    draft-ietf-httpbis-semantics-11
                    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.12. C.13.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 28, April 5, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     1.1.  Purpose . . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     1.2.  Evolution . . . . . . . . . . . . . . . . . . . . . . . .   9
     1.3.  Semantics . . . . . . . . . . . . . . . . . . . . . . . .   9  10
     1.4.  Obsoletes . . . . . . . . . . . . . . . . . . . . . . . .  10
     1.5.  Requirements Notation . .  11
   2.  Conformance . . . . . . . . . . . . . . . .  11
     1.6.  Syntax Notation . . . . . . . . .  12
     2.1.  Syntax Notation . . . . . . . . . . . .  11
       1.6.1.  Whitespace . . . . . . . . .  12
     2.2.  Requirements Notation . . . . . . . . . . . .  12
   2.  Architecture . . . . . .  12
     2.3.  Length Requirements . . . . . . . . . . . . . . . . . .  12
     2.1.  Client/Server Messaging .  13
     2.4.  Error Handling  . . . . . . . . . . . . . . . .  13
     2.2.  Intermediaries . . . . .  14
   3.  Terminology . . . . . . . . . . . . . . . .  15
     2.3.  Caches . . . . . . . . .  14
     3.1.  Resources . . . . . . . . . . . . . . . .  17
     2.4.  Uniform Resource Identifiers . . . . . . . .  14
     3.2.  Connections . . . . . .  18
     2.5.  Resources . . . . . . . . . . . . . . . . .  15
     3.3.  Messages  . . . . . . .  19
       2.5.1.  http URI Scheme . . . . . . . . . . . . . . . . .  15
     3.4.  User Agent  . .  19
       2.5.2.  https URI Scheme . . . . . . . . . . . . . . . . . .  20
       2.5.3.  http and https URI Normalization and Comparison . . .  21
       2.5.4.  Deprecated userinfo  15
     3.5.  Origin Server . . . . . . . . . . . . . . . . .  21
       2.5.5.  Fragment Identifiers on http(s) URI References . . .  22
   3.  Conformance . .  16
     3.6.  Example Request and Response  . . . . . . . . . . . . . .  16
     3.7.  Intermediaries  . . . . . . . . .  22
     3.1.  Implementation Diversity . . . . . . . . . . . .  17
     3.8.  Caches  . . . .  22
     3.2.  Role-based Requirements . . . . . . . . . . . . . . . . .  23
     3.3.  Parsing Elements . . . .  19
   4.  Identifiers . . . . . . . . . . . . . . . .  24
     3.4.  Error Handling . . . . . . . . .  20
     4.1.  URI References  . . . . . . . . . . . .  24
   4.  Extending and Versioning HTTP . . . . . . . . .  20
     4.2.  URI Schemes . . . . . . .  25
     4.1.  Extending HTTP . . . . . . . . . . . . . . . .  21
       4.2.1.  http URI Scheme . . . . .  25
     4.2.  Protocol Versioning . . . . . . . . . . . . . .  22
       4.2.2.  https URI Scheme  . . . . .  26
   5.  Header and Trailer Fields . . . . . . . . . . . . .  22
       4.2.3.  http(s) Normalization and Comparison  . . . . .  27
     5.1.  Field Ordering and Combination . . .  23
       4.2.4.  http(s) Deprecated userinfo . . . . . . . . . .  28
     5.2.  Field Limits . . .  24
       4.2.5.  http(s) References with Fragment Identifiers  . . . .  24
     4.3.  Authoritative Access  . . . . . . . . . . . . . . .  29
     5.3.  Field Names . . .  24
       4.3.1.  URI Origin  . . . . . . . . . . . . . . . . . . . .  30
       5.3.1.  Field Extensibility .  24
       4.3.2.  http origins  . . . . . . . . . . . . . . . .  30
       5.3.2.  Field Name Registry . . . .  25
       4.3.3.  https origins . . . . . . . . . . . . .  31
     5.4.  Field Values . . . . . . .  26
       4.3.4.  https certificate verification  . . . . . . . . . . .  27
   5.  Message Abstraction . . . .  32
       5.4.1.  Common Field Value Components . . . . . . . . . . . .  34
     5.5.  ABNF List Extension: #rule . . . . .  28
     5.1.  Protocol Version  . . . . . . . . . .  37
       5.5.1.  Sender Requirements . . . . . . . . . .  28
     5.2.  Framing . . . . . . .  38
       5.5.2.  Recipient Requirements . . . . . . . . . . . . . . .  38
     5.6.  Trailer Fields . . .  30
     5.3.  Control Data  . . . . . . . . . . . . . . . . . .  39
       5.6.1.  Purpose . . . .  30
       5.3.1.  Request . . . . . . . . . . . . . . . . . . .  39
       5.6.2.  Limitations . . . .  30
       5.3.2.  Response  . . . . . . . . . . . . . . . . .  39
       5.6.3.  Processing . . . . .  30
     5.4.  Header Fields . . . . . . . . . . . . . . . .  40
       5.6.4.  Trailer . . . . . .  30
       5.4.1.  Field Ordering and Combination  . . . . . . . . . . .  32
       5.4.2.  Field Limits  . . . . . .  41
       5.6.5.  TE . . . . . . . . . . . . . .  33
       5.4.3.  Field Names . . . . . . . . . . .  41
     5.7.  Considerations for New HTTP Fields . . . . . . . . . .  33
       5.4.4.  Field Values  .  41
     5.8.  Fields Defined In This Document . . . . . . . . . . . . .  43
   6.  Message Routing . . . . . .  33
     5.5.  Payload . . . . . . . . . . . . . . . . .  44
     6.1.  Identifying a Target Resource . . . . . . . .  35
       5.5.1.  Purpose . . . . . .  44
     6.2.  Determining Origin . . . . . . . . . . . . . . . . .  35
       5.5.2.  Identification  . .  45
     6.3.  Routing Inbound . . . . . . . . . . . . . . . . .  36
       5.5.3.  Payload Metadata  . . . .  45
       6.3.1.  To a Cache . . . . . . . . . . . . . .  37
       5.5.4.  Payload Body  . . . . . . .  46
       6.3.2.  To a Proxy . . . . . . . . . . . . .  37
     5.6.  Trailer Fields  . . . . . . . .  46
       6.3.3.  To the Origin . . . . . . . . . . . . .  37
       5.6.1.  Purpose . . . . . . .  46
     6.4.  Reconstructing the Target URI . . . . . . . . . . . . . .  49
     6.5.  Host . .  38
       5.6.2.  Limitations . . . . . . . . . . . . . . . . . . . . .  38
       5.6.3.  Processing  . . .  50
     6.6.  Message Forwarding . . . . . . . . . . . . . . . . . .  39
     5.7.  Common Rules for Defining Field Values  .  50
       6.6.1.  Via . . . . . . . .  39
       5.7.1.  Lists (#rule ABNF Extension)  . . . . . . . . . . . .  39
       5.7.2.  Tokens  . . . . .  51
       6.6.2.  Transformations . . . . . . . . . . . . . . . . . .  41
       5.7.3.  Whitespace  .  53
     6.7.  Upgrading HTTP . . . . . . . . . . . . . . . . . . . .  41
       5.7.4.  Quoted Strings  .  54
       6.7.1.  Upgrade Protocol Names . . . . . . . . . . . . . . .  56
       6.7.2.  Upgrade Token Registry . . .  42
       5.7.5.  Comments  . . . . . . . . . . . .  56

     6.8.  Connection-Specific Fields . . . . . . . . . .  42
       5.7.6.  Parameters  . . . . .  57
   7.  Representations . . . . . . . . . . . . . . . .  43
       5.7.7.  Date/Time Formats . . . . . . .  59
     7.1.  Representation Data . . . . . . . . . . .  43
   6.  Routing . . . . . . . .  59
       7.1.1.  Media Type . . . . . . . . . . . . . . . . . . .  45
     6.1.  Target Resource . .  60
       7.1.2.  Content Codings . . . . . . . . . . . . . . . . . . .  62
       7.1.3.  Language Tags  45
       6.1.1.  Request Target  . . . . . . . . . . . . . . . . . . .  45
       6.1.2.  Host  .  64
       7.1.4.  Range Units . . . . . . . . . . . . . . . . . . . . .  64
     7.2.  Representation Metadata . .  46
       6.1.3.  Reconstructing the Target URI . . . . . . . . . . . .  47
     6.2.  Routing Inbound . . .  68
       7.2.1.  Content-Type . . . . . . . . . . . . . . . . . .  47
       6.2.1.  To a Cache  . .  69
       7.2.2.  Content-Encoding . . . . . . . . . . . . . . . . . .  70
       7.2.3.  Content-Language .  47
       6.2.2.  To a Proxy  . . . . . . . . . . . . . . . . .  71
       7.2.4.  Content-Length . . . .  48
       6.2.3.  To the Origin . . . . . . . . . . . . . . .  72
       7.2.5.  Content-Location . . . . .  48
     6.3.  Response Correlation  . . . . . . . . . . . . .  73
     7.3.  Payload . . . . .  48
     6.4.  Message Forwarding  . . . . . . . . . . . . . . . . . . .  48
       6.4.1.  Connection  .  75
       7.3.1.  Purpose . . . . . . . . . . . . . . . . . . . .  49
       6.4.2.  Max-Forwards  . . .  75
       7.3.2.  Identification . . . . . . . . . . . . . . . . .  50
       6.4.3.  Via . .  76
       7.3.3.  Payload Body . . . . . . . . . . . . . . . . . . . .  77
       7.3.4.  Content-Range . . .  51
     6.5.  Transformations . . . . . . . . . . . . . . . . .  78
       7.3.5.  Media Type multipart/byteranges . . . .  53
     6.6.  Upgrade . . . . . . .  79
     7.4.  Content Negotiation . . . . . . . . . . . . . . . . . .  54
   7.  Representations .  81
       7.4.1.  Proactive Negotiation . . . . . . . . . . . . . . . .  82
       7.4.2.  Reactive Negotiation . . . . . .  56
     7.1.  Selected Representation . . . . . . . . . .  83
       7.4.3.  Request Payload Negotiation . . . . . . .  57
     7.2.  Data  . . . . . .  84
       7.4.4.  Quality Values . . . . . . . . . . . . . . . . . . .  84
   8.  Request Methods .  57
     7.3.  Metadata  . . . . . . . . . . . . . . . . . . . . . .  85
     8.1.  Overview . .  57
     7.4.  Content-Type  . . . . . . . . . . . . . . . . . . . . . .  85
     8.2.  Common Method Properties  58
       7.4.1.  Media Type  . . . . . . . . . . . . . . . .  86
       8.2.1.  Safe Methods . . . . .  59
       7.4.2.  Charset . . . . . . . . . . . . . . .  87
       8.2.2.  Idempotent Methods . . . . . . . .  59
       7.4.3.  Canonicalization and Text Defaults  . . . . . . . . .  88
       8.2.3.  Methods and Caching  60
       7.4.4.  Multipart Types . . . . . . . . . . . . . . . . .  89
     8.3.  Method Definitions . .  61
     7.5.  Content-Encoding  . . . . . . . . . . . . . . . . .  89
       8.3.1.  GET . . .  61
       7.5.1.  Content Codings . . . . . . . . . . . . . . . . . . .  62
     7.6.  Content-Language  . . .  89
       8.3.2.  HEAD . . . . . . . . . . . . . . . . .  63
       7.6.1.  Language Tags . . . . . . .  90
       8.3.3.  POST . . . . . . . . . . . . .  64
     7.7.  Content-Length  . . . . . . . . . . .  91
       8.3.4.  PUT . . . . . . . . . .  65
     7.8.  Content-Location  . . . . . . . . . . . . . . .  92
       8.3.5.  DELETE . . . . .  66
     7.9.  Validators  . . . . . . . . . . . . . . . . . .  95
       8.3.6.  CONNECT . . . . .  68
       7.9.1.  Weak versus Strong  . . . . . . . . . . . . . . . . .  69
       7.9.2.  Last-Modified .  96
       8.3.7.  OPTIONS . . . . . . . . . . . . . . . . . . .  71
       7.9.3.  ETag  . . . .  97
       8.3.8.  TRACE . . . . . . . . . . . . . . . . . . . .  73
       7.9.4.  When to Use Entity-Tags and Last-Modified Dates . . .  76
   8.  Methods .  98
     8.4.  Method Extensibility . . . . . . . . . . . . . . . . . .  99
       8.4.1.  Method Registry . . . . . . . .  77
     8.1.  Overview  . . . . . . . . . . .  99
       8.4.2.  Considerations for New Methods . . . . . . . . . . . 100
   9.  Request Header Fields . .  77
     8.2.  Common Method Properties  . . . . . . . . . . . . . . . .  78
       8.2.1.  Safe Methods  . . 100
     9.1.  Controls . . . . . . . . . . . . . . . . . .  79
       8.2.2.  Idempotent Methods  . . . . . . 100
       9.1.1.  Expect . . . . . . . . . . .  80
       8.2.3.  Methods and Caching . . . . . . . . . . . . 101
       9.1.2.  Max-Forwards . . . . .  81
     8.3.  Method Definitions  . . . . . . . . . . . . . . . 103
     9.2.  Preconditions . . . .  81
       8.3.1.  GET . . . . . . . . . . . . . . . . . . 104
       9.2.1.  Evaluation . . . . . . .  81
       8.3.2.  HEAD  . . . . . . . . . . . . . . 105
       9.2.2.  Precedence . . . . . . . . . .  82
       8.3.3.  POST  . . . . . . . . . . . 106
       9.2.3.  If-Match . . . . . . . . . . . . .  83
       8.3.4.  PUT . . . . . . . . . 107
       9.2.4.  If-None-Match . . . . . . . . . . . . . . . .  84
       8.3.5.  DELETE  . . . . 109
       9.2.5.  If-Modified-Since . . . . . . . . . . . . . . . . . . 110
       9.2.6.  If-Unmodified-Since .  87
       8.3.6.  CONNECT . . . . . . . . . . . . . . . . 112
       9.2.7.  If-Range . . . . . . .  88
       8.3.7.  OPTIONS . . . . . . . . . . . . . . . 113
     9.3.  Range . . . . . . . .  89
       8.3.8.  TRACE . . . . . . . . . . . . . . . . . . 114
     9.4.  Negotiation . . . . . .  90
   9.  Context . . . . . . . . . . . . . . . . . 116
       9.4.1.  Accept . . . . . . . . . .  91
     9.1.  Request Context . . . . . . . . . . . . . 117
       9.4.2.  Accept-Charset . . . . . . . .  91
       9.1.1.  Expect  . . . . . . . . . . . 119
       9.4.3.  Accept-Encoding . . . . . . . . . . . .  92
       9.1.2.  From  . . . . . . . 120
       9.4.4.  Accept-Language . . . . . . . . . . . . . . . . .  94
       9.1.3.  Referer . . 122
     9.5.  Authentication Credentials . . . . . . . . . . . . . . . 123
       9.5.1.  Challenge and Response . . . . . .  95
       9.1.4.  TE  . . . . . . . . . 123
       9.5.2.  Protection Space (Realm) . . . . . . . . . . . . . . 125
       9.5.3.  Authorization . .  96
       9.1.5.  Trailer . . . . . . . . . . . . . . . . . . 126
       9.5.4.  Proxy-Authorization . . . . .  96
       9.1.6.  User-Agent  . . . . . . . . . . . . 126
       9.5.5.  Authentication Scheme Extensibility . . . . . . . . . 127
     9.6.  Request  97
     9.2.  Response Context  . . . . . . . . . . . . . . . . . . . .  98
       9.2.1.  Allow . 129
       9.6.1.  From . . . . . . . . . . . . . . . . . . . . . . .  98
       9.2.2.  Date  . 129
       9.6.2.  Referer . . . . . . . . . . . . . . . . . . . . . . . 130
       9.6.3.  User-Agent  99
       9.2.3.  Location  . . . . . . . . . . . . . . . . . . . . . 131
   10. Response Status Codes . 100
       9.2.4.  Retry-After . . . . . . . . . . . . . . . . . . . 132
     10.1.  Overview of Status Codes . . 101
       9.2.5.  Server  . . . . . . . . . . . . . . 133
     10.2.  Informational 1xx . . . . . . . . . 102
   10. Authentication  . . . . . . . . . . 134
       10.2.1.  100 Continue . . . . . . . . . . . . . 102
     10.1.  Authentication Scheme  . . . . . . . 134
       10.2.2.  101 Switching Protocols . . . . . . . . . . 102
     10.2.  Authentication Parameters  . . . . 135
     10.3.  Successful 2xx . . . . . . . . . . . 103
     10.3.  Challenge and Response . . . . . . . . . . 135
       10.3.1.  200 OK . . . . . . . 103
     10.4.  Credentials  . . . . . . . . . . . . . . . . 135
       10.3.2.  201 Created . . . . . . 104
     10.5.  Protection Space (Realm) . . . . . . . . . . . . . . 136
       10.3.3.  202 Accepted . . 105
     10.6.  Authenticating User to Origin Server . . . . . . . . . . 106
       10.6.1.  WWW-Authenticate . . . . . . . . 136
       10.3.4.  203 Non-Authoritative Information . . . . . . . . . 137
       10.3.5.  204 No Content . 106
       10.6.2.  Authorization  . . . . . . . . . . . . . . . . . . 137
       10.3.6.  205 Reset Content . 107
       10.6.3.  Authentication-Info  . . . . . . . . . . . . . . . . 138
       10.3.7.  206 Partial Content 107
     10.7.  Authenticating Client to Proxy . . . . . . . . . . . . . 108
       10.7.1.  Proxy-Authenticate . . . 138
     10.4.  Redirection 3xx . . . . . . . . . . . . . . 108
       10.7.2.  Proxy-Authorization  . . . . . . 141
       10.4.1.  300 Multiple Choices . . . . . . . . . . 108
       10.7.3.  Proxy-Authentication-Info  . . . . . . 144
       10.4.2.  301 Moved Permanently . . . . . . . 109
   11. Content Negotiation . . . . . . . . 145
       10.4.3.  302 Found . . . . . . . . . . . . . 109
     11.1.  Proactive Negotiation  . . . . . . . . 145
       10.4.4.  303 See Other . . . . . . . . . 110
       11.1.1.  Shared Negotiation Features  . . . . . . . . . . 146
       10.4.5.  304 Not Modified . . 111
       11.1.2.  Accept . . . . . . . . . . . . . . . . 146
       10.4.6.  305 Use Proxy . . . . . . . 113
       11.1.3.  Accept-Charset . . . . . . . . . . . . 147
       10.4.7.  306 (Unused) . . . . . . . 115
       11.1.4.  Accept-Encoding  . . . . . . . . . . . . . 147
       10.4.8.  307 Temporary Redirect . . . . . 116
       11.1.5.  Accept-Language  . . . . . . . . . . 147
       10.4.9.  308 Permanent Redirect . . . . . . . . 117
     11.2.  Reactive Negotiation . . . . . . . 148
     10.5.  Client Error 4xx . . . . . . . . . . . 119
       11.2.1.  Vary . . . . . . . . . 148
       10.5.1.  400 Bad Request . . . . . . . . . . . . . . . 120
     11.3.  Request Payload Negotiation  . . . 148
       10.5.2.  401 Unauthorized . . . . . . . . . . . 121
   12. Conditional Requests  . . . . . . . 148
       10.5.3.  402 Payment Required . . . . . . . . . . . . . 121
     12.1.  Preconditions  . . . 149
       10.5.4.  403 Forbidden . . . . . . . . . . . . . . . . . . 122
       12.1.1.  If-Match . 149
       10.5.5.  404 Not Found . . . . . . . . . . . . . . . . . . . 149
       10.5.6.  405 Method Not Allowed . . 122
       12.1.2.  If-None-Match  . . . . . . . . . . . . . 150
       10.5.7.  406 Not Acceptable . . . . . . 124
       12.1.3.  If-Modified-Since  . . . . . . . . . . . 150
       10.5.8.  407 Proxy Authentication Required . . . . . . 125
       12.1.4.  If-Unmodified-Since  . . . 150
       10.5.9.  408 Request Timeout . . . . . . . . . . . . . 127
       12.1.5.  If-Range . . . 150
       10.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . 128
     12.2.  Evaluation . 151
       10.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 151
       10.5.12. 411 Length Required 129
     12.3.  Precedence . . . . . . . . . . . . . . . . 151
       10.5.13. 412 Precondition Failed . . . . . . . 130
   13. Range Requests  . . . . . . . 152
       10.5.14. 413 Payload Too Large . . . . . . . . . . . . . . . 152
       10.5.15. 414 URI Too Long . . 131
     13.1.  Range Units  . . . . . . . . . . . . . . . . 152
       10.5.16. 415 Unsupported Media Type . . . . . . 132
       13.1.1.  Range Specifiers . . . . . . . 152
       10.5.17. 416 Range Not Satisfiable . . . . . . . . . . . 133
       13.1.2.  Byte Ranges  . . 153
       10.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 153
       10.5.19. 418 (Unused) . . . 134
     13.2.  Range  . . . . . . . . . . . . . . . . . 154
       10.5.20. 422 Unprocessable Payload . . . . . . . . 135
     13.3.  Accept-Ranges  . . . . . 154
       10.5.21. 426 Upgrade Required . . . . . . . . . . . . . . . . 154
     10.6.  Server Error 5xx 137
     13.4.  Content-Range  . . . . . . . . . . . . . . . . . . . . 155
       10.6.1.  500 Internal Server Error . 137
     13.5.  Media Type multipart/byteranges  . . . . . . . . . . . . 155
       10.6.2.  501 Not Implemented 139
   14. Status Codes  . . . . . . . . . . . . . . . . 155
       10.6.3.  502 Bad Gateway . . . . . . . . 141
     14.1.  Overview of Status Codes . . . . . . . . . . 155
       10.6.4.  503 Service Unavailable . . . . . . 142
     14.2.  Informational 1xx  . . . . . . . . 155
       10.6.5.  504 Gateway Timeout . . . . . . . . . . . 142
       14.2.1.  100 Continue . . . . . 156
       10.6.6.  505 HTTP Version Not Supported . . . . . . . . . . . 156
     10.7.  Status Code Extensibility . . . . 142
       14.2.2.  101 Switching Protocols  . . . . . . . . . . . 156
       10.7.1.  Status Code Registry . . . 143
     14.3.  Successful 2xx . . . . . . . . . . . . . 156
       10.7.2.  Considerations for New Status Codes . . . . . . . . 157
   11. Response Header Fields 143
       14.3.1.  200 OK . . . . . . . . . . . . . . . . . . . 158
     11.1.  Control Data . . . . 143
       14.3.2.  201 Created  . . . . . . . . . . . . . . . . . . 158
       11.1.1.  Date . . 144
       14.3.3.  202 Accepted . . . . . . . . . . . . . . . . . . . . 144
       14.3.4.  203 Non-Authoritative Information  . . 158
       11.1.2.  Location . . . . . . . 145
       14.3.5.  204 No Content . . . . . . . . . . . . . . . 159
       11.1.3.  Retry-After . . . . 145
       14.3.6.  205 Reset Content  . . . . . . . . . . . . . . . . 161
       11.1.4.  Vary . 146
       14.3.7.  206 Partial Content  . . . . . . . . . . . . . . . . 146
     14.4.  Redirection 3xx  . . . . . . . 161
     11.2.  Validators . . . . . . . . . . . . . 149
       14.4.1.  300 Multiple Choices . . . . . . . . . . 162
       11.2.1.  Weak versus Strong . . . . . . 152
       14.4.2.  301 Moved Permanently  . . . . . . . . . . . 163
       11.2.2.  Last-Modified . . . . 153
       14.4.3.  302 Found  . . . . . . . . . . . . . . . 165
       11.2.3.  ETag . . . . . . 153
       14.4.4.  303 See Other  . . . . . . . . . . . . . . . . . . 167
       11.2.4.  When to Use Entity-Tags and Last-Modified Dates . 154
       14.4.5.  304 Not Modified . 170
     11.3.  Authentication Challenges . . . . . . . . . . . . . . . 171
       11.3.1.  WWW-Authenticate . . 154
       14.4.6.  305 Use Proxy  . . . . . . . . . . . . . . . . 172
       11.3.2.  Proxy-Authenticate . . . 155
       14.4.7.  306 (Unused) . . . . . . . . . . . . . . 173
       11.3.3.  Authentication-Info . . . . . . 155
       14.4.8.  307 Temporary Redirect . . . . . . . . . . 173
       11.3.4.  Proxy-Authentication-Info . . . . . 155
       14.4.9.  308 Permanent Redirect . . . . . . . . 174
     11.4.  Response Context . . . . . . . 156
     14.5.  Client Error 4xx . . . . . . . . . . . . . 175
       11.4.1.  Accept-Ranges . . . . . . . 156
       14.5.1.  400 Bad Request  . . . . . . . . . . . . 175
       11.4.2.  Allow . . . . . . 156
       14.5.2.  401 Unauthorized . . . . . . . . . . . . . . . . . 175
       11.4.3.  Server . 156
       14.5.3.  402 Payment Required . . . . . . . . . . . . . . . . 157
       14.5.4.  403 Forbidden  . . . . . . 176
   12. Security Considerations . . . . . . . . . . . . . 157
       14.5.5.  404 Not Found  . . . . . . 177
     12.1.  Establishing Authority . . . . . . . . . . . . . 157
       14.5.6.  405 Method Not Allowed . . . . 177
     12.2.  Risks of Intermediaries . . . . . . . . . . . 158
       14.5.7.  406 Not Acceptable . . . . . 178
     12.3.  Attacks Based on File and Path Names . . . . . . . . . . 179
     12.4.  Attacks Based on Command, Code, or Query Injection . . 158
       14.5.8.  407 Proxy Authentication Required  . 179
     12.5.  Attacks via Protocol Element Length . . . . . . . . 158
       14.5.9.  408 Request Timeout  . . 180
     12.6.  Attacks using Shared-dictionary Compression . . . . . . 180
     12.7.  Disclosure of Personal Information . . . . . . . . 158
       14.5.10. 409 Conflict . . . 181
     12.8.  Privacy of Server Log Information . . . . . . . . . . . 181
     12.9.  Disclosure of Sensitive Information in URIs . . . . . . 182
     12.10. Disclosure of Fragment after Redirects 159
       14.5.11. 410 Gone . . . . . . . . . 182
     12.11. Disclosure of Product Information . . . . . . . . . . . 183
     12.12. Browser Fingerprinting . . 159
       14.5.12. 411 Length Required  . . . . . . . . . . . . . . . 183
     12.13. Validator Retention . 159
       14.5.13. 412 Precondition Failed  . . . . . . . . . . . . . . 160
       14.5.14. 413 Payload Too Large  . . . 184
     12.14. Denial-of-Service Attacks Using Range . . . . . . . . . 184
     12.15. Authentication Considerations . . . 160
       14.5.15. 414 URI Too Long . . . . . . . . . . 185
       12.15.1.  Confidentiality of Credentials . . . . . . . . 160
       14.5.16. 415 Unsupported Media Type . . 185
       12.15.2.  Credentials and Idle Clients . . . . . . . . . . . 186
       12.15.3.  Protection Spaces 160
       14.5.17. 416 Range Not Satisfiable  . . . . . . . . . . . . . 161
       14.5.18. 417 Expectation Failed . . . . 186
       12.15.4.  Additional Response Fields . . . . . . . . . . . 161
       14.5.19. 418 (Unused) . 187
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . 162
       14.5.20. 422 Unprocessable Payload  . . 187
     13.1.  URI Scheme Registration . . . . . . . . . . . 162
       14.5.21. 426 Upgrade Required . . . . . 187
     13.2.  Method Registration . . . . . . . . . . . 162
     14.6.  Server Error 5xx . . . . . . . 187
     13.3.  Status Code Registration . . . . . . . . . . . . . 163
       14.6.1.  500 Internal Server Error  . . . 187
     13.4.  HTTP Field Name Registration . . . . . . . . . . 163
       14.6.2.  501 Not Implemented  . . . . 188
     13.5.  Authentication Scheme Registration . . . . . . . . . . . 189
     13.6.  Content Coding Registration . 163
       14.6.3.  502 Bad Gateway  . . . . . . . . . . . . . 189
     13.7.  Range Unit Registration . . . . . 163
       14.6.4.  503 Service Unavailable  . . . . . . . . . . . 189
     13.8.  Media Type Registration . . . 163
       14.6.5.  504 Gateway Timeout  . . . . . . . . . . . . . 189
     13.9.  Port Registration . . . 164
       14.6.6.  505 HTTP Version Not Supported . . . . . . . . . . . 164
   15. Extending HTTP  . . . . . 189
   14. References . . . . . . . . . . . . . . . . . . 164
     15.1.  Method Extensibility . . . . . . . 189
     14.1.  Normative References . . . . . . . . . . . 165
       15.1.1.  Method Registry  . . . . . . . 189
     14.2.  Informative References . . . . . . . . . . . 165
       15.1.2.  Considerations for New Methods . . . . . . 191
   Appendix A.  Collected ABNF . . . . . 165
     15.2.  Status Code Extensibility  . . . . . . . . . . . . . . 197
   Appendix B.  Changes from previous RFCs . 166
       15.2.1.  Status Code Registry . . . . . . . . . . . . 202
     B.1.  Changes from RFC 2818 . . . . 166
       15.2.2.  Considerations for New Status Codes  . . . . . . . . 166
     15.3.  Field Name Extensibility . . . . . . 202
     B.2.  Changes from RFC 7230 . . . . . . . . . . 167
       15.3.1.  Field Name Registry  . . . . . . . . 202
     B.3.  Changes from RFC 7231 . . . . . . . . 167
       15.3.2.  Considerations for New Field Names . . . . . . . . . 168
       15.3.3.  Considerations for New Field Values  . 203
     B.4.  Changes from RFC 7232 . . . . . . . 169
     15.4.  Authentication Scheme Extensibility  . . . . . . . . . . 171
       15.4.1.  Authentication Scheme Registry . 204
     B.5.  Changes from RFC 7233 . . . . . . . . . . 171
       15.4.2.  Considerations for New Authentication Schemes  . . . 171
     15.5.  Range Unit Extensibility . . . . . 205
     B.6.  Changes from RFC 7235 . . . . . . . . . . . 172
       15.5.1.  Range Unit Registry  . . . . . . . 205
     B.7.  Changes from RFC 7538 . . . . . . . . . 172
       15.5.2.  Considerations for New Range Units . . . . . . . . . 205
     B.8.  Changes from RFC 7615 173
     15.6.  Content Coding Extensibility . . . . . . . . . . . . . . 173
       15.6.1.  Content Coding Registry  . . . . 205
     B.9.  Changes from RFC 7694 . . . . . . . . . . 173
       15.6.2.  Considerations for New Content Codings . . . . . . . 173
     15.7.  Upgrade Token Registry . 205
   Appendix C.  Change Log . . . . . . . . . . . . . . . . 174
   16. Security Considerations . . . . . 205
     C.1.  Between RFC723x and draft 00 . . . . . . . . . . . . . . 205
     C.2.  Since draft-ietf-httpbis-semantics-00 174
     16.1.  Establishing Authority . . . . . . . . . . 206
     C.3.  Since draft-ietf-httpbis-semantics-01 . . . . . . . 175
     16.2.  Risks of Intermediaries  . . . 206
     C.4.  Since draft-ietf-httpbis-semantics-02 . . . . . . . . . . 208
     C.5.  Since draft-ietf-httpbis-semantics-03 . . . 176
     16.3.  Attacks Based on File and Path Names . . . . . . . 208
     C.6.  Since draft-ietf-httpbis-semantics-04 . . . 176
     16.4.  Attacks Based on Command, Code, or Query Injection . . . 177
     16.5.  Attacks via Protocol Element Length  . . . . 209
     C.7.  Since draft-ietf-httpbis-semantics-05 . . . . . . 177
     16.6.  Attacks using Shared-dictionary Compression  . . . . 210
     C.8.  Since draft-ietf-httpbis-semantics-06 . . 178
     16.7.  Disclosure of Personal Information . . . . . . . . . . 211
     C.9.  Since draft-ietf-httpbis-semantics-07 . 178
     16.8.  Privacy of Server Log Information  . . . . . . . . . . 212
     C.10. Since draft-ietf-httpbis-semantics-08 . 179
     16.9.  Disclosure of Sensitive Information in URIs  . . . . . . 179
     16.10. Disclosure of Fragment after Redirects . . . 214
     C.11. Since draft-ietf-httpbis-semantics-09 . . . . . . 180
     16.11. Disclosure of Product Information  . . . . . . . 215
     C.12. Since draft-ietf-httpbis-semantics-10 . . . . 180
     16.12. Browser Fingerprinting . . . . . . 215
   Acknowledgments . . . . . . . . . . . 181
     16.13. Validator Retention  . . . . . . . . . . . . . . 217
   Authors' Addresses . . . . 182
     16.14. Denial-of-Service Attacks Using Range  . . . . . . . . . 182
     16.15. Authentication Considerations  . . . . . . . . . . 217

1.  Introduction

1.1.  Purpose

   The Hypertext Transfer Protocol (HTTP) is a family . . . 183
       16.15.1.  Confidentiality of stateless,
   application-level, request/response protocols that share a generic
   interface, extensible semantics, Credentials  . . . . . . . . . . 183
       16.15.2.  Credentials and self-descriptive messages to
   enable flexible interaction with network-based hypertext information
   systems. 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 hides the details of how a service is implemented by presenting
   a uniform interface to clients that is independent of the types 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

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 (HTTP/0.9). pathname.  This original protocol is now referred to as
   HTTP/0.9.

   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 2.5), 3.1), regardless of its type, nature, or implementation, by
   sending messages that manipulate or transfer representations
   (Section 7).

   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 (Section 9), fields, status codes that describe the response
   (Section 10), 14), and other control data and resource metadata that might
   be given in response fields (Section 11). 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 7.4). 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].

1.5.  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 ---------

                                 Table 1

   [*] This document only when, they appear in all
   capitals, as shown here.

   Conformance criteria obsoletes the portions of RFC 7230 that are
   independent of the HTTP/1.1 messaging syntax and considerations regarding error handling connection
   management; the remaining bits of RFC 7230 are
   defined in Section 3.

1.6. obsoleted by "HTTP/1.1
   Messaging" [Messaging].

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 5.5, 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 5.4.1 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].

1.6.1.  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.

   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

2.  Architecture

   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.

2.1.  Client/Server Messaging

   HTTP is a stateless request/response protocol that operates by
   exchanging messages across 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.  The term
   "user agent" refers to any of the various client programs that
   initiate a request, including (but not limited to) browsers, spiders
   (web-based robots), command-line tools, custom applications, and
   mobile apps.  The term "origin server" refers to the program that can
   originate authoritative responses 5.7 defines some generic syntactic components for a given target resource. field
   values.

   The rule below is defined in [Messaging];

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

   This specification uses the terms "sender" "character", "character encoding
   scheme", "charset", and "recipient" refer "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 any implementation that sends
   or receives a given message, respectively.

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

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

   Additional (social) requirements are placed on implementations,
   resource owners, and protocol element registrations when they apply
   beyond the origin server
   (O).

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

                                  Figure 1

   Each major version of HTTP defines its own syntax for the
   communication scope of messages.  Nevertheless, a common abstraction single communication.

   The verb "generate" is
   that each message contains some form used instead of envelope/framing with self-
   descriptive control data "send" where a requirement
   applies only to implementations that indicates its semantics and routing, create the protocol element,
   rather than an implementation that forwards a
   potential set received element
   downstream.

   An implementation is considered conformant if it complies with all of named fields up front (a header section), a
   potential body, and potential fields sent after
   the body begins
   (trailer sections).

   A client sends requests to a server requirements associated with the roles it partakes in HTTP.

   Conformance includes both the form of a request message
   with a method (Section 8) and request target.  The request might also
   contain header fields for request modifiers, client information, syntax and
   representation metadata (Section 5), semantics of protocol
   elements.  A sender MUST NOT generate protocol elements that convey a payload body (Section 7.3.3)
   meaning that is known by that sender to be processed in accordance with the method, and trailer fields for
   metadata collected while sending the payload. false.  A server responds to a client's request sender MUST NOT
   generate protocol elements that do not match the grammar defined by sending one or more
   response messages, each including
   the corresponding ABNF rules.  Within a status code (Section 10).  The
   response might also contain header fields for server information,
   resource metadata, and representation metadata (Section 5), given message, a payload
   body (Section 7.3.3) sender MUST
   NOT generate protocol elements or syntax alternatives that are only
   allowed to be interpreted generated by participants in accordance with other roles (i.e., a role
   that the status
   code, and trailer fields sender does not have for metadata collected while sending the
   payload.

   One of the functions of message framing that message).

2.3.  Length Requirements

   When a received protocol element is parsed, the recipient MUST be
   able to assure parse any value of reasonable length that messages
   are complete.  A message is considered complete when all of applicable to
   the
   octets indicated by its framing are available.  Note that, when no
   explicit framing is used, a response message recipient's role and that is ended matches the grammar defined by the
   transport connection's close is considered complete even though it
   corresponding ABNF rules.  Note, however, that some received protocol
   elements might not be indistinguishable from parsed.  For example, an incomplete response, unless intermediary
   forwarding a
   transport-level error indicates that it is 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 complete.

   A connection have specific length limitations for many of its
   protocol elements because the lengths that might be used for multiple request/response exchanges.
   The mechanism used to correlate between request appropriate will
   vary widely, depending on the deployment context and response messages
   is version dependent; some versions of HTTP use implicit ordering purpose of
   messages, while others use an explicit identifier.

   Responses (both final the
   implementation.  Hence, interoperability between senders and interim) can be sent at any time after a
   request is received, even if it
   recipients depends on shared expectations regarding what is not yet complete.  However,
   clients (including intermediaries) might abandon a request if the
   response
   reasonable length for each protocol element.  Furthermore, what is not forthcoming within
   commonly understood to be a reasonable period of time.

   The following example illustrates a typical message exchange length 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.

2.2.  Intermediaries

   HTTP enables some protocol
   elements has changed over the use of intermediaries to satisfy requests through a
   chain course of connections.  There are three common forms the past two decades of HTTP
   intermediary: proxy, gateway,
   use and tunnel.  In some cases, is expected to continue changing in the future.

   At a single
   intermediary might act 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, 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 server that publishes very long URI references to
   its own resources needs to be able to parse and origin server. process those same
   references when received as a target URI.

2.4.  Error Handling

   A request or response message that
   travels recipient MUST interpret a received protocol element according to
   the whole chain will pass through four separate connections.
   Some HTTP communication options 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 apply only to disregard the connection
   with contents of a received
   Accept-Encoding header field if inspection of the nearest, non-tunnel neighbor, only User-Agent header
   field indicates a specific implementation version that is known to the endpoints
   fail on receipt of the
   chain, or certain content codings.

   Unless noted otherwise, a recipient MAY attempt to all connections along the chain.  Although 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 diagram
   is linear, each participant might be engaged in multiple,
   simultaneous communications. protocol
   require different error handling strategies.  For example, B a Web
   browser might be receiving
   requests wish to transparently recover from many clients other than A, and/or forwarding requests a response where the
   Location header field doesn't parse according to servers other than C, at the same time that it is handling A's
   request.  Likewise, later requests ABNF, whereas a
   systems control client might consider any form of error recovery to
   be sent through dangerous.

   Some requests can be automatically retried by a different
   path client in the event
   of connections, often based on dynamic configuration an underlying connection failure, as described in Section 8.2.2.

3.  Terminology

   HTTP was created for load
   balancing.

   The terms "upstream" the World Wide Web (WWW) architecture and "downstream" are used has
   evolved over time to describe
   directional requirements support the scalability needs of a worldwide
   hypertext system.  Much of that architecture is reflected in relation to the message flow: all
   messages flow from upstream
   terminology and syntax productions used to downstream. define HTTP.

3.1.  Resources

   The terms "inbound" and
   "outbound" are 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 describe directional requirements interact with resources.  Most resources are
   identified by a Uniform Resource Identifier (URI), as described in relation
   Section 4.

   One design goal of HTTP is to separate resource identification from
   request semantics, which is made possible by vesting the request route: "inbound" means toward
   semantics in the origin server request method (Section 8) and
   "outbound" means toward the user agent.

   A "proxy" is a message-forwarding agent that few request-
   modifying header fields.  If there is selected a conflict between the method
   semantics and any semantic implied 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 itself, as described in
   Section 8.2.1, the method semantics take precedence.

   HTTP interface.  Some
   translations are minimal, such as for proxy requests for "http" URIs,
   whereas other requests might require translation relies upon the Uniform Resource Identifier (URI) standard
   [RFC3986] to indicate the target resource (Section 6.1) and from entirely
   different application-level protocols.  Proxies are often used to
   group an organization's
   relationships between resources.

3.2.  Connections

   HTTP requests through is a common intermediary
   for the sake of security, annotation services, or shared caching.
   Some proxies are designed to apply transformations to selected
   messages client/server protocol that operates over a reliable
   transport- or payloads while they are being forwarded, as described in
   Section 6.6.2.

   A "gateway" (a.k.a. "reverse proxy") session-layer "connection".

   An HTTP "client" is an intermediary a program that acts as
   an origin server for the outbound establishes a connection but translates received
   requests and forwards them inbound to another a
   server for the purpose of sending one or servers.
   Gateways are often used more HTTP requests.  An HTTP
   "server" is a program that accepts connections in order to encapsulate legacy or untrusted
   information services, service
   HTTP requests by sending HTTP responses.

   The terms "client" and "server" refer only to improve the roles that these
   programs perform for a particular connection.  The same program might
   act as a client on some connections and a server performance through
   "accelerator" caching, 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 enable partitioning any implementation that sends or load balancing
   of HTTP services across multiple machines.

   All HTTP requirements applicable receives a given message,
   respectively.

   A client sends requests to an origin a server also apply to in the outbound communication form of a gateway.  A gateway communicates request message
   with
   inbound servers using any protocol that it desires, including private
   extensions 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 HTTP that are outside be processed in accordance with the scope of this specification.
   However, an HTTP-to-HTTP gateway that wishes method,
   and trailer fields (Section 5.6) for metadata collected while sending
   the payload.

