Indicating Character Encoding and Language for HTTP Header Field Parameters
draft-ietf-httpbis-rfc5987bis-04



HTTP Working Group                                            J. Reschke
Internet-Draft                                                greenbytes
Obsoletes: 5987 (if approved)                            January 9, 2017
Intended status: Standards Track
Expires: July 13, 2017

    Indicating Character Encoding and Language for HTTP Header Field
                               Parameters
                    draft-ietf-httpbis-rfc5987bis-04

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.

Editorial Note (To be removed by RFC Editor before publication)

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

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

   The changes in this draft are summarized in Appendix C.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 13, 2017.

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
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  Comparison to RFC 2231 and Definition of the Encoding  . . . .  5
     3.1.  Parameter Continuations  . . . . . . . . . . . . . . . . .  5
     3.2.  Parameter Value Character Encoding and Language
           Information  . . . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  Definition . . . . . . . . . . . . . . . . . . . . . .  6
       3.2.2.  Historical Notes . . . . . . . . . . . . . . . . . . .  7
       3.2.3.  Examples . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Language Specification in Encoded Words  . . . . . . . . .  8
   4.  Guidelines for Usage in HTTP Header Field Definitions  . . . .  9
     4.1.  When to Use the Extension  . . . . . . . . . . . . . . . .  9
     4.2.  Error Handling . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Changes from RFC 5987 . . . . . . . . . . . . . . . . 13
   Appendix B.  Implementation Report . . . . . . . . . . . . . . . . 14
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 14
     C.1.  Since RFC5987  . . . . . . . . . . . . . . . . . . . . . . 14
     C.2.  Since draft-reschke-rfc5987bis-00  . . . . . . . . . . . . 14
     C.3.  Since draft-reschke-rfc5987bis-01  . . . . . . . . . . . . 15
     C.4.  Since draft-reschke-rfc5987bis-02  . . . . . . . . . . . . 15
     C.5.  Since draft-reschke-rfc5987bis-03  . . . . . . . . . . . . 15
     C.6.  Since draft-reschke-rfc5987bis-04  . . . . . . . . . . . . 15
     C.7.  Since draft-reschke-rfc5987bis-05  . . . . . . . . . . . . 15
     C.8.  Since draft-reschke-rfc5987bis-06  . . . . . . . . . . . . 15
     C.9.  Since draft-ietf-httpbis-rfc5987bis-00 . . . . . . . . . . 15
     C.10. Since draft-ietf-httpbis-rfc5987bis-01 . . . . . . . . . . 15
     C.11. Since draft-ietf-httpbis-rfc5987bis-02 . . . . . . . . . . 15
     C.12. Since draft-ietf-httpbis-rfc5987bis-03 . . . . . . . . . . 16
   Appendix D.  Acknowledgements  . . . . . . . . . . . . . . . . . . 16

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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" 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.

   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; they are therefore 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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This specification uses the ABNF (Augmented Backus-Naur Form)
   notation defined in [RFC5234].  The following core rules are included

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   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" (below abbreviated to "character
   encoding"), "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),

   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

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   left out.  The encoding defined by this specification does not allow
   that.

3.2.1.  Definition

   The presence of extended parameter values usually is indicated by a
   parameter name ending in an asterisk character.  Note however that
   this is just a convention, and that it needs 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:

     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

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   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 [RFC2978], Section 2.3 and [RFC5646], Section
   2.1.1).

   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].

   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,

      *  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 ("{" 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

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   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

   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, 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, then
   percent-encoded.  Likewise, the Unicode euro sign character U+20AC
   was encoded into the octet sequence E2 82 AC, 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

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   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.

   For instance:

     foo         = token ";" LWSP title-param
     title-param = "title" LWSP "=" LWSP value
                 / "title*" LWSP "=" LWSP ext-value
     ext-value   = <see draft-ietf-httpbis-rfc5987bis, Section 3.2>

   [[pub: Upon publication as RFC, the string
   "draft-ietf-httpbis-rfc5987bis" needs to be replaced with the RFC
   name, and this comment needs to be removed.]]

      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 the use of 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 are able to carry language information.  Thus,
   the ext-value production ought to be always used when the parameter
   value is of textual nature and its language is known.

