Handling of Bi-directional Texts in MIME

Network Working Group                                      H. Nussbacher
Request for Comments: 1556                      Israeli Inter-University
Category: Informational                                  Computer Center
                                                           December 1993


                Handling of Bi-directional Texts in MIME

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

   This document describes the format and syntax of the "direction"
   keyword to be used with bi-directional texts in MIME.

Description

   The MIME standards (RFC 1521 and 1522) defined methods for
   transporting non-ASCII data via a standard RFC822 e-mail system.
   Specifically, the Content-type field allows for the inclusion of any
   ISO language such as Arabic (ISO-8859-6) or Hebrew (ISO-8859-8).  The
   problem is that the these two languages are read from right to left
   and can have bi-directional data such as mixed Hebrew and English on
   the same line.

   Fortunately, ECMA (European Computer Manufacturers Association) has
   tackled this problem previously and has issued a technical report
   called "Handling of Bi-Directional Texts".  ECMA TR/53, as it is
   called, was used to update the Standard ECMA-48 which in turn was
   used as the basis for ISO/IEC 6429 which was adopted under a special
   "fast track procedure". It is based on this information that a new
   character set is being defined which will indicate that the bi-
   directional message is either encoded in implicit mode or explicit
   mode.  The default is visual mode which requires no special character
   set other than the standard ones previously defined by ISO-8859.

   Examples of new character sets for bi-directionality support:

            Content-type: text/plain; charset=ISO-8859-6-e
            Content-type: text/plain; charset=ISO-8859-6-i
            Content-type: text/plain; charset=ISO-8859-8-e
            Content-type: text/plain; charset=ISO-8859-8-i

   The "i" suffix refers to implicit mode and the "e" suffix refers to
   explicit mode.

Implicit

   Implicit directionality is a presentation method in which the
   direction is determined by an algorithm according to the type of
   characters and their position relative to the adjacent characters and
   according to their primary direction.   The complete algorithm is
   quite complex and sites wishing to implement it should refer to the
   ECMA Technical Report for further details.

Explicit

   Explicit directionality is a presentation method in which the
   direction is explicitly defined by using control sequences which are
   interleaved within the text and are used for direction determination.
   This presentation method is also defined in ECMA TR/53, which defines
   three new control functions and updates 22 existing control functions
   in the ECMA-48 standard.

Visual

   Visual directionality is a presentation method that displays text
   according to the primary display direction only, which is left to
   right.  All text is viewed in the same direction which is the primary
   display direction.  The displaying application is not aware of the
   contents direction and displays the text as if it were a uni-
   directional text.  The composing application needs to prepare the
   text in such a way that it will be displayed correctly.  No control
   characters or algorithms are used to determine how the data is to be
   displayed. This is the simplest of all methods and the default method
   for use with MIME encoded texts.

References

   [ECMA TR/53] Handling of Bi-Directional Texts, European Computer
                Manufacturers Association, 114 Rue du Rhone, CH-1204,
                Geneva, Switzerland, June 1992.

   [ISO-6429]   Information Technology - Control Functions for Coded
                Character Sets, 3rd edition, December 15, 1992.

   [ISO-8859]   Information Processing -- 8-bit Single-Byte Coded
                Graphic Character Sets, Part 6: Arabic alphabet, ISO
                8859-6, 1988.

   [ISO-8859]   Information Processing -- 8-bit Single-Byte Coded
                Graphic Character Sets, Part 8: Latin/Hebrew alphabet,
                ISO 8859-8, 1988.

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

   [RFC1521]    Borenstein N., and N. Freed, "MIME (Multipurpose
                Internet Mail Extensions) Part One: Mechanisms for
                Specifying and Describing the Format of Internet
                Message Bodies", Bellcore, Innosoft, September 1993.

   [RFC1522]    Moore K., "MIME Part Two: Message Header Extensions for
                Non-ASCII Text", University of Tennessee,
                September 1993.

Security Considerations

   Security issues are not discussed in this memo.

Author's Address

   Hank Nussbacher
   Computer Center
   Tel Aviv University
   Ramat Aviv
   Israel

   Fax: +972 3 6409118
   Phone: +972 3 6408309
   EMail: hank@vm.tau.ac.il