IDNA Update for Unicode 7.0.0

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Network Working Group                                         J. Klensin
Updates: 5892, 5894 (if approved)                           P. Faltstrom
Intended status: Standards Track                                  Netnod
Expires: July 10, 2015                                   January 6, 2015

                     IDNA Update for Unicode 7.0.0


   The current version of the IDNA specifications anticipated that each
   new version of Unicode would be reviewed to verify that no changes
   had been introduced that required adjustments to the set of rules
   and, in particular, whether new exceptions or backward compatibility
   adjustments were needed.  That review was conducted for Unicode 7.0.0
   and identified a potentially problematic new code point.  This
   specification discusses that code point and associated issues and
   updates RFC 5892 accordingly.  It also applies an editorial
   clarification that was the subject of an earlier erratum.  In
   addition, the discussion of the specific issue updates RFC 5894.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Description . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  IDNA assumptions about Unicode normalization  . . . . . .   5
     2.2.  New code point U+08A1, decomposition, and language
           dependency  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Other examples of the same behavior . . . . . . . . . . .   7
     2.4.  Hamza and Combining Sequences . . . . . . . . . . . . . .   8
   3.  Proposed/ Alternative Changes to RFC 5892 for new character
       U+08A1  . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Disallow This New Code Point  . . . . . . . . . . . . . .   9
     3.2.  Disallow the combining sequences for these characters . .  10
     3.3.  Do Nothing Other Than Warn  . . . . . . . . . . . . . . .  11
     3.4.  Normalization Form IETF (or DNS)  . . . . . . . . . . . .  11
   4.  Editorial clarification to RFC 5892 . . . . . . . . . . . . .  11
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  15
     A.1.  Changes from version -00 to -01 . . . . . . . . . . . . .  15
     A.2.  Changes from version -01 to -02 . . . . . . . . . . . . .  15
     A.3.  Changes from version -02 to -03 . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The current version of the IDNA specifications, known as "IDNA2008"
   [RFC5890], anticipated that each new version of Unicode would be
   reviewed to verify that no changes had been introduced that required
   adjustments to IDNA's rules and, in particular, whether new
   exceptions or backward compatibility adjustments were needed.  When
   that review was carefully conducted for Unicode 7.0.0 [Unicode7],
   comparing it to prior versions including the text in Unicode 6.2
   [Unicode62], it identified a problematic new code point (U+08A1,
   ARABIC LETTER BEH WITH HAMZA ABOVE).  The specific problem is
   discussed in detail in Section 2.  The behavior of that code point,
   while non-optimal for IDNA, follows that of a few code points that

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   predate Unicode 7.x and even the IDNA 2008 specifications and Unicode
   6.0.  Those existing code points make the question of what, if
   anything, to do about this new one exceedingly problematic because
   different reasonable criteria yield different decisions,

   o  To disallow it as an IDNA exception case creates inconsistencies
      with how those earlier code points were handled.

   o  To disallow it and the similar code points as well would
      necessitate invalidating some potential labels that would have
      been valid under IDNA2008 until this time.  However, there is
      reason to believe that no such labels exist.

   o  To permit the new code point to be treated as PVALID creates a
      situation in which it is possible, within the same script, to
      compose the same character symbol (glyph) in two different ways
      that do not compare equal even after normalization.  That
      condition would then apply to it and the earlier code points with
      the same behavior.  That situation contradicts a fundamental
      assumption of IDNA that is discussed in more detail below.


      This working draft discusses four alternatives, including, for
      illustration, a radical idea that seems too drastic to be
      considered now although it would have been appropriate to discuss
      when the IDNA2008 specifications were being developed.  The
      authors suggest that the community discuss the relevant tradeoffs
      and make a decision and that the document then be revised to
      reflect that decision, with the other alternatives discussed as
      options not chosen.  Because there is no ideal choice, the
      discussion of the issues in Section 2, is probably as or more
      important than the particular choice of how to handle this code
      point.  In addition to providing information for this document,
      that section should be considered as an updating addendum to RFC
      5894 [RFC5894] and should be incorporated into any future revision
      of that document.

