One document matched: draft-alvestrand-idna-bidi-02.txt
Differences from draft-alvestrand-idna-bidi-01.txt
Network Working Group H. Alvestrand, Ed.
Internet-Draft Google
Intended status: Standards Track C. Karp, Ed.
Expires: July 12, 2008 Swedish Museum of Natural History
Jan 9, 2008
An IDNA problem in right-to-left scripts
draft-alvestrand-idna-bidi-02
Status of this Memo
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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, including one resulting from a constraint on the
use of combining characters at the end of an RTL domain label,
causing some words to be declared invalid as IDN labels, and proposes
a means for ameliorating this problem.
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Table of Contents
1. Introduction and problem description . . . . . . . . . . . . . 3
2. Detailed examples . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Dhivehi . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Yiddish . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Strings with numbers . . . . . . . . . . . . . . . . . . . 5
3. An expanded justification for the bidi rule . . . . . . . . . 6
4. Modification to RFC 3454 . . . . . . . . . . . . . . . . . . . 6
4.1. Alternative approach . . . . . . . . . . . . . . . . . . . 7
5. Other issues in need of resolution . . . . . . . . . . . . . . 8
6. Backwards compatibility considerations . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 11
A.1. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 11
A.2. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction and problem description
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 visually presented 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.
The problem may be addressed by more carefully considering the bidi
algorithm in Unicode Standard Annex #9 [UAX9] which states in section
3.3.3 W1: "Examine each non-spacing mark (NSM) in the level run, and
change the type of the NSM to the type of the previous character."
(See below for some terminology).
Section 3 of UAX9 contains several instructions for determining the
directionality of the characters in a string. Some of them (for
instance those using explicit embedding) are irrelevant to IDNA
because the corresponding codes are not permitted as IDNA input, so a
slightly simplified version should be enough for IDNA purposes.
A note on 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.
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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)
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 IDNA 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 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 used to
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differentiate between them. Finally, some base characters can
indicate several different vowels, which are also disambiguated by
combining marks. Marked 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 all Yiddish strings and there is thus
greater latitude here than there is with Dhivehi.
The "YIVO Institute for Jewish Research" is widely known by the
acronym of its Yiddish name. This organization maintains a primary
reference standard for modern Standard Yiddish orthography, that is
also commonly referred to by the same acronym (as the "YIVO Rules").
YIVO 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 IDNA algorithm to reject the
string. (It may also be noted that the requisite combined characters
also exist in precomposed form at separate positions in the Unicode
chart. However, Stringprep 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.
Considering the string ALEF 5 (HEBREW LETTER ALEF + DIGIT FIVE), if
we specify that UAX#9 is used to find the directionality of
characters, this string will have a consistent direction (R).
However, the string 5 ALEF, when embedded in an LTR context, will
have the same display order, with a different direction assigned to
the number 5. These two display strings are confusable, so we need a
rule that permits only one of these in a domain name label.
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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, for which we think
it is possible to test whether or not the modified rule fulfils the
justification.
The justification proposed is this:
o No two labels, when presented in visual 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 visual presentation of a string of labels, the characters of
each label should remain grouped between the dots delimiting the
label components.
o This property should hold true both when the string is embedded in
a RTL context and when it's embedded in a LTR context.
o This property should hold true without adding extra formatting,
for example bidi control characters, to the string.
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 a RTL context
than in a LTR context.
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.
4. Modification to RFC 3454
If the following modification is made to RFC 3454 section 6,
paragraph 4, we believe that the usefulness of the specification for
languages written with right-to-left scripts will be significantly
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improved:
Old text:
[Unicode3.2] defines several bidirectional categories; each
character has one bidirectional category assigned to it. For the
purposes of the requirements below, an "RandALCat character" is a
character that has Unicode bidirectional categories "R" or "AL";
an "LCat character" is a character that has Unicode bidirectional
category "L".
New text:
[Unicode3.2] defines several bidirectional categories; each
character has one bidirectional category assigned to it.
For characters that have category "R", "AL" or "L", the category
is fixed (UAX#9 defines them as having "strong" category); for
characters in category EN, ES, ET, AN, CS, NSM, BN, B, S, WS and
ON, the category is determined by applying the algorithm described
in UAX#9 section 3.3 to the string.
