INTERNET-DRAFT
Network Working Group                                      H. Nussbacher
Request for Comments: nnnn                      Israeli Inter-University
                                                         Computer Center
                                                             August 1993


             Handling of Bi-directional Texts in MIME


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Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  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 1341 and 1342) 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.

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   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 parameter is being defined for MIME
   Content-type. This new field is called "direction=":

   Content-type: text/plain; charset=ISO-8859-6; direction=drct
   or
   Content-type: text/plain; charset=ISO-8859-8; direction=drct

   This new Content-type parameter is valid only for "text/plain".
   The value of drct can be either implicit, explicit or visual.

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.

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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.

   [RFC1341] Borenstein N., and N. Freed, "MIME (Multipurpose
   Internet Mail Extensions): Mechanisms for Specifying and
   Describing the Format of Internet Message Bodies", Bellcore,
   Innosoft, June 1992.

   [RFC1342] Moore K., "Representation of Non-ASCII Text in Internet
   Message Headers", University of Tennessee, June 1992.

Security Considerations

   Security issues are not discussed in this memo.

Authors' Addresses

   Hank Nussbacher
   Computer Center
   Tel Aviv University
   Ramat Aviv
   Israel

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



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