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: [email protected]




















Nussbacher                                                      [Page 3]


You are viewing proxied material from gopher.fnord.one. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.