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RFC 5894

 
 
 

Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale

Part 2 of 2, p. 22 to 43
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5.  IDNs and the Robustness Principle

   The "Robustness Principle" is often stated as "Be conservative about
   what you send and liberal in what you accept" (see, e.g., Section
   1.2.2 of the applications-layer Host Requirements specification
   [RFC1123]).  This principle applies to IDNA.  In applying the
   principle to registries as the source ("sender") of all registered
   and useful IDNs, registries are responsible for being conservative
   about what they register and put out in the Internet.  For IDNs to
   work well, zone administrators (registries) must have and require
   sensible policies about what is registered -- conservative policies
   -- and implement and enforce them.

   Conversely, lookup applications are expected to reject labels that
   clearly violate global (protocol) rules (no one has ever seriously
   claimed that being liberal in what is accepted requires being
   stupid).  However, once one gets past such global rules and deals
   with anything sensitive to script or locale, it is necessary to
   assume that garbage has not been placed into the DNS, i.e., one must
   be liberal about what one is willing to look up in the DNS rather
   than guessing about whether it should have been permitted to be
   registered.

   If a string cannot be successfully found in the DNS after the lookup
   processing described here, it makes no difference whether it simply
   wasn't registered or was prohibited by some rule at the registry.
   Application implementers should be aware that where DNS wildcards are
   used, the ability to successfully resolve a name does not guarantee
   that it was actually registered.

6.  Front-end and User Interface Processing for Lookup

   Domain names may be identified and processed in many contexts.  They
   may be typed in by users themselves or embedded in an identifier such
   as an email address, URI, or IRI.  They may occur in running text or
   be processed by one system after being provided in another.  Systems
   may try to normalize URLs to determine (or guess) whether a reference
   is valid or if two references point to the same object without
   actually looking the objects up (comparison without lookup is
   necessary for URI types that are not intended to be resolved).  Some
   of these goals may be more easily and reliably satisfied than others.

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   While there are strong arguments for any domain name that is placed
   "on the wire" -- transmitted between systems -- to be in the zero-
   ambiguity forms of A-labels, it is inevitable that programs that
   process domain names will encounter U-labels or variant forms.

   An application that implements the IDNA protocol [RFC5891] will
   always take any user input and convert it to a set of Unicode code
   points.  That user input may be acquired by any of several different
   input methods, all with differing conversion processes to be taken
   into consideration (e.g., typed on a keyboard, written by hand onto
   some sort of digitizer, spoken into a microphone and interpreted by a
   speech-to-text engine, etc.).  The process of taking any particular
   user input and mapping it into a Unicode code point may be a simple
   one: if a user strikes the "A" key on a US English keyboard, without
   any modifiers such as the "Shift" key held down, in order to draw a
   Latin small letter A ("a"), many (perhaps most) modern operating
   system input methods will produce to the calling application the code
   point U+0061, encoded in a single octet.

   Sometimes the process is somewhat more complicated: a user might
   strike a particular set of keys to represent a combining macron
   followed by striking the "A" key in order to draw a Latin small
   letter A with a macron above it.  Depending on the operating system,
   the input method chosen by the user, and even the parameters with
   which the application communicates with the input method, the result
   might be the code point U+0101 (encoded as two octets in UTF-8 or
   UTF-16, four octets in UTF-32, etc.), the code point U+0061 followed
   by the code point U+0304 (again, encoded in three or more octets,
   depending upon the encoding used) or even the code point U+FF41
   followed by the code point U+0304 (and encoded in some form).  These
   examples leave aside the issue of operating systems and input methods
   that do not use Unicode code points for their character set.

   In every case, applications (with the help of the operating systems
   on which they run and the input methods used) need to perform a
   mapping from user input into Unicode code points.

   IDNA2003 used a model whereby input was taken from the user, mapped
   (via whatever input method mechanisms were used) to a set of Unicode
   code points, and then further mapped to a set of Unicode code points
   using the Nameprep profile [RFC3491].  In this procedure, there are
   two separate mapping steps: first, a mapping done by the input method
   (which might be controlled by the operating system, the application,
   or some combination) and then a second mapping performed by the
   Nameprep portion of the IDNA protocol.  The mapping done in Nameprep
   includes a particular mapping table to re-map some characters to
   other characters, a particular normalization, and a set of prohibited
   characters.

