ISO-2022-JP-2: Multilingual Extension of ISO-2022-JP

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.

Introduction

This memo describes a text encoding scheme: "ISO-2022-JP-2", which is used experimentally for electronic mail [RFC822] and network news [RFC1036] messages in several Japanese networks. The encoding is a multilingual extension of "ISO-2022-JP", the existing encoding for Japanese [2022JP]. The encoding is supported by an Emacs based multilingual text editor: MULE [MULE].

The name, "ISO-2022-JP-2", is intended to be used in the "charset" parameter field of MIME headers (see [MIME1] and [MIME2]).

Description

The text with "ISO-2022-JP-2" starts in ASCII [ASCII], and switches to other character sets of ISO 2022 [ISO2022] through limited combinations of escape sequences. All the characters are encoded with 7 bits only.

At the beginning of text, the existence of an announcer sequence: "ESC 2/0 4/1 ESC 2/0 4/6 ESC 2/0 5/10" is (though omitted) assumed. Thus, characters of 94 character sets are designated to G0 and invoked as GL. C1 control characters are represented with 7 bits. Characters of 96 character sets are designated to G2 and invoked with SS2 (single shift two, "ESC 4/14" or "ESC N").

For example, the escape sequence "ESC 2/4 2/8 4/3" or "ESC $ ( C" indicates that the bytes following the escape sequence are Korean KSC characters, which are encoded in two bytes each. The escape sequence "ESC 2/14 4/1" or "ESC . A" indicates that ISO 8859-1 is designated to G2. After the designation, the single shifted sequence "ESC 4/14 4/1" or "ESC N A" is interpreted to represent a character "A with acute".

The following table gives the escape sequences and the character sets used in "ISO-2022-JP-2" messages. The reg# is the registration number in ISO's registry [ISOREG].

                              94 character sets
      reg#  character set      ESC sequence                designated to
      ------------------------------------------------------------------
      6     ASCII              ESC 2/8 4/2      ESC ( B    G0
      42    JIS X 0208-1978    ESC 2/4 4/0      ESC $ @    G0
      87    JIS X 0208-1983    ESC 2/4 4/2      ESC $ B    G0
      14    JIS X 0201-Roman   ESC 2/8 4/10     ESC ( J    G0
      58    GB2312-1980        ESC 2/4 4/1      ESC $ A    G0
      149   KSC5601-1987       ESC 2/4 2/8 4/3  ESC $ ( C  G0
      159   JIS X 0212-1990    ESC 2/4 2/8 4/4  ESC $ ( D  G0
      
                              96 character sets
      reg#  character set      ESC sequence                designated to
      ------------------------------------------------------------------
      100   ISO8859-1          ESC 2/14 4/1     ESC . A    G2
      126   ISO8859-7(Greek)   ESC 2/14 4/6     ESC . F    G2

For further information about the character sets and the escape sequences, see [ISO2022] and [ISOREG].

If there is any G0 designation in text, there must be a switch to ASCII or to JIS X 0201-Roman before a space character (but not necessarily before "ESC 4/14 2/0" or "ESC N ' '") or control characters such as tab or CRLF. This means that the next line starts in the character set that was switched to before the end of the previous line. Though the designation to JIS X 0201-Roman is allowed for backward compatibility to "ISO-2022-JP", its use is discouraged. Applications such as pagers and editors which randomly seek within a text file encoded with "ISO-2022-JP-2" may assume that all the lines begin with ASCII, not with JIS X 0201-Roman.

At the beginning of a line, information on G2 designation of the previous line is cleared. New designation must be given before a character in 96 character sets is used in the line.

The text must end in ASCII designated to G0.

As the "ISO-2022-JP", and thus, "ISO-2022-JP-2", is designed to represent English and modern Japanese, left-to-right directionality is assumed if the text is displayed horizontally.

Users of "ISO-2022-JP-2" must be aware that some common transport such as old Bnews can not relay a 7-bit value 7/15 (decimal 127), which is used to encode, say, "y with diaeresis" of ISO 8859-1.

Other restrictions are given in the Formal Syntax section below.

