Network Working Group J. Klensin

Network Working Group J. Klensin
Request for Comments: 5242
Category: Informational H. Alvestrand
Google
1 April 2008

A Generalized Unified Character Code: Western European and CJK Sections

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

IESG Note

This is not an IETF document. Readers should be aware of RFC 4690,
"Review and Recommendations for Internationalized Domain Names
(IDNs)", and its references.

This document is not a candidate for any level of Internet Standard.
The IETF disclaims any knowledge of the fitness of this document for
any purpose, and in particular notes that it has not had IETF review
for such things as security, congestion control, or inappropriate
interaction with deployed protocols. The RFC Editor has chosen to
publish this document at its discretion. Readers of this document
should exercise caution in evaluating its value for implementation
and deployment.

Abstract

Many issues have been identified with the use of general-purpose
character sets for internationalized domain names and similar
purposes. This memo describes a fully unified coded character set
for scripts based on Latin, Greek, Cyrillic, and Chinese (CJK)
characters. It is not a complete specification of that character
set.

Klensin & Alvestrand Informational [Page 1]

RFC 5242 Unified CCS April 2008

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Types of Characters . . . . . . . . . . . . . . . . . . . . . 4
2.1. Base Character . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Nonspacing Marks . . . . . . . . . . . . . . . . . . . . . 4
2.3. Case Indicators . . . . . . . . . . . . . . . . . . . . . 4
2.4. Joining Indicators . . . . . . . . . . . . . . . . . . . . 5
2.5. Character-Matrix Positioning Indicators . . . . . . . . . 5
2.6. Position Shaping Controls . . . . . . . . . . . . . . . . 6
2.7. Repetition Indicators . . . . . . . . . . . . . . . . . . 6
2.8. Control Characters . . . . . . . . . . . . . . . . . . . . 7
3. Code Assigment Groupings . . . . . . . . . . . . . . . . . . . 7
4. Canonical Form . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Examples of Graphic Element Codes . . . . . . . . . . . . . . 8
6. Composite Characters and Unicode Equivalences . . . . . . . . 10
7. Ideographic Characters . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . . 13

Klensin & Alvestrand Informational [Page 2]

RFC 5242 Unified CCS April 2008

1. Introduction

Many issues have been identified with the use of general-purpose
character sets for internationalized domain names and similar
purposes. This memo specifies a fully unified coded character set
for scripts based on Latin, Greek, Cyrillic, and Chinese characters.

There are four important principles in this work:

1. If it looks alike, it is alike. The number of base characters
and marks should be minimized. Glyphs are more important than
character abstractions.

2. If it is the same thing, it is the same thing. Two symbols that
have the same semantic meaning in all contexts should be encoded
in a way that allows their identity to be discovered by removing
modifiers, rather than having to resort to external equivalence
tables.

3. For simplicity, when a character form can be evaluated on the
basis of either serif or sanserif fonts, the sanserif font is
always preferred.

4. The use of combining characters and modifiers is preferred to
adding more base characters.

Based on these principles, it becomes obvious that:

o Ligatures, digraphs, and final forms are constructed with special
modifiers so that relationships to basic forms are obvious.

o Symbols consisting of multiple marks are always constructed from
combining characters and positional modifiers; thus, the "i"
character is constructed from the vertical line symbol followed by
a combining dot above. Similarly "f" is composed of a centered
vertical line, a right hook in the top position, and an
appropriately-positioned composing hyphen.

This document draws strongly from the design and terminology of
Unicode [Unicode] but represents a radically different approach.

1.1. Terminology

All special-use terms in this document, including descriptions of
behaviors and related relationships, are used with their common-sense
meanings.

Klensin & Alvestrand Informational [Page 3]

RFC 5242 Unified CCS April 2008

1.2. Discussion

Questions to, and contributions for, this coding system should be
addressed to the mailing list
[email protected].

2. Types of Characters

This document defines several types of characters. Note that these
definitions are not the same as the Unicode definitions for similar
or identical terms.