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   Note that the result of the two-step mapping process means that the
   mapping chosen by the operating system or application in the first
   step might differ significantly from the mapping supplied by the
   Nameprep profile in the second step.  This has advantages and
   disadvantages.  Of course, the second mapping regularizes what gets
   looked up in the DNS, making for better interoperability between
   implementations that use the Nameprep mapping.  However, the
   application or operating system may choose mappings in their input
   methods, which when passed through the second (Nameprep) mapping
   result in characters that are "surprising" to the end user.

   The other important feature of IDNA2003 is that, with very few
   exceptions, it assumes that any set of Unicode code points provided
   to the Nameprep mapping can be mapped into a string of Unicode code
   points that are "sensible", even if that means mapping some code
   points to nothing (that is, removing the code points from the
   string).  This allowed maximum flexibility in input strings.

   The present version of IDNA (IDNA2008) differs significantly in
   approach from the original version.  First and foremost, it does not
   provide explicit mapping instructions.  Instead, it assumes that the
   application (perhaps via an operating system input method) will do
   whatever mapping it requires to convert input into Unicode code
   points.  This has the advantage of giving flexibility to the
   application to choose a mapping that is suitable for its user given
   specific user requirements, and avoids the two-step mapping of the
   original protocol.  Instead of a mapping, IDNA2008 provides a set of
   categories that can be used to specify the valid code points allowed
   in a domain name.

   In principle, an application ought to take user input of a domain
   name and convert it to the set of Unicode code points that represent
   the domain name the user intends.  As a practical matter, of course,
   determining user intent is a tricky business, so an application needs
   to choose a reasonable mapping from user input.  That may differ
   based on the particular circumstances of a user, depending on locale,
   language, type of input method, etc.  It is up to the application to
   make a reasonable choice.

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7.  Migration from IDNA2003 and Unicode Version Synchronization

7.1.  Design Criteria

   As mentioned above and in the IAB review and recommendations for IDNs
   [RFC4690], two key goals of the IDNA2008 design are:

   o  to enable applications to be agnostic about whether they are being
      run in environments supporting any Unicode version from 3.2
      onward.

   o  to permit incrementally adding new characters, character groups,
      scripts, and other character collections as they are incorporated
      into Unicode, doing so without disruption and, in the long term,
      without "heavy" processes (an IETF consensus process is required
      by the IDNA2008 specifications and is expected to be required and
      used until significant experience accumulates with IDNA operations
      and new versions of Unicode).

7.1.1.  Summary and Discussion of IDNA Validity Criteria

   The general criteria for a label to be considered valid under IDNA
   are (the actual rules are rigorously defined in the Protocol
   [RFC5891] and Tables [RFC5892] documents):

   o  The characters are "letters", marks needed to form letters,
      numerals, or other code points used to write words in some
      language.  Symbols, drawing characters, and various notational
      characters are intended to be permanently excluded.  There is no
      evidence that they are important enough to Internet operations or
      internationalization to justify expansion of domain names beyond
      the general principle of "letters, digits, and hyphen".
      (Additional discussion and rationale for the symbol decision
      appears in Section 7.6.)

   o  Other than in very exceptional cases, e.g., where they are needed
      to write substantially any word of a given language, punctuation
      characters are excluded.  The fact that a word exists is not proof
      that it should be usable in a DNS label, and DNS labels are not
      expected to be usable for multiple-word phrases (although they are
      certainly not prohibited if the conventions and orthography of a
      particular language cause that to be possible).

   o  Characters that are unassigned (have no character assignment at
      all) in the version of Unicode being used by the registry or
      application are not permitted, even on lookup.  The issues
      involved in this decision are discussed in Section 7.7.

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   o  Any character that is mapped to another character by a current
      version of NFKC is prohibited as input to IDNA (for either
      registration or lookup).  With a few exceptions, this principle
      excludes any character mapped to another by Nameprep [RFC3491].

   The principles above drive the design of rules that are specified
   exactly in the Tables document.  Those rules identify the characters
   that are valid under IDNA.  The rules themselves are normative, and
   the tables are derived from them, rather than vice versa.

