Indicating Character Encoding and Language for HTTP Header Field Parameters
RFC 8187



Internet Engineering Task Force (IETF)                        J. Reschke
Request for Comments: 8187                                    greenbytes
Obsoletes: 5987                                           September 2017
Category: Standards Track
ISSN: 2070-1721

    Indicating Character Encoding and Language for HTTP Header Field
                               Parameters

Abstract

   By default, header field values in Hypertext Transfer Protocol (HTTP)
   messages cannot easily carry characters outside the US-ASCII coded
   character set.  RFC 2231 defines an encoding mechanism for use in
   parameters inside Multipurpose Internet Mail Extensions (MIME) header
   field values.  This document specifies an encoding suitable for use
   in HTTP header fields that is compatible with a simplified profile of
   the encoding defined in RFC 2231.

   This document obsoletes RFC 5987.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8187.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Comparison to RFC 2231 and Definition of the Encoding . . . .   3
     3.1.  Parameter Continuations . . . . . . . . . . . . . . . . .   4
     3.2.  Parameter Value Character Encoding and Language
           Information . . . . . . . . . . . . . . . . . . . . . . .   4
       3.2.1.  Definition  . . . . . . . . . . . . . . . . . . . . .   4
       3.2.2.  Historical Notes  . . . . . . . . . . . . . . . . . .   6
       3.2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Language Specification in Encoded Words . . . . . . . . .   7
   4.  Guidelines for Usage in HTTP Header Field Definitions . . . .   7
     4.1.  When to Use the Extension . . . . . . . . . . . . . . . .   8
     4.2.  Error Handling  . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Changes from RFC 5987  . . . . . . . . . . . . . . .  12
   Appendix B.  Implementation Report  . . . . . . . . . . . . . . .  12
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Use of characters outside the US-ASCII coded character set
   ([RFC0020]) in HTTP header fields ([RFC7230]) is non-trivial:

   o  The HTTP specification discourages use of non-US-ASCII characters
      in field values, placing them into the "obs-text" Augmented
      Backus-Naur Form (ABNF) production ([RFC7230], Section 3.2).

   o  Furthermore, it stays silent about default character encoding
      schemes for field values, so any use of non-US-ASCII characters
      would need to be specific to the field definition or would require
      some other kind of out-of-band information.

   o  Finally, some APIs assume a default character encoding scheme in
      order to map from the octet sequences (obtained from the HTTP
      message) to character sequences: for instance, the XMLHttpRequest
      API ([XMLHttpRequest]) uses the Interface Definition Language type
      "ByteString", effectively resulting in the ISO-8859-1 character
      encoding scheme ([ISO-8859-1]) being used.

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   On the other hand, RFC 2231 defines an encoding mechanism for
   parameters inside MIME header fields ([RFC2231]), which, as opposed
   to HTTP messages, do need to be sent over non-binary transports.
   This document specifies an encoding suitable for use in HTTP header
   fields that is compatible with a simplified profile of the encoding
   defined in RFC 2231.  It can be applied to any HTTP header field that
   uses the common "parameter" ("name=value") syntax.

   This document obsoletes [RFC5987] and moves it to "Historic" status;
   the changes are summarized in Appendix A.

      Note: In the remainder of this document, RFC 2231 is only
      referenced for the purpose of explaining the choice of features
      that were adopted; therefore, they are purely informative.

      Note: This encoding does not apply to message payloads transmitted
      over HTTP, such as when using the media type "multipart/form-data"
      ([RFC7578]).

2.  Notational Conventions

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

   This specification uses the ABNF notation defined in [RFC5234].  The
   following core rules are included by reference, as defined in
   [RFC5234], Appendix B.1: ALPHA (letters), DIGIT (decimal 0-9), HEXDIG
   (hexadecimal 0-9/A-F/a-f), and LWSP (linear whitespace).

   This specification uses terminology defined in [RFC6365], namely:
   "character encoding scheme" (abbreviated to "character encoding"
   below), "charset", and "coded character set".

   Note that this differs from RFC 2231, which uses the term "character
   set" for "character encoding scheme".

3.  Comparison to RFC 2231 and Definition of the Encoding

   RFC 2231 defines several extensions to MIME.  The sections below
   discuss if and how they apply to HTTP header fields.

   In short:

   o  Parameter Continuations aren't needed (Section 3.1),

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   o  Character Encoding and Language Information are useful, therefore
      a simple subset is specified (Section 3.2), and

   o  Language Specifications in Encoded Words aren't needed
      (Section 3.3).

3.1.  Parameter Continuations

   Section 3 of [RFC2231] defines a mechanism that deals with the length
   limitations that apply to MIME headers.  These limitations do not
   apply to HTTP ([RFC7231], Appendix A.6).

