Internet DRAFT - draft-alvestrand-idna-bidi
draft-alvestrand-idna-bidi
Network Working Group H. Alvestrand, Ed.
Internet-Draft Google
Intended status: Standards Track C. Karp, Ed.
Expires: August 17, 2008 Swedish Museum of Natural History
Feb 14, 2008
An updated IDNA criterion for right-to-left scripts
draft-alvestrand-idna-bidi-04
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
The use of right-to-left scripts in internationalized domain names
has presented several challenges. This memo discusses some problems
with these scripts, and some shortcomings in the 2003 IDNA BIDI
criterion. Based on this discussion, it proposes a new BIDI
criterion for IDNA labels.
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Table of Contents
1. Introduction and problem description . . . . . . . . . . . . . 3
1.1. Purpose and applicability . . . . . . . . . . . . . . . . 3
1.2. Background and history . . . . . . . . . . . . . . . . . . 3
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Detailed examples . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Dhivehi . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Yiddish . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Strings with numbers . . . . . . . . . . . . . . . . . . . 6
3. An expanded justification for the bidi rule . . . . . . . . . 7
4. A replacement for the RFC 3454 criterion . . . . . . . . . . . 10
5. Other issues in need of resolution . . . . . . . . . . . . . . 11
6. Compatibility considerations . . . . . . . . . . . . . . . . . 11
6.1. Backwards compatibility considerations . . . . . . . . . . 11
6.2. Forward compatibiltiy considerations . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 14
A.1. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 14
A.2. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 14
A.3. Changes from -02 to -03 . . . . . . . . . . . . . . . . . 14
A.4. Changes from -03 to -04 . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
10.1. Normative references . . . . . . . . . . . . . . . . . . . 15
10.2. Informative references . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction and problem description
1.1. Purpose and applicability
This document's purpose is to establish a test that can be applied to
Internationalized Domain Name (IDN) labels in Unicode form (U-labels)
containing right-to-left characters.
When labels pass the test, they can be used with a minimal chance of
these labels being displayed in a confusing way by a bidirectional
display algorithm. In order to achieve this stability, it is also
necessary that the test be applied to labels occuring before or after
the label containing right-to-left characters, which prohibits some
LDH-labels that are permitted in other contexts.
1.2. Background and history
The IDNA specification "Stringprep", [RFC3454] makes the following
statement in its section 6 on the bidi algorithm, :
3) If a string contains any RandALCat character, a RandALCat
character MUST be the first character of the string, and a
RandALCat character MUST be the last character of the string.
(A RandAlCat character is a character with unambiguously right-to-
left directionality.)
The reasoning behind this prohibition was to ensure that every
component of a displayed domain name has an unambiguously preferred
direction. However, this makes certain words in languages written
with right-to-left scripts invalid as IDN labels, and in at least one
case means that all the words of an entire language are forbidden as
IDN labels.
This will be illustrated below with examples taken from the Dhivehi
and Yiddish languages, as written with the Thaana and Hebrew scripts,
respectively.
In investigating this problem, it was realized that the RFC 3454
specification did not exactly specify what the requirement to be
fulfilled was, and therefore, it was impossible to tell whether a
simple relaxation of the rule would continue to fulfil the
requirement. A further investigation led to the conclusion that for
one reasonable set of requirements, IDNA2003's BIDI restriction did
not fulfil the requirements. This document therefore proposes
replacing the RFC 3454 BIDI requirement in its entirety.
While the document proposes completely new text, most reasonable
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labels that were allowed under the old criterion will also be allowed
under the new criterion, so the operational impact of the rule change
is limited.
1.3. Terminology
In this memo, we use "network order" to describe the sequence of
characters as transmitted on the wire or stored in a file; the terms
"first", "next" and "previous" are used to refer to the relationship
of characters in network order.
We use "display order" to talk about the sequence of characters as
imaged on a display medium; the terms "left" and "right" are used to
refer to the relationship of characters in display order.
Most of the time, the examples use the abbreviations for the Unicode
Bidi classes to denote the directionality of the characters; in some
examples, the convention that uppercase characters are of class R or
AL, and lowercase characters are of class L is used - thus, the
example string ABC.abc would consist of 3 right-to-left characters
and 3 left-to-right characters.
The other terminology used to describe IDNA concepts is defined in
[I-D.klensin-idnabis-issues]
2. Detailed examples
2.1. Dhivehi
Dhivehi, the official language of the Maldives, is written with the
Thaana script. This displays some of the characteristics of Arabic
script, including its directional properties, and the indication of
vowels by the diacritical marking of consonantal base characters.
