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Network Working Group                                      H. NussbacherRequest for Comments: 1556                      Israeli Inter-UniversityCategory: Informational                                  Computer Center                                                           December 1993Handling of Bi-directional Texts in MIMEStatus 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 standardRFC822 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-iNussbacher                                                      [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.ilNussbacher                                                      [Page 3]