7.1.2.  Labels in Registration

   Any label registered in a DNS zone must be validated -- i.e., the
   criteria for that label must be met -- in order for applications to
   work as intended.  This principle is not new.  For example, since the
   DNS was first deployed, zone administrators have been expected to
   verify that names meet "hostname" requirements [RFC0952] where those
   requirements are imposed by the expected applications.  Other
   applications contexts, such as the later addition of special service
   location formats [RFC2782] imposed new requirements on zone
   administrators.  For zones that will contain IDNs, support for
   Unicode version-independence requires restrictions on all strings
   placed in the zone.  In particular, for such zones (the exact rules
   appear in Section 4 of the Protocol document [RFC5891]):

   o  Any label that appears to be an A-label, i.e., any label that
      starts in "xn--", must be valid under IDNA, i.e., they must be
      valid A-labels, as discussed in Section 2 above.

   o  The Unicode tables (i.e., tables of code points, character
      classes, and properties) and IDNA tables (i.e., tables of
      contextual rules such as those that appear in the Tables
      document), must be consistent on the systems performing or
      validating labels to be registered.  Note that this does not
      require that tables reflect the latest version of Unicode, only
      that all tables used on a given system are consistent with each
      other.

   Under this model, registry tables will need to be updated (both the
   Unicode-associated tables and the tables of permitted IDN characters)
   to enable a new script or other set of new characters.  The registry
   will not be affected by newer versions of Unicode, or newly
   authorized characters, until and unless it wishes to support them.
   The zone administrator is responsible for verifying validity for IDNA
   as well as its local policies -- a more extensive set of checks than
   are required for looking up the labels.  Systems looking up or

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   resolving DNS labels, especially IDN DNS labels, must be able to
   assume that applicable registration rules were followed for names
   entered into the DNS.

7.1.3.  Labels in Lookup

   Any application processing a label through IDNA so it can be looked
   up in a DNS zone is required to (the exact rules appear in Section 5
   of the Protocol document [RFC5891]):

   o  Maintain IDNA and Unicode tables that are consistent with regard
      to versions, i.e., unless the application actually executes the
      classification rules in the Tables document [RFC5892], its IDNA
      tables must be derived from the version of Unicode that is
      supported more generally on the system.  As with registration, the
      tables need not reflect the latest version of Unicode, but they
      must be consistent.

   o  Validate the characters in labels to be looked up only to the
      extent of determining that the U-label does not contain
      "DISALLOWED" code points or code points that are unassigned in its
      version of Unicode.

   o  Validate the label itself for conformance with a small number of
      whole-label rules.  In particular, it must verify that:

      *  there are no leading combining marks,

      *  the Bidi conditions are met if right-to-left characters appear,

      *  any required contextual rules are available, and

      *  any contextual rules that are associated with joiner characters
         (and CONTEXTJ characters more generally) are tested.

   o  Do not reject labels based on other contextual rules about
      characters, including mixed-script label prohibitions.  Such rules
      may be used to influence presentation decisions in the user
      interface, but not to avoid looking up domain names.

   To further clarify the rules about handling characters that require
   contextual rules, note that one can have a context-required character
   (i.e., one that requires a rule), but no rule.  In that case, the
   character is treated the same way DISALLOWED characters are treated,
   until and unless a rule is supplied.  That state is more or less
   equivalent to "the idea of permitting this character is accepted in
   principle, but it won't be permitted in practice until consensus is
   reached on a safe way to use it".

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   The ability to add a rule more or less exempts these characters from
   the prohibition against reclassifying characters from DISALLOWED to
   PVALID.

   And, obviously, "no rule" is different from "have a rule, but the
   test either succeeds or fails".

   Lookup applications that follow these rules, rather than having their
   own criteria for rejecting lookup attempts, are not sensitive to
   version incompatibilities with the particular zone registry
   associated with the domain name except for labels containing
   characters recently added to Unicode.

   An application or client that processes names according to this
   protocol and then resolves them in the DNS will be able to locate any
   name that is registered, as long as those registrations are valid
   under IDNA and its version of the IDNA tables is sufficiently up to
   date to interpret all of the characters in the label.  Messages to
   users should distinguish between "label contains an unallocated code
   point" and other types of lookup failures.  A failure on the basis of
   an old version of Unicode may lead the user to a desire to upgrade to
   a newer version, but will have no other ill effects (this is
   consistent with behavior in the transition to the DNS when some hosts
   could not yet handle some forms of names or record types).

7.2.  Changes in Character Interpretations

   As a consequence of the elimination of mapping, the current version
   of IDNA changes the interpretation of a few characters relative to
   its predecessors.  This subsection outlines the issues and discusses
   possible transition strategies.

