Network Working Group                                       D. Goldsmith
Request for Comments: 1641                                      M. Davis
Category: Experimental                                    Taligent, Inc.
                                                               July 1994


                        Using Unicode with MIME

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Distribution of this memo is unlimited.

Abstract

   The Unicode Standard, version 1.1, and ISO/IEC 10646-1:1993(E)
   jointly define a 16 bit character set (hereafter referred to as
   Unicode) which encompasses most of the world's writing systems.
   However, Internet mail (STD 11, RFC 822) currently supports only 7-
   bit US ASCII as a character set. MIME (RFC 1521 and RFC 1522) extends
   Internet mail to support different media types and character sets,
   and thus could support Unicode in mail messages. MIME neither defines
   Unicode as a permitted character set nor specifies how it would be
   encoded, although it does provide for the registration of additional
   character sets over time.

   This document specifies the usage of Unicode within MIME.

Motivation

   Since Unicode is starting to see widespread commercial adoption,
   users will want a way to transmit information in this character set
   in mail messages and other Internet media. Since MIME was expressly
   designed to allow such extensions and is on the standards track for
   the Internet, it is the most appropriate means for encoding Unicode.
   RFC 1521 and RFC 1522 do not define Unicode as an allowed character
   set, but allow registration of additional character sets.

   In addition to allowing use of Unicode within MIME bodies, another
   goal is to specify a way of using Unicode that allows text which
   consists largely, but not entirely, of US-ASCII characters to be
   represented in a way that can be read by mail clients who do not
   understand Unicode. This is in keeping with the philosophy of MIME.
   Such an encoding is described in another document, "UTF-7: A Mail
   Safe Transformation Format of Unicode" [UTF-7].

Overview

   Several ways of using Unicode are possible. This document specifies
   both guidelines for use of Unicode within MIME, and a specific usage.
   The usage specified in this document is a straightforward use of
   Unicode as specified in "The Unicode Standard, Version 1.1".

   This encoding is intended for situations where sender and recipient
   do not want to do a lot of processing, when the text does not consist
   primarily of characters from the US-ASCII character set, or when
   sender and receiver are known in advance to support Unicode.

   Another encoding is intended for situations where the text consists
   primarily of US-ASCII, with occasional characters from other parts of
   Unicode. This encoding allows the US-ASCII portion to be read by all
   recipients without having to support Unicode. This encoding is
   specified in another document, "UTF-7: A Mail Safe Transformation
   Format of Unicode" [UTF-7].

   Finally, in keeping with the principles set forth in RFC 1521, text
   which can be represented using the US-ASCII or ISO-8859-x character
   sets should be so represented where possible, for maximum
   interoperability.

Definitions

   The definition of character set Unicode:

      The 16 bit character set Unicode is defined by "The Unicode
      Standard, Version 1.1". This character set is identical with the
      character repertoire and coding of the international standard
      ISO/IEC 10646-1:1993(E); Coded Representation Form=UCS-2;
      Subset=300; Implementation Level=3.

      Note. Unicode 1.1 further specifies the use and interaction of
      these character codes beyond the ISO standard. However, any valid
      10646 BMP (Basic Multilingual Plane) sequence is a valid Unicode
      sequence, and vice versa; Unicode supplies interpretations of
      sequences on which the ISO standard is silent as to
      interpretation.

      This character set is encoded as sequences of octets, two per 16-
      bit character, with the most significant octet first. Text with an
      odd number of octets is ill-formed.

      Rationale. ISO/IEC 10646-1:1993(E) specifies that when characters
      in the UCS-2 form are serialized as octets, that the most
      significant octet appear first.  This is also in keeping with

      common network practice of choosing a canonical format for
      transmission.

General Specification of Unicode Character Sets Within MIME

   The Unicode Standard is currently at version 1.1. Although new
   versions should be compatible with old implementations if an
   implementation is compliant with the standard, some implementations
   may choose to check the version of the character set that is being
   used. In order to allow some implementations to check the version
   number and allow others to ignore it, all registrations of Unicode
   variants and versions for MIME usage should have MIME charset names
   which conform to one of the two following patterns:

      UNICODE-major-minor
      UNICODE-major-minor-variant

   Where major and minor are strings of decimal digits (0 through 9)
   specifying the major and minor version number of the Unicode standard
   to which the text in question conforms. In the interests of
   interoperability, the lowest version number compatible with the text
   should be used. The lowest acceptable version number is UNICODE-1-1,
   corresponding to "The Unicode Standard, Version 1.1". The optional
   trailing string "variant" describes the particular transformation
   format of Unicode that the registration describes; its content is up
   to the particular registration. If there is no trailing variant
   string, the charset name refers to the basic two octet form of
   Unicode as described in "The Unicode Standard".