For the purposes of the requirements below, an "RandALCat
character" is a character that, after this determination, has
Unicode bidirectional categories "R" or "AL"; an "LCat character"
is a character that has Unicode bidirectional category "L".
Note that Unicode 5.0 is the current version of Unicode. This fix
refers to Unicode 3.2 only, to maintain consistency with the rest of
RFC 3454. Nothing here should affect the relationship between
Unicode versions and IDNA.
Also, as noted in the introduction, the Unicode UAX#9 algorithm is
quite complex. For the purposes of IDNA, a simpler algorithm may be
defind that yields the same result within the constraints of this
context, but may be easier for people to implement consistently.
Such an algorithm may be included in later versions of this memo.
4.1. Alternative approach
The editors are not entirely happy with the text above. We are
considering, instead, a complete replacement for section 6 of RFC
3454.
A first draft of such a section is below.
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Conceptually, to verify suitability as a domain name label, one
constructs the string consisting of the label preceded and followed
by a full stop (U+002E), and executes the Unicode bidirectional
algorithm twice, once with <sor> (start of run) and <eor> (end of
run) having direction L, and once with them having direction R. (The
full stop, being of bidi class CS, is used because it seems likely to
show up any problems, and occurs next to labels a lot of the time.
Other times, a label is adjacent to an @ sign, a space or another
character.)
The following conditions MUST be true in both resulting strings for
the string to be acceptable:
o The leftmost and rightmost character of the resulting string in
display order must be a full stop (U+002E)
o No non-spacing mark (NSM) can occur in the second position of the
string (leftmost in L order, rightmost in R order); that is, no
mark can be allowed to attach to the delimiting characters.
o The direction of the leftmost and rightmost characters in the
string (the periods) must be either L or R
Note that there is no requirement that the character sequence be the
same in the two cases.
All RTL strings permitted by RFC 3454 section 6 will pass this test.
Strings that consist of such a string with NSM characters appended to
it will also pass this test.
[[NOTE: Not sure if the ALEF 5 vs 5 ALEF issue will be solved by this
rule. Test needed.]]
[[NOTE: do we need to require something for the sor=L, eor=R and
sor=R, eor=L cases?]]
5. Other issues in need of resolution
This is not the only issue with right-to-left scripts. Retaining
Yiddish for the purposes of further exemplification, its alphabet
includes three digraphs that can be encoded both as consecutive
instances of the two component characters, and as precomposed
ligatures. One of these digraphs also requires additional combined
marking. For example, the HEBREW LIGATURE YIDDISH DOUBLE VAV
(U+05F0) is orthographically equivalent to, and typographically
utterly confusable with, a sequence of two HEBREW LETTER VAV
(U+05D5). However, the ligature has no canonical decomposition and
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is therefore preserved by the IDNA algorithm. These digraphs need to
be enumerated and the one form either made invalid for input in the
IDNA context, or normalized to the other.
We believe that there is a clear likelihood of similar issues
existing with other scripts and languages that are not currently used
extensively with IDNs. Careful consideration of all the languages
written in a given script, in consultation with all of the
corresponding speech communities, is therefore needed before we can
say with any degree of certainty that using that script for IDNs is
unproblematic.
Another set of issues concerns the proper display of IDNs with a
mixture of LTR and RTL labels, or only RTL labels; it is not clear to
these authors what the proper display order of the components of a
domain name are if the directiion of the components (in network
order) is, for instance, FirstRTL.SecondRTL.LTR - is it
LTRtsriF.LTRdnoceS.LTR or LTRdnoceS.LTRtsrif.LTR? Again, this memo
does not attempt to suggest a solution to this problem.
6. 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.
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.
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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.
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 current Stringprep
prohibitions, it is possible that the possible problems noted under
"backwards compatibility" could cause new kinds of confusion.
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, and Paul Hoffmann
for pointing out the need to be explicit about backwards
compatibility considerations.
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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.
10. References
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[UAX9] 0, "Unicode Standard Annex #9: The Bidirectional
Algorithm, revision 15", 03 2005.
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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