2.1. Base Character

Any character that is used as an atomic shape, rather than being
assembled from such a character in combination with combining
(overstriking) marks, symbols, or specially-designed base characters.
When used alone, base characters always take up space. For example,
a, c, l,...

2.2. Nonspacing Marks

Marks, symbols, and character components that are used to form
characters when used in combination with base characters. They do
not occupy separate character positions when displayed.

For example, the special combining symbols LeftUpperHook and
RightLowerHook, described in Section 5, are nonspacing marks.

2.3. Case Indicators

In scripts with case, only the lower-case characters are base
characters. Upper-case forms are represented by using the UC
modifier. So the traditional "A" character is represented by
"a<UC>". Note that this means that case-independent comparisons are
made simply by ignoring the <UC> modifiers rather than by complicated
mapping operations.

The initial set of case modifiers consists exclusively of:

UC Upper-case, code value 1 (hexadecimal)

The code values two through four are reserved for the impending
encoding of scripts with more than two cases; five is reserved for
expansion in case a script with more than four cases is identified.

Klensin & Alvestrand Informational [Page 4]

RFC 5242 Unified CCS April 2008

2.4. Joining Indicators

Zero-width joiners are used to build characters, not only to separate
or join words. As compared to Unicode, a richer set of joiners is
used to distinguish between the inter-word and ligature-forming
(including half-character forming) cases. Unicode ZWJ and ZWNJ are
supplemented by ZWCJ, OJ, and ONJ. ZWCJ is used to modify a spacing
basic character into a nonspacing role. For example, there is no "w"
character, but only "u<ZWCJ>u". Upper-case "W" is coded as
u<ZWCJ>u<UC> -- the CWCJ binds more tightly than the UC modifier.

The initial set of joining indicators consists exclusively of:

ZWCJ Character joiner (also known as "ligature joiner"), code value
6 (hexadecimal).

OJ Overlay joiner (permits use of a subsequent character that would
normally be spacing as nonspacing), code value 7 (hexadecimal).

ONJ Overlay non-joiner (turns a nonspacing mark into a standalone
character), code value 8 (hexadecimal). This joiner should not be
necessary, and is normally prohibited by the "shortest string"
rule. But there may be unanticipated cases.

ZWJ Zero-width joiner for words or word-like constructions, code
value 9 (hexadecimal).

ZWNJ Zero-width non-joiner for words or word-like constructions,
code value A (hexadecimal).

2.5. Character-Matrix Positioning Indicators

Many characters are defined by constructed glyphs using nonspacing
marks. For example, the characters "b" and "d" are coded as
o<VerticalLine><PositionLeft> and o<VerticalLine><PositionRight>,
respectively. The Catalan ligature that has caused some difficulties
in Internationalizing Domain Names in Applications (IDNA) [RFC3490]
is coded as l<ZWCJ><.><PositionVMiddle><ZWCJ>l

Klensin & Alvestrand Informational [Page 5]

RFC 5242 Unified CCS April 2008

The initial table of positioning indicators is:

+-------------------+-----------+
| Name | Hex value |
+-------------------+-----------+
| PositionLeft | 20 |
| PositionCenter | 21 |
| PositionRight | 22 |
| PositionTop | 30 |
| PositionVMiddle | 31 |
| PositionBottom | 32 |
| PositionDescender | 33 |
+-------------------+-----------+

2.6. Position Shaping Controls

These controls designate character form changes for initial or final-
form characters. Where the distinction is important, medial-form
characters are the default when no qualification occurs. As with
case comparisons, comparisons are performed by ignoring these control
functions.

+-------------+-----------+
| Name | Hex value |
+-------------+-----------+
| InitialForm | 71 |
| FinalForm | 72 |
+-------------+-----------+

2.7. Repetition Indicators

For compactness of coding, two repetition indicators are introduced
for double (Repeat2) and triple (Repeat3) characters that may be
treated as ligatures or special cases. Two consecutive uses of a
character compare equal to the character followed by <Repeat2>. The
interpretation of u<ZWCJ>u<Repeat3> is left as an exercise for the
reader.

