HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                            J. Reschke, Ed.
Updates: 2817 (if approved)                                   greenbytes
Intended status: Standards Track                         October 4, 2012                       February 23, 2013
Expires: April 7, August 27, 2013

     Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
                   draft-ietf-httpbis-p2-semantics-21
                   draft-ietf-httpbis-p2-semantics-22

Abstract

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

Editorial Note (To be removed by RFC Editor)

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

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

   The changes in this draft are summarized in Appendix F.41. E.2.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .   7 .  6
     1.1.  Conformance and Error Handling . . . . . . . . . . . . .   7 .  6
     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .   7  6
   2.  Resource  Resources  . . . . . . . . . . . . . . . . . . . . . . . . . .   8  7
   3.  Representation  Representations  . . . . . . . . . . . . . . . . . . . . . . .   8  7
     3.1.  Representation Metadata  . . . . . . . . . . . . . . . . .  8
       3.1.1.  Processing the Data Type . . . . .  . . . . . . . . . . . . . . . . .   9  8
       3.1.2.  Data  Encoding for Compression or Integrity  . . . . . . . . . . . . . . . . . . . .  12 11
       3.1.3.  Audience Language  . . . . . . . . . . . . . . . . . .  14 13
       3.1.4.  Identification . . . . . . . . . . . . . . . . . . .  15 . 14
     3.2.  Representation Data  . . . . . . . . . . . . . . . . . . .  18 17
     3.3.  Payload Semantics  . . . . . . . . . . . . . . . . . . . .  18 17
     3.4.  Content Negotiation  . . . . . . . . . . . . . . . . . . .  19 18
       3.4.1.  Proactive Negotiation  . . . . . . . . . . . . . . . .  20 18
       3.4.2.  Reactive Negotiation . . . . . . . . . . . . . . . .  21
   4.  Product Tokens  . . . . . . . . . . . . . . . . . . . . . . .  22
   5. 19
   4.  Request Methods  . . . . . . . . . . . . . . . . . . . . . . .  22
     5.1. 20
     4.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . .  22
     5.2. . 20
     4.2.  Common Method Properties . . . . . . . . . . . . . . . .  24
       5.2.1. . 22
       4.2.1.  Safe Methods . . . . . . . . . . . . . . . . . . . .  24
       5.2.2. . 22
       4.2.2.  Idempotent Methods . . . . . . . . . . . . . . . . .  25
       5.2.3. . 23
       4.2.3.  Cacheable Methods  . . . . . . . . . . . . . . . . . .  25
     5.3. 23
     4.3.  Method Definitions . . . . . . . . . . . . . . . . . . .  25
       5.3.1. . 23
       4.3.1.  GET  . . . . . . . . . . . . . . . . . . . . . . . . .  25
       5.3.2. 24
       4.3.2.  HEAD . . . . . . . . . . . . . . . . . . . . . . . .  26
       5.3.3. . 24
       4.3.3.  POST . . . . . . . . . . . . . . . . . . . . . . . .  27
       5.3.4. . 25
       4.3.4.  PUT  . . . . . . . . . . . . . . . . . . . . . . . . .  28
       5.3.5. 26
       4.3.5.  DELETE . . . . . . . . . . . . . . . . . . . . . . .  30
       5.3.6. . 28
       4.3.6.  CONNECT  . . . . . . . . . . . . . . . . . . . . . . .  30
       5.3.7. 29
       4.3.7.  OPTIONS  . . . . . . . . . . . . . . . . . . . . . . .  32
       5.3.8. 31
       4.3.8.  TRACE  . . . . . . . . . . . . . . . . . . . . . . . .  33
   6. 32
   5.  Request Header Fields  . . . . . . . . . . . . . . . . . . . .  33
     6.1. 32
     5.1.  Controls . . . . . . . . . . . . . . . . . . . . . . . .  33
       6.1.1.  Max-Forwards . 32
       5.1.1.  Expect . . . . . . . . . . . . . . . . . . .  34
       6.1.2.  Expect . . . . . 33
       5.1.2.  Max-Forwards . . . . . . . . . . . . . . . . . .  34
     6.2. . . . 36
     5.2.  Conditionals . . . . . . . . . . . . . . . . . . . . . .  37
     6.3. . 36
     5.3.  Content Negotiation  . . . . . . . . . . . . . . . . . . .  38
       6.3.1. 37
       5.3.1.  Quality Values . . . . . . . . . . . . . . . . . . .  38
       6.3.2. . 37
       5.3.2.  Accept . . . . . . . . . . . . . . . . . . . . . . . . 38
       6.3.3.
       5.3.3.  Accept-Charset . . . . . . . . . . . . . . . . . . .  41
       6.3.4. . 40
       5.3.4.  Accept-Encoding  . . . . . . . . . . . . . . . . . . . 41
       6.3.5.
       5.3.5.  Accept-Language  . . . . . . . . . . . . . . . . . . . 42
     6.4.
     5.4.  Authentication Credentials . . . . . . . . . . . . . . .  44
     6.5.  Context . . . . 43
     5.5.  Request Context  . . . . . . . . . . . . . . . . . . . . .  44
       6.5.1. 43
       5.5.1.  From . . . . . . . . . . . . . . . . . . . . . . . . . 44
       6.5.2.
       5.5.2.  Referer  . . . . . . . . . . . . . . . . . . . . . . .  45
       6.5.3. 44
       5.5.3.  User-Agent . . . . . . . . . . . . . . . . . . . . . . 45
   7.
   6.  Response Status Codes  . . . . . . . . . . . . . . . . . . . . 46
     7.1.
     6.1.  Overview of Status Codes . . . . . . . . . . . . . . . . . 47
     7.2.
     6.2.  Informational 1xx  . . . . . . . . . . . . . . . . . . . . 49
       7.2.1.
       6.2.1.  100 Continue . . . . . . . . . . . . . . . . . . . . . 49
       7.2.2.
       6.2.2.  101 Switching Protocols  . . . . . . . . . . . . . . .  49
     7.3. 50
     6.3.  Successful 2xx . . . . . . . . . . . . . . . . . . . . . . 50
       7.3.1.
       6.3.1.  200 OK . . . . . . . . . . . . . . . . . . . . . . . . 50
       7.3.2.
       6.3.2.  201 Created  . . . . . . . . . . . . . . . . . . . . .  50
       7.3.3. 51
       6.3.3.  202 Accepted . . . . . . . . . . . . . . . . . . . . . 51
       7.3.4.
       6.3.4.  203 Non-Authoritative Information  . . . . . . . . . . 51
       7.3.5.
       6.3.5.  204 No Content . . . . . . . . . . . . . . . . . . .  51
       7.3.6. . 52
       6.3.6.  205 Reset Content  . . . . . . . . . . . . . . . . . . 52
     7.4.
     6.4.  Redirection 3xx  . . . . . . . . . . . . . . . . . . . . .  52
       7.4.1. 53
       6.4.1.  300 Multiple Choices . . . . . . . . . . . . . . . . . 54
       7.4.2.
       6.4.2.  301 Moved Permanently  . . . . . . . . . . . . . . . .  54
       7.4.3. 55
       6.4.3.  302 Found  . . . . . . . . . . . . . . . . . . . . . . 55
       7.4.4.
       6.4.4.  303 See Other  . . . . . . . . . . . . . . . . . . . .  55
       7.4.5. 56
       6.4.5.  305 Use Proxy  . . . . . . . . . . . . . . . . . . . . 56
       7.4.6.
       6.4.6.  306 (Unused) . . . . . . . . . . . . . . . . . . . . . 56
       7.4.7.
       6.4.7.  307 Temporary Redirect . . . . . . . . . . . . . . .  56
     7.5. . 57
     6.5.  Client Error 4xx . . . . . . . . . . . . . . . . . . . .  56
       7.5.1. . 57
       6.5.1.  400 Bad Request  . . . . . . . . . . . . . . . . . . .  56
       7.5.2. 57
       6.5.2.  402 Payment Required . . . . . . . . . . . . . . . .  56
       7.5.3. . 57
       6.5.3.  403 Forbidden  . . . . . . . . . . . . . . . . . . . . 57
       7.5.4.
       6.5.4.  404 Not Found  . . . . . . . . . . . . . . . . . . . .  57
       7.5.5. 58
       6.5.5.  405 Method Not Allowed . . . . . . . . . . . . . . .  57
       7.5.6. . 58
       6.5.6.  406 Not Acceptable . . . . . . . . . . . . . . . . .  57
       7.5.7. . 58
       6.5.7.  408 Request Timeout  . . . . . . . . . . . . . . . . .  58
       7.5.8. 59
       6.5.8.  409 Conflict . . . . . . . . . . . . . . . . . . . .  58
       7.5.9. . 59
       6.5.9.  410 Gone . . . . . . . . . . . . . . . . . . . . . .  58
       7.5.10. . 59
       6.5.10. 411 Length Required  . . . . . . . . . . . . . . . . .  59
       7.5.11. 60
       6.5.11. 413 Request Representation Payload Too Large  . . . . . . . .  59
       7.5.12. . . . . . . . . 60
       6.5.12. 414 URI Too Long . . . . . . . . . . . . . . . . . .  59
       7.5.13. . 60
       6.5.13. 415 Unsupported Media Type . . . . . . . . . . . . .  59
       7.5.14. . 60
       6.5.14. 417 Expectation Failed . . . . . . . . . . . . . . .  60
       7.5.15. . 61
       6.5.15. 426 Upgrade Required . . . . . . . . . . . . . . . .  60
     7.6. . 61
     6.6.  Server Error 5xx . . . . . . . . . . . . . . . . . . . .  60
       7.6.1. . 61
       6.6.1.  500 Internal Server Error  . . . . . . . . . . . . . .  60
       7.6.2. 61
       6.6.2.  501 Not Implemented  . . . . . . . . . . . . . . . . .  60
       7.6.3. 61
       6.6.3.  502 Bad Gateway  . . . . . . . . . . . . . . . . . . .  61
       7.6.4. 62
       6.6.4.  503 Service Unavailable  . . . . . . . . . . . . . . .  61
       7.6.5. 62
       6.6.5.  504 Gateway Timeout  . . . . . . . . . . . . . . . . .  61
       7.6.6. 62
       6.6.6.  505 HTTP Version Not Supported . . . . . . . . . . .  61
   8. . 62
   7.  Response Header Fields . . . . . . . . . . . . . . . . . . .  61
     8.1. . 62
     7.1.  Control Data . . . . . . . . . . . . . . . . . . . . . .  62
       8.1.1. . 63
       7.1.1.  Origination Date . . . . . . . . . . . . . . . . . .  62
       8.1.2. . 63
       7.1.2.  Location . . . . . . . . . . . . . . . . . . . . . .  65
       8.1.3. . 66
       7.1.3.  Retry-After  . . . . . . . . . . . . . . . . . . . . .  66
     8.2.  Selected Representation Header Fields 68
       7.1.4.  Vary . . . . . . . . . . .  67
       8.2.1.  Vary . . . . . . . . . . . . . . 68
     7.2.  Validator Header Fields  . . . . . . . . . . .  67
     8.3.  Authentication Challenges . . . . . . 69
     7.3.  Authentication Challenges  . . . . . . . . . . .  68
     8.4.  Informative . . . . . 70
     7.4.  Response Context . . . . . . . . . . . . . . . . . .  68
       8.4.1. . . . 70
       7.4.1.  Allow  . . . . . . . . . . . . . . . . . . . . . . . .  69
       8.4.2. 71
       7.4.2.  Server . . . . . . . . . . . . . . . . . . . . . . .  69
   9. . 71
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  70
     9.1. 72
     8.1.  Method Registry  . . . . . . . . . . . . . . . . . . . . .  70
       9.1.1. 72
       8.1.1.  Procedure  . . . . . . . . . . . . . . . . . . . . . .  70
       9.1.2. 72
       8.1.2.  Considerations for New Methods . . . . . . . . . . .  70
       9.1.3. . 72
       8.1.3.  Registrations  . . . . . . . . . . . . . . . . . . . .  71
     9.2. 73
     8.2.  Status Code Registry . . . . . . . . . . . . . . . . . .  71
       9.2.1. . 73
       8.2.1.  Procedure  . . . . . . . . . . . . . . . . . . . . . .  71
       9.2.2. 73
       8.2.2.  Considerations for New Status Codes  . . . . . . . . .  71
       9.2.3. 73
       8.2.3.  Registrations  . . . . . . . . . . . . . . . . . . . .  72
     9.3. 74
     8.3.  Header Field Registry  . . . . . . . . . . . . . . . . . .  73
       9.3.1. 75
       8.3.1.  Considerations for New Header Fields . . . . . . . .  74
       9.3.2. . 76
       8.3.2.  Registrations  . . . . . . . . . . . . . . . . . . . .  75

     9.4. 78
     8.4.  Content Coding Registry  . . . . . . . . . . . . . . . . .  76
       9.4.1. 78
       8.4.1.  Procedure  . . . . . . . . . . . . . . . . . . . . . .  76
       9.4.2. 78
       8.4.2.  Registrations  . . . . . . . . . . . . . . . . . . . .  77
   10. 79
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  77
     10.1. Transfer of Sensitive 79
     9.1.  Attacks Based On File and Path Names . . . . . . . . . . . 79
     9.2.  Personal Information . . . . . . . . . . . .  77
     10.2. Encoding . . . . . . . 80
     9.3.  Sensitive Information in URIs  . . . . . . . . .  78
     10.3. Location Header Fields: Spoofing and . . . . . 80
     9.4.  Product Information
           Leakage  . . . . . . . . . . . . . . . . . . . 81
     9.5.  Fragment after Redirects . . . . . .  79
     10.4. Security Considerations for CONNECT . . . . . . . . . . .  79
     10.5. Privacy Issues Connected to Accept Header Fields 81
     9.6.  Browser Fingerprinting . . . .  79
   11. . . . . . . . . . . . . . . 81
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  80
   12. 82
   11. References . . . . . . . . . . . . . . . . . . . . . . . . .  80
     12.1. . 82
     11.1. Normative References . . . . . . . . . . . . . . . . . .  80
     12.2. . 82
     11.2. Informative References . . . . . . . . . . . . . . . . .  81 . 84
   Appendix A.  Differences between HTTP and MIME . . . . . . . . .  83 . 85
     A.1.  MIME-Version . . . . . . . . . . . . . . . . . . . . . .  84 . 86
     A.2.  Conversion to Canonical Form . . . . . . . . . . . . . .  84 . 86
     A.3.  Conversion of Date Formats . . . . . . . . . . . . . . .  84 . 86
     A.4.  Introduction  Conversion of Content-Encoding . . . . . . . . . . . .  85
     A.5.  No Content-Transfer-Encoding  . . . . . 87
     A.5.  Conversion of Content-Transfer-Encoding  . . . . . . . . .  85 87
     A.6.  MHTML and Line Length Limitations  . . . . . . . . . . . .  85 87
   Appendix B.  Additional Features  . . . . . . . . . . . . . . . .  85
   Appendix C.  Changes from RFC 2616 . . . . . . . . . . . . . . .  86 . 87
   Appendix D. C.  Imported ABNF . . . . . . . . . . . . . . . . . . .  88 . 90
   Appendix E. D.  Collected ABNF  . . . . . . . . . . . . . . . . . . .  88 90
   Appendix F. E.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . .  91
     F.1. 93
     E.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . .  91
     F.2.  Since draft-ietf-httpbis-p2-semantics-00  . . . . . . . .  91
     F.3.  Since draft-ietf-httpbis-p3-payload-00  . . . . . . . . .  92
     F.4.  Since draft-ietf-httpbis-p2-semantics-01  . . . . . . . .  93
     F.5.  Since draft-ietf-httpbis-p3-payload-01  . . . . . . . . .  93
     F.6.  Since draft-ietf-httpbis-p2-semantics-02  . . . . . . . . 93
     F.7.
     E.2.  Since draft-ietf-httpbis-p3-payload-02 draft-ietf-httpbis-p2-semantics-21 . . . . . . . . . 94
     F.8.  Since draft-ietf-httpbis-p2-semantics-03  . . . . . . . .  95
     F.9.  Since draft-ietf-httpbis-p3-payload-03  . . . . . . . . .  95
     F.10. Since draft-ietf-httpbis-p2-semantics-04  . . . . . . . .  95
     F.11. Since draft-ietf-httpbis-p3-payload-04  . . . . . . . . .  96
     F.12. Since draft-ietf-httpbis-p2-semantics-05  . . . . . . . .  96
     F.13. Since draft-ietf-httpbis-p3-payload-05  . . . . . . . . .  96
     F.14. Since draft-ietf-httpbis-p2-semantics-06  . . . . . . . .  97
     F.15. Since draft-ietf-httpbis-p3-payload-06  . . . . . . . . .  97
     F.16. Since draft-ietf-httpbis-p2-semantics-07  . . . . . . . .  97
     F.17. Since draft-ietf-httpbis-p3-payload-07  . . . . . . . . .  98
     F.18. Since draft-ietf-httpbis-p2-semantics-08  . . . . . . . .  99
     F.19. Since draft-ietf-httpbis-p3-payload-08  . . . . . . . . .  99
     F.20. Since draft-ietf-httpbis-p2-semantics-09  . . . . . . . .  99
     F.21. Since draft-ietf-httpbis-p3-payload-09  . . . . . . . . .  99
     F.22. Since draft-ietf-httpbis-p2-semantics-10  . . . . . . . . 100
     F.23. Since draft-ietf-httpbis-p3-payload-10  . . . . . . . . . 100
     F.24. Since draft-ietf-httpbis-p2-semantics-11  . . . . . .
   Index  . . 101
     F.25. Since draft-ietf-httpbis-p3-payload-11 . . . . . . . . . 101
     F.26. Since draft-ietf-httpbis-p2-semantics-12 . . . . . . . . 101
     F.27. Since draft-ietf-httpbis-p3-payload-12 . . . . . . . . . 103
     F.28. Since draft-ietf-httpbis-p2-semantics-13 . . . . . . . . 103
     F.29. Since draft-ietf-httpbis-p3-payload-13  . . . . . . . . . 103
     F.30. Since draft-ietf-httpbis-p2-semantics-14  . . . . . . . . 103
     F.31. Since draft-ietf-httpbis-p3-payload-14  . . . . . . . . . 104
     F.32. Since draft-ietf-httpbis-p2-semantics-15  . . . . . . . . 104
     F.33. Since draft-ietf-httpbis-p3-payload-15  . . . . . . . . . 104
     F.34. Since draft-ietf-httpbis-p2-semantics-16  . . . . . . . . 104
     F.35. Since draft-ietf-httpbis-p3-payload-16  . . . . . . . . . 104
     F.36. Since draft-ietf-httpbis-p2-semantics-17  . . . . . . . . 105
     F.37. Since draft-ietf-httpbis-p3-payload-17  . . . . . . . . . 105
     F.38. Since draft-ietf-httpbis-p2-semantics-18  . . . . . . . . 105
     F.39. Since draft-ietf-httpbis-p3-payload-18  . . . . . . . . . 106
     F.40. Since draft-ietf-httpbis-p2-semantics-19 and
           draft-ietf-httpbis-p3-payload-19  . . . . . . . . . . . . 106
     F.41. Since draft-ietf-httpbis-p2-semantics-20  . . . . . . . . 107
   Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 94

1.  Introduction

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

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

   HTTP semantics include the intentions defined by each request method
   (Section 5), 4), extensions to those semantics that might be described in
   request header fields (Section 6), 5), the meaning of status codes to
   indicate a machine-readable response (Section 7), 6), and the meaning of
   other control data and resource metadata that might be given in
   response header fields (Section 8). 7).

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

1.1.  Conformance and Error Handling

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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

1.2.  Syntax Notation

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

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

2.  Resource  Resources

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

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

   One design goal of HTTP is to separate resource identification from
   request semantics, which is made possible by vesting the request
   semantics in the request method (Section 5) 4) and a few request-
   modifying header fields (Section 6). 5).  Resource owners SHOULD NOT
   include request semantics within a URI, such as by specifying an
   action to invoke within the path or query components of the effective
   request URI, unless those semantics are disabled when they are
   inconsistent with the request method.

3.  Representation  Representations

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

   For the purposes of HTTP, a representation "representation" is information that
   reflects the current 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, consisting and that consists of a set of representation metadata and a
   potentially unbounded stream of representation data.

3.1.  Representation Metadata

   Representation header fields provide metadata about

   An origin server might be provided with, or capable of generating,
   multiple representations that are each intended to reflect the
   representation.
   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.  We refer to that one representation as the "selected
   representation" and use its particular data and metadata for
   evaluating conditional requests [Part4] and constructing the payload
   for 200 (OK) and 304 (Not Modified) responses to GET (Section 4.3.1).

3.1.  Representation Metadata

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

   The following header fields are defined to convey representation
   metadata:

   +-------------------+------------------------+

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

3.1.1.  Processing the Data Type

3.1.1.1.  Media Types Type

   HTTP uses Internet Media Types [RFC2046] in the Content-Type
   (Section 3.1.1.5) and Accept (Section 6.3.2) 5.3.2) header fields in order
   to provide open and extensible data typing and type negotiation.
   Media types define both a data format and various processing models:
   how to process that data in accordance with each context in which it
   is received.

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

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

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

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

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

   Media-type values  For example, the following
   examples are registered with all equivalent, but the Internet Assigned Number
   Authority (IANA).  The media type registration process first is outlined in
   [RFC4288].  Use of non-registered preferred for
   consistency:

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

   Internet media types is discouraged. ought to be registered with IANA according to
   the procedures defined in [BCP13].

3.1.1.2.  Character Encodings (charset)  Charset

   HTTP uses charset names to indicate or negotiate the character
   encoding scheme of a textual representation. representation [RFC6365].  A character encoding charset is
   identified by a case-insensitive token.  The
   complete set of tokens is defined by the IANA Character Set registry
   (<http://www.iana.org/assignments/character-sets>).

     charset = token

   Although HTTP allows an arbitrary token

   Charset names ought to be used as a charset
   value, any token that has a predefined value within the registered in IANA Character Set registry MUST represent the character encoding defined
   by that registry.  Applications SHOULD limit their use of character
   encodings
   (<http://www.iana.org/assignments/character-sets>) according to those defined within the IANA registry.

   HTTP uses charset
   procedures defined in two contexts: within an Accept-Charset request
   header field (in which the charset value is an unquoted token) [RFC2978].

3.1.1.3.  Canonicalization and as
   the value of Text Defaults

   Internet media types are registered with a parameter in a Content-Type header field (within a
   request or response), canonical form in which case the parameter value of the
   charset parameter can be quoted.

