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Versions: (draft-alvestrand-idna-bidi) 00 01 02 03 04 05 06 07 RFC 5893

Network Working Group                                 H. Alvestrand, Ed.
Internet-Draft                                                    Google
Intended status: Standards Track                                 C. Karp
Expires: June 3, 2009                  Swedish Museum of Natural History
                                                       November 30, 2008


          An updated IDNA criterion for right-to-left scripts
                       draft-ietf-idnabis-bidi-03

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on June 3, 2009.

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 rule for
   IDNA labels.









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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Purpose and applicability  . . . . . . . . . . . . . . . .  3
     1.2.  Background and history . . . . . . . . . . . . . . . . . .  3
     1.3.  Layout of the rest of this document  . . . . . . . . . . .  4
     1.4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  A replacement for the RFC 3454 BIDI rule . . . . . . . . . . .  6
   3.  A requirement set for the BIDI rule  . . . . . . . . . . . . .  7
   4.  Examples of issues found . . . . . . . . . . . . . . . . . . . 10
     4.1.  Dhivehi  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  Yiddish  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     4.3.  Strings with numbers . . . . . . . . . . . . . . . . . . . 12
   5.  Troublesome situations and guidelines  . . . . . . . . . . . . 12
   6.  Other issues in need of resolution . . . . . . . . . . . . . . 13
   7.  Compatibility considerations . . . . . . . . . . . . . . . . . 14
     7.1.  Backwards compatibility considerations . . . . . . . . . . 14
     7.2.  Forward compatibility considerations . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1. Normative references . . . . . . . . . . . . . . . . . . . 16
     11.2. Informative references . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Change log  . . . . . . . . . . . . . . . . . . . . . 17
     A.1.  Changes from draft-alvestrand-00 to -01  . . . . . . . . . 17
     A.2.  Changes from alvestrand-01 to -02  . . . . . . . . . . . . 17
     A.3.  Changes from alvestrand-02 to -03  . . . . . . . . . . . . 17
     A.4.  Changes from alvestrand-03 to -04  . . . . . . . . . . . . 17
     A.5.  Changes from draft-alvestrand-04 to draft-ietf -00 . . . . 18
     A.6.  Changes from idnabis -00 to -01  . . . . . . . . . . . . . 18
     A.7.  Changes from idnabis -01 to -02  . . . . . . . . . . . . . 18
     A.8.  Changes from idnabis -02 to -03  . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20
















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1.  Introduction

1.1.  Purpose and applicability

   This document's purpose is to establish a rule that can be applied to
   Internationalized Domain Name (IDN) labels in Unicode form (U-labels)
   containing right-to-left characters.

   When labels satisfy the rule, and when certain other conditions are
   satisfied, they can be used with a minimal chance of these labels
   being displayed in a confusing way by a bidirectional display
   algorithm.

   This specification is not intended to place any requirements on
   domain names that do not contain right-to-left characters.

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 is 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 explicitly state the requirement to be
   fulfilled, 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.




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   While the document proposes completely new text, most reasonable
   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.  Layout of the rest of this document

   Section 2 defines a test, the "BIDI test", that can be used on a
   domain name label to check how safe it is to use in a domain name of
   possibly mixed directionality.- one user of that test is the IDNA2008
   protocol[I-D.ietf-idnabis-protocol].

   Section 3 sets out the requirements for defining a BIDI rule.

   Section 4 gives detailed examples that serve as justification for the
   change proposed here.

   Section 5 to Section 9 describe various situations that can occur
   when dealing with domain names with characters of different
   directionality.

   Only Section 1.4 and Section 2 are normative.

1.4.  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", "previous", "before" and "after" are used to refer
   to the relationship of characters and labels 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 and labels 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 directionality of such examples is determined by context - for
   instance, in the sentence "ABC.abc is displayed as CBA.abc", the
   first example string is in network order, the second example string
   is in display order.