   A server responds to interoperate with
   third-party HTTP servers ought 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 conform 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 requirements
   on is the gateway's inbound connection.

   A "tunnel" acts as general-purpose Web
   browser, but that's only a blind relay between two connections without
   changing the messages.  Once active, small percentage of implementations.
   Other common user agents include spiders (web-traversing robots),
   command-line tools, billboard screens, household appliances, scales,
   light bulbs, firmware update scripts, mobile apps, and communication
   devices in a tunnel is multitude of shapes and sizes.

   Being a user agent does not considered imply that there is a
   party to human user directly
   interacting with the HTTP communication, though software agent at the tunnel might have been
   initiated by an HTTP request.  A tunnel ceases to exist when both
   ends time of the relayed connection are closed.  Tunnels are used to
   extend a virtual connection through an intermediary, such as when
   Transport Layer Security (TLS, [RFC8446]) request.  In
   many cases, a user agent is used installed or configured to establish
   confidential communication through a shared firewall proxy.

   The above categories run in the
   background and save its results for intermediary later inspection (or save only consider a
   subset of those acting as
   participants in the HTTP communication.  There are also
   intermediaries results that can act on lower layers of the network protocol
   stack, filtering or redirecting HTTP traffic without the knowledge might be interesting or
   permission of message senders.  Network intermediaries erroneous).
   Spiders, for example, are
   indistinguishable (at typically given a protocol level) from an on-path attacker,
   often introducing security flaws or interoperability problems due start URI and configured
   to
   mistakenly violating HTTP semantics.

   For example, an "interception proxy" [RFC3040] (also commonly known follow certain behavior while crawling the Web as a "transparent proxy" [RFC1919] hypertext
   graph.

   Many user agents cannot, or "captive portal") differs from
   an HTTP proxy because 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 not selected 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 client.  Instead, an
   interception proxy filters user before
   proceeding might be met via advance configuration choices, run-time
   options, 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 simple avoidance of
   enforcing account subscription prior to allowing use the unsafe action; confirmation does
   not imply any specific user interface or interruption of non-local
   Internet services, and within corporate firewalls normal
   processing if the user has already made that choice.

3.5.  Origin Server

   The term "origin server" refers to enforce network
   usage policies.

   HTTP is defined as a stateless protocol, meaning program 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, originate
   authoritative responses for a given target resource.

   The most familiar form of origin server MUST NOT assume that two requests on the same connection are
   from the same large public websites.
   However, like user agent unless the connection agents being equated with browsers, it is secured and
   specific easy to
   be misled into thinking that agent.  Some non-standard HTTP extensions (e.g.,
   [RFC4559]) have been known to violate this requirement, resulting in
   security 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 interoperability problems.

2.3.  Caches

   A "cache" is video-on-demand
   platforms.

3.6.  Example Request and Response

   Most HTTP communication consists of a local store retrieval request (GET) for a
   representation of previous response messages and some resource identified by a URI.  In the
   subsystem that controls its message storage, retrieval, and deletion.
   A cache stores cacheable responses in order to reduce simplest
   case, this might be accomplished via a single bidirectional
   connection (===) between the response
   time user agent (UA) and network bandwidth consumption on future, equivalent
   requests.  Any client or the origin server MAY employ a cache, though a cache
   cannot be used by
   (O).

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

                                  Figure 1

   The following example illustrates a server while it is acting as typical message exchange for a tunnel.

   The effect of
   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 cache is that trailing CRLF.

3.7.  Intermediaries

   HTTP enables the request/response chain is shortened
   if one use of the participants along the chain has a cached response
   applicable intermediaries to that request.  The following illustrates the resulting
   chain if B has satisfy requests through a cached copy
   chain of an earlier response from O (via C)
   for connections.  There are three common forms of HTTP
   intermediary: proxy, gateway, and tunnel.  In some cases, a request that has not been cached by UA single
   intermediary might act as an origin server, proxy, gateway, or A.
   tunnel, switching behavior based on the nature of each request.

            >             >             >             >
       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

   The figure above shows three intermediaries (A, B, and configurations of caches
   deployed across C) between the World Wide Web
   user agent 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 origin server.  A request or high-latency environments, and so on.

2.4.  Uniform Resource Identifiers

   Uniform Resource Identifiers (URIs) [RFC3986] are used throughout
   HTTP as the means for identifying resources (Section 2.5).  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 response message that can contain a non-empty path
   component.  (This rule differs slightly from
   travels the path-abempty rule of
   RFC 3986, which allows for an empty path whole chain will pass through four separate connections.
   Some HTTP communication options might apply only to be used in references,
   and path-absolute rule, which does not allow paths that begin the connection
   with
   "//".)  A "partial-URI" rule is defined for protocol elements that
   can contain a relative URI but not a fragment component.

     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), nearest, non-tunnel neighbor, only to the path and optional query components, or some
   combination endpoints of the above.  Unless otherwise indicated, URI references
   are parsed relative
   chain, or to all connections along the target URI (Section 6.1).

   It chain.  Although the diagram
   is RECOMMENDED that all senders and recipients support, at a
   minimum, URIs with lengths of 8000 octets linear, each participant might be engaged in protocol elements.  Note
   that this implies some structures and on-wire representations (for multiple,
   simultaneous communications.  For example, the request line in HTTP/1.1) will necessarily B might be larger in
   some cases.

2.5.  Resources

   The target of an HTTP request is called a "resource".  HTTP does not
   limit receiving
   requests from many clients other than A, and/or forwarding requests
   to servers other than C, at the nature of a resource; it merely defines an interface 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 interact with resources.  Most resources are
   identified by a Uniform Resource Identifier (URI), as described describe
   directional requirements in
   Section 2.4.

   One design goal of HTTP is relation to separate resource identification from
   request semantics, which is made possible by vesting the request
   semantics 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 method (Section 8) route: "inbound" means toward the origin server and a few request-
   modifying header fields (Section 9).  If there
   "outbound" means toward the user agent.

   A "proxy" is a conflict between
   the method semantics and any semantic implied message-forwarding agent that is chosen by the URI itself, as
   described in Section 8.2.1, the method semantics take precedence.

   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 client,
   usually via local configuration rules, to
   HTTP servers:

    ------------ ------------------------------------ -------
     URI Scheme   Description                          Ref.
    ------------ ------------------------------------ -------
     http         Hypertext Transfer Protocol          2.5.1
     https        Hypertext Transfer Protocol Secure   2.5.2
    ------------ ------------------------------------ -------

                             Table 2

   Note that the presence receive requests for some
   type(s) of an "http" or "https" absolute 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 attempt to a resource.  The delegated nature of registered names
   and IP addresses creates a federated namespace whether or not an HTTP
   server is present.

2.5.1.  http URI Scheme

   The "http" URI scheme is hereby defined for minting identifiers
   within satisfy those requests via
   translation through the hierarchical namespace governed by a potential HTTP origin
   server listening interface.  Some translations are
   minimal, such as for TCP ([RFC0793]) connections on a given port.

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

   The origin server proxy requests 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 URIs, whereas other
   requests might require translation to respond
   authoritatively and from entirely different
   application-level protocols.  Proxies are often used to group an
   organization's HTTP requests that target through a common intermediary for the identified resource,
   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
   defined described in Section 6.3.3.1. 6.5.

   A sender MUST NOT generate an "http" URI with "gateway" (a.k.a. "reverse proxy") is an empty host
   identifier.  A recipient intermediary that processes such a URI reference MUST
   reject it acts as invalid.

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

2.5.2.  https URI Scheme

   The "https" URI scheme is hereby defined server for minting identifiers
   within the hierarchical namespace governed by a potential origin outbound connection but translates received
   requests and forwards them inbound to another server listening for TCP connections on a given port or servers.
   Gateways are often used to encapsulate legacy or untrusted
   information services, to improve server performance through
   "accelerator" caching, and capable to enable partitioning or load balancing
   of
   establishing a TLS ([RFC8446]) connection that has been secured for HTTP communication.  In this context, "secured" specifically means
   that the services across multiple machines.

   All HTTP requirements applicable to an origin server has been authenticated as acting on behalf of also apply to
   the
   identified authority and all HTTP outbound communication of a gateway.  A gateway communicates with
   inbound servers using any protocol that server has
   been protected for confidentiality and integrity through it desires, including private
   extensions to HTTP that are outside the use scope of
   strong encryption.

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

   The origin server for this specification.
   However, an "https" URI is identified by HTTP-to-HTTP gateway that wishes to interoperate with
   third-party HTTP servers ought to conform to user agent requirements
   on the authority
   component, which includes gateway's inbound connection.

   A "tunnel" acts as a host identifier and optional port number
   ([RFC3986], Section 3.2.2).  If blind relay between two connections without
   changing the port subcomponent messages.  Once active, a tunnel is empty or not
   given, TCP port 443 (the reserved port for considered a
   party to the HTTP over TLS) is communication, though the
   default.  The origin determines who has tunnel might have been
   initiated by an HTTP request.  A tunnel ceases to exist when both
   ends of the right relayed connection are closed.  Tunnels are used to respond
   authoritatively
   extend a virtual connection through an intermediary, such as when
   Transport Layer Security (TLS, [RFC8446]) is used to requests that target the identified resource, establish
   confidential communication through a shared firewall proxy.

   The above categories for intermediary only consider those acting as
   defined
   participants in Section 6.3.3.2.

   A sender MUST NOT generate 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 "https" URI with on-path attacker,
   often introducing security flaws or interoperability problems due to
   mistakenly violating HTTP semantics.

   For example, an empty host
   identifier.  A recipient that processes such "interception proxy" [RFC3040] (also commonly known
   as a URI reference MUST
   reject "transparent proxy" [RFC1919] or "captive portal") differs from
   an HTTP proxy because it as invalid.

   The hierarchical path component and optional query component identify is not chosen by the target resource within that origin server's name space.

   A client MUST ensure that its HTTP requests for client.  Instead, an "https" resource
   interception proxy filters or redirects outgoing TCP port 80 packets
   (and occasionally other common port traffic).  Interception proxies
   are secured, commonly found on public network access points, as a means of
   enforcing account subscription prior to being communicated, allowing use of non-local
   Internet services, 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 within corporate firewalls to enforce network
   usage policies.

   HTTP that is defined
   to apply to all origins with the same host, such as the Cookie
   protocol [RFC6265], a stateless protocol, meaning that each request
   message can allow information set by one service be understood in isolation.  Many implementations depend
   on HTTP's stateless design in order to
   impact communication with other services within reuse proxied connections or
   dynamically load balance requests across multiple servers.  Hence, a matching group of
   host domains.

2.5.3.  http and https URI Normalization and Comparison

   Since the "http" and "https" schemes conform to
   server MUST NOT assume that two requests on the URI generic
   syntax, such URIs same connection are normalized and compared according to the
   algorithm defined in Section 6 of [RFC3986], using
   from the defaults
   described above for each scheme.

   If same user agent unless the port connection is equal secured and
   specific to the default port for 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 scheme, local store of previous response messages and the normal
   form is
   subsystem that controls its message storage, retrieval, and deletion.
   A cache stores cacheable responses in order to omit reduce the port subcomponent.  When not being 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 the
   target a tunnel.

   The effect of an OPTIONS request, an empty path component a cache is equivalent
   to an absolute path of "/", so that the normal form request/response chain is to provide a path shortened
   if one 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: participants along the
   normal form is chain has a cached response
   applicable 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

2.5.4.  Deprecated userinfo that request.  The URI generic syntax following illustrates the resulting
   chain if B has a cached copy of an earlier response from O (via C)
   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" request that has not been cached by UA or A.

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

                                  Figure 3

   A response is "cacheable" if a cache is deprecated.

   Some implementations make use allowed to store a copy of
   the userinfo component response message for internal
   configuration of authentication information, such as within command
   invocation options, configuration files, or bookmark lists, even
   though such usage might expose use in answering subsequent requests.  Even
   when a user identifier response is cacheable, there might be additional constraints
   placed by the client or password.

   A sender MUST NOT generate by the userinfo subcomponent (and its "@"
   delimiter) origin server on when an "http" or "https" URI reference is generated
   within that cached
   response can be used for a message as particular request.  HTTP requirements for
   cache behavior and cacheable responses are defined in Section 2 of
   [Caching].

   There is a target URI or field value.

   Before making use wide variety of an "http" 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 "https" URI reference received from
   an untrusted source, a recipient SHOULD parse
   multicast cache entries, archives of pre-fetched cache entries for userinfo
   use in off-line or high-latency environments, and treat
   its presence as an error; it is likely being so on.

4.  Identifiers

   Uniform Resource Identifiers (URIs) [RFC3986] are used to obscure throughout
   HTTP as the
   authority means for the sake of phishing attacks.

2.5.5.  Fragment Identifiers on http(s) identifying resources (Section 3.1).

4.1.  URI References

   Fragment identifiers allow for indirect identification of a secondary
   resource, independent

   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 scheme, as generic syntax.  An "absolute-path" rule is
   defined in Section 3.5 of
   [RFC3986].  Some for protocol elements that refer to a URI allow
   inclusion of can contain a fragment, while others do not.  They are distinguished
   by use of non-empty path
   component.  (This rule differs slightly from the ABNF path-abempty rule of
   RFC 3986, which allows for elements where fragment is allowed;
   otherwise, a specific rule that excludes fragments is an empty path to be used (see
   Section 6.1).

      |  *Note:* the fragment identifier component is in references,
   and path-absolute rule, which does not part of the
      |  actual scheme definition allow paths that begin with
   "//".)  A "partial-URI" rule is defined for protocol elements that
   can contain a relative URI scheme (see but not a fragment component.

     URI-reference = <URI-reference, see [RFC3986], Section 4.3 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
      |  [RFC3986]), thus does not appear reference (URI-reference), only a URI in absolute form (absolute-
   URI), only the ABNF definitions for
      |  the "http" path and "https" URI schemes optional query components, or some
   combination of the above.

3.  Conformance

3.1.  Implementation Diversity

   When considering  Unless otherwise indicated, URI references
   are parsed relative to the design of HTTP, it target URI (Section 6.1).

   It is easy to fall into RECOMMENDED that all senders and recipients support, at a trap
   minimum, URIs with lengths of thinking 8000 octets in protocol elements.  Note
   that all user agents are general-purpose browsers this implies some structures and all
   origin servers are large public websites.  That is not on-wire representations (for
   example, the case request line in
   practice.  Common HTTP user agents include household appliances,
   stereos, scales, firmware update scripts, command-line programs,
   mobile apps, and communication devices HTTP/1.1) will necessarily be larger in a multitude
   some cases.

4.2.  URI Schemes

   IANA maintains the registry of shapes and
   sizes.  Likewise, common 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 origin servers include home automation
   units, configurable networking components, office machines,
   autonomous robots, news feeds, traffic cameras, ad selectors, and
   video-delivery platforms.

   The term "user agent" servers:

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

                             Table 2

   Note that the presence of an "http" or "https" URI does not imply
   that there is a human user
   directly interacting with the software agent always an HTTP server 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 identified origin
   listening for later inspection (or
   save only connections.  Anyone can mint a subset of those results that might be interesting URI, whether or
   erroneous).  Spiders, for example, are typically given not a start URI
   server exists and configured whether or not that server currently maps that
   identifier to follow certain behavior while crawling the Web as a
   hypertext graph. resource.  The implementation diversity delegated nature of HTTP means that not all user agents
   can make interactive suggestions to their user or provide adequate
   warning for security registered names
   and IP addresses creates a federated namespace whether or privacy concerns.  In the few cases where
   this specification requires reporting of errors to the user, it not an HTTP
   server is
   acceptable present.

4.2.1.  http URI Scheme

   The "http" URI scheme is hereby defined 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.

3.2.  Role-based Requirements

   This specification targets conformance criteria according to minting identifiers
   within the role
   of hierarchical namespace governed by a participant in HTTP communication.  Hence, potential HTTP 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.
   Additional (social) requirements are placed
   server listening for TCP ([RFC0793]) connections on implementations,
   resource owners, and protocol element registrations when they apply
   beyond the scope of a single communication. given port.

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

   The verb "generate" origin server for an "http" URI is used instead of "send" where a requirement
   differentiates between creating identified by the authority
   component, which includes a protocol element host identifier and merely
   forwarding a received element downstream.

   An implementation 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 considered conformant if it complies with all of the requirements associated with
   default.  The origin determines who has the roles it partakes in HTTP.

   Conformance includes both right to respond
   authoritatively to requests that target the syntax and semantics of protocol
   elements. identified resource, as
   defined in Section 4.3.2.

   A sender MUST NOT generate protocol elements an "http" URI with an empty host
   identifier.  A recipient that convey processes such a
   meaning that is known by that sender to be false.  A sender URI reference MUST NOT
   generate protocol elements that do not match
   reject it as invalid.

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

4.2.2.  https URI Scheme

   The "https" URI scheme is hereby defined by for minting identifiers
   within the corresponding ABNF rules.  Within hierarchical namespace governed by a given message, potential origin
   server listening for TCP connections on a sender MUST
   NOT generate protocol elements or syntax alternatives that are only
   allowed to be generated by participants in other roles (i.e., given port and capable of
   establishing a role TLS ([RFC8446]) connection that the sender does not have has been secured for
   HTTP communication.  In this context, "secured" specifically means
   that message).

3.3.  Parsing Elements

   When a received protocol element is parsed, the recipient MUST be
   able to parse any value server has been authenticated as acting on behalf of reasonable length that is applicable to the recipient's role
   identified authority and all HTTP communication with 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 server has
   been protected for confidentiality and
   field value components, but then forward the field without further
   parsing inside integrity through the field value.

   HTTP does not have specific length limitations for many use of its
   protocol elements because the lengths that might be appropriate will
   vary widely, depending on
   strong encryption.

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

   The origin server for an "https" URI is identified by the deployment context authority
   component, which includes a host identifier and purpose of optional port number
   ([RFC3986], Section 3.2.2).  If the
   implementation.  Hence, interoperability between senders and
   recipients depends on shared expectations regarding what is a
   reasonable length for each protocol element.  Furthermore, what port subcomponent is
   commonly understood to be a reasonable length empty or not
   given, TCP port 443 (the reserved port for some protocol
   elements has changed over the course of the past two decades of HTTP
   use and over TLS) 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
   default.  The origin determines who has the values right to respond
   authoritatively to requests that it
   generates for those same protocol elements target the identified resource, as
   defined in other messages.  For
   example, Section 4.3.3.

   A sender MUST NOT generate an origin server "https" URI with an empty host
   identifier.  A recipient that publishes very long processes such a URI references to
   its own resources needs to be able to parse and process those same
   references when received reference MUST
   reject it as a invalid.

   The hierarchical path component and optional query component identify
   the target URI.

3.4.  Error Handling resource within that origin server's name space.

   A recipient client MUST interpret a received protocol element according to
   the semantics defined ensure that its HTTP requests for it by this specification, including
   extensions an "https" resource
   are secured, prior to this specification, unless the recipient has determined
   (through experience or configuration) being communicated, and that the sender incorrectly
   implements what is implied by it only accepts
   secured responses to those semantics.  For example, an
   origin server might disregard requests.

   Resources made available via the contents of a received
   Accept-Encoding header field if inspection of "https" scheme have no shared
   identity with the User-Agent header
   field indicates a specific implementation version "http" scheme.  They are distinct origins with
   separate namespaces.  However, an extension to HTTP that is known defined
   to
   fail on receipt of certain content codings.

   Unless noted otherwise, a recipient MAY attempt apply 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 all origins with the same host, such as the Cookie
   protocol
   require different error handling strategies.  For example, a Web
   browser might wish [RFC6265], can allow information set by one service to transparently recover from
   impact communication with other services within a response where matching group of
   host domains.

4.2.3.  http(s) Normalization and Comparison

   Since the
   Location header field doesn't parse according "http" and "https" schemes conform to the ABNF, whereas a
   systems control client might consider any form of error recovery URI generic
   syntax, such URIs are normalized and compared according to
   be dangerous.

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

4.  Extending and Versioning HTTP

   While HTTP's core semantics don't change between protocol versions,
   the expression 6 of them "on [RFC3986], using the wire" can change, and so defaults
   described above for each scheme.

   If the HTTP
   version number changes when incompatible changes are made port is equal to the wire
   format.  Additionally, HTTP allows incremental, backwards-compatible
   changes to be made default port for a scheme, the normal
   form is to omit the protocol without changing its version
   through port subcomponent.  When not being used as the use of defined extension points.

4.1.  Extending HTTP

   HTTP defines a number
   target of generic extension points that can be used to
   introduce capabilities an OPTIONS request, an empty path component is equivalent
   to an absolute path of "/", so the protocol without introducing normal form is to provide a new
   version, including methods (Section 8.4), status codes
   (Section 10.7), header path
   of "/" instead.  The scheme and trailer fields (Section 5.7), host are case-insensitive and further
   extensibility points within defined fields (such as Cache-Control
   normally provided in lowercase; all other components are compared in
   a case-sensitive manner.  Characters other than those in
   Section 5.2.3 of [Caching]).  Because the semantics of HTTP are not
   versioned, these extension points
   "reserved" set are persistent; equivalent to their percent-encoded octets: the version
   normal form is to not encode them (see Sections 2.1 and 2.2 of
   [RFC3986]).

   For example, the
   protocol in use does not affect their semantics.

   Version-independent extensions following three URIs are discouraged from depending on or
   interacting with equivalent:

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

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 specific version URI.  In that subcomponent, the use
   of the protocol in use.  When
   this format "user:password" is unavoidable, careful consideration needs to be given to how deprecated.

   Some implementations make use of the extension can interoperate across versions.

   Additionally, specific versions userinfo component for internal
   configuration of HTTP might have their own
   extensibility points, authentication information, such as transfer-codings in HTTP/1.1
   (Section 6.1 of [Messaging]) and HTTP/2 ([RFC7540]) SETTINGS within command
   invocation options, configuration files, or frame
   types.  These extension points are specific to the version of the
   protocol they occur within.

   Version-specific extensions cannot override bookmark lists, even
   though such usage might expose a user identifier or modify password.

   A sender MUST NOT generate the semantics
   of a version-independent mechanism userinfo subcomponent (and its "@"
   delimiter) when an "http" or extension point (like "https" URI reference is generated
   within a method message as a target URI or header field) without explicitly 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 allowed by that protocol
   element.  For example, the CONNECT method (Section 8.3.6) allows
   this.

   These guidelines assure that used to obscure the protocol operates correctly and
   predictably, even when parts of
   authority for the path implement different versions sake of HTTP.

4.2.  Protocol Versioning

   The HTTP version number consists phishing attacks.

4.2.5.  http(s) References with Fragment Identifiers

   Fragment identifiers allow for indirect identification of two decimal digits separated by a
   "." (period or decimal point).  The first digit ("major version")
   indicates the HTTP messaging syntax, whereas the second digit ("minor
   version") indicates secondary
   resource, independent of the highest minor version within URI scheme, as defined in Section 3.5 of
   [RFC3986].  Some protocol elements that major
   version refer to which a URI allow
   inclusion of a fragment, while others do not.  They are distinguished
   by use of the sender is conformant and able to understand ABNF rule for
   future communication.

   The protocol version as elements where fragment is allowed;
   otherwise, a whole indicates specific rule that excludes fragments is used (see
   Section 6.1).

      |  *Note:* the sender's conformance
   with fragment identifier component is not part of the set
      |  actual scheme definition for a URI scheme (see Section 4.3 of requirements laid out
      |  [RFC3986]), thus does not appear in that version's corresponding
   specification of HTTP.  For example, the version "HTTP/1.1" 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
   defined by the combined specifications triple of this document, "HTTP
   Caching" [Caching], scheme, host, and "HTTP/1.1 Messaging" [Messaging].

   The minor version advertises port
   after normalizing the sender's communication capabilities
   even when scheme and host to lowercase and normalizing
   the sender port to remove any leading zeros.  If port is only using a backwards-compatible subset of elided from the protocol, thereby letting
   URI, the recipient know default port for that more advanced
   features scheme is used.  For example, the URI
      https://Example.Com/happy.js

   would have the origin

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

   which can also be used in response (by servers) or in future requests
   (by clients).

   A client SHOULD send a request version equal 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 highest version
   origin responds to which valid requests, consistently over time, determines
   the client is conformant semantics that users will associate with a URI, and whose major version is no
   higher than the highest version supported by the server, if this
   usefulness of those semantics is
   known.  A client MUST NOT send what ultimately transforms these
   mechanisms into a version "resource" for users 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 reference and determined
   from access in the response status code
   future.

   Two origins are distinct if they differ in scheme, host, or header fields (e.g., Server) port.
   Even when it can be verified that the server improperly handles higher request versions.

   A server SHOULD send a response version equal to the highest version
   to which same entity controls two
   distinct origins, the two namespaces under those origins are distinct
   unless explicitly aliased by a server is conformant authoritative for that has a major version less than
   or equal to origin.

   Origin is also used within HTML and related Web protocols, beyond the one received
   scope of this document, as described in the request.  A server MUST NOT send
   a version to which it [RFC6454].

4.3.2.  http origins

   Although HTTP is not conformant.  A server can send a 505
   (HTTP Version Not Supported) response if it wishes, for any reason,
   to refuse service independent of the client's major protocol version.

   HTTP's major version number is incremented when an incompatible
   message syntax is introduced.  The minor number transport protocol, the "http"
   scheme (Section 4.2.1) is incremented when
   changes made specific to associating authority with
   whomever controls the protocol have origin server listening for TCP connections on
   the effect indicated port of adding to whatever host is identified within the message
   semantics or implying additional capabilities
   authority component.  This is a very weak sense of the sender.

   When authority because
   it depends on both client-specific name resolution mechanisms and
   communication that might not be secured from an HTTP message on-path attacker.
   Nevertheless, it is received with a major version number that the
   recipient implements, but sufficient minimum for binding "http"
   identifiers to an origin server for consistent resolution within a higher minor version number than what the
   recipient implements, the recipient SHOULD process the message as if
   it were in
   trusted environment.

   If the highest minor version within that major version to
   which host identifier is provided as an IP address, the recipient origin
   server is conformant.  A recipient can assume the listener (if any) on the indicated TCP port at that IP
   address.  If host is a
   message registered name, the registered name is an
   indirect identifier for use with a higher minor version, when sent name resolution service, such as
   DNS, to a recipient that
   has not yet indicated support find an address for that higher version, is
   sufficiently backwards-compatible to be safely processed by any
   implementation of the same major version. an appropriate origin server.

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

5.  Header and Trailer Fields

   HTTP messages use key/value pairs calls for access to convey data about the message,
   its payload,
   the target indicated 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 client MAY attempt access by resolving the
   host identifier to some named
   fields specifically as an IP address, establishing a "header field" when they are only allowed TCP connection to
   be sent in the header section.

   Fields
   that are sent/received after the header section has ended
   (usually after address on the indicated port, and sending an HTTP request
   message body begins to stream) are referred the server containing the URI's identifying data.

   If the server responds to as
   "trailer fields" (or just "trailers", colloquially) and located
   within such 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 request with a "field name" (see non-interim HTTP
   response message, as described in Section 5.3) identifying the field, and a "field
   line value" 14, then that conveys data for response is
   considered an authoritative answer to 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 client's request.

   Note, however, that section, concatenated together and
   separated with commas.  See Section 5.1 for further discussion of the
   semantics of field ordering and combination in messages, and
   Section 5.4 above is not the only means for more discussion of field values. obtaining an
   authoritative response, nor does it imply that an authoritative
   response is always necessary (see [Caching]).  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 Alt-
   Svc header field value [RFC7838] allows an origin server to identify other
   services that are also authoritative 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 that origin.  Access to
   "http" identified resources might also be provided by protocols
   outside the same major HTTP version, though scope of this document.

4.3.3.  https origins

   The "https" scheme (Section 4.2.2) associates authority based on the default behavior
   ability of a
   recipient in server to use the absence of such private key corresponding to a field can change.  Unless
   specified otherwise, fields are defined for all versions of HTTP.  In
   particular,
   certificate that the Host and Connection fields ought client considers 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 trustworthy for the protocol version if
   their defined semantics allow them
   identified origin server.  The client usually relies upon a chain of
   trust, conveyed from some prearranged or configured trust anchor, to be safely ignored by recipients
   that do not recognize them; see Section 5.3.1.

5.1.  Field Ordering
   deem a certificate trustworthy (Section 4.3.4).

   In HTTP/1.1 and Combination

   The order in which field lines with differing names earlier, a client will only attribute authority to a
   server when they are received in communicating over a
   message is not significant.  However, it is good practice successfully established
   and secured connection specifically to send
   header fields that contain control data first, such URI origin's host.  The
   connection establishment and certificate verification are used as Host on
   requests
   proof of authority.

   In HTTP/2 and Date on responses, so that implementations can decide
   when not HTTP/3, a client will attribute authority to handle a message as early as possible.  A server MUST NOT
   apply
   when they are communicating over 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 successfully established and
   secured connection if the semantics URI origin's host matches any of the message,
   by appending each subsequent field line value to the initial field
   line value hosts
   present in order, separated by a comma the server's certificate and OWS (optional
   whitespace).  For consistency, use comma SP.

   The order in which field lines with the same name are received is
   therefore significant client believes that it
   could open a connection to the interpretation of the field value; that host for that URI.  In practice, a
   proxy MUST NOT change the order of these field line values when
   forwarding
   client will make a message.

   This means that, aside from DNS query to check that the well-known exception noted below, a
   sender MUST NOT generate multiple field lines with origin's host contains
   the same name in a
   message (whether in server IP address as the headers or trailers), or append a field line
   when a field line of 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 name already exists in server.  It is the message,
   unless
   origin's responsibility to ensure that field's definition allows multiple field line values any services provided with
   control over its certificate's private key are equally responsible
   for managing the corresponding "https" namespaces, or at least
   prepared to reject requests that appear to have been misdirected.  A
   server might be recombined unwilling to serve as the origin for some hosts even
   when they have the authority to do so.

   For example, if a comma-separated list [i.e., network attacker causes connections for port N to
   be received at least one
   alternative of port Q, checking the field's definition allows a comma-separated list,
   such as an ABNF rule of #(values) defined in Section 5.5].

      |  *Note:* In practice, target URI is necessary to ensure
   that the "Set-Cookie" header field ([RFC6265])
      |  often appears in a response message across multiple field lines
      |  and 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 use the list syntax, violating the above
      |  requirements rely on multiple field lines with TCP and the same field name.
      |  Since it connected
   port number for associating authority, since both are outside the
   secured communication and thus cannot be combined into a single field value,
      |  recipients ought to handle "Set-Cookie" trusted as a special case while
      |  processing fields.  (See Appendix A.2.3 of [Kri2001] for
      |  details.)

5.2.  Field Limits definitive.
   Hence, the HTTP does not 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 predefined limit on context that calls for access to
   the length of each field
   line, field value, or 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 length of 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 header or trailer section as request with a whole, non-interim HTTP
   response message, as described in Section 3.  Various ad hoc limitations on
   individual lengths are found in practice, often depending on 14, then that response is
   considered an authoritative answer to the
   specific field's semantics.

   A server client's request.

   Note, however, that receives a request header field line, field value, or
   set of fields larger than the above is not the only means for obtaining an
   authoritative response, nor does it wishes imply that an authoritative
   response is always necessary (see [Caching]).

4.3.4.  https certificate verification

   To establish a secured connection to process dereference a URI, a client MUST respond with
   verify that the service's identity is an
   appropriate 4xx (Client Error) status code.  Ignoring such header
   fields would increase acceptable match for the server's vulnerability
   URI's origin server.  Certificate verification is used to request smuggling
   attacks (Section 11.2 of [Messaging]).

   A client MAY discard prevent
   server impersonation by an on-path attacker or truncate received field lines by an attacker that are larger
   than the client wishes to
   controls name resolution.  This process if the field semantics are such requires that the dropped value(s) can a client be safely ignored without changing the
   message framing or response semantics.

5.3.  Field Names

   The field-name token labels the corresponding field value as having
   the semantics defined by that field.  For example,
   configured with a set of trust anchors.

   In general, a client MUST verify the Date header
   field is service identity using the
   verification process defined in Section 11.1.1 as containing the origination
   timestamp for 6 of [RFC6125] (for a
   reference identifier of type URI-ID) unless the message in which it appears.

     field-name     = token

   Field names are case-insensitive and ought client has been
   specifically configured to be registered within
   the "Hypertext Transfer Protocol (HTTP) Field Name Registry"; see
   Section 5.3.2.

   Authors accept some other form of specifications defining new fields are advised to choose verification.
   For example, a
   short but descriptive field name.  Short names avoid needless data
   transmission; descriptive names avoid confusion and "squatting" on
   names that client might have broader uses.

   To that end, limited-use fields (such as a header confined be connecting to a
   single application or use case) server whose address
   and hostname are encouraged to use a name dynamic, with an expectation that
   includes its name the service will
   present a specific certificate (or an abbreviation) as a prefix; for example, if certificate matching some
   externally defined reference identity) rather than one matching the Foo Application needs a Description field,
   dynamic URI's origin server identifier.

   In special cases, it might use "Foo-
   Desc"; "Description" is too generic, and "Foo-Description" is
   needlessly long.

   While the field-name syntax is defined be appropriate for a client to allow any token character,
   in practice some implementations place limits on simply
   ignore the characters they
   accept in field-names.  To server's identity, but it must be interoperable, new field names SHOULD
   constrain themselves understood that this
   leaves a connection open to alphanumeric characters, "-", and ".", and
   SHOULD begin with an alphanumeric character.

   Field names ought active attack.

   If the certificate is not be prefixed with "X-"; see [BCP178] for further
   information.

   Other prefixes are sometimes used in HTTP field names; valid for example,
   "Accept-" is used in many content negotiation headers.  These
   prefixes are only the URI's origin server, a user
   agent MUST either notify the user (user agents MAY give the user an aid
   option to recognizing continue with the purpose of connection in any case) or terminate the
   connection with a field, bad certificate error.  Automated clients MUST log
   the error to an appropriate audit log (if available) and
   do not trigger automatic processing.

5.3.1.  Field Extensibility

   There is no limit on SHOULD
   terminate the introduction of new field names, each
   presumably defining new semantics.

   New fields can be defined such that, when they are understood by connection (with a
   recipient, they might override or enhance the interpretation 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
   previously defined fields, define preconditions on request
   evaluation, or refine HTTP defines its own syntax for the meaning
   communication of responses. messages.  However, they share a common data
   abstraction.

   A proxy MUST forward unrecognized message consists of control data to describe and route the message,
   optional header fields unless that modify or extend the field name
   is listed in message semantics,
   describe the Connection header field (Section 6.8) or sender, the proxy
   is specifically configured to block, payload, or otherwise transform, such
   fields.  Other recipients SHOULD ignore unrecognized header provide additional context, a
   potentially unbounded stream of payload data, and optional trailer fields.  These requirements allow HTTP's functionality
   fields for metadata collected while sending the payload.

   Messages are intended to be
   enhanced 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 prior update an understanding of deployed intermediaries.

5.3.2.  Field Name Registry

   The "Hypertext Transfer the sender's current
   application state (established via prior messages).

5.1.  Protocol (HTTP) Field Name Registry" defines Version

   While HTTP's core semantics don't change between protocol versions,
   the namespace for HTTP field names.

   Any party can request registration expression of a them "on the wire" can change, and so the HTTP field.  See Section 5.7
   for considerations to take into account
   version number changes when creating a new incompatible changes are made to the wire
   format.  Additionally, HTTP
   field.

   The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is
   located at <https://www.iana.org/assignments/http-fields/>.
   Registration requests can allows incremental, backwards-compatible
   changes to be made by following the instructions
   located there or by sending an email to the "ietf-http-wg@ietf.org"
   mailing list.

   Field names are registered on protocol without changing its version
   through the advice use of defined extension points (Section 15).

   The protocol version as a Designated Expert
   (appointed by whole indicates the IESG or their delegate).  Fields sender's conformance
   with the status
   'permanent' are Specification Required ([RFC8126], Section 4.6).

   Registration requests consist set of at least requirements laid out in that version's corresponding
   specification of HTTP.  For example, the following information:

   Field name: 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 requested field name.  It MUST conform minor number is incremented when
   changes made to the field-name
      syntax defined in Section 5.3, and SHOULD be restricted protocol have the effect of adding to just
      letters, digits, hyphen ('-') and underscore ('_') characters,
      with the first character being a letter.

   Status:
      "permanent" message
   semantics or "provisional".

   Specification document(s):
      Reference to implying additional capabilities of the document that specifies sender.

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

   A client SHOULD send a copy of request version equal to the
      document.  An indication of highest version
   to which the relevant section(s) can also be
      included, but 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 required.

   And, optionally:

   Comments:  Additional information, such as about reserved entries.

   The Expert(s) can define additional
   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 be collected in the
   registry, highest version
   to which the server is conformant that has a major version less than
   or equal to the one received in consultation with the community.

   Standards-defined names have request.  A server MUST NOT send
   a status of "permanent".  Other names version to which it is not conformant.  A server can also be registered as permanent, send a 505
   (HTTP Version Not Supported) response if it wishes, for any reason,
   to refuse service of the Expert(s) find that they
   are in use, in consultation client's major protocol version.

   When an HTTP message is received with a major version number that the community.  Other names should
   be registered as "provisional".

   Provisional entries can be removed by
   recipient implements, but a higher minor version number than what the Expert(s)
   recipient implements, the recipient SHOULD process the message as if -
   it were in
   consultation with the community - the Expert(s) find highest minor version within that they are
   not in use.  The Experts major version to
   which the recipient is conformant.  A recipient can change assume that a provisional entry's status
   message with a higher minor version, when sent to
   permanent at any time.

   Note a recipient that names can be registered by third parties (including the
   Expert(s)), if the Expert(s) determines
   has not yet indicated support for that an unregistered name higher version, is
   widely deployed and not likely
   sufficiently backwards-compatible to be registered in safely processed by any
   implementation of the same major version.

   When a timely manner
   otherwise.

5.4.  Field Values major version of HTTP field values typically have their syntax defined using ABNF
   ([RFC5234]), using does not define any minor versions, the extension defined in Section 5.5 as necessary,
   minor version "0" is implied and are usually constrained is used when referring to the range of US-ASCII characters.
   Fields needing that
   protocol within a greater range of characters can use an encoding protocol element that requires sending a minor
   version.

5.2.  Framing

   // Message framing defines how each message begins and ends, 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
   // that the ISO-8859-1
   charset [ISO-8859-1], supporting message can be distinguished from other charsets only through use message (or
   // noise) on the same connection.  Framing is specific to each major
   // version of
   [RFC2047] encoding.  In practice, most HTTP field values use only a
   subset HTTP.

   One of the US-ASCII charset [USASCII].  Newly defined fields
   SHOULD limit their values functions of message framing is to US-ASCII octets. assure that messages
   are complete.  A recipient SHOULD
   treat other message is considered complete when all of the
   octets in field content (obs-text) as opaque data.

   Field values containing control (CTL) characters such as CR or LF indicated by its framing are
   invalid; recipients MUST either reject available.  Note that, when no
   explicit framing is used, a field value containing
   control characters, or convert them to SP before processing or
   forwarding response message that is ended by the message.

   Leading and trailing whitespace in raw field values
   transport connection's close is removed upon
   field parsing (e.g., Section 5.1 of [Messaging]).  Field definitions
   where leading or trailing whitespace in values 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 significant will
   have to use a container syntax such as quoted-string
   (Section 5.4.1.2).

   Commas (",") often are used combination of request semantics and a target
   resource upon which to separate members in field values. apply those semantics.

5.3.2.  Response

5.4.  Header Fields that allow multiple members are referred

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

   Fields that only anticipate a single member are sent/received before the message body are referred to
   as singleton fields.

   Because commas "header fields" (or just "headers", colloquially) and are used as located
   within the "header section" of a generic delimiter between members, they
   need message.  We refer to be treated with care if some named
   fields specifically as a "header field" when they are only allowed as data within a
   member.  This is true for both list-based and singleton fields, since
   a singleton field might to
   be sent with multiple members erroneously;
   being able to detect this condition improves interoperability. in the header section.

   Fields that expect are sent/received after the header section has ended
   (usually after the message body begins to contain a comma stream) are referred to as
   "trailer fields" (or just "trailers", colloquially) and located
   within a member, such as an
   HTTP-date "trailer section".  One or URI-reference element, ought to be defined with
   delimiters around that element to distinguish 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 comma within that
   data from potential list separators.

   For example, number of "field lines", each with
   a textual date "field name" (see Section 5.4.3) identifying the field, and a URI (either of which might contain
   a comma) could be safely carried
   "field line value" that conveys data for the field.

   Each field name present in list-based a section has a corresponding "field
   value" for that section, composed from all field line 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 with
   that double-quote delimiters almost always are used given field name in that section, concatenated together and
   separated with commas.  See Section 5.4.1 for further discussion of
   the
   quoted-string production; using a different syntax inside double-
   quotes will likely cause unnecessary confusion.

   Many fields (such as Content-Type, defined semantics of field ordering and combination in messages, and
   Section 7.2.1) use a
   common syntax 5.4.4 for parameters that allows more discussion of field values.

   For example, this section:

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

   contains two field lines, both unquoted (token) and
   quoted (quoted-string) syntax for with the field name "Example-Field".
   The first field line has a parameter field line value
   (Section 5.4.1.4).  Use of common syntax allows recipients to reuse
   existing parser components.  When allowing both forms, "Foo, Bar", while the meaning
   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
   a parameter value HTTP.  In
   particular, the Host and Connection fields ought to be the same implemented by
   all HTTP/1.x implementations whether it was received as a
   token or a quoted string.

   Historically, HTTP field values could not they advertise
   conformance with HTTP/1.1.

   New fields can be extended over multiple lines introduced without changing the protocol version if
   their defined semantics allow them to be safely ignored by preceding each extra line with at least one space or horizontal
   tab (obs-fold).  This document assumes recipients
   that any such obsolete line
   folding has been replaced with one or more SP octets prior to
   interpreting do not recognize them; see Section 15.3.

   A proxy MUST forward unrecognized header fields unless 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 name
   is listed 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 Connection header field
      |  values (i.e., after (Section 6.4.1) or the field value has been extracted by a
      |  generic field parser). 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.