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

3.1.1.3.  Canonicalization and Text Defaults

   Internet media types are registered interoperable among systems with a canonical form.  A
   representation varying native encoding formats.
   Representations selected or transferred via HTTP messages MUST ought to be in the
   appropriate
   canonical form prior form, for many of the same reasons described by the
   Multipurpose Internet Mail Extensions (MIME) [RFC2045].  However, the
   performance characteristics of email deployments (i.e., store and
   forward messages to its transmission except peers) are significantly different from those
   common to HTTP and the Web (server-based information services).
   Furthermore, MIME's constraints for
   "text" types, as defined in the next paragraph.

   When in sake of compatibility with
   older mail transfer protocols do not apply to HTTP (see Appendix A).

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

   If a representation is encoded with
   that has been assigned a content-coding, the underlying
   data MUST be in "text" media type; it does not apply to
   "multipart" types or HTTP elements outside the payload body (e.g.,
   header fields).

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

3.1.1.4.  Multipart Types

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

   In general, HTTP treats a multipart message body no differently than
   any other media type: strictly as payload. parts.

   HTTP message framing does not use the multipart boundary as an
   indicator of message body length.  In all
   other respects, an HTTP user agent SHOULD follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart type.
   The MIME header fields within each body-part of a multipart message
   body do not have any significance to HTTP beyond that defined length, though it might be used by
   their MIME semantics.

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

      Note: The
   implementations that generate or process the payload.  For example,
   the "multipart/form-data" type has been specifically defined is often used for carrying form data suitable for processing via the POST
      request method,
   in a request, as described in [RFC2388]. [RFC2388], and the "multipart/
   byteranges" type is defined by this specification for use in some 206
   (Partial Content) responses [Part5].

3.1.1.5.  Content-Type

   The "Content-Type" header field indicates the media type of the
   associated representation: either the representation enclosed in the
   message payload or the selected representation, which as determined by the
   message semantics.  The indicated media type defines both the data
   format and how that data
   SHOULD is intended to be processed by the recipient (within a recipient,
   within the scope of the request
   method semantics) received message semantics, after any content
   codings indicated by Content-Encoding is are decoded.  For
   responses to the HEAD method, the media type is that which would have
   been sent had the request been a GET.

     Content-Type = media-type

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

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

   A sender that generates a message containing a payload body SHOULD include
   generate a Content-Type header field in a that message
   containing a payload body, defining unless the
   intended media type of the enclosed
   representation, unless the intended media type representation is unknown to the
   sender.  If a Content-Type header field is not present, recipients
   MAY either assume a media type of "application/octet-stream"
   ([RFC2046], Section 4.5.1) or examine the representation data to determine its type.

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

3.1.2.  Data  Encoding for Compression or Integrity

3.1.2.1.  Content Codings

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

     content-coding   = token

   All content-coding values are case-insensitive and SHOULD ought to be
   registered within the HTTP Content Coding registry, as defined in
   Section 9.4. 8.4.  They are used in the Accept-Encoding (Section 6.3.4) 5.3.4)
   and Content-Encoding (Section 3.1.2.2) header fields.

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

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

      deflate: See Section 4.2.2 of [Part1].

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

3.1.2.2.  Content-Encoding

   The "Content-Encoding" header field indicates what content codings
   have been applied to the representation, beyond those inherent in the
   media type, and thus what decoding mechanisms have to be applied in
   order to obtain data in the media type referenced by the Content-Type
   header field.  Content-Encoding is primarily used to allow a
   representation's data to be compressed without losing the identity of
   its underlying media type.

     Content-Encoding = 1#content-coding

   An example of its use is

     Content-Encoding: gzip

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

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

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

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

   If the content-coding of a representation in a request message is not
   acceptable to the

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

3.1.3.  Audience Language

3.1.3.1.  Language Tags

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

   In summary, a

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

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

     language-tag = <Language-Tag, defined in [RFC5646], Section 2.1> subtags.  White space
   is not allowed within the tag and all tags are case-
   insensitive.  The name space of language subtags is administered by
   the IANA (see
   <http://www.iana.org/assignments/language-subtag-registry>). case-insensitive.
   Example tags include:

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

   See [RFC5646] for further information.

3.1.3.2.  Content-Language

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

     Content-Language = 1#language-tag

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

     Content-Language: da

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

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

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

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

3.1.4.  Identification

3.1.4.1.  Identifying a Representation

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

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

   For a request message:

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

   o  Otherwise, the payload is unidentified.

   The

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

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

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

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

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

   5.  Otherwise, the payload is unidentified.

3.1.4.2.  Content-Location

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

     Content-Location = absolute-URI / partial-URI

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

   If Content-Location is included in a 2xx (Successful) response
   message and its value refers (after conversion to absolute form) to a
   URI that is the same as the effective request URI, then the response recipient
   MAY consider the payload SHOULD to be considered a current representation of that
   resource.
   resource at the time indicated by the message origination date.  For
   a GET or HEAD request, this is the same as the default semantics when
   no Content-Location is provided by the server.  For a state-changing
   request like PUT or POST, it implies that the server's response
   contains the new representation of that resource, thereby
   distinguishing it from representations that might only report about
   the action (e.g., "It worked!").  This allows authoring applications
   to update their local copies without the need for a subsequent GET
   request.

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

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

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

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

   If

   A user agent that sends Content-Location is included in a request message, then it MAY be
   interpreted by the origin server as an indication of message is
   stating that its value refers to where the user agent originally
   obtained the content of the enclosed representation (prior to any subsequent modification of the content
   modifications made by that user agent).  In other words, the user
   agent is providing a back link to the same
   representation metadata that it received with source of the original
   representation.  However,

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

   For example, if a client makes a PUT request on a negotiated resource
   and the origin server accepts that PUT (without redirection), then
   the new set state of values for that resource is expected to be consistent with the
   one representation supplied in that PUT; the Content-Location cannot
   be used as a form of reverse content selection that identifies identifier to update
   only one of the negotiated representations to be updated. representations.  If the user agent had
   wanted the latter semantics, it would have applied the PUT directly
   to the Content-Location URI.

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

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

3.2.  Representation Data

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

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

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

3.3.  Payload Semantics

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

   The purpose of a payload in a request is defined by the method
   semantics.  In a response, the payload's purpose is defined by both
   the request method and the response status code.  For example, a representation in the payload of a PUT
   request (Section 5.3.4) 4.3.4) represents the desired state of the target
   resource if the request is successfully applied, whereas a
   representation in the payload of a POST request (Section 5.3.3) 4.3.3)
   represents an anonymous resource for providing data to be processed,
   such as the information that a user entered within an HTML form.

   Likewise,

   In a response, the payload's purpose is defined by both the request
   method and the response status code.  For example, the payload of a
   200 (OK) response to GET (Section 5.3.1)
   contains a representation 4.3.1) represents the current state
   of the target resource, as observed at the time of the message
   origination date (Section 8.1.1.2), 7.1.1.2), whereas the payload of the same
   status code in a response to POST might contain represent either a
   representation of the
   processing result or a current representation the new state of the target resource after
   applying the processing.  Response messages with an error status code
   usually contain a representation payload that represents the error condition, such
   that it describes the error state and what next steps are suggested
   for resolving it.

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

   +-------------------+--------------------------+
   | Header Field Name | Defined in...            |
   +-------------------+--------------------------+
   | Content-Length    | Section 3.3.2 of [Part1] |
   | Content-Range     | Section 5.2 4.2 of [Part5]   |
   | Transfer-Encoding | Section 3.3.1 of [Part1] |
   +-------------------+--------------------------+

3.4.  Content Negotiation

   HTTP

   When responses include a representation which contains information
   for interpretation, convey payload information, whether by indicating a human user
   success or for further
   processing.  Often, an error, the origin server often has different ways of
   representing the
   same that information; for example, in different formats,
   languages, or
   using different character encodings.

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

   This specification defines two patterns of content negotiation; negotiation that
   can be made visible within the protocol: "proactive", where the
   server selects the representation based upon the client's user agent's stated
   preferences, and "reactive" negotiation, where the server provides a
   list of representations for the client user agent to choose from, based upon their metadata.  In addition, there are other
   patterns: some applications use an "active content" pattern, where
   the server returns active from.  Other
   patterns of content which runs on negotiation include "conditional content", where
   the client and, representation consists of multiple parts that are selectively
   rendered based on client available user agent parameters, selects "active content", where the
   representation contains a script that makes additional resources to
   invoke. (more
   specific) requests based on the user agent characteristics, and
   "Transparent Content Negotiation" ([RFC2295]) has also been
   proposed. ([RFC2295]), where content
   selection is performed by an intermediary.  These patterns are all widely used, not
   mutually exclusive, and have each has trade-offs in applicability and
   practicality.  In particular, when the number of
   preferences or capabilities to be expressed by a client are large
   (such as when many different formats are supported by a user-agent),
   proactive negotiation becomes unwieldy, and might not be appropriate.
   Conversely, when the number of representations to choose from is very
   large, reactive negotiation might not be appropriate.

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

3.4.1.  Proactive Negotiation

   If the selection of

   When content negotiation preferences are sent by the best representation for a response is made by user agent in a
   request in order to encourage an algorithm located at the server, server to
   select the preferred representation, it is called proactive
   negotiation.
   negotiation (a.k.a., server-driven negotiation).  Selection is based
   on the available representations of
   the for a response (the dimensions over
   which it can vary; e.g., might vary, such as language, content-coding, etc.) and the contents of particular header fields compared
   to various information supplied in the request message or on other information pertaining to request, including both the request
   (such
   explicit negotiation fields of Section 5.3 and implicit
   characteristics, such as the client's network address or parts of the client).
   User-Agent field.

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

   Proactive negotiation has serious disadvantages:

   1.

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

   2. paper?);

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

   3. privacy;

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

   4. request; and,

   o  It might limit a public cache's ability to use limits the same response reusability of responses for multiple user's requests.

   Proactive negotiation allows the shared caching.

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

   HTTP/1.1 includes the following header fields for enabling proactive
   negotiation through description of user agent capabilities and user
   preferences: Accept (Section 6.3.2), Accept-Charset (Section 6.3.3),
   Accept-Encoding (Section 6.3.4), Accept-Language (Section 6.3.5), and
   User-Agent (Section 6.5.3).  However, an

   An origin server is not limited
   to these dimensions and MAY vary the response based on any aspect of
   the request, including aspects of the connection (e.g., IP address)
   or information within extension header fields not defined by this
   specification.

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

   The generate a Vary header field (Section 8.2.1) can be used to express the
   parameters the server uses to select a representation 7.1.4) in
   responses that is are subject to proactive negotiation.

3.4.2.  Reactive Negotiation

   With reactive negotiation, negotiation to indicate what
   parameters of request information might be used in its selection
   algorithm, thereby providing a means for recipients to determine the
   reusability of that same response for user agents with differing
   request information.

3.4.2.  Reactive Negotiation

   With reactive negotiation (a.k.a., agent-driven negotiation),
   selection of the best representation for a response is performed by
   the user agent after receiving an initial response from the origin server.  Selection is based on
   server with a list of alternative resources.  If the
   available representations user agent is
   not satisfied by the initial response, it can perform a GET request
   on one or more of the response alternative resources, selected based on
   metadata included within in the header
   fields or body list, to obtain a different form of the initial response, with each representation
   identified by its own URI.
   representation.  Selection from among the representations
   can of alternatives might be performed
   automatically (if by the user agent is capable of doing
   so) or manually by the user selecting
   from a generated (possibly hypertext) menu.

   A server can send a 300 (Multiple Choices) response to indicate that
   reactive negotiation by the user agent is desired, or a 406 (Not
   Acceptable) status code to indicate that proactive negotiation has
   failed.  In both cases, the response ought to include information
   about the available representations so that the user or user agent
   can react by making a selection.

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

   Reactive negotiation suffers from the disadvantage disadvantages of transmitting a
   list of alternatives to the user agent, which degrades user-perceived
   latency if transmitted in the header section, and needing a second
   request to obtain the best an alternate representation.  This
   second request is only efficient when caching is used.  In addition,  Furthermore, this
   specification does not define any a mechanism for supporting automatic
   selection, though it also does not prevent any such a mechanism from being
   developed as an extension and used within
   HTTP/1.1.

   This specification defines extension.

4.  Request Methods

4.1.  Overview

   The request method token is the 300 (Multiple Choices) and 406 (Not
   Acceptable) status codes primary source of request semantics;
   it indicates the purpose for enabling reactive negotiation when which the
   server is unwilling or unable to provide a varying response using
   proactive negotiation.

4.  Product Tokens

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

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

   Examples:

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

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

5.  Request Methods

5.1.  Overview

   The request method token is the primary source of request semantics;
   it indicates the purpose for which the client has made this request
   and what client has made this request
   and what is expected by the client as a successful result.  The
   request semantics MAY might be further specialized by the semantics of
   some header fields when present in a request (Section 6) 5) if those
   additional semantics do not conflict with the method.

     method = token

   HTTP was originally designed to be usable as an interface to
   distributed object systems.  The request method was envisioned as
   applying semantics to a target resource in much the same way as
   invoking a defined method on an identified object would apply
   semantics.  The method token is case-sensitive because it might be
   used as a gateway to object-based systems with case-sensitive method
   names.

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

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

   +---------+-------------------------------------------------+-------+
   | Method  | Description                                     | Sec.  |
   +---------+-------------------------------------------------+-------+
   | GET     | Transfer a current representation of the target | 5.3.1 4.3.1 |
   |         | resource.                                       |       |
   | HEAD    | Same as GET, but do not include a message body only transfer the status line  | 5.3.2 4.3.2 |
   |         | in the response. and header block.                               |       |
   | POST    | Perform resource-specific processing on the     | 5.3.3 4.3.3 |
   |         | request payload.                                |       |
   | PUT     | Replace all current representations of the      | 5.3.4 4.3.4 |
   |         | target resource with the request payload.       |       |
   | DELETE  | Remove all current representations of the       | 5.3.5 4.3.5 |
   |         | target resource.                                |       |
   | CONNECT | Establish a tunnel to the server identified by  | 5.3.6 4.3.6 |
   |         | the target resource.                            |       |
   | OPTIONS | Describe the communication options for the      | 5.3.7 4.3.7 |
   |         | target resource.                                |       |
   | TRACE   | Perform a message loop-back test along the path | 5.3.8 4.3.8 |
   |         | to the target resource.                         |       |
   +---------+-------------------------------------------------+-------+

   The

   All general-purpose servers MUST support the methods GET and HEAD MUST be supported by all general-purpose
   servers. HEAD.
   All other methods are OPTIONAL.  When OPTIONAL; when implemented, a server MUST
   implement the above methods according to the semantics defined for
   them in Section 5.3. 4.3.

   Additional methods MAY be used in HTTP; many have already been
   standardized methods, outside the scope of this specification and specification, have
   been standardized for use in HTTP.  All such methods ought to be
   registered within the HTTP Method Registry maintained by IANA, as
   defined in Section 9.1. 8.1.

   The set of methods allowed by a target resource can be listed in an
   Allow header field (Section 8.4.1). 7.4.1).  However, the set of allowed
   methods can change dynamically.  When a request message method is received
   that is unrecognized or not implemented by an origin server, the
   origin server SHOULD respond with the 501 (Not Implemented) status
   code.  When a request message method is received that is known by an origin
   server but not allowed for the target resource, the origin server
   SHOULD respond with the 405 (Method Not Allowed) status code.

5.2.

   A client can send conditional request header fields (Section 5.2) to
   make the requested action conditional on the current state of the
   target resource ([Part4]).

4.2.  Common Method Properties

5.2.1.

4.2.1.  Safe Methods

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

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

   The

   Of the request methods defined by this specification, the GET, HEAD,
   OPTIONS, and TRACE request methods are defined to be safe.

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

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

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

5.2.2.

4.2.2.  Idempotent Methods

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

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

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

5.2.3.

4.2.3.  Cacheable Methods

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

5.3.

4.3.  Method Definitions

5.3.1.
4.3.1.  GET

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

   If  GET is the target 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.

   It is tempting to think of resource identifiers as remote filesystem
   pathnames, and of representations as being a copy of the contents of
   such files.  In fact, that is how many resources are implemented (see
   Section 9.1 for related security considerations).  However, there are
   no such limitations in practice.  The HTTP interface for a data-producing process, it resource
   is the
   produced data which shall just as likely to be returned implemented as the representation in the
   response and not the source text a tree of content objects, a
   programmatic view on various database records, or a gateway to other
   information systems.  Even when the process, unless that text
   happens URI mapping mechanism is tied to
   a filesystem, an origin server might be configured to execute the
   files with the request as input and send the output of as the process.

   The semantics of
   representation, rather then transfer the GET method change files directly.  Regardless,
   only the origin server needs to know how each of its resource
   identifiers correspond to an implementation, and how each
   implementation manages to select and send a "conditional GET" if current representation of
   the
   request message includes an If-Modified-Since, If-Unmodified-Since,
   If-Match, If-None-Match, or If-Range header field ([Part4]). target resource in a response to GET.

   A
   conditional GET requests that client can alter the representation semantics of GET to be transferred a "range request",
   requesting transfer of only
   under some part(s) of the circumstances described selected
   representation, by the conditional sending a Range header field(s).
   The conditional GET field in the request is intended to reduce unnecessary network
   usage by allowing cached representations to be refreshed without
   requiring multiple requests or transferring data already held by the
   client.

   The semantics of the GET method change to a "partial GET" if the
   request message includes a Range header field
   ([Part5]).

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

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

   The response to a GET request is cacheable and cacheable; a cache MAY be used use it to
   satisfy subsequent GET and HEAD requests (see unless otherwise indicated
   by the Cache-Control header field (Section 7.2 of [Part6]).

   See Section 10.2 for security considerations when used for forms.

5.3.2.

4.3.2.  HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   return
   send a message body in the response.  The metadata contained in response (i.e., the
   HTTP response terminates at
   the end of the header block).  Aside from the payload header fields
   (Section 3.3), the server SHOULD send the same header fields in
   response to a HEAD request SHOULD be identical
   to the information as it would have sent in response to if the request had
   been a GET request. GET.  This method can be used for obtaining metadata about the
   selected representation implied
   by the request without transferring the representation data.
   This method is often used for testing hypertext links for validity,
   accessibility, and recent modification.

   The response to a HEAD request is cacheable and MAY be used to
   satisfy a subsequent HEAD request.  It also has potential side
   effects on previously stored responses to GET; see Section 5 of
   [Part6].

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

5.3.3.

   The response to a HEAD request is cacheable; a cache MAY use it to
   satisfy subsequent HEAD requests unless otherwise indicated by the
   Cache-Control header field (Section 7.2 of [Part6]).  A HEAD response
   might also have an effect on previously cached responses to GET; see
   Section 5 of [Part6].

4.3.3.  POST

   The POST method requests that the origin server accept target resource process the
   representation enclosed in the request as data according to be processed by the
   target resource. resource's
   own specific semantics.  For example, POST is designed to allow a uniform method to cover used for the following functions:
   functions (among others):

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

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

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

   o  Extending a database through an append operation.

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

   The action performed by the POST method might not result in  Creating a new resource that can has yet to be identified by a URI.  In this case, either 200
   (OK) or 204 (No Content) is the appropriate
      origin server; and

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

   An origin server indicates response semantics by choosing an
   appropriate status code, code depending on whether or not the response includes a representation
   that describes result of processing the result.

   If
   POST request; almost all of the status codes defined by this
   specification might be received in a resource response to POST (the exceptions
   being 206, 304, and 416).

   If one or more resources has been created on the origin server, server as a
   result of successfully processing a POST request, the response origin server
   SHOULD be send a 201 (Created) response containing a Location header
   field that provides an identifier for the primary resource created
   (Section 7.1.2) and contain a representation which that describes the status of the
   request and refers while referring to the new resource, and a
   Location header field (see Section 8.1.2). resource(s).

   Responses to POST requests are only cacheable when they include
   explicit freshness information (see Section 4.1.1 of [Part6]).  A
   cached
   However, POST response with a Content-Location header field (see
   Section 3.1.4.2) whose value caching is not widely implemented.  For cases where an
   origin server wishes the effective Request URI MAY be used client to satisfy subsequent GET be able to cache the result of a
   POST in a way that can be reused by a later GET, the origin server
   MAY send a 200 (OK) response containing the result and HEAD (not POST) requests.

   Note a Content-
   Location header field that has the same value as the POST's effective
   request URI (Section 3.1.4.2).

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

5.3.4. user
   agent does not already have the representation cached.

4.3.4.  PUT

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

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

   Unrecognized

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

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

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

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

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

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

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

   The fundamental difference between the POST and PUT methods is
   highlighted by the different intent for the target resource.  The
   target resource in a POST request is intended to handle the enclosed
   representation as

   An origin server MUST NOT send a data-accepting process, validator header field
   (Section 7.2), such as for a gateway to
   some other protocol an ETag or a document that accepts annotations.  In
   contrast, the target resource Last-Modified field, in a PUT request is intended
   successful response to take PUT unless the enclosed request's representation as a data
   was saved without any transformation applied to the body (i.e., the
   resource's new or replacement value.  Hence, representation data is identical to the representation
   data received in the PUT request) and the validator field value
   reflects the new representation.  This requirement allows a user
   agent to know when the representation body it has in memory remains
   current as a result of the PUT, thus not in need of retrieving again
   from the origin server, and that the new validator(s) received in the
   response can be used for future conditional requests in order to
   prevent accidental overwrites (Section 5.2).

   The fundamental difference between the POST and PUT methods is
   highlighted by the different intent for the enclosed representation.
   The target resource in a POST request is intended to handle the
   enclosed representation according to the resource's own semantics,
   whereas the enclosed representation in a PUT request is defined as
   replacing the state of the target resource.  Hence, the intent of PUT
   is idempotent and visible to intermediaries, even though the exact
   effect is only known by the origin server.

   Proper interpretation of a PUT request presumes that the user agent
   knows what which target resource is desired.  A service that is intended to
   select selects a
   proper URI on behalf of the client, after receiving a state-
   changing state-changing
   request, SHOULD be implemented using the POST method rather than PUT.
   If the origin server will not make the requested PUT state change to
   the target resource and instead wishes to have it applied to a
   different resource, such as when the resource has been moved to a
   different URI, then the origin server MUST send a 301 (Moved
   Permanently) an appropriate 3xx
   (Redirection) response; the user agent MAY then make its own decision
   regarding whether or not to redirect the request.

   A PUT request applied to the target resource MAY have side-effects on
   other resources.  For example, an article might have a URI for
   identifying "the current version" (a resource) which that is separate from
   the URIs identifying each particular version (different resources
   that at one point shared the same state as the current version
   resource).  A successful PUT request on "the current version" URI
   might therefore create a new version resource in addition to changing
   the state of the target resource, and might also cause links to be
   added between the related resources.

   An origin server SHOULD reject any PUT request that contains a
   Content-Range header field (Section 5.2 4.2 of [Part5]), since it might
   be misinterpreted as partial content (or might be partial content
   that is being mistakenly PUT as a full representation).  Partial
   content updates are possible by targeting a separately identified
   resource with state that overlaps a portion of the larger resource,
   or by using a different method that has been specifically defined for
   partial updates (for example, the PATCH method defined in [RFC5789]).

