path: root/doc/rfc/rfc1502.txt

Network Working Group                                      H. Alvestrand
Request for Comments: 1502                                  SINTEF DELAB
                                                             August 1993


                  X.400 Use of Extended Character Sets

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

1.  Introduction

   Since 1988, X.400 has had the capacity for carrying a large number of
   different character sets in a message by using the body part
   "GeneralText" defined by ISO/IEC 10021-7.

   Since 1992, the Internet also has the means of passing around
   messages containing multiple character sets, by using the mechanism
   defined in RFC-MIME.

   This RFC defines a suggested method of using "GeneralText" in order
   to harmonize as much as possible the usage of this body part.

2.  General principles

2.1.  Goals

   The target of this memo is to define a way of using existing
   standards to achieve:

    (1)  in the short term, a standard for sending E-mail in the
         European languages (Latin letters with European accents,
         Greek and Cyrillic)

    (2)  in the medium term, extending this to cover the Hebrew and
         Arabic character sets

    (3)  in the long term, opening up true international E-mail by
         allowing the full character set specified in ISO-10646 to be
         used.






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   The author believes that this document gives a specification that can
   easily accomodate the use of any character set in the ISO registry,
   and, by giving guidance rules for choosing character sets, will help
   interworking.

2.2.  Families of character sets

2.2.1.  ISO 6937/T.61

   ISO 6937 is a code technique used and recommended in T.51 and T.101
   (Teletex and Videotex service) and in X.500, providing a repertoire
   of 333 characters from the Latin script by use of non- spacing
   diacritical marks. It corresponds closely to CCITT recommendation
   T.61.

   The problem with that technique is that the character stream comes in
   two modes, i.e., some characters are coded with one byte and some
   with two (composite characters). This makes information processing
   systems such as an E-mail UA or GW more complex.

   It is also not extensible to other languages like Korean or Chinese,
   or even Greek, without invoking the character set switching
   techniques of ISO 2022.

2.2.2.  ISO 8859

   ISO 8859 defines a set of character sets, each suitable for use in
   some group of languages. Each character in ISO 8859 is coded in a
   single byte.

   There are currently 11 parts of ISO 8859, plus a "supplementary" set,
   registered as ISO IR 154. Most languages using single-byte characters
   can be written in one or another of the ISO 8859 sets.  There are
   sets covering Greek, Hebrew and Arabic, but there is still
   controversy over the problem of the rendering direction for Hebrew
   and Arabic.

   All the ISO 8859 sets include US-ASCII as a subset. All use 8 bits.

   ISO 8859 is regarded by many as a solution; for instance, the X
   windows system now comes with ISO-8859-1 as the "standard" character
   set, with the possibility of specifying others. But since the same
   applications often do not support character set switching within
   text, it is problematic to use these in a truly multilingual
   environment.  (Also, most fonts claiming to be "ISO- 8859-1" in X11R5
   are actually 7-bit fonts. The implied lie is very unfortunate.)





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   It turns out to work fine, however, if the second language is
   English, since this can be written in all ISO 8859 sets.

   The parts 3 and 4 have not seen wide acceptance, and it is expected
   that they will be discarded. They should therefore not be used.

   Note that an ISO 8859 set is actually 2 sets in the ISO sense: US-
   ASCII in the G0 set and another character set in the G1 set.  The
   overloading of the word "character set" is unfortunate, but
   traditional.

2.2.3.  ISO 10646

   At the moment of writing, ISO 10646 has just been accepted as an
   International Standard. It is basically a 32-bit character set, with
   all of the currently used characters being numbered by the first 16
   bits, leaving some room for expansion.

   It is not possible to use ISO 10646 as a normal character set,
   because it does not conform to the rules for usage of byte values set
   down in ISO 2022 and other places; it uses the "control space" for
   (parts of) graphic character codes.

   There are a number of ways to encode ISO 10646 characters "on the
   wire". There are methods within the ISO 2022 standard to switch to
   these, either as "other coding system without return" or as "other
   coding system with return" (that is, you can go back from it to the
   one you came from using an ISO 2022 escape sequence).

   The following registrations have been made:

    ISO 10646 UCS-2 Level 1 has been registered with ESC 2/5 2/15 4/0,
    ISO 10646 UCS-4 Level 1 has been registered with ESC 2/5 2/15 4/1,

   The following are applied for:

    Reg# Escape sequence  Standard/Sponsor   Description
    174  ESC 2/5 2/15 F   ISO/IEC 10646      UCS-2, Level 2
    175  ESC 2/5 2/15 F   ISO/IEC 10646      UCS-4, Level 2
    176  ESC 2/5 2/15 F   ISO/IEC 10646      UCS-2, Level 3
    177  ESC 2/5 2/15 F   ISO/IEC 10646      UCS-4, Level 3
    178  ESC 2/5 F        ISO/IEC 10646      UTF-1

    << NOTE: The registration numbers for UCS-2 level 1 and UCS-4
    level 1 are not known. Neither are the assigned final characters
    for the other sets. Information requested!>>





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   This character set will become very important in the future, but at
   the moment, few systems are able to support this directly.