7.2.1.  Character Changes: Eszett and Final Sigma

   In those scripts that make case distinctions, there are a few
   characters for which an obvious and unique uppercase character has
   not historically been available to match a lowercase one, or vice
   versa.  For those characters, the mappings used in constructing the
   Stringprep tables for IDNA2003, performed using the Unicode
   toCaseFold operation (see Section 5.18 of the Unicode Standard
   [Unicode52]), generate different characters or sets of characters.
   Those operations are not reversible and lose even more information
   than traditional uppercase or lowercase transformations, but are more
   useful than those transformations for comparison purposes.  Two
   notable characters of this type are the German character Eszett
   (Sharp S, U+00DF) and the Greek Final Form Sigma (U+03C2).  The
   former is case folded to the ASCII string "ss", the latter to a
   medial (lowercase) Sigma (U+03C3).

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7.2.2.  Character Changes: Zero Width Joiner and Zero Width Non-Joiner

   IDNA2003 mapped both ZERO WIDTH JOINER (ZWJ, U+200D) and ZERO WIDTH
   NON-JOINER (ZWNJ, U+200C) to nothing, effectively dropping these
   characters from any label in which they appeared and treating strings
   containing them as identical to strings that did not.  As discussed
   in Section 3.1.2 above, those characters are essential for writing
   many reasonable mnemonics for certain scripts.  However, treating
   them as valid in IDNA2008, even with contextual restrictions, raises
   approximately the same problem as exists with Eszett and Final Sigma:
   strings that were valid under IDNA2003 have different interpretations
   as labels, and different A-labels, than the same strings under this
   newer version.

7.2.3.  Character Changes and the Need for Transition

   The decision to eliminate mandatory and standardized mappings,
   including case folding, from the IDNA2008 protocol in order to make
   A-labels and U-labels idempotent made these characters problematic.
   If they were to be disallowed, important words and mnemonics could
   not be written in orthographically reasonable ways.  If they were to
   be permitted as distinct characters, there would be no information
   loss and registries would have more flexibility, but IDNA2003 and
   IDNA2008 lookups might result in different A-labels.

   With the understanding that there would be incompatibility either way
   but a judgment that the incompatibility was not significant enough to
   justify a prefix change, the Working Group concluded that Eszett and
   Final Form Sigma should be treated as distinct and Protocol-Valid
   characters.

   Since these characters are interpreted in different ways under the
   older and newer versions of IDNA, transition strategies and policies
   will be necessary.  Some actions can reasonably be taken by
   applications' client programs (those that perform lookup operations
   or cause them to be performed), but because of the diversity of
   situations and uses of the DNS, much of the responsibility will need
   to fall on registries.

   Registries, especially those maintaining zones for third parties,
   must decide how to introduce a new service in a way that does not
   create confusion or significantly weaken or invalidate existing
   identifiers.  This is not a new problem; registries were faced with
   similar issues when IDNs were introduced (potentially, and especially
   for Latin-based scripts, in conflict with existing labels that had
   been rendered in ASCII characters by applying more or less
   standardized conventions) and when other new forms of strings have
   been permitted as labels.

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7.2.4.  Transition Strategies

   There are several approaches to the introduction of new characters or
   changes in interpretation of existing characters from their mapped
   forms in the earlier version of IDNA.  The transition issue is
   complicated because the forms of these labels after the
   ToUnicode(ToASCII()) translation in IDNA2003 not only remain valid
   but do not provide strong indications of what the registrant
   intended: a string containing "ss" could have simply been intended to
   be that string or could have been intended to contain an Eszett; a
   string containing lowercase Sigma could have been intended to contain
   Final Sigma (one might make heuristic guesses based on position in a
   string, but the long tradition of forming labels by concatenating
   words makes such heuristics unreliable), and strings that do not
   contain ZWJ or ZWNJ might have been intended to contain them.
   Without any preference or claim to completeness, some of these, all
   of which have been used by registries in the past for similar
   transitions, are:

   1.  Do not permit use of the newly available character at the
       registry level.  This might cause lookup failures if a domain
       name were to be written with the expectation of the IDNA2003
       mapping behavior, but would eliminate any possibility of false
       matches.

   2.  Hold a "sunrise"-like arrangement in which holders of labels
       containing "ss" in the Eszett case, lowercase Sigma in that case,
       or that might have contained ZWJ or ZWNJ in context, are given
       priority (and perhaps other benefits) for registering the
       corresponding string containing Eszett, Final Sigma, or the
       appropriate zero-width character respectively.

