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rfc:rfc1556

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

Nussbacher [Page 1] RFC 1556 Bi-directional Texts December 1993

 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.

Nussbacher [Page 2] RFC 1556 Bi-directional Texts December 1993

 [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

Nussbacher [Page 3]

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