   The GeneralText body part can be used for carrying any of these
   character sets.

2.3.  Body parts that can be used in X.400

   At the moment, no established way of transferring a full set of
   characters in X.400-based E-mail exists.  In the future, it is likely
   that a new body part, based in ISO 10646, will be available, or
   GeneralText may be able to use ISO 10646, but this matter has not yet
   been clarified.

   In the short term, the deployed and available body parts are:

    (1)  IA5Text

    (2)  For X.400/84: ISO6937Text and Teletex

    (3)  For X.400/88: GeneralText

   IA5Text is the method of choice for E-mail that contains only
   characters from IA5 (equivalent to US-ASCII).

   The ISO6937Text body part is defined in the ISO DIS documents
   corresponding to X.400(84) [10]; these never became a standard, so
   they are now quite difficult to find. It is in principle limited to
   using text that can be presented in ISO 6937, but since ISO 6937
   refers to the ISO 2022 method of changing character sets, it is
   theoretically possible to use any ISO registered character set, but
   there is no facility for announcing the character sets used. This
   makes interworking with equipment that does not support the same
   character sets complex.

   It is still, however, the only body part suitable for carrying non-
   paginated text in non-basic character sets in X.400(84).

   Teletex, which is identical in all versions of the X.400 standard,
   has the same problem of implicit ISO6937, but has the added problem
   that it also specifies a page format, with, for instance, a left
   margin of 5 character positions. This is often not desirable.

   The details of Teletex are specified in recommendation T.51 and its
   relatives.

   GeneralText is defined in ISO 10021-8, the part of [9] that
   corresponds to CCITT recommendation [11]. It is an Extended body



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   part, so no modification to CCITT implementations is needed to carry
   it.

   GeneralText is suitable for interchange, since it has got proper
   announcement facilities. It can use any number of character sets, and
   announces them both in the Encoded Information Types of the X.400
   envelope and the parameters of the body part.

   We recommend this body part for carrying unformatted text in
   X.400/88.

3.  GUIDELINES FOR THE GENERATION OF GENERALTEXT

3.1.  Formal definition of GeneralText

   A GeneralText message is a byte stream that contains characters and
   character switching sequences according to [12].

   The X.400 ASN.1 definition of the GeneralText body part is:

    general-text-body-part EXTENDED-BODY-PART-TYPE
        PARAMETERS GeneralTextParameters IDENTIFIED BY id-ep-general-text
        DATA       GeneralTextData
        ::= id-et-general-text

    GeneralTextParameters ::= SET OF CharacterSetRegistration

    CharacterSetRegistration ::= INTEGER (1..32767)

    GeneralTextData ::= GeneralString

   The definition is from ISO/IEC 10021-7 [9], Annex I, with
   modifications made in the MHS Implementor' Guide, version 8, chapter
   3.6.3, bullet F130. It does not appear in the CCITT version of the
   standards.

3.2.  Brief description of ISO 2022 character set switching

   There are 4 graphic character sets active at any time in a
   GeneralText message, called G0, G1, G2 and G3. In addition, there are
   2 control character sets, called C0 and C1.

   At any moment, one of the sets G0-G3 is active in code positions 2/1
   to 7/14, and another is active in code positions 10/0 to 15/15. The
   setting is achieved by so-called "locking shift" sequences.

   (Formally, code positions 2/0 and 7/15 are reserved for "space" and
   "DEL" respectively, and only 94-character character sets can be used



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   in G0. In practice, this restriction is sometimes ignored)

   Single characters from the non-active sets may be invoked by the use
   of "single shift" sequences.

   The control character sets always occupy the code positions 0/0 to
   1/15 (C0) and 8/0 to 9/15 (C1).

   The character sets currently active as G0-G3 and C0-C1 may be changed
   using "character set designating sequences".

   At the beginning of a GeneralText message, one must always assume
   that set 2 (IA5) is active as G0, shifted into the lower half, that
   set 1 (standard) is active as C0, and that no G1-G3 or C1 set is
   invoked. This is specified in the definition of "GeneralString" in
   [5], the definition of ASN.1 encoding (section 23.5.2).