   Example. A hypothetical charset which referred to the UTF-8
   transformation format of Unicode/10646 (also known as UTF-2 or UTF-
   FSS) might be named UNICODE-1-1-UTF-8.

Encoding Character Set Unicode Within MIME

   Character set Unicode uses 16 bit characters, and therefore would
   normally be used with the Binary or Base64 content transfer encodings
   of MIME. In header fields, it would normally be used with the B
   content transfer encoding. The MIME character set identifier is
   UNICODE-1-1.

   Example. Here is a text portion of a MIME message containing the
   Japanese word "nihongo" (hexadecimal 65E5,672C,8A9E) written in Han
   characters.

   Content-Type: text/plain; charset=UNICODE-1-1
   Content-Transfer-Encoding: base64

   ZeVnLIqe

   Example. Here is a text portion of a MIME message containing the
   Unicode sequence "A<NOT IDENTICAL TO><ALPHA>." (hexadecimal
   0041,2262,0391,002E)

   Content-Type: text/plain; charset=UNICODE-1-1
   Content-Transfer-Encoding: base64

   AEEiYgORAC4=

Acknowledgements

   Many thanks to the following people for their contributions,
   comments, and suggestions. If we have omitted anyone it was through
   oversight and not intentionally.

         Glenn Adams
         Harald T. Alvestrand
         Nathaniel Borenstein
         Lee Collins
         Jim Conklin
         Dave Crocker
         Steve Dorner
         Dana S. Emery
         Ned Freed
         John H. Jenkins
         John C. Klensin
         Valdis Kletnieks
         Keith Moore
         Masataka Ohta
         Einar Stefferud

Security Considerations

   Security issues are not discussed in this memo.

References

[UNICODE 1.1] "The Unicode Standard, Version 1.1": Version 1.0, Volume 1
              (ISBN 0-201-56788-1), Version 1.0, Volume 2 (ISBN 0-201-
              60845-6), and "Unicode Technical Report #4, The Unicode
              Standard, Version 1.1" (available from The Unicode
              Consortium, and soon to be published by Addison-Wesley).

[ISO 10646]   ISO/IEC 10646-1:1993(E) Information Technology--Universal
              Multiple-octet Coded Character Set (UCS).

[UTF-7]       Goldsmith, D., and M. Davis, "UTF-7: A Mail Safe
              Transformation Format of Unicode", RFC 1642, Taligent,
              Inc., July 1994.

[US-ASCII]    Coded Character Set--7-bit American Standard Code for
              Information Interchange, ANSI X3.4-1986.

[ISO-8859]    Information Processing -- 8-bit Single-Byte Coded Graphic
              Character Sets -- Part 1: Latin Alphabet No. 1, ISO 8859-
              1:1987.  Part 2: Latin alphabet No.  2, ISO 8859-2, 1987.
              Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.  Part 4:
              Latin alphabet No.  4, ISO 8859-4, 1988.  Part 5:
              Latin/Cyrillic alphabet, ISO 8859-5, 1988.  Part 6:
              Latin/Arabic alphabet, ISO 8859-6, 1987.  Part 7:
              Latin/Greek alphabet, ISO 8859-7, 1987.  Part 8:
              Latin/Hebrew alphabet, ISO 8859-8, 1988.  Part 9: Latin
              alphabet No. 5, ISO 8859-9, 1990.

[RFC822]      Crocker, D., "Standard for the Format of ARPA Internet
              Text Messages", STD 11, RFC 822, UDEL, August 1982.

[RFC-1521]    Borenstein N., and N. Freed, "MIME (Multipurpose Internet
              Mail Extensions) Part One:  Mechanisms for Specifying and
              Describing the Format of Internet Message Bodies", RFC
              1521, Bellcore, Innosoft, September 1993.

[RFC-1522]    Moore, K., "Representation of Non-Ascii Text in Internet
              Message Headers" RFC 1522, University of Tennessee,
              September 1993.

[UTF-8]       X/Open Company Ltd., "File System Safe UCS Transformation
              Format (FSS_UTF)", X/Open Preliminary Specification,
              Document Number: P316. This information also appears in
              Unicode Technical Report #4, and in a forthcoming annex to
              ISO/IEC 10646.

Authors' Addresses

   David Goldsmith
   Taligent, Inc.
   10201 N. DeAnza Blvd.
   Cupertino, CA 95014-2233

   Phone: 408-777-5225
   Fax: 408-777-5081
   EMail: david_goldsmith@taligent.com


   Mark Davis
   Taligent, Inc.
   10201 N. DeAnza Blvd.
   Cupertino, CA 95014-2233

   Phone: 408-777-5116
   Fax: 408-777-5081
   EMail: mark_davis@taligent.com