Formal Syntax

The notational conventions used here are identical to those used in STD 11, 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.

   message             = headers 1*(CRLF text)
                                          ; see also [MIME1] "body-part"
                                          ; note: must end in ASCII
   
   text                = *(single-byte-char /
                           g2-desig-seq /
                           single-shift-char)
                          [*segment
                           reset-seq
                           *(single-byte-char /
                             g2-desig-seq /
                             single-shift-char ) ]
                                          ; note: g2-desig-seq must
                                          ; precede single-shift-char
   
   headers             = <see [RFC822] "fields" and [MIME1] "body-part">
   
   segment             = single-byte-segment / double-byte-segment
   
   single-byte-segment = single-byte-seq
                         *(single-byte-char /
                           g2-desig-seq /
                           single-shift-char )
   
   double-byte-segment = double-byte-seq
                         *((one-of-94 one-of-94) /
                           g2-desig-seq /
                           single-shift-char )
   
   reset-seq           = ESC "(" ( "B" / "J" )
   
   single-byte-seq     = ESC "(" ( "B" / "J" )
   
   double-byte-seq     = (ESC "$" ( "@" / "A" / "B" )) /

(ESC "$" "(" ( "C" / "D" ))

   g2-desig-seq        = ESC "." ( "A" / "F" )
   
   single-shift-seq    = ESC "N"
   
   single-shift-char   = single-shift-seq one-of-96
   
   CRLF                = CR LF
   
                                                    ; ( 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.)
   
   CR                  = <ASCII CR, carriage return>; (    15,      13.)
   
   LF                  = <ASCII LF, linefeed>       ; (    12,      10.)
   
   one-of-94           = <any one of 94 values>     ; (41-176, 33.-126.)
   
   one-of-96           = <any one of 96 values>     ; (40-177, 32.-127.)
   
   7BIT                = <any 7-bit value>          ; ( 0-177,  0.-127.)
   
   single-byte-char    = <any 7BIT, including bare CR & bare LF, but NOT
                          including CRLF, and not including ESC, SI, SO>

MIME Considerations

The name given to the character encoding is "ISO-2022-JP-2". This name is intended to be used in MIME messages as follows:

      Content-Type: text/plain; charset=iso-2022-jp-2

The "ISO-2022-JP-2" 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 non-MIME-compliant software.

"ISO-2022-JP-2" may also be used in MIME headers. Both "B" and "Q" encoding could be useful with "ISO-2022-JP-2" text.

References

   [ASCII] American National Standards Institute, "Coded character set
   
           -- 7-bit American national standard code for information
           interchange", ANSI X3.4-1986.

[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".

   [MIME1] Borenstein, N., and N. Freed, "MIME  (Multipurpose Internet
           Mail Extensions) Part One: Mechanisms for Specifying and
           Describing the Format of Internet Message Bodies", RFC 1521,
           September 1993.
   
   [MIME2] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
           Two: Message Header Extensions for Non-ASCII Text", RFC 1522,
           September 1993.
   
   [RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text
            Messages", STD 11, RFC 1522, UDEL, August 1982.
   
   [RFC1036] Horton M., and R. Adams, "Standard for Interchange of
             USENET Messages", RFC 1036, AT&T Bell Laboratories, Center
             for Seismic Studies, December 1987.
   
   [2022JP] Murai, J., Crispin, M., and E. van der Poel, "Japanese
            Character Encoding for Internet Messages", RFC 1468, June
            1993.
   
   [MULE] Nishikimi, M., Handa, K., and S. Tomura, "Mule: MULtilingual
          Enhancement to GNU Emacs", Proc. of INET'93, August, 1993.

Acknowledgements

This memo is the result of discussion between various people in a news group: fj.kanji and is reviewed by a mailing list: jp-msg @iij.ad.jp. The Authors wish to thank in particular Prof. Eiichi Wada for his suggestions based on profound knowledge in ISO 2022 and related standards.

Security Considerations

Security issues are not discussed in this memo.

Authors' Addresses

   Masataka Ohta
   Tokyo Institute of Technology
   2-12-1, O-okayama, Meguro-ku,
   Tokyo 152, JAPAN
   
   Phone: +81-3-5499-7084
   Fax: +81-3-3729-1940
   EMail: mohta@cc.titech.ac.jp
   
   Ken'ichi Handa
   Electrotechnical Laboratory
   Umezono 1-1-4, Tsukuba,
   Ibaraki 305, JAPAN
   
   Phone: +81-298-58-5916
   Fax: +81-298-58-5918
   EMail: handa@etl.go.jp