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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5987

Network Working Group                                         J. Reschke
Internet-Draft                                                greenbytes
Intended status: Standards Track                         August 15, 2008
Expires: February 16, 2009


                 Applicability of RFC 2231 Encoding to
               Hypertext Transfer Protocol (HTTP) Headers
                    draft-reschke-rfc2231-in-http-00

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Abstract

   By default, message header parameters in Hypertext Transfer Protocol
   (HTTP) messages can not carry characters outside the ISO-8859-1
   character set.  RFC 2231 defines an escaping mechanism for use in
   Multipurpose Internet Mail Extensions (MIME) headers.  This document
   specifies a profile of that encoding suitable for use in HTTP.

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

   There are multiple HTTP headers that already use RFC 2231 encoding in
   practice (Content-Disposition) or might use it in the future (Link).
   The purpose of this document is to provide a single place where the



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   generic aspects of RFC 2231 encoding in HTTP headers are defined.

   Distribution of this document is unlimited.  Although this is not a
   work item of the HTTPbis Working Group, comments should be sent to
   the Hypertext Transfer Protocol (HTTP) mailing list at
   ietf-http-wg@w3.org [1], which may be joined by sending a message
   with subject "subscribe" to ietf-http-wg-request@w3.org [2].

   Discussions of the HTTPbis Working Group are archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

   XML versions, latest edits and the issues list for this document are
   available from
   <http://greenbytes.de/tech/webdav/#draft-reschke-rfc2231-in-http>.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  A Profile of RFC 2231 for Use in HTTP  . . . . . . . . . . . .  3
     3.1.  Parameter Continuations  . . . . . . . . . . . . . . . . .  4
     3.2.  Parameter Value Character Set and Language Information . .  4
       3.2.1.  Examples . . . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Language specification in Encoded Words  . . . . . . . . .  6
   4.  Guidelines for Usage in HTTP Header Definitions  . . . . . . .  6
     4.1.  When to Use the Extension  . . . . . . . . . . . . . . . .  6
     4.2.  Error Handling . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  8
   Intellectual Property and Copyright Statements . . . . . . . . . . 10















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1.  Introduction

   By default, message header parameters in HTTP ([RFC2616]) messages
   can not carry characters outside the ISO-8859-1 character set
   ([ISO-8859-1]).  RFC 2231 ([RFC2231]) defines an escaping mechanism
   for use in MIME headers.  This document specifies a profile of that
   encoding for use in HTTP.


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 augmented BNF notation defined in Section
   2.1 of [RFC2616], including its rules for linear whitespace (LWS).
   [[LWS: This needs to be checked.]]

   Non-ASCII characters used in prose for examples are encoded using the
   format "Backslash-U with Delimiters", defined in Section 5.1 of
   [RFC5137].

   Note that this specification uses the term "character set" for
   consistency with other IETF specifications such as RFC 2277 (see
   [RFC2277], Section 3).  A more accurate term would be "character
   encoding" (a mapping of code points to octet sequences).


3.  A Profile of RFC 2231 for Use in HTTP

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

   In short:

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

   o  Character Set 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).








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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 ([RFC2616], Section 19.4.7).

   Thus in HTTP, senders MUST NOT use parameter continuations, and
   therefore recipients do not need to support them.

3.2.  Parameter Value Character Set 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 parameters.

   However, RFC 2231 does not specify mandatory-to-implement character
   encoding, making it hard for senders to decide which character set to
   use.  Thus, recipients implementing this specification MUST support
   the character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].