5.4.1.  Common  Field Value Components

   Many HTTP Ordering and Combination

   The order in which field values are defined using common syntax components,
   separated by whitespace or specific delimiting characters.
   Delimiters lines with differing names are chosen from the set of US-ASCII visual characters not
   allowed received in a token (DQUOTE
   message is not significant.  However, it is good practice to send
   header fields that contain control data first, such as Host on
   requests and "(),/:;<=>?@[\]{}").

5.4.1.1.  Tokens

   Tokens are short textual identifiers Date on responses, so that do 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 whitespace conditionals, authentication credentials, or delimiters.

     token          = 1*tchar

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

5.4.1.2.  Quoted Strings deliberately
   misleading duplicate header fields that would impact request
   processing.

   A string recipient MAY combine multiple field lines with the same field name
   into one field line, without changing the semantics of text is parsed as a single the message,
   by appending each subsequent field line 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 to the initial field
   line value in order, separated by a single-octet quoting
   mechanism within quoted-string comma and comment constructs.  Recipients
   that process OWS (optional
   whitespace).  For consistency, use comma SP.

   The order in which field lines with the value same name are received is
   therefore significant to the interpretation of the field value; a quoted-string
   proxy 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 change the order of these field line values when
   forwarding a quoted-pair in message.

   This means that, aside from the well-known exception noted below, a quoted-string except
   where necessary to quote DQUOTE and backslash octets occurring within
   that string.  A
   sender SHOULD MUST NOT generate a quoted-pair multiple field lines with the same name in a comment
   except where necessary to quote parentheses ["(" and ")"] and
   backslash octets occurring within that comment.

5.4.1.3.  Comments

   Comments can be included
   message (whether 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

5.4.1.4.  Parameters

   Parameters are zero headers or more instances of trailers), or append a name=value pair; they are
   often used 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 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 recombined as a comma-separated list [i.e., at least one
   alternative of the
   parameter name.  Examples field's definition allows a comma-separated list,
   such as an ABNF rule of parameters and some equivalent forms can
   be seen #(values) defined in media types (Section 7.1.1) and Section 5.7.1].

      |  *Note:* In practice, the Accept "Set-Cookie" header field
   (Section 9.4.1).

   A parameter value that matches the token production can be
   transmitted either as a token or within ([RFC6265])
      |  often appears in a quoted-string.  The quoted
   and unquoted values are equivalent. response message across multiple field lines
      |  *Note:* Parameters do  and does not allow whitespace (not even "bad" use the list syntax, violating the above
      |  whitespace) around  requirements on multiple field lines with the "=" character.

5.4.1.5.  Date/Time Formats

   Prior to 1995, there were three different formats commonly used by
   servers same field name.
      |  Since it cannot be combined into a single field value,
      |  recipients ought to communicate timestamps.  For compatibility with old
   implementations, all three are defined here.  The preferred format is handle "Set-Cookie" as a fixed-length and single-zone subset special case while
      |  processing fields.  (See Appendix A.2.3 of [Kri2001] for
      |  details.)

5.4.2.  Field Limits

   HTTP does not place a predefined limit on the date and time
   specification used by the Internet Message Format [RFC5322].

     HTTP-date    = IMF-fixdate / obs-date

   An example length of each field
   line, field value, or on the preferred format is

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

   Examples length of the two obsolete formats a header or trailer section as
   a whole, as described in Section 2.  Various ad hoc limitations on
   individual lengths 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 found in practice, often depending on the
   specific field's semantics.

   A recipient server that parses receives a timestamp value in an HTTP request header field line, field value, or
   set of fields larger than it wishes to process MUST
   accept all three HTTP-date formats.  When a sender generates a 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 contains one or more timestamps defined as HTTP-date, are larger
   than the sender
   MUST generate those timestamps in client wishes to process if the IMF-fixdate format.

   An HTTP-date 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 represents time as an instance of Coordinated
   Universal Time (UTC).  The first two formats indicate UTC having
   the semantics defined by that field.  For example, the
   three-letter abbreviation Date header
   field is defined in Section 9.2.2 as containing the origination
   timestamp for Greenwich Mean Time, "GMT", a
   predecessor of the UTC name; values message in the asctime format which it appears.

     field-name     = token

   Field names are assumed
   to be in UTC.  A sender that generates HTTP-date values from a local
   clock case-insensitive and 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 be registered within
   the format
     ; "Hypertext Transfer Protocol (HTTP) Field Name Registry"; see
   Section 3.3 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 [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 US-ASCII
   characters.  Fields needing a greater range of characters can use an
   encoding such as the one defined in [RFC8187].

     field-value    = rfc850-date / asctime-date
     rfc850-date *field-content
     field-content  = day-name-l "," SP date2 SP time-of-day field-vchar
                      [ 1*( 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" HTAB / %s"Sunday"

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

   HTTP-date is case sensitive. 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
   subset of the US-ASCII charset [USASCII].  Newly defined fields
   SHOULD limit their values to US-ASCII octets.  A sender MUST NOT generate additional
   whitespace recipient SHOULD
   treat other octets in an HTTP-date beyond that specifically included 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 in before processing or
   forwarding the grammar.  The semantics of day-name, day, month, year, message.

   Leading and
   time-of-day are the same as those defined for the Internet Message
   Format constructs with the corresponding name ([RFC5322], trailing whitespace in raw field values is removed upon
   field parsing (e.g., Section 3.3).

   Recipients 5.1 of [Messaging]).  Field definitions
   where leading or trailing whitespace in values is significant will
   have to use a timestamp value container syntax such as quoted-string (Section 5.7.4).

   Commas (",") often are used to separate members in rfc850-date format, which uses field values.
   Fields that allow multiple members are referred to as list-based
   fields.  Fields that only anticipate a
   two-digit year, MUST interpret single member are referred to
   as singleton fields.

   Because commas are used as a timestamp 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 appears expect to contain a comma within a member, such as an
   HTTP-date or URI-reference element, ought to be more
   than 50 years 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 future
   quoted-string production; using a different syntax inside double-
   quotes will likely cause unnecessary confusion.

   Many fields (such as representing the most recent year Content-Type, defined in
   the past Section 7.4) use a
   common syntax for parameters that had 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 same last two digits.

   Recipients meaning of timestamp values are encouraged a parameter
   value ought to be robust in parsing
   timestamps unless otherwise restricted by the field definition.  For
   example, messages are occasionally forwarded over HTTP from same whether it was received as a non- token or a
   quoted string.

   Historically, HTTP source 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 might generate any of the date and time
   specifications defined by such obsolete line
   folding has been replaced with one or more SP octets prior to
   interpreting the Internet Message Format. 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
      |  *Note:* HTTP requirements for the date/time stamp format apply  editions (published before [RFC7230]).  Instead, ABNF rules are
      |  only  named according to their usage within each registered field name, wherein the protocol stream. rule
      |  Implementations are not required to use these formats  defines the valid grammar for user that field's corresponding field
      |  presentation, request logging, etc.

5.5.  ABNF List Extension: #rule

   A #rule extension to  values (i.e., after the ABNF rules of [RFC5234] is used to improve
   readability in field value has been extracted by a
      |  generic field parser).

5.5.  Payload

   Some HTTP messages transfer a complete or partial representation as
   the definitions of message "payload".  In some list-based field values.

   A construct "#" is defined, similar cases, a payload might contain only
   the associated representation's header fields (e.g., responses to "*", for defining comma-
   delimited lists
   HEAD) or only some part(s) of elements. the representation data (e.g., the 206
   (Partial Content) status code).

5.5.1.  Purpose

   The full form purpose of a payload in a request is "<n>#<m>element"
   indicating at least <n> and at most <m> elements, each separated defined by a
   single comma (",") and optional whitespace (OWS).

5.5.1.  Sender Requirements

   In any production that uses the list construct, method
   semantics.  For example, a sender MUST NOT
   generate empty list elements.  In other words, representation in the payload of a sender MUST generate
   lists that satisfy PUT
   request (Section 8.3.4) represents 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 desired state of the collected ABNF for senders after target
   resource if the list
   constructs have been expanded.

5.5.2.  Recipient Requirements

   Empty elements do not contribute to request is successfully applied, whereas a
   representation in the count payload of elements present.  A
   recipient MUST parse and ignore a reasonable number of empty list
   elements: enough POST request (Section 8.3.3)
   represents information to handle common mistakes by senders that merge
   values, but not so much that they could be used as a denial-of-
   service mechanism. processed by the target resource.

   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, response, the RFC 5234 ABNF cannot enforce payload's purpose is defined by both the cardinality of list elements, request
   method and consequently all cases are mapped is if there was no cardinality
   specified. the response status code.  For example, given these ABNF productions:

     example-list      = 1#example-list-elmt
     example-list-elmt = token ; see Section 5.4.1.1

   Then the following are valid values for example-list (not including payload of a
   200 (OK) response to GET (Section 8.3.1) represents the double quotes, which are present for delimitation only):

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

   In contrast, current state
   of the following values would be invalid, since target resource, as observed at least
   one non-empty element is required by the example-list production:

     ""
     ","
     ",   ,"

5.6.  Trailer Fields

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 time of the message
   origination date (Section 9.2.2), whereas the payload body), such as a message integrity check,
   digital signature, or post-processing status.  For example, of the
   chunked coding same
   status code in HTTP/1.1 allows a trailer section after response to POST might represent either the payload
   body (Section 7.1.2
   processing result or the new state of [Messaging]) which can the target resource after
   applying the processing.  Response messages with an error status code
   usually contain trailer fields:
   field names and values a payload that share represents the same syntax and namespace as
   header fields but error condition, such
   that are received after it describes the header section.

   Trailer fields ought to be processed error state and stored separately from the
   fields what next steps are suggested
   for resolving it.

5.5.2.  Identification

   When a complete or partial representation is transferred in a message
   payload, it is often desirable for the header section sender to avoid contradicting message semantics
   known at supply, or the time
   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 section was complete.  The presence or
   absence of certain header fields might impact choices made for field, then the
   routing or processing of
      sender asserts that the message as payload is a whole before representation of the trailers
   are received; those choices cannot be unmade
      resource identified by the later discovery
   of trailer fields.

5.6.2.  Limitations

   Many fields Content-Location field value.  However,
      such an assertion cannot be processed outside 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 header section because
   their evaluation payload is necessary prior to receiving unidentified.

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

   1.  If the message body,
   such as those that describe message framing, routing, authentication, request modifiers, method is GET or HEAD and the response controls, status code
       is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not
       Modified), the payload format.  A sender
   MUST NOT generate is a trailer field unless the sender knows the
   corresponding header field name's definition permits representation of the field to be
   sent in trailers.

   Trailer fields can be difficult to process resource
       identified by intermediaries that
   forward messages from one protocol version to another. the target URI (Section 6.1).

   2.  If the entire
   message can be buffered in transit, some intermediaries could merge
   trailer fields into request method is GET or HEAD and the header section (as appropriate) before it response status code
       is
   forwarded.  However, in most cases, 203 (Non-Authoritative Information), the trailers are simply
   discarded.  A recipient MUST NOT merge a trailer field into payload is a header
   section unless
       potentially modified or enhanced representation of the recipient understands target
       resource as provided by an intermediary.

   3.  If the corresponding response has a Content-Location header field definition and that definition explicitly permits and defines
   how trailer its field values can be safely merged.

   The presence
       value is a reference to the same URI as the target URI, the
       payload is a representation of the keyword "trailers" in target resource.

   4.  If the TE response has a Content-Location header field
   (Section 5.6.5) indicates and its field
       value is a reference to a URI different from the target URI, then
       the sender asserts that the client payload is willing to accept
   trailer fields, on behalf a representation of itself and any downstream clients.  For
   requests from the
       resource identified by the Content-Location field value.
       However, such an intermediary, assertion cannot be trusted unless it can be
       verified by other means (not defined by this implies specification).

   5.  Otherwise, the payload is unidentified.

5.5.3.  Payload Metadata

   Header fields that all downstream
   clients specifically describe the payload, rather than the
   associated representation, are willing referred to accept trailer 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 forwarded data of a request or response.  Note that  This is
   distinct from the presence message body (e.g., Section 6 of "trailers" does not mean that [Messaging]),
   which is how the client(s) will process any particular trailer field in payload body is transferred "on the
   response; only that wire", and might
   be encoded, depending on the trailer section(s) will not HTTP version in use.

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

   Because wire."

   The presence of a payload body in a request depends on whether the potential
   request method used defines semantics for trailer fields to be discarded it.

   The presence of a payload body in
   transit, a server SHOULD NOT generate trailer fields that it believes
   are necessary for response depends on both the user agent
   request method to receive.

5.6.3.  Processing

   Like header fields, trailer fields with which it is responding and the same name are processed
   in response status code
   (Section 14).

   Responses to the order received; multiple trailer field lines with HEAD request method (Section 8.3.2) never include a
   payload body because the same
   name associated response header fields indicate
   only what their values would have been if the equivalent semantics as appending the multiple values
   as request method had been
   GET (Section 8.3.1).

   2xx (Successful) responses to a list of members, even when CONNECT request method
   (Section 8.3.6) switch 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 connection to tunnel mode instead of
   having a list value even
   if each member value is only processed once per field line received.

   Trailer fields are expected (but payload body.

   All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
   responses do not required) to be processed one
   trailer section at include a time.  That is, for each trailer section
   received, payload body.

   All other responses do include a recipient payload body, although that is looking for trailer fields will parse body
   might be of zero length.

5.6.  Trailer Fields
5.6.1.  Purpose

   In some HTTP versions, additional metadata can be sent after the received
   initial header section into fields, invoke any associated processing
   for those fields at that point in has been completed (during or after
   transmission of the payload body), such as a message processing, and then
   append those fields to integrity check,
   digital signature, or post-processing status.  For example, the set of
   chunked coding in HTTP/1.1 allows a trailer fields received for section after the
   overall message.

   This behavior allows for iterative processing payload
   body (Section 7.1.2 of [Messaging]) which can contain trailer fields fields:
   field names and values that
   contain incremental signatures or mid-stream status information, share the same syntax and namespace as
   header fields but that might refer to each other's values within are received after the same header section.  However, there is no guarantee that trailer sections won't
   shift in relation

   Trailer fields ought to the message body stream, or won't be recombined
   (or dropped) in transit, so trailer processed and stored separately from the
   fields that refer to data outside in the present trailer header 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, avoid contradicting message semantics
   known at the end of a message, a recipient MAY treat time the entire
   set header section was complete.  The presence or
   absence of received trailer certain header fields as one data structure to be considered
   as might impact choices made for the message concludes.  Additional
   routing or processing expectations, if
   any, can be defined within the field specification for a field
   intended for use in trailers.

5.6.4.  Trailer

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

     Trailer = #field-name

   For example, a sender might indicate that a message integrity check
   will trailers
   are received; those choices cannot be computed as the payload is being streamed and provide unmade by the
   final signature as a later discovery
   of trailer field.  This allows a recipient to
   perform fields.

5.6.2.  Limitations

   Many fields cannot be processed outside the same check on header section because
   their evaluation is necessary prior to receiving the fly message body,
   such as the those that describe message framing, routing, authentication,
   request modifiers, response controls, or payload data is received. format.  A sender that intends to generate one or more trailer fields in a
   message SHOULD
   MUST NOT generate a Trailer header trailer field in unless the sender knows the
   corresponding header section
   of that message field name's definition permits the field to indicate which fields might be present
   sent in the trailers.

5.6.5.  TE

   The "TE" header field in a request

   Trailer fields can be used difficult to indicate process by intermediaries that
   forward messages from one protocol version to another.  If the
   sender will not discard entire
   message can be buffered in transit, some intermediaries could merge
   trailer fields when into the header section (as appropriate) before it contains 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"
   member, as described in Section 5.6.

   Additionally, specific HTTP versions can use it to indicate the
   transfer codings 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.

   The TE field-value consists
   response; only that the trailer section(s) will not be dropped by any
   of a list the clients.

   Because of tokens, each allowing the potential for
   optional parameters (as described trailer fields to be discarded in Section 5.4.1.4).

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

5.7.  Considerations for New HTTP Fields

   See Section 5.3 for a general requirements for field names, and
   Section 5.4 for
   transit, a discussion of field values.

   Authors of specifications defining new server SHOULD NOT generate trailer fields that it believes
   are advised to consider
   documenting:

   o  Whether necessary for the field has a singleton or list-based value (see
      Section 5.4).

      If it is a singleton field, document how user agent to treat messages where receive.

5.6.3.  Processing

   Like header fields, trailer fields with the multiple members same name are present (a sensible default would be to
      ignore the field, but this might not always be processed
   in the right choice).

      Note that intermediaries and software libraries might combine order received; multiple trailer field instances into a single one, despite lines with the field
      being defined as a singleton.  A robust format enables recipients
      to discover these situations (good example: "Content-Type", same
   name have the equivalent semantics as appending the
      comma can only appear inside quoted strings; bad example:
      "Location", multiple values
   as a comma can occur inside a URI).

   o  Under what conditions list of members, even when the field can be used; e.g., only in
      responses or requests, lines are received in all messages,
   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 on responses processed once per field line received.

   Trailer fields are expected (but not required) to be processed one
   trailer section at a
      particular request method, etc.

   o  What the scope of applicability time.  That is, for each trailer section
   received, a recipient that is looking for trailer fields will parse
   the information conveyed in
      the field is.  By default, received section into fields, invoke any associated processing
   for those fields apply only to at that point in the message they
      are associated with, but some response processing, and then
   append those fields are designed to
      apply to all representations the set of a resource, trailer fields received for the resource itself,
   overall message.

   This behavior allows for iterative processing of trailer fields that
   contain incremental signatures or an even broader scope.  Specifications mid-stream status information, and
   fields that expand the scope of
      a response field will need might refer to carefully consider issues such as
      content negotiation, each other's values within the time period of applicability, and (in
      some cases) multi-tenant server deployments.

   o  Whether same
   section.  However, there is no guarantee that trailer sections won't
   shift in relation to the field should message body stream, or won't be stored by origin servers recombined
   (or dropped) in transit, so trailer fields that
      understand it upon a PUT request.

   o  Whether refer to data outside
   the field semantics are further refined by present trailer section need to use self-descriptive references
   (i.e., refer to the context,
      such as data by existing request methods name, ordinal position, or status codes.

   o  Whether an octet
   range) rather than assume it is appropriate to list the field name in data most recently received.

   Likewise, at the Connection
      header field (i.e., if end of a message, a recipient MAY treat the field is entire
   set of received trailer fields as one data structure to be hop-by-hop; see
      Section 6.8).

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

   o  Whether it is appropriate to list message concludes.  Additional processing expectations, if
   any, can be defined within the field name in specification for a Vary
      response header field (e.g., when
   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 request header field ABNF rules of [RFC5234] is used
      by an origin server's content selection algorithm; see
      Section 11.1.4).

   o  Whether the field is allowable to improve
   readability in trailers (see Section 5.6).

   o  Whether the definitions of some list-based field ought values.

   A construct "#" is defined, similar to be preserved across redirects.

   o  Whether it introduces any additional security considerations, such
      as disclosure "*", for defining comma-
   delimited lists of privacy-related data.

5.8.  Fields Defined In This Document elements.  The following fields are defined by this document:

    --------------------------- ------------ --------
     Field Name                  Status       Ref.
    --------------------------- ------------ --------
     Accept                      standard     9.4.1
     Accept-Charset              deprecated   9.4.2
     Accept-Encoding             standard     9.4.3
     Accept-Language             standard     9.4.4
     Accept-Ranges               standard     11.4.1
     Allow                       standard     11.4.2
     Authentication-Info         standard     11.3.3
     Authorization               standard     9.5.3
     Connection                  standard     6.8
     Content-Encoding            standard     7.2.2
     Content-Language            standard     7.2.3
     Content-Length              standard     7.2.4
     Content-Location            standard     7.2.5
     Content-Range               standard     7.3.4
     Content-Type                standard     7.2.1
     Date                        standard     11.1.1
     ETag                        standard     11.2.3
     Expect                      standard     9.1.1
     From                        standard     9.6.1
     Host                        standard     6.5
     If-Match                    standard     9.2.3
     If-Modified-Since           standard     9.2.5
     If-None-Match               standard     9.2.4
     If-Range                    standard     9.2.7
     If-Unmodified-Since         standard     9.2.6
     Last-Modified               standard     11.2.2
     Location                    standard     11.1.2
     Max-Forwards                standard     9.1.2
     Proxy-Authenticate          standard     11.3.2
     Proxy-Authentication-Info   standard     11.3.4
     Proxy-Authorization         standard     9.5.4
     Range                       standard     9.3
     Referer                     standard     9.6.2
     Retry-After                 standard     11.1.3
     Server                      standard     11.4.3
     TE                          standard     5.6.5
     Trailer                     standard     5.6.4
     Upgrade                     standard     6.7
     User-Agent                  standard     9.6.3
     Vary                        standard     11.1.4
     Via                         standard     6.6.1
     WWW-Authenticate            standard     11.3.1
    --------------------------- ------------ --------

                         Table 3

   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 11.1.4).

    ------------ ---------- ------ ------------
     Field Name   Status     Ref.   Comments
    ------------ ---------- ------ ------------
     *            standard   5.8    (reserved)
    ------------ ---------- ------ ------------

                      Table 4

6.  Message Routing

   HTTP request message routing full form is determined by "<n>#<m>element"
   indicating at least <n> and at most <m> elements, each client based on
   the target resource, the client's proxy configuration, separated by a
   single comma (",") and
   establishment or reuse of an inbound connection.  The corresponding
   response routing follows the same connection chain back to optional whitespace (OWS).

5.7.1.1.  Sender Requirements

   In any production that uses the
   client.

6.1.  Identifying a Target Resource

   HTTP is used in list construct, a wide variety of applications, ranging from general-
   purpose computers to home appliances. sender MUST NOT
   generate empty list elements.  In some cases, communication
   options are hard-coded in other words, a client's configuration.  However, most
   HTTP clients rely on sender MUST generate
   lists that satisfy the same resource identification mechanism following syntax:

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

   and:

     #element => [ 1#element ]

   and
   configuration techniques as general-purpose Web browsers.

   HTTP communication is initiated by a user agent for some purpose.
   The purpose is a combination 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 request semantics elements present.  A
   recipient MUST parse and ignore a target
   resource upon which reasonable number of empty list
   elements: enough to apply those semantics.  The "request target"
   is 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 protocol following syntax:

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

   Note that identifies the "target resource".

   Typically, because of the request target is a URI reference (Section 2.4) which
   a user agent would resolve to its absolute form in order to obtain potential presence of empty list elements,
   the "target URI".  The target URI excludes RFC 5234 ABNF cannot enforce the reference's fragment
   component, if any, since fragment identifiers cardinality of list elements,
   and consequently all cases are reserved for
   client-side processing ([RFC3986], Section 3.5).

   However, 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 two special, method-specific forms allowed valid values for example-list (not including
   the
   request target in specific circumstances:

   o  For CONNECT (Section 8.3.6), double quotes, which are present for delimitation only):

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

   In contrast, the request target following values would be invalid, since at least
   one non-empty element is required by the host name
      and port number of the tunnel destination, 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 colon.

   o 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 OPTIONS (Section 8.3.7), protocol elements where optional whitespace is
   preferred to improve readability, a sender SHOULD generate the request target can be
   optional whitespace as a single
      asterisk ("*").

   See the respective method definitions for details.  These forms MUST 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 used replaced with other methods.

6.2.  Determining Origin

   The "origin" for a given URI single SP
   before interpreting it or forwarding the message downstream.

   The BWS rule is used where the triple of scheme, host, 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 port
   after normalizing remove
   it before interpreting the scheme and host protocol element.

   BWS has no semantics.  Any content known to lowercase and normalizing be defined as BWS MAY be
   removed before interpreting it or forwarding the port to remove any leading zeros.  If port 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 elided from the
   URI, the default port for that scheme parsed as a single value if it is used.  For example, the URI

      https://Example.Com/happy.js

   would have the origin

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

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

      https://example.com:443

   Each origin defines its own namespace and controls how identifiers a single-octet quoting
   mechanism within quoted-string and comment constructs.  Recipients
   that namespace are mapped to resources.  In turn, how process the
   origin responds to valid requests, consistently over time, determines value of a quoted-string MUST handle a quoted-pair
   as if it were replaced by the semantics that users will associate with octet following the backslash.

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

   A sender SHOULD NOT generate a URI, quoted-pair in a quoted-string except
   where necessary to quote DQUOTE and the
   usefulness of those semantics is what ultimately transforms these
   mechanisms into backslash octets occurring within
   that string.  A sender SHOULD NOT generate a "resource" for users quoted-pair in a comment
   except where necessary to reference quote parentheses ["(" and access ")"] and
   backslash octets occurring within that comment.

5.7.5.  Comments

   Comments can be included in some HTTP fields by surrounding the
   future.

   Two origins
   comment text with parentheses.  Comments are distinct if they differ only allowed in scheme, host, 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

5.7.6.  Parameters

   Parameters are zero or port.
   Even when it can be verified that the same entity controls two
   distinct origins, the two namespaces under those origins more instances of a name=value pair; they are distinct
   unless explicitly aliased by
   often used in field values as a server authoritative common syntax for that origin.

   Origin appending auxiliary
   information to an item.  Each parameter is also used within HTML and related Web protocols, beyond 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
   scope semantics of this document, as described in [RFC6454].

6.3.  Routing Inbound

   Once the target URI
   parameter name.  Examples of parameters and some equivalent forms can
   be seen in media types (Section 7.4.1) and its origin are determined, a client decides
   whether a network request is necessary to accomplish the desired
   semantics and, if so, where Accept header field
   (Section 11.1.2).

   A parameter value that request is to matches the token production can be directed.

6.3.1.  To
   transmitted either as a Cache

   If the client has token or within a cache [Caching] quoted-string.  The quoted
   and unquoted values are equivalent.

      |  *Note:* Parameters do not allow whitespace (not even "bad"
      |  whitespace) around the request can be satisfied
   by it, then the request is usually directed "=" character.

5.7.7.  Date/Time Formats

   Prior to 1995, there first.

6.3.2.  To a Proxy

   If the request is not satisfied were three different formats commonly used by a cache, then a typical client
   will check its configuration
   servers to determine whether a proxy communicate timestamps.  For compatibility with old
   implementations, all three are defined here.  The preferred format is to be
   used to satisfy
   a fixed-length and single-zone subset of the request.  Proxy configuration is implementation-
   dependent, but is often based on URI prefix matching, selective
   authority matching, or both, date and time
   specification used by the proxy itself Internet Message Format [RFC5322].

     HTTP-date    = IMF-fixdate / obs-date

   An example of the preferred format is usually
   identified by

     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" or "https" URI.  If HTTP field MUST
   accept all three HTTP-date formats.  When a proxy is applicable, sender generates a field
   that contains one or more timestamps defined as HTTP-date, the client connects inbound sender
   MUST generate those timestamps in the IMF-fixdate format.

   An HTTP-date value represents time as an instance of Coordinated
   Universal Time (UTC).  The first two formats indicate UTC by establishing (or reusing) the
   three-letter abbreviation for Greenwich Mean Time, "GMT", a connection
   predecessor of the UTC name; values in the asctime format are assumed
   to be in UTC.  A sender that proxy.

6.3.3.  To 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 Origin

   If no proxy 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"

     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 applicable, a typical client will invoke a handler
   routine, usually specific to the target URI's scheme, to connect
   directly to case sensitive.  A sender MUST NOT generate additional
   whitespace in an origin for the target resource.  How HTTP-date beyond that is
   accomplished is dependent on the target URI scheme and defined by its
   associated specification.

6.3.3.1.  http origins

   Although HTTP is independent of the transport protocol, the "http"
   scheme (Section 2.5.1) 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 specifically included as SP in
   the
   authority component.  This is a very weak sense grammar.  The semantics of authority because
   it depends on both client-specific name resolution mechanisms day-name, day, month, year, 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
   time-of-day are the host identifier is provided same 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 those defined for use the Internet Message
   Format constructs with a the corresponding name resolution service, such as
   DNS, to find an address for an appropriate origin server.

   When an "http" URI is used within ([RFC5322],
   Section 3.3).

   Recipients of a context that calls for access to
   the indicated resource, timestamp value in rfc850-date format, which uses a client MAY attempt access by resolving the
   host identifier to an IP address, establishing
   two-digit year, MUST interpret a TCP connection to timestamp that address on the indicated port, and sending an HTTP request
   message appears to be more
   than 50 years in the server containing the URI's identifying data
   (Section 2.1).

   If the server responds to such a request with a non-interim HTTP
   response message, future as described representing the most recent year in Section 10, then that response is
   considered an authoritative answer to
   the client's request.

   Note, however, past that had the above is not same last two digits.

   Recipients of timestamp values are encouraged to be robust in parsing
   timestamps unless otherwise restricted by the only means for obtaining an
   authoritative response, nor does it imply that an authoritative
   response is always necessary (see [Caching]). field definition.  For
   example, the Alt-
   Svc header field [RFC7838] allows an origin server to identify other
   services that messages are also authoritative for occasionally forwarded over HTTP from a non-
   HTTP source that origin.  Access to
   "http" identified resources might also be provided by protocols
   outside the scope generate any of this document.

   See Section 12.1 for security considerations related to establishing
   authority.

6.3.3.2.  https origins

   The "https" scheme (Section 2.5.2) associates authority based on the
   ability of a server to use date and time
   specifications defined by the private key corresponding Internet Message Format.

      |  *Note:* HTTP requirements for the date/time stamp format apply
      |  only to a
   certificate that their usage within the client considers protocol stream.
      |  Implementations are not required to be trustworthy use these formats for the
   identified origin server.  The client usually relies upon user
      |  presentation, request logging, etc.

6.  Routing

   HTTP is used in a chain wide variety of
   trust, conveyed applications, ranging from some prearranged or configured trust anchor, general-
   purpose computers to
   deem a certificate trustworthy (Section 6.3.3.3). home appliances.  In HTTP/1.1 and earlier, a client will only attribute authority to a
   server when they some cases, communication
   options are communicating over hard-coded in a successfully established
   and secured connection specifically to that URI origin's host.  The
   connection establishment client's configuration.  However, most
   HTTP clients rely on the same resource identification mechanism and certificate verification are used
   configuration techniques as
   proof of authority.

   In HTTP/2 and HTTP/3, a general-purpose Web browsers.

   HTTP request message routing is determined by each 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 based on
   the hosts
   present in target resource, the server's certificate client's proxy configuration, and
   establishment or reuse of an inbound connection.  The corresponding
   response routing follows the client believes that it
   could open a same connection chain back to the
   client.

6.1.  Target Resource

6.1.1.  Request Target

   The "request target" is the protocol element that host for that URI.  In practice, identifies the
   "target resource".

   Typically, the request target is a
   client will make URI reference (Section 4) which a DNS query
   user agent would resolve to check that 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 origin's host contains name
      and port number of the same server IP address as tunnel destination, separated by a colon.

   o  For OPTIONS (Section 8.3.7), the established connection.  This
   restriction request target can be removed by a single
      asterisk ("*").

   See the origin server sending an equivalent
   ORIGIN frame [RFC8336]. 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 target's provides the host and port value are passed within each HTTP
   request, identifying the origin and distinguishing it
   information 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
   for managing target URI, enabling the corresponding "https" namespaces, or at least
   prepared to reject requests that appear to have been misdirected.  A origin server might be unwilling to serve as the origin
   distinguish among resources while servicing requests for some hosts even
   when they have the authority to do so.

   For example, if multiple
   host names on a network attacker causes connections for port N to
   be received at single IP address.

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

   Since the Host field value is critical information for handling a
   request, a user agent SHOULD generate Host as the target URI is necessary to ensure
   that first field in the attacker can't cause "https://example.com:N/foo"
   header section.

   For example, a GET request 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 origin server for associating authority, since both are outside the
   secured communication and thus cannot be trusted as definitive.
   Hence,
   <http://www.example.org/pub/WWW/> would begin with:

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

   Since the HTTP communication might take place over any channel that
   has been secured, Host header field acts as defined in Section 2.5.2, including protocols
   that don't use TCP.

   When an "https" URI application-level routing
   mechanism, it is used within a context that calls frequent target for access malware seeking to
   the indicated resource, poison a client MAY attempt access by resolving the
   host identifier
   shared cache or redirect a request to an IP address, establishing a TCP connection 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 address on
   the indicated port, securing intercepted connection is targeting a valid IP address for that
   host.

6.1.3.  Reconstructing the Target URI

   Once an inbound connection end-to-
   end by successfully initiating TLS over TCP with confidentiality and
   integrity protection, and sending is obtained, the client sends an HTTP
   request message over that
   connection containing message.

   Depending on the URI's identifying data (Section 2.1).

   If nature of the server responds to such request, the client's target URI might
   be split into components and transmitted (or implied) within various
   parts of a request with a non-interim HTTP
   response message, as described message.  These parts are recombined by each
   recipient, in Section 10, then that response is
   considered an authoritative answer accordance with their local configuration and incoming
   connection context, to determine the client's target URI.  Appendix of
   [Messaging] defines how a server determines the target URI for an
   HTTP/1.1 request.

   Note, however, that

   Once the above is target URI has been reconstructed, an origin server needs to
   decide whether or not the only means to provide service for obtaining an
   authoritative response, nor does it imply that an authoritative
   response is always necessary (see [Caching]).

6.3.3.3.  https certificate verification

   To establish a secured URI via the
   connection to dereference a URI, a client MUST
   verify in which the request was received.  For example, the
   request might have been misdirected, deliberately or accidentally,
   such that the service's identity 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 acceptable match 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
   URI's recomposed target
      |  URI as a distinct concept, the effective request URI.

6.2.  Routing Inbound

   Once the target URI and its origin server.  Certificate verification are determined, a client decides
   whether a network request is used necessary to prevent
   server impersonation by an on-path attacker or by an attacker that
   controls name resolution.  This process requires accomplish the desired
   semantics and, if so, where that a client request is to be
   configured with a set of trust anchors.

   In general, directed.

6.2.1.  To a Cache

   If the client MUST verify has a cache [Caching] and the service identity using request can be satisfied
   by it, then the
   verification process defined in Section 6 of [RFC6125] (for request is usually directed there first.

6.2.2.  To a
   reference identifier of type URI-ID) unless Proxy

   If the request is not satisfied by a cache, then a typical client has been
   specifically configured
   will check its configuration to accept some other form of verification.
   For example, determine whether a client might proxy is to be connecting
   used to a server whose address satisfy the request.  Proxy configuration is implementation-
   dependent, but is often based on URI prefix matching, selective
   authority matching, or both, and hostname are dynamic, with an expectation that the service will
   present a specific certificate (or proxy itself is usually
   identified by an "http" or "https" URI.  If a certificate matching some
   externally defined reference identity) rather than one matching proxy is applicable,
   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 connects inbound by establishing (or reusing) a connection open
   to active attack.

   If that proxy.

6.2.3.  To the certificate Origin

   If no proxy is not valid for the URI's origin server, applicable, a user
   agent MUST either notify the user (user agents MAY give typical client will invoke a handler
   routine, usually specific to the user an
   option target URI's scheme, to continue with connect
   directly to an origin for the connection in any case) or terminate 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 with a bad certificate error.  Automated clients MUST log
   the error 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 appropriate audit log (if available) explicit identifier.

   Responses (both final and SHOULD
   terminate the connection (with interim) can be sent at any time after a bad certificate error).  Automated
   request is received, even if it is not yet complete.  However,
   clients MAY provide a configuration setting that disables this check,
   but MUST provide (including intermediaries) might abandon a setting which enables it.

6.4.  Reconstructing request if the Target URI

   Once an inbound connection
   response is obtained, 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 client sends 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
   request message (Section 2.1).

   Depending on stream has characteristics similar to a pipe-and-filter
   architecture, there are no inherent limits to the nature extent an
   intermediary can enhance (or interfere) with either direction of the request,
   stream.

   An intermediary not acting as a tunnel MUST implement the client's target URI might
   be split into components Connection
   header field, as specified in Section 6.4.1, and transmitted (or implied) within various
   parts of exclude fields from
   being forwarded that are only intended for the incoming connection.

   An intermediary MUST NOT forward a message to itself unless it is
   protected from an infinite request message.  These parts are recombined by each
   recipient, in accordance with their loop.  In general, an intermediary
   ought to recognize its own server names, including any aliases, local configuration
   variations, or literal IP addresses, and incoming
   connection context, respond to determine 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 target URI.  Appendix sake of
   [Messaging] defines how a server determines network
   efficiency, security checks, or payload transformations.

6.4.1.  Connection

   The "Connection" header field allows the target URI sender to list desired
   control options for an
   HTTP/1.1 request.

   Once the target URI has been reconstructed, an origin server needs to
   decide whether or not current connection.

   When a field aside from Connection is used to provide service supply control
   information for that URI via or about the
   connection in which current connection, the request was received.  For example, sender MUST list
   the
   request might have been misdirected, deliberately or accidentally,
   such corresponding field name within the Connection header field.
   Note that some versions of HTTP prohibit the information within use of fields for such
   information, and therefore do not allow the Connection field.

   Intermediaries MUST parse a received Host Connection header field differs before a
   message is forwarded and, for each connection-option in this field,
   remove any header or trailer field(s) from the host or port upon which message with the connection has been made.  If same
   name as the connection is from a trusted gateway, that inconsistency might be
   expected; otherwise, connection-option, and then remove the Connection header
   field itself (or replace it might indicate an attempt to bypass security
   filters, trick with the server into delivering non-public content, or
   poison a cache.  See Section 12 intermediary's own connection
   options for security considerations regarding
   message routing.

      |  *Note:* previous specifications defined the recomposed target
      |  URI as a distinct concept, forwarded message).

   Hence, the effective request URI.

6.5.  Host

   The "Host" Connection header field in a request provides a declarative way of
   distinguishing fields that are only intended for the host and port
   information immediate
   recipient ("hop-by-hop") from those fields that are intended for all
   recipients on the target URI, chain ("end-to-end"), enabling the origin server message to
   distinguish among resources while servicing requests for multiple
   host names on 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 single IP address.

     Host = uri-host [ ":" port ] ; Section 2.4

   Since the Host field value Connection option, after applying those fields'
   semantics.  This includes but is critical information for handling 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)
   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
   request, connection option corresponding to a user agent SHOULD generate Host as the first field in
   that is intended for all recipients of the
   header section. payload.  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
   Cache-Control is never appropriate as a frequent target for malware seeking connection option
   (Section 5.2 of [Caching]).

   The connection options do not always correspond to poison a
   shared cache or redirect a request to an unintended server.  An
   interception proxy is particularly vulnerable if it relies on field present in
   the
   Host message, since a connection-specific field value for redirecting requests to internal servers, or for
   use as might not be needed if
   there are no parameters associated with a cache key in connection option.  In
   contrast, a shared cache, connection-specific field that is received without first verifying a
   corresponding connection option usually indicates that the intercepted field has
   been improperly forwarded by an intermediary and ought to be ignored
   by the recipient.

   When defining new connection is targeting a valid IP address for options, specification authors ought to
   document it as reserved field name and register that
   host.

6.6.  Message Forwarding

   As described definition in Section 2.2, intermediaries can serve
   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 variety of
   roles in mechanism with the processing of HTTP requests TRACE
   (Section 8.3.8) and responses.  Some
   intermediaries are used to improve performance or availability.
   Others are used for access control or OPTIONS (Section 8.3.7) request methods to filter content.  Since an
   HTTP stream has characteristics similar limit
   the number of times that the request is forwarded by proxies.  This
   can be useful when the client is attempting to trace a pipe-and-filter
   architecture, there are no inherent limits request that
   appears to be failing or looping mid-chain.

     Max-Forwards = 1*DIGIT

   The Max-Forwards value is a decimal integer indicating the extent an
   intermediary can enhance (or interfere) with either direction remaining
   number of the
   stream.

   An times this request message can be forwarded.

   Each intermediary not acting as that receives a tunnel MUST implement the Connection TRACE or OPTIONS request containing
   a Max-Forwards header field, as specified in Section 6.8, field MUST check and exclude fields from
   being forwarded that are only intended for update its value prior to
   forwarding the request.  If the received value is zero (0), the incoming connection.

   An
   intermediary MUST NOT forward a message to itself unless it is
   protected from an infinite request loop.  In general, an the request; instead, the intermediary
   ought to recognize its own server names, including any aliases, local
   variations, or literal IP addresses, and
   MUST 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 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 forwarding for with a field value that
   is the sake lesser of network
   efficiency, security checks, a) the received value decremented by one (1) or payload transformations.

6.6.1. 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 6.7 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 4.2. 5.1.
   For brevity, the protocol-name is omitted when the received protocol
   is HTTP.

   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.

6.6.2.

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)
   selected
   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 7.3) 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 10.3.4). 14.3.4).

   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.

6.7.  Upgrading HTTP

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.

   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 6.8) 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).
   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 10.4).

6.7.1.  Upgrade Protocol Names

   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 4.2 and future updates to this
   specification.  Additional protocol names ought to be registered
   using the registration procedure defined in Section 6.7.2.

    ------ ------------------- ------------------------- ------
     Name   Description         Expected Version Tokens   Ref.
    ------ ------------------- ------------------------- ------
     HTTP   Hypertext           any DIGIT.DIGIT (e.g,     4.2
            Transfer Protocol   "2.0")
    ------ ------------------- ------------------------- ------

                              Table 5

6.7.2.  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.

6.8.  Connection-Specific Fields

   The "Connection" header field allows the sender an appropriate response
     (as defined by new protocol) to list desired
   control options for the current connection. "GET /hello" request ...]

   When a field aside from Connection Upgrade is used to supply control
   information for or about the current connection, sent, the sender MUST list
   the corresponding field name within the also send a Connection header field.
   Note
   field (Section 6.4.1) that some versions of HTTP prohibit the use of fields for such
   information, and therefore do contains an "upgrade" connection option,
   in order to prevent Upgrade from being accidentally forwarded by
   intermediaries that might not allow implement the Connection field.

   Intermediaries listed protocols.  A
   server MUST parse a received Connection ignore an Upgrade header field before a
   message that is forwarded and, for each connection-option received in this field,
   remove any header or trailer field(s) from an
   HTTP/1.0 request.