This marking is obligatory, and both double vowels and syllable-final
consonants are indicated by the marking of special unvoiced
characters. Every Dhivehi word therefore ends with a combining mark.
The word for "computer", which is romanized as "konpeetaru", is
written with the following sequence of Unicode code points:
U+0786 THAANA LETTER KAAFU (AL)
U+07AE THAANA OBOFILI (NSM)
U+0782 THAANA LETTER NOONU (AL)
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U+07B0 THAANA SUKUN (NSM)
U+0795 THAANA LETTER PAVIYANI (AL)
U+07A9 THAANA LETTER EEBEEFILI (AL)
U+0793 THAANA LETTER TAVIYANI (AL)
U+07A6 THAANA ABAFILI (NSM)
U+0783 THAANA LETTER RAA (AL)
U+07AA THANAA UBIUFILI (NSM)
The directionality class of U+07AA in the Unicode database is NSM
(non-spacing mark), which is not R or AL; a conformant implementation
of the IDNA2003 algorithm will say that "this is not in RandALCat",
and refuse to encode the string.
2.2. Yiddish
Yiddish is one of several languages written with the Hebrew script
(others include Hebrew and Ladino). This is basically a consonantal
alphabet (also termed an "abjad") but Yiddish is written using an
extended form that is fully vocalic. The vowels are indicated in
several ways, of which one is by repurposing letters that are
consonants in Hebrew. Other letters are used both as vowels and
consonants, with combining marks, called "points", used to
differentiate between them. Finally, some base characters can
indicate several different vowels, which are also disambiguated by
combining marks. Pointed characters can appear in word-final
position and may therefore also be needed at the end of labels. This
is not an invariable attribute of a Yiddish string and there is thus
greater latitude here than there is with Dhivehi.
The organization now known as the "YIVO Institute for Jewish
Research" developed orthographic rules for modern Standard Yiddish
during the 1930s on the basis of work conducted in several venues
since earlier in that century. These are given in, "The Standardized
Yiddish Orthography: Rules of Yiddish Spelling, 6th ed., YIVO
Institute for Jewish Research, New York, 1999, ISBN 0-914512-25-0",
("SYO") and are taken as normatively descriptive of modern Standard
Yiddish in any context where that notion is deemed relevant. They
have been applied exclusively in all Yiddish dictionaries published
since their establishment, and are similarly dominant in academic and
bibliographic regards.
It therefore appears appropriate for this repertoire also to be
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supported fully by IDNA. This presents no difficulty with characters
in initial and medial positions, but pointed characters are regularly
used in final position as well. All of the characters in the SYO
repertoire appear in both marked and unmarked form with one
exception: the HEBREW LETTER PE (U+05E4). The SYO only permits this
with a HEBREW POINT DAGESH (U+05BC), providing the Yiddish equivalent
to the Latin letter "p", or a HEBREW POINT RAFE (U+05BF), equivalent
to the Latin letter "f". There is, however, a separate unpointed
allograph, the HEBREW LETTER FINAL PE (U+05E3), for the latter
character when it appears in final position. The constraint on the
use of the SYO repertoire resulting from the proscription of
combining marks at the end of RTL strings thus reduces to nothing
more, or less, than the equivalent of saying that a string of Latin
characters cannot end with the letter "p". It must also be noted
that the HEBREW LETTER PE with HEBREW POINT DAGESH is characteristic
of almost all traditional Yiddish orthographies that predate (or
remain in use in parallel to) the SYO, being the first pointed
character to appear in any of them.
A more general instantiation of the basic problem can be seen in the
representation of the YIVO acronym. This is written with the Hebrew
letters YOD YOD HIRIQ VAV VAV ALEF QAMATS, where HIRIQ and QAMATS are
combining points:
U+05D9 HEBREW LETTER YOD (R)
U+05B4 HEBREW POINT HIRIQ (NSM)
U+05D5 HEBREW LETTER VAV (R)
U+05D0 HEBREW LETTER ALEF (R)
U+05B8 HEBREW POINT QAMATS (NSM)
The directionality class of U+05B8 HEBREW POINT QAMATS in the Unicode
database is NSM, which again causes the IDNA2003 algorithm to reject
the string.
It may also be noted that all of the combined characters mentioned
above exist in precomposed form at separate positions in the Unicode
chart. However, by invoking Stringprep, the IDNA2003 algorithm also
rejects those codepoints, for reasons not discussed here.