   Thus, parameter continuations are not part of the encoding defined by
   this specification.

3.2.  Parameter Value Character Encoding and Language Information

   Section 4 of [RFC2231] specifies how to embed language information
   into parameter values and also how to encode non-ASCII characters,
   dealing with restrictions both in MIME and HTTP header field
   parameters.

   However, RFC 2231 does not specify a mandatory-to-implement character
   encoding, making it hard for senders to decide which encoding to use.
   Thus, recipients implementing this specification MUST support the
   "UTF-8" character encoding [RFC3629].

   Furthermore, RFC 2231 allows the character encoding information to be
   left out.  The encoding defined by this specification does not allow
   that.

3.2.1.  Definition

   The presence of extended parameter values is usually indicated by a
   parameter name ending in an asterisk character.  However, note that
   this is just a convention, and that the extended parameter values
   need to be explicitly specified in the definition of the header field
   using this extension (see Section 4).

   The ABNF for extended parameter values is specified below:

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     ext-value     = charset  "'" [ language ] "'" value-chars
                   ; like RFC 2231's <extended-initial-value>
                   ; (see [RFC2231], Section 7)

     charset       = "UTF-8" / mime-charset

     mime-charset  = 1*mime-charsetc
     mime-charsetc = ALPHA / DIGIT
                   / "!" / "#" / "$" / "%" / "&"
                   / "+" / "-" / "^" / "_" / "`"
                   / "{" / "}" / "~"
                   ; as <mime-charset> in Section 2.3 of [RFC2978]
                   ; except that the single quote is not included
                   ; SHOULD be registered in the IANA charset registry

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

     value-chars   = *( pct-encoded / attr-char )

     pct-encoded   = "%" HEXDIG HEXDIG
                   ; see [RFC3986], Section 2.1

     attr-char     = ALPHA / DIGIT
                   / "!" / "#" / "$" / "&" / "+" / "-" / "."
                   / "^" / "_" / "`" / "|" / "~"
                   ; token except ( "*" / "'" / "%" )

   The value part of an extended parameter (ext-value) is a token that
   consists of three parts:

   1.  the REQUIRED character encoding name (charset),

   2.  the OPTIONAL language information (language), and

   3.  a character sequence representing the actual value (value-chars),
       separated by single quote characters.

   Note that both character encoding names and language tags are
   restricted to the US-ASCII coded character set and are matched case-
   insensitively (see Section 2.3 of [RFC2978] and Section 2.1.1 of
   [RFC5646]).

   Inside the value part, characters not contained in attr-char are
   encoded into an octet sequence using the specified character
   encoding.  That octet sequence is then percent-encoded as specified
   in Section 2.1 of [RFC3986].

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   Producers MUST use the "UTF-8" ([RFC3629]) character encoding.
   Extension character encodings (mime-charset) are reserved for future
   use.

      Note: Recipients should be prepared to handle encoding errors,
      such as malformed or incomplete percent escape sequences, or
      non-decodable octet sequences, in a robust manner.  This
      specification does not mandate any specific behavior; for
      instance, the following strategies are all acceptable:

      *  ignoring the parameter,

      *  stripping a non-decodable octet sequence, and

      *  substituting a non-decodable octet sequence by a replacement
         character, such as the Unicode character U+FFFD (Replacement
         Character).

3.2.2.  Historical Notes

   The RFC 7230 token production ([RFC7230], Section 3.2.6) differs from
   the production used in RFC 2231 (imported from Section 5.1 of
   [RFC2045]) in that curly braces (i.e., "{" and "}") are excluded.
   Thus, these two characters are excluded from the attr-char production
   as well.

   The <mime-charset> ABNF defined here differs from the one in
   Section 2.3 of [RFC2978] in that it does not allow the single quote
   character (see also RFC Errata ID 1912 [Err1912]).  In practice, no
   character encoding names using that character have been registered at
   the time of this writing.

   For backwards compatibility with RFC 2231, the encoding defined by
   this specification deviates from common parameter syntax in that the
   quoted-string notation is not allowed.  Implementations using generic
   parser components might not be able to detect the use of quoted-
   string notation and thus might accept that format, although invalid,
   as well.

   [RFC5987] did require support for ISO-8859-1 ([ISO-8859-1]), too; for
   compatibility with legacy code, recipients are encouraged to support
   this encoding as well.

