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