The initial table of repetition indicators is:

+---------+-----------+
| Name | Hex value |
+---------+-----------+
| Repeat2 | 50 |
| Repeat3 | 51 |
| Repeat1 | 52 |
+---------+-----------+

Klensin & Alvestrand Informational [Page 6]

RFC 5242 Unified CCS April 2008

For larger repeats, these repeats can be combined; the sequence
<Repeat2><Repeat3> represents six repeats, while the
<Repeat3><Repeat2> represents five repeats. Following the "shortest
string" principle (see Section 4), Repeat1 must not ever appear
except in combination with Repeat2 and/or Repeat3. The generation of
other numbers is left as an exercise for the reader.

2.8. Control Characters

Because it is intended primarily for domain names, this specification
has no provision for control or spacing characters.

3. Code Assigment Groupings

Following the reasoning used in Unicode [Unicode], every character
occupies exactly 23 bits (conventionally stored as three octets, with
the leading bit always zero). This value is chosen because both 3
and 23 are prime numbers, unlike 42.

The code point value zero is permanently reserved and will not be
used unless it is necessary to expand the code space.

Code values between 1 and 255 (decimal) are reserved for the special
character formation codes described in Section 2.3 through
Section 2.7.

Code values between 256 and 511 (decimal) are reserved for character
formation marks for non-ideographic characters. Most, but not all,
of these are nonspacing (combining) characters.

Code values between 512 and 1023 are reserved on general principles
and in case it is necessary to invent new rules and make them
retroactive.

Code values of 1024 and above are to be allocated for characters,
glyphs, and other character elements.

4. Canonical Form

When glyphs are constructed using the mechanisms described here,
there is a single canonical form for representing any given glyph.
There are no exceptions to that form, and any sequence of characters
and qualifiers that is not consistent with the form is invalid. If
there are two possible ways to represent a given character, the
shorter one (in octet count) is the only permitted form. If there
are two possible ways that are of the same length, the only permitted
form is the one that has the smaller value when the numeric values of
all of the octets in each are summed.

Klensin & Alvestrand Informational [Page 7]

RFC 5242 Unified CCS April 2008

The ordering rules are as follows:

1. A base character or composite character (see below) must come
first.

2. The base character may be followed by ZWCJ or OJ, but not both,
followed by a base or nonspacing character or mark.

3. If ZWCJ appears, the next character must be a base character or
nonspacing mark.

4. If OJ appears, the next character must be a base character, since
the function of OJ is to make a spacing base character into a
nonspacing (overlay) character.

5. That character can be followed by positional qualifiers that
apply to it. Vertical positional qualifiers precede horizontal
positional qualifiers.

6. That sequence of characters may be followed by a case qualifier.

7. That entire sequence of characters forms a composite character.
When the composite character is non-trivial, the rules may be
applied to it recursively. If grouping is needed to distinguish
between one composite character and the next, ZWNCJ may be used
at the beginning of a composite character to identify a group
boundary.

5. Examples of Graphic Element Codes

The initial lists of positioning and combining controls appear above.
This section shows codes for some base characters. Names in upper
case are the Unicode names for the characters. These are followed,
for information, by the Unicode code point designations. The code
point list is informative, not normative, and may not be complete
(especially since additional matching code points may be added to
Unicode over time). Note that several Unicode characters that are
considered different by Unicode are assigned the same code sequence
in the system specified here.

