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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.
Alvestrand [Page 1]
RFC 1502 X.400 Use of Extended Character Sets August 1993
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.)
Alvestrand [Page 2]
RFC 1502 X.400 Use of Extended Character Sets August 1993
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!>>
Alvestrand [Page 3]
RFC 1502 X.400 Use of Extended Character Sets August 1993
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
Alvestrand [Page 4]
RFC 1502 X.400 Use of Extended Character Sets August 1993
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
Alvestrand [Page 5]
RFC 1502 X.400 Use of Extended Character Sets August 1993
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.
Alvestrand [Page 6]
RFC 1502 X.400 Use of Extended Character Sets August 1993
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.
Alvestrand [Page 7]
RFC 1502 X.400 Use of Extended Character Sets August 1993
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