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   Furthermore, the extension ought to also be used whenever the
   parameter value needs to carry characters not present in the US-ASCII
   ([RFC0020]) coded character set (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.

   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 relating 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 useful as an attack depends on the parameter
   specified.

6.  IANA Considerations

   There are no IANA Considerations related to this specification.

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7.  References

7.1.  Normative References

   [RFC0020]         Cerf, V., "ASCII format for network interchange",
                     STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969,
                     <http://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,
                     <http://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,
                     <http://www.rfc-editor.org/info/rfc2978>.

   [RFC3629]         Yergeau, F., "UTF-8, a transformation format of ISO
                     10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
                     November 2003,
                     <http://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,
                     <http://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,
                     <http://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,
                     <http://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,
                     <http://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,

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                     June 2014,
                     <http://www.rfc-editor.org/info/rfc7231>.

7.2.  Informative References

   [Err1912]         RFC Errata, "Errata ID 1912, RFC 2978",
                     October 2009, <http://www.rfc-editor.org>.

   [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,
                     <http://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,
                     <http://www.rfc-editor.org/info/rfc2047>.

   [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,
                     <http://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,
                     <http://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,
                     <http://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,
                     <http://www.rfc-editor.org/info/rfc5987>.

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   [RFC5988]         Nottingham, M., "Web Linking", RFC 5988,
                     DOI 10.17487/RFC5988, October 2010,
                     <http://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,
                     <http://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,
                     <http://www.rfc-editor.org/info/rfc6365>.

   [RFC7578]         Masinter, L., "Returning Values from Forms:
                     multipart/form-data", RFC 7578, DOI 10.17487/
                     RFC7578, July 2015,
                     <http://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,
                     <http://www.rfc-editor.org/info/rfc7616>.

   [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  The document does not attempt to re-define a generic "parameter"
      ABNF anymore (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 wasn't accurate anymore.

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Appendix B.  Implementation Report

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

   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] got 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.

   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.

Appendix C.  Change Log (to be removed by RFC Editor before publication)

C.1.  Since RFC5987

   Only editorial changes for the purpose of starting the revision
   process (obs5987).

C.2.  Since draft-reschke-rfc5987bis-00

   Resolved issues "iso-8859-1" and "title" (title simplified).  Added
   and resolved issue "historic5987".

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C.3.  Since draft-reschke-rfc5987bis-01

   Added issues "httpbis", "parmsyntax", "terminology" and
   "valuesyntax".  Closed issue "impls".

C.4.  Since draft-reschke-rfc5987bis-02

   Resolved issue "terminology".

C.5.  Since draft-reschke-rfc5987bis-03

   In Section 3.2, pull historical notes into a separate subsection.
   Resolved issues "valuesyntax" and "parmsyntax".

C.6.  Since draft-reschke-rfc5987bis-04

   Update status of Firefox support in HTTP Link Header field.

C.7.  Since draft-reschke-rfc5987bis-05

   Update status of Firefox support in HTTP Link Header field.

C.8.  Since draft-reschke-rfc5987bis-06

   Update status with respect to Safari 6.

   Started work on update with respect to RFC 723x.

C.9.  Since draft-ietf-httpbis-rfc5987bis-00

   Editorial changes; introducing non-ASCII characters into author's
   address, acknowledgements, and examples.

C.10.  Since draft-ietf-httpbis-rfc5987bis-01

   Removed mention of RFC 2616 from Abstract and Introduction.

   Reference RFC 20 for US-ASCII.

   Do not attempt to define a generic parameter ABNF; just concentrate
   on the parameter value syntax.

C.11.  Since draft-ietf-httpbis-rfc5987bis-02

   RFC 2388 -> RFC 7578.

   Expand on the motivation (see
   <https://github.com/httpwg/http-extensions/issues/213>).

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   Mention RFC 7616 in implementation report.

C.12.  Since draft-ietf-httpbis-rfc5987bis-03

   Fixed one editorial issue.  Updated XHR reference.

   Fixed <https://github.com/httpwg/http-extensions/issues/266>: use of
   now undefined term "parmname".

   Include WG into Acknowledgements for this revision.

   Mention RFC 5987 in the abstract
   (<https://github.com/httpwg/http-extensions/issues/271>).

Appendix D.  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 implementer's 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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