      As the result of this version of the document containing several
      alternate proposals, some of the text is also a little bit
      redundant.  That will be corrected in future versions.

   As anticipated when IDNA2008, and RFC 5892 in particular, were
   written, exceptions and explicit updates are likely to be needed only
   if there is disagreement between the Unicode Consortium's view about
   what is best for the Standard and the IETF's view of what is best for
   IDNs, the DNS, and IDNA.  It was hoped that a situation would never

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   arise in which the the two perspectives would disagree, but the
   possibility was anticipated and considerable mechanism added to RFC
   5890 and 5982 as a result.  It is probably important to note that a
   disagreement in this context does not imply that anyone is "wrong",
   only that the two different groups have different needs and therefore
   criteria about what is acceptable.  For that reason, the IETF has, in
   the past, allowed some characters for IDNA that active Unicode
   Technical Committee members suggested be disallowed to avoid a change
   in derived tables [RFC6452].  This document describes a case where
   the IETF should disallow a character or characters that the various
   properties would otherwise treat as PVALID.

   This document provides the "flagging for the IESG" specified by
   Section 5.1 of RFC 5892.  As specified there, the change itself
   requires IETF review because it alters the rules of Section 2 of that

   Readers of this document are expected to be familiar with Unicode
   terminology [Unicode62] and the IETF conventions for representing
   Unicode code points [RFC5137].

   As a convenience to readers of RFC 5892 and to reduce the risks of
   confusion, this document also formally applies the content of an
   erratum to the text of the RFC (see Section 4) and so brings that RFC
   up to date with all agreed changes.

      [[RFC Editor: please remove the following comment and note if they
      get to you.]]

      [[IESG: It might not be a bad idea to incorporate some version of
      the following into the Last Call announcement.]]

      NOTE IN DRAFT to IETF Reviewers: The issues in this document, and
      particularly the choices among options for either adding exception
      cases to RFC 5892 or ignoring the issue, warning people, and
      hoping the results do not include serious problems, are fairly
      esoteric.  Understanding them requires that one have at least some
      understanding of how the Arabic Script works and the reasons the
      Unicode Standard gives various Arabic Script characters a fairly
      extended discussion [Unicode62-Arabic].  It also requires
      understanding of a number of Unicode principles, including the
      Normalization Stability rules [UAX15-Versioning] as applied to new
      precomposed characters and guidelines for adding new characters.
      There is considerable discussion of the issues in Section 2 and
      references are provided for those who want to pursue them, but
      potential reviewers should assume that the background needed to
      understand the reasons for this change is no less deep in the
      subject matter than would be expected of someone reviewing a

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      proposed change in, e.g., the fundamentals of BGP, TCP congestion
      control, or some cryptographic algorithm.  Put more bluntly, one's
      ability to read or speak languages other than English, or even one
      or more languages that use the Arabic script, does not make one an
      expert in these matters.

2.  Problem Description

2.1.  IDNA assumptions about Unicode normalization

   IDNA makes several assumptions about Unicode, Unicode "characters",
   and the effects of normalization.  Those assumptions were based on
   careful reading of the Unicode Standard at the time [Unicode5],
   guided by advice and commitments by members of the Unicode Technical
   Committee.  Those assumptions, and the associated requirements, are
   necessitated by three properties of DNS labels that do not apply to
   blocks of running text:

   1.  There is no language context for a label.  While particular DNS
       zones may impose restrictions, including language or script
       restrictions, on what labels can be registered, neither the DNS
       nor IDNA impose either type of restriction or give the user of a
       label any indication about the registration or other restrictions
       that may have been imposed.

   2.  Labels are often mnemonics rather than words in any language.
       They may be abbreviations or acronyms or contain embedded digits
       and have other characteristics that are not typical of words.