   If this is not a suitable initial state, a message must always start
   with the necessary announcers and escape sequences to designate and
   invoke the character sets that are actually used.  The character sets
   in use may be changed later in the message by use of escape
   sequences.

   The parameters of a GeneralText message always list all the character
   sets used, by quoting their ISO reference numbers.

   It is impossible to use a character set not registered with ISO in a
   GeneralText message.

   It is also impossible to decide on the true meaning of a byte in a
   GeneralText message without scanning the whole message for shift and
   escape sequences.

3.3.  How to use the character sets

   RECOMMENDATION:

   When the text to be rendered is representable in one of the character
   sets of ISO-8859, the G0 set should be set to ISO 646 International
   Reference Version (1991), also called US-ASCII, ISO-IR-6.

   The older character set ISO-IR-2, ISO 646 IRV(1983), should NOT be
   used.  This means that the escape sequence ESC 2/8 4/2 (designating
   US-ASCII as G0) should always occur at the beginning of the message.

   The G1 set should be set to the character set identified by the
   relevant ISO-8859 part. G2 and G3 are not used.




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   This corresponds to the first level of ISO 4873 usage.

   For the currently defined parts of ISO 8859, the character set
   designations for the G1 set are (relative to ISO 8859:1987):

    Part    ISO IR name             Escape sequence Remarks
                                    for G1 use

    1       ISO-IR-100              Esc 2D 41    West Europe (Latin-1)
    2       ISO-IR-101              Esc 2D 42    East European (Latin-2)
    3       ISO-IR-109              Esc 2D 43    (Latin-3)
    4       ISO-IR-110              Esc 2D 44    (Latin-4)
    5       ISO-IR-144              Esc 2D 4C    Cyrillic
    6       ISO-IR-127              Esc 2D 47    Arabic
    7       ISO-IR-126              Esc 2D 46    Greek
    8       ISO-IR-138              Esc 2D 48    Hebrew
    9       ISO-IR-148              Esc 2D 4D    Turkish (Latin-5)
    10      ISO-IR-157              Not listed   Sami (Latin-6)

   The escape sequence for 8859-10 (Latin-6) is not listed in RFC 1345.

   NOTE: The use of ISO 8859-3 and ISO 8859-4 is NOT recommended if
   other possibilities exist.

   NOTE: There is a debate about the Arabic and Hebrew character sets.
   These languages are normally read right to left, but encodings have
   been done in both "visual" (left to right) and "phonetic" (right to
   left) ordering, there is significant disagreement about what the
   "right" way to do it is, and the character sets mentioned do not
   specify it. So, one should be careful not to use these character sets
   until a standard is agreed upon, or the result will probably be
   unreadable (siht ekil).

   (Note that there is some confusion as to what parts are actually
   standardized; the Norwegian standards institute reports that only
   part 1, 2, 3, 4, 6, 7 and 8 are currently standards. Other reports
   claim that both 8859-10 and 8859-11 are standards, and I definitely
   think that 8859-9 is.)

   NOTE: ISO has not ruled out the possibility of changing the ISO 8859
   standard. This would involve changing the registry information in
   this table, so this should be assumed valid for ISO 8859 versions
   that are current in 1993.

   The G1 set should be permanently shifted into the upper half of the
   code page.





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   When the text is not representable in one of the ISO-8859 character
   sets, the following rules may be applied:

    (1)  If any Latin characters are used, keep IA5 as the G0 set.

    (2)  If a mainstream character set is used (Greek, Cyrillic,
         Hebrew, Arabic), designate this as the G1 character set,
         and permanently shift this into the upper half of the code
         page (LS1R).
         EXCEPTION: The Japanese community has a long tradition of
         switching between the Japanese 16-bit character set
         ISO-IR-87 and US-ASCII as the G0 set. See [7]
         for details. If ISO-IR-87 is used, that technique should be
         used instead of the one recommended here.

    (3)  If occasional extensions to a character set that is
         basically Latin occur (like accents, national variants
         and so on), and these are available in a single character
         set, designate the relevant set as G2 and use single
         shift (SS2) to invoke characters from this character set.

         The ISO 8859 supplementary set, ISO-IR-154, is recommended
         for this purpose.

         This corresponds to the ISO 4873 "second level" application.

    (4)  If two non-Latin character sets are used, the second should
         be designated as G3, and shifted into the upper half of the
         code page by the use of Locking Shift 3 Right (LS3R).

         This corresponds to the ISO 4873 "third level" application.

    (5)  If avoidable, use of character sets with floating accents,
         like ISO 6937, should be avoided.