   A client cannot begin using an upgraded protocol on the connection
   until it has completely sent the request message with (i.e., the same
   name as client
   can't change the connection-option, and then remove protocol it is sending in the Connection middle of a message).

   If a server receives both an Upgrade and an Expect header field itself (or replace it with
   the intermediary's own connection
   options for the forwarded message).

   Hence, "100-continue" expectation (Section 9.1.1), the Connection server MUST send
   a 100 (Continue) response before sending a 101 (Switching Protocols)
   response.

   The Upgrade 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 applies to switching protocols on top
   of 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 existing connection; it cannot be blindly forwarded by
   older intermediaries.

   Furthermore, intermediaries SHOULD remove or replace field(s) whose
   semantics are known used 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 5.6.5)

   o  Trailer (Section 5.6.4)

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

   o  Upgrade (Section 6.7)

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

     Connection        = #connection-option
     connection-option = token

   Connection options are case-insensitive.

   A sender MUST NOT send a
   underlying connection option corresponding (transport) protocol, nor to a field
   that is intended for all recipients of switch the payload.
   existing communication to a different connection.  For example,
   Cache-Control those
   purposes, it is never more appropriate as a connection option
   (Section 5.2 of [Caching]).

   The connection options do not always correspond to use 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, 3xx (Redirection) response
   (Section 14.4).

   This specification authors ought to
   document it as reserved field only defines the protocol name and register that definition in "HTTP" for use by
   the family of Hypertext Transfer Protocol (HTTP) Field Name Registry
   (Section 5.3.2), Protocols, as defined by the HTTP
   version rules of Section 5.1 and future updates to this
   specification.  Additional protocol names ought to avoid collisions. be registered
   using the registration procedure defined in Section 15.7.

7.  Representations

   Considering

   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 could state, rather than transferring the resource itself.
   This allows the resource identified by a URI to be anything, and
   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 provided by HTTP is similar to a window through which one can
   observe and act upon such a thing only through the communication of
   messages to some an independent actor on the other side, an side.  A shared
   abstraction is needed to represent ("take the place of") the current
   or desired state of that thing in our communications.  That
   abstraction is called  When a
   representation [REST].

   For the purposes of HTTP, a "representation" is information that is
   intended to reflect hypertext, it can provide both a past, current, or desired state representation of a given
   resource, in a format that can be readily communicated via
   the
   protocol, resource state and processing instructions that consists of a set of representation metadata and a
   potentially unbounded stream of representation data. help guide the
   recipient's future interactions.

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 9.2) 12.1)
   and constructing the payload for 200 (OK), 206 (Partial Content), and
   304 (Not Modified) responses to GET (Section 8.3.1).

7.1.  Representation

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 ) )

7.1.1.  Media Type

   HTTP uses media types [RFC2046] in the Content-Type (Section 7.2.1)
   and Accept (Section 9.4.1)

7.3.  Metadata

   Representation header fields in order to provide open and
   extensible data typing and type negotiation.  Media types define both metadata about the
   representation.  When a data format and various processing models: message includes a payload body, the
   representation header fields describe how to process that interpret the
   representation 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 5.4.1.4) enclosed in the form of name=value pairs.  The presence or
   absence of payload body.  In a parameter might be significant response 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
   HEAD request, the parameter name.

   For example, representation header fields describe the following media types are equivalent in describing
   HTML text
   representation 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 that would have been enclosed 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].

7.1.1.1.  Charset

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

     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 GET.

   The following header fields convey representation metadata:

    ------------------ ------
     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 time media type of this writing includes braces (see
      |  [Err5433]).

7.1.1.2.  Canonicalization and Text Defaults

   Media types are registered with a canonical form the
   associated representation: either the representation enclosed in order to be
   interoperable among systems with varying native encoding formats.
   Representations selected the
   message payload or transferred via HTTP ought to be in
   canonical form, for many of the same reasons described selected representation, as determined by the
   Multipurpose Internet Mail Extensions (MIME) [RFC2045].  However,
   message semantics.  The indicated media type defines both the
   performance characteristics of email deployments (i.e., store data
   format and
   forward messages to peers) are significantly different from those
   common how that data is intended to HTTP and the Web (server-based information services).
   Furthermore, MIME's constraints for be processed by a recipient,
   within the sake scope of compatibility with
   older mail transfer protocols do not apply to HTTP (see Appendix B 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
   [Messaging]).

   MIME's canonical form requires the field is

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

   A sender that media subtypes of generates a message containing a payload body SHOULD
   generate a Content-Type header field in that message unless the "text"
   intended media type
   use CRLF as of the text line break.  HTTP allows enclosed representation is unknown to the transfer of text
   media with plain CR or LF alone representing
   sender.  If a line break, when such
   line breaks are consistent for an entire representation.  An HTTP
   sender Content-Type header field is not present, the recipient
   MAY generate, and either assume a recipient MUST be able to parse, line
   breaks in text media that consist type of CRLF, bare CR, "application/octet-stream"
   ([RFC2046], Section 4.5.1) or bare LF. examine the data to determine its type.

   In
   addition, text media in HTTP is practice, resource owners do not limited always properly configure their
   origin server to charsets that use
   octets 13 and 10 provide the correct Content-Type for CR and LF, respectively. a given
   representation.  Some user agents examine a payload's content and, in
   certain cases, override the received type (for example, see
   [Sniffing]).  This flexibility
   regarding line breaks applies "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 text within provide a representation
   that has been assigned means to disable such sniffing.

   Furthermore, although Content-Type is defined as a "text" media type; singleton field,
   it does not apply is sometimes incorrectly generated multiple times, resulting in a
   combined field value that appears to
   "multipart" types 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 elements outside uses media types [RFC2046] in the payload body (e.g., Content-Type (Section 7.4) and
   Accept (Section 11.1.2) header fields).

   If a representation is encoded with fields in order to provide open and
   extensible data typing and type negotiation.  Media types define both
   a content-coding, the underlying data ought 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 a the form defined above prior to being encoded.

7.1.1.3.  Multipart Types

   MIME provides for a number of "multipart" types - encapsulations of
   one name=value pairs.  The presence or more representations within a single message body.  All
   multipart types share a common syntax, as defined in Section 5.1.1
   absence of
   [RFC2046], and include a boundary parameter as part might be significant to the processing of a
   media type, depending on its definition within 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
   registry.  Parameter values might or might not use be case-sensitive,
   depending on the multipart boundary as an
   indicator semantics of message body length, though it might be used by
   implementations that generate or process the payload. parameter name.

   For example, the "multipart/form-data" type is often used for carrying form data following media types are equivalent in a request, as described describing
   HTML text data encoded in [RFC7578], and the "multipart/
   byteranges" type is defined by this specification for use in some 206
   (Partial Content) responses (see Section 10.3.7).

7.1.2.  Content Codings

   Content coding values indicate an UTF-8 character 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, scheme, but
   the representation first is stored in coded form, transmitted directly, and
   only decoded by the final recipient.

     content-coding   = token

   All content codings are preferred for consistency (the "charset" parameter value
   is defined as being case-insensitive and 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
   within with IANA according to the "HTTP Content Coding Registry", as
   procedures defined in
   Section 7.1.2.4

   Content-coding values are used in the Accept-Encoding (Section 9.4.3)
   and Content-Encoding (Section 7.2.2) header fields.

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

    ------------ ------------------------------------------- ---------
     Name         Description                                 Ref.
    ------------ ------------------------------------------- ---------
     compress     UNIX "compress" data format [Welch]         7.1.2.1
     deflate      "deflate" compressed data ([RFC1951])       7.1.2.2
                  inside the "zlib" data format ([RFC1950])
     gzip         GZIP file format [RFC1952]                  7.1.2.3
     identity     Reserved                                    9.4.3
     x-compress   Deprecated (alias for compress)             7.1.2.1
     x-gzip       Deprecated (alias for gzip)                 7.1.2.3
    ------------ ------------------------------------------- ---------

                                 Table 6

7.1.2.1.  Compress Coding

   The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
   [Welch] that is commonly produced by [BCP13].

7.4.2.  Charset

   HTTP uses charset names to indicate or negotiate the UNIX file compression
   program "compress". character
   encoding scheme of a textual representation [RFC6365].  A recipient SHOULD consider "x-compress" to be
   equivalent to "compress".

7.1.2.2.  Deflate Coding

   The "deflate" coding charset is
   identified by a "zlib" data format [RFC1950] containing a
   "deflate" compressed data stream [RFC1951] that uses a combination of case-insensitive token.

     charset = token

   Charset names ought to be registered in the Lempel-Ziv (LZ77) compression algorithm and Huffman coding. IANA "Character Sets"
   registry (<https://www.iana.org/assignments/character-sets>)
   according to the procedures defined in Section 2 of [RFC2978].

      |  *Note:* Some non-conformant implementations send In theory, charset names are defined by the "deflate" "mime-
      |  compressed data without  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 zlib wrapper.

7.1.2.3.  Gzip Coding

   The "gzip" coding is an LZ77 coding time of this writing includes braces (see
      |  [Err5433]).

7.4.3.  Canonicalization and Text Defaults

   Media types are registered 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" canonical form in order to be
   equivalent
   interoperable among systems with varying native encoding formats.
   Representations selected or transferred via HTTP ought to "gzip".

7.1.2.4.  Content Coding Registry

   The "HTTP Content Coding Registry", maintained be in
   canonical form, for many of the same reasons described 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
   Multipurpose Internet Mail Extensions (MIME) [RFC2045].  However, the
   performance characteristics of email deployments (i.e., store and
   forward messages to specification text

   Names peers) are significantly different from those
   common to HTTP and the Web (server-based information services).
   Furthermore, MIME's constraints for the sake of content codings MUST NOT overlap compatibility with names of
   older mail transfer
   codings (Section 7 protocols do not apply to HTTP (see Appendix B of
   [Messaging]).

   MIME's canonical form requires that media subtypes of [Messaging]), unless the encoding
   transformation is identical (as is "text" type
   use CRLF as the case for text line break.  HTTP allows the compression
   codings defined in Section 7.1.2).

   Values to be added to this namespace require IETF Review (see
   Section 4.8 transfer of [RFC8126]) 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 conform be able to the purpose parse, line
   breaks in text media that consist of content
   coding defined CRLF, bare CR, or bare LF.  In
   addition, text media in Section 7.1.2.

   New content codings ought HTTP is not limited to be self-descriptive whenever possible,
   with optional parameters discoverable charsets that use
   octets 13 and 10 for CR and LF, respectively.  This flexibility
   regarding line breaks applies only to text within the coding format
   itself, rather than rely on external metadata a representation
   that might be lost
   during transit.

7.1.3.  Language Tags

   A language tag, as defined in [RFC5646], identifies has been assigned a natural
   language spoken, written, or otherwise conveyed by human beings for
   communication of information "text" media type; it does not apply to other human beings.  Computer
   languages are explicitly excluded.
   "multipart" types or HTTP uses language tags within elements outside the Accept-Language and
   Content-Language payload body (e.g.,
   header fields.  Accept-Language uses fields).

   If a representation is encoded with a content-coding, the broader
   language-range production defined underlying
   data ought to be in Section 9.4.4, whereas
   Content-Language uses the language-tag production a form defined below.

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

   A language tag is above prior to being encoded.

7.4.4.  Multipart Types

   MIME provides for a sequence number of "multipart" types - encapsulations of
   one or more case-insensitive subtags,
   each separated by representations within a hyphen character ("-", %x2D).  In most cases, single message body.  All
   multipart types share a
   language tag consists common syntax, as defined in Section 5.1.1 of
   [RFC2046], and include a primary language subtag that identifies a
   broad family boundary parameter as part of related languages (e.g., "en" = English), which the media type
   value.  The message body is
   optionally followed by itself a series of subtags that refine or narrow that
   language's range (e.g., "en-CA" = protocol element; a sender MUST
   generate only CRLF to represent line breaks between body parts.

   HTTP message framing does not use the variety of English multipart boundary 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.

7.1.4.  Range Units

   Representation data can an
   indicator of message body length, though it might be partitioned into subranges when there are
   addressable structural units inherent to used by
   implementations that data's content coding generate or media type. process the payload.  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" "multipart/form-data" type is often used for carrying form data
   in a request, as described in [RFC7578], and the Accept-Ranges
   (Section 11.4.1) response header field to advertise support "multipart/
   byteranges" type is defined by this specification for range
   requests, the Range (Section 9.3) request 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 delineate the parts of a representation that are requested, and representation, beyond those inherent in the
   Content-Range (Section 7.3.4) payload header field to describe which
   part of a representation is being transferred.

     range-unit       = token

   All range unit names are case-insensitive
   media type, and ought thus what decoding mechanisms have to be registered
   within the "HTTP Range Unit Registry", as defined applied in Section 7.1.4.4

   The following range unit names are defined by this document:

    ----------------- ---------------------------------- ---------
     Range Unit Name   Description                        Ref.
    ----------------- ---------------------------------- ---------
     bytes             a range of octets                  7.1.4.2
     none              reserved as keyword
   order to indicate    11.4.1
                       range requests are not supported
    ----------------- ---------------------------------- ---------

                               Table 7

7.1.4.1.  Range Specifiers

   Ranges are expressed obtain data 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 media type referenced by the Content-Type
   header field.  Content-Encoding is expected primarily used to specify
   requirements on when int-range, suffix-range, and other-range are
   allowed.

   A range request can specify a single range or allow a set
   representation's data to be compressed without losing the identity of ranges within
   a single representation.

     ranges-specifier = range-unit "=" range-set
     range-set        = 1#range-spec
     range-spec
   its underlying media type.

     Content-Encoding = int-range
                      / suffix-range
                      / other-range #content-coding

   An int-range example of its use is a range expressed as two non-negative integers or as

     Content-Encoding: gzip

   If one non-negative integer through or more encodings have been applied to a representation, the end of
   sender that applied the representation
   data.  The range unit specifies what encodings MUST generate a Content-Encoding
   header field that lists the integers mean (e.g., they
   might indicate unit offsets from content codings in 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 order in which
   they were applied.  Note that the last-pos value coding named "identity" is present reserved
   for its special role in Accept-Encoding, and less
   than thus SHOULD NOT be
   included.

   Additional information about the first-pos.

   A suffix-range is a range expressed as encoding parameters can be provided
   by other header fields not defined by this specification.

   Unlike Transfer-Encoding (Section 6.1 of [Messaging]), the codings
   listed in Content-Encoding are a suffix characteristic of the
   representation; the representation
   data with is defined in terms of the provided non-negative integer maximum length (in range
   units).  In coded
   form, and all other words, the last N units of metadata about the representation data.

     suffix-range  = "-" suffix-length
     suffix-length = 1*DIGIT

   To provide for extensibility, is about the other-range rule
   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 mostly
   unconstrained grammar data
   format that allows application-specific or future
   range units to define additional range specifiers.

     other-range   = 1*( %x21-2B / %x2D-7E )
                   ; 1*(VCHAR excluding comma)

7.1.4.2.  Byte Ranges

   The "bytes" range unit is used always compressed, then that encoding would not be
   restated in Content-Encoding even if it happens to express subranges be the same
   algorithm as one of the content codings.  Such a
   representation data's octet sequence.  Each byte range is expressed
   as an integer range at content coding would
   only be listed if, for some offset, relative bizarre reason, it is applied a second
   time to either the beginning
   (int-range) or end (suffix-range) of form the representation data.  Byte
   ranges do not use representation.  Likewise, an origin server might
   choose to publish the other-range specifier.

   The first-pos value same data as multiple representations that
   differ only in a bytes int-range gives whether the offset coding is defined as part of the
   first byte Content-Type
   or Content-Encoding, since some user agents will behave differently
   in their handling of each response (e.g., open a range.  The last-pos value gives the offset "Save as ..." dialog
   instead of the
   last byte 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 range; that is, the byte positions specified are
   inclusive.  Byte offsets start at zero.

   If the representation data request message has a content
   coding applied, each byte
   range that is calculated with respect to the encoded sequence of bytes, not the sequence of underlying bytes acceptable.

7.5.1.  Content Codings

   Content coding values indicate an encoding transformation 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 has
   been or can limit be applied to a representation.  Content codings are
   primarily used to allow a representation to be compressed or
   otherwise usefully transformed without losing the number identity of bytes requested its
   underlying media type and without knowing the
   size loss of information.  Frequently,
   the selected representation.  If the last-pos value representation is
   absent, or if stored in coded form, transmitted directly, and
   only decoded by the value is greater than or equal final recipient.

     content-coding   = token

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

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

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

    ------------ ------------------------------------------- ---------
     Name         Description                                 Ref.
    ------------ ------------------------------------------- ---------
     compress     UNIX "compress" data format [Welch]         7.5.1.1
     deflate      "deflate" compressed data ([RFC1951])       7.5.1.2
                  inside the byte range "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 interpreted as an adaptive Lempel-Ziv-Welch (LZW) coding
   [Welch] that is commonly produced by the remainder 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 representation (i.e., Lempel-Ziv (LZ77) compression algorithm and Huffman coding.

      |  *Note:* Some non-conformant implementations send the server replaces "deflate"
      |  compressed data without the
   value of last-pos zlib wrapper.

7.5.1.3.  Gzip Coding

   The "gzip" coding is an LZ77 coding with a value 32-bit Cyclic Redundancy
   Check (CRC) that is one less than the current
   length of commonly produced by the selected representation). gzip file compression
   program [RFC1952].  A client can request recipient SHOULD consider "x-gzip" to be
   equivalent to "gzip".

7.6.  Content-Language

   The "Content-Language" header field describes the last N bytes (N > 0) natural language(s)
   of the selected
   representation using a suffix-range.  If intended audience for the selected representation
   is shorter than representation.  Note that this
   might not be equivalent to all the specified suffix-length, languages used within the entire
   representation
   representation.

     Content-Language = #language-tag

   Language tags are defined in Section 7.6.1.  The primary purpose of
   Content-Language is used.

   Additional examples, assuming to allow a representation of length 10000:

   o  The final 500 bytes (byte offsets 9500-9999, inclusive):

           bytes=-500

      Or:

           bytes=9500-

   o  The first user to identify and last bytes differentiate
   representations according to the users' own preferred language.
   Thus, if the content is intended 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 for a Danish-literate audience,
   the second 500
      bytes (byte offsets 500-999, inclusive):

           bytes=500-600,601-999
           bytes=500-700,601-999 appropriate field is

     Content-Language: da

   If a valid bytes range-set includes at least one range-spec with a
   first-pos that no Content-Language is less than the current length of the representation,
   or at least one suffix-range with a non-zero suffix-length, then specified, the
   bytes range-set default is satisfiable.  Otherwise, that the bytes range-set content
   is
   unsatisfiable.

   If intended for all language audiences.  This might mean that the selected representation has zero length,
   sender does not consider it to be specific to any natural language,
   or that the only satisfiable
   form of range-spec 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 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 rendition of octets.  Since there is no predefined
   limit to the length "Treaty of a payload, recipients MUST anticipate
   potentially large decimal numerals and prevent parsing errors due to
   integer conversion overflows.

7.1.4.3.  Other Range Units

   Other range units, such as format-specific boundaries like pages,
   sections, records, rows, or time, are potentially usable
   Waitangi", presented simultaneously in HTTP the original Maori and English
   versions, would call for
   application-specific purposes, but

     Content-Language: mi, en

   However, just because multiple languages are present within a
   representation does not commonly used in practice.
   Implementors of alternative range units ought to consider how they mean that it is intended for multiple
   linguistic audiences.  An example would work with content codings and general-purpose intermediaries.

   Range units are be a beginner's language
   primer, such as "A First Lesson in Latin", which is clearly intended
   to be extensible.  New range units ought to used by an English-literate audience.  In this case, the
   Content-Language would properly only include "en".

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

7.6.1.  Language Tags

   A language tag, as defined in Section 7.1.4.4.

7.1.4.4.  Range Unit Registry

   The "HTTP Range Unit Registry" defines the namespace [RFC5646], identifies a natural
   language spoken, written, or otherwise conveyed by human beings for the range
   unit names and refers to their corresponding specifications.  It is
   maintained at <https://www.iana.org/assignments/http-parameters>.

   Registration
   communication of an HTTP Range Unit MUST include the following fields:

   o  Name

   o  Description

   o  Pointer to specification text

   Values to be added information to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

7.2.  Representation Metadata

   Representation header fields provide metadata about the
   representation.  When a message includes a payload body, other human beings.  Computer
   languages are explicitly excluded.

   HTTP uses language tags within the
   representation Accept-Language and
   Content-Language header fields describe how to interpret fields.  Accept-Language uses the
   representation data enclosed broader
   language-range production defined in Section 11.1.5, whereas
   Content-Language uses the payload body. 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 response to
   language tag consists of a
   HEAD request, the representation header fields describe the
   representation data primary language subtag that would have been enclosed in the payload body
   if identifies a
   broad family of related languages (e.g., "en" = English), which is
   optionally followed by a series of subtags that refine or narrow that
   language's range (e.g., "en-CA" = the same request had been variety of English as
   communicated in Canada).  Whitespace is not allowed within a GET.

   The following header fields convey representation metadata:

    ------------------ -------
     Field Name         Ref.
    ------------------ -------
     Content-Type       7.2.1
     Content-Encoding   7.2.2
     Content-Language   7.2.3 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     7.2.4
     Content-Location   7.2.5
    ------------------ -------

             Table 8

7.2.1.  Content-Type

   The "Content-Type" "Content-Length" header field indicates the media type of the associated representation: either the representation enclosed in the
   message payload or the selected representation,
   representation's data length as determined by the
   message semantics.  The indicated media type defines both a decimal non-negative integer number
   of octets.  When transferring a representation in a message, Content-
   Length refers specifically to the amount of data
   format and how enclosed so that data is intended to it
   can be processed by a recipient,
   within the scope used to delimit framing of the received message semantics, after any content
   codings indicated 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 Content-Encoding are decoded.

     Content-Type
   recipients to estimate transfer time or compare to previously stored
   representations.

     Content-Length = media-type

   Media types are defined in Section 7.1.1. 1*DIGIT

   An example of the field is

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

     Content-Length: 3495

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

   A user agent SHOULD
   generate send a Content-Type header field Content-Length in that a request message unless the
   intended media type of when
   no Transfer-Encoding is sent and the request method defines a meaning
   for an enclosed representation payload body.  For example, a Content-Length header
   field is unknown to normally sent in a POST request even when the
   sender.  If value is 0
   (indicating an empty payload body).  A user agent SHOULD NOT send a Content-Type
   Content-Length header field is not present, when the recipient
   MAY either assume request message does not contain
   a media type of "application/octet-stream"
   ([RFC2046], Section 4.5.1) or examine payload body and the data to determine its type.

   In practice, resource owners method semantics do not always properly configure their
   origin server to provide the correct Content-Type for anticipate such a given
   representation.  Some user agents examine
   body.

   A server MAY send 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 Content-Length header field in processing semantics.  Implementers are
   encouraged to provide a means response to disable such sniffing.

   Furthermore, although Content-Type is defined as a singleton field,
   it is sometimes incorrectly generated multiple times, resulting
   HEAD request (Section 8.3.2); a server MUST NOT send Content-Length
   in such a
   combined response unless its field value that appears to be a list.  Recipients often
   attempt to handle this error by using equals the last syntactically valid
   member decimal number
   of the list, but note octets that some implementations might have
   different error handling behaviors, leading to interoperability and/
   or security issues.

7.2.2.  Content-Encoding

   The "Content-Encoding" header field indicates what content codings would have been applied to the representation, beyond those inherent sent in the
   media type, and thus what decoding mechanisms have to be applied in
   order to obtain data in payload body of a response
   if the media type referenced by same request had used the Content-Type GET method.

   A server MAY send a Content-Length header field.  Content-Encoding is primarily used to allow field in a
   representation's data 304 (Not
   Modified) response to be compressed without losing the identity of a conditional GET request (Section 14.4.5); a
   server MUST NOT send Content-Length in such a response unless its underlying media type.

     Content-Encoding = #content-coding

   An example
   field value equals the decimal number of its use is

     Content-Encoding: gzip

   If one or more encodings octets that would have been applied to a representation,
   sent in the
   sender that applied payload body of a 200 (OK) response to the encodings same request.

   A server MUST generate NOT send a Content-Encoding Content-Length 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 any response
   with a status code of 1xx (Informational) or 204 (No Content).  A
   server MUST NOT be
   included.

   Additional information about the encoding parameters can be provided
   by other send a Content-Length header fields not defined by this specification.

   Unlike Transfer-Encoding (Section 6.1 of [Messaging]), the codings
   listed field in Content-Encoding are any 2xx
   (Successful) response to a characteristic of the
   representation; CONNECT request (Section 8.3.6).

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

   Any Content-Length field value greater than or equal to zero is only decoded just prior
   valid.  Since there is no predefined limit to rendering
   or analogous usage. 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 media type includes an inherent encoding, such same decimal value as a data
   format comma-separated list
   (Section 5.7.1) - for example, "Content-Length: 42, 42" - indicating
   that is always compressed, 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 encoding would not be
   restated in Content-Encoding even if it happens
   decimal value prior to be determining the same
   algorithm as one of message body length or
   forwarding the content codings.  Such message.

7.8.  Content-Location

   The "Content-Location" header field references a content coding would
   only URI that can be listed if, used
   as an identifier for some bizarre reason, it is applied a second
   time specific resource corresponding to form the representation.  Likewise, an origin server might
   choose
   representation in this message's payload.  In other words, if one
   were to publish perform a GET request on this URI at the time of this
   message's generation, then a 200 (OK) response would contain the same data as multiple representations
   representation that
   differ only in whether the coding is defined enclosed as part of Content-Type
   or Content-Encoding, since some user agents will behave differently payload 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 this message.

     Content-Location = absolute-URI / partial-URI

   The field value is either an absolute-URI or a status code of 415 (Unsupported
   Media Type) if partial-URI.  In the
   latter case (Section 4), the referenced URI is relative to the target
   URI ([RFC3986], Section 5).

   The Content-Location value is not a representation in replacement for the request message has a content
   coding that target URI
   (Section 6.1).  It is not acceptable.

7.2.3.  Content-Language

   The "Content-Language" representation metadata.  It has the same
   syntax and semantics as the 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

   Language tags are same name defined for
   MIME body parts in Section 7.1.3.  The primary purpose 4 of
   Content-Language [RFC2557].  However, its appearance
   in an HTTP message has some special implications for HTTP recipients.

   If Content-Location is to allow included in a user to identify 2xx (Successful) response
   message and differentiate
   representations according its value refers (after conversion to absolute form) 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
   URI that is specified, the default is that same as the content
   is intended for all language audiences.  This might mean that target URI, then the
   sender does not recipient MAY
   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 payload to be listed for content that is intended for
   multiple audiences.  For example, a rendition current representation of that resource
   at the "Treaty of
   Waitangi", presented simultaneously in time indicated by the original Maori and English
   versions, would call for

     Content-Language: mi, en

   However, just because multiple languages are present within message origination date.  For a
   representation does not mean that it GET
   (Section 8.3.1) or HEAD (Section 8.3.2) request, this is intended for multiple
   linguistic audiences.  An example would be a beginner's language
   primer, such the same as "A First Lesson in Latin", which
   the default semantics when no Content-Location is clearly intended
   to be used provided 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 -
   server.  For a state-changing request like PUT (Section 8.3.4) or
   POST (Section 8.3.3), it is not limited
   to textual documents.

7.2.4.  Content-Length

   The "Content-Length" header field indicates implies that 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 server's response contains
   the amount new representation of data enclosed so that resource, thereby distinguishing it
   can be used to delimit framing of
   from representations that might only report about the message body action (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
   "It worked!").  This allows authoring applications 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 update their
   local copies without the need for a Content-Length subsequent GET request.

   If Content-Location is included in a request 2xx (Successful) response
   message when
   no Transfer-Encoding is sent and the request method defines its field value refers to a meaning URI that differs from the
   target URI, then the origin server claims that the URI is an
   identifier for an a different resource corresponding to the enclosed payload body.
   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 example, a Content-Length header
   field is normally sent in response to a POST request even when the value GET or HEAD request, this is 0
   (indicating an empty payload body).  A user agent SHOULD NOT send a
   Content-Length header field when indication
      that the request message does not contain target URI refers to a payload body resource that is subject to
      content negotiation and the method semantics do not anticipate such a
   body.

   A server MAY send a Content-Length header Content-Location field in value is a more
      specific identifier for the selected representation.

   o  For a 201 (Created) response to a
   HEAD request (Section 8.3.2); a server MUST NOT send Content-Length
   in such state-changing method, a response unless its
      Content-Location field value equals the decimal number
   of octets that would have been sent in is identical to the Location
      field value indicates that this payload body is a current
      representation of the newly created resource.

   o  Otherwise, such a response
   if Content-Location indicates that this payload is
      a representation reporting on the requested action's status and
      that the same request had used report is available (for future access with GET) at
      the GET method.

   A server MAY send a Content-Length header field in given URI.  For example, a 304 (Not
   Modified) response to purchase transaction made via a conditional GET
      POST request (Section 10.4.5); a
   server MUST NOT send Content-Length in such might include a response unless its
   field value equals the decimal number of octets that would have been
   sent in receipt document as the payload body of a
      the 200 (OK) response to response; the same request.

   A server MUST NOT send a Content-Length header Content-Location field in any response
   with value provides
      an identifier for retrieving a status code copy of 1xx (Informational) or 204 (No Content). that same receipt in the
      future.

   A
   server MUST NOT send a Content-Length header field user agent that sends Content-Location in any 2xx
   (Successful) response to a CONNECT request (Section 8.3.6).

   Aside from message is
   stating that its value refers to where the cases defined above, in user agent originally
   obtained the absence content 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 enclosed representation (prior to measure
   transfer progress, know when a received message is complete, and
   potentially reuse any
   modifications made by that user agent).  In other words, the connection for additional requests.

   Any Content-Length field value greater than or equal to zero is
   valid.  Since there user
   agent is no predefined limit providing a back link to the length source of a
   payload, a recipient MUST anticipate potentially large decimal
   numerals and prevent parsing errors due to integer conversion
   overflows (Section 12.5).

   If a message is received the original
   representation.

   An origin server that has receives a Content-Length header Content-Location field value
   consisting of the same decimal value as in a comma-separated list
   (Section 5.5) - for example, "Content-Length: 42, 42" - indicating
   that duplicate Content-Length header fields have been generated or
   combined by an upstream request
   message processor, then MUST treat the information as transitory request context
   rather than as metadata to be saved verbatim as part of the recipient MUST
   either reject
   representation.  An origin server MAY use that context to guide in
   processing the message request or to save it for other uses, such as invalid within
   source links or replace versioning metadata.  However, an origin server MUST
   NOT use such context information to alter the duplicated field
   values with request semantics.

   For example, if a single valid Content-Length field containing client makes a PUT request on a negotiated resource
   and the origin server accepts that
   decimal value prior PUT (without redirection), then
   the new state of that resource is expected to determining be consistent with the message body length or
   forwarding
   one representation supplied in that PUT; the message.

7.2.5. Content-Location

   The "Content-Location" header field references a URI that can cannot
   be used as an identifier for a specific resource corresponding 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 in this message's payload. (Section 7).  In other words, if one
   were responses to perform a GET request safe requests, validator
   fields describe the selected representation chosen by the origin
   server while handling the response.  Note that, depending on this URI at the time of this
   message's generation, then
   status code semantics, the selected representation for a 200 (OK) given
   response would contain is not necessarily the same as the 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. response payload.

   In the
   latter case (Section 2.4), the referenced URI is relative a successful response to the
   target URI ([RFC3986], Section 5).

   The Content-Location value is not a replacement for state-changing request, validator
   fields describe the target URI
   (Section 6.1).  It is new representation metadata.  It that has replaced the same
   syntax and semantics prior
   selected representation as a result of processing the header request.

   For example, an ETag field in a 201 (Created) response communicates
   the entity-tag of the same name defined for
   MIME body parts newly created resource's representation, so
   that it can be used in later conditional requests to prevent the
   "lost update" problem Section 4 12.1.

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

            Table 5

   This specification defines two forms of [RFC2557].  However, its appearance
   in an HTTP message has some special implications metadata that are commonly
   used to observe resource state and test for HTTP recipients.

   If Content-Location is included in a 2xx (Successful) response
   message preconditions:
   modification dates (Section 7.9.2) and its 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 refers (after conversion to absolute form) is
   referred to as a
   URI that "validator" when it is the same as the target URI, then the recipient MAY
   consider the payload 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 be a current representation of generate efficiently.  Rather than impose
   that all forms of 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 adhere to the same as strength of validator,
   HTTP exposes the default semantics type of validator in use and imposes restrictions on
   when no Content-Location weak validators can be used as preconditions.

   A "strong validator" is provided by the
   server.  For representation metadata that changes value
   whenever a state-changing request like PUT (Section 8.3.4) or
   POST (Section 8.3.3), it implies change occurs to the representation data that would be
   observable in the server's payload body of a 200 (OK) response contains to GET.

   A strong validator might change for reasons other than a change to
   the new representation data, such as when a semantically significant part
   of that resource, thereby distinguishing it
   from representations that might only report about the action representation metadata is changed (e.g.,
   "It worked!").  This allows authoring applications to update their
   local copies without the need for a subsequent GET request.

   If Content-Location Content-Type), but
   it is included in a 2xx (Successful) response
   message and its field value refers to a URI that differs from the
   target URI, then best interests of 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 change the target URI refers to a resource that
   value when it is subject necessary to
      content negotiation and invalidate the Content-Location field value is a more
      specific identifier stored responses held by
   remote caches and authoring tools.

   Cache entries might persist for the selected representation.

   o  For arbitrarily long periods, regardless
   of expiration times.  Thus, a 201 (Created) response cache might attempt to validate an
   entry using a state-changing method, a
      Content-Location field value validator that is identical to it obtained in the Location
      field value indicates that this payload distant past.  A
   strong validator is a current
      representation unique across all versions of the newly created resource.

   o  Otherwise, such all representations
   associated with a Content-Location indicates that this payload particular resource over time.  However, there is
      a representation reporting on the requested action's status and
      that
   no implication of uniqueness across representations of different
   resources (i.e., the same report is available (for future access with GET) at
      the given URI.  For example, a purchase transaction made via a
      POST request strong validator might include a receipt document as the payload be in use for
   representations of multiple resources at the 200 (OK) response; the Content-Location field value provides
      an identifier for retrieving same time and does not
   imply that those representations are equivalent).

   There are a copy variety of that same receipt strong validators used in the
      future.

   A user agent that sends Content-Location practice.  The best
   are based on strict revision control, wherein each change to a
   representation always results in a request message unique node name and revision
   identifier being assigned before the representation is
   stating that its value refers made
   accessible to GET.  A collision-resistant hash function applied 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, data is also sufficient if the user
   agent data is providing a back link available
   prior to the source of the original
   representation.

   An origin server that receives a Content-Location field in a request
   message MUST treat response header fields being sent and the information as transitory request context
   rather than as metadata digest does
   not need to be saved verbatim as part of the
   representation.  An origin server MAY use recalculated every time a validation request is
   received.  However, if a resource has distinct representations that context to guide
   differ only in
   processing the request or to save it for other uses, their metadata, such as within
   source links or versioning metadata.  However, an origin server MUST
   NOT use such context information might occur with content
   negotiation over media types that happen to alter share the request semantics.

   For example, if a client makes a PUT request on a negotiated resource
   and same data
   format, then the origin server accepts that PUT (without redirection), then needs to incorporate additional
   information in the new state of that resource validator to distinguish those representations.

   In contrast, a "weak validator" is expected representation metadata that might
   not change for every change to be consistent with the
   one representation supplied data.  This
   weakness might be due to limitations in that PUT; how the Content-Location cannot
   be used value is calculated,
   such as a form of reverse content selection identifier clock resolution, an inability to update
   only one ensure uniqueness for all
   possible representations 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.

7.3.  Payload

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

   Header fields that specifically describe the payload,
   equivalency rather than the
   associated representation, are referred to as "payload header
   fields".  Payload header fields are defined in other parts unique sequences of this
   specification, due data.  An origin server
   SHOULD change a weak entity-tag whenever it considers prior
   representations to their impact on message parsing.

    ------------------- ----------------------------
     Field Name          Ref.
    ------------------- ----------------------------
     Content-Range       7.3.4
     Trailer             5.6.4
     Transfer-Encoding   Section 6.1 of [Messaging]
    ------------------- ----------------------------

                        Table 9

7.3.1.  Purpose

   The purpose of be unacceptable as a payload in substitute for the current
   representation.  In other words, a request is defined by weak entity-tag ought to change
   whenever the method
   semantics. origin server wants caches to invalidate old responses.

   For example, a representation in the payload representation of a PUT
   request (Section 8.3.4) represents the desired state weather report that changes in
   content every second, based on dynamic measurements, might be grouped
   into sets of equivalent representations (from the target
   resource if origin server's
   perspective) with the request is successfully applied, whereas a
   representation same weak validator in the payload 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 POST request (Section 8.3.3)
   represents information 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 processed by the target resource.

   In modified twice during a response, the payload's purpose single second
   and retrieved between those modifications.

   Likewise, a validator is defined weak if it is shared by both the request
   method and the response status code.  For example, the payload two or more
   representations of a
   200 (OK) response to GET (Section 8.3.1) represents the current state
   of the target resource, as observed given resource at the time of the message
   origination date (Section 11.1.1), whereas same time, unless those
   representations have identical representation data.  For example, if
   the payload of origin server sends the same
   status code in validator for a response to POST might represent either the
   processing result or the new state of the target resource after
   applying the processing.  Response messages representation with an error status code
   usually contain
   a payload gzip content coding applied as it does for a representation with no
   content coding, then that represents validator is weak.  However, two
   simultaneous representations might share the error condition, same strong validator if
   they differ only in the representation metadata, such
   that it describes as when two
   different media types are available for the error state and what next steps same representation data.

   Strong validators are suggested usable for resolving it.

7.3.2.  Identification

   When a complete or 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 is transferred in data,
   such as when validating a message
   payload, it is often desirable for the sender to supply, cache entry or the
   recipient to determine, an identifier for limiting a resource corresponding web traversal to
   that representation.

   For
   recent changes.

7.9.2.  Last-Modified

   The "Last-Modified" header field in a request message:

   o  If the request has response provides a Content-Location header field, then timestamp
   indicating the
      sender asserts that date and time at which the payload is a representation of origin server believes the
      resource identified by
   selected representation was last modified, as determined at 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,
   conclusion of handling the payload request.

     Last-Modified = HTTP-date

   An example of its use is unidentified.

   For

     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 response message, the following rules are applied last modification date can be reasonably
   and consistently determined, since its use in conditional requests
   and evaluating cache freshness ([Caching]) results in order
   until a match is found:

   1.  If substantial
   reduction of HTTP traffic on the request method is GET or HEAD Internet and the response status code
       is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not
       Modified), the payload is can be a significant
   factor in improving service scalability and reliability.

   A representation is typically the sum of many parts behind the
   resource
       identified by the target URI (Section 6.1).

   2.  If interface.  The last-modified time would usually be the request method most
   recent time that any of those parts were changed.  How that value is GET or HEAD and the response status code
   determined for any given resource is 203 (Non-Authoritative Information), an implementation detail beyond
   the payload scope of this specification.  What matters to HTTP is a
       potentially modified or enhanced representation how
   recipients of the target
       resource as provided by an intermediary.

   3.  If the response has a Content-Location Last-Modified header field and can use its field value is a reference to
   make conditional requests and test the same URI validity of locally cached
   responses.

   An origin server SHOULD obtain the Last-Modified value of the
   representation as close as possible to the target URI, time that it generates the
       payload is
   Date field value for its response.  This allows a representation recipient to make
   an accurate assessment of the target resource.

   4.  If representation's modification time,
   especially if the representation changes near the time that the
   response has a Content-Location header field and its field
       value is generated.

   An origin server with a reference to clock MUST NOT send a URI different 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
   metadata that evaluates to some time in the target URI, future, according to the
   origin server's clock, then the sender asserts origin server MUST replace that value
   with the payload is message origination date.  This prevents a representation of 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 identified
   by the Content-Location field value.
       However, such an assertion cannot be trusted 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 can be
       verified by other means (not defined by this specification).

   5.  Otherwise, the payload is unidentified.

7.3.3.  Payload Body

   The payload body contains the data of a request or response.  This possible to deduce that it is
   distinct from strong,
   using the message body (e.g., Section 6 of [Messaging]),
   which following rules:

   o  The validator is how being compared by an origin server to the payload body is transferred "on actual
      current validator for the wire", and might
   be encoded, depending on representation and,

   o  That origin server reliably knows that the HTTP version in use.

   It is also distinct from a request or response's associated
      representation data
   (Section 7.1), which can be inferred from protocol operation, rather
   than necessarily appearing "on did not change twice during the wire."

   The presence of a payload body in a request depends on whether second covered by
      the
   request method used defines semantics for it. presented validator.

   or

   o  The presence of validator is about to be used by a payload body client in an
      If-Modified-Since, If-Unmodified-Since, or If-Range header field,
      because the client has a response depends on both cache entry for the
   request method to associated
      representation, and

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

   o  The presented Last-Modified time is responding 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 response status code
   (Section 10).

   Responses to original response, and

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

   This method (Section 8.3.2) never include a
   payload body because relies on the associated response header fields indicate
   only what their values would have been fact that if two different responses were
   sent by the request method origin server during the same second, but both 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
   same Last-Modified time, then at least one of
   having a payload body.

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

   All other those responses do include would
   have a payload body, although Date value equal to its Last-Modified time.  The arbitrary
   60-second limit guards against the possibility that body
   might be the Date and
   Last-Modified values are generated from different clocks or at
   somewhat different times during the preparation of zero length.

7.3.4.  Content-Range 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 "Content-Range" header "ETag" field is sent in a single part 206
   (Partial Content) response to indicate provides the partial range of current entity-tag for
   the selected representation enclosed representation, as determined at the message payload, sent in each
   part conclusion of a multipart 206 response to indicate
   handling the range enclosed
   within each body part, and sent in 416 (Range Not Satisfiable)
   responses 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 provide information about resource state changes over time, content negotiation
   resulting in multiple representations being valid at the selected representation.

