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rfc:rfc8187



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

Reschke Standards Track [Page 1] RFC 8187 Charset/Language Encoding in HTTP September 2017

 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>.

Reschke Standards Track [Page 9] RFC 8187 Charset/Language Encoding in HTTP September 2017

 [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>.

Reschke Standards Track [Page 10] RFC 8187 Charset/Language Encoding in HTTP September 2017

 [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>.

Reschke Standards Track [Page 11] RFC 8187 Charset/Language Encoding in HTTP September 2017

 [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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