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

5.3.5.

4.3.5.  DELETE

   The DELETE method requests that the origin server delete remove the
   association between the target
   resource.  This resource and its current
   functionality.  In effect, this method MAY be overridden by human intervention (or
   other means) on the origin server.  The client cannot be guaranteed
   that is similar to the rm command
   in UNIX: it expresses a deletion operation has been carried out, even if on the status code
   returned from URI mapping of the
   origin server indicates server, rather than an expectation that the action has been
   completed successfully.  However, the server SHOULD NOT indicate
   success unless, at the time the response is given, it intends to
   delete previously
   associated information be deleted.

   If the target resource has one or move it to an inaccessible location.

   A successful response SHOULD more current representations, they
   might or might not be 200 (OK) if destroyed by the response includes a
   representation describing origin server, and the status,
   associated storage might or might not be reclaimed, depending
   entirely on the nature of the resource and its implementation by the
   origin server (which are beyond the scope of this specification).
   Likewise, other implementation aspects of a resource might need to be
   deactivated or archived as a result of a DELETE, such as database or
   gateway connections.  In general, it is assumed that the origin
   server will only allow DELETE on resources for which it has a
   prescribed mechanism for accomplishing the deletion.

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

   If a DELETE method is successfully applied, the origin server SHOULD
   send a 202 (Accepted) status code if the action seems okay but has
   not yet been enacted, or a 204 (No Content) status code if the action
   has been enacted but and no further information is to be supplied, or a
   200 (OK) status code if the action has been enacted and the response does not include
   message includes a representation. representation describing the status.

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

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

5.3.6.

4.3.6.  CONNECT

   The CONNECT method requests that the proxy recipient establish a tunnel to
   the destination origin server identified by the request-target and,
   if successful, thereafter restrict its behavior to blind forwarding
   of packets packets, in both directions, until the connection is closed.

   When using CONNECT, the request-target MUST use the authority form
   (Section 5.3 of [Part1]); i.e., the request-target consists of

   CONNECT is intended only
   the host name and port for use in requests to a proxy.  An origin
   server that receives a CONNECT request for itself MAY respond with a
   2xx status code to indicate that a connection is established.

   However, most origin servers do not implement CONNECT.

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

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

   Any 2xx (Successful) response to a CONNECT request indicates that the

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

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

   Any response other than

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

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

     CONNECT tunnel.  For example,

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

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

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

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

   When a direct or tunnel
   connection established to the authority.

   If at any point intermediary detects that either one of the peers gets disconnected, side has closed its
   connection, any outstanding data that came from that peer side will first
   be passed sent to the other
   one, side and after that also then the other connection intermediary will be terminated by
   the proxy. close both
   connections.  If there is outstanding data to that peer left undelivered, that
   data will be discarded.

   An origin server which receives

   A payload within a CONNECT request for itself MAY
   respond with message has no defined semantics;
   sending a 2xx status code to indicate that payload body on a connection is
   established.  However, most origin servers do CONNECT request might cause some existing
   implementations to reject the request.

   Responses to the CONNECT method are not implement CONNECT.

5.3.7. cacheable.

4.3.7.  OPTIONS

   The OPTIONS method requests information about the communication
   options available on the request/response chain identified by the
   effective request URI.  This method allows a client to determine the
   options and/or requirements associated with a resource, or the
   capabilities of a server, without implying a resource action or
   initiating a resource retrieval.

   Responses to the OPTIONS method are not cacheable.

   If the action.

   An OPTIONS request includes a payload, then the media type MUST
   be indicated by a Content-Type field.  Although this specification
   does not define any use for such a body, future extensions to HTTP
   might use the OPTIONS body to make more detailed queries on the
   server.

   If with an asterisk ("*") as the request-target
   (Section 5.3 of [Part1]) is an asterisk ("*"),
   the OPTIONS request is intended to apply applies to the server in general rather than
   to a specific resource.  Since a server's communication options
   typically depend on the resource, the "*" request is only useful as a
   "ping" or "no-op" type of method; it does nothing beyond allowing the
   client to test the capabilities of the server.  For example, this can
   be used to test a proxy for HTTP/1.1 conformance (or lack thereof).

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

   A 200 (OK) server generating a successful response to OPTIONS SHOULD include send any
   header fields that might indicate optional features implemented by
   the server and applicable to that the target resource (e.g., Allow), possibly
   including potential extensions not defined by this specification.
   The response payload, if any, SHOULD might also
   include information about describe the communication options.  The
   options in a machine or human-readable representation.  A standard
   format for such a payload representation is not defined by this
   specification, but might be defined by future extensions to HTTP.  Content negotiation MAY be
   used to select the appropriate representation.  If no payload body is
   included, the response  A
   server MUST include generate a Content-Length field with a
   field-value value of "0".

   The "0" if no
   payload body is to be sent in the response.

   A client MAY send a Max-Forwards header field MAY be used in an OPTIONS request
   to target a specific proxy recipient in the request chain (see
   Section 6.1.1).  If no 5.1.2).  A proxy MUST NOT generate a Max-Forwards header
   field
   is present in the request, then the forwarded while forwarding a request MUST NOT
   include unless that request was received
   with a Max-Forwards field.

5.3.8.

   A client that generates an OPTIONS request containing a payload body
   MUST send a valid Content-Type header field describing the
   representation media type.  Although this specification does not
   define any use for such a payload, future extensions to HTTP might
   use the OPTIONS body to make more detailed queries about the target
   resource.

   Responses to the OPTIONS method are not cacheable.

4.3.8.  TRACE

   The TRACE method requests a remote, application-level loop-back of
   the request message.  The final recipient of the request SHOULD
   reflect the message received received, excluding some fields described below,
   back to the client as the message body of a 200 (OK) response. response with a
   Content-Type of "message/http" (Section 7.3.1 of [Part1]).  The final
   recipient is either the origin server or the first proxy server to receive
   a Max-Forwards value of zero (0) in the request (see Section 6.1.1). (Section 5.1.2).

   A TRACE request client MUST NOT include send header fields in a message body. TRACE allows the request containing
   sensitive data that might be disclosed by the response.  For example,
   it would be foolish for a user agent to send stored user credentials
   [Part7] or cookies [RFC6265] in a TRACE request.  The final recipient
   SHOULD exclude any request header fields from the response body that
   are likely to contain sensitive data.

   TRACE allows the client to see what is being received at the other
   end of the request chain and use that data for testing or diagnostic
   information.  The value of the Via header field (Section 5.7 5.7.1 of
   [Part1]) is of particular interest, since it acts as a trace of the
   request chain.  Use of the Max-Forwards header field allows the
   client to limit the length of the request chain, which is useful for
   testing a chain of proxies forwarding messages in an infinite loop.

   If the request is valid, the response SHOULD have a Content-Type of
   "message/http" (see Section 7.3.1 of [Part1]) and contain

   A client MUST NOT send a message body that encloses in a copy of the entire request message. TRACE request.

   Responses to the TRACE method are not cacheable.

6.

5.  Request Header Fields

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

6.1.

5.1.  Controls

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

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Cache-Control     | Section 7.2 of [Part6] |
   | Expect            | Section 5.1.1          |
   | Host              | Section 5.4 of [Part1] |
   | Max-Forwards      | Section 6.1.1 5.1.2          |
   | Expect Pragma            | Section 6.1.2 7.4 of [Part6] |
   | Range             | Section 5.4 3.1 of [Part5] |
   | TE                | Section 4.3 of [Part1] |
   +-------------------+------------------------+

6.1.1.  Max-Forwards

5.1.1.  Expect

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

     Max-Forwards client.

     Expect       = 1*DIGIT

   The Max-Forwards value is a decimal integer indicating 1#expectation

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

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

   If all received Expect header field(s) are syntactically valid but
   contain an expectation that the remaining
   number of times this request message can be forwarded.

   Each recipient of a TRACE or OPTIONS request containing a Max-
   Forwards header field MUST check and update its value prior to
   forwarding the request.  If the received value is zero (0), the
   recipient MUST NOT forward the request; instead, it MUST respond as
   the final recipient.  If the received Max-Forwards value is greater
   than zero, then the forwarded message MUST contain an updated Max-
   Forwards field with a value decremented by one (1).

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

6.1.2.  Expect

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

     Expect       = 1#expectation

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

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

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

   The only expectation defined by this specification is:

   100-continue

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

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

   The Expect mechanism is hop-by-hop: the above requirements apply to
   any server, including proxies.  However, the Expect header field
   itself is end-to-end; it MUST be forwarded if the request is
   forwarded.

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

6.1.2.1.

5.1.1.1.  Use of the 100 (Continue) Status

   The only expectation defined by this specification is:

   100-continue

      The request includes a payload body and the client will wait for a
      100 (Continue) response after sending the request header section
      but before sending the payload body.  The 100-continue expectation
      does not use any expect-params.

   The primary purpose of the 100 (Continue) status code (Section 7.2.1) 6.2.1)
   is to allow a client that is sending a request message with a payload
   to determine if the origin server is willing to accept the request
   (based on the request header fields) before the client sends the
   payload body.  In some cases, it might either be inappropriate or
   highly inefficient for the client to send the payload body if the
   server will reject the message without looking at the body.

   Requirements for HTTP/1.1 clients:

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

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

   Because of the presence of older implementations, the protocol allows
   ambiguous situations in which a client might send "Expect: 100-
   continue" without receiving either a 417 (Expectation Failed) or a
   100 (Continue) status code.  Therefore, when a client sends this
   header field to an origin server (possibly via a proxy) from which it
   has never seen a 100 (Continue) status code, the client SHOULD NOT
   wait for an indefinite period before sending the payload body.

   Requirements for HTTP/1.1 origin servers:

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

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

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

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

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

   Requirements for HTTP/1.1 proxies:

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

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

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

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

6.2.  Conditionals 6.2.1).

5.1.2.  Max-Forwards

   The "Max-Forwards" header field provides a mechanism with the TRACE
   (Section 4.3.8) and OPTIONS (Section 4.3.7) methods to limit the
   number of times that the request is forwarded by proxies.  This can
   be useful when the client is attempting to trace a request that
   appears to be failing or looping mid-chain.

     Max-Forwards = 1*DIGIT

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

   Each recipient of a TRACE or OPTIONS request containing a Max-
   Forwards header field MUST check and update its value prior to
   forwarding the request.  If the received value is zero (0), the
   recipient MUST NOT forward the request; instead, it MUST respond as
   the final recipient.  If the received Max-Forwards value is greater
   than zero, then the forwarded message MUST contain an updated Max-
   Forwards field with a value decremented by one (1).

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

5.2.  Conditionals are

   The HTTP conditional request header fields that indicate [Part4] allow a client to
   place a precondition on the state of the target resource, so that the
   action corresponding to be tested before applying the method semantics to will not be applied if
   the target
   resource. precondition evaluates to false.  Each precondition is based on metadata that is expected defined by
   this specification consists of a comparison between a set of
   validators obtained from prior representations of the target resource
   to
   change if the current state of validators for the selected representation
   (Section 7.2).  Hence, these preconditions evaluate whether the state
   of the target resource is
   changed. has changed since a given state known by the
   client.  The HTTP/1.1 conditional request mechanisms are effect of such an evaluation depends on the method
   semantics and choice of conditional, as defined in Section 5 of
   [Part4].

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

6.3.

5.3.  Content Negotiation

   The following request header fields are sent by a user agent to
   engage in proactive negotiation of the response content, as defined
   in Section 3.4.1.  The preferences sent in these fields apply to any
   content in the response, including representations of the target
   resource, representations of error or processing status, and
   potentially even the miscellaneous text strings that might appear
   within the protocol.

   +-------------------+---------------+
   | Header Field Name | Defined in... |
   +-------------------+---------------+
   | Accept            | Section 6.3.2 5.3.2 |
   | Accept-Charset    | Section 6.3.3 5.3.3 |
   | Accept-Encoding   | Section 6.3.4 5.3.4 |
   | Accept-Language   | Section 6.3.5 5.3.5 |
   +-------------------+---------------+

6.3.1.

5.3.1.  Quality Values

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

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

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

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

6.3.2.

5.3.2.  Accept

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

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

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

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

   Each media-range MAY might be followed by one zero or more accept-params,
   beginning with the applicable media
   type parameters (e.g., charset), an optional "q" parameter for
   indicating a relative weight, as
   defined in Section 6.3.1. weight (Section 5.3.1), and then zero or more
   extension parameters.  The first "q" parameter (if any) separates
   the media-range parameter(s) from is necessary if any accept-
   ext are present, since it acts as a separator between the accept-params. two
   parameter sets.

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

   The example

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

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

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

   A more elaborate example is

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

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

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

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

   have the following precedence:

   1.  text/plain;format=flowed

   2.  text/plain

   3.  text/*

   4.  */*

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

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

   would cause the following values to be associated:

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

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

6.3.3.

5.3.3.  Accept-Charset

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

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

   Character encoding values (a.k.a., charsets)

   Charset names are described defined in Section 3.1.1.2.  Each charset  A user agent MAY be given an associated
   associate a quality value which represents with each charset to indicate the user's
   relative preference for that charset, as defined in Section 6.3.1. 5.3.1.
   An example is

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

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

   A request without any Accept-Charset header field implies that the
   user agent will accept any character encoding charset in response.  Most general-purpose
   user agents do not send Accept-Charset, unless specifically
   configured to do so, because a detailed list of supported charsets
   makes it easier for a server to identify an individual by virtue of
   the user agent's request characteristics (Section 9.6).

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

6.3.4.

5.3.4.  Accept-Encoding

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

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

   Each codings value MAY be given an associated quality value which
   represents
   representing the preference for that encoding, as defined in
   Section 6.3.1. 5.3.1.  The asterisk "*" symbol in an Accept-Encoding field
   matches any available content-coding not explicitly listed in the
   header field.

   For example,

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

   A request without an Accept-Encoding header field implies that the
   user agent has no preferences regarding content-codings.  Although
   this allows the server to use any content-coding in a response, it
   does not imply that the user agent will be able to correctly process
   all encodings.

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

   1.  The special "*" symbol in an  If no Accept-Encoding field matches is in the request, any
       available content-coding not explicitly listed in
       is considered acceptable by the header
       field. user agent.

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

   3.  If the representation's content-coding is one of the content-
       codings listed in the Accept-Encoding field, then it is
       acceptable unless it is accompanied by a qvalue of 0.  (As
       defined in Section 6.3.1, 5.3.1, a qvalue of 0 means "not acceptable".)

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

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

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

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

6.3.5.

5.3.5.  Accept-Language

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

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

   Each language-range can be given an associated quality value which
   represents
   representing an estimate of the user's preference for the languages
   specified by that range, as defined in Section 6.3.1. 5.3.1.  For example,

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

   would mean: "I prefer Danish, but will accept British English and
   other types of English". (see

   Note that some recipients treat the order in which language tags are
   listed as an indication of descending priority, particularly for tags
   that are assigned equal quality values (no value is the same as q=1).
   However, this behavior cannot be relied upon.  For consistency and to
   maximize interoperability, many user agents assign each language tag
   a unique quality value while also listing them in order of decreasing
   quality.  Additional discussion of language priority lists can be
   found in Section 2.3 of [RFC4647]) [RFC4647].

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

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

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

   As request (Section 9.6).

   Since intelligibility is highly dependent on the individual user, it is
   recommended that client applications make
   user agents need to allow user control over the choice of linguistic
   preference available
   preference.  A user agent that does not provide such control to the user.  If the choice is not made
   available, then the Accept-Language header field
   user MUST NOT be given in
   the request. send an Accept-Language header field.

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

6.4.

5.4.  Authentication Credentials

   Two header fields are used for carrying authentication credentials,
   as defined in [Part7].  Note that various custom mechanisms for user
   authentication use the Cookie header field for this purpose, as
   defined in [RFC6265].

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

6.5.

5.5.  Request Context

   +-------------------+------------------------+

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

   +-------------------+---------------+
   | Header Field Name | Defined in... |
   +-------------------+------------------------+
   +-------------------+---------------+
   | From              | Section 6.5.1 5.5.1 |
   | Referer           | Section 6.5.2          |
   | TE                | Section 4.3 of [Part1] 5.5.2 |
   | User-Agent        | Section 6.5.3 5.5.3 |
   +-------------------+------------------------+

6.5.1.
   +-------------------+---------------+

5.5.1.  From

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

     From    = mailbox

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

   An example is:

     From: webmaster@example.org

   This

   The From header field MAY be used for logging purposes and as a means for
   identifying the source of invalid or unwanted requests.  It is rarely sent by non-robotic user agents.  A
   user agent SHOULD NOT be used as an insecure form of access protection.  The
   interpretation of this send a From header field is that the request is being performed
   on behalf of without explicit
   configuration by the person given, who accepts responsibility for user, since that might conflict with the
   method performed.  In particular, robot user's
   privacy interests or their site's security policy.

   Robotic user agents SHOULD include this send a valid From header field so that the
   person responsible for running the robot can be contacted if problems
   occur on servers, such as if the receiving end.

   The Internet e-mail address in this field MAY be separate from the
   Internet host which issued the request.  For example, when a request robot is passed through a proxy the original issuer's address SHOULD be
   used.

   The client sending excessive,
   unwanted, or invalid requests.

   Servers SHOULD NOT send use the From header field without the user's
   approval, as it might conflict with the user's privacy interests for access control or
   their site's security policy.  It is strongly recommended
   authentication, since most recipients will assume that the
   user be able to disable, enable, and modify the value of this field
   at any time prior to a request.

6.5.2.
   value is public information.

5.5.2.  Referer

   The "Referer" [sic] header field allows the client user agent to specify the a
   URI
   of reference for the resource from which the target URI was obtained (the
   (i.e., the "referrer", although though the header field name is misspelled.).

   The misspelled).  A user
   agent MUST exclude any fragment or userinfo components [RFC3986] when
   generating the Referer header field value.

     Referer = absolute-URI / partial-URI

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

   Example:

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

   If the target URI was obtained from a source that does not have its
   own URI (e.g., input from the user keyboard), keyboard, or an entry within the Referer field
   user's bookmarks/favorites), the user agent MUST either be sent with the value "about:blank", exclude
   Referer or not be sent at all.
   Note that this requirement does not apply to sources send it with non-HTTP
   URIs (e.g., FTP). a value of "about:blank".

   The Referer = absolute-URI / partial-URI

   Example:

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

   If the field value has the potential to reveal information about the
   request context or browsing history of the user, which is a relative URI, it SHOULD privacy
   concern if the referring resource's identifier reveals personal
   information (such as an account name) or a resource that is supposed
   to be interpreted
   relative confidential (such as behind a firewall or internal to a
   secured service).  Most general-purpose user agents do not send the effective request
   Referer header field when the referring resource is a local "file" or
   "data" URI.  The URI MUST  A user agent SHOULD NOT include send a
   fragment. Referer header field in
   an unsecured HTTP request if the referring page was received with a
   secure protocol.  See Section 10.2 9.3 for additional security
   considerations.

6.5.3.

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

5.5.3.  User-Agent

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

   Typically, it request, which is often used for statistical purposes, by servers to help
   identify the tracing scope of
   protocol violations, and tailoring reported interoperability problems, to work
   around or tailor responses to avoid particular user agent limitations.

   The
   limitations, and for analytics regarding browser or operating system
   use.  A user agent SHOULD send a User-Agent field can contain multiple in each request
   unless specifically configured not to do so.

     User-Agent = product tokens (Section 4) and *( RWS ( product / comment ) )

   The User-Agent field-value consists of one or more product
   identifiers, each followed by zero or more comments (Section 3.2 of [Part1]) identifying
   [Part1]), which together identify the user agent software and its
   significant subproducts.  By convention, the product tokens identifiers are
   listed in decreasing order of their significance for identifying the
   application.

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

   Likewise, implementations are encouraged not to use the
   optional version.

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

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

   Example:

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

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

   Likewise, implementations are encouraged not to use the product
   tokens of other implementations in order to declare compatibility
   with them, as this circumvents the purpose of the field.  Finally,  If a user
   agent masquerades as a different user agent, recipients can assume
   that the user intentionally desires to see responses tailored for
   that identified user agent, even if they are encouraged might not to use comments to identify products; doing
   so makes work as well for
   the field value more difficult to parse.

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

   Example:

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

7. actual user agent being used.

6.  Response Status Codes

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

   HTTP status codes are extensible.  HTTP applications clients are not required to
   understand the meaning of all registered status codes, though such
   understanding is obviously desirable.  However, applications clients MUST
   understand the class of any status code, as indicated by the first
   digit, and treat any an unrecognized response status code as being equivalent to
   the x00 status code of that class, with the exception that a response
   with an unrecognized response status code MUST NOT be cached.

   For example, if an unrecognized status code of 431 471 is received by the a
   client, it the client can
   safely assume that there was something wrong with its
   request and treat the response as if it had received a 400 status
   code.  In such
   cases, user agents SHOULD present to the user the  The response message will usually contain a representation
   enclosed with the response, since
   that representation is likely to
   include human-readable information which will explain explains the unusual status.

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

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

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

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

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

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

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

7.1.