     Content-Range same time,
   or both.  An entity-tag consists of an opaque quoted string, possibly
   prefixed by a weakness indicator.

     ETag       = range-unit SP
                           ( range-resp / unsatisfied-range )

     range-resp entity-tag

     entity-tag = incl-range "/" ( complete-length / "*" )
     incl-range [ weak ] opaque-tag
     weak       = first-pos "-" last-pos
     unsatisfied-range %s"W/"
     opaque-tag = "*/" complete-length

     complete-length DQUOTE *etagc DQUOTE
     etagc      = 1*DIGIT

   If a 206 (Partial Content) response contains a Content-Range header
   field with a range unit (Section 7.1.4) that the recipient does not
   understand, the recipient MUST NOT attempt %x21 / %x23-7E / obs-text
                ; VCHAR except double quotes, plus obs-text

      |  *Note:* Previously, opaque-tag was defined to recombine it with a
   stored representation.  A proxy that receives such a message SHOULD
   forward it downstream.

   For byte ranges, be 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 quoted-
      |  string ([RFC2616], Section 3.11); thus, some recipients might
      |  perform backslash unescaping.  Servers therefore ought to determine.  An asterisk
   character ("*") avoid
      |  backslash characters 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 entity tags.

   An entity-tag can be 1234 bytes:

     Content-Range: bytes 42-1233/1234

   and this second example illustrates when the complete length more reliable for validation than a modification
   date in situations where it is
   unknown:

     Content-Range: bytes 42-1233/*

   A Content-Range field value inconvenient to store modification
   dates, where the one-second resolution of HTTP date values is invalid if it contains a range-resp
   that has a last-pos value less than its first-pos value, not
   sufficient, or where modification dates are not consistently
   maintained.

   Examples:

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

   An entity-tag can be either a
   complete-length value less than weak or equal to its last-pos value.  The
   recipient of an invalid Content-Range MUST NOT attempt to recombine
   the received content strong validator, with a stored representation.

   A strong
   being the default.  If an origin server generating a 416 (Range Not Satisfiable) response to a byte-
   range request SHOULD send a Content-Range header field with provides an
   unsatisfied-range value, as in the following example:

     Content-Range: bytes */1234

   The complete-length in entity-tag for a 416 response indicates
   representation and the current length generation of
   the selected representation.

   The Content-Range header field has no meaning for status codes that
   do entity-tag does 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 satisfy
   all of Content-Range values in which the
   selected representation contains a total characteristics 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

   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

7.3.5.  Media Type multipart/byteranges

   When a 206 (Partial Content) response message includes strong validator (Section 7.9.1),
   then the content of
   multiple ranges, they are transmitted origin server MUST mark the entity-tag as body parts weak by prefixing
   its opaque value with "W/" (case-sensitive).

   A sender MAY send the Etag field in a multipart
   message body ([RFC2046], trailer section (see
   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 5.6).  However, since trailers are often ignored, it is
   preferable to
   separate each body part.

   Implementation Notes:

   1.  Additional CRLFs might precede send Etag as a header field unless the first boundary string in entity-tag is
   generated while sending the message body.

   2.  Although [RFC2046] permits

7.9.3.1.  Generation

   The principle behind entity-tags is that only the boundary string to be quoted, some
       existing implementations handle a quoted boundary string
       incorrectly.

   3.  A number service author
   knows the implementation of clients and servers were coded a resource well enough to an early draft of select the byteranges specification most
   accurate and efficient validation mechanism for 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 resource, and
   that any such mechanism can be mapped to byte ranges.  The following example uses an "exampleunit"
   range unit:

     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 a simple sequence of octets
   for easy comparison.  Since the registration form value is opaque, there is no need 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 12

   Interoperability considerations:  N/A

   Published specification:  This specification (see Section 7.3.5).

   Applications 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 this media type:  HTTP components supporting
      multiple ranges in an internal revision number, perhaps
   combined with a single request.

   Fragment variance identifier considerations:  N/A

   Additional information:  Deprecated alias names for this type:  N/A

                            Magic number(s):  N/A

                            File extension(s):  N/A

                            Macintosh file type code(s):  N/A

   Person and email address content negotiation, 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

7.4.  Content Negotiation

   When responses convey payload information, whether indicating
   accurately differentiate between representations.  Other
   implementations might use a
   success collision-resistant hash of
   representation content, a combination of various file attributes, or an error, the
   a modification timestamp that has sub-second resolution.

   An origin server often has different ways of
   representing that information; SHOULD send an ETag for any selected representation
   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 detection of changes can be best to deliver.  For this reason, 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 provides mechanisms 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".

   The example below shows the results for content negotiation.

   This specification defines three patterns 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 that
   can be made visible within the protocol: "proactive" negotiation,
   (Section 11), and where the server selects 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 based upon the user
   agent's stated preferences, "reactive" negotiation, where the server
   provides that does use gzip content coding would
   be:

   >> 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 list property of representations for the user agent to choose from,
   and "request payload" negotiation, where the user agent selects the representation
      |  data, so a strong entity-tag for a future request based upon content-encoded
      |  representation has to be distinct from the server's stated
   preferences in past responses.  Other patterns entity tag of content negotiation
   include "conditional content", where the an
      |  unencoded representation consists to prevent potential conflicts during
      |  cache updates and range requests.  In contrast, transfer
      |  codings (Section 7 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, [Messaging]) apply only during message
      |  transfer and "Transparent Content Negotiation"
   ([RFC2295]), where content selection is performed by an intermediary.
   These patterns are do not mutually exclusive, and each has trade-offs result in
   applicability distinct entity-tags.

7.9.4.  When to Use Entity-Tags and practicality.

   Note that, in all cases, HTTP is not aware of the resource semantics.
   The consistency with which Last-Modified Dates

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

   o  SHOULD send an entity-tag validator unless it is not feasible to requests,
   over time and over the varying dimensions
      generate one.

   o  MAY send a weak entity-tag instead of content negotiation, and
   thus a strong entity-tag, if
      performance considerations support the "sameness" use of a resource's observed representations over
   time, is determined entirely by whatever entity or algorithm selects weak entity-tags, or generates those responses.

7.4.1.  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,
      if it is called proactive negotiation (a.k.a.,
   server-driven negotiation).  Selection is based on the available
   representations for unfeasible to send a response (the dimensions over which strong entity-tag.

   o  SHOULD send a Last-Modified value if it might
   vary, such as language, content-coding, etc.) compared to various
   information supplied in the request, including both the explicit
   negotiation fields of Section 9.4 and implicit characteristics, such
   as the client's network address or parts of the User-Agent field.

   Proactive negotiation is advantageous when feasible to send one.

   In other words, the algorithm preferred behavior for
   selecting from among the available representations an origin server is difficult to
   describe
   send both a strong entity-tag and a Last-Modified value in successful
   responses to a user agent, retrieval request.

   A client:

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

   o  MAY send its
   "best guess" to 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 along with
      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 first response (hoping
      origin server.  This allows both HTTP/1.0 and HTTP/1.1 caches to avoid
      respond appropriately.

8.  Methods

8.1.  Overview

   The request method token is the round trip delay primary source of a subsequent request if semantics;
   it indicates the "best
   guess" is good enough purpose for which the user).  In order to improve the
   server's guess, a user agent MAY send request header fields that
   describe its preferences.

   Proactive negotiation client has serious disadvantages:

   o  It made this request
   and what is impossible for expected by the server to accurately determine what client as a successful result.

   The request method's semantics might be "best" for any given user, since that would require complete
      knowledge of both further specialized by the capabilities
   semantics 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?);

   o  Having the user agent describe its capabilities some header fields when present in every request
      can be both very inefficient (given that only a small percentage
      of responses have multiple representations) and request if those
   additional semantics do not conflict with the method.  For example, a potential risk
   client can send conditional request header fields (Section 12.1) to
   make the user's privacy;

   o  It complicates requested action conditional on the implementation current state of an origin server and the
      algorithms for generating responses
   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 request; and,

   o  It limits target resource in much the reusability of responses for shared caching.

   A user agent cannot rely same way as
   invoking a defined method on proactive negotiation preferences being
   consistently honored, since the origin server might not implement
   proactive negotiation for the requested resource or an identified object would apply
   semantics.

   The method token is case-sensitive because it might decide that
   sending be used as a response that doesn't conform
   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 user agent's
   preferences is standardized request methods in HTTP
   are not resource-specific, since uniform interfaces provide for
   better than sending a 406 (Not Acceptable) response.

   A Vary header field (Section 11.1.4) is often sent visibility and reuse in network-based systems [REST].  Once
   defined, a response
   subject standardized method ought to proactive negotiation have the same semantics when
   applied to indicate what parts any resource, though each resource determines for itself
   whether those semantics are implemented or allowed.

   This specification defines a number of the
   request information were standardized methods that are
   commonly used in HTTP, as outlined by the selection algorithm.

7.4.2.  Reactive Negotiation

   With reactive negotiation (a.k.a., agent-driven negotiation),
   selection following table.

    --------- -------------------------------------------- -------
     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 best response representation (regardless   8.3.4
               target resource with the request payload.
     DELETE    Remove all current representations of the
   status code) is performed    8.3.5
               target resource.
     CONNECT   Establish a tunnel to the server             8.3.6
               identified by the user agent after receiving an
   initial response from target resource.
     OPTIONS   Describe the origin server that contains a list of
   resources communication options for alternative representations.  If the user agent is not
   satisfied   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 initial response representation, it set of allowed
   methods can perform change dynamically.  When a
   GET request on one method is received
   that is unrecognized or more of not implemented by an origin server, the alternative resources, selected
   based on metadata included in
   origin server SHOULD respond with the list, to obtain 501 (Not Implemented) status
   code.  When a different form of
   representation for request method is received that response.  Selection of alternatives might be
   performed automatically is known by an origin
   server but not allowed for the user agent or manually by target resource, the user
   selecting from a generated (possibly hypertext) menu.

   Note that origin server
   SHOULD respond with the above refers to representations of 405 (Method Not Allowed) status code.

   Additional methods, outside the response, in
   general, not representations scope of this specification, have
   been specified for use in HTTP.  All such methods ought to be
   registered within the resource.  The alternative
   representations "Hypertext Transfer Protocol (HTTP) Method
   Registry", as described in Section 15.1.

8.2.  Common Method Properties
8.2.1.  Safe Methods

   Request methods are only considered representations of the target
   resource "safe" if the response in which those alternatives their defined semantics are provided has
   essentially read-only; i.e., the semantics of being client does not request, and does
   not expect, any state change on the origin server as a representation result of the target resource (e.g.,
   applying a 200 (OK) response safe method 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 resource.  Likewise, reasonable
   use of a GET request).

   A server might choose safe method is not expected to send an initial representation, other
   than cause any harm, loss of
   property, or unusual burden on the list origin server.

   This definition of alternatives, and thereby indicate safe methods does not prevent an implementation
   from including behavior 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 potentially harmful, that the user is not
   entirely read-only, or user agent can react by
   making that causes side effects while invoking a selection.

   Reactive negotiation safe
   method.  What is advantageous when the response would vary
   over commonly used dimensions (such as type, language, or encoding),
   when the origin server important, however, is unable to determine a user agent's
   capabilities from examining that the request, client did not
   request that additional behavior and generally when public
   caches are used cannot be held accountable for
   it.  For example, most servers append request information to distribute server load and reduce network usage.

   Reactive negotiation suffers from access
   log files at the disadvantages completion of transmitting a
   list every response, regardless of alternatives to the user agent, which degrades user-perceived
   latency if transmitted in
   method, and that is considered safe even though the header section, log storage might
   become full and needing crash the server.  Likewise, a second safe 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 initiated
   by selecting an extension.

7.4.3.  Request Payload Negotiation

   When content negotiation preferences are sent in a server's response, advertisement on the listed preferences are called request payload negotiation because
   they intend to influence selection Web will often have the side
   effect of charging an appropriate payload for
   subsequent requests to that resource.  For example, advertising account.

   Of the
   Accept-Encoding field (Section 9.4.3) can request methods defined by this specification, the GET, HEAD,
   OPTIONS, and TRACE methods are defined to be sent in a response safe.

   The purpose of distinguishing between safe and unsafe methods is to
   indicate preferred content codings for subsequent requests
   allow automated retrieval processes (spiders) and cache performance
   optimization (pre-fetching) to that
   resource [RFC7694].

      |  Similarly, Section 3.1 work without fear of [RFC5789] defines the "Accept-Patch"
      |  response header field which causing harm.  In
   addition, it allows discovery of which content
      |  types are accepted in PATCH requests.

7.4.4.  Quality Values

   The content negotiation fields defined by this specification use a
   common parameter, named "q" (case-insensitive), to assign a relative
   "weight" user agent to apply appropriate constraints on
   the preference for that associated kind automated use of unsafe methods when processing potentially
   untrusted content.  This
   weight is referred

   A user agent SHOULD distinguish between safe and unsafe methods when
   presenting potential actions to as a "quality value" (or "qvalue") because user, such that the
   same parameter name user can be
   made aware of an unsafe action before it is often used within server configurations to
   assign requested.

   When a weight to resource is constructed such that parameters within the relative quality of target
   URI have the various
   representations that can be selected for a resource.

   The weight effect of selecting an action, it is normalized the resource
   owner's responsibility to a real number in ensure that the range 0 through 1,
   where 0.001 action is consistent with
   the least preferred and 1 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 most preferred; a
   value purpose of 0 means "not acceptable".  If no "q" parameter such a
   resource is present, to perform an unsafe action, then the default weight is 1.

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

   A sender of qvalue resource owner MUST NOT generate more than three digits after the
   decimal point.  User configuration of these values ought
   disable or disallow that action when it is accessed using a safe
   request method.  Failure to be
   limited do so will result in unfortunate side
   effects when automated processes perform a GET on every URI reference
   for the same fashion.

8.  Request sake of link maintenance, pre-fetching, building a search
   index, etc.

8.2.2.  Idempotent Methods

8.1.  Overview

   The

   A request method token is considered "idempotent" if the primary source intended effect on
   the server of request semantics;
   it indicates multiple identical requests with that method is the purpose
   same as the effect for which a single such request.  Of the client has made this 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 is expected has been requested by the client as user; a successful result.

   The server is free to log each
   request method's semantics might be further specialized by the
   semantics of some header fields when present in separately, retain a revision control history, or implement
   other non-idempotent side effects for each idempotent request.

   Idempotent methods are distinguished because the request (Section 9) can be
   repeated automatically if those additional semantics do not conflict with a communication failure occurs before the method.
   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 send conditional establish a new
   connection and retry the idempotent request.  It knows that repeating
   the request header fields
   (Section 9.2) to make will have the requested action conditional on same intended effect, even if the current
   state of original
   request succeeded, though the target resource. response might differ.

   A client SHOULD NOT automatically retry a request with a non-
   idempotent method = token

   HTTP was originally designed unless it has some means to be usable as an interface know that the request
   semantics are actually idempotent, regardless of the method, or some
   means to
   distributed object systems.  The detect that the original request method was envisioned as
   applying semantics never applied.

   For example, a user agent that knows (through design or
   configuration) that a POST request to a target given resource in much the same way as
   invoking is safe can
   repeat that request automatically.  Likewise, a defined method user agent designed
   specifically to operate on an identified object would apply
   semantics.

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

   Unlike distributed objects, recover from partial failure conditions by checking the standardized 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 methods in HTTP
   are 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 resource-specific, since uniform interfaces provide for
   better visibility recommended.

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

8.2.3.  Methods and reuse in network-based systems [REST].  Once
   defined, Caching

   For a standardized method ought cache to have store and use a response, the same semantics when
   applied associated method needs
   to any resource, though each resource determines for itself
   whether those semantics are implemented or allowed. 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 a number of standardized methods that are
   commonly used in HTTP, as outlined by caching semantics for GET, HEAD, and POST,
   although the following table.

    --------- -------------------------------------------- ------- overwhelming majority of cache implementations only
   support GET and HEAD.

8.3.  Method    Description                                  Ref.
    --------- -------------------------------------------- ------- Definitions

8.3.1.  GET       Transfer

   The GET method requests transfer of a current selected representation of
   for 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

   GET is the request payload.
     PUT       Replace all current representations primary mechanism of information retrieval and the   8.3.4
               target resource with the request payload.
     DELETE    Remove focus
   of almost all current representations performance optimizations.  Hence, when people speak of the    8.3.5
               target resource.
     CONNECT   Establish a tunnel
   retrieving some identifiable information via HTTP, they are generally
   referring to making a GET request.  A successful response reflects
   the server             8.3.6 quality of "sameness" identified by the target resource.
     OPTIONS   Describe the communication options URI.  In turn,
   constructing applications such that they produce a URI for the   8.3.7
               target resource.
     TRACE     Perform each
   important resource results in more resources being available for
   other applications, producing a message loop-back test along network effect that promotes further
   expansion of the   8.3.8
               path Web.

   It is tempting to the target resource.
    --------- -------------------------------------------- -------

                               Table 10

   All general-purpose servers MUST support the methods GET think of resource identifiers as remote file system
   pathnames and HEAD.
   All other methods of representations as being a copy of the contents of
   such files.  In fact, that is how many resources are OPTIONAL. implemented (see
   Section 16.3 for related security considerations).  However, there
   are no such limitations in practice.

   The set of methods allowed by HTTP interface for a target resource can is just as likely to be listed in an
   Allow header field (Section 11.4.2).  However, the set implemented
   as a tree of allowed
   methods can change dynamically.  When content objects, a request method is received
   that is unrecognized programmatic view on various database
   records, or not implemented by an origin server, a gateway to other information systems.  Even when the
   URI mapping mechanism is tied to a file system, an origin server SHOULD respond
   might be configured to execute the files with the 501 (Not Implemented) status
   code.  When a request method is received that is known by an as input
   and send the output as the representation rather than transfer the
   files directly.  Regardless, only the origin server but not allowed for 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, resource in a response to GET.

   A client can alter the origin server
   SHOULD respond with semantics of GET to be a "range request",
   requesting transfer of only some part(s) of the 405 (Method Not Allowed) status code.

8.2.  Common Method Properties

                   --------- ------ ------------ -------
                    Method    Safe   Idempotent   Ref.
                   --------- ------ ------------ -------
                    CONNECT   no     no           8.3.6
                    DELETE    no     yes          8.3.5 selected
   representation, by sending a Range header field in the request
   (Section 13.2).

   A client SHOULD NOT generate a body in a 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 request.  A payload
   received in a GET request has no     yes          8.3.4
                    TRACE     yes    yes          8.3.8
                   --------- ------ ------------ -------

                                  Table 11

8.2.1.  Safe Methods

   Request methods are considered "safe" if their defined semantics are
   essentially read-only; i.e., semantics, cannot alter the
   meaning or target of the client does not request, and does
   not expect, any state change on might lead some implementations
   to reject the origin server request and close the connection because of its
   potential as a result request smuggling attack (Section 11.2 of
   applying a safe method
   [Messaging]).

   The response to a target resource.  Likewise, reasonable
   use of a safe method GET request is not expected cacheable; a cache MAY use it to cause any harm, loss of
   property, or unusual burden on
   satisfy subsequent GET and HEAD requests unless otherwise indicated
   by the origin server.

   This definition Cache-Control header field (Section 5.2 of safe methods does not prevent an implementation
   from including behavior [Caching]).  A
   cache that receives a payload in a GET request is potentially harmful, likely to ignore
   that payload and cache regardless of the payload contents.

   When information retrieval is not
   entirely read-only, or that causes side effects while invoking performed with a safe
   method.  What is important, however, is mechanism that
   constructs a target URI from user-provided information, such as the client did not
   request
   query fields of a form using GET, potentially sensitive data might be
   provided that additional behavior and cannot would not be held accountable appropriate for
   it.  For example, most servers append request information to access
   log files at disclosure within a URI
   (see Section 16.9).  In some cases, the completion of every 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, regardless using the POST method (Section 8.3.3) instead of GET will
   usually transmit such information in the
   method, and that request body rather than
   construct a new URI.

8.3.2.  HEAD

   The HEAD method is considered safe even though the log storage might
   become full and crash identical to GET except that the server.  Likewise, server MUST NOT
   send a safe request initiated
   by selecting an advertisement on message body in the Web will often have response (i.e., the side
   effect response terminates at
   the end of charging an advertising account.

   Of the header section).  The server SHOULD send the same
   header fields in response to a HEAD request methods defined by this specification, as it would have sent if
   the request had been a GET, HEAD,
   OPTIONS, 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 TRACE methods are 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 be safe. reject the request.

   The purpose of distinguishing between safe and unsafe methods is response to
   allow automated retrieval processes (spiders) and a HEAD request is cacheable; a cache performance
   optimization (pre-fetching) to work without fear of causing harm.  In
   addition, MAY use it allows a user agent to apply appropriate constraints on
   satisfy subsequent HEAD requests unless otherwise indicated by the automated use
   Cache-Control header field (Section 5.2 of unsafe methods when processing potentially
   untrusted content. [Caching]).  A user agent SHOULD distinguish between safe and unsafe methods when
   presenting potential actions HEAD
   response might also have an effect on previously cached responses to a user, such that the user can be
   made aware
   GET; see Section 4.3.5 of an unsafe action before it is requested.

   When a resource is constructed such [Caching].

8.3.3.  POST

   The POST method requests that parameters within the target
   URI have the effect of selecting an action, it is the resource
   owner's responsibility to ensure that process the action is consistent with
   representation enclosed in the request method according to the resource's
   own specific semantics.  For example, it POST is common used for Web-
   based content editing software to use actions within query
   parameters, the following
   functions (among others):

   o  Providing a block of data, such as "page?do=delete".  If the purpose 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 such articles;

   o  Creating a new resource is that has yet to perform an unsafe action, then be identified by the resource owner MUST
   disable or disallow that action when it is accessed using a safe
   request method.  Failure
      origin server; and

   o  Appending data to do so will result in unfortunate side
   effects when automated processes perform a GET resource's existing representation(s).

   An origin server indicates response semantics by choosing an
   appropriate status code depending on every URI reference
   for the sake result of link maintenance, pre-fetching, building a search
   index, etc.

8.2.2.  Idempotent Methods

   A request method is considered "idempotent" if processing the intended effect
   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 of multiple identical requests with that method is the
   same as a
   result of successfully processing a POST request, the effect for origin server
   SHOULD send a single such request.  Of 201 (Created) response containing a Location header
   field that provides an identifier for the request methods
   defined by this specification, PUT, DELETE, primary resource created
   (Section 9.2.3) and safe request methods
   are idempotent.

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

   Idempotent methods are distinguished 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].

   If the result of processing a POST would be equivalent to a
   representation of an existing resource, an origin server MAY redirect
   the request can be
   repeated automatically if user agent to that resource by sending a communication failure occurs before 303 (See Other) response
   with the
   client is able to read existing resource's identifier in the server's response.  For example, if a
   client sends a PUT request and Location field.  This
   has the underlying connection is closed
   before any response is received, then benefits of providing the client can establish user agent a new
   connection resource identifier
   and retry transferring the idempotent request.  It knows that repeating representation via a method more amenable to
   shared caching, though at the cost of an extra request will have the same intended effect, even if the original
   request succeeded, though user
   agent does not already have the response might differ.

   A client SHOULD NOT automatically retry a request with a non-
   idempotent representation cached.

8.3.4.  PUT

   The PUT method unless it has some means to know requests that the request
   semantics are actually idempotent, regardless state of the method, target resource be
   created or some
   means to detect that replaced with the state defined by the representation
   enclosed in the original request was never applied.

   For example, message payload.  A successful PUT of a user agent that knows (through design or
   configuration) given
   representation would suggest that a POST request to a given subsequent GET on that same
   target resource will result in an equivalent representation being
   sent in a 200 (OK) response.  However, there is safe can
   repeat no guarantee that request automatically.  Likewise, a user agent designed
   specifically to operate on
   such a version control repository might state change will be able
   to recover from partial failure conditions by checking observable, since the target resource revision(s) after a failed connection, reverting
   might be acted upon by other user agents in parallel, or fixing
   any changes that were partially applied, and then automatically
   retrying the requests that failed.

   Some clients use weaker signals might be
   subject to initiate automatic retries.  For
   example, when a POST request is sent, but dynamic processing by the underlying transport
   connection is closed origin server, before any part of the response
   subsequent GET 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.

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 successful 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 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 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
   PUT successfully creates one, then the quality of "sameness" identified origin server MUST inform the
   user agent by sending a 201 (Created) response.  If 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 does have a network effect current representation and that promotes further
   expansion representation
   is successfully modified in accordance with the state of the Web.

   It is tempting enclosed
   representation, then the origin server MUST send either a 200 (OK) or
   a 204 (No Content) response to think indicate successful completion of resource identifiers as remote file system
   pathnames the
   request.

   An origin server SHOULD ignore unrecognized header and of representations as being trailer fields
   received in a copy PUT request (i.e., do not save them as part of the contents of
   such files.  In fact,
   resource state).

   An origin server SHOULD verify that the PUT representation is how many resources are implemented (see
   Section 12.3 for related security considerations).  However, there
   are no such limitations in practice.

   The HTTP interface
   consistent with any constraints the server has for a the target
   resource is just as likely to be implemented
   as a tree of content objects, a programmatic view on various database
   records, that cannot or a gateway to other information systems.  Even when will not be changed by the
   URI mapping mechanism PUT.  This is tied to a file system, an
   particularly important when the origin server
   might be configured uses internal
   configuration information related to execute the files with the request as input
   and send the output as URI in order to set the
   values for representation rather than transfer metadata on GET responses.  When a PUT
   representation is inconsistent with the
   files directly.  Regardless, only target resource, the origin
   server needs to know how
   each of its SHOULD either make them consistent, by transforming the
   representation or changing the resource identifiers corresponds to configuration, or respond
   with an implementation and
   how each implementation manages appropriate error message containing sufficient information
   to select and send a current explain why the representation of is unsuitable.  The 409 (Conflict)
   or 415 (Unsupported Media Type) status codes are suggested, with the target resource in a response
   latter being specific to GET.

   A client can alter constraints on Content-Type values.

   For example, if the semantics of GET target resource is configured to be always have a "range request",
   requesting transfer of only some part(s)
   Content-Type of "text/html" and the selected
   representation, by sending representation being PUT has a Range header field in
   Content-Type of "image/jpeg", the request
   (Section 9.3).

   A client SHOULD NOT generate a body in a GET request.  A payload
   received in a GET request has no defined semantics, cannot alter origin server ought to do one of:

   a.  reconfigure the
   meaning or target of the request, and might lead some implementations resource to reject reflect the request and close new media type;

   b.  transform 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 PUT representation 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 format consistent with that payload and cache regardless
       of the payload contents.

   When information retrieval is performed resource before saving it as the new resource state; or,

   c.  reject the request with a mechanism 415 (Unsupported Media Type) response
       indicating that
   constructs a target URI from user-provided information, such as the
   query fields of target resource is limited to "text/html",
       perhaps including a form using GET, potentially sensitive data might be
   provided link to a different resource that would not be appropriate a
       suitable target for disclosure within the new representation.

   HTTP does not define exactly how a URI
   (see Section 12.9).  In some cases, PUT method affects the data state of an
   origin server beyond what 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 expressed by the POST method (Section 8.3.3) instead intent of GET will
   usually transmit such information in the user
   agent request body rather than
   construct a new URI.

8.3.2.  HEAD

   The HEAD method is identical to GET except that and the semantics of the origin server MUST NOT
   send response.  It
   does not define what a message body resource might be, in any sense of that word,
   beyond the response (i.e., 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 response terminates at origin server
   translates resource state into representations.  Generally speaking,
   all implementation details behind the end of resource interface are
   intentionally hidden by the header section).  The server.

   An origin server SHOULD MUST NOT send the same a validator header fields field
   (Section 7.9), such as an ETag or Last-Modified field, in a
   successful response to a HEAD request as it would have sent if
   the request had been a GET, except that the payload header fields
   (Section 7.3) MAY be omitted.  This method can be used for obtaining
   metadata about PUT unless the selected request's representation data
   was saved without transferring any transformation applied to the body (i.e., the
   resource's new 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 identical to reject the request.

   The response to a HEAD request is cacheable; representation
   data received in the PUT request) and the validator field value
   reflects the new representation.  This requirement allows a cache MAY use it user
   agent to
   satisfy subsequent HEAD requests unless otherwise indicated by know when the
   Cache-Control header field (Section 5.2 representation body it has in memory remains
   current as a result of [Caching]).  A HEAD
   response might also have an effect on previously cached responses to
   GET; see Section 4.3.5 the PUT, thus not in need of [Caching].

8.3.3.  POST

   The POST method requests that being retrieved
   again from the target resource process origin server, and that the
   representation enclosed new validator(s) received
   in the request according response can be used for future conditional requests in order
   to prevent accidental overwrites (Section 12.1).

   The fundamental difference between the resource's
   own specific semantics.  For example, POST and PUT methods is used
   highlighted by the different intent for the following
   functions (among others):

   o  Providing enclosed representation.
   The target resource in a block of data, such as the fields entered into an HTML
      form, POST request is intended to a data-handling process;

   o  Posting a message handle the
   enclosed representation according to the resource's own semantics,
   whereas the enclosed representation in a bulletin board, newsgroup, mailing list,
      blog, or similar group PUT request is defined as
   replacing the state of articles;

   o  Creating a new resource that has yet to be identified by the
      origin server; target resource.  Hence, the intent of PUT
   is idempotent and

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

   An origin server indicates response semantics intermediaries, even though the exact
   effect is only known by choosing an
   appropriate status code depending on the result origin server.

   Proper interpretation of processing a PUT request presumes that the
   POST request; almost all user agent
   knows which target resource is desired.  A service that selects a
   proper URI on behalf of the status codes defined by this
   specification could be received in client, after receiving a response to state-changing
   request, SHOULD be implemented using the POST (the exceptions
   being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not
   Satisfiable)). method rather than PUT.
   If one or more resources has been created on the origin server as will not make the requested PUT state change to
   the target resource and instead wishes to have it applied to a
   result of successfully processing
   different resource, such as when the resource has been moved to a POST request,
   different URI, then the origin server
   SHOULD MUST send a 201 (Created) response containing a Location header
   field that provides an identifier for the primary resource created
   (Section 11.1.2) and a representation that describes appropriate 3xx
   (Redirection) response; the status of user agent MAY then make its own decision
   regarding whether or not to redirect the request.

   A PUT request while referring applied 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 target resource can have side effects on
   other resources.  For example, an article might have a
   Content-Location header field URI for
   identifying "the current version" (a resource) that has is separate from
   the URIs identifying each particular version (different resources
   that at one point shared the same value state as the POST's
   target URI (Section 7.2.5). current version
   resource).  A cached POST response can be reused to
   satisfy a later GET or HEAD request, but not successful PUT request on "the current version" URI
   might therefore create a POST request, since
   POST is required to be written through new version resource in addition to changing
   the origin server, because
   it is unsafe; see Section 4 state of [Caching].

   If the result of processing a POST would be equivalent to a
   representation of an existing target resource, an and might also cause links to be
   added between the related resources.

   An origin server MAY redirect
   the user agent to that allows PUT on a given target resource by sending MUST send
   a 303 (See Other) 400 (Bad Request) response
   with the existing resource's identifier in to a PUT request that contains a
   Content-Range header field (Section 13.4), since the Location field.  This payload is
   likely to be partial content that has the benefits 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 providing
   the user agent larger resource, or by using a resource identifier
   and transferring different method that has been
   specifically defined for partial updates (for example, the representation via a PATCH
   method more amenable defined in [RFC5789]).

   Responses to
   shared caching, though at the cost of an extra request if the user
   agent does PUT method are not already have the representation cached.

8.3.4. 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]).

8.3.5.  DELETE

   The PUT DELETE method requests that the state of origin server remove the
   association between the target resource be
   created or replaced with the state defined by and its current
   functionality.  In effect, this method is similar to the representation
   enclosed rm command
   in the request message payload.  A successful PUT of a given
   representation would suggest that UNIX: it expresses a subsequent GET deletion operation on that same
   target resource will result in the URI mapping of the
   origin server rather than an equivalent representation being
   sent in a 200 (OK) response.  However, there is no guarantee expectation that
   such a state change will the previously
   associated information be observable, since deleted.

   If the target resource has one or more current representations, they
   might be acted upon by other user agents in parallel, or might not be
   subject to dynamic processing destroyed by the origin server, before any
   subsequent GET is received.  A successful response only implies that and the user agent's intent was achieved at
   associated storage might or might not be reclaimed, depending
   entirely on the time nature of the resource and its processing implementation by the
   origin server.

   If server (which are beyond the target scope of this specification).
   Likewise, other implementation aspects of a resource does not have might need to be
   deactivated or archived as a current representation and the
   PUT successfully creates one, then result of a DELETE, such as database or
   gateway connections.  In general, it is assumed that the origin
   server MUST inform 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 agent by sending 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.  If the target
   resource does have response to a current representation and POST request, might allow a
   corresponding DELETE request to undo those actions.  Similarly,
   custom user agent implementations that representation 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 modified in accordance with the state of the enclosed
   representation, then applied, the origin server MUST SHOULD
   send either

   o  a 200 (OK) or 202 (Accepted) status code if the action will likely succeed but
      has not yet been enacted,

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

   o  a 200 (OK) status code if the
   request.

   An origin server SHOULD ignore unrecognized header action has been enacted and trailer fields 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 PUT DELETE request (i.e., do not save them as part has no defined semantics, cannot alter
   the meaning or target of the
   resource state).

   An origin server SHOULD verify that request, and might lead some
   implementations to reject the PUT representation is
   consistent with any constraints request.

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

8.3.6.  CONNECT

   The CONNECT method requests that the PUT.  This is
   particularly important when recipient establish a tunnel to
   the destination origin server uses internal
   configuration information related to identified by the URI in order request target and,
   if successful, thereafter restrict its behavior to set blind forwarding
   of data, in both directions, until the
   values for representation metadata on GET responses.  When a PUT
   representation tunnel is inconsistent with 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 target resource, 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 SHOULD either make them consistent, by transforming the
   representation or changing the resource configuration, or that receives a CONNECT request for itself MAY respond with an appropriate error message containing sufficient information a
   2xx (Successful) status code to explain why the representation indicate that a connection is unsuitable.  The 409 (Conflict)
   or 415 (Unsupported Media Type) status codes are suggested, with
   established.  However, most origin servers do not implement CONNECT.

   A client sending a CONNECT request MUST send the
   latter being specific to constraints on Content-Type values.

   For example, if authority component
   (described in Section 3.2 of [RFC3986]) as the request target; i.e.,
   the request target resource is configured to always have a
   Content-Type consists of "text/html" and only the representation being PUT has a
   Content-Type host name and port number of "image/jpeg",
   the origin server ought 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 do one of:

   a.  reconfigure the request target resource or, if configured to reflect use another
   proxy, by forwarding the new media type;

   b.  transform CONNECT request to the PUT representation next inbound proxy.
   Any 2xx (Successful) response indicates that the sender (and all
   inbound proxies) will switch to a format consistent with tunnel mode immediately after the
   blank line that
       of concludes the resource before saving it as successful response's header section;
   data received after that blank line is from the new resource state; or,

   c.  reject server identified by
   the request with target.  Any response other than a 415 (Unsupported Media Type) successful response
       indicating
   indicates that the tunnel has not yet been formed and that the
   connection remains governed by HTTP.

   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 target resource is limited authority to "text/html",
       perhaps including
   create a link 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 different resource well-known or
   reserved TCP port that would be a
       suitable target for the new representation.

   HTTP does is not define exactly how intended for Web traffic.  For example,
   a PUT method affects the state of an
   origin server beyond what can be expressed by the intent of the user
   agent request and CONNECT to "example.com:25" would suggest that the semantics of proxy connect to
   the origin server response.  It
   does not define what a resource might be, in any sense of reserved port for SMTP traffic; if allowed, that word,
   beyond could trick the interface provided via HTTP.  It does not define how
   resource state is "stored", nor how such storage might change as
   proxy into relaying spam email.  Proxies that support CONNECT SHOULD
   restrict its use to a
   result limited set of known ports or 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 configurable
   whitelist of safe request targets.

   A server MUST NOT send a validator header field
   (Section 11.2), such as an ETag any Transfer-Encoding or Last-Modified field, Content-Length header
   fields in a
   successful 2xx (Successful) response to PUT unless the request's representation data
   was saved without CONNECT.  A client MUST
   ignore any transformation applied Content-Length or Transfer-Encoding header fields received
   in a successful response to the CONNECT.

   A payload within a CONNECT request message has no defined semantics;
   sending a payload body (i.e., on a CONNECT request might cause some existing
   implementations to reject the
   resource's new representation data is identical request.

   Responses to the representation
   data received in CONNECT method are not cacheable.

8.3.7.  OPTIONS

   The OPTIONS method requests information about the PUT request) and communication
   options available for the validator field value
   reflects target resource, at either the new representation. origin
   server or an intervening intermediary.  This requirement method allows a user
   agent client
   to know when determine the representation body it has in memory remains
   current as options and/or requirements associated with a result of
   resource, or the PUT, thus not in need capabilities of being retrieved
   again from the origin a server, and that without implying a
   resource action.

   An OPTIONS request with an asterisk ("*") as the new validator(s) received
   in request target
   (Section 6.1) applies to the response can be used for future conditional requests server in order general rather than to prevent accidental overwrites (Section 9.2).

   The fundamental difference between the POST and PUT methods is
   highlighted by a
   specific resource.  Since a server's communication options typically
   depend on the different intent for resource, the enclosed representation.
   The target resource in a POST "*" request is intended to handle only useful as a "ping" or
   "no-op" type of method; it does nothing beyond allowing the
   enclosed representation according client to
   test the resource's own semantics,
   whereas capabilities of the enclosed representation in server.  For example, this can be used
   to test a PUT proxy for HTTP/1.1 conformance (or lack thereof).

   If the request target is defined as
   replacing not an asterisk, the state of OPTIONS request applies
   to the options that are available when communicating with the target
   resource.  Hence,

   A server generating a successful response to OPTIONS SHOULD send any
   header that might indicate optional features implemented by the intent of PUT
   is idempotent
   server and visible applicable to intermediaries, even though the exact
   effect 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 only known 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 origin server.

   Proper interpretation of request chain (see
   Section 6.4.2).  A proxy MUST NOT generate a Max-Forwards header
   field while forwarding a PUT request presumes unless that the user agent
   knows which target resource is desired. request was received
   with a Max-Forwards field.

   A service client that selects generates an OPTIONS request containing a
   proper URI on behalf of payload body
   MUST send a valid Content-Type header field describing the client, after receiving
   representation media type.  Note that this specification does not
   define any use for such a state-changing
   request, SHOULD be implemented using payload.

   Responses to the POST OPTIONS method rather than PUT.
   If the origin server will are not make cacheable.

8.3.8.  TRACE

   The TRACE method requests a remote, application-level loop-back of
   the requested PUT state change to request message.  The final recipient of the target resource and instead wishes to have it applied request SHOULD
   reflect the message received, excluding some fields described below,
   back to a
   different resource, such the client as when the resource has been moved to message body of a
   different URI, then 200 (OK) response with a
   Content-Type of "message/http" (Section 10.1 of [Messaging]).  The
   final recipient is either the origin server MUST send an appropriate 3xx
   (Redirection) response; the user agent MAY then make its own decision
   regarding whether or not the first server to redirect
   receive a Max-Forwards value of zero (0) in the request. request
   (Section 6.4.2).

   A PUT client MUST NOT generate fields in a TRACE request applied to containing
   sensitive data that might be disclosed by the target resource can have side effects on
   other resources. response.  For example, an article might have a URI
   it would be foolish 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 user agent to changing
   the state send stored user credentials
   Section 10 or cookies [RFC6265] in a TRACE request.  The final
   recipient of the target resource, and might also cause links request SHOULD exclude any request fields that are
   likely to be
   added between the related resources.

   An origin server contain sensitive data when that allows PUT on a given target resource MUST send
   a 400 (Bad Request) recipient generates the
   response body.

   TRACE allows the client to a PUT see what is being received at the other
   end of the request chain and use that contains a
   Content-Range data for testing or diagnostic
   information.  The value of the Via header field (Section 7.3.4), since the payload 6.4.3) is
   likely to be partial content that has been mistakenly PUT of
   particular interest, since it acts as a full
   representation.  Partial content updates are possible by targeting a
   separately identified resource with state that overlaps a portion trace of the larger resource, or by using a different method that has been
   specifically defined for partial updates (for example, request chain.
   Use of the PATCH
   method defined 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 [RFC5789]). an infinite loop.

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

   Responses to the PUT TRACE method are not cacheable.  If a successful PUT

9.  Context

9.1.  Request Context

   A client sends request passes through a cache that has one or header fields to provide more stored responses
   for the target URI, those stored responses will be invalidated (see
   Section 4.4 of [Caching]).

8.3.5.  DELETE

   The DELETE method requests that information
   about the origin server remove request context, make the
   association between request conditional based on the
   target resource and its current
   functionality.  In effect, this method is 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 rm command
   in UNIX: it expresses a deletion operation
   parameters on the URI mapping of the
   origin server rather than an expectation that the previously
   associated a programming language method invocation.

   The following request header fields provide additional information be deleted.

   If
   about the target resource has one or more current representations, they
   might or might not be destroyed by request context, including information about the origin server, user, user
   agent, and resource behind the
   associated storage might or might not be reclaimed, depending
   entirely on the nature 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

9.1.1.  Expect

   The "Expect" header field in a request indicates a certain set of the resource and its implementation
   behaviors (expectations) that need to be supported by the
   origin server (which are beyond the scope of in
   order to properly handle this specification).
   Likewise, 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 implementation aspects of than 100-continue MAY respond with a resource might need 417 (Expectation Failed)
   status code to indicate that the unexpected expectation cannot be
   deactivated or archived as
   met.

   A 100-continue expectation informs recipients that the client is
   about to send a result of (presumably large) message body in this request and
   wishes to receive a DELETE, such as database 100 (Continue) interim response if the method,
   target URI, and header fields are not sufficient to cause an
   immediate success, redirect, or
   gateway connections.  In general, error response.  This allows the
   client to wait for an indication that it is assumed that worthwhile to send the origin
   server will only allow DELETE on resources for
   message body before actually doing so, which it has a
   prescribed mechanism for accomplishing can improve efficiency
   when the deletion.

