IDNA Bidi: Right-to-Left Scripts for Internationalized Domain Names for Applications (IDNA)

Internet Engineering Task Force (IETF)                H. Alvestrand, Ed.
Request for Comments: 5893                                        Google
Category: Standards Track                                        C. Karp
ISSN: 2070-1721                        Swedish Museum of Natural History
                                                             August 2010


                       Right-to-Left Scripts for
         Internationalized Domain Names for Applications (IDNA)

Abstract

   The use of right-to-left scripts in Internationalized Domain Names
   (IDNs) has presented several challenges.  This memo provides a new
   Bidi rule for Internationalized Domain Names for Applications (IDNA)
   labels, based on the encountered problems with some scripts and some
   shortcomings in the 2003 IDNA Bidi criterion.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5893.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Purpose and Applicability  . . . . . . . . . . . . . . . .  2
     1.2.  Background and History . . . . . . . . . . . . . . . . . .  3
     1.3.  Structure of the Rest of This Document . . . . . . . . . .  3
     1.4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  The Bidi Rule  . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  The Requirement Set for the Bidi Rule  . . . . . . . . . . . .  6
   4.  Examples of Issues Found with RFC 3454 . . . . . . . . . . . .  9
     4.1.  Dhivehi  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     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.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 17

1.  Introduction

1.1.  Purpose and Applicability

   The purpose of this document is to establish a rule that can be
   applied to Internationalized Domain Name (IDN) labels in Unicode form
   (U-labels) containing characters from scripts that are written from
   right to left.  It is part of the revised IDNA protocol [RFC5891].

   When labels satisfy the rule, and when certain other conditions are
   satisfied, there is only a minimal chance of these labels being
   displayed in a confusing way by the Unicode bidirectional display
   algorithm.

   The other normative documents in the IDNA2008 document set establish
   criteria for valid labels, including listing the permitted
   characters.  This document establishes additional validity criteria
   for labels in scripts normally written from right to left.

   This specification is not intended to place any requirements on
   domain names that do not contain characters from such scripts.

1.2.  Background and History

   The "Stringprep" specification [RFC3454], part of IDNA2003, made 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 made certain words in languages written
   with right-to-left scripts invalid as IDN labels, and in at least one
   case (Dhivehi) meant that all the words of an entire language were
   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.

   RFC 3454 did not explicitly state the requirement to be fulfilled.
   Therefore, it is impossible to determine whether a simple relaxation
   of the rule would continue to fulfill the requirement.

   While this document specifies rules quite different from RFC 3454,
   most reasonable labels that were allowed under RFC 3454 will also be
   allowed under this specification (the most important example of
   non-permitted labels being labels that mix Arabic and European digits
   (AN and EN) inside an RTL label, and labels that use AN in an LTR
   label -- see Section 1.4 for terminology), so the operational impact
   of using the new rule in the updated IDNA specification is limited.

1.3.  Structure of the Rest of This Document

   Section 2 defines a rule, the "Bidi rule", which can be used on a
   domain name label to check how safe it is to use in a domain name of
   possibly mixed directionality.  The primary initial use of this rule
   is as part of the IDNA2008 protocol [RFC5891].

   Section 3 sets out the requirements for defining the Bidi rule.

   Section 4 gives detailed examples that serve as justification for the
   new rule.

   Section 5 to Section 8 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

   The terminology used to describe IDNA concepts is defined in the
   Definitions document [RFC5890].

   The terminology used for the Bidi properties of Unicode characters is
   taken from the Unicode Standard [Unicode52].

   The Unicode Standard specifies a Bidi property for each character.
   That property controls the character's behavior in the Unicode
   bidirectional algorithm [Unicode-UAX9].  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; this encompasses the Arabic-Indic numbers, but
      not the Extended Arabic-Indic numbers

   o  CS - Common Number Separator (. , / : et al)

   o  NSM - Nonspacing Mark - most combining accents

   o  BN - Boundary Neutral - control characters (ZWNJ, ZWJ, and others)

   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.

   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", "beginning", "end", "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; the
   example string CS L consists of one character of class CS and one
   character of class L.  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 three right-to-left characters and three 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 [Unicode-UAX9].  It means "a block of text that has an
   overall direction, either left to right or right to left",
   approximately; see the "Unicode Bidirectional Algorithm"
   [Unicode-UAX9] for details.

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

   An RTL label is a label that contains at least one character of type
   R, AL, or AN.

   An LTR label is any label that is not an RTL label.

   A "Bidi domain name" is a domain name that contains at least one RTL
   label.  (Note: This definition includes domain names containing only
   dots and right-to-left characters.  Providing a separate category of
   "RTL domain names" would not make this specification simpler, so it
   has not been done.)