6.1.  Overview of Status Codes

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

   +-------------+------------------------------+----------------------+

   +------+-------------------------------+------------------------+
   | status-code code | reason-phrase                 | Defined in...          |
   +-------------+------------------------------+----------------------+
   +------+-------------------------------+------------------------+
   | 100  | Continue                      | Section 7.2.1 6.2.1          |
   | 101  | Switching Protocols           | Section 7.2.2 6.2.2          |
   | 200  | OK                            | Section 7.3.1 6.3.1          |
   | 201  | Created                       | Section 7.3.2 6.3.2          |
   | 202  | Accepted                      | Section 7.3.3 6.3.3          |
   | 203  | Non-Authoritative            | Section 7.3.4        |
   |             | Information | Section 6.3.4          |
   | 204  | No Content                    | Section 7.3.5 6.3.5          |
   | 205  | Reset Content                 | Section 7.3.6 6.3.6          |
   | 206  | Partial Content               | Section 3.1 4.1 of       |
   |             |                              | [Part5] |
   | 300  | Multiple Choices              | Section 7.4.1 6.4.1          |
   | 301  | Moved Permanently             | Section 7.4.2 6.4.2          |
   | 302  | Found                         | Section 7.4.3 6.4.3          |
   | 303  | See Other                     | Section 7.4.4 6.4.4          |
   | 304  | Not Modified                  | Section 4.1 of       |
   |             |                              | [Part4] |
   | 305  | Use Proxy                     | Section 7.4.5 6.4.5          |
   | 307  | Temporary Redirect            | Section 7.4.7 6.4.7          |
   | 400  | Bad Request                   | Section 7.5.1 6.5.1          |
   | 401  | Unauthorized                  | Section 3.1 of       |
   |             |                              | [Part7] |
   | 402  | Payment Required              | Section 7.5.2 6.5.2          |
   | 403  | Forbidden                     | Section 7.5.3 6.5.3          |
   | 404  | Not Found                     | Section 7.5.4 6.5.4          |
   | 405  | Method Not Allowed            | Section 7.5.5 6.5.5          |
   | 406  | Not Acceptable                | Section 7.5.6 6.5.6          |
   | 407  | Proxy Authentication Required | Section 3.2 of       |
   |             | Required                     | [Part7] |
   | 408  | Request Time-out              | Section 7.5.7 6.5.7          |
   | 409  | Conflict                      | Section 7.5.8 6.5.8          |
   | 410  | Gone                          | Section 7.5.9 6.5.9          |
   | 411  | Length Required               | Section 7.5.10 6.5.10         |
   | 412  | Precondition Failed           | Section 4.2 of       |
   |             |                              | [Part4] |
   | 413  | Request Representation Payload Too   | Section 7.5.11       |
   |             | Large             | Section 6.5.11         |
   | 414  | URI Too Long                  | Section 7.5.12 6.5.12         |
   | 415  | Unsupported Media Type        | Section 7.5.13 6.5.13         |
   | 416  | Requested range not Range Not Satisfiable         | Section 3.2 4.4 of       |
   |             | satisfiable                  | [Part5] |
   | 417  | Expectation Failed            | Section 7.5.14 6.5.14         |
   | 426  | Upgrade Required              | Section 7.5.15 6.5.15         |
   | 500  | Internal Server Error         | Section 7.6.1 6.6.1          |
   | 501  | Not Implemented               | Section 7.6.2 6.6.2          |
   | 502  | Bad Gateway                   | Section 7.6.3 6.6.3          |
   | 503  | Service Unavailable           | Section 7.6.4 6.6.4          |
   | 504  | Gateway Time-out              | Section 7.6.5 6.6.5          |
   | 505  | HTTP Version not supported Not Supported    | Section 7.6.6 6.6.6          |
   +-------------+------------------------------+----------------------+
   +------+-------------------------------+------------------------+

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

7.2.

   Responses with status codes that are defined as cacheable by default
   (e.g., 200, 203, 206, 300, 301, and 410 in this specification) can be
   reused by a cache with heuristic expiration unless otherwise
   indicated by the method definition or explicit cache controls
   [Part6]; all other status codes are not cacheable by default.

6.2.  Informational 1xx

   This

   The 1xx (Informational) class of status code indicates an interim
   response for communicating connection status or request progress
   prior to completing the requested action and sending a provisional response,
   consisting only final
   response.  All 1xx responses consist of only the status-line and
   optional header fields, and is thus are terminated by an the empty line.  There are no required header fields for
   this class line at
   the end of status code. the header block.  Since HTTP/1.0 did not define any 1xx
   status codes, servers MUST NOT send a 1xx response to an HTTP/1.0
   client except under experimental conditions.

   A client MUST be prepared to accept one or more 1xx status responses
   prior to a regular final response, even if the client does not expect a 100
   (Continue) status message.  Unexpected one.  A
   user agent MAY ignore unexpected 1xx status responses MAY be
   ignored by a user agent. responses.

   Proxies MUST forward 1xx responses, unless the connection between the
   proxy and its client has been closed, or unless the proxy itself
   requested the generation of the 1xx response.  (For  For example, if a
   proxy adds an "Expect: 100-continue" field when it forwards a
   request, then it need not forward the corresponding 100 (Continue)
   response(s).)

7.2.1.
   response(s).

6.2.1.  100 Continue

   The client SHOULD continue with its request.  This interim response
   is used to inform the client 100 (Continue) status code indicates that the initial part of the a
   request has been received and has not yet been rejected by the
   server.  The
   client SHOULD continue by sending the remainder of the request or, if
   the request has already been completed, ignore this response.  The server MUST intends to send a final response after the
   request has been
   completed.  See Section 6.1.2.1 for detailed discussion of fully received and acted upon.

   When the use request contains an Expect header field that includes a 100-
   continue expectation, the 100 response indicates that the server
   wishes to receive the request payload body, as described in
   Section 5.1.1.1.  The client ought to continue sending the request
   and handling of discard the 100 response.

   If the request did not contain an Expect header field containing the
   100-continue expectation, the client can simply discard this status code.

7.2.2. interim
   response.

6.2.2.  101 Switching Protocols

   The 101 (Switching Protocols) status code indicates that the server
   understands and is willing to comply with the client's request, via
   the Upgrade message header field (Section 6.3 6.7 of [Part1]), for a change in
   the application protocol being used on this connection.  The server will switch protocols to those defined by the
   response's
   MUST generate an Upgrade header field in the response that indicates
   which protocol(s) will be switched to immediately after the empty
   line
   which that terminates the 101 response.

   The protocol SHOULD be switched

   It is assumed that the server will only agree to switch protocols
   when it is advantageous to do so.  For example, switching to a newer
   version of HTTP is might be advantageous over older versions, and
   switching to a real-time, synchronous protocol might be advantageous
   when delivering resources that use such features.

7.3.

6.3.  Successful 2xx

   This

   The 2xx (Successful) class of status code indicates that the client's
   request was successfully received, understood, and accepted.

7.3.1.

6.3.1.  200 OK

   The
   200 (OK) status code indicates that the request has succeeded.  The
   payload returned with the sent in a 200 response is
   dependent depends on the method used in request method.  For
   the request, for example: methods defined by this specification, the intended meaning of
   the payload can be summarized as:

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

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

   POST  a representation describing of the status of, or containing results obtained from,
      the result action;

   PUT, DELETE  a representation of the status of the action;

   OPTIONS  a representation of the communications options;

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

   Caches MAY use a heuristic (see Section 4.1.2 of [Part6])

   Aside from responses to
   determine freshness for CONNECT, a 200 responses.

7.3.2.  201 Created

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

   Newly created resources are typically linked to from the response always has a payload, with the most relevant URI also being carried in the
   Location header field.
   though an origin server MAY generate a payload body of zero length.
   If the newly created resource's URI no payload is the
   same as the Effective Request URI, this information can be omitted
   (e.g., in the case of a response to a PUT request).

   The desired, an origin server MUST create the resource(s) before returning the
   201 status code.  If ought to send 204 (No
   Content) instead.  For CONNECT, no payload is allowed because the action cannot be carried out immediately,
   successful result is a tunnel, which begins immediately after the server SHOULD respond with 202 (Accepted) 200
   response instead. header block.

   A 201 response MAY contain an ETag 200 response header field indicating
   the current value of the entity-tag for is cacheable unless otherwise indicated by the representation method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.3.2.  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 identified created by the request is identified
   by either a Location header field or, in case the response or, if no Location header
   field was omitted, is received, by the Effective Request URI (see effective request URI.

   The 201 response payload typically describes and links to the
   resource(s) created.  See Section 2.3 7.2 for a discussion of the meaning
   and purpose of [Part4]).

7.3.3. validator header fields, such as ETag and Last-
   Modified, in a 201 response.

6.3.3.  202 Accepted

   The 202 (Accepted) status code indicates that the request has been
   accepted for processing, but the processing has not been completed.
   The request might or might not eventually be acted upon, as it might
   be disallowed when processing actually takes place.  There is no
   facility in HTTP for re-sending a status code from an asynchronous operation such as this.
   operation.

   The 202 response is intentionally non-committal.  Its purpose is to
   allow a server to accept a request for some other process (perhaps a
   batch-oriented process that is only run once per day) without
   requiring that the user agent's connection to the server persist
   until the process is completed.  The representation returned sent with this
   response SHOULD include an indication of ought to describe the request's current status and either a pointer point to
   (or embed) a status monitor or some that can provide the user with an
   estimate of when the user can expect the request to will be fulfilled.

7.3.4.

6.3.4.  203 Non-Authoritative Information

   The representation in the response 203 (Non-Authoritative Information) status code indicates that
   the request was successful but the enclosed payload has been transformed or otherwise modified
   from that of the origin server's 200 (OK) response by a transforming
   proxy (Section 2.3 5.7.2 of [Part1]).  Note that
   the behavior of transforming intermediaries is controlled by the no-
   transform Cache-Control directive (Section 7.2 of [Part6]).  This status code is only appropriate allows the proxy
   to notify recipients when a transformation has been applied, since
   that knowledge might impact later decisions regarding the content.
   For example, future cache validation requests for the content might
   only be applicable along the same request path (through the same
   proxies).

   The 203 response status code
   would have been 200 (OK) otherwise.  When is similar to the status Warning code before
   transformation would have been different, the of 214 Transformation
   Applied warn-code (Section 7.5 of [Part6]) [Part6]), which has the advantage of being
   applicable to responses with any status code.

   A 203 response is appropriate.

   Caches MAY use a heuristic cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]) to
   determine freshness for 203 responses.

7.3.5. [Part6]).

6.3.5.  204 No Content

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

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

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

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

   The

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

7.3.6.
   fields because it cannot contain a message body.

   A 204 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.3.6.  205 Reset Content

   The 205 (Reset Content) status code indicates that the server has
   fulfilled the request and desires that the user agent SHOULD reset the document view
   "document view", which caused the request to be sent. sent, to its original
   state as received from the origin server.

   This response is primarily intended to allow input for actions to take place via
   user input, followed by support a clearing of the form in which common data entry use case
   where the input is
   given user receives content that supports data entry (a form,
   notepad, canvas, etc.), enters or manipulates data in that space,
   causes the entered data to be submitted in a request, and then the
   data entry mechanism is reset for the next entry so that the user can
   easily initiate another input action.

   The

   Since the 205 status code implies that no additional content will be
   provided in the payload, the server MUST send a message body included with of zero
   length.  In other words, the response server MUST be empty.  Note that
   receivers still need to parse the send a "Content-Length: 0"
   field in a 205 response according to or close the algorithm
   defined in Section 3.3 of [Part1].

7.4. connection immediately after
   sending the blank line terminating the header section.

6.4.  Redirection 3xx

   This

   The 3xx (Redirection) class of status code indicates that further
   action needs to be taken by the user agent in order to fulfill the
   request.  If the
   required action involves a subsequent HTTP request, it MAY be carried
   out by Location header field (Section 7.1.2) is provided, the
   user agent without interaction with MAY automatically redirect its request to the user if and only
   if URI
   referenced by the method used in Location field value, even if the second request specific status
   code is not understood.  Automatic redirection needs to done with
   care for methods not known to be "safe", safe, as defined in Section 5.2.1. 4.2.1,
   since the user might not wish to redirect an unsafe request.

   There are several types of redirects:

   1.  Redirects of that indicate the request to another resource might be available at a
       different URI, either temporarily or
       permanently.  The new URI is specified in as provided by the Location header
       field.  In this specification, field, as in the
       status codes 301 (Moved Permanently), 302 (Found), and 307
       (Temporary Redirect) fall
       under this category. Redirect).

   2.  Redirection that offers a choice of matching resources, each
       capable of representing the original request target, as in the
       300 (Multiple Choices) status code.

   3.  Redirection to a new location different resource, identified by the Location
       field, that represents can represent an indirect response to the request, such as
       in the result of a POST operation
       to be retrieved with a subsequent GET request.  This is status
       code 303 (See Other).

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

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

      Note: In HTTP/1.0, only the status codes 301 (Moved Permanently) and
      302 (Found) were defined for the first type of redirect, and
      the second type did not exist at all redirect
      ([RFC1945], Section 9.3).
      However it turned out that web forms using POST expected redirects
      to change the operation for  Early user agents split on whether the subsequent request
      method applied to retrieval
      (GET).  To address this use case, HTTP/1.1 introduced the second
      type of redirect with target would be the status code same as the
      original request or would be rewritten as GET.  Although HTTP
      originally defined the former semantics for 301 and 302 (to match
      its original implementation at CERN), and defined 303 (See Other) ([RFC2068],
      Section 10.3.4).  As user agents did not change their behavior
      to
      maintain backwards compatibility, match the latter semantics, prevailing practice gradually
      converged on the latter semantics for 301 and 302 as well.  The
      first revision of HTTP/1.1 added yet another status code, 307 (Temporary Redirect), for which Redirect) to
      indicate the backwards compatibility problems did not apply ([RFC2616],
      Section 10.3.8). former semantics without being impacted by divergent
      practice.  Over 10 years later, most user agents still do method
      rewriting for status codes 301 and 302, therefore 302; therefore, this specification makes
      that behavior conformant in case when the original request was is POST.

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

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

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

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

7.4.1.

6.4.1.  300 Multiple Choices

   The 300 (Multiple Choices) status code indicates that the target
   resource has more than one representation, each with its own more
   specific location, identifier, and reactive negotiation information
   (Section 3.4) about the alternatives is being
   provided so that the user (or user agent) can select a preferred
   representation by redirecting its request to that
   location.

   Unless it was a HEAD request, one or more of those
   identifiers.  In other words, the response server desires that the user agent
   engage in reactive negotiation to select the most appropriate
   representation(s) for its needs (Section 3.4).

   If the server has a preferred choice, the server SHOULD include generate a
   representation
   Location header field containing a preferred choice's URI reference.
   The user agent MAY use the Location field value for automatic
   redirection.

   For request methods other than HEAD, the server SHOULD generate a
   payload in the 300 response containing a list of representation
   metadata and
   location(s) URI reference(s) from which the user or user agent can
   choose the one most
   appropriate.  Depending upon the format and the capabilities of the preferred.  The user agent, agent MAY make a selection of
   from that list automatically, depending upon the most appropriate choice MAY be performed
   automatically.  However, list format, but
   this specification does not define any a standard for such automatic
   selection.

   If

   A 300 response is cacheable unless otherwise indicated by the server has a preferred choice method
   definition or explicit cache controls (see Section 4.1.2 of representation, it SHOULD
   include [Part6]).

      Note: The original proposal for 300 defined the specific URI for header field
      as providing a list of alternative representations, such that representation it
      would be usable for 200, 300, and 406 responses and be transferred
      in responses to the Location
   field; user agents MAY use HEAD method.  However, lack of deployment and
      disagreement over syntax led to both URI and Alternates (a
      subsequent proposal) being dropped from this specification.  It is
      possible to communicate the Location field value for automatic
   redirection.

   Caches MAY use list using a heuristic (see Section 4.1.2 set of [Part6]) to
   determine freshness for 300 responses.

7.4.2. Link header fields
      [RFC5988], each with a relationship of "alternate", though
      deployment is a chicken-and-egg problem.

6.4.2.  301 Moved Permanently

   The 301 (Moved Permanently) status code indicates that the target
   resource has been assigned a new permanent URI and any future
   references to this resource SHOULD ought to use one of the returned enclosed URIs.
   Clients with link editing capabilities ought to automatically re-link
   references to the effective request URI to one or more of the new
   references returned sent by the server, where possible.

   Caches MAY use

   The server SHOULD generate a heuristic (see Section 4.1.2 of [Part6]) to
   determine freshness Location header field in the response
   containing a preferred URI reference for 301 responses.

   The the new permanent URI SHOULD be given by URI.  The
   user agent MAY use the Location field in the
   response.  A value for automatic
   redirection.  The server's response payload can contain usually contains a short
   hypertext note with a hyperlink to the new URI(s).

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

7.4.3.

   A 301 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.4.3.  302 Found

   The 302 (Found) status code indicates that the target resource
   resides temporarily under a different URI.  Since the redirection
   might be altered on occasion, the client SHOULD ought to continue to use the
   effective request URI for future requests.

   The temporary URI server SHOULD be given by the generate a Location header field in the
   response.  A 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 can contain usually contains a short hypertext note with a
   hyperlink to the new different URI(s).

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

7.4.4.

6.4.4.  303 See Other

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

   This status code is generally applicable to any HTTP method.  It is primarily
   used to allow the output of a POST action to redirect the user agent
   to a selected resource, since doing so provides the information
   corresponding to the POST response in a form that can be separately
   identified, bookmarked, and cached independent of the original
   request.

   A 303 response to a GET request indicates that the requested resource origin server does
   not have a representation of its own the target resource that can be
   transferred by the server over HTTP.  The  However, the Location URI indicates field
   value refers to a resource that is descriptive of the target
   resource, such that the follow-on
   representation making a retrieval request on that other resource
   might be result in a representation that is useful to recipients without
   implying that it
   adequately represents the original target resource.  Note that
   answers to the questions of what can be represented, what
   representations are adequate, and what might be a useful description
   are outside the scope of HTTP and thus entirely determined by the URI owner(s). HTTP.

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

7.4.5. header field.

6.4.5.  305 Use Proxy

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

7.4.6. deprecated (Appendix B).

6.4.6.  306 (Unused)

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

7.4.7.

6.4.7.  307 Temporary Redirect

   The 307 (Temporary Redirect) status code indicates that the target
   resource resides temporarily under a different URI and the user agent
   MUST NOT change the request method if it performs an automatic
   redirection to that URI.  Since the redirection can change over time,
   the client SHOULD continue ought to
   use continue using the original effective request URI
   for future requests.

   The temporary URI server SHOULD be given by the generate a Location header field in the
   response.  A response payload can contain a short hypertext note with
   containing a hyperlink to URI reference for the new URI(s).

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

7.5.

6.5.  Client Error 4xx

   The 4xx (Client Error) class of status code is intended for cases in which indicates that the client
   seems to have erred.  Except when responding to a HEAD request, the
   server SHOULD include send a representation containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition.  These status codes are applicable to any request method.
   User agents SHOULD display any included representation to the user.

7.5.1.

6.5.1.  400 Bad Request

   The 400 (Bad Request) status code indicates that the server cannot or
   will not process the request, due request because the received syntax is invalid,
   nonsensical, or exceeds some limitation on what the server is willing
   to a client
   error (e.g., malformed syntax).

7.5.2. process.

6.5.2.  402 Payment Required

   This

   The 402 (Payment Required) status code is reserved for future use.

7.5.3.

6.5.3.  403 Forbidden

   The 403 (Forbidden) status code indicates that the server understood
   the request, request but refuses to authorize it.
   Providing different user authentication credentials might be
   successful, but any credentials that were provided in the request are
   insufficient.  The request SHOULD NOT be repeated with the same
   credentials.

   If the request method was not HEAD and the  A server that wishes to
   make public why the request has not been fulfilled, it SHOULD forbidden can describe the that
   reason for the refusal in the representation. response payload (if any).

   If authentication credentials were provided in the request, the
   server does not
   wish considers them insufficient to make this information available grant access.  The client
   SHOULD NOT repeat the request with the same credentials.  The client
   MAY repeat the request with new or different credentials.  However, a
   request might be forbidden for reasons unrelated to the client, credentials.

   An origin server that wishes to "hide" the current existence of a
   forbidden target resource MAY instead respond with a status code of
   404 (Not Found) MAY be used instead.

7.5.4. Found).

6.5.4.  404 Not Found

   The 404 (Not Found) status code indicates that the origin server has did
   not found anything matching find a current representation for the effective request URI.
   No indication target resource or is given of not
   willing to disclose that one exists.  A 404 status does not indicate
   whether the condition this lack of representation is temporary or
   permanent.  The permanent; the
   410 (Gone) status code SHOULD be used is preferred over 404 if the origin server
   knows, presumably through some internally configurable mechanism, means, that an old
   resource is permanently unavailable and has no forwarding address.
   This status code is commonly used when the server does not wish condition
   is likely to
   reveal exactly why the request has been refused, or when no other be permanent.

   A 404 response is applicable.

7.5.5. cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.5.  405 Method Not Allowed

   The 405 (Method Not Allowed) status code indicates that the method
   specified in the request-line is known by the origin server but not allowed for
   supported by the target resource.  The response origin server MUST include generate an
   Allow header field in a 405 response containing a list of valid methods for the requested resource.

7.5.6. target
   resource's currently supported methods.

   A 405 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.6.  406 Not Acceptable

   The resource identified by 406 (Not Acceptable) status code indicates that the request is only capable of generating
   response representations which have content characteristics target
   resource does not have a current representation that would be
   acceptable to the user agent, according to the Accept and Accept-* proactive negotiation
   header fields sent received in the request.

   Unless it was a HEAD request, request (Section 5.3), and the response server
   is unwilling to supply a default representation.

   The server SHOULD include generate a
   representation payload containing a list of available
   representation characteristics and location(s) corresponding resource identifiers
   from which the user or user agent can choose the one most
   appropriate.  Depending upon the format and the
   capabilities of the  A user agent, selection of agent MAY automatically select the most
   appropriate choice MAY be performed automatically. from that list.  However, this specification does
   not define any standard for such automatic selection.

      Note: HTTP/1.1 servers are allowed to return responses which are
      not acceptable according to the accept header fields sent selection, as described in
   Section 6.4.1.

6.5.7.  408 Request Timeout

   The 408 (Request Timeout) status code indicates that the
      request.  In some cases, this might even be preferable to sending
      a 406 response.  User agents are encouraged to inspect the header
      fields of an incoming response to determine if it is acceptable.

   If the response could be unacceptable, a user agent SHOULD
   temporarily stop receipt of more data and query the user for a
   decision on further actions.

7.5.7.  408 Request Timeout

   The client server did
   not produce receive a complete request message within the time that the server it was
   prepared to wait.  The  A server SHOULD send the close connection option
   (Section 6.1 of [Part1]) in the response, since 408 implies that the
   server has decided to close the connection rather than continue
   waiting.  If the client has an outstanding request in transit, the
   client MAY repeat the that request without
   modifications at any later time.

7.5.8. on a new connection.

6.5.8.  409 Conflict

   The 409 (Conflict) status code indicates that the request could not
   be completed due to a conflict with the current state of the
   resource.  This code is only allowed used in situations where
   it is expected that the user might be
   able to resolve the conflict and resubmit the request.  The payload server
   SHOULD include generate a payload that includes enough information for the a user
   to recognize the source of the conflict.
   Ideally, the response representation would include enough information
   for the user or user agent to fix the problem; however, that might
   not be possible and is not required.

   Conflicts are most likely to occur in response to a PUT request.  For
   example, if versioning were being used and the representation being
   PUT included changes to a resource which that conflict with those made by
   an earlier (third-party) request, the origin server might use the a 409
   response to indicate that it can't complete the request.  In this
   case, the response representation would likely contain a list of information
   useful for merging the differences between based on the two versions.

7.5.9. revision history.

6.5.9.  410 Gone

   The 410 (Gone) status code indicates that access to the target
   resource is no longer available at the origin server and no
   forwarding address is known.  This that this
   condition is expected likely to be
   considered permanent.  Clients with link editing capabilities SHOULD
   delete references to the effective request URI after user approval.  If the origin server does not
   know, or has no facility to determine, whether or not the condition
   is permanent, the status code 404 (Not Found)
   SHOULD ought to be used
   instead.