2.3. Strings with numbers
RFC 3454, in its insistence that the first or last character of a
string be category R or AL, prohibited strings that contained right-
to-left characters and numbers at the end.
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Consider the strings ALEF 5 (HEBREW LETTER ALEF + DIGIT FIVE) and 5
ALEF. Displayed in a LTR context, the first one will be displayed
from left to right as 5 ALEF (with the 5 being considered right-to-
left because of the leading ALEF), while 5 ALEF will be displayed in
exactly the same order (5 taking the direction from context).
Clearly, only one of those should be permitted as a registered label.
3. An expanded justification for the bidi rule
One issue with RFC 3454 was that it did not give an explicit
justification for the bidi rule, thus it was hard to tell if a
modified rule would continue to fulfil the purpose for which the RFC
3454 rule was written.
This document proposes an explicit justification, by stating a set of
requirements for which it is possible to test whether or not the
modified rule fulfils the requirement.
All the text in this document assumes that text containing the labels
under consideration will be displayed using the Unicode bidirectional
algorithm [UAX9].
The justification proposed is this:
o No two labels, when presented in display order, should have the
same sequence of characters without also having the same sequence
of characters in network order. (This is the criterion that is
explicit in RFC 3454).
o In a display of a string of labels, the characters of each label
should remain grouped between the characters delimiting the
labels.
o These properties should hold true both when the string is embedded
in a paragraph with LTR direction and when it's embedded in a
paragraph with RTL direction, as long as explicit directional
controls are not used within the same paragraph.
Several stronger statements were considered and rejected, because
they seem to be impossible to fulfil within the constraints of the
Unicode bidirectional algorithm. These include:
o The appearance of a label should be unaffected by its embedding
context. This proved impossible even for ASCII labels; the label
"123-456" will have a different display order in an RTL context
than in a LTR context.
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o The sequence of labels should be consistent with network order.
This proved impossible - a domain name consisting of the labels
(in network order) L1.R1.R2.L2 will be displayed as L1.R2.R1.L2 in
an LTR context.
o The "remain grouped" property should remain true when directional
controls (LRE, RLE, RLO, LRO, PDF) are used in the same paragraph
(outside of the labels). Because these controls affect
presentation order in non-obvious ways, by affecting the "sor" and
"eor" properties of the Unicode BIDI algorithm, the conditions
above would be very hard to satisfy for an useful set of strings
if this was true. As long as these controls have no influence
over the display of the domain name, no problem will be caused,
but the exact criterion for "will not influence" is hard to
codify.
o The "no two labels display the same" should hold true between LTR
paragraphs and RTL paragraphs. This was shown to be unsound.
o No two domain names should be displayed the same, even under
differing directionality. This was shown to be unsound, since the
domain name (network) ABC.abc will have display order CBA.abc in
an LTR context and abc.CBA in an RTL context, while the domain
name (network) abc.ABC will display as abc.CBA in an LTR context
and as CBA.abc in an RTL context.
For reference, here are the values that the Unicode BIDI property can
have:
o L - Left-to-right - most letters in LTR scripts
o R - Right-to-left - most letters in non-Arabic RTL scripts
o AL - Arabic letters - most letters in the Arabic script
o EN - European Number (0-9)
o ES - European Number Separator (+ and -)
o ET - European Number Terminator (currency symbols, the hash sign,
the percent sign and so on)
o AN - Arabic Number
o CS - Common Number Separator (. , / : et al)
o NSM - Nonspacing Mark - most combining accents
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o BN - Boundary Neutral - control characters
o B - Paragraph Separator
o S - Segment Separator
o WS - Whitespace, including the SPACE character
o ON - Other Neutrals, including @, &, parentheses, MIDDLE DOT
o LRE, LRO, RLE, RLO, PDF - these are "directional control
characters", and are not used in IDNA labels.
The "remain grouped" property can be more formally stated as:
o Let "Delimiter chars" be a set of characters with the Unicode BIDI
properties CS, WS, ON. (These are commonly used to delimit labels
- both the FULL STOP and the space are included.)
* ET, though it commonly occurs next to domain names in practice,
is problematic: the context R CS L EN ET (for instance A.a1%)
makes the label L EN grow unstable.