   3.  Labels are, in practice, usually short.  Even when they are the
       maximum length allowed by the DNS and IDNA, they are typically
       too short to provide significant context.  Statements that
       suggest that languages can almost always be determined from
       relatively short paragraphs or equivalent bodies of text do not
       apply to DNS labels because of their typical short length and
       because, as noted above, they are not required to be formed
       according to language-based rules.

   At the same time, because the DNS is an exact-match system, there
   must be no ambiguity about whether two labels are equal.  Although
   there have been extensive discussions about "confusingly similar"
   characters, labels, and strings, such tests between scripts are
   always somewhat subjective: they are affected by choices of type
   styles and by what the user expects to see.  In spite of the fact
   that the glyphs that represent many characters in different scripts
   are identical in appearance (e.g., basic Latin "a" (U+0061) and the
   identical-appearing Cyrillic character (U+0430), the most important

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   test is that, if two glyphs are the same within a given script, they
   must represent the same character no matter how they are formed.

   Unicode normalization, as explained in [UAX15], is expected to
   resolve those "same script, same glyph, different formation methods"
   issues.  Within the Latin script, the code point sequence for lower
   case "o" (U+006F) and combining diaeresis (U+0308) will, when
   normalized using the "NFC" method required by IDNA, produce the
   precombined small letter o with diaeresis (U+00F6) and hence the two
   ways of forming the character will compare equal (and the combining
   sequence is effectively prohibited from U-labels).

   NFC was preferred over other normalization methods for IDNA because
   it is more compact, more likely to be produced on keyboards on which
   the relevant characters actually appeared, and because it does not
   lose substantive information (e.g., some types of compatibility
   equivalence involves judgment calls as to whether two characters are
   actually the same -- they may be "the same" in some contexts but not
   others -- while canonical equivalence is about different ways to
   produce the glyph for the same abstract character).

   IDNA also assumed that the extensive Unicode stability rules would be
   applied and work as specified when new code points were added.  Those
   rules, as described in The Unicode Standard and the normative annexes
   identified below, provide that:

   1.  New code points representing precombined characters that can be
       formed from combining sequences will not be added to Unicode
       unless neither the relevant base character nor required combining
       character are part of the Standard within the relevant script

   2.  If circumstances require that principle be violated,
       normalization stability requires that the newly-added character
       decompose (even under NFC) to the previously-available combining
       sequence [UAX15-Exclusion].

   There is no explicit provision in the Standard's discussion of
   conditions for adding new code points, nor of normalization
   stability, for an exception based on different languages using the
   same script.

2.2.  New code point U+08A1, decomposition, and language dependency

   Unicode 7.0.0 introduces the new code point U+08A1, ARABIC LETTER BEH
   WITH HAMZA ABOVE.  As can be deduced from the name, it is visually
   identical to the glyph that can be formed from a combining sequence
   consisting of the code point for ARABIC LETTER BEH (U+0628) and the

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   code point for Combining Hamza Above (U+0654).  The two rules
   summarized above suggest that either the new code point should not be
   allocated at all or that it should have a decomposition to

   Had the issues outlined in this document been better understood at
   the time, it probably would have been wise for RFC 5892 to disallow
   either the precomposed character or the combining sequence of each
   pair in those cases in which Unicode normalization rules do not cause
   the right thing to happen, i.e., the combining sequence and
   precomposed character to be treated as equivalent.  Failure to do so
   at the time places an extra burden on registries to be sure that
   conflicts (and the potential for confusion and attacks) do not exist.
   Oddly, had the exclusion been made part of the specification at that
   time, the preference for precombined forms noted above would probably
   have dictated excluding the combining sequence, something not
   otherwise done in IDNA2008 because the NFC requirement serves the
   same purpose.  Today, the only thing that can be excluded without the
   potential disruption of disallowing a previously-PVALID combining
   sequence is the to exclude the newly-added code point so whatever is
   done, or might have been contemplated with hindsight, will be
   somewhat inconsistent.