   Relatively few resources allow message body is huge or when the DELETE method - its primary use client anticipates that an
   error is likely (e.g., when sending a state-changing method, for remote authoring environments, where the user has some direction
   regarding its effect.
   first time, without previously verified authentication credentials).

   For example, a resource request that was previously
   created using a begins with

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

   allows the Location header
   field after a 201 (Created) response to a POST request, might allow a
   corresponding DELETE request origin server to undo those actions.  Similarly,
   custom user agent implementations that implement immediately respond with an authoring
   function, error
   message, such as revision control clients using HTTP for remote
   operations, might use DELETE based on an assumption that 401 (Unauthorized) or 405 (Method Not Allowed),
   before the server's
   URI space has been crafted to correspond to a version repository.

   If a DELETE method is successfully applied, client starts filling the origin server SHOULD
   send pipes with an unnecessary data
   transfer.

   Requirements for clients:

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

   o 100-continue expectation in a 204 (No Content) status code if the action has been enacted and
      no further information is to be supplied, or

   o request
      that does not include a 200 (OK) status code if the action has been enacted and the
      response message includes a representation describing the status. body.

   o  A client SHOULD NOT generate that will wait for a 100 (Continue) response before
      sending the request message body in MUST send an Expect header field
      containing a DELETE request. 100-continue expectation.

   o  A payload
   received in client that sends a DELETE request has no defined semantics, cannot alter
   the meaning or target of the request, and might lead some
   implementations 100-continue expectation is not required to reject the request.

   Responses
      wait for any specific length of time; such a client MAY proceed to
      send the DELETE method are message body even if it has not cacheable.  If yet received a successful
   DELETE request passes response.
      Furthermore, since 100 (Continue) responses cannot be sent through
      an HTTP/1.0 intermediary, such a cache that has one or more stored
   responses client SHOULD NOT wait for an
      indefinite period before sending the target URI, those stored responses will be
   invalidated (see Section 4.4 of [Caching]).

8.3.6.  CONNECT

   The CONNECT method requests message body.

   o  A client that the recipient establish receives a tunnel to
   the destination origin server identified by the request target and,
   if successful, thereafter restrict its behavior to blind forwarding
   of data, 417 (Expectation Failed) status code in both directions, until the tunnel is closed.  Tunnels are
   commonly used
      response 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 a request containing a 100-continue expectation SHOULD
      repeat that request without a 100-continue expectation, since the request/response nature of an HTTP
   connection, specific HTTP versions might have different ways of
   mapping its semantics into
      417 response merely indicates that the protocol's wire format.

   CONNECT is intended only response chain does not
      support expectations (e.g., it passes through an HTTP/1.0 server).

   Requirements for use in requests to a proxy.  An origin servers:

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

   o  A server MAY respond with omit sending a
   2xx (Successful) status code to indicate 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
   established.  However, most origin servers do not implement CONNECT. closed prematurely.

   o  A client sending server that responds with a CONNECT final status code before reading the
      entire request MUST send payload body SHOULD indicate whether it intends to
      close the authority component
   (described in connection (e.g., see Section 3.2 9.6 of [RFC3986]) as the request target; i.e., [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 consists of only the host name URI, and port number of
   the tunnel destination, separated by complete header section that
   contains a colon.  For example,

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

   The recipient proxy can establish 100-continue expectation and indicates a tunnel request message
   body will follow, either send an immediate response with a final
   status code, if that status can be determined by directly
   connecting 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.

   A proxy MUST, upon receiving an HTTP/1.1 (or later) request that has
   a method, target or, 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 configured to use another
   proxy, that status can be determined by examining just the method, target
   URI, and header fields, or begin forwarding the CONNECT request toward the
   origin server by sending a corresponding request-line and header
   section to the next inbound proxy.
   Any 2xx (Successful) response indicates server.  If the proxy believes (from
   configuration or past interaction) that the sender (and all next inbound proxies) will switch server only
   supports HTTP/1.0, the proxy MAY generate an immediate 100 (Continue)
   response to tunnel mode immediately after encourage the
   blank line that concludes client to begin sending the successful response's message body.

      |  *Note:* The Expect header section;
   data received field was added after that blank line is from the server identified by original
      |  publication of HTTP/1.1 [RFC2068] as both the means to request target.  Any response other than a successful
      |  an interim 100 (Continue) response
   indicates that and the tunnel general mechanism
      |  for indicating must-understand extensions.  However, the
      |  extension mechanism has not yet been formed used by clients and that the
   connection remains governed must-
      |  understand requirements have not been implemented by HTTP.

   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 many
      |  servers, rendering the closed side extension mechanism useless.  This
      |  specification has removed the extension mechanism in order to
      |  simplify the other
   side, close both connections, definition and then discard any remaining data
   left undelivered.

   Proxy authentication might be used to establish processing of 100-continue.

9.1.2.  From

   The "From" header field contains an Internet email address for a
   human user who controls the authority requesting user agent.  The address ought
   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 be machine-usable, as defined by "mailbox" in establishing a tunnel to arbitrary
   servers, particularly when the destination 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 well-known or
   reserved TCP port From header field without explicit
   configuration by the user, since that is not intended for Web traffic.  For example, might conflict with the user's
   privacy interests or their site's security policy.

   A robotic user agent SHOULD send a CONNECT to "example.com:25" would suggest valid From header field so that
   the proxy connect to
   the reserved port person responsible for SMTP traffic; running the robot can be contacted if
   problems occur on servers, such as 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 robot is sending excessive,
   unwanted, or a configurable
   whitelist of safe request targets. invalid requests.

   A server MUST SHOULD NOT send any Transfer-Encoding or Content-Length use the From header
   fields in a 2xx (Successful) response to CONNECT.  A client MUST
   ignore any Content-Length field for access control 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
   authentication, since most recipients will assume that the CONNECT method are not cacheable.

8.3.7.  OPTIONS field
   value is public information.

9.1.3.  Referer

   The OPTIONS method requests information about "Referer" [sic] header field allows the communication
   options available user agent to specify a
   URI reference for the resource from which the target resource, at either URI was obtained
   (i.e., the origin
   server or an intervening intermediary.  This method allows a client
   to determine "referrer", though the options and/or requirements associated with a
   resource, or field name is misspelled).  A user
   agent MUST NOT include the capabilities fragment and userinfo components of a server, without implying a
   resource action.

   An OPTIONS request with an asterisk ("*") as the request target
   (Section 6.1) applies to
   URI reference [RFC3986], if any, when generating the server in general rather than to a
   specific resource.  Since Referer field
   value.

     Referer = absolute-URI / partial-URI

   The field value is either an absolute-URI or a server's communication options typically
   depend on partial-URI.  In the resource,
   latter case (Section 4), the "*" request referenced URI is only useful as a "ping" or
   "no-op" type of method; it does nothing beyond allowing the client relative to
   test the capabilities of the server.  For example, this can be used 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 test a proxy be found for HTTP/1.1 conformance (or lack thereof).

   If
   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 is URI was obtained from a source that does not have its
   own URI (e.g., input from the user keyboard, or an asterisk, entry within the OPTIONS request applies
   to
   user's bookmarks/favorites), the options that are available when communicating with user agent MUST either exclude the target
   resource.

   A server generating a successful response to OPTIONS SHOULD
   Referer field or send any
   header that might indicate optional features implemented by it with a value of "about:blank".

   The Referer field has the
   server and applicable potential to reveal information about the target resource (e.g., Allow), including
   potential extensions not defined by this specification.  The response
   payload, if any, might also describe
   request context or browsing history of the communication options in user, which is a
   machine privacy
   concern if the referring resource's identifier reveals personal
   information (such as an account name) or human-readable representation.  A standard format for such a representation resource that is not defined by this specification, but might supposed
   to be
   defined by future extensions confidential (such as behind a firewall or internal to HTTP.

   A client MAY send a Max-Forwards
   secured service).  Most general-purpose user agents do not send the
   Referer header field in an OPTIONS request
   to target a specific recipient in when the request chain (see
   Section 9.1.2). referring resource is a local "file" or
   "data" URI.  A proxy user agent MUST NOT generate send a Max-Forwards Referer header field while forwarding a request unless that in an
   unsecured HTTP request if the referring page was received with a Max-Forwards field.

   A client
   secure protocol.  See Section 16.9 for additional security
   considerations.

   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 generates an OPTIONS request containing a payload body
   MUST send a valid Content-Type 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 describing
   when the
   representation media type.  Note 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 this specification does the
   sender will not
   define any use for such discard trailer fields when it contains a payload.

   Responses "trailers"
   member, as described in Section 5.6.

   Additionally, specific HTTP versions can use it to indicate the OPTIONS method are not cacheable.

8.3.8.  TRACE
   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 TRACE method requests "Trailer" header field provides a remote, application-level loop-back list of field names that the request
   sender anticipates sending as trailer fields within that message.  The final
   This allows a recipient to prepare for receipt of the request SHOULD
   reflect indicated
   metadata before it starts processing the body.

     Trailer = #field-name

   For example, a sender might indicate that a message received, excluding some fields described below,
   back to the client integrity check
   will be computed as the message body of payload is being streamed and provide the
   final signature as a 200 (OK) response with trailer field.  This allows a
   Content-Type of "message/http" (Section 10.1 of [Messaging]).  The
   final recipient is either to
   perform the origin server or same check on the first server to
   receive a Max-Forwards value of zero (0) in fly as the request
   (Section 9.1.2).

   A client MUST NOT generate fields in a TRACE request containing
   sensitive payload data is received.

   A sender that might be disclosed by the response.  For example,
   it would be foolish for a user agent intends to send stored user credentials
   Section 9.5 generate one or cookies [RFC6265] more trailer fields in a TRACE request.  The final
   recipient of the request
   message 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 generate a Trailer header field (Section 6.6.1) is of
   particular interest, since it acts as a trace of in the request chain.
   Use header section
   of that message to indicate which fields might be present in the Max-Forwards
   trailers.

9.1.6.  User-Agent

   The "User-Agent" header field allows the client to limit contains information about the
   length of user
   agent originating the request chain, request, 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 often used by servers to the TRACE method are not cacheable.

8.4.  Method Extensibility

   Additional methods, outside help
   identify the scope of this specification, have
   been specified for use in HTTP.  All such methods ought reported interoperability problems, to be
   registered within the "Hypertext Transfer Protocol (HTTP) Method
   Registry".

8.4.1.  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" work
   around 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).

8.4.2.  Considerations for New Methods

   Standardized methods are generic; that is, they are potentially
   applicable tailor responses to any resource, not just one avoid particular media type, kind
   of resource, user agent
   limitations, and for analytics regarding browser or application.  As such, it is preferred that new
   methods be registered in operating system
   use.  A user agent SHOULD send a document that isn't specific User-Agent field in each request
   unless specifically configured not to a single
   application do so.

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

   The User-Agent field value consists of one or data format, since orthogonal technologies deserve
   orthogonal specification.

   Since message parsing more product
   identifiers, each followed by zero or more comments (Section 6 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 [Messaging]) needs to be
   independent their significance for identifying the user agent
   software.  Each product identifier consists of method semantics (aside from responses a name and optional
   version.

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

   A sender SHOULD limit generated product identifiers to HEAD),
   definitions of new methods cannot change what is
   necessary to identify the parsing algorithm product; a sender MUST NOT generate
   advertising or
   prohibit other nonessential information within the presence of product
   identifier.  A sender SHOULD NOT generate information in
   product-version that is not a message body on either version identifier (i.e., successive
   versions of the request or same product name ought to differ only in the
   response message.  Definitions
   product-version portion of new methods can specify that only the product identifier).

   Example:

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

   A user agent SHOULD NOT generate a
   zero-length message body is allowed User-Agent field containing
   needlessly fine-grained detail and SHOULD limit the addition of
   subproducts by requiring a Content-Length
   header third parties.  Overly long and detailed User-Agent
   field with a value values increase request latency and the risk 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 a user being
   identified against their wishes ("fingerprinting").

   Likewise, implementations are encouraged not to be associated with use the payload
   body if any is present product
   tokens of other implementations in order to declare compatibility
   with them, as this circumvents the request and what refinements purpose of the method
   makes to header field or status code semantics. field.  If the new method is
   cacheable, its definition ought to describe how, and under what
   conditions, a cache can store a response and use it to satisfy user
   agent masquerades as a
   subsequent request.  The new method ought to describe whether it different user agent, recipients can
   be made conditional (Section 9.2) and, if so, how a server responds
   when the condition is false.  Likewise, if assume
   that the new method might have
   some use for partial response semantics (Section 9.3), it ought user intentionally desires to
   document this, too.

      |  *Note:* Avoid defining a method name that starts with "M-",
      |  since see responses tailored for
   that prefix identified user agent, even if they might be misinterpreted not work as having well for
   the actual user agent being used.

9.2.  Response Context

   Response header fields can supply control data that supplements the
   status code, directs caching, or instructs the
      |  semantics assigned to it by [RFC2774].

9.  Request Header Fields

   A client sends request where to go
   next.

   The response header fields allow the server to provide more pass additional
   information about the request context, make response beyond the request conditional based on status code.  These header
   fields give information about the
   target resource state, suggest preferred formats for server, about further access to the response,
   supply authentication credentials,
   target resource, or modify 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 expected request
   processing.  These fields act as request modifiers, similar to the
   parameters on a programming language method invocation.

9.1.  Controls

   Controls are request and/or response status code.

   The remaining response header fields that direct specific handling of provide more information about
   the request.

    --------------- -------------------------- target resource for potential use in later requests.

    ------------- -------
     Field Name    Ref.
    --------------- --------------------------
     Cache-Control   Section 5.2 of [Caching]
     Expect          9.1.1
     Host            6.5
     Max-Forwards    9.1.2
     Pragma          Section 5.4 of [Caching]
     TE              5.6.5
    --------------- --------------------------
    ------------- -------
     Allow         9.2.1
     Date          9.2.2
     Location      9.2.3
     Retry-After   9.2.4
     Server        9.2.5
    ------------- -------

           Table 12

9.1.1.  Expect 9

9.2.1.  Allow

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

     Expect =      #expectation
     expectation = token [ "=" ( token / quoted-string ) parameters ] target resource.  The Expect purpose of this field value is case-insensitive.
   strictly to inform the recipient of valid request methods associated
   with the resource.

     Allow = #method

   Example of use:

     Allow: GET, HEAD, PUT

   The only expectation defined by this specification actual set of allowed methods is "100-continue"
   (with no defined parameters).

   A by the origin server that receives at
   the time of each request.  An origin server MUST generate an Expect Allow
   field value containing in a member
   other than 100-continue 405 (Method Not Allowed) response and MAY respond with a 417 (Expectation Failed)
   status code to indicate that the unexpected expectation cannot be
   met.

   A 100-continue expectation informs recipients do so in any
   other response.  An empty Allow field value indicates that the client is
   about to send a (presumably large) message body
   resource allows no methods, which might occur in this request and
   wishes to receive a 100 (Continue) interim 405 response if
   the method,
   target URI, and resource has been temporarily disabled by configuration.

   A proxy MUST NOT modify the Allow header fields are field - it does not sufficient need to cause an
   immediate success, redirect, or error response.  This allows
   understand all of the
   client indicated methods in order to wait for an indication that it is worthwhile handle them
   according to send the generic message body before actually doing so, handling rules.

9.2.2.  Date

   The "Date" header field represents the date and time at which can improve efficiency
   when the
   message body is huge or when was originated, having the client anticipates that same semantics as the Origination
   Date Field (orig-date) defined in Section 3.6.1 of [RFC5322].  The
   field value is an
   error HTTP-date, as defined in Section 5.7.7.

     Date = HTTP-date

   An example is likely (e.g., when sending a state-changing method, for the
   first time, without previously verified authentication credentials).

   For example,

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

   When 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 Date header field is generated, the origin server to immediately respond with an error
   message, such sender SHOULD generate its
   field value as 401 (Unauthorized) or 405 (Method Not Allowed),
   before the client starts filling best available approximation of 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 date and time
   of message body.

   o  A client that will wait for a 100 (Continue) response generation.  In theory, the date ought to represent the
   moment just before
      sending the request payload is generated.  In practice, the date
   can be generated at any time during message body origination.

   An origin server MUST NOT send an Expect a Date 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 does not yet received
   have a response.
      Furthermore, since 100 (Continue) responses cannot be sent through
      an HTTP/1.0 intermediary, such clock capable of providing a client SHOULD NOT wait for an
      indefinite period before sending reasonable approximation of the message body.

   o  A client that receives a 417 (Expectation Failed) status code
   current instance in
      response to a request containing a 100-continue expectation SHOULD
      repeat that request without Coordinated Universal Time.  An origin server MAY
   send a 100-continue expectation, since Date header field if the
      417 response merely indicates that is in the response chain does not
      support expectations (e.g., it passes through an HTTP/1.0 server).

   Requirements for servers:

   o  A 1xx
   (Informational) or 5xx (Server Error) class of status codes.  An
   origin server that receives a 100-continue expectation in an HTTP/1.0
      request MUST ignore that expectation.

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

   o other cases.

   A server recipient with a clock that sends receives a 100 (Continue) response MUST ultimately send message without a final status code, once
   Date header field MUST record the message body is time it was received and
      processed, unless append a
   corresponding Date header field to the connection message's header section if it
   is closed prematurely.

   o cached or forwarded downstream.

   A server that responds with user agent MAY send a final status code before reading the
      entire request payload body SHOULD indicate whether Date header field in a request, though
   generally will not do so unless it intends is believed to convey useful
   information to
      close the connection (e.g., see Section 9.6 server.  For example, custom applications of [Messaging]) or
      continue reading HTTP
   might convey a Date if the payload body.

   An origin server MUST, upon receiving an HTTP/1.1 (or later) is expected to adjust its
   interpretation of the user's request
   that has a method, target URI, based on differences between the
   user agent and complete server clocks.

9.2.3.  Location

   The "Location" header section that
   contains field is used in some responses to refer to a 100-continue expectation
   specific resource in relation to the response.  The type of
   relationship is defined by the combination of request method and indicates
   status code semantics.

     Location = URI-reference

   The field value consists of a request message
   body will follow, either send an immediate response with single URI-reference.  When it has the
   form of a relative reference ([RFC3986], Section 4.2), the final
   status code, if that status can be determined
   value is computed by examining just resolving it against the
   method, target URI, and header fields, or send an immediate 100
   (Continue) response URI ([RFC3986],
   Section 5).

   For 201 (Created) responses, the Location value refers to encourage the client primary
   resource created by the request.  For 3xx (Redirection) responses,
   the Location value refers to send the request's
   message body.  The origin server MUST NOT wait preferred target resource for
   automatically redirecting the message body
   before sending request.

   If the 100 (Continue) response.

   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 Location value provided in a request message body will
   follow, either send an immediate 3xx (Redirection) response with does
   not have a final status code, fragment component, a user agent MUST process the
   redirection as if that status can be determined by examining just the method, value inherits the fragment component of the
   URI reference used to generate the target
   URI, and header fields, or begin forwarding URI (i.e., the redirection
   inherits the original reference's fragment, if any).

   For example, a GET request toward generated for the
   origin server by sending URI reference
   "http://www.example.org/~tim" might result in a corresponding request-line and 303 (See Other)
   response containing the header
   section 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 next inbound server.  If URI reference
   "http://www.example.org/index.html#larry" might result in a 301
   (Moved Permanently) response containing the proxy believes (from
   configuration or past interaction) header field:

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

   which suggests that the next inbound server only
   supports HTTP/1.0, user agent redirect to
   "http://www.example.net/index.html#larry", preserving the proxy MAY generate an immediate 100 (Continue) 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 encourage the client provide a URI that
   is specific to begin sending the message body. created resource.

      |  *Note:* The Expect header field was added after 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 original
      |  publication  sake of HTTP/1.1 [RFC2068] as both robustness.  A Location field value cannot allow a list
      |  of members because the means to request comma list separator is a valid data
      |  character within a URI-reference.  If an interim 100 (Continue) response and the general mechanism invalid message is
      |  for indicating must-understand extensions.  However, the  sent with multiple Location field instances, a recipient along
      |  extension mechanism has not been used by clients and  the must- path might combine the field instances into one value.
      |  understand requirements have  Recovery of a valid Location field value from that situation is
      |  difficult and not been implemented by many interoperable across implementations.

      |  servers, rendering the extension mechanism useless.  This  *Note:* The Content-Location header field (Section 7.8) differs
      |  specification has removed the extension mechanism  from Location in order that the Content-Location refers to the most
      |  simplify  specific resource corresponding to the definition and processing of 100-continue.

9.1.2.  Max-Forwards

   The "Max-Forwards" header field provides enclosed representation.
      |  It is therefore possible for a mechanism with response to contain both the TRACE
   (Section 8.3.8)
      |  Location and OPTIONS (Section 8.3.7) request methods Content-Location header fields.

9.2.4.  Retry-After

   Servers send the "Retry-After" header field to limit indicate how long the number of times that
   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 request service is forwarded by proxies.  This
   can expected to be useful when the client is attempting unavailable to trace a request the client.
   When sent with any 3xx (Redirection) response, Retry-After indicates
   the minimum time that
   appears the user agent is asked to be failing or looping mid-chain.

     Max-Forwards = 1*DIGIT wait before issuing
   the redirected request.

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

   Each intermediary that receives a TRACE either an HTTP-date or OPTIONS request containing a Max-Forwards header field MUST check and update its value prior number of
   seconds to
   forwarding the request.  If delay after the received response is received.

     Retry-After = HTTP-date / delay-seconds

   A delay-seconds value is zero (0), 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
   intermediary MUST NOT forward latter example, the request; instead, delay is 2 minutes.

9.2.5.  Server

   The "Server" header field contains information about the intermediary
   MUST respond as software
   used by the final recipient.  If origin server to handle the received Max-Forwards
   value request, which is greater than zero, often used
   by clients to help identify the intermediary MUST 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 an updated
   Max-Forwards a Server field in the forwarded message with a its
   responses.

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

   The Server field value that
   is the lesser consists 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.

9.2.  Preconditions

   A conditional request is an HTTP request with one more product identifiers,
   each followed by zero or more request
   header fields that indicate a precondition to be tested before
   applying comments (Section 5.7.5), which
   together identify the request method to origin server software and its significant
   subproducts.  By convention, the target resource.  Section 9.2.1
   defines when preconditions product identifiers are applied.  Section 9.2.2 defines the listed in
   decreasing order of evaluation when more than one precondition is present.

   Conditional GET requests are the most efficient mechanism their significance 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 identifying the state origin
   server software.  Each product identifier consists of the
   target resource as a whole (its current value set) or the state name and
   optional version, as
   observed defined in Section 9.1.6.

   Example:

     Server: CERN/3.0 libwww/2.17

   An origin server SHOULD NOT generate 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 Server field containing
   needlessly fine-grained detail and SHOULD limit the mapping addition of requests
   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 selected representation
   (Section 7) will be consistent over time if the server intends to
   take advantage of conditionals.  Regardless, if the mapping is
   inconsistent general framework for access control and the
   authentication, via an extensible set of challenge-response
   authentication schemes, which can be used by a server is unable to select the appropriate
   representation, then no harm will result when the precondition
   evaluates to false.

   The following challenge a
   client request header fields allow and by a client to place provide authentication information.
   It uses a
   precondition on the state of the target resource, so that the action
   corresponding case-insensitive token to identify the method semantics will authentication
   scheme

     auth-scheme    = token

   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 applied if registered within the
   precondition evaluates to false.  Each precondition
   "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".
   For example, the "basic" and "digest" authentication schemes are
   defined by this
   specification consists of 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 comparison between
   comma-separated list of parameters or a set single sequence of validators
   obtained from prior representations characters
   capable of holding base64-encoded information.

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

   The token68 syntax allows the target resource to 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
   current state of validators 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 selected representation
   (Section 11.2).  Hence, these preconditions evaluate whether
   authentication scheme.

   For backwards compatibility, authentication scheme definitions can
   restrict the
   state format for senders to one of the target resource has changed since a given state two variants.  This can
   be important when it is known by
   the client.  The effect that deployed implementations will fail
   when encountering one of such an evaluation depends on the method
   semantics two formats.

10.3.  Challenge and choice of conditional, as defined in Section 9.2.1.

    --------------------- -------
     Field Name            Ref.
    --------------------- -------
     If-Match              9.2.3
     If-None-Match         9.2.4
     If-Modified-Since     9.2.5
     If-Unmodified-Since   9.2.6
     If-Range              9.2.7
    --------------------- -------

               Table 13

9.2.1.  Evaluation

   Except when excluded below, a recipient cache or Response

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

   A server MUST ignore all received preconditions if
   its 407 (Proxy Authentication Required) response message is used by a
   proxy to challenge the same request without those conditions, prior authorization of a client, including a
   Proxy-Authenticate header field containing at least one challenge
   applicable to
   processing the request body, would have been a status code other than proxy for the requested resource.

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

      |  *Note:* Many clients fail to parse a 2xx (Successful) or 412 (Precondition Failed).  In other words,
   redirects and failures challenge that can be detected before significant
   processing occurs take precedence over the evaluation of
   preconditions. contains an
      |  unknown scheme.  A server that workaround for this problem is not the to list well-
      |  supported schemes (such as "basic") first.

   A user agent that wishes to authenticate itself with an origin server for the target resource and
   cannot act as
   - usually, but not necessarily, after receiving a cache for requests on the target resource MUST NOT
   evaluate the conditional request header fields defined 401 (Unauthorized)
   - can do so by this
   specification, and it MUST forward them if the request is forwarded,
   since including an Authorization header field with the generating
   request.

   A client intends that they be evaluated by wishes to authenticate itself with a
   server that can provide proxy - usually,
   but not necessarily, after receiving a current representation.  Likewise, 407 (Proxy Authentication
   Required) - can do so by including a server
   MUST ignore the conditional request Proxy-Authorization header fields defined by this
   specification when received field
   with a request method that does not
   involve the selection or modification of a selected representation,
   such as CONNECT, OPTIONS, or TRACE.

   Note that protocol extensions can modify request.

10.4.  Credentials

   Both the conditions under which
   revalidation is triggered.  For example, Authorization field value and the "immutable" cache
   directive (defined by [RFC8246]) instructs caches to forgo
   revalidation of fresh responses even when requested by Proxy-Authorization field
   value contain the client.

   Conditional request header fields that are defined by extensions to
   HTTP might place conditions on all recipients, on client's credentials for the state realm of the
   target resource in general, or on
   being requested, based upon a group of resources.  For
   instance, the "If" header field challenge received in WebDAV can make a request
   conditional on various aspects of multiple resources, such as locks,
   if response
   (possibly at some point in the recipient understands and implements that field ([RFC4918],
   Section 10.4).

   Although conditional request header fields are defined as being
   usable with past).  When creating their values,
   the HEAD method (to keep HEAD's semantics consistent user agent ought to do so by selecting the challenge with
   those of GET), there is no point in sending a conditional HEAD
   because a successful response is around what it
   considers to be the same size most secure auth-scheme that it understands,
   obtaining credentials from the user as a 304 (Not
   Modified) response and more useful than a 412 (Precondition Failed)
   response.

9.2.2.  Precedence

   When more than one conditional request appropriate.  Transmission of
   credentials within header field is present in a
   request, the order in which values implies significant security
   considerations regarding the fields are evaluated becomes
   important.  In practice, confidentiality of the fields defined in this document are
   consistently implemented underlying
   connection, as described in Section 16.15.1.

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

   Upon receipt of a single, logical order, since "lost
   update" preconditions have more strict requirements than cache
   validation, request for a validated cache is more efficient than protected resource that omits
   credentials, contains invalid credentials (e.g., a bad password) or
   partial
   response, and entity tags are presumed to be credentials (e.g., when the authentication scheme requires
   more accurate than date
   validators.

   A recipient cache or one round trip), an origin server MUST evaluate 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.

   Likewise, upon receipt of a request
   preconditions defined by this specification in the following order:

   1.  When recipient is 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 origin proxy.

   A 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 9.2.3)

   2.  When recipient is the origin server, If-Match is receives valid credentials that are not present, and
       If-Unmodified-Since is present, evaluate the If-Unmodified-Since
       precondition:

       o  if true, continue adequate to
   gain access ought 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 9.2.6)

   3.  When If-None-Match is present, evaluate with the If-None-Match
       precondition:

       o  if true, continue 403 (Forbidden) status code
   (Section 14.5.4).

   HTTP does not restrict applications to step 5

       o  if false for GET/HEAD, respond 304 (Not Modified)

       o  if false this simple challenge-response
   framework for other methods, respond 412 (Precondition Failed)

   4.  When access authentication.  Additional mechanisms can be
   used, such as authentication at the method is GET transport level or HEAD, If-None-Match is via message
   encapsulation, and with additional header fields specifying
   authentication information.  However, such additional mechanisms are
   not present, defined by this specification.

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

10.5.  Protection Space (Realm)

   The "realm" authentication parameter is present, evaluate the If-Modified-Since
       precondition:

       o  if true, continue reserved for use by
   authentication schemes that wish to step 5

       o  if false, respond 304 (Not Modified)

   5.  When the method indicate a scope of protection.

   A protection space is GET and both Range and If-Range are present,
       evaluate the If-Range precondition:

       o  if defined by the validator matches canonical root URI (the scheme
   and authority components of the Range specification is
          applicable to target URI; see Section 6.1) of the selected representation, respond 206
          (Partial Content)

   6.  Otherwise,

       o  all conditions are met, so perform
   server being accessed, in combination with the requested action and
          respond according 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 success or failure.

   Any extension to HTTP own
   authentication scheme and/or authorization database.  The realm value
   is a string, generally assigned by the origin server, that defines can have
   additional conditional request
   header fields ought semantics specific to define its own expectations regarding the
   order for evaluating such fields in relation to those defined in this
   document and other conditionals authentication scheme.  Note
   that might be found in practice.

9.2.3.  If-Match a response can have multiple challenges with the same auth-
   scheme but with different realms.

   The "If-Match" header field makes protection space determines the domain over which credentials can
   be automatically applied.  If a prior request method conditional on
   the recipient origin server either having at least one current
   representation of has been authorized,
   the target resource, when user agent MAY reuse the field value is "*",
   or having same credentials for all other requests
   within that protection space for a current representation period of time determined by the target resource that has an
   entity-tag matching
   authentication scheme, parameters, and/or user preferences (such as a member of
   configurable inactivity timeout).  Unless specifically allowed by the list of entity-tags provided in
   authentication scheme, a single protection space cannot extend
   outside the field value.

   An origin server scope of its server.

   For historical reasons, a sender MUST use the strong comparison function when
   comparing entity-tags for If-Match (Section 11.2.3.2), since only generate the
   client intends this precondition quoted-string
   syntax.  Recipients might have to prevent the method from being
   applied if there support both token and quoted-
   string syntax for maximum interoperability with existing clients that
   have been any changes 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 representation data.

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

   Examples:

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

   If-Match is most often used with state-changing methods (e.g., POST,
   PUT, DELETE) authentication
   scheme(s) and parameters applicable to prevent accidental overwrites when multiple user
   agents might be acting in parallel on the same resource (i.e., 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
   prevent indicate that supplying credentials (or different
   credentials) might affect the "lost update" problem).  It can also be used with response.

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

   User agents are advised to abort a request if take special care in parsing the selected representation does not
   match field
   value, as it might contain more than one that the client has already stored (or partially stored)
   from challenge, and each
   challenge can contain a prior request.

   An origin server that receives an If-Match comma-separated list of authentication
   parameters.  Furthermore, the header field MUST evaluate
   the condition as per Section 9.2.1 prior itself can occur multiple
   times.

   For instance:

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

   This header field contains two challenges; one for the method.

   To evaluate "Newauth"
   scheme with a received If-Match header field:

   1.  If 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 value is "*", the condition is true if line might not be interoperable.

      |  *Note:* The challenge grammar production uses the origin
       server has list syntax
      |  as well.  Therefore, a current representation for sequence of comma, whitespace, and comma
      |  can be considered either as applying to the target resource.

   2.  If preceding
      |  challenge, or to be an empty entry in the field value is a list of entity-tags, the condition is
       true if any of the listed tags match challenges.
      |  In practice, this ambiguity does not affect the entity-tag semantics of
      |  the
       selected representation.

   3.  Otherwise, the condition header field value and thus is false.

   An origin server MUST NOT perform the requested method if harmless.

10.6.2.  Authorization

   The "Authorization" header field allows a received
   If-Match condition evaluates user agent to false.  Instead, the origin server
   MAY indicate that the conditional request failed by responding authenticate
   itself 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 an origin server MAY respond
   with - usually, but not necessarily, after
   receiving a 2xx (Successful) status code (i.e., 401 (Unauthorized) response.  Its value consists of
   credentials containing the change requested by authentication information of the user
   agent has already succeeded, but for the user agent might not be
   aware realm 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 resource being requested.

     Authorization = credentials

   If 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., authenticated and a non-atomic increment) realm specified, the same
   credentials are likely presumed to
   collide and potentially lose important state transitions.  For those
   kinds of resources, an origin server is better off being stringent in
   sending 412 be valid for every failed precondition on an unsafe method.  In all other cases, excluding the ETag field from a success response might
   encourage requests within
   this realm (assuming that the user agent authentication scheme itself does not
   require otherwise, such as credentials that vary according to perform a GET as its next
   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
   eliminate confusion about handling of the resource's current state.

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

   Note that an If-Match
   HTTP caches.

10.6.3.  Authentication-Info

   HTTP authentication schemes can use the Authentication-Info response
   header field with a list value containing "*"
   and other values (including other instances of "*") is unlikely to be
   interoperable.

9.2.4.  If-None-Match

   The "If-None-Match" header field makes communicate information after the request method conditional
   on client's
   authentication credentials have been accepted.  This information can
   include a recipient cache or origin server either not having any current
   representation of the target resource, when finalization message from the server (e.g., it can contain
   the server authentication).

   The field value is "*",
   or having a selected representation with an entity-tag that does not
   match any list of those listed parameters (name/value pairs), using the
   "auth-param" syntax defined in Section 10.3.  This specification only
   describes the field value.

   A recipient MUST use generic format; authentication schemes using
   Authentication-Info will define the weak comparison function when comparing
   entity-tags individual parameters.  The
   "Digest" Authentication Scheme, for If-None-Match (Section 11.2.3.2), since weak entity-
   tags instance, defines multiple
   parameters in Section 3.5 of [RFC7616].

     Authentication-Info = #auth-param

   The Authentication-Info header field 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 any HTTP
   response, independently of
   transaction overhead.  When a client desires to update one or more
   stored responses that have entity-tags, request method and status code.  Its
   semantics are defined by the client SHOULD generate an
   If-None-Match authentication scheme indicated by the
   Authorization header field containing a list (Section 10.6.2) of those entity-tags
   when making a GET request; this allows recipient servers to send the corresponding
   request.

   A proxy forwarding a
   304 (Not Modified) response is not allowed to indicate when one of those stored
   responses matches modify the selected representation.

   If-None-Match field
   value in any way.

   Authentication-Info can also be used with sent as a value of "*" to prevent an
   unsafe request method (e.g., PUT) from inadvertently modifying an
   existing representation of the target resource trailer field (Section 5.6) when
   the client
   believes 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 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 authentication scheme(s) and parameters
   applicable to create an
   initial representation for the target resource.

   An origin server that receives an If-None-Match proxy for this request.  A proxy MUST send at least
   one Proxy-Authenticate header field MUST
   evaluate in each 407 (Proxy Authentication
   Required) response that it generates.

     Proxy-Authenticate = #challenge

   Unlike WWW-Authenticate, the condition as per Section 9.2.1 prior Proxy-Authenticate header field applies
   only to performing the
   method.

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

   1.  If next outbound client on the field value response chain.  This is "*",
   because only the condition client that chose a given proxy is false if likely to have
   the origin
       server has a current representation credentials necessary for authentication.  However, when multiple
   proxies are used within the target resource.

   2.  If the field value is same administrative domain, such as
   office and regional caching proxies within 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 large corporate network,
   it is true.

   An origin server MUST NOT perform the requested method if the
   condition evaluates common for credentials to false; instead, be generated by the origin server MUST respond
   with either a) user agent and
   passed through the 304 (Not Modified) status code hierarchy until consumed.  Hence, in such a
   configuration, it will appear as if the request
   method Proxy-Authenticate is GET or HEAD or b) being
   forwarded because each proxy will send the 412 (Precondition Failed) status code same challenge set.

   Note that the parsing considerations for all other request methods.

   Requirements on cache handling of a received If-None-Match WWW-Authenticate apply to
   this header field are defined in as well; see Section 4.3.2 of [Caching].

   Note that an If-None-Match 10.6.1 for details.

10.7.2.  Proxy-Authorization

   The "Proxy-Authorization" header field with allows the client to identify
   itself (or its user) to a list proxy that requires authentication.  Its
   value consists of credentials containing
   "*" and other values (including other instances the authentication
   information of "*") is unlikely
   to be interoperable.

9.2.5.  If-Modified-Since

   The "If-Modified-Since" the client for the proxy and/or realm of the resource
   being requested.

     Proxy-Authorization = credentials

   Unlike Authorization, the Proxy-Authorization header field makes a GET or HEAD request
   method conditional on applies
   only to the selected representation's modification date
   being more recent than next inbound proxy that demanded authentication using the date provided
   Proxy-Authenticate field.  When multiple proxies are used in a chain,
   the Proxy-Authorization header 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 consumed by the field is:

     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT first inbound
   proxy that was expecting to receive credentials.  A recipient MUST ignore If-Modified-Since if proxy MAY relay
   the credentials from the client request contains an
   If-None-Match header field; to the condition in If-None-Match next proxy if that is
   considered to be a more accurate replacement for
   the condition in If-
   Modified-Since, and mechanism by which the two 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 only combined for defined by the sake of
   interoperating with older intermediaries that might not implement
   If-None-Match.

   A recipient MUST ignore authentication
   scheme indicated by the If-Modified-Since Proxy-Authorization header field if
   (Section 10.7.2) of the
   received field value is not a valid HTTP-date, corresponding request:

     Proxy-Authentication-Info = #auth-param

   However, unlike Authentication-Info, the Proxy-Authentication-Info
   header field value has
   more than one member, or if applies only to the request method is neither GET nor
   HEAD.

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

   If-Modified-Since response
   chain.  This 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 because only the scope of client that chose a web traversal given proxy is
   likely to
   resources that have recently changed.

   When used the credentials necessary for cache updates, authentication.
   However, when multiple proxies are used within the same
   administrative domain, such as office and regional caching proxies
   within a cache will typically use large corporate network, it is common for credentials to be
   generated by the value of user agent and passed through the cached message's Last-Modified field to generate 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
   of If-Modified-Since.  This behavior is most interoperable for cases
   where clocks are poorly synchronized value.

11.  Content Negotiation

   When responses convey payload information, whether indicating a
   success or when an error, the origin server often has chosen to
   only honor exact timestamp matches (due to a problem with Last-
   Modified dates different ways of
   representing that appear to go "back information; for example, in time" when the origin
   server's clock is corrected different formats,
   languages, 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 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
   cached message or protocol: "proactive" negotiation,
   where the local clock time that server selects the message was received,
   particularly when representation based upon the cached message does not contain user
   agent's stated preferences, "reactive" negotiation, where the server
   provides a Last-Modified
   field.

   When used list of representations for limiting the scope of retrieval user agent to a recent time
   window, a choose from,
   and "request payload" negotiation, where the user agent will generate an If-Modified-Since field value
   based on either its own local clock or selects the
   representation for a Date header field received
   from future request based upon the server server's stated
   preferences in a prior response.  Origin servers past responses.  Other patterns of content negotiation
   include "conditional content", where the representation consists of
   multiple parts that choose an
   exact timestamp match are selectively rendered 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
   parameters, "active content", where the specified window.

   An origin server representation contains a
   script that receives an If-Modified-Since header field
   SHOULD evaluate makes additional (more specific) requests based on the condition as per Section 9.2.1 prior to
   performing
   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 method. resource semantics.
   The consistency with which an origin server SHOULD NOT perform responds to requests,
   over time and over the
   requested method if varying dimensions of content negotiation, and
   thus the selected representation's last modification
   date "sameness" of a resource's observed representations over
   time, is earlier than determined entirely by whatever entity or equal to algorithm selects
   or generates those responses.

11.1.  Proactive Negotiation

   When content negotiation preferences are sent by the date provided user agent in a
   request to encourage an algorithm located at 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].

9.2.6.  If-Unmodified-Since

   The "If-Unmodified-Since" header field makes to select the request method
   conditional
   preferred representation, it is called proactive negotiation (a.k.a.,
   server-driven negotiation).  Selection is based on the selected representation's last modification date
   being earlier than or equal available
   representations for a response (the dimensions over which it might
   vary, such as language, content-coding, etc.) compared to the date provided various
   information supplied in the field value.
   This field accomplishes request, including both the same purpose explicit
   negotiation fields below and implicit characteristics, such 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
   client's network address or parts of the field is:

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

   A recipient MUST ignore If-Unmodified-Since if User-Agent field.

   Proactive negotiation is advantageous when the request contains
   an If-Match header field; algorithm for
   selecting from among the condition in If-Match available representations is considered difficult to
   describe to
   be a more accurate replacement for the condition in If-Unmodified-
   Since, and user agent, or when the two are only combined for server desires to send its
   "best guess" to the sake of interoperating user agent along with older intermediaries that might not implement If-Match.

   A recipient MUST ignore the If-Unmodified-Since header field first response (hoping
   to avoid the round trip delay of a subsequent request if the
   received field value "best
   guess" is not a valid HTTP-date (including when good enough for the
   field value appears user).  In order to be a list of dates).

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

   If-Unmodified-Since guess, a user agent MAY send request header fields that
   describe its preferences.