   The term "paragraph" is used in the sense of the Unicode BIDI
   specification [UAX9] - it means "a block of text that has an overall



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   direction, either left-to-right or right-to-left", approximately; see
   UAX 9 for the details.

   "LTR" and "RTL" are abbreviations for "right to left" and "left to
   right", respectively.

   The terminology used for the BIDI properties of Unicode characters is
   taken from the Unicode Standard[Unicode]

   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, and Extended Arabic-Indic numbers)

   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 - Non spacing Mark - most combining accents

   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 other terminology used to describe IDNA concepts is defined in
   [I-D.ietf-idnabis-defs]





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2.  A replacement for the RFC 3454 BIDI rule

   The following test has been developed for BIDI domain name labels.
   The requirements that this test satisifes are described in Section 3.

   A label containing a character of type R, AL or AN MUST satisfy all 7
   of the rules below.

   The main bullets give the rule, subordinate bullets (if any) give
   justifications or examples of things that break if this rule is not
   present.

   1.  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 does not satisfy the
          Character Grouping requirement.

       *  CS is excluded because the string L CS does not satisfy the
          Character Grouping requirement.

   2.  ES and ON are not allowed in the first position

       *  ES R and ON R do not satisfy the Character Grouping
          requirement.

   3.  ES and ON, followed by zero or more NSM, is not allowed in the
       last position

       *  L ON and L ES both fail the Character Grouping requirement.

   4.  If an R, AL or AN is present, no L may be present.

   5.  If an EN is present, no AN may be present, and vice versa.

   6.  The first character may not be an NSM

   7.  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 fail the Character Grouping requirement.



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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..

       *  We could achieve stability by barring numbers at the end of
          labels, but this may be more disruptive in practice.

   The following guarantees can be made based on the above:

   o  In a domain name consisting of only labels that pass the test, the
      requirements of Section 3 are satisfied.

   o  In a domain name consisting of only LDH-labels and labels that
      pass the test, the requirements of Section 3 are satisfied as long
      as a label that starts with an ASCII digit does not come after a
      right-to-left label that ends in a digit.

   No guarantee is given for other combinations.


3.  A requirement set 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 requirements proposed are these:

   o  Label Uniqueness: No two labels, when presented in display order
      in the same paragraph, should have the same sequence of characters
      without also having the same sequence of characters in network
      order, both when the paragraph has LTR direction and when the
      paragraph has RTL direction.  (This is the criterion that is
      explicit in RFC 3454).  (Note that a label displayed in an RTL
      paragraph may display the same as a different label displayed in a
      LTR paragraph, and still satisfy this criterion.)

   o  Character Grouping: When displaying a string of labels, using the
      Unicode BIDI algorithm to reorder the characters for display, the



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      characters of each label should remain grouped between the
      characters delimiting the labels, both when the string is embedded
      in a paragraph with LTR direction and when it's embedded in a
      paragraph with RTL direction.

   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.

   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.  (In a RTL context, it will be displayed as
      L2.R2.R1.L1).

   o  The Label Uniqueness property 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 have display order abc.CBA in an LTR
      context and CBA.abc in an RTL context.

   One specific requirement was thought to be problematic, but turned
   out to be satisfied by a string that obeys the proposed rules:

   o  The Character Grouping requirement should be satisfied 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 require extra testing in order to figure out
      whether or not they influence the display of the domain name.
      Testing found that for the strings allowed under the rule
      presented in this document, directional controls do not influence
      the display of the domain name.

   In the following descriptions, first-level bullets are used to
   indicate rules or normative statements; second-level bullets are
   commentary.




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   The Character Grouping requirement can be more formally stated as:

   o  Let "Delimiterchars" 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 not satisfy the character grouping
         requirement.

      *  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 "unproblematic label" be a label that either satisfies the
      requirements, or does not contain any character with the BIDI
      properties R, AL or AN, and does not begin with a character with
      the BIDI property EN.  (Informally, "it does not start with a
      number".)