2.3.  Other examples of the same behavior

   One of the things that complicates the issue with the new U+08A1 code
   point is that there are several other Arabic-script code points that
   behave in the same way for similar language-specific reasons.

   In particular, at least three other grapheme clusters that have been
   present for many version of Unicode can be seen as involving issues
   similar to those for the newly-added ARABIC LETTER BEH WITH HAMZA
   REH WITH HAMZA ABOVE (U+076C) do not have decomposition forms and are
   preferred over combining sequences using HAMZA ABOVE (U+0654)
   [Unicode62-Hamza].  By contrast, ARABIC LETTER ALEF WITH HAMZA ABOVE
   (U+0623) decomposes into \u'0627'\u'0654' and ARABIC LETTER YEH WITH
   HAMZA ABOVE (U+0626) decomposes into \u'064A'\u'0654' so the
   precomposed character and combining sequences compare equal when both
   are normalized, as this specification prefers.

   There are other variations in which a precomposed character involving
   HAMZA ABOVE has a decomposition to a combining sequence that can form
   it.  For example, ARABIC LETTER U WITH HAMZA ABOVE (U+0677) has a
   compatibility (???) decomposition into the combining sequence

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2.4.  Hamza and Combining Sequences

   As the Unicode Standard points out at some length [Unicode62-Arabic],
   Hamza is a problematic abstract character and the "Hamza Above"
   construction even more so [Unicode62-Hamza].  Those sections explain
   a distinction made by Unicode between the use of a Hamza mark to
   denote a glottal stop and one used as a diacritic mark to denote a
   separate letter.  In the first case, the combining sequence is used.
   In the second, a precombined character is assigned.

   Unlike Unicode generally and because of concerns about identifier
   spoofing and attacks based on similarities, character distinctions in
   IDNA are based much more strictly on the appearance of characters;
   language and pronunciation distinctions within a script are not
   considered.  So, for IDNA, BEH WITH HAMZA ABOVE is not-quite-
   tautologically the same as BEH WITH HAMZA ABOVE, even if one of them
   is written as U+08A1 (new to Unicode 7.0.0) and the other as the
   sequence \u'0628'\u'0654' (feasible with Unicode 7.0.0 but also
   available in versions of Unicode going back at least to the version
   [Unicode32] used in the original version of IDNA [RFC3490].  Because
   the precomposed form and combining sequence are, for IDNA purposes,
   the same, IDNA expects that normalization (specifically the
   requirement that all U-labels be in NFC form) will cause them to
   compare equal.

   If Unicode also considered them the same, then the principle would
   apply that new precomposed ("composition") forms are not added unless
   one of the code points that could be used to construct it did not
   exist in an earlier version (and even then is
   discouraged)[UAX15-Versioning].  When exceptions are made, they are
   expected to conform to the rules and classes in the "Composition
   Exclusion Table", with class 2 being relevant to this case
   [UAX15-Exclusion].  That rule essentially requires that the
   normalization for the old combining sequence to itself be retained
   (for stability) but that the newly-added character be treated as
   canonically decomposable and decompose back to the older sequence
   even under NFC.  That was not done for this particular case,
   presumably because of the distinction about pronunciation modifiers
   versus separate letters noted above.  Because, for IDNA and the DNS,
   there is a possibility that the composing sequence \u'0628'\u'0654'
   already appears in labels, the only choice other than allowing an
   otherwise-identical, and identically-appearing, label with U+08A1
   substituted to identify a different DNS entry is to DISALLOW the new

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3.  Proposed/ Alternative Changes to RFC 5892 for new character U+08A1

   NOTE IN DRAFT: See the comments in the Introduction, Section 1 and
   the first paragraph of each Subsection below for the status of the
   Subsections that follow.  Each one, in combination with the material
   in Section 2 above, also provides information about the reasons why
   that particular strategy is appropriate.