   3.  Adopt some sort of "variant" approach in which registrants obtain
       labels with both character forms.

   4.  Adopt a different form of "variant" approach in which
       registration of additional strings that would produce the same
       A-label if interpreted according to IDNA2003 is either not
       permitted at all or permitted only by the registrant who already
       has one of the names.

   5.  Ignore the issue and assume that the marketplace or other
       mechanisms will sort things out.

   In any event, a registry (at any level of the DNS tree) that chooses
   to permit labels to be registered that contains these characters, or
   considers doing so, will have to address the relationship with
   existing, possibly conflicting, labels in some way, just as

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   registries that already had a considerable number of labels did when
   IDNs were first introduced.

7.3.  Elimination of Character Mapping

   As discussed at length in Section 6, IDNA2003, via Nameprep (see
   Section 7.5), mapped many characters into related ones.  Those
   mappings no longer exist as requirements in IDNA2008.  These
   specifications strongly prefer that only A-labels or U-labels be used
   in protocol contexts and as much as practical more generally.
   IDNA2008 does anticipate situations in which some mapping at the time
   of user input into lookup applications is appropriate and desirable.
   The issues are discussed in Section 6 and specific recommendations
   are made in the Mapping document [IDNA2008-Mapping].

7.4.  The Question of Prefix Changes

   The conditions that would have required a change in the IDNA ACE
   prefix ("xn--", used in IDNA2003) were of great concern to the
   community.  A prefix change would have clearly been necessary if the
   algorithms were modified in a manner that would have created serious
   ambiguities during subsequent transition in registrations.  This
   section summarizes the working group's conclusions about the
   conditions under which a change in the prefix would have been
   necessary and the implications of such a change.

7.4.1.  Conditions Requiring a Prefix Change

   An IDN prefix change would have been needed if a given string would
   be looked up or otherwise interpreted differently depending on the
   version of the protocol or tables being used.  This IDNA upgrade
   would have required a prefix change if, and only if, one of the
   following four conditions were met:

   1.  The conversion of an A-label to Unicode (i.e., a U-label) would
       have yielded one string under IDNA2003 and a different string
       under IDNA2008.

   2.  In a significant number of cases, an input string that was valid
       under IDNA2003 and also valid under IDNA2008 would have yielded
       two different A-labels with the different versions.  This
       condition is believed to be essentially equivalent to the one
       above except for a very small number of edge cases that were not
       found to justify a prefix change (see Section 7.2).

       Note that if the input string was valid under one version and not
       valid under the other, this condition would not apply.  See the
       first item in Section 7.4.2, below.

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   3.  A fundamental change was made to the semantics of the string that
       would be inserted in the DNS, e.g., if a decision were made to
       try to include language or script information in the encoding in
       addition to the string itself.

   4.  A sufficiently large number of characters were added to Unicode
       so that the Punycode mechanism for block offsets would no longer
       reference the higher-numbered planes and blocks.  This condition
       is unlikely even in the long term and certain not to arise in the
       next several years.

7.4.2.  Conditions Not Requiring a Prefix Change

   As a result of the principles described above, none of the following
   changes required a new prefix:

   1.  Prohibition of some characters as input to IDNA.  Such a
       prohibition might make names that were previously registered
       inaccessible, but did not change those names.

   2.  Adjustments in IDNA tables or actions, including normalization
       definitions, that affected characters that were already invalid
       under IDNA2003.

   3.  Changes in the style of the IDNA definition that did not alter
       the actions performed by IDNA.

7.4.3.  Implications of Prefix Changes

   While it might have been possible to make a prefix change, the costs
   of such a change are considerable.  Registries could not have
   converted all IDNA2003 ("xn--") registrations to a new form at the
   same time and synchronize that change with applications supporting
   lookup.  Unless all existing registrations were simply to be declared
   invalid (and perhaps even then), systems that needed to support both
   labels with old prefixes and labels with new ones would be required
   to first process a putative label under the IDNA2008 rules and try to
   look it up and then, if it were not found, would be required to
   process the label under IDNA2003 rules and look it up again.  That
   process would probably have significantly slowed down all processing
   that involved IDNs in the DNS, especially since a fully-qualified
   name might contain a mixture of labels that were registered with the
   old and new prefixes.  That would have made DNS caching very
   difficult.  In addition, looking up the same input string as two
   separate A-labels would have created some potential for confusion and
   attacks, since the labels could map to different targets and then
   resolve to different entries in the DNS.