   The 410 response is primarily intended to assist the task of web
   maintenance by notifying the recipient that the resource is
   intentionally unavailable and that the server owners desire that
   remote links to that resource be removed.  Such an event is common
   for limited-time, promotional services and for resources belonging to
   individuals no longer working at associated with the origin server's site.  It
   is not necessary to mark all permanently unavailable resources as
   "gone" or to keep the mark for any length of time -- that is left to
   the discretion of the server owner.

   Caches MAY use a heuristic

   A 410 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]) to
   determine freshness for 410 responses.

7.5.10. [Part6]).

6.5.10.  411 Length Required

   The server refuses to accept 411 (Length Required) status code indicates that the server
   refuses to accept the request without a defined Content-
   Length. Content-Length
   (Section 3.3.2 of [Part1]).  The client MAY repeat the request if it
   adds a valid Content-Length header field containing the length of the
   message body in the request message.

7.5.11.

6.5.11.  413 Request Representation Payload Too Large

   The 413 (Payload Too Large) status code indicates that the server is
   refusing to process a request because the request
   representation payload is larger
   than the server is willing or able to process.  The server MAY close
   the connection to prevent the client from continuing the request.

   If the condition is temporary, the server SHOULD include generate a Retry-
   After header field to indicate that it is temporary and after what
   time the client MAY try again.

7.5.12.

6.5.12.  414 URI Too Long

   The 414 (URI Too Long) status code indicates that the server is
   refusing to service the request because the effective
   request URI request-target (Section
   5.3 of [Part1]) is longer than the server is willing to interpret.
   This rare condition is only likely to occur when a client has
   improperly converted a POST request to a GET request with long query
   information, when the client has descended into a URI "black hole" of
   redirection (e.g., a redirected URI prefix that points to a suffix of
   itself), or when the server is under attack by a client attempting to
   exploit potential security holes present in some servers using fixed-length
   buffers for reading or manipulating holes.

   A 414 response is cacheable unless otherwise indicated by the request-target.

7.5.13. method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.5.13.  415 Unsupported Media Type

   The 415 (Unsupported Media Type) status code indicates that the
   origin server is refusing to service the request because the request payload
   is in a format not supported by this request method on the target resource.

7.5.14. resource for this method.
   The format problem might be due to the request's indicated Content-
   Type or Content-Encoding, or as a result of inspecting the data
   directly.

6.5.14.  417 Expectation Failed

   The 417 (Expectation Failed) status code indicates that the
   expectation given in an the request's Expect header field (see Section 6.1.2)
   could not be met by this server, or, if the server is a proxy, the
   server has unambiguous evidence that the request
   (Section 5.1.1) could not be met by at least one of the next-hop server.

7.5.15. inbound
   servers.

6.5.15.  426 Upgrade Required

   The 426 (Upgrade Required) status code indicates that the server
   refuses to perform the request can not using the current protocol but might
   be completed without willing to do so after the client upgrades to a prior protocol upgrade.
   This response different
   protocol.  The server MUST include send an Upgrade header field (Section 6.3 of
   [Part1]) specifying in a 426
   response to indicate the required protocols. protocol(s) (Section 6.7 of
   [Part1]).

   Example:

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

     This service requires use of the HTTP/3.0 protocol.

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

7.6.

6.6.  Server Error 5xx

   Response

   The 5xx (Server Error) class of status codes beginning with the digit "5" indicate cases in
   which code indicates that the server
   is aware that it has erred or is incapable of performing the request.
   requested method.  Except when responding to a HEAD request, the
   server SHOULD include send a representation containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition.  User agents SHOULD display any included representation to
   the user.  These response codes are applicable to any request method.

7.6.1.

6.6.1.  500 Internal Server Error

   The 500 (Internal Server Error) status code indicates that the server
   encountered an unexpected condition which that prevented it from fulfilling
   the request.

7.6.2.

6.6.2.  501 Not Implemented

   The 501 (Not Implemented) status code indicates that the server does
   not support the functionality required to fulfill the request.  This
   is the appropriate response when the server does not recognize the
   request method and is not capable of supporting it for any resource.

7.6.3.

   A 501 response is cacheable unless otherwise indicated by the method
   definition or explicit cache controls (see Section 4.1.2 of [Part6]).

6.6.3.  502 Bad Gateway

   The 502 (Bad Gateway) status code indicates that the server, while
   acting as a gateway or proxy, received an invalid response from the upstream an
   inbound server it accessed in while attempting to fulfill the request.

7.6.4.

6.6.4.  503 Service Unavailable

   The 503 (Service Unavailable) status code indicates that the server
   is currently unable to handle the request due to a temporary overloading overload
   or maintenance of the server.

   The implication is that this is a temporary condition scheduled maintenance, which will likely be alleviated after some
   delay.  If known, the length of the delay  The server MAY
   be indicated in send a Retry-After header field
   (Section 8.1.3).  If no
   Retry-After is given, 7.1.3) to suggest an appropriate amount of time for the
   client SHOULD handle to wait before retrying the response as it
   would for a 500 (Internal Server Error) response. request.

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

7.6.5.

6.6.5.  504 Gateway Timeout

   The 504 (Gateway Timeout) status code indicates that the server,
   while acting as a gateway or proxy, did not receive a timely response
   from the an upstream server specified by the URI (e.g.,
   HTTP, FTP, LDAP) or some other auxiliary server (e.g., DNS) it needed to access in attempting order to complete the
   request.

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

7.6.6.

6.6.6.  505 HTTP Version Not Supported

   The 505 (HTTP Version Not Supported) status code indicates that the
   server does not support, or refuses to support, the protocol version
   that was used in the request message.  The server is indicating that
   it is unable or unwilling to complete the request using the same
   major version as the client, as described in Section 2.6 of [Part1],
   other than with this error message.  The response server SHOULD contain generate a
   representation describing for the 505 response that describes why that version
   is not supported and what other protocols are supported by that
   server.

8.

7.  Response Header Fields

   The response header fields allow the server to pass additional
   information about the response which cannot be beyond what is placed in the status-
   line.  These header fields give information about the server and server, about
   further access to the target resource (Section 5.5 resource, or about related resources.

   Although each response header field has a defined meaning, in
   general, the precise semantics might be further refined by the
   semantics of [Part1]).

8.1. the request method and/or response status code.

7.1.  Control Data

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

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | Age               | Section 7.1 of [Part6] |
   | Cache-Control     | Section 7.2 of [Part6] |
   | Expires           | Section 7.3 of [Part6] |
   | Date              | Section 8.1.1.2 7.1.1.2        |
   | Location          | Section 8.1.2 7.1.2          |
   | Retry-After       | Section 8.1.3 7.1.3          |
   | Vary              | Section 7.1.4          |
   | Warning           | Section 7.5 of [Part6] |
   +-------------------+------------------------+

8.1.1.

7.1.1.  Origination Date

8.1.1.1.

7.1.1.1.  Date/Time Formats

   HTTP applications have historically allowed

   Prior to 1995, there were three different formats
   for date/time stamps.  However, the commonly used by
   servers to communicate timestamps.  For compatibility with old
   implementations, all three are defined here.  The preferred format is
   a fixed-
   length fixed-length and single-zone subset of that defined the date and time
   specification used by [RFC1123]: the Internet Message Format [RFC5322].

     HTTP-date    = IMF-fixdate / obs-date

   An example of the preferred format is

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

   The other IMF-fixdate

   Examples of the two obsolete formats are described here only for compatibility with
   obsolete implementations.

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

   HTTP/1.1 clients and servers

   A recipient that parse parses a date timestamp value in an HTTP header field
   MUST accept all three formats (for compatibility with HTTP/1.0), though they formats.  A sender MUST
   only generate the RFC 1123 IMF-
   fixdate format for representing when sending an HTTP-date values value in a header fields.

   All HTTP date/time stamps MUST be represented in Greenwich Mean Time
   (GMT), without exception.  For the purposes field.

   An HTTP-date value represents time as an instance of HTTP, GMT is exactly
   equal to UTC (Coordinated Coordinated
   Universal Time).  This is indicated in the Time (UTC).  The first two formats indicate UTC by the inclusion of "GMT" as the
   three-letter abbreviation for time zone, and MUST be assumed when reading Greenwich Mean Time, "GMT", a
   predecessor of the
   asctime format.  HTTP-date is case sensitive and MUST NOT include
   additional whitespace beyond that specifically included as SP UTC name; values in the
   grammar. asctime format are assumed
   to be in UTC.  A sender that generates HTTP-date    = rfc1123-date / obs-date values from a local
   clock ought to use NTP ([RFC1305]) or some similar protocol to
   synchronize its clock to UTC.

   Preferred format:

     rfc1123-date

     IMF-fixdate  = day-name "," SP date1 SP time-of-day SP GMT
     ; fixed length length/zone subset of the format defined in
     ; Section 5.2.14 3.3 of [RFC1123] [RFC5322]

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

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

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

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

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

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

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

   Obsolete formats:

     obs-date     = rfc850-date / asctime-date

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

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

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

      Note: Recipients of date  2

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

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

   Recipients of timestamp values are encouraged to be robust in
      accepting date values parsing
   timestamps unless otherwise restricted by the field definition.  For
   example, messages are occasionally forwarded over HTTP from a non-
   HTTP source that might have been sent generate any of the date and time
   specifications defined by non-HTTP
      applications, as is sometimes the case when retrieving or posting
      messages via proxies/gateways to SMTP or NNTP. Internet Message Format.

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

8.1.1.2.

7.1.1.2.  Date

   The "Date" header field represents the date and time at which the
   message was originated, having the same semantics as the Origination
   Date Field (orig-date) defined in Section 3.6.1 of [RFC5322].  The
   field value is an HTTP-date, as defined in Section 8.1.1.1; it MUST
   be sent in rfc1123-date format. 7.1.1.1.

     Date = HTTP-date

   An example is

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

   Origin servers MUST include

   When a Date header field in all responses,
   except in these cases:

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

   2.  If best available approximation of the response status code conveys a server error, e.g., 500
       (Internal Server Error) or 503 (Service Unavailable), date and it is
       inconvenient or impossible time
   of message generation.  In theory, the date ought to generate a valid Date.

   3.  If represent the
   moment just before the payload is generated.  In practice, the date
   can be generated at any time during message origination.

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

   A received message recipient with a clock that does not have receives a response message without a
   Date header field MUST be
   assigned one by the recipient if the message will be cached by that
   recipient.

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

   The HTTP-date sent in cached or forwarded downstream.

   A user agent MAY send a Date header field SHOULD NOT represent in a date
   and time subsequent to the generation of the message.  It SHOULD
   represent the best available approximation of the date and time of
   message generation, request, though
   generally will not do so unless it is believed to convey useful
   information to the implementation has no means server.  For example, custom applications of
   generating HTTP
   might convey a reasonably accurate date and time.  In theory, the date
   ought to represent the moment just before Date if the payload server is generated.
   In practice, expected to adjust its
   interpretation of the date can be generated at any time during user's request based on differences between the message
   origination without affecting its semantic value.

8.1.2.
   user agent and server clocks.

7.1.2.  Location

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

     Location = URI-reference

   For 201 (Created) responses, the Location is the URI of the new
   resource which was created by the request.  For 3xx (Redirection)
   responses, the location SHOULD indicate the server's preferred URI
   for automatic redirection to the resource.

   The field value consists of a single URI-reference.  When it has the
   form of a relative reference ([RFC3986], Section 4.2), the final
   value is computed by resolving it against the effective request URI
   ([RFC3986], Section 5).  If

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

   When Location is provided in a 3xx (Redirection) response and the URI
   reference that the user agent, did contain agent used to generate the request target
   contains a fragment identifier, and the final user agent SHOULD process the
   redirection as if the Location field value
   does not, then inherits the original URI's
   fragment.  In other words, if the Location does not have a fragment identifier is added to
   component, the
   final value. user agent SHOULD interpret the Location reference as
   if it had the original reference's fragment.

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

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

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

   An original URI "http://www.example.org/index.html#larry", combined
   with a field value given as: 303 (See Other)
   response containing the header field:

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

   would /People.html#tim

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

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

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

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

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

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

      Note: Some recipients attempt to recover from Location header field specifies the fields that
      are not valid URI references.  This specification does not mandate
      or define such processing, but does allow it for the entire created resource. sake of
      robustness.

      Note: The Content-Location header field (Section 3.1.4.2) differs
      from Location in that the Content-Location identifies refers to the most
      specific resource corresponding to the enclosed representation.
      It is therefore possible for a response to contain header fields
      for both the
      Location and Content-Location.

8.1.3.  Retry-After

   The Content-Location header fields.

7.1.3.  Retry-After

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

   The value of this field can be either an HTTP-date or an integer
   number of seconds (in decimal) after the time of the response.

     Retry-After = HTTP-date / delta-seconds

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

     delta-seconds  = 1*DIGIT

   Two examples of its use are

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

   In the latter example, the delay is 2 minutes.

8.2.  Selected Representation Header Fields

   We use the term "selected representation" to refer to the the current
   representation

7.1.4.  Vary

   The "Vary" header field describes what parts of a target resource that would have been selected in
   a successful response if the same request had used message,
   aside from the method GET and
   excluded any conditional request header fields.

   Additional header fields define metadata about the selected
   representation, which target, might differ from the representation included
   in influence the message origin
   server's process for responses to some state-changing methods. selecting and representing the response.  The
   following header fields are defined as selected representation
   metadata:

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | ETag              | Section 2.3
   value consists of [Part4] |
   | Last-Modified     | Section 2.2 either a single asterisk ("*") or a list of [Part4] |
   | header
   field names (case-insensitive).

     Vary              | Section 8.2.1          |
   +-------------------+------------------------+

8.2.1. = "*" / 1#field-name

   A Vary

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

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

   In uncacheable or stale responses, the Vary field value advises the
   user agent anything about the criteria that were used to select request
   might play a role in selecting the
   representation.

     Vary = "*" / 1#field-name

   The set of header fields named by response representation, possibly
   including elements outside the Vary field value is known as message syntax (e.g., the selecting header fields.

   A server SHOULD include client's
   network address), and thus a Vary header field with any cacheable recipient will not be able to determine
   whether this response that is subject to proactive negotiation.  Doing so allows appropriate for a
   cache to properly interpret future requests on that resource and
   informs later request without
   forwarding the user agent about request to the presence of negotiation on that
   resource. origin server.  A server MAY include proxy MUST NOT
   generate a Vary header field with a non-
   cacheable response that is subject to proactive negotiation, since
   this might provide the user agent with useful information about the
   dimensions over which the response varies at the time of the
   response. "*" value.

   A Vary field value consisting of "*" signals a comma-separated list of names
   indicates that unspecified parameters not
   limited to the named request header fields (e.g., the network address of fields, known as the
   client), play
   selecting header fields, might have a role in selecting the selection of the response representation;
   therefore, a cache cannot determine whether this response is
   appropriate.  A proxy MUST NOT generate the "*" value.
   representation.  The field-names given potential selecting header fields are not
   limited to the set of standard header
   fields those defined by this specification.  Field names are case-
   insensitive.

8.3.  Authentication Challenges

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

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

8.4.  Informative

   The remaining

   For example, a response header fields provide more information about that contains

     Vary: accept-encoding, accept-language

   indicates that the target resource for potential use in later requests.

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

8.4.1.  Allow

   The "Allow" header field lists origin server might have used the set of methods advertised request's
   Accept-Encoding and Accept-Language fields (or lack thereof) as
   supported by
   determining factors while choosing the target resource.  The purpose content for this response.

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

   1.  To inform cache recipients that they MUST NOT use this field is
   strictly response
       to inform satisfy a later request unless the recipient of valid later request methods associated
   with has the resource.

     Allow = #method

   Example of use:

     Allow: GET, HEAD, PUT

   The actual set of allowed methods is defined by same
       values for the origin server at listed fields as the time original request (Section 4.3
       of each request.

   A proxy MUST NOT modify the Allow header field -- it does not need to
   understand all [Part6]).  In other words, Vary expands the methods specified in order cache key required
       to handle them
   according match a new request to the generic message handling rules.

8.4.2.  Server

   The "Server" stored cache entry.

   2.  To inform user agent recipients that this response is subject to
       content negotiation (Section 5.3) and that a different
       representation might be sent in a subsequent request if
       additional parameters are provided in the listed header fields
       (proactive negotiation).

   An origin server SHOULD send a Vary header field contains information about when its algorithm
   for selecting a representation varies based on aspects of the software
   used by request
   message other than the method and request target, unless the variance
   cannot be crossed or the origin server has been deliberately
   configured to handle prevent cache transparency.  For example, there is no
   need to send the request.

   The Authorization field can contain multiple product tokens name in Vary because reuse
   across users is constrained by the field definition (Section 4) and
   comments 4.1 of
   [Part7]).  Likewise, an origin server might use Cache-Control
   directives (Section 3.2 7.2 of [Part1]) identifying [Part6]) to supplant Vary if it considers
   the server and any variance less significant subproducts.  The product tokens are listed in order than the performance cost of
   their significance for identifying Vary's
   impact on caching.

7.2.  Validator Header Fields

   Validator header fields convey metadata about the application.

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

   Example:

     Server: CERN/3.0 libwww/2.17

   If selected
   representation (Section 3).  In responses to safe requests, validator
   fields describe the response is being forwarded through a proxy, selected representation chosen by the proxy
   application MUST NOT modify origin
   server while handling the Server header field.  Instead, it
   MUST include a Via field (as described in Section 5.7 of [Part1]).

      Note: Revealing response.  Note that, depending on the specific software version of
   status code semantics, the server might
      allow selected representation for a given
   response is not necessarily the server machine to become more vulnerable same as the representation enclosed
   as response payload.

   In a successful response to attacks
      against software a state-changing request, validator
   fields describe the new representation that is known to contain security holes.  Server
      implementers are encouraged to make this field has replaced the prior
   selected representation as a configurable
      option.

9.  IANA Considerations

9.1.  Method Registry

   The HTTP Method Registry defines result of processing the name space for request.

   For example, an ETag header field in a 201 response communicates the request
   method token (Section 5).  The method registry is maintained at
   <http://www.iana.org/assignments/http-methods>.

9.1.1.  Procedure

   HTTP method registrations MUST include
   entity-tag of the following fields:

   o  Method Name (see Section 5)

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

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

   o  Pointer to specification text

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

9.1.2.  Considerations for New Methods

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

   Since message parsing (Section 3.3 of [Part1]) needs to be
   independent of method semantics (aside from responses later conditional requests to HEAD),
   definitions of new methods cannot change the parsing algorithm or
   prohibit prevent the presence "lost
   update" problem [Part4].

   +-------------------+------------------------+
   | Header Field Name | Defined in...          |
   +-------------------+------------------------+
   | ETag              | Section 2.3 of a message body on either the request or the
   response message.  Definitions of new methods can specify that only a
   zero-length message body is allowed by requiring a Content-Length
   header field with a value [Part4] |
   | Last-Modified     | Section 2.2 of "0".

   New method definitions need to [Part4] |
   +-------------------+------------------------+

7.3.  Authentication Challenges

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

9.1.3.  Registrations

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

   +---------+------+------------+---------------+
   | Method  | Safe | Idempotent | Reference     |
   +---------+------+------------+---------------+
   | CONNECT | no   | no         | Section 5.3.6 |
   | DELETE  | no   | yes        | Section 5.3.5 provide authentication credentials in future requests.

   +--------------------+------------------------+
   | Header Field Name  | GET Defined in...          | yes
   +--------------------+------------------------+
   | yes WWW-Authenticate   | Section 5.3.1 |
   | HEAD 4.4 of [Part7] | yes
   | yes Proxy-Authenticate | Section 5.3.2 |
   | OPTIONS | yes  | yes 4.2 of [Part7] | Section 5.3.7
   +--------------------+------------------------+

7.4.  Response Context

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

   +-------------------+------------------------+
   | Header Field Name | POST Defined in...          | no
   +-------------------+------------------------+
   | no Accept-Ranges     | Section 5.3.3 |
   | PUT 2.3 of [Part5] | no
   | yes Allow             | Section 5.3.4 |
   | TRACE 7.4.1          | yes
   | yes Server            | Section 5.3.8 7.4.2          |
   +---------+------+------------+---------------+

9.2.  Status Code Registry
   +-------------------+------------------------+

7.4.1.  Allow

   The HTTP Status Code Registry defines "Allow" header field lists the name space for set of methods advertised as
   supported by the response
   status-code token (Section 7). target resource.  The status code registry purpose of this field is
   maintained at <http://www.iana.org/assignments/http-status-codes>.

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

9.2.1.  Procedure

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

9.2.2.  Considerations for New Status Codes

   When it resource.

     Allow = #method

   Example of use:

     Allow: GET, HEAD, PUT

   The actual set of allowed methods is necessary to express semantics for a response that are not defined by current status codes, a new status code can be registered.
   HTTP status codes are generic; they are potentially applicable to any
   resource, not just one particular media type, "type" the origin server at
   the time of resource, or
   application.  As such, it is preferred that new status codes be
   registered each request.  An origin server MUST generate an Allow
   field in a document 405 (Method Not Allowed) response and MAY do so in any
   other response.  An empty Allow field value indicates that isn't specific to the
   resource allows no methods, which might occur in a single application.

   New status codes are required 405 response if
   the resource has been temporarily disabled by configuration.

   A proxy MUST NOT modify the Allow header field -- it does not need to fall under one
   understand all of the categories
   defined indicated methods in Section 7.  To allow existing parsers order to properly handle
   them, new status codes cannot disallow a payload, although they can
   mandate a zero-length payload body.

   A definition for a new status code ought them
   according to explain the request
   conditions that produce generic message handling rules.

7.4.2.  Server

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

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

   The Server field-value consists of request header fields and/or method(s)) along with
   any dependencies on response header fields (e.g., what fields are
   required one or more product identifiers,
   each followed by zero or more comments (Section 3.2 of [Part1]),
   which together identify the origin server software and what fields can modify its
   significant subproducts.  By convention, the semantics).  A response that
   can transfer product identifiers are
   listed in decreasing order of their significance for identifying the
   origin server software.  Each product identifier consists of a payload ought to specify expected cache behavior
   (e.g., cacheability name
   and freshness criteria, optional version, as described defined in [Part6]) Section 5.5.3.

   Example:

     Server: CERN/3.0 libwww/2.17

   An origin server SHOULD NOT generate a Server field containing
   needlessly fine-grained detail and whether SHOULD limit the payload has any implied association with an
   identified resource (Section 3.1.4.1).

9.2.3.  Registrations addition of
   subproducts by third parties.  Overly long and detailed Server field
   values increase response latency and potentially reveal internal
   implementation details that might make it (slightly) easier for
   attackers to find and exploit known security holes.

8.  IANA Considerations

8.1.  Method Registry

   The HTTP Status Code Method Registry shall be updated with defines the name space for the request
   method token (Section 4).  The method registry is maintained at
   <http://www.iana.org/assignments/http-methods>.

