Handling of Bi-directional Texts in MIME (RFC1556)
Original Publication Date: 1993-Dec-01
Included in the Prior Art Database: 2000-Sep-12
Internet Society Requests For Comment (RFCs)
This document describes the format and syntax of the "direction" keyword to be used with bi-directional texts in MIME.
Network Working Group H. Nussbacher
Request for Comments: 1556 Israeli Inter-University
Category: Informational Computer Center
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.
This document describes the format and syntax of the "direction"
keyword to be used with bi-directional texts in MIME.
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
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 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 directionality is a presentation...