2.  The Bidi Rule

   The following rule, consisting of six conditions, applies to labels
   in Bidi domain names.  The requirements that this rule satisfies are
   described in Section 3.  All of the conditions must be satisfied for
   the rule to be satisfied.

   1.  The first character must be a character with Bidi property L, R,
       or AL.  If it has the R or AL property, it is an RTL label; if it
       has the L property, it is an LTR label.

   2.  In an RTL label, only characters with the Bidi properties R, AL,
       AN, EN, ES, CS, ET, ON, BN, or NSM are allowed.

   3.  In an RTL label, the end of the label must be a character with
       Bidi property R, AL, EN, or AN, followed by zero or more
       characters with Bidi property NSM.

   4.  In an RTL label, if an EN is present, no AN may be present, and
       vice versa.

   5.  In an LTR label, only characters with the Bidi properties L, EN,
       ES, CS, ET, ON, BN, or NSM are allowed.

   6.  In an LTR label, the end of the label must be a character with
       Bidi property L or EN, followed by zero or more characters with
       Bidi property NSM.

   The following guarantees can be made based on the above:

   o  In a domain name consisting of only labels that satisfy the rule,
      the requirements of Section 3 are satisfied.  Note that even LTR
      labels and pure ASCII labels have to be tested.

   o  In a domain name consisting of only LDH labels (as defined in the
      Definitions document [RFC5890]) and labels that satisfy the rule,
      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.

   No guarantee is given for other combinations.

3.  The Requirement Set for the Bidi Rule

   This document, unlike RFC 3454 [RFC3454], provides an explicit
   justification for the Bidi rule, and states a set of requirements for
   which it is possible to test whether or not the modified rule
   fulfills the requirement.

   All the text in this document assumes that text containing the labels
   under consideration will be displayed using the Unicode bidirectional
   algorithm [Unicode-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
      an 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
      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 is embedded in a
      paragraph with RTL direction.

   Several stronger statements were considered and rejected, because
   they seem to be impossible to fulfill 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-A" will have a different display order in an RTL context than
      in an LTR context.  (This particular example is, however,
      disallowed anyway.)

   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.R2.R3.L4 will be displayed as L1.R3.R2.L4 in
      an LTR context.  (In an RTL context, it will be displayed as
      L4.R3.R2.L1).

   o  No two domain names should be displayed the same, even under
      differing directionality.  This was shown to be unsound, since the
      domain name (in network order) 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 possible 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.

   This is still not stated as a requirement, since it did not seem as
   important as the stated requirements, but it is useful to know that
   Bidi domain names where the labels satisfy the rule have this
   property.

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

   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.  They are not
      allowed in domain labels.)

      *  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 X satisfies the Character Grouping requirement when, for any
   Delimiter Character D1 and D2, and for any label S1 and S2 that is an
   unproblematic label or an empty string, the following holds true:

   If the string formed by concatenating S1, D1, X, D2, and S2 is
   reordered according to the Bidi algorithm, then all the characters of
   X 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.

   Note about the position of BN: The Unicode bidirectional algorithm
   specifies that a BN has an effect on the adjoining characters in
   network order, not in display order, and are therefore treated as if
   removed during Bidi processing ([Unicode-UAX9], Section 3.3.2, rule
   X9 and Section 5.3).  Therefore, the question of "what position does
   a BN have after reordering" is not meaningful.  It has been ignored
   while developing the rules here.

   The Label Uniqueness requirement can be formally stated as:

   If two non-identical labels X and Y, embedded as for the test above,
   displayed in paragraphs with the same directionality, are reordered
   by the Bidi algorithm into the same sequence of code points, the
   labels X and Y cannot both be legal.

4.  Examples of Issues Found with RFC 3454

4.1.  Dhivehi

   Dhivehi, the official language of the Maldives, is written with the
   Thaana script.  This script displays some of the characteristics of
   the 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 two consecutive
   vowels and syllable-final consonants are indicated with unvoiced
   combining marks.  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 UBUFILI (NSM)

   The directionality class of U+07AA in the Unicode database
   [Unicode52] is NSM (Nonspacing 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, one of which 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" [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 formal 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 the 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 acronym is written with the
   Hebrew letters YOD YOD HIRIQ VAV VAV ALEF QAMATS, where HIRIQ and
   QAMATS are combining points.  The Unicode code points are:

      U+05D9 HEBREW LETTER YOD (R)

      U+05B4 HEBREW POINT HIRIQ (NSM)

      U+05D5 HEBREW LETTER VAV (R)

      U+05D0 HEBREW LETTER ALEF (R)

      U+05B8 HEBREW POINT QAMATS (NSM)

   The directionality class of U+05B8 HEBREW POINT QAMATS in the Unicode
   database is NSM, which again causes the IDNA2003 algorithm to reject
   the string.