3.1.  Disallow This New Code Point

   If chosen by the community, this subsection would update the portion
   of the IDNA2008 specification that identifies rules for what
   characters are permitted [RFC5892] to disallow that code point.

   With the publication of this document, Section 2.6 ("Exceptions (F)")
   of RFC 5892 [RFC5892] is updated by adding 08A1 to the rule in
   Category F so that the rule itself reads:

      F: cp is in {00B7, 00DF, 0375, 03C2, 05F3, 05F4, 0640, 0660,
                   0661, 0662, 0663, 0664, 0665, 0666, 0667, 0668,
                   0669, 06F0, 06F1, 06F2, 06F3, 06F4, 06F5, 06F6,
                   06F7, 06F8, 06F9, 06FD, 06FE, 07FA, 08A1, 0F0B,
                   3007, 302E, 302F, 3031, 3032, 3033, 3034, 3035,
                   303B, 30FB}

   and then add to the subtable designated
   "DISALLOWED -- Would otherwise have been PVALID"
   after the line that begins "07FA", the additional line:


   This has the effect of making the cited code point DISALLOWED
   independent of application of the rest of the IDNA rule set to the
   current version of Unicode.  Those wishing to create domain name
   labels containing Beh with Hamza Above may continue to use the

      followed by


   which was valid for IDNA purposes in Unicode 5.0 and earlier and
   which continues to be valid.

   In principle, much the same thing could be accomplished by using the
   IDNA "BackwardCompatible" category (IDNA Category G, RFC 5892
   Section 5.3).  However, that category is described as applying only

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   when "property values in versions of Unicode after 5.2 have changed
   in such a way that the derived property value would no longer be
   PVALID or DISALLOWED".  Because U+08A1 is a newly-added code point in
   Unicode 7.0.0 and no property values of code points in prior versions
   have changed, category G does not apply.  If that section of RFC 5892
   were to be replaced in the future, perhaps consideration should be
   given to adding Normalization Stability and other issues to that
   description but, at present, it is not relevant.

3.2.  Disallow the combining sequences for these characters

   If chosen by the community, this subsection would update the portion
   of the IDNA2008 specification that identifies contextual rules
   [RFC5892] to prohibit (combining) Hamza Above (U+0654) in conjunction
   with Arabic BEH (U+0628), HAH (U+062D), and REH (U+0631).  Note that
   the choice of this option is consistent with the general preference
   for precomposed characters discussed above but would ban some labels
   that are valid today and that might, in principle, be in use.

   The required prohibition could be imposed by creating a new
   contextual rule in RFC 5892 to constrain combining sequences
   containing Hamza Above.

   As the Unicode Standard points out at some length [Unicode62-Arabic],
   Hamza is a problematic abstract character and the "Hamza Above"
   construction even more so.  IDNA has historically associated
   characters whose use is reasonable in some contexts but not others
   with the special derived property "CONTEXTO" and then specified
   specific, context-dependent, rules about where they may be used.
   Because Hamza Above is problematic (and spawns edge cases, as
   discussed in the Unicode Standard section cited above), it was
   suggested that a contextual rule might be appropriate.  There are at
   least two reasons why a contextual rule would not be suitable for the
   present situation.

   1.  As discussed above, the present situation is a normalization
       stability and predictability problem, not a contextual one.  Had
       the same issues arisen with a newly-added precomposed character
       that could previously be constructed from non-problematic base
       and combining characters, it would be even more clearly a
       normalization issue and, following the principles discussed there
       and particularly in UAX 15 [UAX15-Exclusion], might not have been
       assigned at all.

   2.  The contextual rule sets are designed around restricting the use
       of code points to a particular script or adjacent to particular
       characters within that script.  Neither of these cases applies to
       the newly-added character even if one could imagine rules for the

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       use of Hamza Above (U+0654) that would reflect the considerations
       of Chapter 8 of Unicode 6.2.  Even had the latter been desired,
       it would be somewhat late now -- Hamza Above has been present as
       a combining character (U+0654) in many versions of Unicode.
       While that section of the Unicode Standard describes the issues,
       it does not provide actionable guidance about what to do about it
       for cases going forward or when visual identity is important.

