RFC 2152 (rfc2152) - Page 1 of 15

UTF-7 A Mail-Safe Transformation Format of Unicode

Alternative Format: Original Text Document
Next >
Network Working Group                                       D. Goldsmith
Request for Comments: 2152                          Apple Computer, Inc.
Obsoletes: RFC 1642                                             M. Davis
Category: Informational                                   Taligent, Inc.
                                                                May 1997


                                 UTF-7

              A Mail-Safe Transformation Format of Unicode

Status of this Memo

   This memo provides information 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 2.0, and ISO/IEC 10646-1:1993(E) (as
   amended) jointly define a 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 2045 through 2049) 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 describes a transformation format of Unicode that
   contains only 7-bit ASCII octets and is intended to be readable by
   humans in the limiting case that the document consists of characters
   from the US-ASCII repertoire. It also specifies how this
   transformation format is used in the context of MIME and RFC 1641,
   "Using Unicode with MIME".

Motivation

   Although other transformation formats of Unicode exist and could
   conceivably be used in this context (most notably UTF-8, also known
   as UTF-2 or UTF-FSS), they suffer the disadvantage that they use
   octets in the range decimal 128 through 255 to encode Unicode
   characters outside the US-ASCII range. Thus, in the context of mail,
   those octets must themselves be encoded. This requires putting text
   through two successive encoding processes, and leads to a significant
   expansion of characters outside the US-ASCII range, putting non-
   English speakers at a disadvantage. For example, using UTF-8 together



Goldsmith & Davis            Informational
Next >