    (6)  The shifts changing the lower half of the code table (SI/SO,
         LS2 and LS3) should NOT be used.

   RATIONALE: Keeping the G0 set reserved for US-ASCII will ensure that
   text in US-ASCII has the same bit representation always.

   The use of the upper code page for other scripts ensures that both
   text in these languages and text of this type mixed with English can
   be represented without the use of shift sequences.

   If the language and/or content of a text is completely unknown,
   chapter 5 gives an algorithm that may be used to decide upon the
   character sets. This might, for instance, be suitable for use at



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   automatic mail gateways.

   NOTE: At the time of this writing, few applications that use ISO 4873
   level 2 and level 3 encoding exist. It has been estimated that
   implementing them in an application that already uses a rich
   repertoire of characters is a matter of programmer-days, not
   programmer-months, but this has not been proven.

4.  GUIDELINES FOR THE RENDERING OF GENERALTEXT

   As a basic rule, one should NOT assume that any of the rules above
   are followed.

   An user agent capable of rendering GeneralText should:

    (1)  ALWAYS be able to identify and render characters in IA5, no
         matter how they are designated and invoked.

    (2)  ALWAYS be able to identify and render characters in the
         "native" character sets, no matter how they are designated
         and invoked.

    (3)  ALWAYS indicate the presence of characters that cannot be
         adequately represented on the current output device.

    (4)  NEVER render a character in an unknown or unrepresentable
         character set by displaying the character in the same bit
         position in the native character set.

    (5)  PREFERABLY be able to identify and render characters that are
         the same as characters in the "native" character sets, even
         though they are designated and invoked as part of other
         character sets.  This applies in particular to the
         "invariant" part of ISO 8859, parts 1 through 6.

    (6)  PREFERABLY be able to combine the floating accents of ISO
         6937 with their base characters for suitable rendering using
         the capabilities of the current output device.

    (7)  PREFERABLY be able to display text both in a mode using
         fallbacks for nonrenderable characters and in a mode
         designating nonrenderable characters as such.

    (8)  PREFERABLY be able to save the content of a GeneralText
         message to a file or other suitable media, saving all
         character set information, for later processing by other
         means.  It is not illegal to render the character set
         information into a different format; however, it should be



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         noted that it is easy to lose vital information if the format
         chosen for representing character sets does not offer the
         possibility of referencing all character sets in the ISO
         registry of character sets.

   These requirements also apply to gateways that transform the message
   into some other format, for example a gateway that transforms a
   message into MIME using [7] for the purpose.

5.  RECOMMENDATION FOR SELECTION OF CHARACTER SETS

5.1.  Algorithm for selection of character sets

   When one has text in which characters from several character sets
   occurs, and wants to process this into a GeneralText document, it is
   often hard to guess right at the character sets to select.

   The following paragraphs give an algorithm that can be started at the
   beginning of a message, and at the end of it, return a set of
   character sets that can be used as G0..G3 character sets, OR an
   indication that the task is impossible.

    VARIABLES:

    UsedSets
         The set of character sets that MUST be used for this message

    UsableSets
         The set of character sets that MAY be used for this message.
         Each set also contains a counter for each character position.

    PossibleSets
         The set of all the character sets known to be usable in the
         destination format.

         ALGORITHM:

    1)   Add IA5 (ISO-IR-6) to the UsedSets (as G0).

    2)   Get the next character of the text.  If the text is
         completely analyzed, go to FINISHED

    3)   If it is in the UsedSets, go to 2).

    4)   Find the set of character sets from PossibleSets in which the
         character occurs. If it does not occur in any, report
         failure.




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    5)   If it is in a single character set in PossibleSets only, add
         this set to UsedSets, and go to 2).

    6)   If it is in more than one character set, add these to
         PossibleSets (if not already present), and increment the
         counter for that character in all the sets. Go to 2).

    FINISHED)

    1)   (FINAL SELECTION) Remove any character set in UsedSets from
         PossibleSets.

         Zero the counters for any character in PossibleSets that also
         occurs in UsedSets.
         WHILE (more characters left)
           Select one character set and move it from PossibleSets to
           UsedSqets.
           Zero the counters for all characters in this set in the other
           PossibleSets.
         END WHILE
         This step can be "tuned" any way you want, for instance by
         choosing the character sets most likely to be understood at
         the destination first, choosing the character sets covering
         the most characters first, avoiding multi-byte character sets
         as long as possible, or any other scheme suitable for the
         application.