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

   The syntax for parameters is defined in Section 3.6 of [RFC2616]:

     parameter     = attribute "=" value


     attribute     = token
     value         = token | quoted-string

     quoted-string = <quoted-string, defined in [RFC2616], Section 2.2>
     token         = <token, defined in [RFC2616], Section 2.2>

   This specification extends the grammar to:















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     parameter     = reg-parameter | ext-parameter

     reg-parameter = attribute "=" value
     ext-parameter = attribute "*=" ext-value

     ext-value     = charset  "'" [ language ] "'" value-chars

     charset       = "UTF-8" | "ISO-8859-1" | ext-charset
                   ; NOTE: case-insensitive

     ext-charset   = token ; see IANA charset registry
                   ; (<http://www.iana.org/assignments/character-sets>)

     language      = <Language-Tag, defined in [RFC4646], Section 2.1>

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

     pct-encoded   = "%" HEXDIG HEXDIG

     attr-char     = ALPHA | DIGIT
                   | "-" | "." | "_" | "~" | ":"
                   | "!" | "$" | "&" | "+"

     ALPHA         = %x41-5A | %x61-7A
                   ; A-Z | a-z
     DIGIT         = %x30-39
                   ; any US-ASCII digit "0".."9"
     HEXDIG        = DIGIT | "A" | "B" | "C" | "D" | "E" | "F"
                   ; NOTE: case-insensitive

3.2.1.  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*=iso-8859-1'en'%A3%20rates

   Note: the Unicode pound sign character \u'00A3' was encoded using
   ISO-8859-1 into the single octet A3, then percent-encoded.  Also note
   that the space character was encoded as %20, as attr-char does not
   contain it.



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   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 using
   UTF-8 into the octet sequence C2 A3, 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 lower-case and upper-case character, so recipients must
   understand both, and that the language information is optional, while
   the character set 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.  Although the
   HTTP/1.1 does refer to RFC 2047 ([RFC2616], Section 2.2), it's not
   clear to which header field exactly it applies, and whether it is
   implemented in practice (see
   <http://tools.ietf.org/wg/httpbis/trac/ticket/111> for details).

   Thus, the RFC 2231 profile defined by this specification does not
   include this feature.


4.  Guidelines for Usage in HTTP Header Definitions

   Specifications of HTTP headers that use the extensions defined in
   Section 3.2 should clearly state that.  The best way to achieve this
   is to normatively reference this specification, and to include the
   ext-parameter production into the ABNF for that header.

4.1.  When to Use the Extension

   Section 4.2 of [RFC2277] requires that protocol elements containing
   text can carry language information.  Thus, the ext-parameter
   production should always be used when the parameter value is of
   textual nature.

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






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4.2.  Error Handling

   Header specifications that include parameters should also specify
   whether same-named parameters can occur multiple times.  If
   repetitions are not allowed (and this is believed to be the common
   case), the specification should state whether regular or the extended
   syntax takes precedence.  In the latter case, this could be used by
   senders to use both formats without breaking recipients that do not
   understand the syntax.

   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 recipient, but also includes an internationalized version for
   recipients understanding this specification -- the latter obviously
   should prefer the new syntax over the old one.


5.  Security Considerations

   This document does not discuss security issues and is not believed to
   raise any security issues not already endemic in HTTP.


6.  IANA Considerations

   There are no IANA Considerations related to this specification.


7.  Acknowledgements

   Thanks to Frank Ellermann for help figuring out BNF details.


8.  References

8.1.  Normative References

   [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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate



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              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
              Word Extensions:
              Character Sets, Languages, and Continuations", RFC 2231,
              November 1997.

   [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.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 3629, STD 63, November 2003.

   [RFC4646]  Phillips, A. and M. Davis, "Tags for Identifying
              Languages", BCP 47, RFC 4646, September 2006.

8.2.  Informative References

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2277]  Alvestrand, H., "IETF Policy on Character Sets and
              Languages", BCP 18, RFC 2277, January 1998.

   [RFC5137]  Klensin, J., "ASCII Escaping of Unicode Characters",
              BCP 137, RFC 5137, February 2008.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information
              Interchange", ANSI X3.4, 1986.

URIs

   [1]  <mailto:ietf-http-wg@w3.org>

   [2]  <mailto:ietf-http-wg-request@w3.org?subject=subscribe>













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Author's Address

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

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









































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