8.1.1.  Procedure

   HTTP method registrations
   below:

   +-------+----------------------------------+----------------+
   | Value | Description                      | Reference      |
   +-------+----------------------------------+----------------+
   | 100   | Continue                         | Section 7.2.1  |
   | 101   | Switching Protocols              | Section 7.2.2  |
   | 200   | OK                               | Section 7.3.1  |
   | 201   | Created                          | Section 7.3.2  |
   | 202   | Accepted                         | Section 7.3.3  |
   | 203   | Non-Authoritative Information    | Section 7.3.4  |
   | 204   | No Content                       | Section 7.3.5  |
   | 205   | Reset Content                    | Section 7.3.6  |
   | 300   | Multiple Choices                 | Section 7.4.1  |
   | 301   | Moved Permanently                | Section 7.4.2  |
   | 302   | Found                            | Section 7.4.3  |
   | 303   | See Other                        | Section 7.4.4  |
   | 305   | Use Proxy                        | Section 7.4.5  |
   | 306   | (Unused)                         | Section 7.4.6  |
   | 307   | Temporary Redirect               | Section 7.4.7  |
   | 400   | Bad Request                      | Section 7.5.1  |
   | 402   | Payment Required                 | Section 7.5.2  |
   | 403   | Forbidden                        | Section 7.5.3  |
   | 404   | Not Found                        | Section 7.5.4  |
   | 405   | MUST include the following fields:

   o  Method Not Allowed               | Section 7.5.5  |
   | 406   | Not Acceptable                   | Name (see Section 7.5.6  |
   | 408   | Request Timeout                  | 4)

   o  Safe ("yes" or "no", see Section 7.5.7  |
   | 409   | Conflict                         | 4.2.1)

   o  Idempotent ("yes" or "no", see Section 7.5.8  |
   | 410   | Gone                             | 4.2.2)

   o  Pointer to specification text

   Values to be added to this name space require IETF Review (see
   [RFC5226], Section 7.5.9  |
   | 411   | Length Required                  | Section 7.5.10 |
   | 413   | Request Representation Too Large | Section 7.5.11 |
   | 414   | URI Too Long                     | Section 7.5.12 |
   | 415   | Unsupported Media Type           | Section 7.5.13 |
   | 417   | Expectation Failed               | Section 7.5.14 |
   | 426   | Upgrade Required                 | Section 7.5.15 |
   | 500   | Internal Server Error            | Section 7.6.1  |
   | 501   | Not Implemented                  | Section 7.6.2  |
   | 502   | Bad Gateway                      | Section 7.6.3  |
   | 503   | Service Unavailable              | Section 7.6.4  |
   | 504   | Gateway Timeout                  | Section 7.6.5  |
   | 505   | HTTP Version Not Supported       | Section 7.6.6  |
   +-------+----------------------------------+----------------+

9.3.  Header Field Registry

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

9.3.1. 4.1).

8.1.2.  Considerations for New Header Fields

   Header fields Methods

   Standardized methods are key:value pairs generic; that can is, they are potentially
   applicable to any resource, not just one particular media type, kind
   of resource, or application.  As such, it is preferred that new
   methods be used registered in a document that isn't specific to communicate a single
   application or data about the message, its payload, format, since orthogonal technologies deserve
   orthogonal specification.

   Since message parsing (Section 3.3 of [Part1]) needs to be
   independent of method semantics (aside from responses to HEAD),
   definitions of new methods cannot change the target resource, parsing algorithm or
   prohibit the
   connection (i.e., control data).  See Section 3.2 presence of [Part1] for a
   general definition message body on either the request or the
   response message.  Definitions of new methods can specify that only a
   zero-length message body is allowed by requiring a Content-Length
   header field syntax in HTTP messages.

   The requirements for header field names are defined in Section 4.1 of
   [RFC3864].  Authors with a value of specifications defining "0".

   A new fields are advised
   to keep the name as short as practical, and not method definition needs to prefix them with
   "X-" if they indicate whether it is safe
   (Section 4.2.1), idempotent (Section 4.2.2), cacheable
   (Section 4.2.3), what semantics are to be registered (either immediately or associated with the payload
   body if any is present in the
   future).

   New request, and what refinements the
   method makes to header field values typically have their syntax defined using
   ABNF ([RFC5234]), using or status code semantics.  If the extension defined in Appendix B of
   [Part1] as necessary, and are usually constrained new
   method is cacheable, its definition ought to the range of
   ASCII characters.  Header fields needing describe how, and under
   what conditions, a greater range of
   characters cache can store a response and use an encoding such as the one defined in [RFC5987].

   Because commas (",") are used as it to satisfy a generic delimiter between field-
   values, they need
   subsequent request.  The new method ought to describe whether it can
   be treated with care made conditional (Section 5.2) and, if they are allowed in the
   field-value's payload.  Typically, components that might contain so, how a
   comma are protected with double-quotes using server responds
   when the quoted-string ABNF
   production (Section 3.2.4 of [Part1]).

   For example, a textual date and a URI (either of which condition is false.  Likewise, if the new method might contain
   a comma) could have
   some use for partial response semantics ([Part5]), it ought to
   document this too.

8.1.3.  Registrations

   The HTTP Method Registry shall be safely carried in field-values like these:

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

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

   Many header fields use a format including (case-insensitively) named
   parameters (for instance, Content-Type, defined in registrations
   below:

   +---------+------+------------+---------------+
   | Method  | Safe | Idempotent | Reference     |
   +---------+------+------------+---------------+
   | CONNECT | no   | no         | Section 3.1.1.5).
   Allowing both unquoted (token) and quoted (quoted-string) syntax 4.3.6 |
   | DELETE  | no   | yes        | Section 4.3.5 |
   | GET     | yes  | yes        | Section 4.3.1 |
   | HEAD    | yes  | yes        | Section 4.3.2 |
   | OPTIONS | yes  | yes        | Section 4.3.7 |
   | POST    | no   | no         | Section 4.3.3 |
   | PUT     | no   | yes        | Section 4.3.4 |
   | TRACE   | yes  | yes        | Section 4.3.8 |
   +---------+------+------------+---------------+

8.2.  Status Code Registry

   The HTTP Status Code Registry defines the name space for the parameter value enables recipients to use existing parser
   components.  When allowing both forms, response
   status-code token (Section 6).  The status code registry is
   maintained at <http://www.iana.org/assignments/http-status-codes>.

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

8.2.1.  Procedure

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

8.2.2.  Considerations for New Status Codes

   When it (for an
   example, see the notes on parameter handling is necessary to express semantics for media types in
   Section 3.1.1.1).

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

   o  Whether the field is not
   defined by current status codes, a single value, or whether it new status code can be a list
      (delimited by commas; see Section 3.2 of [Part1]).

      If it does not use the list syntax, document how to treat messages
      where the header field occurs multiple times (a sensible default
      would be registered.

   Status codes are generic; they are potentially applicable to ignore the header field, but this might any
   resource, not always just one particular media type, kind of resource, or
   application of HTTP.  As such, it is preferred that new status codes
   be
      the right choice).

      Note registered in a document that intermediaries and software libraries might combine
      multiple header field instances into isn't specific to a single one, despite
   application.

   New status codes are required to fall under one of the
      header field not allowing this.  A robust format enables
      recipients categories
   defined in Section 6.  To allow existing parsers to discover these situations (good example: "Content-
      Type", as process the comma can only appear inside quoted strings; bad
      example: "Location", as
   response message, new status codes cannot disallow a comma payload,
   although they can occur inside mandate a URI).

   o  Under what conditions the header field can be used; e.g., only in
      responses or requests, in all messages, only on responses zero-length payload body.

   Proposals for new status codes that are not yet widely deployed ought
   to avoid allocating a
      particular request method.

   o  Whether it specific number for the code until there is appropriate
   clear consensus that it will be registered; instead, early drafts can
   use a notation such as "4NN", or "3N0" .. "3N9", to list the field-name in indicate the Connection
      header field (i.e., if
   class of the header field is proposed status code(s) without consuming a number
   prematurely.

   The definition of a new status code ought to be hop-by-hop, see
      Section 6.1 explain the request
   conditions that would cause a response containing that status code
   (e.g., combinations of [Part1]).

   o  Under request header fields and/or method(s)) along
   with any dependencies on response header fields (e.g., what conditions intermediaries fields
   are allowed to required, what fields can modify the
      header field's value, insert or delete it.

   o  How the semantics, and what header
   field might interact semantics are further refined when used with caching (see [Part6]).

   o  Whether the header field is useful new status
   code).

   The definition of a new status code ought to specify whether or allowable not
   it is cacheable.  Note that all status codes can be cached if the
   response they occur in trailers (see
      Section 4.1 has explicit freshness information; however,
   status codes that are defined as being cacheable are allowed to be
   cached without explicit freshness information.  Likewise, the
   definition of [Part1]).

   o  Whether a status code can place constraints upon cache
   behaviour.  See [Part6] for more information.

   Finally, the header field definition of a new status code ought to be preserved across redirects.

9.3.2. indicate
   whether the payload has any implied association with an identified
   resource (Section 3.1.4.1).

8.2.3.  Registrations

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

   +-------------------+----------+----------+-----------------+
   | Header Field Name registrations
   below:

   +-------+-------------------------------+----------------+
   | Protocol Value | Status Description                   | Reference      |
   +-------------------+----------+----------+-----------------+
   | Accept
   +-------+-------------------------------+----------------+
   | http 100   | standard Continue                      | Section 6.3.2   | 6.2.1  | Accept-Charset
   | http 101   | standard Switching Protocols           | Section 6.3.3   | 6.2.2  | Accept-Encoding
   | http 200   | standard OK                            | Section 6.3.4 6.3.1  |
   | Accept-Language   | http 201   | standard Created                       | Section 6.3.5   | 6.3.2  | Allow
   | http 202   | standard Accepted                      | Section 8.4.1 6.3.3  |
   | Content-Encoding 203   | http Non-Authoritative Information | standard Section 6.3.4  |
   | 204   | No Content                    | Section 3.1.2.2 6.3.5  |
   | Content-Language 205   | http Reset Content                 | standard Section 6.3.6  |
   | 300   | Multiple Choices              | Section 3.1.3.2 6.4.1  |
   | Content-Location 301   | http Moved Permanently             | standard Section 6.4.2  |
   | 302   | Found                         | Section 3.1.4.2 6.4.3  |
   | Content-Type 303   | http See Other                     | standard Section 6.4.4  |
   | 305   | Use Proxy                     | Section 3.1.1.5 6.4.5  |
   | Date 306   | http (Unused)                      | standard Section 6.4.6  |
   | 307   | Temporary Redirect            | Section 8.1.1.2 6.4.7  |
   | Expect 400   | http Bad Request                   | standard Section 6.5.1  |
   | 402   | Payment Required              | Section 6.1.2 6.5.2  |
   | From 403   | http Forbidden                     | standard Section 6.5.3  |
   | 404   | Not Found                     | Section 6.5.1 6.5.4  |
   | Location 405   | http Method Not Allowed            | standard Section 6.5.5  |
   | 406   | Not Acceptable                | Section 8.1.2 6.5.6  |
   | MIME-Version 408   | http Request Timeout               | standard Section 6.5.7  | Appendix A.1
   | 409   | Max-Forwards Conflict                      | http Section 6.5.8  | standard
   | 410   | Gone                          | Section 6.1.1 6.5.9  |
   | Referer 411   | http Length Required               | standard Section 6.5.10 |
   | 413   | Payload Too Large             | Section 6.5.2 6.5.11 |
   | Retry-After 414   | http URI Too Long                  | standard Section 6.5.12 |
   | 415   | Unsupported Media Type        | Section 6.5.13 |
   | 417   | Expectation Failed            | Section 6.5.14 |
   | 426   | Upgrade Required              | Section 8.1.3 6.5.15 |
   | 500   | Internal Server Error         | http Section 6.6.1  | standard
   | 501   | Not Implemented               | Section 8.4.2 6.6.2  |
   | User-Agent 502   | http Bad Gateway                   | Section 6.6.3  | standard
   | 503   | Service Unavailable           | Section 6.5.3 6.6.4  |
   | Vary 504   | http Gateway Timeout               | standard Section 6.6.5  |
   | 505   | HTTP Version Not Supported    | Section 8.2.1 6.6.6  |
   +-------------------+----------+----------+-----------------+

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

9.4.  Content Coding
   +-------+-------------------------------+----------------+

8.3.  Header Field Registry

   The

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

9.4.1.  Procedure

   Content Coding registrations MUST include the following fields:

   o  Name

   o  Description

   o  Pointer <http://www.iana.org/assignments/message-headers/
   message-header-index.html>, as defined by [BCP90].

8.3.1.  Considerations for New Header Fields

   Header fields are key:value pairs that can be used to specification text

   Names of content codings MUST NOT overlap with names of transfer
   codings (Section 4 of [Part1]), unless communicate
   data about the encoding transformation is
   identical (as is message, its payload, the case for target resource, or the compression codings
   connection (i.e., control data).  See Section 3.2 of [Part1] for a
   general definition of header field syntax in HTTP messages.

   The requirements for header field names are defined in
   Section 4.2 [BCP90].
   Authors of [Part1]).

   Values to be added specifications defining new fields are advised to this keep the
   name space require IETF Review (see
   Section 4.1 of [RFC5226]), as short as practical and MUST conform to not prefix the purpose of content
   coding defined name with "X-"
   unless the header field will never be used on the Internet.  (The
   "x-" prefix idiom has been extensively misused in this section.

9.4.2.  Registrations

   The HTTP Content Codings Registry shall practice; it was
   intended to only be updated used as a mechanism for avoiding name collisions
   inside proprietary software or intranet processing, since the prefix
   would ensure that private names never collide with a newly registered
   Internet name.)

   New header field values typically have their syntax defined using
   ABNF ([RFC5234]), using the
   registrations below:

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

10.  Security Considerations

   This section is meant to inform application developers, information
   providers, necessary, and users of the security limitations in HTTP/1.1 as
   described by this document.  The discussion does not include
   definitive solutions are usually constrained to the problems revealed, though it does make
   some suggestions for reducing security risks.

10.1.  Transfer range of Sensitive Information

   Like any generic data transfer protocol, HTTP cannot regulate the
   content
   ASCII characters.  Header fields needing a greater range of
   characters can use an encoding such as the data that one defined in [RFC5987].

   Leading and trailing whitespace in raw field values is transferred, nor removed upon
   field parsing (Section 3.2.4 of [Part1]).  Field definitions where
   leading or trailing whitespace in values is there any significant will have to
   use a priori
   method of determining the sensitivity of any particular piece of
   information within the context of any given request.  Therefore,
   applications SHOULD supply container syntax such as much control over this information quoted-string.

   Because commas (",") are used as
   possible a generic delimiter between field-
   values, they need to the provider of that information.  Four header fields be treated with care if they are
   worth special mention allowed in this context: Server, Via, Referer and From.

   Revealing the specific software version of the server might allow the
   server machine to become more vulnerable to attacks against software
   field-value.  Typically, components that is known to might contain security holes.  Implementers SHOULD make
   the Server header field a configurable option.

   Proxies which serve as comma are
   protected with double-quotes using the quoted-string ABNF production
   (Section 3.2.6 of [Part1]).

   For example, a portal through textual date and a network firewall SHOULD
   take special precautions regarding the transfer URI (either of header information which might contain
   a comma) could be safely carried in field-values like these:

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

   Note that identifies the hosts behind the firewall.  In particular, they
   SHOULD remove, or replace double-quote delimiters almost always are used with sanitized versions, any Via fields
   generated behind the firewall.

   The Referer
   quoted-string production; using a different syntax inside double-
   quotes will likely cause unnecessary confusion.

   Many header field allows reading patterns to be studied and
   reverse links drawn.  Although it can be very useful, its power can
   be abused if user details are not separated from the information
   contained fields use a format including (case-insensitively) named
   parameters (for instance, Content-Type, defined in Section 3.1.1.5).
   Allowing both unquoted (token) and quoted (quoted-string) syntax for
   the Referer.  Even when the personal information has
   been removed, parameter value enables recipients to use existing parser
   components.  When allowing both forms, the Referer header field might indicate meaning of a private
   document's URI whose publication would parameter
   value ought to be inappropriate.

   The information sent independent of the syntax used for it (for an
   example, see the notes on parameter handling for media types in
   Section 3.1.1.1).

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

   o  Whether the From field might conflict with the user's
   privacy interests is a single value, or their site's security policy, and hence whether it
   SHOULD NOT can be transmitted without the user being able to disable,
   enable, and modify the contents a list
      (delimited by commas; see Section 3.2 of [Part1]).

      If it does not use the field.  The user MUST list syntax, document how to treat messages
      where the field occurs multiple times (a sensible default would be able
      to set ignore the contents of field, but this field within a user preference or
   application defaults configuration.

   We suggest, though do might not require, always be the right
      choice).

      Note that intermediaries and software libraries might combine
      multiple header field instances into a convenient toggle interface
   be provided for single one, despite the user
      field's definition not allowing the list syntax.  A robust format
      enables recipients to enable or disable discover these situations (good example:
      "Content-Type", as the sending of From and
   Referer information.

   The User-Agent (Section 6.5.3) or Server (Section 8.4.2) header
   fields comma can sometimes be used to determine that only appear inside quoted
      strings; bad example: "Location", as a specific client or
   server has comma can occur inside a particular security hole which might be exploited.
   Unfortunately, this same information is often used for other valuable
   purposes for which HTTP currently has no better mechanism.

   Furthermore,
      URI).

   o  Under what conditions the User-Agent header field might contain enough entropy
   to can be used, possibly used; e.g., only in conjunction with other material,
      responses or requests, in all messages, only on responses to uniquely
   identify a
      particular request method, etc.

   o  Whether the user.

   Some field semantics are further refined by the context,
      such as by existing request methods, like TRACE (Section 5.3.8), expose information
   that was sent methods or status codes.

   o  Whether it is appropriate to list the field-name in request the Connection
      header fields within field (i.e., if the body of their
   response.  Clients SHOULD header field is to be careful with sensitive information, like
   Cookies, Authorization credentials, and other header fields that
   might be used to collect data from the client.

10.2.  Encoding Sensitive Information in URIs

   Because the source hop-by-hop; see
      Section 6.1 of a link might be private information or might
   reveal an otherwise private information source, it is strongly
   recommended that the user be able [Part1]).

   o  Under what conditions intermediaries are allowed to select whether insert,
      delete, or not modify the
   Referer field is sent.  For example, a browser client could have a
   toggle switch for browsing openly/anonymously, which would
   respectively enable/disable field's value.

   o  How the sending of Referer and From
   information.

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

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

10.3.  Location Header Fields: Spoofing and Information Leakage

   If a single server supports multiple organizations that do not trust
   one another, then it MUST check interact with caching (see [Part6]).

   o  Whether the values of Location and Content-
   Location header fields field is useful or allowable in responses that are generated under control trailers (see
      Section 4.1 of said organizations to make sure that they do not attempt to
   invalidate resources over which they have no authority.

   Furthermore, appending [Part1]).

   o  Whether the fragment identifier from one URI to
   another one obtained from a Location header field might leak
   confidential information ought to be preserved across redirects.

8.3.2.  Registrations

   The Message Header Field Registry shall be updated with the target server -- although the
   fragment identifier is not transmitted in the final request, it might
   be visible to the user agent through other means, such as scripting.

10.4.  Security Considerations for CONNECT

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

10.5.  Privacy Issues Connected to Accept following
   permanent registrations:

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

   An approach that limits the loss of privacy would be for a user agent
   to omit the sending of Accept-Language header fields by default, and
   to ask the user whether or not to start sending            | http     | standard | Section 5.3.2   |
   | Accept-Charset    | http     | standard | Section 5.3.3   |
   | Accept-Encoding   | http     | standard | Section 5.3.4   |
   | Accept-Language
   header fields to a server if it detects, by looking for any Vary
   header fields generated by   | http     | standard | Section 5.3.5   |
   | Allow             | http     | standard | Section 7.4.1   |
   | Content-Encoding  | http     | standard | Section 3.1.2.2 |
   | Content-Language  | http     | standard | Section 3.1.3.2 |
   | Content-Location  | http     | standard | Section 3.1.4.2 |
   | Content-Type      | http     | standard | Section 3.1.1.5 |
   | Date              | http     | standard | Section 7.1.1.2 |
   | Expect            | http     | standard | Section 5.1.1   |
   | From              | http     | standard | Section 5.5.1   |
   | Location          | http     | standard | Section 7.1.2   |
   | MIME-Version      | http     | standard | Appendix A.1    |
   | Max-Forwards      | http     | standard | Section 5.1.2   |
   | Referer           | http     | standard | Section 5.5.2   |
   | Retry-After       | http     | standard | Section 7.1.3   |
   | Server            | http     | standard | Section 7.4.2   |
   | User-Agent        | http     | standard | Section 5.5.3   |
   | Vary              | http     | standard | Section 7.1.4   |
   +-------------------+----------+----------+-----------------+

   The change controller for the server, that such sending could
   improve above registrations is: "IETF
   (iesg@ietf.org) - Internet Engineering Task Force".

8.4.  Content Coding Registry

   The HTTP Content Coding Registry defines the quality name space for content
   coding names (Section 4.2 of service.

   Elaborate user-customized accept header fields sent in every request,
   in particular if these include quality values, can be used by servers
   as relatively reliable and long-lived user identifiers.  Such user
   identifiers would allow [Part1]).  The content providers coding registry
   is maintained at <http://www.iana.org/assignments/http-parameters>.

8.4.1.  Procedure

   Content Coding registrations MUST include the following fields:

   o  Name

   o  Description
   o  Pointer to do click-trail tracking,
   and would allow collaborating specification text

   Names of content providers to match cross-server
   click-trails or form submissions codings MUST NOT overlap with names of individual users.  Note that for
   many users not behind a proxy, the network address transfer
   codings (Section 4 of [Part1]), unless the host
   running encoding transformation is
   identical (as is the user agent will also serve as a long-lived user
   identifier.  In environments where proxies are used to enhance
   privacy, user agents ought case for the compression codings defined in
   Section 4.2 of [Part1]).

   Values to be conservative in offering accept
   header field configuration options added to this name space require IETF Review (see
   Section 4.1 of [RFC5226]), and MUST conform to end users.  As an extreme
   privacy measure, proxies could filter the accept header fields in
   relayed requests.  General purpose user agents which provide a high
   degree of header field configurability SHOULD warn users about content
   coding defined in this section.

8.4.2.  Registrations

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

   +----------+----------------------------------------+---------------+
   | Name     | Description                            | Reference     |
   +----------+----------------------------------------+---------------+
   | compress | UNIX "compress" program method         | Section 4.2.1 |
   |          |                                        | of privacy which can be involved.

11.  Acknowledgments

   See [Part1]    |
   | deflate  | "deflate" compression mechanism        | Section 9 4.2.2 |
   |          | ([RFC1951]) used inside the "zlib"     | of [Part1].