3.2.3.  Examples

   Non-extended notation, using "token":

     foo: bar; title=Economy

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   Non-extended notation, using "quoted-string":

     foo: bar; title="US-$ rates"

   Extended notation, using the Unicode character U+00A3 ("£", POUND
   SIGN):

     foo: bar; title*=utf-8'en'%C2%A3%20rates

   Note: The Unicode pound sign character U+00A3 was encoded into the
   octet sequence C2 A3 using the UTF-8 character encoding, and then
   percent-encoded.  Also, note that the space character was encoded as
   %20, as it is not contained in attr-char.

   Extended notation, using the Unicode characters U+00A3 ("£", POUND
   SIGN) and U+20AC ("€", EURO SIGN):

     foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates

   Note: The Unicode pound sign character U+00A3 was encoded into the
   octet sequence C2 A3 using the UTF-8 character encoding, and then
   percent-encoded.  Likewise, the Unicode euro sign character U+20AC
   was encoded into the octet sequence E2 82 AC, and then percent-
   encoded.  Also note that HEXDIG allows both lowercase and uppercase
   characters, so recipients must understand both, and that the language
   information is optional, while the character encoding is not.

3.3.  Language Specification in Encoded Words

   Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
   also support language specification in encoded words.  RFC 2616, the
   now-obsolete HTTP/1.1 specification, did refer to RFC 2047
   ([RFC2616], Section 2.2).  However, it wasn't clear to which header
   field it applied.  Consequently, the current revision of the HTTP/1.1
   specification has deprecated use of the encoding forms defined in RFC
   2047 (see Section 3.2.4 of [RFC7230]).

   Thus, this specification does not include this feature.

4.  Guidelines for Usage in HTTP Header Field Definitions

   Specifications of HTTP header fields that use the extensions defined
   in Section 3.2 ought to clearly state that.  A simple way to achieve
   this is to normatively reference this specification and to include
   the ext-value production into the ABNF for specific header field
   parameters.

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   For instance:

     foo         = token ";" LWSP title-param
     title-param = "title" LWSP "=" LWSP value
                 / "title*" LWSP "=" LWSP ext-value
     ext-value   = <see RFC 8187, Section 3.2>

      Note: The Parameter Value Continuation feature defined in
      Section 3 of [RFC2231] makes it impossible to have multiple
      instances of extended parameters with identical names, as the
      processing of continuations would become ambiguous.  Thus,
      specifications using this extension are advised to disallow this
      case for compatibility with RFC 2231.

      Note: This specification does not automatically assign a new
      interpretation to parameter names ending in an asterisk.  As
      pointed out above, it's up to the specification for the
      non-extended parameter to "opt in" to the syntax defined here.
      That being said, some existing implementations are known to
      automatically switch to using this notation when a parameter name
      ends with an asterisk; thus, using parameter names ending in an
      asterisk for something else is likely to cause interoperability
      problems.

4.1.  When to Use the Extension

   Section 4.2 of [RFC2277] requires that protocol elements containing
   human-readable text be able to carry language information.  Thus, the
   ext-value production ought to always be used when the parameter value
   is of a textual nature and its language is known.

   Furthermore, the extension ought to also be used whenever the
   parameter value needs to carry characters not present in the US-ASCII
   coded character set ([RFC0020]); note that it would be unacceptable
   to define a new parameter that would be restricted to a subset of the
   Unicode character set.

4.2.  Error Handling

   Header field specifications need to define whether multiple instances
   of parameters with identical names are allowed and how they should be
   processed.  This specification suggests that a parameter using the
   extended syntax takes precedence.  This would allow producers to use
   both formats without breaking recipients that do not understand the
   extended syntax yet.

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   Example:

     foo: bar; title="EURO exchange rates";
               title*=utf-8''%e2%82%ac%20exchange%20rates

   In this case, the sender provides an ASCII version of the title for
   legacy recipients, but also includes an internationalized version for
   recipients understanding this specification -- the latter obviously
   ought to prefer the new syntax over the old one.

5.  Security Considerations

   The format described in this document makes it possible to transport
   non-ASCII characters, and thus enables character "spoofing" scenarios
   in which a displayed value appears to be something other than it is.

   Furthermore, there are known attack scenarios related to decoding
   UTF-8.

   See Section 10 of [RFC3629] for more information on both topics.

   In addition, the extension specified in this document makes it
   possible to transport multiple language variants for a single
   parameter, and such use might allow spoofing attacks where different
   language versions of the same parameter are not equivalent.  Whether
   this attack is effective as an attack depends on the parameter
   specified.

6.  IANA Considerations

   This document does not require any IANA actions.

7.  References

7.1.  Normative References

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <https://www.rfc-editor.org/info/rfc2978>.

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   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

7.2.  Informative References

   [Err1912]  RFC Errata, "Erratum ID 1912, RFC 2978",
              <https://www.rfc-editor.org/errata/eid1912>.