* ES commonly occurs in labels as HYPHEN-MINUS, but could also be
used as a delimiter (for instance, the plus sign). It is left
out here.
o Let "Position" be the position of a character in a string (in
network order)
o Let "Bidi position" be the position computed by the Unicode Bidi
algorithm
In a paragraph with an embedded string formed from the substrings A B
L C D, where A and D are (possibly zero-length) legal labels, and B
and C are single "Delimiter chars", the label L is a legal label if,
for all A, B, C and D, the bidi position of all characters in L is
within the range of positions for the characters of L in the string,
for both the LTR and RTL paragraph direction.
(The "zero-length" case represents the case where a domain name is
next to something that isn't a domain name, separated by a delimiter
character).
The "No two labels" property can be formally stated as:
If two labels L and L', embedded as for the test above, displayed in
a paragraph with the same directionality, are rearranged into the
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same sequence of codepoints, neither L nor L' is a legal label.
4. A replacement for the RFC 3454 criterion
A set of rules that satisfies the tests above is as follows. The
main bullets give the rule, subordinate bullets (if any) give
justifications or examples of things that break if this rule is not
present. The term "unstable" means that it fails to satisfy the
"remain grouped" property defined above.
Exhaustive testing has verified that strings that satisfy this
criterion satisfy both the requirements above at least for all
strings up to 6 characters.
o Only characters with the BIDI properties L, R, AL, AN, EN, ES, BN,
ON and NSM are allowed.
* B, S and WS are excluded because they are separators or spaces.
* LRE, LRO, RLE, RLO, PDF are excluded because they are bidi
controls.
* ET is excluded because the string L ET is unstable.
* CS is excluded because the string L CS is unstable.
o ES and ON are not allowed in the first position
* ES R and ON R are both unstable.
o ES and ON, followed by zero or more NSM, is not allowed in the
last position
* L ON and L ES are both unstable.
o If an L is present, no R, AL or AN may be present, and vice versa.
o If an EN is present, no AN may be present, and vice versa.
o The first character may not be an NSM
o The first character may not be an EN (European Number) or an AN
(Arabic Number).
* If the character on both sides of a CS is an EN or an AN, the
labels turn unstable.
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* Some domain names where some of the labels use leading EN and
AN may be problem-free, but there's no way of verifying this
while looking at a single label in isolation.
* NOTE: This is a restriction on ASCII labels when used together
with IDNA labels. This is a change from the existing rules for
ASCII labels.
* We could achieve stability by barring numbers at the end of
labels, but this may be more disruptive in practice.
5. Other issues in need of resolution
This document concerns itself only with the rules that are needed
when dealing with domain names with characters that have differing
Bidi properties, and considers characters only in terms of their Bidi
properties. All other issues with these scripts have to be
considered in other contexts.
Another set of issues concerns the proper display of IDNs with a
mixture of LTR and RTL labels, or only RTL labels.
It is unrealistic to expect that domain names will be written using
embedded formatting codes between their labels; thus, the display
order will be determined by the bidirectional algorithm. Thus, a
sequence (in network order) of R1.R2.ltr will be displayed in the
order 2R.1R.ltr in a LTR context, which might surprise someone
expecting to see labels displayed in hierarchical order. Again, this
memo does not attempt to suggest a solution to this problem.
6. Compatibility considerations
6.1. Backwards compatibility considerations
As with any change to an existing standard, it is important to
consider what happens with existing implementations when the change
is introduced. The following troublesome cases have been noted:
o Old program used to input the newly allowed string. If the old
program checks the input against RFC 3454, the string will not be
allowed, and that domain name will remain inaccessible.
o Old program is asked to display the newly allowed string, and
checks it against RFC 3454 before displaying. The program will
perform some kind of fallback, most likely displaying the Punycode
form of the string.
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o Old program tries to display the newly allowed string. If the old
program has code for displaying the last character of a string
that is different from the code used to display the characters in
the middle of the string, display may be inconsistent and cause
confusion.
One particular example of the last case is if a program chooses to
examine the last character (in network order) of a string in order to
determine its directionality, rather than its first; if it finds an
NSM character and tries to display the string as if it was a left-to-
right string, the resulting display may be interesting, but not
useful.
The editors believe that these cases will have less harmful impact in
practice than continuing to deny the use of words from the languages
for which these strings are necessary as IDN labels.
This specification forbids using leading European numbers in ASCII-
only labels; this is in conflict with a large installed base of such
labels. The harm resulting from violating this rule is seen when a
label at the next level down in the hierarchy ends with a number
(Arabic or European). Zone managers, both registries and private
zone managers, can check for this particular condition before they
allow registration of any string with right-to-left characters in it;
generally it is best to not allow registration of any right-to-left
strings in a zone where the label at the level above begins with a
digit.