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   Consequently, a prefix change should have been, and was, avoided if
   at all possible, even if it means accepting some IDNA2003 decisions
   about character distinctions as irreversible and/or giving special
   treatment to edge cases.

7.5.  Stringprep Changes and Compatibility

   The Nameprep specification [RFC3491], a key part of IDNA2003, is a
   profile of Stringprep [RFC3454].  While Nameprep is a Stringprep
   profile specific to IDNA, Stringprep is used by a number of other
   protocols.  Were Stringprep to have been modified by IDNA2008, those
   changes to improve the handling of IDNs could cause problems for
   non-DNS uses, most notably if they affected identification and
   authentication protocols.  Several elements of IDNA2008 give
   interpretations to strings prohibited under IDNA2003 or prohibit
   strings that IDNA2003 permitted.  Those elements include the new
   inclusion information in the Tables document [RFC5892], the reduction
   in the number of characters permitted as input for registration or
   lookup (Section 3), and even the changes in handling of right-to-left
   strings as described in the Bidi document [RFC5893].  IDNA2008 does
   not use Nameprep or Stringprep at all, so there are no side-effect
   changes to other protocols.

   It is particularly important to keep IDNA processing separate from
   processing for various security protocols because some of the
   constraints that are necessary for smooth and comprehensible use of
   IDNs may be unwanted or undesirable in other contexts.  For example,
   the criteria for good passwords or passphrases are very different
   from those for desirable IDNs: passwords should be hard to guess,
   while domain names should normally be easily memorable.  Similarly,
   internationalized Small Computer System Interface (SCSI) identifiers
   and other protocol components are likely to have different
   requirements than IDNs.

7.6.  The Symbol Question

   One of the major differences between this specification and the
   original version of IDNA is that IDNA2003 permitted non-letter
   symbols of various sorts, including punctuation and line-drawing
   symbols, in the protocol.  They were always discouraged in practice.
   In particular, both the "IESG Statement" about IDNA and all versions
   of the ICANN Guidelines specify that only language characters be used
   in labels.  This specification disallows symbols entirely.  There are
   several reasons for this, which include:

   1.  As discussed elsewhere, the original IDNA specification assumed
       that as many Unicode characters as possible should be permitted,
       directly or via mapping to other characters, in IDNs.  This

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       specification operates on an inclusion model, extrapolating from
       the original "hostname" rules (LDH, see the Definitions document
       [RFC5890]) -- which have served the Internet very well -- to a
       Unicode base rather than an ASCII base.

   2.  Symbol names are more problematic than letters because there may
       be no general agreement on whether a particular glyph matches a
       symbol; there are no uniform conventions for naming; variations
       such as outline, solid, and shaded forms may or may not exist;
       and so on.  As just one example, consider a "heart" symbol as it
       might appear in a logo that might be read as "I love...".  While
       the user might read such a logo as "I love..." or "I heart...",
       considerable knowledge of the coding distinctions made in Unicode
       is needed to know that there is more than one "heart" character
       (e.g., U+2665, U+2661, and U+2765) and how to describe it.  These
       issues are of particular importance if strings are expected to be
       understood or transcribed by the listener after being read out
       loud.

   3.  Design of a screen reader used by blind Internet users who must
       listen to renderings of IDN domain names and possibly reproduce
       them on the keyboard becomes considerably more complicated when
       the names of characters are not obvious and intuitive to anyone
       familiar with the language in question.

   4.  As a simplified example of this, assume one wanted to use a
       "heart" or "star" symbol in a label.  This is problematic because
       those names are ambiguous in the Unicode system of naming (the
       actual Unicode names require far more qualification).  A user or
       would-be registrant has no way to know -- absent careful study of
       the code tables -- whether it is ambiguous (e.g., where there are
       multiple "heart" characters) or not.  Conversely, the user seeing
       the hypothetical label doesn't know whether to read it -- try to
       transmit it to a colleague by voice -- as "heart", as "love", as
       "black heart", or as any of the other examples below.

   5.  The actual situation is even worse than this.  There is no
       possible way for a normal, casual, user to tell the difference
       between the hearts of U+2665 and U+2765 and the stars of U+2606
       and U+2729 without somehow knowing to look for a distinction.  We
       have a white heart (U+2661) and few black hearts.  Consequently,
       describing a label as containing a heart is hopelessly ambiguous:
       we can only know that it contains one of several characters that
       look like hearts or have "heart" in their names.  In cities where
       "Square" is a popular part of a location name, one might well
       want to use a square symbol in a label as well and there are far
       more squares of various flavors in Unicode than there are hearts
       or stars.