   Proactive negotiation has serious disadvantages:

   o  It is most often used with state-changing methods
   (e.g., POST, PUT, DELETE) impossible for the server to prevent accidental overwrites when
   multiple user agents accurately determine what might
      be acting in parallel on a resource "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 not supply entity-tags with its representations (i.e., the user want to
   prevent view it
      on screen or print it on paper?);

   o  Having the "lost update" problem).  It user agent describe its capabilities in every request
      can also be used with any
   method to abort both very inefficient (given that only a request if small percentage
      of responses have multiple representations) and a potential risk
      to the selected representation does not
   match one that user's privacy;

   o  It complicates the client already stored (or partially stored) from a
   prior request.

   An implementation of an origin server that receives an If-Unmodified-Since header field
   MUST evaluate and the condition as per Section 9.2.1 prior
      algorithms for generating responses to performing
   the method.

   If the selected representation has a last modification date, 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 MUST NOT perform might not implement
   proactive negotiation for the requested method if resource or might decide that date
   sending a response that doesn't conform to the user agent's
   preferences is
   more recent 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 date provided
   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 field value.  Instead, target
   resource, representations of error or processing status, and
   potentially even the
   origin server MAY indicate miscellaneous text strings that might appear
   within the conditional request failed by
   responding with 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
11.1.1.1.  Absence

   For each of these header fields, 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., does not contain the change requested by
   field implies that the user agent has already succeeded,
   but the user agent might not be aware no preference on that axis of it, perhaps because
   negotiation.  If 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 header field is present in a change request appears to have already been applied is more efficient and none of
   the available representations 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 the response can be considered
   acceptable according to it, the origin server is better off being
   stringent in can either honor the
   header field by sending 412 for every failed precondition on an unsafe
   method.

   The If-Unmodified-Since a 406 (Not Acceptable) response or disregard
   the header field can be ignored by caches and
   intermediaries because treating the response as if it is not applicable subject to a stored response.

9.2.7.  If-Range

   The "If-Range" header field provides a special conditional request
   mechanism
   content negotiation for that is similar to the If-Match and If-Unmodified-Since request header fields but field.  This does not
   imply, however, that instructs the recipient client will be able to ignore use the Range
   representation.

   *Note:* Sending these header field if the validator doesn't match, resulting in transfer fields makes it easier for a server to
   identify an individual by virtue of the new selected representation instead of a 412 (Precondition
   Failed) response.

   If user agent's request
   characteristics (Section 16.12).

11.1.1.2.  Quality Values

   The content negotiation fields defined by this specification use a client has
   common parameter, named "q" (case-insensitive), to assign a partial copy relative
   "weight" to the preference for that associated kind of content.  This
   weight is referred to as a representation and wishes "quality value" (or "qvalue") because the
   same parameter name is often used within server configurations to
   assign a weight to have
   an up-to-date copy of the entire representation, it could use relative quality of the
   Range header field with various
   representations that can be selected for a conditional GET (using either or both of
   If-Unmodified-Since resource.

   The weight is normalized to a real number in the range 0 through 1,
   where 0.001 is the least preferred and If-Match.)  However, if 1 is the precondition
   fails because most preferred; a
   value of 0 means "not acceptable".  If no "q" parameter is present,
   the representation has been modified, 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 client would
   then have to make a second request
   decimal point.  User configuration of these values ought to obtain be
   limited in the entire current
   representation.

   The "If-Range" same fashion.

11.1.1.3.  Wildcard Values

   Most of these header field allows fields, where indicated, define a client wildcard value
   ("*") to "short-circuit" the
   second request.  Informally, its meaning is as follows: if the
   representation select unspecified values.  If no wildcard is unchanged, send me the part(s) that I am requesting present, all
   values not explicitly mentioned in Range; otherwise, send me the entire representation.

     If-Range = entity-tag / HTTP-date

   A client MUST NOT generate an If-Range header field are considered "not
   acceptable" to the client.

   *Note:* In practice, using wildcards in a 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 that
   does not contain a Range header field.  A server MUST ignore an If-
   Range header field received in 406 (Not Acceptable)
   response if a request that does more preferred format is not contain available by sending
   Accept: */*;q=0, but they still need to be able to handle a
   Range header field.  An origin different
   response, since the server MUST is allowed to ignore an If-Range their preference.

11.1.2.  Accept

   The "Accept" header field received in a request for a target resource that does not
   support Range requests.

   A client MUST NOT generate an If-Range can be used by user agents to specify their
   preferences regarding response media types.  For example, Accept
   header field containing an
   entity-tag fields can be used to indicate 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 request is
   specifically limited to a
   strong validator in the sense defined by Section 11.2.2.2.

   A server that evaluates an If-Range precondition MUST use the strong
   comparison function when comparing entity-tags (Section 11.2.3.2) and
   MUST evaluate the condition small set of desired types, as false if an HTTP-date validator is
   provided that is not a strong validator in the sense defined by
   Section 11.2.2.2.  A valid entity-tag can be distinguished from case
   of a
   valid HTTP-date by examining the first two characters request for an in-line image.

   When sent by a DQUOTE.

   If the validator given server in a response, Accept provides information
   about what content types are preferred in the If-Range header field matches the
   current validator for the selected representation payload of a subsequent
   request to 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 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 this comparison type.  The media-range can include media type
   parameters that are applicable to that range.

   Each media-range might be followed by exact match,
   including when the validator is an HTTP-date, differs from the
   "earlier than zero or equal to" comparison used when evaluating more applicable media
   type parameters (e.g., charset), an
   If-Unmodified-Since conditional.

9.3.  Range

   The "Range" header field on optional "q" parameter for
   indicating a GET request modifies the method
   semantics to request transfer of only one relative weight (Section 11.1.1.2), and then zero or
   more subranges of the
   selected representation data (Section 7.1), rather than the entire
   selected representation.

     Range = ranges-specifier

   Clients often encounter interrupted data transfers extension parameters.  The "q" parameter is necessary if any
   extensions (accept-ext) are present, since it acts as a result separator
   between the two parameter sets.

      |  *Note:* Use of
   canceled requests or dropped connections.  When a client has stored 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
   partial representation, it media range, such an
      |  event is desirable believed to request be unlikely given the remainder lack of
   that representation any "q"
      |  parameters 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 IANA media type registry and the rare usage
      |  of a larger
   representation, such 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 a single page of a very large document, or "I prefer audio/basic, but send me any audio type
   if it is the dimensions of best available after an embedded image.

   Range requests 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 an OPTIONAL feature of HTTP, designed so that
   recipients
   the equally preferred media types, but if they do not implementing this feature (or exist, then
   send the text/x-dvi representation, and if that does not supporting it for exist, send
   the target resource) text/plain representation".

   Media ranges can respond as if it is a normal GET request
   without impacting interoperability.  Partial responses are indicated be overridden by a distinct status code more specific media ranges or
   specific media types.  If more than one media range applies to not be mistaken for full responses 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
   caches finding the media range with the highest precedence
   that might not implement matches the feature.

   A server MAY ignore 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

   would cause the Range header field.  However, origin servers
   and intermediate caches ought following values to support byte ranges when possible,
   since they support efficient recovery from partially failed transfers
   and partial retrieval of large representations. 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 server MUST ignore
   a Range header field received user agent might be provided with a request method other than GET.

   Although the range request mechanism is designed to allow for
   extensible range types, this specification only defines requests default set of quality
   values for
   byte certain media ranges.

   An origin server MUST ignore  However, unless the user agent is a Range header field
   closed system that contains a
   range unit it does not understand.  A proxy MAY discard a Range 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 that contains a range unit it does not understand.

   A server that supports range requests MAY ignore or reject can be sent by a Range
   header user agent to
   indicate its preferences for charsets in textual response content.
   For example, this field that consists allows user agents capable of understanding
   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 comprehensive or a deliberate
   denial-of-service attack (Section 12.14).  A client SHOULD NOT
   request multiple ranges that are inherently less efficient special-purpose charsets to process
   and transfer than a single range signal that encompasses the same data.

   A
   capability to an origin 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 capable of zero length). representing
   information in those charsets.

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

   Charset names are defined in Section 7.4.2.  A client user agent MAY
   associate a quality value with each charset to indicate the user's
   relative preference for that is requesting multiple ranges SHOULD list those ranges charset, as defined in ascending order (the order 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 which they would typically be
   received the Accept-Charset field,
   matches every charset that is not mentioned elsewhere in a complete representation) unless there the Accept-
   Charset field.

   *Note:* Accept-Charset is deprecated because UTF-8 has become nearly
   ubiquitous and sending a specific
   need 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 request a later part earlier.  For example, do so.

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
   processing in a large representation with an internal catalog 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 parts
   might need to a subsequent request later parts first, particularly if to the
   representation consists of pages stored same resource.

   An "identity" token is used as a synonym for "no encoding" in reverse order and the user
   agent wishes
   to transfer one page at a time.

   The Range header field communicate when no encoding is evaluated after evaluating preferred.

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

   Each codings value MAY be given an associated quality value
   representing the precondition
   header fields preference for that encoding, as defined in
   Section 9.2, and only if the result 11.1.1.2.  The asterisk "*" symbol in
   absence of the Range header an Accept-Encoding
   field would be a 200 (OK) response.  In
   other words, Range is ignored when a conditional GET would result matches any available content-coding not explicitly listed in
   a 304 (Not Modified) response.

   The If-Range header field (Section 9.2.7) can be used as a
   precondition to applying
   the Range header field.

   If all of the preconditions are true, the

   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 supports the Range
   header field tests whether a content-coding for the target resource, and the specified range(s) are
   valid and satisfiable (as defined a given representation is
   acceptable using these rules:

   1.  If no Accept-Encoding field is in Section 7.1.4.2), the server
   SHOULD send a 206 (Partial Content) response with a payload
   containing one or more partial representations that correspond to request, any content-coding
       is considered acceptable by the
   satisfiable ranges requested. user agent.

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

   3.  If the target resource, and representation's content-coding is one of the specified range(s) are
   invalid or unsatisfiable, content-
       codings listed in the server SHOULD send a 416 (Range Not
   Satisfiable) response.

9.4.  Negotiation

   The following request header fields can be sent Accept-Encoding field value, then it is
       acceptable unless it is accompanied by a user agent to
   engage in proactive negotiation qvalue of the response content, as 0.  (As
       defined in Section 7.4.1.  The preferences sent in these fields apply to any
   content in the response, including representations of the target
   resource, representations 11.1.1.2, a qvalue of error or processing status, and
   potentially even 0 means "not
       acceptable".)

   4.  If multiple content-codings are acceptable, then the miscellaneous text strings that might appear
   within acceptable
       content-coding with the protocol.

    ----------------- -------
     Field Name        Ref.
    ----------------- -------
     Accept            9.4.1
     Accept-Charset    9.4.2 highest non-zero qvalue is preferred.

   An Accept-Encoding   9.4.3
     Accept-Language   9.4.4
    ----------------- -------

             Table 14

   For each of these header fields, field with a request field value that does not contain it is empty
   implies that the user agent has no preference on that axis of
   negotiation. does not want any content-coding in
   response.  If the an Accept-Encoding header field is present in a request
   and none of the available representations for the response can be considered
   acceptable according to it, have a
   content-coding that is listed as acceptable, the origin server can either honor SHOULD
   send a response without any content-coding.

   When the Accept-Encoding header field by sending is present in a 406 (Not Acceptable) response or disregard response, it
   indicates what content codings the header resource was willing to accept in
   the associated request.  The field by treating value is evaluated the response same way as if it
   in a request.

   Note that this information is not subject 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 negotiation for codings and
   media types.  In order to avoid confusion with issues related to
   media types, servers that fail a request header field.  This does not
   imply, however, that the client will be able with a 415 status for
   reasons unrelated to use content codings MUST NOT include the
   representation.

   *Note:* Sending these Accept-
   Encoding header fields makes it easier for field.

   The most common use of Accept-Encoding is in responses with a server 415
   (Unsupported Media Type) status code, in response to
   identify an individual by virtue of the user agent's request
   characteristics (Section 12.12).

   Each optimistic use
   of these header fields defines a wildcard value (often, "*") to
   select unspecified values.  If no wildcard is present, all values not
   explicitly mentioned in content coding by clients.  However, the header field are considered "not acceptable" can also
   be used to
   the client.

   *Note:* In practice, using wildcards in indicate to clients that content negotiation has
   limited practical value, because it is seldom useful codings are supported, to say, for
   optimize future interactions.  For example, "I prefer image/* more or less than (some other specific
   value)".  Clients can explicitly request a 406 (Not Acceptable)
   response if resource might include
   it in a more preferred format is not available by sending
   Accept: */*;q=0, but they still need to be able 2xx (Successful) response when the request payload was big
   enough to handle justify use of a different
   response, since compression coding but the server is allowed to ignore their preference.

9.4.1.  Accept 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" "Accept-Language" 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 natural languages that are preferred in the payload of a subsequent
   request to the same resource.

     Accept
   response.  Language tags are defined in Section 7.6.1.

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

     media-range    = ( "*/*"
                        / ( type "/" "*" )
                        / ( type "/" subtype )
                      ) parameters
     accept-params  = weight *( accept-ext ] )
     accept-ext
     language-range  = 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.
               <language-range, see [RFC4647], Section 2.1>

   Each media-range might language-range can be followed by zero or more applicable media
   type parameters (e.g., charset), given an optional "q" parameter associated quality value
   representing an estimate of the user's preference for
   indicating a relative weight (Section 7.4.4), 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 then zero or more
   extension parameters.  The "q" parameter is necessary if any
   extensions (accept-ext)
   other types of English".

   Note that some recipients treat the order in which language tags are present, since it acts
   listed as a separator
   between the two parameter sets.

      |  *Note:* Use an indication of the "q" parameter name to separate media type
      |  parameters from Accept extension parameters descending priority, particularly for tags
   that are assigned equal quality values (no value is due to
      |  historical practice.  Although the same as q=1).
   However, this prevents any media type
      |  parameter named "q" from being used with behavior cannot be relied upon.  For consistency and to
   maximize interoperability, many user agents assign each language tag
   a media range, such an
      |  event 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 believed identical to the matching scheme that
   was previously defined for HTTP in Section 14.4 of [RFC2616].

   It might be unlikely given contrary to the lack privacy expectations of any "q"
      |  parameters in the IANA media type registry and user to send
   an Accept-Language header field with the rare usage
      | complete linguistic
   preferences 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 user in quality".

   A more elaborate example every request (Section 16.12).

   Since intelligibility 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 highly dependent on the equally preferred media types, but if they do not exist, then
   send individual user,
   user agents need to allow user control over the text/x-dvi representation, and if 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 exist, send provide such control to 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 user MUST NOT send an Accept-
   Language header field.

      |  *Note:* User agents ought to provide guidance to users when
      |  setting a
   given type, preference, since users are rarely familiar with the most specific reference has precedence.
      |  details of language matching as described above.  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
      |  users might assume that on selecting "en-gb", they will be
      |  served any kind of English document if British English is
   determined by finding the media range with not
      |  available.  A user agent might suggest, in such a case, to add
      |  "en" to the highest precedence
   that matches list for better matching behavior.

11.2.  Reactive Negotiation

   With reactive negotiation (a.k.a., agent-driven negotiation),
   selection of 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

   would cause best response representation (regardless of the
   status code) is performed by 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 15

   *Note:* A user agent might be provided with after receiving an
   initial response from the origin server that contains a default set list of quality
   values
   resources for certain media ranges.  However, unless alternative representations.  If the user agent is a
   closed system that cannot interact with other rendering agents, this
   default set ought to be configurable not
   satisfied by the user.

9.4.2.  Accept-Charset

   The "Accept-Charset" header field initial response representation, it can be sent by perform a user agent
   GET request on one or more of the alternative resources, selected
   based on metadata included in the list, to
   indicate its preferences obtain a different form of
   representation for charsets in textual response content.
   For example, this field allows user agents capable that response.  Selection of understanding
   more comprehensive alternatives might be
   performed automatically by the user agent or special-purpose charsets to signal manually by the user
   selecting from a generated (possibly hypertext) menu.

   Note that
   capability the above refers to an origin server that is capable representations of representing
   information the response, in those charsets.

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

   Charset names
   general, not representations of the resource.  The alternative
   representations are defined only considered representations of the target
   resource if the response in Section 7.1.1.1.  A user agent MAY
   associate which those alternatives are provided has
   the semantics of being a quality value with each charset representation of the target resource (e.g.,
   a 200 (OK) response to indicate a GET request) or has the user's
   relative preference 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 charset, as defined in Section 7.4.4.
   An example reactive
   negotiation by the user agent is

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

   The special value "*", if present preferred.  For example, the
   alternatives listed in responses with the Accept-Charset field,
   matches every charset 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 not mentioned elsewhere in advantageous when the Accept-
   Charset field.

   *Note:* Accept-Charset response would vary
   over commonly used dimensions (such as type, language, or encoding),
   when the origin server is deprecated because UTF-8 has become nearly
   ubiquitous and sending unable to determine a detailed list of user-preferred charsets
   wastes bandwidth, increases latency, and makes passive fingerprinting
   far too easy (Section 12.12).  Most general-purpose user agents do
   not send Accept-Charset, unless specifically configured to do so.

9.4.3.  Accept-Encoding

   The "Accept-Encoding" header field can be agent's
   capabilities from examining the request, and generally when public
   caches are used to indicate
   preferences regarding distribute server load and reduce network usage.

   Reactive negotiation suffers from the use disadvantages of content codings (Section 7.1.2).

   When sent by transmitting a
   list of alternatives to the user agent agent, which degrades user-perceived
   latency if transmitted in a request, Accept-Encoding indicates the
   content codings acceptable in header section, and needing a second
   request to obtain an alternate representation.  Furthermore, this
   specification does not define a response.

   When sent by mechanism for supporting automatic
   selection, though it does not prevent such a server mechanism from being
   developed as an extension.

11.2.1.  Vary

   The "Vary" header field in a response, Accept-Encoding provides
   information about response describes what content codings are preferred in the payload parts of a subsequent
   request to message, aside from the same resource.

   An "identity" token is used as a synonym method and target URI, might
   influence the origin server's process for "no encoding" in order
   to communicate when no encoding is preferred.

     Accept-Encoding selecting and representing
   this response.

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

   Each codings value MAY be given an associated quality / field-name )

   A Vary field value
   representing the preference for that encoding, as defined in
   Section 7.4.4.  The asterisk "*" symbol in an Accept-Encoding is a list of request field
   matches any available content-coding not explicitly listed in names, known as the
   selecting 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 fields, that might have a given representation is
   acceptable using these rules:

   1.  If no Accept-Encoding field is role in selecting the request, any content-coding
       is considered acceptable
   representation for this response.  Potential selecting header fields
   are not limited to those defined by the user agent.

   2. this specification.

   If the representation has no content-coding, then list contains "*", it is
       acceptable by default unless specifically excluded by signals that other aspects of the Accept-
       Encoding field stating either "identity;q=0" or "*;q=0" without
   request might play a
       more specific entry for "identity".

   3.  If role in selecting the representation's content-coding is one of response representation,
   possibly including elements outside the content-
       codings listed in message syntax (e.g., the Accept-Encoding field value, then it is
       acceptable unless it
   client's network address).  A recipient will not be able to determine
   whether this response is accompanied by appropriate for a qvalue of 0.  (As
       defined later request without
   forwarding the request to the origin server.  A proxy MUST NOT
   generate "*" in Section 7.4.4, a qvalue of 0 means "not acceptable".)

   4.  If multiple content-codings are acceptable, then Vary field value.

   For example, a response that contains

     Vary: accept-encoding, accept-language

   indicates that the acceptable
       content-coding with origin server might have used the highest non-zero qvalue is preferred.

   An request's
   Accept-Encoding header field 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 field value that is empty
   implies 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 does not want any content-coding in
   response.  If an Accept-Encoding header field recipients that this response is present subject to
       content negotiation (Section 11) and that a different
       representation might be sent in a subsequent request
   and none of the available representations for if
       additional parameters are provided in the response have a
   content-coding that is listed as acceptable, the header fields
       (proactive negotiation).

   An origin server SHOULD send a response without any content-coding.

   When the Accept-Encoding Vary header field is present in when its algorithm
   for selecting a response, it
   indicates what content codings representation varies based on aspects of the resource was willing to accept in request
   message other than the associated request.  The field value is evaluated method and target URI, unless the same way as
   in a request.

   Note that this information variance
   cannot be crossed or the origin server has been deliberately
   configured to prevent cache transparency.  For example, there is specific no
   need to send the associated request; the
   set of supported encodings might be different for other resources on Authorization field name in Vary because reuse
   across users is constrained by the same field definition (Section 10.6.2).
   Likewise, an origin server and could change over time or depend on other aspects might use Cache-Control response
   directives (Section 5.2 of [Caching]) to supplant Vary if it
   considers the request (such as variance less significant than the request method).

   Servers that fail 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 due payload negotiation because
   they intend to influence selection of an unsupported content coding
   ought appropriate payload for
   subsequent requests to respond with a 415 (Unsupported Media Type) status and
   include an that resource.  For example, the
   Accept-Encoding header field (Section 11.1.4) can be sent in that response, allowing
   clients to distinguish between issues related a response to
   indicate preferred 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 subsequent requests to content codings MUST NOT include that
   resource [RFC7694].

      |  Similarly, Section 3.1 of [RFC5789] defines the Accept-
   Encoding "Accept-Patch"
      |  response header field.

   The most common use field which allows discovery of Accept-Encoding is which content
      |  types are accepted in responses PATCH requests.

12.  Conditional Requests

   A conditional request is an HTTP request with one or more request
   header fields that indicate a 415
   (Unsupported Media Type) status code, in response precondition to optimistic use 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 a content coding by clients.  However, evaluation when more than one precondition is present.

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

   Conditional request preconditions are supported, 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
   optimize future interactions.  For example, a resource might include
   it in a 2xx (Successful) response select the appropriate
   representation, then no harm will result when the precondition
   evaluates to false.

12.1.  Preconditions

   The following request payload was big
   enough header fields allow a client to justify use of place a compression coding but
   precondition on the client failed
   do so.

      |  *Note:* Most HTTP/1.0 applications do not recognize or obey
      |  qvalues associated with content-codings.  This means state of the target resource, so that
      |  qvalues might not work and are the action
   corresponding to the method semantics will not permitted with x-gzip or
      |  x-compress.

9.4.4.  Accept-Language

   The "Accept-Language" header field can be used by user agents to
   indicate applied if the
   precondition evaluates to false.  Each precondition defined by this
   specification consists of a comparison between a set of natural languages that are preferred in the
   response.  Language tags are defined in Section 7.1.3.

     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 validators
   obtained from prior representations of the user's preference target resource to the
   current state of validators for the languages
   specified selected representation
   (Section 7.9).  Hence, these preconditions evaluate whether the state
   of the target resource has changed since a given state known by that range, the
   client.  The effect of such an evaluation depends on the method
   semantics and choice of conditional, as defined in Section 7.4.4.  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 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 English".

   Note that some recipients treat the order in which language tags are
   listed as target resource, when the field value is "*",
   or having a current representation of the target resource that has an indication
   entity-tag matching a member of descending priority, particularly the list of entity-tags provided in
   the field value.

   An origin server MUST use the strong comparison function when
   comparing entity-tags for tags
   that are assigned equal quality values (no value is If-Match (Section 7.9.3.2), since the same as q=1).
   However,
   client intends this behavior cannot be relied upon.  For consistency and precondition to
   maximize interoperability, many 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: *

   If-Match is most often used with state-changing methods (e.g., POST,
   PUT, DELETE) to prevent accidental overwrites when multiple 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 might be
   found acting in Section 2.3 of [RFC4647].

   For matching, Section 3 of [RFC4647] defines several matching
   schemes.  Implementations can offer parallel on the most appropriate matching
   scheme for their requirements.  The "Basic Filtering" scheme
   ([RFC4647], Section 3.3.1) is identical same resource (i.e., to
   prevent the matching scheme that
   was previously defined for HTTP in Section 14.4 of [RFC2616]. "lost update" problem).  It might can also be contrary used with any
   method to abort a request if the privacy expectations of selected representation does not
   match one that the user to send client has already stored (or partially stored)
   from a prior request.

   An origin server that receives an Accept-Language If-Match header field with MUST evaluate
   the complete linguistic
   preferences 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 user in every request (Section 12.12).

   Since intelligibility condition is highly dependent on
       true if any of the individual user,
   user agents need to allow user control over listed tags match the linguistic preference
   (either through configuration entity-tag of the user agent itself or by
   defaulting to a user controllable system setting).  A user agent that
   does not provide such control to
       selected representation.

   3.  Otherwise, the user condition is false.

   An origin server MUST NOT send an Accept-
   Language header field.

      |  *Note:* User agents ought to provide guidance to users when
      |  setting perform the requested method if a preference, since users are rarely familiar with received
   If-Match condition evaluates to false.  Instead, the
      |  details of language matching as described above.  For example,
      |  users might assume origin server
   MAY indicate that on selecting "en-gb", they will be
      |  served any kind of English document the conditional request failed by responding with a
   412 (Precondition Failed) status code.  Alternatively, if British English the request
   is not
      |  available.  A user agent might suggest, in such a case, state-changing operation that appears to add
      |  "en" have already been
   applied to the list for better matching behavior.

9.5.  Authentication Credentials

   HTTP provides selected representation, the origin server MAY respond
   with a general framework for access control and
   authentication, via an extensible set of challenge-response
   authentication schemes, which can 2xx (Successful) status code (i.e., the change requested by
   the user agent has already succeeded, but the user agent might not be used
   aware of it, perhaps because the prior response was lost or an
   equivalent change was made by a some other user agent).

   Allowing an origin server to challenge send a
   client request and by success response when a client change
   request appears to provide authentication information.

   Two header fields are used have already been applied is more efficient for carrying authentication credentials.
   Note
   many authoring use cases, but comes with some risk if multiple user
   agents are making change requests that various custom mechanisms for are very similar but not
   cooperative.  For example, multiple user authentication use the
   Cookie header field for this purpose, agents writing to a common
   resource as defined in [RFC6265].

    --------------------- -------
     Field Name            Ref.
    --------------------- -------
     Authorization         9.5.3
     Proxy-Authorization   9.5.4
    --------------------- -------

               Table 16

9.5.1.  Challenge and Response

   HTTP provides a simple challenge-response authentication framework
   that can be used by semaphore (e.g., a server non-atomic increment) are likely to challenge a client request
   collide and by 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
   client success response might
   encourage the user agent to provide authentication information.  It uses perform a case-
   insensitive token GET as a means its next request to identify
   eliminate confusion about the authentication scheme,
   followed by additional information necessary for achieving
   authentication via that scheme. resource's current state.

   The latter If-Match header field can be either ignored by caches and intermediaries
   because it is not applicable to a comma-
   separated list of parameters or stored response.

   Note that an If-Match header field with a single sequence of characters
   capable list value containing "*"
   and other values (including other instances of holding base64-encoded information.

   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-scheme    = token

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

     token68        = 1*( ALPHA / DIGIT /
                          "-" / "." / "_" / "~" / "+" / "/" ) *"=" unlikely to be
   interoperable.

12.1.2.  If-None-Match

   The token68 syntax allows "If-None-Match" header field makes the 66 unreserved URI characters
   ([RFC3986]), plus a few others, so that it can hold request method conditional
   on a base64,
   base64url (URL and filename safe alphabet), base32, or base16 (hex)
   encoding, with recipient cache or without padding, but excluding whitespace
   ([RFC4648]).

   A 401 (Unauthorized) response message is used by an origin server to
   challenge the authorization either not having any current
   representation of a user agent, including a
   WWW-Authenticate header field containing at least one challenge
   applicable to the requested resource.

   A 407 (Proxy Authentication Required) response message target resource, when the field value is used by "*",
   or having a
   proxy to challenge the authorization selected representation with an entity-tag that does not
   match any of a client, including a
   Proxy-Authenticate header those listed in the field containing at least one challenge
   applicable to value.

   A recipient MUST use the proxy 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 requested resource.

     challenge representation data.

     If-None-Match = auth-scheme [ 1*SP ( token68 "*" / #auth-param ) ]

      |  *Note:* Many clients fail #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 parse enable
   efficient updates of cached information with a challenge minimum amount of
   transaction overhead.  When a client desires to update one or more
   stored responses that contains have entity-tags, the client SHOULD generate an
      |  unknown scheme.  A workaround for
   If-None-Match header field containing a list of those entity-tags
   when making a GET request; this problem is allows recipient servers to list well-
      |  supported schemes (such as "basic") first.

   A user agent that wishes send a
   304 (Not Modified) response to authenticate itself 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 origin server
   - usually, but
   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 necessarily, after receiving have a 401 (Unauthorized)
   - can do so by including 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 Authorization
   initial representation for the target resource.

   An origin server that receives an If-None-Match header field with MUST
   evaluate the
   request.

   A client that wishes condition as per Section 12.2 prior to authenticate itself with a proxy - usually,
   but not necessarily, after receiving a 407 (Proxy Authentication
   Required) - can do so by including performing the
   method.

   To evaluate a Proxy-Authorization received If-None-Match header field
   with the request.

   Both field:

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

   2.  If the Proxy-Authorization field value contain is a list of entity-tags, the client's credentials for condition is
       false if one of the realm listed tags matches the entity-tag of the resource
   being requested, based upon a challenge received in a response
   (possibly at some point in
       selected representation.

   3.  Otherwise, the past).  When creating their values, condition is true.

   An origin server MUST NOT perform the user agent ought requested method if the
   condition evaluates to do so by selecting false; instead, the challenge origin server MUST respond
   with what it
   considers to be either a) the most secure auth-scheme that it understands,
   obtaining credentials from 304 (Not Modified) status code if the user as appropriate.  Transmission request
   method is GET or HEAD or b) the 412 (Precondition Failed) status code
   for all other request methods.

   Requirements on cache handling of
   credentials within a received If-None-Match header
   field values implies significant security
   considerations regarding the confidentiality of the underlying
   connection, as described are defined in Section 12.15.1.

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

   Upon receipt 4.3.2 of a request for a protected resource [Caching].

   Note 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 If-None-Match header field with at least one (possibly new) challenge applicable to the
   requested resource.

   Likewise, upon receipt 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 request that omits proxy credentials or
   contains invalid GET or partial proxy credentials, a proxy that requires
   authentication SHOULD generate a 407 (Proxy Authentication Required)
   response that contains a Proxy-Authenticate header HEAD request
   method conditional on the selected representation's modification date
   being more recent than the date provided in the field with at
   least one (possibly new) challenge applicable to value.
   Transfer of the proxy.

   A server that receives valid credentials selected representation's data is avoided if that are
   data has not adequate to
   gain access ought to respond with changed.

     If-Modified-Since = HTTP-date

   An example of the 403 (Forbidden) status code
   (Section 10.5.4).

   HTTP does not restrict applications to this simple challenge-response
   framework for access authentication.  Additional mechanisms can be
   used, such as authentication at field is:

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

   A recipient MUST ignore If-Modified-Since if the transport level or via message
   encapsulation, and with additional request contains an
   If-None-Match header fields specifying
   authentication information.  However, such additional mechanisms are
   not defined by this specification.

9.5.2.  Protection Space (Realm)

   The "realm" authentication parameter is reserved for use by
   authentication schemes that wish field; the condition in If-None-Match is
   considered to indicate be a scope of protection.

   A protection space is defined by more accurate replacement for the canonical root URI (the scheme condition in If-
   Modified-Since, and authority components of the target URI; see Section 6.1) of two are only combined for the
   server being accessed, in combination sake of
   interoperating with older intermediaries that might not implement
   If-None-Match.

   A recipient MUST ignore the realm value If-Modified-Since header field 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
   received field value is not a string, generally assigned by 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, server's clock.

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

   When used for cache updates, a prior request has been authorized, cache will typically use the user agent MAY reuse value of
   the same credentials cached message's Last-Modified field to generate the field value
   of If-Modified-Since.  This behavior is most interoperable for all other requests
   within 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 protection space for appear to go "back in time" when the origin
   server's clock is corrected or a period 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 determined by that the
   authentication scheme, parameters, and/or user preferences (such as a
   configurable inactivity timeout).  Unless specifically allowed by message was received,
   particularly when the
   authentication scheme, cached message does not contain a single protection space cannot extend
   outside Last-Modified
   field.

   When used for limiting the scope of its server.

   For historical reasons, retrieval to a sender MUST only recent time
   window, a user agent will 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 an If-Modified-Since field value
   based on either its own local clock or a long time.

9.5.3.  Authorization

   The "Authorization" Date header field allows 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 authenticate
   itself with 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 - usually, but not necessarily, after
   receiving a 401 (Unauthorized) response.  Its value consists of
   credentials containing SHOULD NOT perform the authentication information of requested
   method if the user
   agent for selected representation's last modification date is
   earlier than or equal to the realm of date provided in the resource being requested.

     Authorization = credentials

   If a request is authenticated and a realm specified, field value;
   instead, the same
   credentials origin server SHOULD generate a 304 (Not Modified)
   response, including only those metadata that are presumed to be valid useful 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
   identifying or using synchronized clocks).

   A proxy forwarding updating a request MUST NOT modify any Authorization fields
   in that request.  See Section 3.3 of [Caching] for details of and
   requirements pertaining to previously cached response.

   Requirements on cache handling of the Authorization a received If-Modified-Since header
   field by
   HTTP caches.

9.5.4.  Proxy-Authorization are defined in Section 4.3.2 of [Caching].

12.1.4.  If-Unmodified-Since

   The "Proxy-Authorization" "If-Unmodified-Since" 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 makes the proxy and/or realm of request method
   conditional on the resource selected representation's last modification date
   being requested.

     Proxy-Authorization = credentials

   Unlike Authorization, the Proxy-Authorization header field applies
   only earlier than or equal to the next inbound proxy that demanded authentication using the
   Proxy-Authenticate field.  When multiple proxies are used date provided in a chain, the Proxy-Authorization header field is consumed by value.
   This field accomplishes the first inbound
   proxy that was expecting to receive credentials.  A proxy MAY relay same purpose as If-Match for cases where
   the credentials from user agent does not have an entity-tag for the client request to representation.

     If-Unmodified-Since = HTTP-date

   An example of the next proxy field is:

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

   A recipient MUST ignore If-Unmodified-Since if that is
   the mechanism by which the proxies cooperatively authenticate a given
   request.

9.5.5.  Authentication Scheme Extensibility

   Aside from request contains
   an If-Match header field; the general framework, this document does not specify any
   authentication schemes.  New and existing authentication schemes are
   specified independently and ought condition in If-Match is considered to
   be registered within the
   "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".
   For example, a more accurate replacement for the "basic" and "digest" authentication schemes are
   defined by RFC 7617 condition in If-Unmodified-
   Since, and RFC 7616, respectively.

9.5.5.1.  Authentication Scheme Registry

   The "Hypertext Transfer Protocol (HTTP) Authentication Scheme
   Registry" defines the namespace two are only combined for the authentication schemes in
   challenges and credentials.  It is maintained at
   <https://www.iana.org/assignments/http-authschemes>.

   Registrations sake of interoperating
   with older intermediaries that might not implement If-Match.

   A recipient MUST include ignore the following fields:

   o  Authentication Scheme Name

   o  Pointer to specification text

   o  Notes (optional)

   Values If-Unmodified-Since header field if the
   received field value is not a valid HTTP-date (including when the
   field value appears to be added to this namespace require IETF Review (see
   [RFC8126], Section 4.8).

9.5.5.2.  Considerations for New Authentication Schemes

   There are certain aspects a list of dates).

   A recipient MUST interpret an If-Unmodified-Since field value's
   timestamp in terms of the HTTP Authentication framework that
   put constraints on how new authentication schemes can work:

   o  HTTP authentication origin server's clock.

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

   An origin server that receives an If-Unmodified-Since header field
   MUST evaluate the
      connection cannot be used by any party other than the
      authenticated user (see Section 2.2).

   o  The authentication parameter "realm" is reserved for defining
      protection spaces condition as described in per Section 9.5.2.  New schemes 12.2 prior to performing
   the method.

   If the selected representation has a last modification date, the
   origin server 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 perform the auth-
      param syntax instead, because otherwise future extensions will be
      impossible.

   o  The parsing of challenges and credentials requested method if that date is defined by this
      specification and cannot be modified by new authentication
      schemes.  When
   more recent than the auth-param syntax is used, all parameters ought
      to support both token and quoted-string syntax, and syntactical
      constraints ought to be defined on date provided in the field value after parsing
      (i.e., quoted-string processing).  This is necessary so value.  Instead, the
   origin server MAY indicate that
      recipients can use the conditional request failed by
   responding with a generic parser 412 (Precondition Failed) status code.
   Alternatively, if the request is a state-changing operation that applies
   appears to have already been applied to all
      authentication schemes.

      *Note:* The fact that the value syntax for selected representation,
   the "realm" parameter
      is restricted to quoted-string was origin server MAY respond with a bad design choice 2xx (Successful) status code
   (i.e., the change requested by the user agent has already succeeded,
   but the user agent might not to be
      repeated for new parameters.

   o  Definitions aware of new schemes ought to define it, perhaps because the treatment of
      unknown extension parameters.  In general, a "must-ignore" rule is
      preferable
   prior response was lost or an equivalent change was made by some
   other user agent).

   Allowing an origin server to send a "must-understand" rule, because otherwise it will
      be hard to introduce new parameters in the presence of legacy
      recipients.  Furthermore, it's good success response when a change
   request appears to describe the policy have already been applied is more efficient for
      defining new parameters (such as "update the specification" or
      "use this registry").

   o  Authentication schemes need to document whether they
   many authoring use cases, but comes with some risk if multiple user
   agents are usable in
      origin-server authentication (i.e., using WWW-Authenticate), and/
      or proxy authentication (i.e., using Proxy-Authenticate).

   o  The credentials carried 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 Authorization unsafe
   method.

   The If-Unmodified-Since header field are
      specific 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 user agent and, therefore, have If-Match and If-Unmodified-Since
   header fields but that instructs the same effect on
      HTTP caches as recipient to ignore the "private" Cache-Control response directive
      (Section 5.2.2.7 of [Caching]), within Range
   header field if the scope validator doesn't match, resulting in transfer 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 selected representation instead of a newly
      defined header field) will need 412 (Precondition
   Failed) response.

   If a client has a partial copy of a representation and wishes to explicitly disallow caching, by
      mandating have
   an up-to-date copy of the entire representation, it could use of Cache-Control response directives (e.g.,
      "private").

   o  Schemes using Authentication-Info, Proxy-Authentication-Info, or
      any other authentication related response the
   Range header field need to
      consider with a conditional GET (using either or both of
   If-Unmodified-Since and document If-Match.)  However, if the related security considerations (see
      Section 12.15.4).

9.6.  Request Context

   The following request header fields provide additional information
   about precondition
   fails because the request context, including information about representation has been modified, the user, user
   agent, and resource behind client would
   then have to make a second request to obtain the request.

    ------------ -------
     Field Name   Ref.
    ------------ -------
     From         9.6.1
     Referer      9.6.2
     User-Agent   9.6.3
    ------------ -------

          Table 17

9.6.1.  From entire current
   representation.

   The "From" "If-Range" header field contains an Internet email address for allows a
   human user who controls the requesting user agent.  The address ought client to be machine-usable, "short-circuit" the
   second request.  Informally, its meaning is as defined by "mailbox" follows: if the
   representation is unchanged, send me the part(s) that I am requesting
   in Section 3.4 of
   [RFC5322]:

     From    = mailbox

     mailbox Range; otherwise, send me the entire representation.

     If-Range = <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. entity-tag / HTTP-date

   A
   user agent SHOULD client MUST NOT send a From generate an If-Range header field without explicit
   configuration by the user, since in a request that might conflict with the user's
   privacy interests or their site's security policy.

   A robotic user agent SHOULD send
   does not contain a valid From Range header field.  A server MUST ignore an If-
   Range header field so received in a request that
   the person responsible does not contain a
   Range header field.  An origin server MUST ignore an If-Range header
   field received in a request for running the robot can be contacted if
   problems occur on servers, such as if the robot is sending excessive,
   unwanted, or invalid a target resource that does not
   support Range requests.

   A server SHOULD client MUST NOT use the From generate an If-Range header field for access control or
   authentication, since most recipients will assume containing an
   entity-tag that the field
   value is public information.

9.6.2.  Referer

   The "Referer" [sic] marked as weak.  A client MUST NOT generate an If-
   Range header field allows the user agent to specify a
   URI reference for the resource from which containing an HTTP-date unless the target URI was obtained
   (i.e., client has no
   entity-tag for the "referrer", though corresponding representation and the field name date is misspelled). a
   strong validator in the sense defined by Section 7.9.2.2.

   A user
   agent server that evaluates an If-Range precondition MUST NOT include use the fragment strong
   comparison function when comparing entity-tags (Section 7.9.3.2) and userinfo components of
   MUST evaluate the
   URI reference [RFC3986], condition as false if any, when generating the Referer field
   value.

     Referer = absolute-URI / partial-URI

   The field value is either an absolute-URI or HTTP-date validator is
   provided that is not a partial-URI.  In strong validator in the
   latter case (Section 2.4), sense defined by
   Section 7.9.2.2.  A valid entity-tag can be distinguished from a
   valid HTTP-date by examining the referenced URI is relative to first two characters for a DQUOTE.

   If the
   target URI ([RFC3986], Section 5).

   The Referer validator given in the If-Range 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 matches the
   current validator for
   maintenance.  Some servers use the Referer selected representation of the target
   resource, then the server SHOULD process the Range 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
   requested.  If the target URI was obtained from a source that validator does not have its
   own URI (e.g., input from match, the user keyboard, or an entry within server MUST ignore
   the
   user's bookmarks/favorites), Range header field.  Note that this comparison by exact match,
   including when the user agent MUST either exclude validator is an HTTP-date, differs from the
   Referer field
   "earlier than or send it with equal to" comparison used when evaluating an
   If-Unmodified-Since conditional.

12.2.  Evaluation

   Except when excluded below, a value of "about:blank".

   The Referer field recipient cache or origin server MUST
   evaluate received request preconditions after it has the potential to reveal information about the successfully
   performed its normal request context or browsing history of the user, which is a privacy
   concern if checks and just before it would process
   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 request body (if any) or internal to a
   secured service).  Most general-purpose user agents do not send perform the
   Referer header field when action associated with the referring resource is a local "file" or
   "data" URI.
   request method.  A user agent server MUST NOT send a Referer header field in an
   unsecured HTTP request if the referring page was ignore all received with a
   secure protocol.  See Section 12.9 for additional security
   considerations.