6.2. Forward compatibiltiy considerations
This text is, intentionally, specified strictly in terms of the
Unicode BIDI properties. The determination that the condition is
sufficient to fulfil the criteria depends on the Unicode BIDI
algorithm; it is unlikely that drastic changes will be made to this
algorithm.
However, the determination of validity for any string depends on the
Unicode BIDI property values, which are not declared immutable by the
Unicode Consortium. Furthermore, the behaviour of the algorithm for
any given character is likely to be linguistically and culturally
sensitive, so that it's not unlikely that later versions of the
Unicode standard may change the bidi properties assigned to certain
Unicode characters.
This memo does not propose a solution for this problem.
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7. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
8. Security Considerations
This modification will allow some strings to be used in Stringprep
contexts that are not allowed today. It is possible that differences
in the interpretation of the specification between old and new
implementations could pose a security risk, but it is difficult to
envision any specific instantiation of this.
Any rational attempt to compute, for instance, a hash over an
identifier processed by Stringprep would use network order for its
computation, and thus be unaffected by the changes proposed here.
While it is not believed to pose a problem, if display routines had
been written with specific knowledge of the RFC 3454 Stringprep
prohibitions, it is possible that the potential problems noted under
"backwards compatibility" could cause new kinds of confusion.
The rule about leading numbers, which is more restrictive than
current practice for domain names, has a peculiar interaction with
the DNAME record; a DNAME record can point to a zone where right-to-
left labels are registered without the knowledge or consent of the
zone owner; if the name of the DNAME begins with a number, this can
cause display of the right-to-left labels in the zone to be
confusing. It is recommended that DNAMEs pointing to zones allowing
right-to-left labels should not start with a digit, but a pointed-to
zone owner has no way of enforcing this.
9. Acknowledgements
While the listed editors held the pen, this document represents the
joint work and conclusions of an ad hoc design team. In addition to
the editors this consisted of, in alphabetic order, Tina Dam, Patrik
Faltstrom, and John Klensin. Many further specific contributions and
helpful comments were received from the people listed below, and
others who have contributed to the development and use of the IDNA
protocols.
The team wishes in particular to thank Roozbeh Pournader for calling
its attention to the issue with the Thaana script, Paul Hoffmann for
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pointing out the need to be explicit about backwards compatibility
considerations, Ken Whistler for suggesting the basis of the
formalized "remain grouped" requirement, and Erik van der Poel for
careful review, comments and verification of the rulesets.
Appendix A. Change log
This appendix is intended to be removed when this document is
published as an RFC.
A.1. Changes from -00 to -01
Suggested a possible new algorithm.
Multiple smaller changes.
A.2. Changes from -01 to -02
Date of publication updated.
Change log added.
A.3. Changes from -02 to -03
Intro changed to reflect addressing the deeper issues with the Bidi
algorithm.
Gave formalized criteria for "valid strings", and documented the new
set of requirements for strings that satisfy the criteria.
Removed most of section 5, "Other problems", and noted that this memo
focuses ONLY on issues that can be evaluated by looking at the bidi
properties of characters.
A.4. Changes from -03 to -04
Added back AN to the list of allowed characters; it had been left out
by accident in -03.
Removed some rules that were redundant.
Added some considerations for backwards compatibility and interaction
with ASCII labels that start with a number.
Mentioned the issue with DNAME pointing to a zone containing RTL
labels in the security considerations section.
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Wording updates in multiple places, including some spelling errors.
Rewrote the introduction section.
Split references into "normative" and "informative".
10. References
10.1. Normative references
[I-D.klensin-idnabis-issues]
Klensin, J., "Internationalizing Domain Names for
Applications (IDNA): Issues, Explanation, and Rationale",
draft-klensin-idnabis-issues-07 (work in progress),
February 2008.
[UAX9] Davis, M., "Unicode Standard Annex #9: The Bidirectional
Algorithm, revision 15", 03 2005.
10.2. Informative references
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
Authors' Addresses
Harald Tveit Alvestrand (editor)
Google
Beddingen 10
Trondheim, 7014
Norway
Email: harald@alvestrand.no
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Cary Karp (editor)
Swedish Museum of Natural History
Frescativ. 40
Stockholm, 10405
Sweden
Phone: +46 8 5195 4055
Fax:
Email: ck@nrm.museum
URI:
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This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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Alvestrand & Karp Expires August 17, 2008 [Page 17]