5.2.  WHAT TO DO ON FAILURE

   Failure will occur in this schema if a character is found that is not
   in the PossibleSets. It may then be handled in one of the following
   ways:

    (1)  Replace the character with the SUB control character

    (2)  Replace the character with Keld Simonsen Mnemonics [8].
         This is a reversible transformation as long as the
         recipient is aware that it has been used, but requires
         passing out-of-band information to indicate this.

    (3)  Replace the lost characters with any suitable fallback or
         mnemonic scheme intended for human understanding

    (4)  Bounce the message/refuse the conversion/give up.

   The action to be taken may be different based on the percentage of
   "lost" characters.




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   If the message has "controls" like "conversion with loss prohibited",
   only the last possibility may be used.

5.3.  RECOMMENDED CHARACTER SETS

   There are 2 steps in the algorithm above that are left for local
   judgement:

    (1)  Selection of the sets to appear in PossibleSets.

    (2)  The algorithm for deciding which character set to select in
         step 9.

   In the context of generating X.400 GeneralText messages, the
   following is recommended:

    Sets in PossibleSets:
    ISO-IR-6        Esc 28 42 (G0)  US-ASCII, IA5, ISO646
    ISO-IR-100      Esc 2D 41 (G1)  ISO-8859-1   West Europe
    ISO-IR-101      Esc 2D 42 (G1)  ISO-8859-2   Central/Eastern Europe
    ISO-IR-144      Esc 2D 4C (G1)  ISO-8859-5   Cyrillic
    ISO-IR-127      Esc 2D 47 (G1)  ISO-8859-6   Arabic
    ISO-IR-126      Esc 2D 46 (G1)  ISO-8859-7   Greek
    ISO-IR-138      Esc 2D 48 (G1)  ISO-8859-8   Hebrew
    ISO-IR-148      Esc 2D 4D (G1)  ISO-8859-9   Turkish

   The following multi-byte character sets are recommended:

    ISO-IR-87 (Japanese JIS C6226-1983)     Esc 24 29 42 (G1)
    ISO-IR-149 (Korean KS C 5601-1989)      Esc 24 29 43 (G1)
    ISO-IR-58 (Chinese GB 2312-80)          Esc 24 29 41 (G1)

   It is a STRONG recommendation that character sets not listed above,
   which do not add any new characters to the total set of characters
   given by the character sets above, should NOT be used in X.400
   interchange.

   ISO-IR-87 is the Japanese character set that is allowed in a Teletex
   string, such as the subject field.

   NOTE: ISO-IR-87 has been "superseded" by ISO-IR-168, which allows two
   extra Kanji characters. Any application that handles ISO-IR-87 should
   also be able to handle ISO-IR-168.

   Algorithm for selecting character sets:

   Start at the top of the list above, and add each set only if it is
   needed.



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6.  REFERENCES

   [1]  Information technology - ISO 8-bit code for information
        interchange - Structure and rules for implementation, Third
        edition, 1991-12-15.

   [2]  Information technology - 8-bit single-byte coded graphic
        character sets (parts 1-11; the parts have different dates, the
        ones referenced here are from RFC 1345).

   [3]  Information technology - Coded graphic character set for text
        communication (parts 1 and 2; part 2 dated 1983-12-15).

   [4]  Code for the representation of names of languages. 1988 version.

   [5]  CCITT Recommendation X.209(1988): Specification of Basic
        Encoding Rules for Abstract Syntax Notation One (ASN.1).
        Technically aligned with ISO 8825 and ISO 8825/AD 1.

   [6]  Information Technology - Universal Multiple-Octet Coded
        Character Set (UCS) - ISO 10646.

   [7]  Murai, J., Crispin M., and E. van der Poel, "Japanese Character
        Encoding for Internet Message Bodies", RFC 1468, Keio
        University, Panda Programming, June 1993.

   [8]  Simonsen, K., "Character Mnemonics & Character Sets", RFC 1345,
        Rationel Almen Planlaegning, June 1992.

   [9]  Information Technology - Text communication - Message- Oriented
        Text Interchange Systems (MOTIS) - ISO 10021  - October 1988.

   [10] ISO DIS documents describing X.400/84 with slight extensions.
        Now very hard to get copies of, since they failed to become
        ISes.

   [11] CCITT Recommendation X.420 (1988), Interpersonal Messaging
        System.

   [12] International Standard--Information Processing-- ISO 7-bit and
        8-bit coded character sets--Code extension techniques, ISO
        2022:1986.

7.  Security Considerations

   Security issues are not discussed in this memo.





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8.  Author's Address

   Harald Tveit Alvestrand
   SINTEF DELAB
   N-7034 Trondheim
   NORWAY

   EMail: Harald.Alvestrand@delab.sintef.no











































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