Network Working Group Jun Murai
Internet Draft Mark Crispin
Erik van der Poel
1st December 1992
Japanese Character Encoding for Internet Message Bodies
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
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Please check the I-D abstract listing contained in each Internet
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informational document. This document will expire before 1st June
1993. Distribution of this memo is unlimited. Please send comments
to ietf-822@dimacs.rutgers.edu.
Introduction
This document describes the encoding used in electronic mail [RFC822]
and network news [RFC1036] message bodies in several Japanese
networks. It was first specified by and used in JUNET [JUNET]. The
encoding is now also widely used in Japanese IP communities.
This document names the encoding "ISO-2022-JP", which is intended to
be used in the "charset" parameter field of MIME [MIME] messages. The
use of ISO-2022-JP in RFC 1342 [RFC1342] headers is expected to be
the subject of a separate document.
This document only describes the encoding of plain text. The encoding
of other subtypes of text, such as richtext, is not discussed here.
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Description
The message body starts in ASCII [ASCII], and switches to Japanese
characters through an escape sequence. For example, the escape
sequence ESC $ B (three bytes, hexadecimal values: 1B 24 42)
indicates that the bytes following this escape sequence are Japanese
characters, which are encoded in two bytes each. To switch back to
ASCII, the escape sequence ESC ( B is used.
The following table gives the escape sequences and the character sets
used in ISO-2022-JP messages. The ISOREG number is the registration
number in ISO's registry [ISOREG].
Esc Seq Character Set ISOREG
ESC ( B ASCII 6
ESC ( J JIS X 0201-1976 ("Roman" set) 14
ESC $ @ JIS X 0208-1978 42
ESC $ B JIS X 0208-1983 87
Note that JIS X 0208-1983 was called JIS C 6226-1983 until the name
was changed in March 1987. Likewise, JIS C 6220 was renamed JIS X
0201.
The "Roman" character set of JIS X 0201 [JISX0201] is identical to
ASCII except for backslash (\) and tilde (~). The backslash is
replaced by the Yen sign, and the tilde is replaced by macron
(overline). This set is Japan's national variant of ISO 646 [ISO646].
The JIS X 0208 [JISX0208] character sets consist of Kanji, Hiragana,
Katakana and some other symbols and characters. Each character takes
up two bytes.
For further details about the JIS Japanese national character set
standards, refer to [JISX0201] and [JISX0208]. For further
information about the escape sequences, see [ISO2022] and [ISOREG].
If there are JIS X 0208 characters on a line, there must be a switch
to ASCII or to the "Roman" set of JIS X 0201 before the end of the
line (i.e. before the CRLF). This means that the next line starts in
the character set that was switched to before the end of the previous
line.
Also, the message body must end with CRLF, and there must be a switch
to ASCII before the last CRLF (if there are any non-ASCII characters
in the message body).
Other restrictions are given in the Formal Syntax below.
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Formal Syntax
The notational conventions used here are identical to those used in
RFC 822 [RFC822].
The * (asterisk) convention is as follows:
l*m something
meaning at least l and at most m somethings, with l and m taking
default values of 0 and infinity, respectively.
line = *text *1( *segment single-byte-seq *text ) CRLF
segment = single-byte-segment / double-byte-segment
single-byte-segment = single-byte-seq 1*text
double-byte-segment = double-byte-seq 1*( one-of-94 one-of-94 )
single-byte-seq = ESC "(" ( "B" / "J" )
double-byte-seq = ESC "$" ( "@" / "B" )
; ( Octal, Decimal.)
ESC = <ISO 2022 ESC, escape> ; ( 33, 27.)
SI = <ISO 2022 SI, shift-in> ; ( 17, 15.)
SO = <ISO 2022 SO, shift-out> ; ( 16, 14.)
one-of-94 = <any char in 94-char set> ; (41-176, 33.-126.)
CHAR = <any ASCII character> ; ( 0-177, 0.-127.)
text = <any CHAR, including bare CR & bare LF, but NOT
including CRLF, and not including ESC, SI, SO>
MIME Considerations
The name given to the JUNET character encoding is "ISO-2022-JP". This
name is intended to be used in MIME messages as follows:
Content-Type: text/plain; charset=iso-2022-jp
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The ISO-2022-JP encoding is already in 7-bit form, so it is not
necessary to use a Content-Transfer-Encoding header. It should be
noted that applying the Base64 or Quoted-Printable encoding will
render the message unreadable in current JUNET software.