Klensin & Alvestrand Informational [Page 8]

RFC 5242 Unified CCS April 2008

+------------------------+-------+----------------------------------+
| Name | Hex | Comment |
| | value | |
+------------------------+-------+----------------------------------+
| FULL STOP (U+002E) | 110 | Used as both base character (in |
| | | bottom center position) and as |
| | | movable dot with OJ and |
| | | positional qualifiers. |
| HYPHEN-MINUS (U+002D) | 108 | Used as a spacing base character |
| | | (in horizontally and vertically |
| | | centered position) and as a |
| | | movable half-width horizontal |
| | | line with OJ and positional |
| | | qualifiers. In the context of |
| | | this specification, should be |
| | | known as Half Horizontal Line. |
| LOW LINE (U+005F) | 109 | Used as a spacing base character |
| | | (in bottom position) and as a |
| | | movable full-width horizontal |
| | | line with OJ and positional |
| | | qualifiers. In the context of |
| | | this specification, should be |
| | | known as Horizontal Line. |
| VERTICAL LINE (U+007C) | 102 | As with the horizontal lines, |
| | | normally a spacing base |
| | | character (in the middle |
| | | position between left and |
| | | right), but can be used as a |
| | | right to left movable |
| | | full-height vertical line with |
| | | OJ and/or positional qualifiers. |
| HalfHeightVerticalLine | 105 | Similar to VERTICAL LINE, but |
| | | only half height. |
| SOLIDUS (U+002F) | 103 | Used only for character |
| | | formation; forward slash |
| REVERSE SOLIDUS | 104 | Used only for character |
| (U+005C) | | formation; reverse slash |
| RightUpperHook | 131 | Used only for character |
| | | formation; nonspacing mark. |
| LeftUpperHook | 132 | Used only for character |
| | | formation; nonspacing mark. |
| LeftLowerHook | 133 | Used only for character |
| | | formation; nonspacing mark. |
| RightLowerHook | 134 | Used only for character |
| | | formation; nonspacing mark. |
| HalfHeightHoop | 140 | Used only for character |
| | | formation; nonspacing mark. |

Klensin & Alvestrand Informational [Page 9]

RFC 5242 Unified CCS April 2008

| HalfHeightInvertedHoop | 141 | Used only for character |
| | | formation; nonspacing mark. |
| DIGIT ZERO (U+0030) | 400 | |
| DIGIT ONE (U+0031) | 401 | |
| DIGIT TWO (U+0032) | 402 | |
| DIGIT NINE (U+0039) | 409 | |
| LATIN SMALL LETTER A | 40A | |
| (U+0061) | | |
| LATIN SMALL LETTER O | 418 | Unify with Greek Omicron |
| (U+006F, U+03BF) | | |
| LATIN SMALL LETTER C | 40C | Unifying C with Cyrillic ES |
| (U+0063, U+0441) | | |
| GREEK SMALL LETTER | 491 | |
| SIGMA (U+03C3) | | |
+------------------------+-------+----------------------------------+

6. Composite Characters and Unicode Equivalences

This section provides examples of characters that are derived from or
based on others, known as "composite characters".

+------------------+--------------+---------------------------------+
| Name | Hex value | Comment |
+------------------+--------------+---------------------------------+
| LATIN SMALL | 418 007 102 | |
| LETTER B | 020 | |
| (U+0062) | | |
| LATIN SMALL | 418 007 102 | |
| LETTER D | 022 | |
| (U+0064) | | |
| LATIN SMALL | 40C 007 108 | |
| LETTER E | 031 | |
| (U+0065) | | |
| LATIN SMALL | 40A 006 40C | |
| LETTER AE | 007 108 031 | |
| (U+00E6) | | |
| LATIN SMALL | 102 131 030 | Note that 007 is not needed |
| LETTER F | 007 108 | before 131 because hooks are |
| (U+0066) | | exclusively nonspacing |
| | | (combining). |
| LATIN SMALL | 102 020 141 | |
| LETTER H | 021 032 | |
| (U+0068) | | |
| LATIN SMALL | 105 007 110 | |
| LETTER I | 021 030 | |
| (U+0069) | | |

Klensin & Alvestrand Informational [Page 10]

