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INTERNET-DRAFT
Network Working Group H. Nussbacher
Request for Comments: nnnn Israeli Inter-University
Computer Center
August 1993
Handling of Bi-directional Texts in MIME
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups. Note that other
groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted
by other documents at any time. It is not appropriate to use
Internet-Drafts as reference material or to cite them other
than as a ``working draft'' or ``work in progress.''
To learn the current status of any Internet-Draft, please check
the 1id-abstracts.txt listing contained in the Internet-Drafts
Shadow Directories on ds.internic.net, nic.nordu.net,
ftp.nisc.sri.com, or munnari.oz.au.
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. 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 1341 and 1342) 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.
Nussbacher Expires January 15, 1994 [Page 1]
INTERNET-DRAFT Hebrew email encodings June 1993
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 parameter is being defined for MIME
Content-type. This new field is called "direction=":
Content-type: text/plain; charset=ISO-8859-6; direction=drct
or
Content-type: text/plain; charset=ISO-8859-8; direction=drct
This new Content-type parameter is valid only for "text/plain".
The value of drct can be either implicit, explicit or visual.
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.
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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.
[RFC1341] Borenstein N., and N. Freed, "MIME (Multipurpose
Internet Mail Extensions): Mechanisms for Specifying and
Describing the Format of Internet Message Bodies", Bellcore,
Innosoft, June 1992.
[RFC1342] Moore K., "Representation of Non-ASCII Text in Internet
Message Headers", University of Tennessee, June 1992.
Security Considerations
Security issues are not discussed in this memo.
Authors' Addresses
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 Expires January 15, 1994 [Page 3]