   A label L satisfies the Character Grouping requirement when, for any
   Delimiter Character D1 and D2, and for any label S1 and S2 that is
   either a label satisfying the requirements or an unproblematic label,
   the following holds true:

   If the string formed by concatenating S1, D1, L, D2 and S2 is
   reordered according to the BIDI algorithm, then all the characters of
   L in the reordered string are between D1 and D2, and no other
   characters are between D1 and D2, both if the overall paragraph
   direction is LTR and if the overall paragraph direction is RTL.

   Note that the definition is self-referential, since S1 and S2 are
   constrained to be "legal" by this definition; this makes testing
   changes to proposed rules a little complex, but does not create
   problems for testing whether or not a given proposed rule satisfies
   the criterion.

   (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 Label Uniqueness requirement can be formally stated as:

   If two labels L and L', embedded as for the test above, displayed in
   paragraphs with the same directionality, are reordered by the BIDI
   algorithm into the same sequence of codepoints, at most one of the
   labels L and L' is a legal label.



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4.  Examples of issues found

4.1.  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 THAANA UBIUFILI (NSM)

   The directionality class of U+07AA in the Unicode database [Unicode]
   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.

4.2.  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



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   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" [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
   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)




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      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.

4.3.  Strings with numbers

   By requiring that the first or last character of a string be category
   R or AL, RFC 3454 prohibited a string containing right-to-left
   characters from ending with a number.

   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,
   but barring them both seems to require justification.


5.  Troublesome situations and guidelines

   There are situations in which labels that satisfy the rule above will
   be displayed in a surprising fashion; the most important of these is
   the case where a label ending in a character with BIDI property AL,
   AN or R occurs before a label beginning with a character of BIDI
   property EN.  In that case, the number will appear to move into the
   label containing the right-to-left character, violating the Character
   Grouping requirement.

   If the label that occurs after the right-to-left label itself
   satisfies the BIDI criterion, the requirements will be satisfied in
   all cases (this is the reason why the criterion talks about strings
   containing L in some cases).  However, the WG concluded that this
   could not be required for several reasons:






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   o  There is a large current deployment of ASCII domain names starting
      with digits.  These cannot possibly be invalidated.

   o  Domain names are often constructed piecemeal, for instance by
      combining a string with the content of a search list.  This may
      occur after IDNA processing, and thus in part of the code that is
      not IDNA-aware, making detection of the undesirable combination
      impossible.

   o  Even if a label is registered under a "safe" label, there may be a
      DNAME [RFC2672]with an "unsafe" label that points to the "safe"
      label, thus creating seemingly-valid names that would not satisfy
      the criterion.

   o  Wildcards create the odd situation where a label is "valid" (can
      be looked up successfully) without the zone owner's knowing that
      this label exists.  So an owner of a zone whose name starts with a
      digit and contains a wildcard has no way of controlling whether or
      not names with RTL labels in them are looked up in his zone.

   So rather than trying to suggest rules that disallow all such
   undesirable situations, this document merely warns about the
   possibility.


6.  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.

   One such issue is the need to keep numbers separate; several scripts,
   such as Arabic, have multiple sets of numbers.  The algorithm in this
   document disallows occurrences of AN-class characters ("Arabic-Indic
   digits", U+0660 to U+0669) together with EN-class characters (which
   includes "European" digits, U+0030 to U+0039 and "extended Arabic-
   Indic digits", U+06F0 to U+06F9), but does not help in preventing the
   mixing of, for instance, Bengali digits (U+09E6 to U+09EF) and
   Gujarati digits (U+0AE6 to U+0AEF), both of which have BIDI class L.
   A registry or script community that wishes to create rules for the
   mixing of digits in a label will be able to specify these
   restrictions at the registry level; rules can also be specified at
   the protocol level, but while the example above involves right-to-
   left characters, this is not inherently a bidi problem.