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   The consequence of these ambiguities is that symbols are a very poor
   basis for reliable communication.  Consistent with this conclusion,
   the Unicode standard recommends that strings used in identifiers not
   contain symbols or punctuation [Unicode-UAX31].  Of course, these
   difficulties with symbols do not arise with actual pictographic
   languages and scripts which would be treated like any other language
   characters; the two should not be confused.

7.7.  Migration between Unicode Versions: Unassigned Code Points

   In IDNA2003, labels containing unassigned code points are looked up
   on the assumption that, if they appear in labels and can be mapped
   and then resolved, the relevant standards must have changed and the
   registry has properly allocated only assigned values.

   In the IDNA2008 protocol, strings containing unassigned code points
   must not be either looked up or registered.  In summary, the status
   of an unassigned character with regard to the DISALLOWED,
   PROTOCOL-VALID, and CONTEXTUAL RULE REQUIRED categories cannot be
   evaluated until a character is actually assigned and known.  There
   are several reasons for this, with the most important ones being:

   o  Tests involving the context of characters (e.g., some characters
      being permitted only adjacent to others of specific types) and
      integrity tests on complete labels are needed.  Unassigned code
      points cannot be permitted because one cannot determine whether
      particular code points will require contextual rules (and what
      those rules should be) before characters are assigned to them and
      the properties of those characters fully understood.

   o  It cannot be known in advance, and with sufficient reliability,
      whether a newly assigned code point will be associated with a
      character that would be disallowed by the rules in the Tables
      document [RFC5892] (such as a compatibility character).  In
      IDNA2003, since there is no direct dependency on NFKC (many of the
      entries in Stringprep's tables are based on NFKC, but IDNA2003
      depends only on Stringprep), allocation of a compatibility
      character might produce some odd situations, but it would not be a
      problem.  In IDNA2008, where compatibility characters are
      DISALLOWED unless character-specific exceptions are made,
      permitting strings containing unassigned characters to be looked
      up would violate the principle that characters in DISALLOWED are
      not looked up.

   o  The Unicode Standard specifies that an unassigned code point
      normalizes (and, where relevant, case folds) to itself.  If the
      code point is later assigned to a character, and particularly if
      the newly assigned code point has a combining class that

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      determines its placement relative to other combining characters,
      it could normalize to some other code point or sequence.

   It is possible to argue that the issues above are not important and
   that, as a consequence, it is better to retain the principle of
   looking up labels even if they contain unassigned characters because
   all of the important scripts and characters have been coded as of
   Unicode 5.2 (or even earlier), and hence unassigned code points will
   be assigned only to obscure characters or archaic scripts.
   Unfortunately, that does not appear to be a safe assumption for at
   least two reasons.  First, much the same claim of completeness has
   been made for earlier versions of Unicode.  The reality is that a
   script that is obscure to much of the world may still be very
   important to those who use it.  Cultural and linguistic preservation
   principles make it inappropriate to declare the script of no
   importance in IDNs.  Second, we already have counterexamples, e.g.,
   in the relationships associated with new Han characters being added
   (whether in the BMP or in Unicode Plane 2).

   Independent of the technical transition issues identified above, it
   can be observed that any addition of characters to an existing script
   to make it easier to use or to better accommodate particular
   languages may lead to transition issues.  Such additions may change
   the preferred form for writing a particular string, changes that may
   be reflected, e.g., in keyboard transition modules that would
   necessarily be different from those for earlier versions of Unicode
   where the newer characters may not exist.  This creates an inherent
   transition problem because attempts to access labels may use either
   the old or the new conventions, requiring registry action whether or
   not the older conventions were used in labels.  The need to consider
   transition mechanisms is inherent to evolution of Unicode to better
   accommodate writing systems and is independent of how IDNs are
   represented in the DNS or how transitions among versions of those
   mechanisms occur.  The requirement for transitions of this type is
   illustrated by the addition of Malayalam Chillu in Unicode 5.1.0.

