Using Unicode with MIME
RFC 1641

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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].

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RFC 1641                Using Unicode with MIME                July 1994

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

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RFC 1641                Using Unicode with MIME                July 1994

      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