12.  References

12.1.  Normative References [Part1]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Message Syntax and
                 Routing", draft-ietf-httpbis-p1-messaging-21 (work    |
   |          | data format ([RFC1950])                |               |
   | gzip     | Same as GNU zip [RFC1952]              | Section 4.2.3 |
   |          |                                        | of [Part1]    |
   | identity | reserved (synonym for "no encoding" in
                 progress), October 2012.

   [Part4]       Fielding, R., Ed. | Section 5.3.4 |
   |          | Accept-Encoding header field)          |               |
   +----------+----------------------------------------+---------------+

9.  Security Considerations

   This section is meant to inform developers, information providers,
   and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Conditional Requests",
                 draft-ietf-httpbis-p4-conditional-21 (work in
                 progress), October 2012.

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

   [Part6]       Fielding, R., Ed., Nottingham, M., Ed., its use for transferring information over the Internet.

9.1.  Attacks Based On File and J. Reschke,
                 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                 draft-ietf-httpbis-p6-cache-21 (work in progress),
                 October 2012.

   [Part7]       Fielding, R., Ed. Path Names

   Origin servers frequently make use of their local file system to
   manage the mapping from effective request URI to resource
   representations.  Implementors need to be aware that most file
   systems are not designed to protect against malicious file or path
   names, and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Authentication",
                 draft-ietf-httpbis-p7-auth-21 (work in progress),
                 October 2012.

   [RFC1950]     Deutsch, L. thus depend on the origin server to avoid mapping to file
   names, folders, or directories that have special significance to the
   system.

   For example, UNIX, Microsoft Windows, and J-L. Gailly, "ZLIB Compressed Data
                 Format Specification version 3.3", RFC 1950, May 1996.

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

   [RFC1952]     Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., other operating systems use
   ".." as a path component to indicate a directory level above the
   current one, and
                 G. Randers-Pehrson, "GZIP use specially named paths or file format specification
                 version 4.3", RFC 1952, May 1996.

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

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

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

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

   [RFC4647]     Phillips, A., Ed. and M. Davis, Ed., "Matching send data
   to system devices.  Similar naming conventions might exist within
   other types of
                 Language Tags", BCP 47, RFC 4647, September 2006.

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

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

12.2.  Informative References

   [REST]        Fielding, R., "Architectural Styles storage systems.  Likewise, local storage systems have
   an annoying tendency to prefer user-friendliness over security when
   handling invalid or unexpected characters, recomposition of
   decomposed characters, and the Design case-normalization of
                 Network-based Software Architectures", Doctoral
                 Dissertation, University case-insensitive
   names.

   Attacks based on such special names tend to focus on either denial of California, Irvine ,
                 September 2000,
                 <http://roy.gbiv.com/pubs/dissertation/top.htm>.

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

   [RFC1945]     Berners-Lee, T., Fielding, R.,
   service (e.g., telling the server to read from a COM port) or
   disclosure of configuration and H. Nielsen,
                 "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945,
                 May 1996.

   [RFC2049]     Freed, N. source files that are not meant to be
   served.

9.2.  Personal Information

   Clients are often privy to large amounts of personal information,
   including both information provided by the user to interact with
   resources (e.g., the user's name, location, mail address, passwords,
   encryption keys, etc.) and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Five: Conformance Criteria
                 and Examples", RFC 2049, November 1996.

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

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

   [RFC2277]     Alvestrand, H., "IETF Policy information about the user's browsing
   activity over time (e.g., history, bookmarks, etc.).  Implementations
   need to prevent unintentional leakage of personal information.

9.3.  Sensitive Information in URIs

   URIs are intended to be shared, not secured, even when they identify
   secure resources.  URIs are often shown on Character Sets and
                 Languages", BCP 18, RFC 2277, January 1998.

   [RFC2295]     Holtman, K. displays, added to
   templates when a page is printed, and A. Mutz, "Transparent Content
                 Negotiation stored in HTTP", RFC 2295, March 1998.

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

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

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

   [RFC2817]     Khare, R. and S. Lawrence, "Upgrading
   unprotected bookmark lists.  It is therefore unwise to TLS Within
                 HTTP/1.1", RFC 2817, May 2000.

   [RFC3629]     Yergeau, F., "UTF-8, include
   information within a transformation format URI that is sensitive, personally identifiable,
   or a risk to disclose.

   Authors of ISO
                 10646", STD 63, RFC 3629, November 2003.

   [RFC3864]     Klyne, G., Nottingham, M., and J. Mogul, "Registration
                 Procedures services ought to avoid GET-based forms for Message Header Fields", BCP 90,
                 RFC 3864, September 2004.

   [RFC4288]     Freed, N. and J. Klensin, "Media Type Specifications the submission
   of sensitive data because that data will be placed in the request-
   target.  Many existing servers, proxies, and Registration Procedures", BCP 13, RFC 4288,
                 December 2005.

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

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

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

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

   [RFC6151]     Turner, S. and L. Chen, "Updated Security
                 Considerations for user agents log or
   display the MD5 Message-Digest and request-target in places where it might be visible to
   third parties.  Such services ought to use POST-based form submission
   instead.

   Since the HMAC-
                 MD5 Algorithms", RFC 6151, March 2011.

   [RFC6266]     Reschke, J., "Use of Referer header field tells a target site about the Content-Disposition Header
                 Field context
   that resulted in a request, it has the Hypertext Transfer Protocol (HTTP)",
                 RFC 6266, June 2011.

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

Appendix A.  Differences between HTTP and MIME

   HTTP/1.1 uses many of potential to reveal
   information about the constructs defined for Internet Mail
   ([RFC5322]) user's immediate browsing history and the Multipurpose Internet Mail Extensions (MIME
   [RFC2045]) to allow a message body to any
   personal information that might be transmitted found in an open
   variety of representations and with extensible mechanisms.  However,
   RFC 2045 discusses mail, and HTTP has a few features that the referring resource's
   URI.  Limitations on Referer are
   different from those described in MIME.  These differences were
   carefully chosen to optimize performance over binary connections, to
   allow greater freedom in the use of new media types, to make date
   comparisons easier, and Section 5.5.2 to acknowledge the practice of
   address some early
   HTTP servers and clients.

   This appendix describes specific areas where HTTP differs from MIME.
   Proxies and gateways to strict MIME environments SHOULD be aware of
   these differences its security considerations.

9.4.  Product Information

   The User-Agent (Section 5.5.3), Via (Section 5.7.1 of [Part1]), and provide
   Server (Section 7.4.2) header fields often reveal information about
   the appropriate conversions where
   necessary.  Proxies and gateways from MIME environments respective sender's software systems.  In theory, this can make
   it easier for an attacker to HTTP also
   need exploit known security holes; in
   practice, attackers tend to be aware try all potential holes regardless of the differences because some conversions might be
   required.

A.1.  MIME-Version

   HTTP is not
   apparent software versions being used.

   Proxies that serve as a MIME-compliant protocol.  However, HTTP/1.1 messages
   MAY include portal through a single MIME-Version header field to indicate what
   version of the MIME protocol was used network firewall ought to construct
   take special precautions regarding the message.  Use transfer of the MIME-Version header field indicates information
   that might identify hosts behind the message is in
   full conformance with the MIME protocol (as defined in [RFC2045]).
   Proxies/gateways are responsible for ensuring full conformance (where
   possible) when exporting HTTP messages firewall.  The Via header field
   allows intermediaries to strict MIME environments.

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

   MIME version "1.0" is the default for use in HTTP/1.1.  However,
   HTTP/1.1 message parsing and semantics replace sensitive machine names with
   pseudonyms.

9.5.  Fragment after Redirects

   Although fragment identifiers used within URI references are defined by this document
   and not the MIME specification.

A.2.  Conversion sent
   in requests, implementers ought to Canonical Form

   MIME requires be aware that an Internet mail body-part they will be converted to
   canonical form prior visible
   to being transferred, the user agent and any extensions or scripts running as described in Section 4
   of [RFC2049].  Section 3.1.1.3 of this document describes the forms
   allowed for subtypes a result
   of the "text" media type response.  In particular, when transmitted over
   HTTP.  [RFC2046] requires that content with a type of "text"
   represent line breaks as CRLF redirect occurs and forbids the use of CR or LF outside
   original request's fragment identifier is inherited by the new
   reference in Location (Section 7.1.2), this might have the effect of line break sequences.  HTTP allows CRLF, bare CR, and bare LF
   leaking one site's fragment to
   indicate a line break within text content when a message is
   transmitted over HTTP.

   Where another site.  If the first site uses
   personal information in fragments, it is possible, a proxy or gateway from HTTP ought to ensure that redirects
   to other sites include a strict MIME
   environment SHOULD translate all line breaks within the text media
   types described (possibly empty) fragment component in Section 3.1.1.3 order
   to block that inheritance.

9.6.  Browser Fingerprinting

   Browser fingerprinting is a set of this document techniques for identifying a
   specific user agent over time through its unique set of
   characteristics.  These characteristics might include information
   related to its TCP behavior, feature capabilities, and scripting
   environment, though of particular interest here is the RFC 2049
   canonical form set of CRLF.  Note, however, unique
   characteristics that this might be
   complicated by the presence communicated via HTTP.  Fingerprinting
   is considered a privacy concern because it enables tracking of a Content-Encoding and by user
   agent's behavior over time without the fact corresponding controls that HTTP allows
   the use user might have over other forms of some character encodings which do not use
   octets 13 and 10 data collection (e.g.,
   cookies).  Many general-purpose user agents (i.e., Web browsers) have
   taken steps to represent CR and LF, respectively, as is the case
   for some multi-byte character encodings.

   Conversion will break any cryptographic checksums applied reduce their fingerprints.

   There are a number of request header fields that might reveal
   information to the
   original content unless the original content servers that is sufficiently unique to enable
   fingerprinting.  The From header field is already in canonical
   form.  Therefore, the canonical form most obvious, though it
   is recommended for any content expected that uses such checksums in HTTP.

A.3.  Conversion of Date Formats

   HTTP/1.1 uses a restricted set of date formats (Section 8.1.1.1) to
   simplify From will only be sent when self-identification is
   desired by the process of date comparison.  Proxies and gateways from
   other protocols SHOULD ensure user.  Likewise, Cookie header fields are deliberately
   designed to enable re-identification, so we can assume that any Date
   fingerprinting concerns only apply to situations where cookies are
   disabled or restricted by browser configuration.

   The User-Agent header field present in a
   message conforms might contain enough information to one of
   uniquely identify a specific device, usually when combined with other
   characteristics, particularly if the HTTP/1.1 formats and rewrite user agent sends excessive
   details about the date
   if necessary.

A.4.  Introduction user's system or extensions.  However, the source
   of Content-Encoding

   MIME does not include any concept equivalent to HTTP/1.1's Content-
   Encoding header field.  Since this acts as a modifier on unique information that is least expected by users is proactive
   negotiation (Section 5.3), including the media
   type, proxies Accept, Accept-Charset,
   Accept-Encoding, and gateways from HTTP Accept-Language header fields.

   In addition to MIME-compliant protocols MUST
   either change the value fingerprinting concern, detailed use of the Content-Type
   Accept-Language header field or decode
   the representation before forwarding can reveal information the message.  (Some experimental
   applications user might
   consider to be of Content-Type for Internet mail have used a media-type
   parameter private nature, because the understanding of ";conversions=<content-coding>"
   particular languages is often strongly correlated to perform membership in a function
   equivalent to Content-Encoding.  However, this parameter is not part
   of the MIME standards).

A.5.  No Content-Transfer-Encoding

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

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

A.6.  MHTML and Line Length Limitations

   HTTP implementations which share code with MHTML [RFC2557]
   implementations need user agent to be aware omit the sending of MIME line length limitations.
   Since HTTP does not Accept-
   Language except for sites that have this limitation, HTTP does not fold long
   lines.  MHTML messages being transported by HTTP follow all
   conventions of MHTML, including line length limitations and folding,
   canonicalization, etc., since HTTP transports all message-bodies as
   payload (see Section 3.1.1.4) and does not interpret the content or
   any MIME been whitelisted, perhaps via
   interaction after detecting a Vary header lines field that would indicate
   language negotiation might be contained therein.

Appendix B.  Additional Features

   [RFC1945] and [RFC2068] document protocol elements used by some
   existing HTTP implementations, but not consistently and correctly
   across most HTTP/1.1 applications.  Implementers useful.

   In environments where proxies are advised used to enhance privacy, user
   agents ought to be
   aware of these features, but cannot rely upon their presence in, or
   interoperability with, other HTTP/1.1 applications.  Some of these
   describe proposed experimental features, and some describe features
   that experimental deployment found lacking that are now addressed in
   the base HTTP/1.1 specification.

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

Appendix C.  Changes from RFC 2616

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

   Clarify definition of POST.  (Section 5.3.3)

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

   Take over definition fields.  General-purpose user agents that provide a high
   degree of CONNECT method from [RFC2817].
   (Section 5.3.6)

   Restrict "Max-Forwards" header field configurability ought to OPTIONS and TRACE
   (previously, extension methods inform users about
   the loss of privacy that might result if too much detail is provided.
   As an extreme privacy measure, proxies could have used it as well).
   (Section 6.1.1)

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

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

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

   Broadened the definition of 203 (Non-Authoritative Information) to
   include cases relayed requests.

10.  Acknowledgments

   See Section 9 of payload transformations as well.  (Section 7.3.4)

   Status codes 301, 302, [Part1].

11.  References

11.1.  Normative References

   [Part1]       Fielding, R., Ed. and 307: removed the normative requirements on
   both response payloads and user interaction.  (Section 7.4)

   Failed to consider that there are many other request methods that are
   safe to automatically redirect, and further that the user agent is
   able to make that determination based on the request method
   semantics.  Furthermore, allow user agents to rewrite the method from
   POST to GET for status codes 301 J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Message Syntax and 302.  (Sections 7.4.2, 7.4.3
                 Routing", draft-ietf-httpbis-p1-messaging-22 (work in
                 progress), February 2013.

   [Part4]       Fielding, R., Ed. and
   7.4.7)

   Deprecate 305 (Use Proxy) status code, because user agents did not
   implement it.  It used to indicate that the target resource needs to
   be accessed through the proxy given by the Location field.  The
   Location field gave the URI of the proxy.  The recipient was expected
   to repeat this single request via the proxy.  (Section 7.4.5)

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

   Correct syntax of "Location" header field to allow URI references
   (including relative references J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Conditional Requests",
                 draft-ietf-httpbis-p4-conditional-22 (work in
                 progress), February 2013.

   [Part5]       Fielding, R., Ed., Lafon, Y., Ed., and fragments), as referred symbol
   "absoluteURI" wasn't what was expected, J. Reschke, Ed.,
                 "Hypertext Transfer Protocol (HTTP/1.1): Range
                 Requests", draft-ietf-httpbis-p5-range-22 (work in
                 progress), February 2013.

   [Part6]       Fielding, R., Ed., Nottingham, M., Ed., and add some clarifications
   as to when use of fragments would not be appropriate.
   (Section 8.1.2)

   Reclassify "Allow" as response header field, removing the option to
   specify it J. Reschke,
                 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                 draft-ietf-httpbis-p6-cache-22 (work in a PUT request.  Relax the server requirement on the
   contents of the Allow header field progress),
                 February 2013.

   [Part7]       Fielding, R., Ed. and remove requirement on clients
   to always trust the header field value.  (Section 8.4.1)

   In the description of the "Server" header field, the "Via" field was
   described as a SHOULD.  The requirement was and is stated correctly
   in the description of the Via header field J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Authentication",
                 draft-ietf-httpbis-p7-auth-22 (work in Section 5.7 of [Part1].
   (Section 8.4.2)

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

   Remove the default character encoding progress),
                 February 2013.

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

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

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

   [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of "ISO-8859-1" Internet
                 Message Bodies", RFC 2045, November 1996.

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

   [RFC2119]     Bradner, S., "Key words for text media
   types; the default now is whatever the media type definition says.
   (Section 3.1.1.3)

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

   Remove definition of "Content-MD5 header" field because it was
   inconsistently implemented with respect use in RFCs to partial responses, Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [RFC4647]     Phillips, A., Ed. and M. Davis, Ed., "Matching of known deficiencies in the hash algorithm itself (see
   [RFC6151]
                 Language Tags", BCP 47, RFC 4647, September 2006.

   [RFC5234]     Crocker, D., Ed. and P. Overell, "Augmented BNF for details).

   Introduce Method Registry.  (Section 9.1)

   Take over the Status Code Registry, previously defined
                 Syntax Specifications: ABNF", STD 68, RFC 5234,
                 January 2008.

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

   [RFC6365]     Hoffman, P. and J. Klensin, "Terminology Used in Section 7.1
   of [RFC2817].  (Section 9.2)

   Remove reference to non-existant identity transfer-coding value
   tokens.  (Appendix A.5)
   Remove discussion of Content-Disposition header field, it is now
   defined by [RFC6266].  (Appendix B)

Appendix D.  Imported ABNF

   The following core rules are included by reference, as defined
                 Internationalization in
   Appendix B.1 the IETF", BCP 166, RFC 6365,
                 September 2011.

11.2.  Informative References

   [BCP13]       Freed, N., Klensin, J., and T. Hansen, "Media Type
                 Specifications and Registration Procedures", BCP 13,
                 RFC 6838, January 2013.

   [BCP90]       Klyne, G., Nottingham, M., and J. Mogul, "Registration
                 Procedures for Message Header Fields", BCP 90,
                 RFC 3864, September 2004.

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

   [RFC1305]     Mills, D., "Network Time Protocol (Version 3)
                 Specification, Implementation", RFC 1305, March 1992.

   [RFC1945]     Berners-Lee, T., Fielding, R., and
   VCHAR (any visible US-ASCII character).

   The rules below are defined H. Nielsen,
                 "Hypertext Transfer Protocol -- HTTP/1.0", RFC 1945,
                 May 1996.

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

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

   [RFC2295]     Holtman, K. and A. Mutz, "Transparent Content
                 Negotiation in [Part1]:

     BWS           = <BWS, defined in [Part1], Section 3.2.1>
     OWS           = <OWS, defined in [Part1], HTTP", RFC 2295, March 1998.

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

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

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

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

   [RFC2978]     Freed, N. and J. Postel, "IANA Charset Registration
                 Procedures", BCP 19, RFC 2978, October 2000.

   [RFC5226]     Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section 3.2.1>
     RWS           = <RWS, defined in [Part1], Section 3.2.1>
     URI-reference = <URI-reference, defined RFCs", BCP 26,
                 RFC 5226, May 2008.

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

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

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

   [RFC5988]     Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [RFC6265]     Barth, A., "HTTP State Management Mechanism", RFC 6265,
                 April 2011.

   [RFC6266]     Reschke, J., "Use of the Content-Disposition Header
                 Field in [Part1], Section 2.7>
     absolute-URI  = <absolute-URI, defined the Hypertext Transfer Protocol (HTTP)",
                 RFC 6266, June 2011.

   [status-308]  Reschke, J., "The Hypertext Transfer Protocol (HTTP)
                 Status Code 308 (Permanent Redirect)",
                 draft-reschke-http-status-308-07 (work in [Part1], Section 2.7>
     comment       = <comment, defined in [Part1], Section 3.2.4>
     field-name    = <comment, progress),
                 March 2012.

Appendix A.  Differences between HTTP and MIME

   HTTP/1.1 uses many of the constructs defined for the Internet Message
   Format [RFC5322] and the Multipurpose Internet Mail Extensions (MIME)
   [RFC2045] to allow a message body to be transmitted in [Part1], Section 3.2>
     partial-URI   = <partial-URI, defined an open
   variety of representations and with extensible header fields.
   However, RFC 2045 is focused only on email; applications of HTTP have
   many characteristics that differ from email, and hence HTTP has
   features that differ from MIME.  These differences were carefully
   chosen to optimize performance over binary connections, to allow
   greater freedom in [Part1], Section 2.7>
     quoted-string = <quoted-string, defined the use of new media types, to make date
   comparisons easier, and to acknowledge the practice of some early
   HTTP servers and clients.

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

A.1.  MIME-Version

   HTTP is not a MIME-compliant protocol.  However, messages can include
   a single MIME-Version header field to indicate what version of the
   MIME protocol was used to construct the message.  Use of the MIME-
   Version header field indicates that the message is in [Part1], Section 3.2.4>
     token         = <token, full
   conformance with the MIME protocol (as defined in [Part1], Section 3.2.4>
     word          = <word, defined [RFC2045]).
   Senders are responsible for ensuring full conformance (where
   possible) when exporting HTTP messages to strict MIME environments.

A.2.  Conversion to Canonical Form

   MIME requires that an Internet mail body part be converted to
   canonical form prior to being transferred, as described in [Part1], Section 3.2.4>

Appendix E.  Collected ABNF

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

   BWS = <BWS, defined 4
   of [RFC2049].  Section 3.1.1.3 of this document describes the forms
   allowed for subtypes of the "text" media type when transmitted over
   HTTP.  [RFC2046] requires that content with a type of "text"
   represent line breaks as CRLF and forbids the use of CR or LF outside
   of line break sequences.  HTTP allows CRLF, bare CR, and bare LF to
   indicate a line break within text content.

   A proxy or gateway from HTTP to a strict MIME environment ought to
   translate all line breaks within the text media types described in [Part1],
   Section 3.2.1> 3.1.1.3 of this document to the RFC 2049 canonical form of
   CRLF.  Note, however, this might be complicated by the presence of a
   Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
    content-coding ] )
   Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
    language-tag ] )
   Content-Location = absolute-URI / partial-URI
   Content-Type = media-type

   Date = HTTP-date
   Expect = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )

   From = mailbox

   GMT = %x47.4D.54 ; GMT

   HTTP-date = rfc1123-date / obs-date

   Location = URI-reference

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

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

   RWS = <RWS, defined and by the fact that HTTP allows the use of some
   charsets that do not use octets 13 and 10 to represent CR and LF,
   respectively.

   Conversion will break any cryptographic checksums applied to the
   original content unless the original content is already in [Part1], Section 3.2.1>
   Referer = absolute-URI / partial-URI
   Retry-After = HTTP-date / delta-seconds

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

   URI-reference = <URI-reference, defined canonical
   form.  Therefore, the canonical form is recommended for any content
   that uses such checksums in [Part1], Section 2.7>
   User-Agent = product *( RWS ( product / comment ) )

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

   absolute-URI = <absolute-URI, defined HTTP.

A.3.  Conversion of Date Formats

   HTTP/1.1 uses a restricted set of date formats (Section 7.1.1.1) to
   simplify the process of date comparison.  Proxies and gateways from
   other protocols ought to ensure that any Date header field present in [Part1], Section 2.7>
   accept-ext = OWS ";" OWS token [ "=" word ]
   accept-params = weight *accept-ext
   asctime-date = day-name SP date3 SP time-of-day SP year
   attribute = token
   a message conforms to one of the HTTP/1.1 formats and rewrite the
   date if necessary.

