RFC 1641 (rfc1641) - Page 2 of 6

Using Unicode with MIME

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



Goldsmith & Davis