3.3.  Do Nothing Other Than Warn

   The recommendation from UTC is to simply warn registries, at all
   levels of the tree, to be careful with this set of characters, making
   language distinctions within zones.  Because the DNS cannot make or
   enforce language distinctions, this suggestion is problematic but it
   would avoid having the IETF either invalidating label strings that
   are potentially now in use or creating inconsistencies among the
   characters that combine with Hamza Above but that also have
   precomposed forms that do not have decompositions.  The potential
   would still exist for registries to respect the warning and deprecate
   such labels if they existed.

3.4.  Normalization Form IETF (or DNS)

   The most radical possibility would be to decide that none of the
   Unicode Normalization Forms specified in UAX 15 [UAX15] are adequate
   for use with the DNS because, contrary to their apparent
   descriptions, normalization tables are actually determined using
   language information.  However, use of language information is
   unacceptable for IDNA for reasons described elsewhere in this
   document.  The remedy would be to define an IETF-specific (or DNS-
   specific) normalization form, building on NFC but adhering strictly
   to the rule that normalization causes two different forms of the same
   character (glyph image) within the same script to be treated as
   equal.  In practice such a form would be implemented for IDNA
   purposes as an additional rule within RFC 5892 (and its successors)
   that constituted an exception list for the NFC tables.  For this set
   of characters, the special IETF normalization form would be
   equivalent to the exclusion discussed in Section 3.2 above.

4.  Editorial clarification to RFC 5892

   Verified RFC Editor Erratum 3312 [RFC5892Erratum] provides a
   clarification to Appendix A and Section A.1 of RFC 5892.  This
   section of this document updates the RFC to apply that clarification.

   1.  In Appendix A, add a new paragraph after the paragraph that
       begins "The code point...".  The new paragraph should read:

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       "For the rule to be evaluated to True for the label, it MUST be
       evaluated separately for every occurrence of the Code point in
       the label; each of those evaluations must result in True."

   2.  In Appendix A, Section A.1, replace the "Rule Set" by

      Rule Set:
        If Canonical_Combining_Class(Before(cp)) .eq.  Virama Then True;
        If cp .eq. \u200C And
          (Joining_Type:T)*(Joining_Type:{R,D})) Then True;

5.  Acknowledgements

   The Unicode 7.0.0 changes were extensively discussed within the IAB's
   Internationalization Program.  The authors are grateful for the
   discussions and feedback there, especially from Andrew Sullivan and
   David Thaler.  Additional information was requested and received from
   Mark Davis and Ken Whistler and while they probably do not agree with
   the necessity of excluding this code point or taking even more
   drastic action as their responsibility is to look at the Unicode
   Consortium requirements for stability, the decision would not have
   been possible without their input.  Thanks to Bill McQuillan and Ted
   Hardie for reading versions of the document carefully enough to
   identify and report some confusing typographical errors.  Several
   experts and reviewers who prefer to remain anonymous also provided
   helpful input and comments on preliminary versions of this document.

6.  IANA Considerations

   When the IANA registry and tables are updated to reflect Unicode
   7.0.0, changes should be made according to the decisions the IETF
   makes about Section 3.

7.  Security Considerations

   [[CREF1: NOTE IN DRAFT: This section is unchanged in version -01 of
   this document relative to what appeared in -00.  It will need to be
   rewritten once decisions are made about what path to follow.  In
   particular, if "just warn" is chosen, it will need to contain very
   strong warnings.]]