Background Information
The JUNET encoding was described in the JUNET User's Guide [JUNET]
(JUNET Riyou No Tebiki Dai Ippan).
The encoding is based on the particular usage of ISO 2022 announced
by 4/1 (see [ISO2022] for details). However, the escape sequence
normally used for this announcement is not included in ISO-2022-JP
messages.
The so-called half-width (hankaku) Katakana, that is, the Kana set of
JIS X 0201, are not used in ISO-2022-JP messages.
In the past, some systems erroneously used the escape sequence ESC (
H in JUNET messages. This escape sequence is officially registered
for a Swedish character set [ISOREG], and should not be used in ISO-
2022-JP messages.
Some systems do not distinguish between ESC ( B and ESC ( J or
between ESC $ @ and ESC $ B for display. However, when relaying a
message to another system, the escape sequences must not be altered
in any way.
The human user (not implementor) should try to keep lines within 80
display columns, or, preferably, within 75 (or so) columns, to allow
insertion of ">" at the beginning of each line in excerpts. Each JIS
X 0208 character takes up two columns, and the escape sequences do
not take up any columns. The implementor is reminded that JIS X 0208
characters take up two bytes and should not be split in the middle to
break lines for displaying, etc.
The JIS X 0208 standard was revised in 1990, to add two characters at
the end of the table. Although ISO 2022 specifies special additional
escape sequences to indicate the use of revised character sets, it is
suggested here not to make use of this special escape sequence in
ISO-2022-JP text, even if the two characters added to JIS X 0208 in
1990 are used.
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References
[ASCII] American National Standards Institute, "Coded character set
-- 7-bit American national standard code for information
interchange", ANSI X3.4-1968
[ISO646] International Organization for Standardization (ISO),
"Information processing -- ISO 7-bit coded character set for
information interchange", International Standard, Ref. No. ISO 646-
1983 (E)
[ISO2022] International Organization for Standardization (ISO),
"Information processing -- ISO 7-bit and 8-bit coded character sets
-- Code extension techniques", International Standard, Ref. No. ISO
2022-1986 (E)
[ISOREG] International Organization for Standardization (ISO),
"International Register of Coded Character Sets To Be Used With
Escape Sequences"
[JISX0201] Japanese Standards Association, "Code for Information
Interchange", JIS X 0201-1976
[JISX0208] Japanese Standards Association, "Code of the Japanese
graphic character set for information interchange", JIS X 0208-1978,
-1983 and -1990
[JUNET] JUNET Riyou No Tebiki Sakusei Iin Kai (JUNET User's Guide
Drafting Committee), "JUNET Riyou No Tebiki (Dai Ippan)" ("JUNET
User's Guide (First Edition)"), February 1988
[MIME] Nathaniel Borenstein and Ned Freed, "MIME (Multipurpose
Internet Mail Extensions): Mechanisms for Specifying and Describing
the Format of Internet Message Bodies", Proposed (Internet) standard,
June 1992, rfc1341
[RFC822] David H. Crocker, "Standard for the Format of ARPA Internet
Text Messages", Internet standard, August 1982, rfc822
[RFC1036] M. Horton and R. Adams, "Standard for Interchange of USENET
Messages", December 1987, rfc1036
[RFC1342] Keith Moore, "Representation of Non-ASCII Text in Internet
Message Headers", Proposed (Internet) standard, June 1992, rfc1342
Security Considerations
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Security considerations are not discussed in this memo.
Acknowledgements
Many people assisted in drafting this document. The authors wish to
thank in particular Akira Kato, Masahiro Sekiguchi and Ken'ichi
Handa.
Authors' Addresses
Jun Murai
Keio University
5322 Endo, Fujisawa
Kanagawa 252 Japan
Fax: +81 (466) 49-1101
EMail: jun@wide.ad.jp
Mark Crispin
Panda Programming
6158 Lariat Loop NE
Bainbridge Island, WA 98110-2098
USA
Phone: +1 (206) 842-2385
EMail: MRC@PANDA.COM
Erik M. van der Poel
A-105 Park Avenue
4-4-10 Ohta, Kisarazu
Chiba 292 Japan
Phone: +81 (438) 22-5836
Fax: +81 (438) 22-5837
EMail: erik@poel.juice.or.jp
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