Internet DRAFT - 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

Status of this Memo

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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, 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,

   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 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

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:




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   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
   combining points:






   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

   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

   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) will have display order 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 in an RTL context.

   For reference, here are the values that the Unicode BIDI property can

   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

   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

   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

      *  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

   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

   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

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

   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

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

   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

   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

              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)
   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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Internet-Draft                IDNA RTL fix                      Feb 2008

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