   It may also be noted that all of the combined characters mentioned
   above exist in precomposed form at separate positions in the Unicode
   chart.  However, by invoking Stringprep, the IDNA2003 algorithm also
   rejects those code points, for reasons not discussed here.

4.3.  Strings with Numbers

   By requiring that the first or last character of a string be a member
   of category R or AL, the Stringprep specification [RFC3454]
   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 an 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 unnecessary.

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 IDNABIS WG concluded that
   this could not be required for several reasons:

   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 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.

   Rather than trying to suggest rules that disallow all such
   undesirable situations, this document merely warns about the
   possibility, and leaves it to application developers to take whatever
   measures they deem appropriate to avoid problematic situations.

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 scripts that are written from
   right to left must be considered in other contexts.

   One such issue is the need to keep numbers separate.  Several scripts
   are used with multiple sets of numbers -- most commonly they use
   Latin numbers and a script-specific set of numbers, but in the case
   of Arabic, there are two sets of "Arabic-Indic" digits involved.

   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 restricting the mixing of digits in a label
   will be able to specify these restrictions at the registry level.
   Some rules are also specified at the protocol level.

   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 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 Bidi
   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.  People used to working with text that mixes LTR and RTL
   strings might not be so surprised by this.  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.  Some troublesome cases include:

   o  An old program used to input the newly allowed label.  If the old
      program checks the input against RFC 3454, some labels will not be
      allowed, and domain names containing those labels will remain
      inaccessible.

   o  An old program is asked to display the newly allowed label, and
      checks it against RFC 3454 before displaying.  The program will
      perform some kind of fallback, most likely displaying the label in
      A-label form.

   o  An old program tries to display the newly allowed label.  If the
      old program has code for displaying the last character of a label
      that is different from the code used to display the characters in
      the middle of the label, the 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 a 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 digits in
   ASCII-only labels, since this would 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.  Registries and
   private zone managers can check for this particular condition before
   they allow registration of any RTL label.  Generally, it is best to
   disallow 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 fulfill 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 behavior of the algorithm for
   any given character is likely to be linguistically and culturally
   sensitive, so while it should occur rarely, it is possible 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.  Security Considerations

   The display behavior of mixed-direction text can be extremely
   surprising to users who are not used to it; for instance, cut and
   paste of a piece of text can cause the text to display differently at
   the destination, if the destination is in another directionality
   context, and adding a character in one place of a text can cause
   characters some distance from the point of insertion to change their
   display position.  This is, however, not a phenomenon unique to the
   display of domain names.

   The new IDNA protocol, and particularly these new Bidi rules, will
   allow some strings to be used in IDNA contexts that are not allowed
   today.  It is possible that differences in the interpretation of
   labels between implementations of IDNA2003 and IDNA2008 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 IDNA would use network order for its
   computation, and thus be unaffected by the new rules proposed here.

   While it is not believed to pose a problem, if display routines had
   been written with specific knowledge of the RFC 3454 IDNA
   prohibitions, it is possible that the potential problems noted under
   "Backwards Compatibility Considerations" 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 particular formulation of the Bidi rule in Section 2 was
   suggested by Matitiahu Allouche.

   The team wishes, in particular, to thank Roozbeh Pournader for
   calling its attention to the issue with the Thaana script, Paul
   Hoffman 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, Marcos Sanz, Andrew Sullivan, and Pete
   Resnick for reviews, and Vint Cerf for chairing the working group and
   contributing massively to getting the documents finished.

10.  References

10.1.  Normative References

   [RFC5890]      Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Definitions and Document
                  Framework", RFC 5890, August 2010.

   [Unicode-UAX9] The Unicode Consortium, "Unicode Standard Annex #9:
                  Unicode Bidirectional Algorithm", September 2009,
                  <http://www.unicode.org/reports/tr9/>.

   [Unicode52]    The Unicode Consortium.  The Unicode Standard, Version
                  5.2.0, defined by: "The Unicode Standard, Version
                  5.2.0", (Mountain View, CA: The Unicode Consortium,
                  2009. ISBN 978-1-936213-00-9).
                  <http://www.unicode.org/versions/Unicode5.2.0/>.

10.2.  Informative References

   [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.

   [RFC5891]      Klensin, J., "Internationalized Domain Names in
                  Applications (IDNA): Protocol", RFC 5891, August 2010.

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

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@nic.museum