RFC 5242 Unified CCS April 2008

| LATIN SMALL | 105 020 141 | |
| LETTER N | 021 032 | |
| (U+006E) | | |
| LATIN SMALL | 418 007 102 | Unified P, Greek Rho, Cyrillic |
| LETTER P | 033 020 033 | ER |
| (U+0070, U+03C1, | | |
| U+0440) | | |
| LATIN CAPITAL | 40A 001 | |
| LETTER A | | |
| (U+0041) | | |
| LATIN CAPITAL | 418 007 102 | |
| LETTER B | 020 001 | |
| (U+0042) | | |
| LATIN CAPITAL | 40C 001 | |
| LETTER C | | |
| (U+0043) | | |
| LATIN CAPITAL | 418 007 102 | |
| LETTER D | 022 001 | |
| (U+0044) | | |
| GREEK SMALL | 491 072 | |
| LETTER FINAL | | |
| SIGMA (U+03C2) | | |
+------------------+--------------+---------------------------------+

7. Ideographic Characters

Because of the traditional model of forming characters using selected
radicals and strokes in combination, Han-derived ("CJK") characters
are even more naturally represented, with less ambiguity, in the
system specified here than European ones. The mechanisms used in
this specification and represented in the tables (see Section 8) are
similar to those described as "Radicals" and "Strokes" in Section 5.1
and in Section 5.2 ("Ideographic Description Characters") of The
Unicode Standard [Unicode]. Of course, following the same principles
outlined above for European characters, only radicals, stroke, and
description controls would be treated as base characters; no distinct
compound precomposed ideographic characters are registered.

8. IANA Considerations

IANA is requested to keep the actual registry of characters and code
tables. The registry entries consist of a character name (preferably
matching the Unicode character name when one is available), the code
sequence used to represent the character and optional descriptive
information. The characters and codes identified in Section 2,
Section 5, and Section 6 above should be used to initialize the
table. Since the coding system is user-extensible, registrations
should be accepted for new characters as long as they don't look like

Klensin & Alvestrand Informational [Page 11]

RFC 5242 Unified CCS April 2008

old ones. A designated expert with a background in calligraphy or
abstract art, and considerable experience in evaluating claims about
the count of angels on heads of pins, should be selected to advise
IANA on "looks like".

9. Security Considerations

The representation of characters in this format should be a
significant boon for security. It eliminates many possibilities of
phishing attacks, since Principle 1 prevents the existence of two
characters that look alike but are different.

By detaching the encoding of characters for domain names from the
encoding of characters for other purposes, it also guarantees that
reasonable-looking names will have been encoded by competent
entities, thereby providing a significant degree of safety by
obscurity.

Because of the method by which upper-case forms are encoded and
because similarity is sometimes in the mind of the beholder, this
specification will not completely eliminate opportunities for visual
confusion. For example, because the lower-case characters are quite
different, LATIN CAPITAL LETTER A and GREEK CAPITAL LETTER ALPHA will
never compare equal, even though they look alike.

10. Acknowledgments

The authors would like to acknowledge the many contributions of
J.F.C. Morphin for pointing out the inadequacies of trying to address
the challenges of internationalization within the context of existing
engineering principles. His comments and related ones, in
combination with issues encountered in trying to internationalize
domain names based on Unicode, have contributed greatly to the frame
of mind underlying large parts of the proposal documented here. The
theoretical framework for this coding system is based, in part, on
Unicode and its collection of names and sample glyphs but represents
a very different approach to the coding system itself.

Klensin & Alvestrand Informational [Page 12]

RFC 5242 Unified CCS April 2008

11. References

11.1. Normative References

[Unicode] The Unicode Consortium, "The Unicode Standard, Version
5.0", 2007.
Boston, MA, USA: Addison-Wesley. ISBN 0-321-48091-0

11.2. Informative References

[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.

Authors' Addresses

John C Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
USA

Phone: +1 617 491 5735
EMail: [email protected]

Harald Tveit Alvestrand
Google
Beddingen 10
Trondheim, 7014
Norway

EMail: [email protected]

Klensin & Alvestrand Informational [Page 13]

RFC 5242 Unified CCS April 2008

Full Copyright Statement

Copyright (C) The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions
contained in BCP 78 and at http://www.rfc-editor.org/copyright.html,
and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
[email protected].

Klensin & Alvestrand Informational [Page 14]