A.4.  Conversion of Content-Encoding

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

A.5.  Conversion of Content-Transfer-Encoding

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

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

A.6.  MHTML and Line Length Limitations

   HTTP implementations that share code with MHTML [RFC2557]
   implementations need to be aware of MIME line length limitations.
   Since HTTP does not have this limitation, HTTP does not fold long
   lines.  MHTML messages being transported by HTTP follow all
   conventions of MHTML, including line length limitations and folding,
   canonicalization, etc., since HTTP transfers message-bodies as
   payload and, aside from the "multipart/byteranges" type (Appendix A
   of [Part5]), does not interpret the content or any MIME header lines
   that might be contained therein.

Appendix B.  Changes from RFC 2616

   The primary changes in this revision have been editorial in nature:
   extracting the messaging syntax and partitioning HTTP semantics into
   separate documents for the core features, conditional requests,
   partial requests, caching, and authentication.  The conformance
   language has been revised to clearly target requirements and the
   terminology has been improved to distinguish payload from
   representations and representations from resources.  An algorithm has
   been added for determining if a payload is associated with a specific
   identifier (Section 3.1.4.1).

   A new requirement has been added that semantics embedded in a URI
   should be disabled when those semantics are inconsistent with the
   request method, since this is a common cause of interoperability
   failure.

   The default charset = token
   codings = content-coding / "identity" / "*"
   comment = <comment, of ISO-8859-1 for text media types has been
   removed; the default is now whatever the media type definition says
   (Section 3.1.1.3).  Likewise, special treatment of ISO-8859-1 has
   been removed from the Accept-Charset header field (Section 5.3.3).

   The Content-Disposition header field has been removed since it is now
   defined by [RFC6266].

   The definition of Content-Location has been changed to no longer
   affect the base URI for resolving relative URI references, due to
   poor implementation support and the undesirable effect of potentially
   breaking relative links in [Part1], Section 3.2.4>
   content-coding = token

   date1 = day SP month SP year
   date2 = day "-" month "-" 2DIGIT
   date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
   day = 2DIGIT
   day-name = %x4D.6F.6E ; Mon
    / %x54.75.65 ; Tue
    / %x57.65.64 ; Wed
    / %x54.68.75 ; Thu
    / %x46.72.69 ; Fri
    / %x53.61.74 ; Sat
    / %x53.75.6E ; Sun
   day-name-l = %x4D.6F.6E.64.61.79 ; Monday
    / %x54.75.65.73.64.61.79 ; Tuesday
    / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
    / %x54.68.75.72.73.64.61.79 ; Thursday
    / %x46.72.69.64.61.79 ; Friday
    / %x53.61.74.75.72.64.61.79 ; Saturday
    / %x53.75.6E.64.61.79 ; Sunday
   delta-seconds = 1*DIGIT

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

   field-name = <comment, content-negotiated resources
   (Section 3.1.4.2).

   The Content-MD5 header field has been removed because it was
   inconsistently implemented with respect to partial responses.

   To be consistent with the method-neutral parsing algorithm of
   [Part1], the definition of GET has been relaxed so that requests can
   have a body, even though a body has no meaning for GET
   (Section 4.3.1).

   Servers are no longer required to handle all Content-* header fields
   and use of Content-Range has been explicitly banned in PUT requests
   (Section 4.3.4).

   Definition of the CONNECT method has been moved from [RFC2817] to
   this specification (Section 4.3.6).

   The OPTIONS (Section 4.3.7) and TRACE (Section 4.3.8) request methods
   have been defined as being safe.

   The definition of Expect has been both fixed to allow parameters for
   value-less expectations and simplified to allow trailing semicolons
   after "100-continue" (Section 5.1.1).

   The Max-Forwards header field (Section 5.1.2) has been restricted to
   the OPTIONS and TRACE methods; previously, extension methods could
   have used it as well.

   The "about:blank" URI has been suggested as a value for the Referer
   header field when no referring URI is applicable, which distinguishes
   that case from others where the Referer field is not sent or has been
   removed (Section 5.5.2).

   The 201 (Created) status description has been changed to allow for
   the possibility that more than one resource has been created
   (Section 6.3.2).

   The definition of 203 (Non-Authoritative Information) has been
   broadened to include cases of payload transformations as well
   (Section 6.3.4).

   The redirect status codes 301, 302, and 307 no longer have normative
   requirements on response payloads and user interaction (Section 6.4).

   The request methods that are safe to automatically redirect is no
   longer a closed set; user agents are able to make that determination
   based upon the request method semantics (Section 6.4).

   The description of 303 (See Other) status code has been changed to
   allow it to be cached if explicit freshness information is given, and
   a specific definition has been added for a 303 response to GET
   (Section 6.4.4).

   The status codes 301 and 302 (sections 6.4.2 and 6.4.3) have been
   changed to allow user agents to rewrite the method from POST to GET.

   The 305 (Use Proxy) status code has been deprecated due to security
   concerns regarding in-band configuration of a proxy (Section 6.4.5).

   The 400 (Bad Request) status code has been relaxed so that it isn't
   limited to syntax errors (Section 6.5.1).

   The 426 (Upgrade Required) status code has been incorporated from
   [RFC2817] (Section 6.5.15).

   The following status codes are now cacheable (that is, they can be
   stored and reused by a cache without explicit freshness information
   present): 204, 404, 405, 414, 501.

   Allow has been reclassified as a response header field, removing the
   option to specify it in [Part1], Section 3.2>

   hour = 2DIGIT

   language-range = <language-range, a PUT request.  Requirements relating to the
   content of Allow have been relaxed; correspondingly, clients are not
   required to always trust its value (Section 7.4.1).

   The target of requirements on HTTP-date and the Date header field
   have been reduced to those systems generating the date, rather than
   all systems sending a date (Section 7.1.1).

   The syntax of the Location header field has been changed to allow all
   URI references, including relative references and fragments, along
   with some clarifications as to when use of fragments would not be
   appropriate (Section 7.1.2).

   A Method Registry has been defined in [RFC4647], Section 2.1>
   language-tag = <Language-Tag, (Section 8.1).

   The Status Code Registry (Section 8.2) has been redefined by this
   specification; previously, it was defined in [RFC5646], Section 2.1>

   mailbox = <mailbox, 7.1 of
   [RFC2817].

   Registration of Content Codings has been changed to require IETF
   Review (Section 8.4).

Appendix C.  Imported ABNF

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

   obs-date = rfc850-date / asctime-date

   parameter = attribute "=" value
   partial-URI = <partial-URI,
   Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
   CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
   quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
   (line feed), OCTET (any 8-bit sequence of data), SP (space), and
   VCHAR (any visible US-ASCII character).

   The rules below are defined in [Part1], Section 2.7>
   product = token [ "/" product-version ]
   product-version = token

   quoted-string [Part1]:

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

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

   second = 2DIGIT
   subtype = token

   time-of-day 3.2.3>
     OWS           = hour ":" minute ":" second
   token <OWS, defined in [Part1], Section 3.2.3>
     RWS           = <token, <RWS, defined in [Part1], Section 3.2.4>
   type 3.2.3>
     URI-reference = token

   value <URI-reference, defined in [Part1], Section 2.7>
     absolute-URI  = word

   weight <absolute-URI, defined in [Part1], Section 2.7>
     comment       = OWS ";" OWS "q=" qvalue
   word <comment, defined in [Part1], Section 3.2.6>
     field-name    = <word, <comment, defined in [Part1], Section 3.2.4>

   year 3.2>
     partial-URI   = 4DIGIT

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

F.1.  Since RFC 2616

   Extracted relevant partitions from [RFC2616].

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

   Closed issues:

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

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

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

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

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

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

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

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

   Other changes:

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

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

   Closed issues:

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

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

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

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

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

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

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

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

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

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

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

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

   Closed issues:

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

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

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

   o  Move "Product Tokens" section (back) into Part 1, as "token" is
      used [Part1], Section 2.7>
     quoted-string = <quoted-string, defined in the definition of the Upgrade header field.

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

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

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

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

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

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

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/24>: "Requiring
      Allow [Part1], Section 3.2.6>
     token         = <token, defined in 405 responses"

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

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

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

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

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

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

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

   o  Reference RFC 3984, and update header field registrations for
      header fields [Part1], Section 3.2.6>
     word          = <word, defined in this document.

   Ongoing work on [Part1], Section 3.2.6>

Appendix D.  Collected ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

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

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

   Closed issues:

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

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

   Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
    OWS ( media-range [ accept-params ] ) ] ) ]
   Accept-Charset = *( "," OWS ) ( ( charset / "*" ) [ weight ] ) *( OWS
    "," [ OWS ( ( charset / "*" ) [ weight ] ) ] )

   Accept-Encoding = [ ( "," / ( codings [ weight ] ) ) *( OWS "," [ OWS
    ( codings [ weight ] ) ] ) ]
   Accept-Language = *( "," OWS ) ( language-range [ weight ] ) *( OWS
    "," [ OWS ( language-range [ weight ] ) ] )
   Allow header field"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/115>: "missing
      default for qvalue = [ ( "," / method ) *( OWS "," [ OWS method ] ) ]

   BWS = <BWS, defined in description of Accept-Encoding"

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

   o  Reference RFC 3984, and update header field registrations for
      header fields [Part1], Section 3.2.3>

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

   Date = HTTP-date

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

   From = mailbox

   GMT = %x47.4D.54 ; GMT

   HTTP-date = IMF-fixdate / obs-date

   IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT

   Location = URI-reference

   Max-Forwards = 1*DIGIT

   OWS = <OWS, defined in this document.

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

   Closed issues:

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

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

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

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

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

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

   Closed issues:

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

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

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

   Other changes:

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

F.10.  Since draft-ietf-httpbis-p2-semantics-04

   Closed issues:

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

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

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

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

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

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

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

   Closed issues:

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

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

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

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

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

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

   Closed issues:

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

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

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

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

   Closed issues:

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

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

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

   Other changes:

   o  Move definition of quality values into Part 1.

F.14.  Since draft-ietf-httpbis-p2-semantics-06

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/144>: "Clarify when [Part1], Section 3.2.3>

   RWS = <RWS, defined in [Part1], Section 3.2.3>
   Referer is sent"

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

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

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

   Closed issues:

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

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

F.16.  Since draft-ietf-httpbis-p2-semantics-07

   Closed issues:

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

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

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

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

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

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/192>: "fragments = absolute-URI / partial-URI
   Retry-After = HTTP-date / delta-seconds

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

   URI-reference = <URI-reference, defined in
      Location vs status 303"

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

   Partly resolved issues:

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

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

   Closed issues:

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

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/154>: "Content-
      Location base-setting problems"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/188>: "pick IANA
      policy (RFC5226) for Transfer Coding [Part1], Section 2.7>
   User-Agent = product *( RWS ( product / Content Coding"

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

   Partly resolved issues:

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

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

F.18.  Since draft-ietf-httpbis-p2-semantics-08

   Closed issues:

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

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

   Closed issues:

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

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

F.20.  Since draft-ietf-httpbis-p2-semantics-09

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment
      combination comment ) )

   Vary = "*" / precedence during redirects"

   Partly resolved issues:

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

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

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

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/122>: "MIME-Version
      not listed ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
    ) )

   absolute-URI = <absolute-URI, defined in P1, general header fields"

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

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

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

   Partly resolved issues:

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

F.22.  Since draft-ietf-httpbis-p2-semantics-10

   Closed issues:

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
      entity [Part1], Section 2.7>
   accept-ext = OWS ";" OWS token [ "=" word ]
   accept-params = weight *accept-ext
   asctime-date = day-name SP date3 SP time-of-day SP year
   attribute = token

   charset = token
   codings = content-coding / "identity" / "*"
   comment = <comment, defined in [Part1], Section 3.2.6>
   content-coding = token

   date1 = day SP month SP year
   date2 = day "-" month "-" 2DIGIT
   date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
   day = 2DIGIT
   day-name = %x4D.6F.6E ; Mon
    / representation %x54.75.65 ; Tue
    / variant terminology"

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

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

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

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

F.23.  Since draft-ietf-httpbis-p3-payload-10

   Closed issues:

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

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

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
      entity %x57.65.64 ; Wed
    / representation %x54.68.75 ; Thu
    / variant terminology"

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

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

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

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

   Partly resolved issues:

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

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

   Closed issues:

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

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

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

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

   Closed issues:

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

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

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment
      combination %x46.72.69 ; Fri
    / precedence during redirects" (added warning about
      having a fragid on the redirect might cause inconvenience in some
      cases)

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

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

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

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

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

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

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/180>: "Note special
      status of Content-* prefix %x53.61.74 ; Sat
    / %x53.75.6E ; Sun
   day-name-l = %x4D.6F.6E.64.61.79 ; Monday
    / %x54.75.65.73.64.61.79 ; Tuesday
    / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
    / %x54.68.75.72.73.64.61.79 ; Thursday
    / %x46.72.69.64.61.79 ; Friday
    / %x53.61.74.75.72.64.61.79 ; Saturday
    / %x53.75.6E.64.61.79 ; Sunday
   delta-seconds = 1*DIGIT

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

   field-name = <comment, defined in header field registration
      procedures"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/213>: "What is the
      value space of HTTP status codes?" (actually fixed [Part1], Section 3.2>

   hour = 2DIGIT

   language-range = <language-range, defined in
      draft-ietf-httpbis-p2-semantics-11)

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

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

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

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

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

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

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

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

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

   Closed issues:

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

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/277>: "potentially
      misleading MAY [RFC4647], Section 2.1>
   language-tag = <Language-Tag, defined in media-type def"

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

   Closed issues:

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/251>: "message body [RFC5646], Section 2.1>

   mailbox = <mailbox, defined in CONNECT request"

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

   Closed issues:

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

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

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/281>: "confusing
      undefined [RFC5322], Section 3.4>
   media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
    ";" OWS parameter )
   media-type = type "/" subtype *( OWS ";" OWS parameter )
   method = token
   minute = 2DIGIT
   month = %x4A.61.6E ; Jan
    / %x46.65.62 ; Feb
    / %x4D.61.72 ; Mar
    / %x41.70.72 ; Apr
    / %x4D.61.79 ; May
    / %x4A.75.6E ; Jun
    / %x4A.75.6C ; Jul
    / %x41.75.67 ; Aug
    / %x53.65.70 ; Sep
    / %x4F.63.74 ; Oct
    / %x4E.6F.76 ; Nov
    / %x44.65.63 ; Dec

   obs-date = rfc850-date / asctime-date

   parameter = attribute "=" value
   partial-URI = <partial-URI, defined in media range example"

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

   Closed issues:

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

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

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

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

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

   None.

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

   Closed issues:

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

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

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

   Closed issues:

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

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

   Closed issues:

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

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

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

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

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

   Closed issues:

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

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

   Closed issues:

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/255>: "Clarify
      status code for rate limiting" (change backed out because a new
      status code is being [Part1], Section 2.7>
   product = token [ "/" product-version ]
   product-version = token

   quoted-string = <quoted-string, defined in [Part1], Section 3.2.6>
   qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )

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

   second = 2DIGIT
   subtype = token

   time-of-day = hour ":" minute ":" second
   token = <token, defined for this purpose)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/312>: "should there in [Part1], Section 3.2.6>
   type = token

   value = word

   weight = OWS ";" OWS "q=" qvalue
   word = <word, defined in [Part1], Section 3.2.6>

   year = 4DIGIT

Appendix E.  Change Log (to be a permanent variant of 307"

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

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

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

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

   Closed issues:

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

F.38. removed by RFC Editor before publication)

E.1.  Since draft-ietf-httpbis-p2-semantics-18

   Closed issues:

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

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

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/295>: "Applying
      original fragment RFC 2616

   Changes up to 'plain' redirected URI"

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

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/332>: "relax
      requirements on hypertext the first Working Group Last Call draft are summarized
   in 3/4/5xx error responses"

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

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

F.39. <http://trac.tools.ietf.org/html/
   draft-ietf-httpbis-p2-semantics-21#appendix-F>.

E.2.  Since draft-ietf-httpbis-p3-payload-18 draft-ietf-httpbis-p2-semantics-21

   Closed issues:

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/338>: "Content-
      Location doesn't constrain the cardinality of representations"  <http://tools.ietf.org/wg/httpbis/trac/ticket/22>: "ETag (and
      other metadata) in status messages"

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

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

   Closed issues:  <http://tools.ietf.org/wg/httpbis/trac/ticket/96>: "Conditional
      GET text"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/312>: "should there
      be a permanent variant  <http://tools.ietf.org/wg/httpbis/trac/ticket/146>: "Clarify
      description of 307"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/351>: "merge P2 and
      P3" 405 (Not Allowed)"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF
      requirements  <http://tools.ietf.org/wg/httpbis/trac/ticket/223>: "Allowing
      heuristic caching for recipients"

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

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

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

   Closed issues:

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

   Other changes: status codes"

   o  Conformance criteria and considerations regarding error handling
      are now defined in Part 1.  <http://tools.ietf.org/wg/httpbis/trac/ticket/315>: "method
      semantics: retrieval/representation"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/388>: "User
      confirmation for unsafe methods"

   o  Properly explain what HTTP semantics are and why.  Rewrite
      introductory description of methods.  Rewrite definition of "safe"
      to be more operable and weaken the original same-origin
      restrictions to be more consistent with modern UAs.  Rewrite
      definition of "idempotent", add definition of "cacheable".  <http://tools.ietf.org/wg/httpbis/trac/ticket/404>: "Tentative
      Status Codes"

   o  Conneg terminology change: "server-driven" => "proactive" (UA
      sends Accept* fields), "agent-driven" => "reactive" (UA waits for
      300/Alternatives)  <http://tools.ietf.org/wg/httpbis/trac/ticket/418>: "No-Transform"

   o  Move description of "100-continue" from Part 1 over here.  <http://tools.ietf.org/wg/httpbis/trac/ticket/419>: "p2 editorial
      feedback"

   o  Move definition  <http://tools.ietf.org/wg/httpbis/trac/ticket/424>: "Absence of "Vary"
      Accept-Encoding"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/428>: "Accept-
      Language ordering for identical qvalues"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/432>: "Identify
      additional status codes as cacheable by default"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/434>: "mention in
      header field from Part 6 over here.

   o  Rewrite definition of "representation". considerations that leading/trailing WS is lossy"

Index

   1
      1xx Informational (status code class)  49

   2
      2xx Successful (status code class)  50

   3
      3xx Redirection (status code class)  52  53

   4
      4xx Client Error (status code class)  56  57

   5
      5xx Server Error (status code class)  60  61

   1
      100 Continue (status code)  49
      100-continue (expect value)  35  33
      101 Switching Protocols (status code)  49  50

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

   3
      300 Multiple Choices (status code)  54
      301 Moved Permanently (status code)  54  55
      302 Found (status code)  55
      303 See Other (status code)  55  56
      305 Use Proxy (status code)  56
      306 (Unused) (status code)  56
      307 Temporary Redirect (status code)  56  57

   4
      400 Bad Request (status code)  56  57
      402 Payment Required (status code)  56  57
      403 Forbidden (status code)  57
      404 Not Found (status code)  57  58
      405 Method Not Allowed (status code)  57  58
      406 Not Acceptable (status code)  57  58
      408 Request Timeout (status code)  58  59
      409 Conflict (status code)  58  59
      410 Gone (status code)  58  59
      411 Length Required (status code)  59  60
      413 Request Representation Payload Too Large (status code)  59  60
      414 URI Too Long (status code)  59  60
      415 Unsupported Media Type (status code)  59  60
      417 Expectation Failed (status code)  60  61
      426 Upgrade Required (status code)  60  61

   5
      500 Internal Server Error (status code)  60  61
      501 Not Implemented (status code)  60  61
      502 Bad Gateway (status code)  61  62
      503 Service Unavailable (status code)  61  62
      504 Gateway Timeout (status code)  61  62
      505 HTTP Version Not Supported (status code)  61  62

   A
      Accept header field  38
      Accept-Charset header field  41  40
      Accept-Encoding header field  41
      Accept-Language header field  42
      Allow header field  69  71

   C
      cacheable  25  23
      compress (content coding)  12  11
      conditional request  36
      CONNECT method  30  29
      content coding  12  11
      content negotiation  7  6
      Content-Encoding header field  12
      Content-Language header field  14  13
      Content-Location header field  16  15
      Content-Transfer-Encoding header field  85  87
      Content-Type header field  11  10

   D
      Date header field  64  66
      deflate (content coding)  12  11
      DELETE method  30  28

   E
      Expect header field  34  33
      Expect Values
         100-continue  35  33

   F
      From header field  44

   G
      GET method  25  24
      Grammar
         Accept  39  38
         Accept-Charset  41  40
         Accept-Encoding  41
         accept-ext  39  38
         Accept-Language  43  42
         accept-params  39  38
         Allow  69  71
         asctime-date  64  65
         attribute  9  8
         charset  10  9
         codings  41
         content-coding  12  11
         Content-Encoding  13  12
         Content-Language  14  13
         Content-Location  16  15
         Content-Type  11  10
         Date  64  66
         date1  63  64
         day  63  64
         day-name  63  64
         day-name-l  63  64
         delta-seconds  66  68
         Expect  34  33
         expect-name  34  33
         expect-param  34  33
         expect-value  34  33
         expectation  34  33
         From  44
         GMT  63  64
         hour  63  64
         HTTP-date  62  63
         IMF-fixdate  64
         language-range  43  42
         language-tag  14  13
         Location  65  66
         Max-Forwards  34  36
         media-range  39  38
         media-type  9  8
         method  22
         MIME-Version  84  20
         minute  63  64
         month  63  64
         obs-date  63  65
         parameter  9  8
         product  22  46
         product-version  22  46
         qvalue  38  37
         Referer  45  44
         Retry-After  66  68
         rfc850-date  64
         rfc1123-date  63  65
         second  63  64
         Server  69  71
         subtype  9  8
         time-of-day  63  64
         type  9  8
         User-Agent  46  45
         value  9  8
         Vary  67  68
         weight  38  37
         year  63  64
      gzip (content coding)  12  11

   H
      HEAD method  26  24

   I
      idempotent  25  23

   L
      Location header field  65  66

   M
      Max-Forwards header field  34  36
      MIME-Version header field  84  86

   O
      OPTIONS method  32  31

   P
      payload  18  17
      POST method  27  25
      PUT method  28  26

   R
      Referer header field  45  44
      representation  8  7
      Retry-After header field  66  68

   S
      safe  24  22
      selected representation  67  7, 69
      Server header field  69  71
      Status Codes Classes
         1xx Informational  49
         2xx Successful  50
         3xx Redirection  52  53
         4xx Client Error  56  57
         5xx Server Error  60  61

   T
      TRACE method  33  32

   U
      User-Agent header field  45

   V
      Vary header field  67  68

   X
      x-compress (content coding)  12  11
      x-gzip (content coding)  12  11

Authors' Addresses

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

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

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

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