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

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   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,

   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:











   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":






   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

   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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   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

   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

   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

   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

   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

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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

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

   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)
   Beddingen 10
   Trondheim,   7014


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   Cary Karp (editor)
   Swedish Museum of Natural History
   Frescativ. 40
   Stockholm,   10405

   Phone: +46 8 5195 4055

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