   This specification excludes a code point for which the Unicode-
   specified normalization behavior could result in two ways to form a
   visually-identical character within the same script not comparing
   equal.  That behavior could create a dream case for someone intending
   to confuse the user by use of a domain name that looked identical to

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   another one, was entirely in the same script, but was still
   considered different (see, for example, the discussion of false
   negatives in identifier comparison in Section 2.1 of RFC 6943
   [RFC6943]).  This exclusion therefore should improve Internet

8.  References

8.1.  Normative References

   [RFC5137]  Klensin, J., "ASCII Escaping of Unicode Characters", BCP
              137, RFC 5137, February 2008.

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

   [RFC5892]  Faltstrom, P., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, August 2010.

              "RFC5892, "The Unicode Code Points and Internationalized
              Domain Names for Applications (IDNA)", August 2010, Errata
              ID: 3312", Errata ID 3312, August 2012,

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, August 2010.

   [RFC6943]  Thaler, D., "Issues in Identifier Comparison for Security
              Purposes", RFC 6943, May 2013.

   [UAX15]    Davis, M., Ed., "Unicode Standard Annex #15: Unicode
              Normalization Forms", June 2014,

              "Unicode Standard Annex #15: ob. cit., Section 5",

              "Unicode Standard Annex #15, ob. cit., Section 3",

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              The Unicode Consortium, "The Unicode Standard, Version
              5.0", ISBN 0-321-48091-0, 2007.

              Boston, MA, USA: Addison-Wesley.  ISBN 0-321-48091-0.
              This printed reference has now been updated online to
              reflect additional code points.  For code points, the
              reference at the time RFC 5890-5894 were published is to
              Unicode 5.2.

              The Unicode Consortium, "The Unicode Standard, Version
              6.2.0", ISBN 978-1-936213-07-8, 2012,

              Preferred citation: The Unicode Consortium.  The Unicode
              Standard, Version 6.2.0, (Mountain View, CA: The Unicode
              Consortium, 2012.  ISBN 978-1-936213-07-8)

              "The Unicode Standard, Version 6.2.0, ob.cit., Chapter 8",
              Chapter 8, 2012,

              Subsection titled "Encoding Principles", paragraph
              numbered 4, starting on page 251.

              "The Unicode Standard, Version 6.2.0, ob.cit., Chapter 8",
              Chapter 8, 2012,

              Subsection titled "Combining Hamza Above" starting on page

              The Unicode Consortium, "The Unicode Standard, Version
              7.0.0", ISBN 978-1-936213-09-2, 2014,

              Preferred Citation: The Unicode Consortium.  The Unicode
              Standard, Version 7.0.0, (Mountain View, CA: The Unicode
              Consortium, 2014.  ISBN 978-1-936213-09-2)

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8.2.  Informative References

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC6452]  Faltstrom, P. and P. Hoffman, "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA) -
              Unicode 6.0", RFC 6452, November 2011.

              The Unicode Consortium, "The Unicode Standard, Version
              3.2.0", .

              The Unicode Standard, Version 3.2.0 is defined by The
              Unicode Standard, Version 3.0 (Reading, MA, Addison-
              Wesley, 2000.  ISBN 0-201-61633-5), as amended by the
              Unicode Standard Annex #27: Unicode 3.1
              ( and by the Unicode
              Standard Annex #28: Unicode 3.2

Appendix A.  Change Log

   RFC Editor: Please remove this appendix before publication.

A.1.  Changes from version -00 to -01

   o  Version 01 of this document is an extensive rewrite and
      reorganization, reflecting discussions with UTC members and adding
      three more options for discussion to the original proposal to
      simply disallow the new code point.

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

   Corrected a typographical error in which Hamza Above was incorrectly
   listed with the wrong code point.

A.3.  Changes from version -02 to -03

   Corrected a typographical error in the Abstract in which RFC 5892 was
   incorrectly shown as 5982.

Authors' Addresses

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   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140

   Phone: +1 617 245 1457

   Patrik Faltstrom
   Franzengatan 5
   Stockholm  112 51

   Phone: +46 70 6059051

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