Network Working Group
Request for Comments: 1556
Category: Informational
H. Nussbacher
Israeli Inter-University
Computer Center
December 1993
この文書は、インターネット界に対して情報提供をしようとするものです。何等、インターネット標準などの定義を行うものではありません。文書の配布に関しては制限を設けません。
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.
この文書は、MIMEでの双方向書字文書の取り扱いに関して、『書字方向』という用語の形式と文法について記したものです。
This document describes the format and syntax of the "direction" keyword to be used with bi-directional texts in MIME.
非ASCIIデータをRFC822準拠の電子メールで送る方法について、MIME標準文書(「RFC1521」と「RFC1522」)が定義を行っています。より正確に言うと、「Content-type」部には、ISO準拠のどんな言語情報でも記載することができるようになっています。例えばアラビア語(ISO-8859-6)やヘブライ語(ISO-8859-8)などでも。ここで問題になるのが、この2つはどちらも基本的に右から左に向けて読み書きする言語であって、しかも文中に逆向きに記す言語のデータをも含み得るということです。例えば、ヘブライ語の文中に英語を交ぜるというように。
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.
幸いにも、この問題に関してECMA(ヨーロッパコンピュータ協会)が先行して解決に挑んでおり、『双方向書字文書の扱い方』と題した技術レポートをまとめています。「ECMA TR/53」と呼ばれるこのレポートは、「fast track procedure」での使用が認めらている「ISO/IEC 6429」に基づく文書流通方法を定めた標準文書「ECMA-48」の改訂を試みたものです。「ECMA TR/53」は、これらを元に、双方向書字文書の方向性指定を含めた符号化を行う際に、明示指向性の符号化を行うものか暗示指向性の符号化を行うものかを表すための、新しい文字セットを定義したものです。既定の文字セットは、見たまま指向性の符号化として捉えられており、「ISO-8859」に定められている以外に何ら特別な制御文字などを必要としないものです。
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.
双方向書字をサポートするための、新しい文字セットの例をいくつか挙げます。
Examples of new character sets for bi-directionality support:
末尾の「i」は暗示モードであることを示し、「e」は明示モードであることを示します。
The "i" suffix refers to implicit mode and the "e" suffix refers to explicit mode.
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 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.
Handling of Bi-Directional Texts, European Computer Manufacturers Association, 114 Rue du Rhone, CH-1204, Geneva, Switzerland, June 1992.
Information Technology - Control Functions for Coded Character Sets, 3rd edition, December 15, 1992.
Information Processing -- 8-bit Single-Byte Coded Graphic Character Sets, Part 6: Arabic alphabet, ISO 8859-6, 1988.
Information Processing -- 8-bit Single-Byte Coded Graphic Character Sets, Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
Crocker, D., "Standard for the Format of ARPA Internet Text Messages", STD 11, RFC 822, UDEL, August 1982.
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.
Moore K., "MIME Part Two: Message Header Extensions for Non-ASCII Text", University of Tennessee, September 1993.
セキュリティーについてはこの文書の範疇外です。
Security issues are not discussed in this memo.
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1998 内田明 UCHIDA,Akira