   Another set of issues concerns the proper display of IDNs with a



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   mixture of LTR and RTL labels, or only RTL labels.

   It is unrealistic to expect that applications will display domain
   names using embedded formatting codes between their labels (for one
   thing, no reliable algorithms for identifying domain names in running
   text exist); 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 an 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.


7.  Compatibility considerations

7.1.  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 string
      in A-label form.

   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 does not forbid using leading European numbers in



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   ASCII-only labels, since this would in conflict with a large
   installed base of such labels, and would increase the scope of the
   specification from RTL labels to all labels.  The harm resulting from
   this limitation of scope is described in Section 5.  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.

7.2.  Forward compatibility 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.


8.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.


9.  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.



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   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.


10.  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
   pointing out the need to be explicit about backwards compatibility
   considerations, Ken Whistler for suggesting the basis of the
   formalized "character grouping" requirement, Mark Davis for
   commentary, Erik van der Poel for careful review, comments and
   verification of the rulesets, and Marcos Sanz, Andrew Sullivan and
   Pete Resnick for reviews.


11.  References

11.1.  Normative references

   [I-D.ietf-idnabis-defs]
              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and  Document Framework",
              draft-ietf-idnabis-defs-01 (work in progress),
              November 2008.

   [UAX9]     Davis, M., "Unicode Standard Annex #9: The Bidirectional
              Algorithm, revision 15", 03 2005.

   [Unicode]  Unicode, "The Unicode Standard - version 5.1", 2008.

11.2.  Informative references

   [I-D.ietf-idnabis-protocol]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol",
              draft-ietf-idnabis-protocol-06 (work in progress),
              November 2008.



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   [RFC2672]  Crawford, M., "Non-Terminal DNS Name Redirection",
              RFC 2672, August 1999.

   [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [SYO]      "The Standardized Yiddish Orthography: Rules of Yiddish
              Spelling, 6th ed., , New York, ISBN 0-914512-25-0",",
              1999.


Appendix A.  Change log

   This appendix is intended to be removed by the RFC Editor when this
   document is published as an RFC.

A.1.  Changes from draft-alvestrand-00 to -01

   Suggested a possible new algorithm.

   Multiple smaller changes.

A.2.  Changes from alvestrand-01 to -02

   Date of publication updated.

   Change log added.

A.3.  Changes from alvestrand-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 alvestrand-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.




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   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.

   Wording updates in multiple places, including some spelling errors.

   Rewrote the introduction section.

   Split references into "normative" and "informative".

A.5.  Changes from draft-alvestrand-04 to draft-ietf -00

   Changed name of draft.

   Added a couple of "note in draft" statements to remind the WG of open
   issues.

   Noted that BIDI controls in the paragraph are unproblematic with the
   given ruleset.

A.6.  Changes from idnabis -00 to -01

   Added text to section 5 describing issues with mixture of numbers in
   labels

   Addressed some of the issues raised by Mark Davis in March 2008 in
   regard to document clarity.

   Changed the formulation of the label uniqueness requirement to be
   consistent with the text under "Labels with numbers".

   Spell-checked document.

A.7.  Changes from idnabis -01 to -02

   Changed the domain of applicability to be only labels containing RTL
   characters, described the conditions under which harm may result from
   putting RTL labels next to other labels, and how to detect them.

   A number of clarification and formatting changes in response to
   reviews.








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A.8.  Changes from idnabis -02 to -03

   Rearranged section list so that the normative material is collected
   at the front.

   Moved list of BIDI properties into "terminology"

   Clarified that only terminology and the BIDI test is normative

   Changed reference to point to -defs for definitions instead of
   -rationale

   Minor fixes in response to comments, wording cleanups, removed all
   tentative language.


Authors' Addresses

   Harald Tveit Alvestrand (editor)
   Google
   Beddingen 10
   Trondheim,   7014
   Norway

   Email: harald@alvestrand.no


   Cary Karp
   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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