   Some intermediaries have been known preconditions if
   its response 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 same request without those conditions, prior to
   specific edits, such as replacing internal domain names with
   pseudonyms or truncating
   processing the query and/or path components.  An
   intermediary SHOULD NOT modify request body, would have been a status code other than
   a 2xx (Successful) or delete 412 (Precondition Failed).  In other words,
   redirects and failures that can be detected before significant
   processing occurs take precedence over the Referer header field
   when evaluation of
   preconditions.

   A server that is not the field value shares origin server for the same scheme target resource and host
   cannot act as a cache for requests on the target
   URI.

9.6.3.  User-Agent

   The "User-Agent" header field contains information about resource MUST NOT
   evaluate the user
   agent originating conditional request header fields defined by this
   specification, and it MUST forward them if the request, which request is often used forwarded,
   since the generating client intends that they be evaluated by servers to help
   identify a
   server that can provide a current representation.  Likewise, a server
   MUST ignore 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 conditional request header fields defined by this
   specification when received with a User-Agent field in each request
   unless specifically configured method that does not to do so.

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

   The User-Agent field value consists of one
   involve the selection or more product
   identifiers, each followed by zero modification of a selected representation,
   such as CONNECT, OPTIONS, or more comments
   (Section 5.4.1.3), which together identify TRACE.

   Note that protocol extensions can modify the user agent software
   and its significant subproducts.  By convention, conditions under which
   revalidation is triggered.  For example, the product
   identifiers are listed in decreasing order "immutable" cache
   directive (defined by [RFC8246]) instructs caches to forgo
   revalidation of their significance for
   identifying fresh responses even when requested by 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 client.

   Conditional request header fields that are defined by extensions to identify
   HTTP might place conditions on all recipients, on the product; a sender MUST NOT generate
   advertising or other nonessential information within state of the product
   identifier.  A sender SHOULD NOT generate information
   target resource in
   product-version that is not general, or on a version identifier (i.e., successive
   versions group of resources.  For
   instance, the same product name ought to differ only "If" header field 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 WebDAV can make a User-Agent field containing
   needlessly fine-grained detail and SHOULD limit the addition request
   conditional on various aspects of
   subproducts by third parties.  Overly long multiple resources, such as locks,
   if the recipient understands and detailed User-Agent implements that field values increase ([RFC4918],
   Section 10.4).

   Although conditional request latency and header fields are defined as being
   usable with the risk HEAD method (to keep HEAD's semantics consistent with
   those of GET), there is no point in sending a user being
   identified against their wishes ("fingerprinting").

   Likewise, implementations are encouraged not to use conditional HEAD
   because a successful response is around the product
   tokens of other implementations 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 to declare compatibility
   with them, as this circumvents in which the purpose of fields are evaluated becomes
   important.  In practice, the field.  If fields defined in this document are
   consistently implemented in a user
   agent masquerades as single, logical order, since "lost
   update" preconditions have more strict requirements than cache
   validation, 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.

10.  Response Status Codes

   The (response) status code validated cache is more efficient than a three-digit integer code giving the
   result of the attempt to understand partial
   response, and satisfy the request.

   HTTP status codes are extensible.  HTTP clients entity tags are not required presumed to
   understand the meaning of all registered status codes, though such
   understanding is obviously desirable.  However, a client be more accurate than date
   validators.

   A recipient cache or origin server MUST
   understand evaluate the class of any status code, as indicated request
   preconditions defined by this specification in the first
   digit, and treat an unrecognized status code as being equivalent to following order:

   1.  When recipient is the x00 status code of that class.

   For example, if an unrecognized status code of 471 origin server and If-Match is received by a
   client, present,
       evaluate the client If-Match precondition:

       o  if true, continue to step 3

       o  if false, respond 412 (Precondition Failed) unless it can assume be
          determined that there was something wrong with its 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 treat
       If-Unmodified-Since is present, evaluate the response as If-Unmodified-Since
       precondition:

       o  if true, continue to step 3
       o  if false, respond 412 (Precondition Failed) unless it had received a 400 (Bad
   Request) status code.  The response message will usually contain a
   representation can be
          determined that explains the status.

   The first digit of state-changing request has already
          succeeded (see Section 12.1.4)

   3.  When If-None-Match is present, evaluate the status code defines 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 class of response.
   The last two digits do method is GET or HEAD, If-None-Match is not have any categorization role.  There are
   five values for present, and
       If-Modified-Since is present, evaluate the first digit: If-Modified-Since
       precondition:

       o  1xx (Informational): The request was received, continuing process  if true, continue to step 5

       o  2xx (Successful): The request was successfully received,
      understood,  if false, respond 304 (Not Modified)

   5.  When the method is GET and accepted both Range and If-Range are present,
       evaluate the If-Range precondition:

       o  3xx (Redirection): Further action needs to be taken in order  if the validator matches and the Range specification is
          applicable to
      complete the request

   o  4xx (Client Error): The request contains bad syntax or cannot be
      fulfilled selected representation, respond 206
          (Partial Content)

   6.  Otherwise,

       o  5xx (Server Error): The server failed  all conditions are met, so perform the requested action and
          respond according to fulfill an apparently
      valid request

   A single request can have multiple associated responses: zero its success or more
   interim (non-final) responses with status codes in failure.

   Any extension to HTTP that defines additional conditional request
   header fields ought to define its own expectations regarding the
   "informational" (1xx) range, followed by exactly one final response
   with a status code
   order for evaluating such fields in one of the other ranges.

10.1.  Overview of Status Codes

   The status codes listed below are relation to those defined in this specification.  The
   reason phrases listed here are only recommendations - they can
   document and other conditionals that might be
   replaced by local equivalents without affecting 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 protocol.

   Responses with status codes remainder of
   that are defined as heuristically
   cacheable (e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410,
   414, and 501 representation in this specification) can be reused by a cache with
   heuristic expiration unless otherwise indicated by subsequent request rather than transfer the method
   definition
   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 explicit cache controls [Caching]; all other status
   codes are not heuristically cacheable.

             ------- ------------------------------- ---------
              Value   Description                     Ref.
             ------- ------------------------------- ---------
              100     Continue                        10.2.1
              101     Switching Protocols             10.2.2
              200     OK                              10.3.1
              201     Created                         10.3.2
              202     Accepted                        10.3.3
              203     Non-Authoritative Information   10.3.4
              204     No Content                      10.3.5
              205     Reset Content                   10.3.6
              206     Partial Content                 10.3.7
              300     Multiple Choices                10.4.1
              301     Moved Permanently               10.4.2
              302     Found                           10.4.3
              303     See Other                       10.4.4
              304     Not Modified                    10.4.5
              305     Use Proxy                       10.4.6
              306     (Unused)                        10.4.7
              307     Temporary Redirect              10.4.8
              308     Permanent Redirect              10.4.9
              400     Bad Request                     10.5.1
              401     Unauthorized                    10.5.2
              402     Payment Required                10.5.3
              403     Forbidden                       10.5.4
              404     Not Found                       10.5.5
              405     Method Not Allowed              10.5.6
              406     Not Acceptable                  10.5.7
              407     Proxy Authentication Required   10.5.8
              408     Request Timeout                 10.5.9
              409     Conflict                        10.5.10
              410     Gone                            10.5.11
              411     Length Required                 10.5.12
              412     Precondition Failed             10.5.13
              413     Payload Too Large               10.5.14
              414     URI Too Long                    10.5.15
              415     Unsupported Media Type          10.5.16
              416
   the dimensions of an embedded image.

   Range Not Satisfiable           10.5.17
              417     Expectation Failed              10.5.18
              418     (Unused)                        10.5.19
              422     Unprocessable Payload           10.5.20
              426     Upgrade Required                10.5.21
              500     Internal Server Error           10.6.1
              501     Not Implemented                 10.6.2
              502     Bad Gateway                     10.6.3
              503     Service Unavailable             10.6.4
              504     Gateway Timeout                 10.6.5
              505     HTTP Version Not Supported      10.6.6
             ------- ------------------------------- ---------

                                  Table 18

   Note requests are an OPTIONAL feature of HTTP, designed so that
   recipients not implementing this list is feature (or not exhaustive - supporting it does not include extension
   status codes defined in other specifications (Section 10.7).

10.2.  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 target resource) can respond as if it is a final
   response.  1xx normal GET request
   without impacting interoperability.  Partial responses are terminated indicated
   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 a distinct status code to not be mistaken for full responses unless the proxy itself requested
   the generation of by
   caches that might not implement the 1xx response. 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, if octet (a.k.a., byte) boundaries are a proxy adds an
   "Expect: 100-continue" field when it forwards
   structural unit common to all representation data, allowing
   partitions of the data to be identified as a request, then it need
   not forward range of bytes at some
   offset from the corresponding 100 (Continue) response(s).

10.2.1.  100 Continue

   The 100 (Continue) status code indicates start or end of that the initial part data.

   This general notion of a
   request has been received and has not yet been rejected by "range unit" is used in the
   server.  The server intends to send a final Accept-Ranges
   (Section 13.3) response after the
   request has been fully received and acted upon.

   When header field to advertise support for range
   requests, the Range (Section 13.2) request contains an Expect header field that includes a
   100-continue expectation, to delineate
   the 100 response indicates parts of a representation that are requested, and the server
   wishes
   Content-Range (Section 13.4) payload header field to receive 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 request payload body, "HTTP Range Unit Registry", as defined in Section 15.5.1

   Range units are intended to be extensible, as described in
   Section 9.1.1. 15.5.  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 following range unit names are defined by this interim
   response.

10.2.2.  101 Switching Protocols

   The 101 (Switching Protocols) status code indicates that the server
   understands and is willing
   document:

    ----------------- ---------------------------------- --------
     Range Unit Name   Description                        Ref.
    ----------------- ---------------------------------- --------
     bytes             a range of octets                  13.1.2
     none              reserved as keyword to comply indicate    13.3
                       range requests are not supported
    ----------------- ---------------------------------- --------

                               Table 13

13.1.1.  Range Specifiers

   Ranges are expressed in terms of a range unit paired with the client's request, via
   the Upgrade header field (Section 6.7), for a change in the
   application protocol being used on this connection. set of
   range specifiers.  The server MUST
   generate an Upgrade header field in the response that indicates which
   protocol(s) will be switched range unit name determines what kinds of
   range-spec are applicable to immediately after the empty line that
   terminates its own specifiers.  Hence, the 101 response.

   It
   following gramar is assumed that the server will only agree to switch protocols
   when it generic: each range unit is advantageous to do so.  For example, switching expected to a newer
   version of HTTP might be advantageous over older versions, specify
   requirements on when int-range, suffix-range, and
   switching to other-range are
   allowed.

   A range request can specify a real-time, synchronous protocol might be advantageous
   when delivering resources that use such features.

10.3.  Successful 2xx

   The 2xx (Successful) class single range or a set of status code indicates that 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 client's
   request was successfully received, understood, and accepted.

10.3.1.  200 OK

   The 200 (OK) status code indicates that end of the request has succeeded. representation
   data.  The payload sent in a 200 response depends on range unit specifies what the request method.
   For integers mean (e.g., they
   might indicate unit offsets from the methods defined by this specification, 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 intended meaning
   of last-pos value is present and less
   than the payload can be summarized as:

   GET first-pos.

   A suffix-range is a representation of the target resource;

   HEAD  the same representation range expressed as GET, but without the representation
      data;

   POST a representation suffix of the status of, or results obtained from,
      the action;

   PUT, DELETE  a representation of
   data with the status of provided non-negative integer maximum length (in range
   units).  In other words, the action;
   OPTIONS  a representation last N units of the communications options;

   TRACE  a representation of the request message as received by data.

     suffix-range  = "-" suffix-length
     suffix-length = 1*DIGIT

   To provide for extensibility, 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 other-range rule is desired, an origin server ought a mostly
   unconstrained grammar that allows application-specific or future
   range units to send 204 (No
   Content) instead.  For CONNECT, no payload is allowed because the
   successful result define additional range specifiers.

     other-range   = 1*( %x21-2B / %x2D-7E )
                   ; 1*(VCHAR excluding comma)

13.1.2.  Byte Ranges

   The "bytes" range unit is used to express subranges of a tunnel, which begins immediately after the 200
   response header section.

   A 200 response
   representation data's octet sequence.  Each byte range is heuristically cacheable; i.e., unless otherwise
   indicated by expressed
   as an integer range at some offset, relative to either the method definition beginning
   (int-range) or explicit cache controls (see
   Section 4.2.2 end (suffix-range) of [Caching]).

10.3.2.  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 representation data.  Byte
   ranges do not use the request is identified
   by either a Location header field other-range specifier.

   The first-pos value in a bytes int-range gives the response or, if no Location
   field is received, by offset of the target URI.
   first byte in a range.  The 201 response payload typically describes and links to last-pos value gives the
   resource(s) created.  See Section 11.2 for a discussion offset of the
   meaning and purpose of validator header fields, such as ETag and
   Last-Modified,
   last byte in a 201 response.

10.3.3.  202 Accepted

   The 202 (Accepted) status code indicates the range; that is, the request has been
   accepted for processing, but byte positions specified are
   inclusive.  Byte offsets start at zero.

   If the processing representation data 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 content coding applied, each byte
   range is calculated with respect to accept a request for some other process (perhaps a
   batch-oriented process the encoded sequence of bytes,
   not the sequence of underlying bytes that is only run once per day) 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
   requiring that knowing the user agent's connection to
   size of the server persist
   until selected representation.  If the process last-pos value is completed.  The representation sent with this
   response ought to describe
   absent, or if the request's current status and point value is greater than or equal to
   (or embed) a status monitor that can provide the user with an
   estimate current
   length of when the request will be fulfilled.

10.3.4.  203 Non-Authoritative Information

   The 203 (Non-Authoritative Information) status code indicates that representation data, the request was successful but byte range is interpreted as
   the enclosed payload has been modified
   from that remainder of the origin server's 200 (OK) response by a transforming
   proxy (Section 6.6.2).  This status code allows representation (i.e., the proxy to notify
   recipients when server replaces the
   value of last-pos with a transformation has been applied, since value that
   knowledge might impact later decisions regarding the content.  For
   example, future cache validation requests for is one less than the content might only
   be applicable along current
   length of the same selected representation).

   A client can request path (through the same proxies).

   The 203 response last N bytes (N > 0) of the selected
   representation using a suffix-range.  If the selected representation
   is similar to shorter than the Warning code specified suffix-length, the entire
   representation is used.

   Additional examples, assuming a representation of 214 Transformation
   Applied (Section 5.5 length 10000:

   o  The final 500 bytes (byte offsets 9500-9999, inclusive):

           bytes=-500

      Or:

           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 [Caching]), which has the advantage of being
   applicable to responses 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 any status code.

   A 203 response a
   first-pos that is heuristically cacheable; i.e., unless otherwise
   indicated by less than the method definition or explicit cache controls (see
   Section 4.2.2 current length of [Caching]).

10.3.5.  204 No Content

   The 204 (No Content) status code indicates that the server 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
   successfully fulfilled zero length, the request 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 that suffix-length are
   expressed as decimal number of octets.  Since there is no additional
   content predefined
   limit to send in the response payload body.  Metadata in 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
   response Range header fields refer field.  However, origin servers
   and intermediate caches ought to the target resource support byte ranges when possible,
   since they support efficient recovery from partially failed transfers
   and its selected
   representation after the requested action was applied.

   For example, if partial retrieval of large representations.  A server MUST ignore
   a 204 status code is Range header field received in response to with a PUT request and the response method other than GET.

   An origin server MUST ignore a Range header field that contains an ETag field, then the PUT was
   successful and the ETag a
   range unit it does not understand.  A proxy MAY discard a Range
   header field value that contains the entity-tag for the
   new representation 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 target resource.

   The 204 response allows are not listed in ascending order,
   since both are indications of either a server to indicate broken client or a deliberate
   denial-of-service attack (Section 16.14).  A client SHOULD NOT
   request multiple ranges that the action has been
   successfully applied are inherently less efficient to the target resource, while implying process
   and transfer than a single range that encompasses the
   user agent does not need to traverse away from its current "document
   view" (if any).  The same data.

   A server assumes that supports range requests MAY ignore a Range header field
   when the user agent will provide
   some indication of selected representation has no body (i.e., the success to its user, selected
   representation data is of zero length).

   A client that is requesting multiple ranges SHOULD list those ranges
   in accord with its own
   interface, and apply any new or updated metadata ascending order (the order in the response which they would typically be
   received in a complete representation) unless there is a specific
   need to
   its active representation. request a later part earlier.  For example, a 204 status code is commonly used user agent
   processing a large representation with document editing
   interfaces corresponding an internal catalog of parts
   might need to a "save" action, such that request later parts first, particularly if the document
   being saved remains available to
   representation consists of pages stored in reverse order and the user for editing.  It is also
   frequently used with interfaces that expect automated data transfers
   agent wishes to be prevalent, such as within distributed version control systems.

   A 204 response transfer one page at a time.

   The Range header field is terminated by the first empty line evaluated after evaluating the precondition
   header fields because it cannot contain a message body.

   A 204 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see defined in Section 4.2.2 12.1, and only if the result in
   absence of [Caching]).

10.3.6.  205 Reset Content 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 205 (Reset Content) status code indicates that If-Range header field (Section 12.1.5) can be used as a
   precondition to applying the server has
   fulfilled Range header field.

   If all of the request and desires that preconditions are true, the user agent reset server supports the
   "document view", which caused Range
   header field for the request to be sent, to its original
   state as received from target resource, and the origin server.

   This specified range(s) are
   valid and satisfiable (as defined in Section 13.1.2), the server
   SHOULD send a 206 (Partial Content) response is intended to support with a common data entry use case
   where the user receives content that supports data entry (a form,
   notepad, canvas, etc.), enters payload
   containing one or manipulates data in more partial representations that space,
   causes the entered data correspond to be submitted in a request, and then the
   data entry mechanism is reset
   satisfiable ranges requested.

   If all of the preconditions are true, the server supports the Range
   header field for the next entry so that target resource, and the user can
   easily initiate another input action.

   Since specified range(s) are
   invalid or unsatisfiable, the 205 status code implies that no additional content will be
   provided, a server MUST NOT generate a payload in SHOULD send a 205 416 (Range Not
   Satisfiable) response.

10.3.7.  206 Partial Content

13.3.  Accept-Ranges

   The 206 (Partial Content) status code indicates "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 is
   successfully fulfilling 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 by
   transferring one or more parts of MAY send

     Accept-Ranges: none

   to advise the selected representation.

   When client not to attempt a 206 response is generated, the server MUST generate the
   following range request.

13.4.  Content-Range

   The "Content-Range" header fields, field is sent in addition a single part 206
   (Partial Content) response to those required in indicate the
   subsections below, if partial range of the field would have been
   selected representation enclosed as the message payload, sent in each
   part of a 200 (OK) multipart 206 response to indicate the same request: Date, Cache-Control, ETag, Expires,
   Content-Location, range enclosed
   within each body part, and Vary.

   A Content-Length field present 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 indicates contains a Content-Range header
   field with a range unit (Section 13.1) that the number
   of octets in recipient does not
   understand, the body recipient MUST NOT attempt to recombine it with a
   stored representation.  A proxy that receives such a message SHOULD
   forward it downstream.

   For byte ranges, a sender SHOULD indicate the complete length of this message, the
   representation from which the range has been extracted, unless the
   complete length is usually not 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.  Each 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 includes information about 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 selected representation's
   complete length.

   If received content with a 206 is generated in stored representation.

   A server generating a 416 (Range Not Satisfiable) response to a byte-
   range 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 send a prior response containing those Content-Range header fields.
   Otherwise, the sender MUST generate all of field with an
   unsatisfied-range value, as in the representation header
   fields that would have been sent following example:

     Content-Range: bytes */1234

   The complete-length in a 200 (OK) 416 response to indicates the same
   request.

   A 206 response is heuristically cacheable; i.e., unless otherwise
   indicated by explicit cache controls (see Section 4.2.2 current length of
   [Caching]).

10.3.7.1.  Single Part

   If a single part is being transferred,
   the server generating 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
   response MUST generate (Partial Content) and 416 (Range Not Satisfiable) status
   codes describe a Content-Range header field, describing what
   range meaning for Content-Range.

   The following are examples of Content-Range values in which the
   selected representation is enclosed, and contains a payload
   consisting 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

   o  All except for 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 first 500 bytes:

           Content-Range: bytes 21010-47021/47022
     Content-Length: 26012
     Content-Type: image/gif

     ... 26012 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 partial image data ...

10.3.7.2.  Multiple Parts

   If
   multiple parts ranges, they are being transferred, the server generating the
   206 response MUST generate a "multipart/byteranges" payload, transmitted as
   defined body parts in Section 7.3.5, and a Content-Type header field containing multipart
   message body ([RFC2046], Section 5.1) with the media type of
   "multipart/byteranges".

   The multipart/byteranges media type and includes one or more body parts,
   each with its own Content-Type and Content-Range fields.  The
   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 parameter specifies the header area of boundary string used to
   separate each body part part.

   Implementation Notes:

   1.  Additional CRLFs might precede the first boundary string in the multipart payload,
       body.

   2.  Although [RFC2046] permits the server MUST generate a Content-Range header field corresponding boundary string to the range being enclosed in that body part.  If the selected
   representation would have had a Content-Type header field in be quoted, some
       existing implementations handle a 200
   (OK) response, quoted boundary string
       incorrectly.

   3.  A number of clients and servers were coded to an early draft of
       the server SHOULD generate byteranges specification that same Content-Type
   field in the header area used a media type of each body part.  For example: 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:

     HTTP/1.1 206 Partial Content
     Date: Wed, 15 Tue, 14 Nov 1995 06:25:24 GMT
     Last-Modified: Wed, 15 Nov 1995 Tue, 14 July 04:58:08 GMT
     Content-Length: 1741 2331785
     Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

     --THIS_STRING_SEPARATES
     Content-Type: application/pdf video/example
     Content-Range: bytes 500-999/8000 exampleunit 1.2-4.3/25

     ...the first range...
     --THIS_STRING_SEPARATES
     Content-Type: application/pdf video/example
     Content-Range: bytes 7000-7999/8000 exampleunit 11.2-14.3/25

     ...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 following information serves as the received Range
   header field.  Since registration form for 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 type.

   Type name:  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
   order that it requested.

10.3.7.3.  Combining Parts

   A response might transfer

   Subtype name:  byteranges

   Required parameters:  boundary

   Optional parameters:  N/A

   Encoding considerations:  only a subrange of a representation if the
   connection closed prematurely or if the request used one "7bit", "8bit", 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 11.2.1).

   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 "binary" are used for any combined response
   and replace those of the matching stored responses.

   If the most recent response is
      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 206 (Partial Content) response 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
                            Macintosh file type code(s):  N/A

   Person and
   at least one of the matching stored responses 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 200 (OK), then three-digit integer code giving the
   combined response header fields consist
   result of the most recent 200
   response's header fields.  If all of attempt to understand and satisfy the matching stored responses request.

   HTTP status codes are 206 responses, then the stored response with the most recent
   header fields is used as extensible.  HTTP clients are not required to
   understand the source meaning of header fields for the combined
   response, except that the all registered status codes, though such
   understanding is obviously desirable.  However, a 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
   understand the union class of partial
   content ranges in any status code, as indicated by the new response first
   digit, and each of the selected
   responses.  If treat an unrecognized status code as being equivalent to
   the union consists x00 status code of the entire range that class.

   For example, if an unrecognized status code of the
   representation, then 471 is received by a
   client, the client MUST process can assume that there was something wrong with its
   request and treat the combined response as if it were had received a complete 200 (OK) response, including 400 (Bad
   Request) status code.  The response message will usually contain a Content-Length
   header field
   representation that reflects the complete length.  Otherwise, the
   client MUST process explains the set of continuous ranges as one status.

   The first digit of the
   following: an incomplete 200 (OK) response if status code defines the combined response
   is a prefix class of response.
   The last two digits do not have any categorization role.  There are
   five values for 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.

10.4.  Redirection 3xx 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) class of status code indicates that further (Redirection): Further action needs to be taken by the user agent 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

   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
   request.  There are several types of redirects:

   1.  Redirects protocol.

   Responses with status codes that indicate are defined as heuristically
   cacheable (e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410,
   414, and 501 in this resource might specification) can be available at reused by a
       different URI, as provided cache with
   heuristic expiration unless otherwise indicated by the Location field, as in the method
   definition or explicit cache controls [Caching]; all other status
   codes 301 (Moved Permanently), 302 (Found), 307 (Temporary
       Redirect), and 308 (Permanent Redirect).

   2.  Redirection that offers a choice among matching resources capable are not heuristically cacheable.

   Additional status codes, outside the scope of representing this resource, as specification,
   have been specified for use in the 300 (Multiple Choices) HTTP.  All such status code.

   3.  Redirection to a different resource, identified by the Location
       field, that can represent an indirect response codes ought to
   be registered within the request, "Hypertext Transfer Protocol (HTTP) Status
   Code Registry", as described in the 303 (See Other) Section 15.2.

14.2.  Informational 1xx

   The 1xx (Informational) class of status code.

   4.  Redirection to a previously stored result, as in the 304 (Not
       Modified) code indicates an interim
   response for communicating connection status code.

   If a Location header field (Section 11.1.2) is provided, the user
   agent MAY automatically redirect its or request progress
   prior to completing the URI referenced requested action and sending a final
   response.  1xx responses are terminated by the Location field value, even if end of the specific status code is header
   section.  Since HTTP/1.0 did not
   understood.  Automatic redirection needs define any 1xx status codes, a
   server MUST NOT send a 1xx response to an HTTP/1.0 client.

   A client MUST be done with care for
   methods not known able to be safe, as defined in Section 8.2.1, since the
   user might not wish parse one or more 1xx responses received
   prior to redirect an unsafe request.

   When automatically following a redirected request, final response, even if the client does not expect one.  A
   user agent
   SHOULD resend the original request message with the following
   modifications:

   1.  Replace MAY ignore unexpected 1xx responses.

   A proxy MUST forward 1xx responses unless the target URI with proxy itself requested
   the URI referenced by generation of the redirection
       response's Location header 1xx response.  For example, if a proxy adds an
   "Expect: 100-continue" field value after resolving when 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 6.8),

       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 forwards a request, then it need
   not
           limited to) Host,

       4.  Validating header fields that were added by forward the
           implementation's cache (e.g., If-None-Match,
           If-Modified-Since),

       5.  Resource-specific header fields, including (but not limited
           to) Referer, Origin, Authorization, and Cookie.

   3.  Consider removing header fields corresponding 100 (Continue) response(s).

14.2.1.  100 Continue

   The 100 (Continue) status code indicates that were not automatically
       generated by the implementation (i.e., those present in the initial part of a
   request because they were added by the calling context) where
       there are security implications; this includes but is not limited
       to Authorization has been received and Cookie.

   4.  Change has not yet been rejected by the request method according
   server.  The server intends to the redirecting status
       code's semantics, if applicable.

   5.  If send a final response after the
   request method has been changed to GET or HEAD, remove
       content-specific fully received and acted upon.

   When the request contains an Expect 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, field that includes a
   100-continue expectation, the status codes 301 (Moved Permanently)
      |  and 302 (Found) were defined for 100 response indicates that the first type of redirect
      |  ([RFC1945], Section 9.3).  Early user agents split on whether
      | server
   wishes to receive the method applied request payload body, as described in
   Section 9.1.1.  The client ought to continue sending the redirect target would be request and
   discard the same as
      | 100 response.

   If the original request or would be rewritten as GET.  Although
      |  HTTP originally defined did not contain an Expect header field containing the former semantics for 301 and 302
      |  (to match its original implementation at CERN),
   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 defined 303
      |  (See Other) is willing to match comply with the latter semantics, prevailing practice
      |  gradually converged on client's request, via
   the latter semantics Upgrade header field (Section 6.6), for 301 and 302 as
      |  well.  The first revision of HTTP/1.1 added 307 (Temporary
      |  Redirect) to indicate a change in the former semantics of 302 without
   application protocol being
      |  impacted by divergent practice.  For the same reason, 308
      |  (Permanent Redirect) was later used on added this connection.  The server MUST
   generate an Upgrade header field in [RFC7538] the response that indicates which
   protocol(s) will be switched to match
      |  301.  Over 10 years later, most user agents still do method
      |  rewriting for 301 and 302; therefore, [RFC7231] made immediately after the empty line that
      |  behavior conformant when
   terminates the original request 101 response.

   It is POST.

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

      |  *Note:* An earlier version of this specification recommended assumed that the server will only agree to switch protocols
   when it is advantageous to do so.  For example, switching to a
      |  maximum newer
   version of five redirections ([RFC2068], Section 10.3).
      |  Content developers need HTTP might be advantageous over older versions, and
   switching to a real-time, synchronous protocol might be aware advantageous
   when delivering resources that some clients might
      |  implement use such a fixed limitation.

10.4.1.  300 Multiple Choices features.

14.3.  Successful 2xx

   The 300 (Multiple Choices) 2xx (Successful) class of status code indicates that the target
   resource has more than one representation, each with its own more
   specific identifier, client's
   request was successfully received, understood, and information about the alternatives is being
   provided so accepted.

14.3.1.  200 OK

   The 200 (OK) status code indicates that the user (or user agent) 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 select be summarized as:

   GET  a representation of the target resource;

   HEAD  the same representation as GET, but without the representation
      data;

   POST  a preferred representation by redirecting its request to one or more of those
   identifiers.  In other words, the server desires that status of, or results obtained from,
      the user agent
   engage in reactive negotiation to select action;

   PUT, DELETE  a representation of the most appropriate
   representation(s) for its needs (Section 7.4).

   If status of the server has action;
   OPTIONS  a preferred choice, representation of the server SHOULD generate a
   Location header field containing communications options;

   TRACE  a preferred choice's URI reference.
   The user agent MAY use representation of the Location field value for automatic
   redirection.

   For request methods other than HEAD, message as received by the server SHOULD generate end
      server.

   Aside from responses to CONNECT, a
   payload in the 300 200 response containing always has 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 payload,
   though an origin server MAY make generate a selection
   from that list automatically if it understands the provided media
   type.  A specific format for automatic selection payload body of zero length.
   If no payload is not defined by
   this specification because HTTP tries to remain orthogonal desired, an origin server ought to send 204 (No
   Content) instead.  For CONNECT, no payload is allowed because the
   definition of its payloads.  In practice, the representation
   successful result is
   provided in some easily parsed format believed to be acceptable to a tunnel, which begins immediately after the user agent, as determined by shared design or content
   negotiation, or in some commonly accepted hypertext format. 200
   response header section.

   A 300 200 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

      |  *Note:*

14.3.2.  201 Created

   The original proposal for the 300 201 (Created) status code defined
      |  the URI header field as providing a list of alternative
      |  representations, such indicates 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 request has been
   fulfilled and Alternates (a subsequent proposal)
      | has resulted in one or more new resources being dropped from this specification.  It is possible to
      |  communicate
   created.  The primary resource created by the list as request is identified
   by either a Link Location header field value [RFC8288]
      |  whose members have 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 relationship discussion of "alternate", though
      |  deployment is the meaning
   and purpose of validator header fields, such as ETag and
   Last-Modified, in a chicken-and-egg problem.

10.4.2.  301 Moved Permanently 201 response.

14.3.3.  202 Accepted

   The 301 (Moved Permanently) 202 (Accepted) status code indicates that the target
   resource request has been assigned
   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 new permanent URI and any future
   references status code from an asynchronous
   operation.

   The 202 response is intentionally noncommittal.  Its purpose is to this resource ought
   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 use one of the enclosed URIs.
   Clients server persist
   until the process is completed.  The representation sent with link-editing capabilities this
   response ought to automatically re-link
   references to describe the target URI request's current status and point to one or more of
   (or embed) a status monitor that can provide the new references
   sent by user with an
   estimate of when the server, where possible. request will be fulfilled.

14.3.4.  203 Non-Authoritative Information

   The server SHOULD generate a Location header field in 203 (Non-Authoritative Information) status code indicates that
   the response
   containing a preferred URI reference for request was successful but the new permanent URI.  The
   user agent MAY use enclosed payload has been modified
   from that of the Location field value for automatic
   redirection.  The origin server's 200 (OK) response payload usually contains a short
   hypertext note with by a hyperlink 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 new URI(s).

      |  *Note:* content.  For historical reasons, a user agent MAY change
   example, future cache validation requests for the
      | content might only
   be applicable along the same request method from POST to GET for path (through the subsequent request.  If
      |  this behavior same proxies).

   The 203 response is undesired, similar to the 308 (Permanent Redirect) status
      | Warning code can be used instead. of 214 Transformation
   Applied (Section 5.5 of [Caching]), which has the advantage of being
   applicable to responses with any status code.

   A 301 203 response is heuristically cacheable; i.e., unless otherwise
   indicated by the method definition or explicit cache controls (see
   Section 4.2.2 of [Caching]).

10.4.3.  302 Found

14.3.5.  204 No Content

   The 302 (Found) 204 (No Content) 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 has
   successfully fulfilled 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.

10.4.4.  303 See Other

   The 303 (See Other) status code indicates and that the server there is
   redirecting the user agent no additional
   content to a different resource, as indicated by a
   URI send 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 payload body.  Metadata in the Location
   response 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 fields refer to the POST response in a form that can be separately
   identified, bookmarked, target resource and cached, independent of its selected
   representation after the original
   request.

   A 303 requested action was applied.

   For example, if a 204 status code is received in response to a GET PUT
   request indicates that the origin server does
   not have a representation of and the target resource that can be
   transferred by response contains an ETag field, then the server over HTTP.  However, PUT was
   successful and the Location ETag field value refers to a resource that is descriptive of contains the target
   resource, such that making a retrieval request on that other resource
   might result in a entity-tag for the
   new representation of that is useful target resource.

   The 204 response allows a server to recipients without
   implying indicate that it represents the original action has been
   successfully applied to the target resource.  Note resource, while implying that
   answers the
   user agent does not need to traverse away from its current "document
   view" (if any).  The server assumes that the questions user agent will provide
   some indication 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 success to a HEAD request, its user, in accord with its own
   interface, and apply any new or updated metadata in the representation of a 303 response ought to contain
   its active representation.

   For example, a short hypertext note 204 status code is commonly used with document editing
   interfaces corresponding to a hyperlink "save" action, such that the document
   being saved remains available to the same URI reference provided in 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 Location header field.

10.4.5.  304 Not Modified
   fields because it cannot contain a message body.

   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 304 (Not Modified) 205 (Reset Content) status code indicates that a conditional GET
   or HEAD request the server has been received and would have resulted in a 200
   (OK) response if it were not for
   fulfilled the fact request and desires that the condition
   evaluated user agent reset the
   "document view", which caused the request to false.  In other words, there is no need for be sent, to its original
   state as received from the server origin server.

   This response is intended to transfer support a representation of the target resource because common data entry use case
   where the
   request indicates user receives content that supports data entry (a form,
   notepad, canvas, etc.), enters or manipulates data in that space,
   causes the client, which made the request
   conditional, already has entered data to be submitted in a valid representation; request, and then the server
   data entry mechanism is
   therefore redirecting reset for the client to make use of next entry so that stored
   representation as if it were 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 of in a 200 (OK) 205 response.

14.3.7.  206 Partial Content

   The 206 (Partial Content) status code indicates that the server generating is
   successfully fulfilling a range request for the target resource by
   transferring one or more parts of the selected representation.

   When a 304 206 response is generated, the server MUST generate any of the
   following header fields that 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: Cache-Control, Content-Location, Date, Cache-Control, ETag, Expires,
   Content-Location, and Vary.

   Since the goal of

   A Content-Length field present in a 304 206 response indicates the number
   of octets in the body of this message, which is to minimize usually not the
   complete length of the selected representation.  Each Content-Range
   field includes information transfer
   when about the recipient already has one or more cached representations, 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 representation metadata other than the
   above listed representation
   header fields unless said metadata exists for the purpose of
   guiding cache updates (e.g., Last-Modified might be useful if beyond those required, because the
   response does not client is understood
   to already have an ETag field).

   Requirements on a cache that receives a 304 prior response are defined in
   Section 4.3.4 containing those header fields.
   Otherwise, the sender MUST generate all 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 representation header
   fields that would have been sent in a shared proxy, then the proxy SHOULD
   forward the 304 200 (OK) response to that client. the same
   request.

   A 304 206 response cannot contain a message-body; it is always terminated heuristically cacheable; i.e., unless otherwise
   indicated by the first empty line after the header fields.

10.4.6.  305 Use Proxy

   The 305 (Use Proxy) status code was defined in a previous version of
   this specification and is now deprecated (Appendix B explicit cache controls (see Section 4.2.2 of [RFC7231]).

10.4.7.  306 (Unused)

   The 306 status code was defined in
   [Caching]).

14.3.7.1.  Single Part

   If a previous version of this
   specification, is no longer used, and the code single part is reserved.

10.4.8.  307 Temporary Redirect

   The 307 (Temporary Redirect) status code indicates that being transferred, the target
   resource resides temporarily under a different URI and server generating the user agent 206
   response 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 Content-Range header field in field, describing what
   range of the response
   containing selected representation is enclosed, and a URI reference for payload
   consisting of the different URI.  The user agent MAY
   use 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 Location field value for automatic redirection.  The server's server generating the
   206 response payload usually contains MUST generate a short hypertext note with "multipart/byteranges" payload, as
   defined in Section 13.5, and a
   hyperlink to the different URI(s).

10.4.9.  308 Permanent Redirect

   The 308 (Permanent Redirect) status code indicates that Content-Type header field containing
   the target
   resource has been assigned a new permanent URI multipart/byteranges media type and any future
   references to this resource ought to use one of the enclosed URIs.
   Clients its required boundary
   parameter.  To avoid confusion with link editing capabilities ought to automatically re-link
   references to single-part responses, a server
   MUST NOT generate a Content-Range header field in the target URI to one or more HTTP header
   section of the new references a multiple part response (this field will be sent by in each
   part instead).

   Within the header area of each body part in the multipart payload,
   the server, where possible.

   The server SHOULD MUST generate a Location Content-Range header field corresponding
   to the range being enclosed in that body part.  If the response
   containing selected
   representation would have had a preferred URI reference for the new permanent URI.  The
   user agent MAY use the Location Content-Type header field value for automatic
   redirection.  The server's response payload usually contains a short
   hypertext note with in a hyperlink to 200
   (OK) response, the new URI(s).

   A 308 response is heuristically cacheable; i.e., unless otherwise
   indicated by server SHOULD generate that same Content-Type
   field in the method definition or explicit cache controls (see
   Section 4.2.2 header area of [Caching]).

      |  *Note:* This status code each body part.  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-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 much younger (June 2014) smaller
   than its
      |  sibling codes, and thus might not be recognized everywhere.
      |  See Section 4 the overhead of [RFC7538] for deployment considerations.

10.5.  Client Error 4xx

   The 4xx (Client Error) class sending multiple parts, regardless of status code indicates that the client
   seems to have erred.  Except when responding to a HEAD request, order
   in which the
   server SHOULD send a representation containing an explanation 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
   error situation,
   selected representation's media type and whether the chosen boundary
   parameter length, it is a temporary or permanent
   condition.  These status codes are applicable can be less efficient to any request method.
   User agents SHOULD display any included representation transfer many small
   disjoint parts than it is to transfer the user.

10.5.1.  400 Bad Request

   The 400 (Bad Request) status code indicates that the entire selected
   representation.

   A server cannot or
   will not process the request due to something that is perceived MUST NOT generate a multipart response to be a client error (e.g., malformed request syntax, invalid request
   message framing, or deceptive request routing).

10.5.2.  401 Unauthorized

   The 401 (Unauthorized) status code indicates for a
   single range, since a client that the does not request has multiple parts
   might not
   been applied because it lacks valid authentication credentials for
   the target resource.  The support multipart responses.  However, a server generating MAY
   generate a 401 response MUST send multipart/byteranges payload with only a WWW-Authenticate header field (Section 11.3.1) containing at least single body part
   if multiple ranges were requested and only one challenge applicable range was found to the target resource.

   If the request included authentication credentials, then the 401 be
   satisfiable or only one range remained after coalescing.  A client
   that cannot process a multipart/byteranges response indicates MUST NOT generate
   a request that authorization has been refused asks for those
   credentials.  The user agent MAY repeat the request with multiple ranges.

   When a new or
   replaced Authorization header field (Section 9.5.3).  If the 401 multipart response contains the same challenge as the prior response, and the
   user agent has already attempted authentication at least once, then payload is generated, the user agent server SHOULD present
   send the enclosed representation to parts in the
   user, since it usually contains relevant diagnostic information.

10.5.3.  402 Payment Required

   The 402 (Payment Required) status code is reserved for future use.

10.5.4.  403 Forbidden

   The 403 (Forbidden) status code indicates same order that the server understood corresponding range-spec
   appeared in the request but refuses to fulfill it. received Range header field, excluding those ranges
   that were deemed unsatisfiable or that were coalesced into other
   ranges.  A server client that wishes to make
   public why receives a multipart response MUST inspect the request has been forbidden can describe that reason
   Content-Range header field present in
   the response payload (if any).

   If authentication credentials were provided each body part in the request, the
   server considers them insufficient order to grant access.  The
   determine which range is contained in that body part; a client
   SHOULD NOT automatically repeat cannot
   rely on receiving the request with same ranges that it requested, nor the same
   credentials.  The client MAY repeat
   order that it requested.

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 with new used one or different
   credentials.  However, more
   Range specifications.  After several such transfers, a request client might
   have received several ranges of the same representation.  These
   ranges can only be forbidden for reasons
   unrelated to safely combined if they all have in common the credentials.

   An origin server
   same strong validator (Section 7.9.1).

   A client that wishes has received multiple partial responses to "hide" the current existence of GET requests
   on a
   forbidden target resource MAY instead respond with combine those responses into a status code 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
   404 (Not Found).

10.5.5.  404 Not Found

   The 404 (Not Found) status code indicates that the origin server did
   not find a current representation response are used for any combined response
   and replace those of the target resource or