   [ISO-8859-1]
              International Organization for Standardization,
              "Information technology -- 8-bit single-byte coded graphic
              character sets -- Part 1: Latin alphabet No. 1", ISO/
              IEC 8859-1:1998, 1998.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/info/rfc2047>.

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   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions: Character Sets, Languages, and
              Continuations", RFC 2231, DOI 10.17487/RFC2231, November
              1997, <https://www.rfc-editor.org/info/rfc2231>.

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, DOI 10.17487/RFC2277,
              January 1998, <https://www.rfc-editor.org/info/rfc2277>.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <https://www.rfc-editor.org/info/rfc2616>.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, DOI 10.17487/RFC5987, August 2010,
              <https://www.rfc-editor.org/info/rfc5987>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <https://www.rfc-editor.org/info/rfc5988>.

   [RFC6266]  Reschke, J., "Use of the Content-Disposition Header Field
              in the Hypertext Transfer Protocol (HTTP)", RFC 6266,
              DOI 10.17487/RFC6266, June 2011,
              <https://www.rfc-editor.org/info/rfc6266>.

   [RFC6365]  Hoffman, P. and J. Klensin, "Terminology Used in
              Internationalization in the IETF", BCP 166, RFC 6365,
              DOI 10.17487/RFC6365, September 2011,
              <https://www.rfc-editor.org/info/rfc6365>.

   [RFC7578]  Masinter, L., "Returning Values from Forms: multipart/
              form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015,
              <https://www.rfc-editor.org/info/rfc7578>.

   [RFC7616]  Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
              Digest Access Authentication", RFC 7616,
              DOI 10.17487/RFC7616, September 2015,
              <https://www.rfc-editor.org/info/rfc7616>.

   [RFC8053]  Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
              T., and Y. Ioku, "HTTP Authentication Extensions for
              Interactive Clients", RFC 8053, DOI 10.17487/RFC8053,
              January 2017, <https://www.rfc-editor.org/info/rfc8053>.

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   [XMLHttpRequest]
              WhatWG, "XMLHttpRequest", <https://xhr.spec.whatwg.org/>.

Appendix A.  Changes from RFC 5987

   This section summarizes the changes compared to [RFC5987]:

   o  The document title was changed to "Indicating Character Encoding
      and Language for HTTP Header Field Parameters".

   o  The introduction was rewritten to better explain the issues around
      non-ASCII characters in field values.

   o  The requirement to support the "ISO-8859-1" encoding was removed.

   o  This document no longer attempts to redefine a generic "parameter"
      ABNF (it turned out that there really isn't a generic definition
      of parameters in HTTP; for instance, there are subtle differences
      with respect to whitespace handling).

   o  A note about defects in error handling in current implementations
      was removed, as it was no longer accurate.

Appendix B.  Implementation Report

   The encoding defined in this document is currently used in four
   different HTTP header fields:

   o  "Authentication-Control", defined in [RFC8053],

   o  "Authorization" (as used in HTTP Digest Authentication, defined in
      [RFC7616]),

   o  "Content-Disposition", defined in [RFC6266], and

   o  "Link", defined in [RFC5988].

   As the encoding is a profile/clarification of the one defined in
   [RFC2231] in 1997, many user agents already supported it for use in
   "Content-Disposition" when [RFC5987] was published.

   Since the publication of [RFC5987], three more popular desktop user
   agents have added support for this encoding; see
   <http://purl.org/NET/http/content-disposition-tests#encoding-
   2231-char> for details.  At this time, the current versions of all
   major desktop user agents support it.

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   Note that the implementation in Internet Explorer 9 does not support
   the ISO-8859-1 character encoding; this document revision
   acknowledges that UTF-8 is sufficient for expressing all code points
   and removes the requirement to support ISO-8859-1.

   The "Link" header field, on the other hand, was more recently
   specified in [RFC5988].  At the time of this writing, no user agent
   except Firefox supported the "title*" parameter (starting with
   release 15).

   Section 3.4 of [RFC7616] defines the "username*" parameter for use in
   HTTP Digest Authentication.  At the time of writing, no user agent
   implemented this extension.

Acknowledgements

   Thanks to Martin Dürst and Frank Ellermann for help figuring out ABNF
   details, to Graham Klyne and Alexey Melnikov for general review, to
   Chris Newman for pointing out an RFC 2231 incompatibility, and to
   Benjamin Carlyle, Roar Lauritzsen, Eric Lawrence, and James Manger
   for implementers feedback.

   Furthermore, thanks to the members of the IETF HTTP Working Group for
   the feedback specific to this update of RFC 5987.

Author's Address

   Julian F. Reschke
   greenbytes GmbH
   Hafenweg 16
   Münster, NW  48155
   Germany

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

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