7.8.  Other Compatibility Issues

   The 2003 IDNA model includes several odd artifacts of the context in
   which it was developed.  Many, if not all, of these are potential
   avenues for exploits, especially if the registration process permits
   "source" names (names that have not been processed through IDNA and
   Nameprep) to be registered.  As one example, since the character
   Eszett, used in German, is mapped by IDNA2003 into the sequence "ss"
   rather than being retained as itself or prohibited, a string
   containing that character, but that is otherwise in ASCII, is not
   really an IDN (in the U-label sense defined above).  After Nameprep
   maps out the Eszett, the result is an ASCII string and so it does not

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   get an xn-- prefix, but the string that can be displayed to a user
   appears to be an IDN.  IDNA2008 eliminates this artifact.  A
   character is either permitted as itself or it is prohibited; special
   cases that make sense only in a particular linguistic or cultural
   context can be dealt with as localization matters where appropriate.

8.  Name Server Considerations

8.1.  Processing Non-ASCII Strings

   Existing DNS servers do not know the IDNA rules for handling
   non-ASCII forms of IDNs, and therefore need to be shielded from them.
   All existing channels through which names can enter a DNS server
   database (for example, master files (as described in RFC 1034) and
   DNS update messages [RFC2136]) could not be IDNA-aware because they
   predate IDNA.  Other sections of this document provide the needed
   shielding by ensuring that internationalized domain names entering
   DNS server databases through such channels have already been
   converted to their equivalent ASCII A-label forms.

   Because of the distinction made between the algorithms for
   Registration and Lookup in Sections 4 and 5 (respectively) of the
   Protocol document [RFC5891] (a domain name containing only ASCII code
   points cannot be converted to an A-label), there cannot be more than
   one A-label form for any given U-label.

   As specified in clarifications to the DNS specification [RFC2181],
   the DNS protocol explicitly allows domain labels to contain octets
   beyond the ASCII range (0000..007F), and this document does not
   change that.  However, although the interpretation of octets
   0080..00FF is well-defined in the DNS, many application protocols
   support only ASCII labels and there is no defined interpretation of
   these non-ASCII octets as characters and, in particular, no
   interpretation of case-independent matching for them (e.g., see the
   clarification on DNS case insensitivity [RFC4343]).  If labels
   containing these octets are returned to applications, unpredictable
   behavior could result.  The A-label form, which cannot contain those
   characters, is the only standard representation for internationalized
   labels in the DNS protocol.

8.2.  Root and Other DNS Server Considerations

   IDNs in A-label form will generally be somewhat longer than current
   domain names, so the bandwidth needed by the root servers is likely
   to go up by a small amount.  Also, queries and responses for IDNs
   will probably be somewhat longer than typical queries historically,

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   so Extension Mechanisms for DNS (EDNS0) [RFC2671] support may be more
   important (otherwise, queries and responses may be forced to go to
   TCP instead of UDP).

9.  Internationalization Considerations

   DNS labels and fully-qualified domain names provide mnemonics that
   assist in identifying and referring to resources on the Internet.
   IDNs expand the range of those mnemonics to include those based on
   languages and character sets other than Western European and Roman-
   derived ones.  But domain "names" are not, in general, words in any
   language.  The recommendations of the IETF policy on character sets
   and languages (BCP 18 [RFC2277]) are applicable to situations in
   which language identification is used to provide language-specific
   contexts.  The DNS is, by contrast, global and international and
   ultimately has nothing to do with languages.  Adding languages (or
   similar context) to IDNs generally, or to DNS matching in particular,
   would imply context-dependent matching in DNS, which would be a very
   significant change to the DNS protocol itself.  It would also imply
   that users would need to identify the language associated with a
   particular label in order to look that label up.  That knowledge is
   generally not available because many labels are not words in any
   language and some may be words in more than one.

10.  IANA Considerations

   This section gives an overview of IANA registries required for IDNA.
   The actual definitions of, and specifications for, the first two,
   which have been newly created for IDNA2008, appear in the Tables
   document [RFC5892].  This document describes the registries, but it
   does not specify any IANA actions.

10.1.  IDNA Character Registry

   The distinction among the major categories "UNASSIGNED",
   "DISALLOWED", "PROTOCOL-VALID", and "CONTEXTUAL RULE REQUIRED" is
   made by special categories and rules that are integral elements of
   the Tables document.  While not normative, an IANA registry of
   characters and scripts and their categories, updated for each new
   version of Unicode and the characters it contains, are convenient for
   programming and validation purposes.  The details of this registry
   are specified in the Tables document.

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10.2.  IDNA Context Registry

   IANA has created and now maintains a list of approved contextual
   rules for characters that are defined in the IDNA Character Registry
   list as requiring a Contextual Rule (i.e., the types of rules
   described in Section 3.1.2).  The details for those rules appear in
   the Tables document.

10.3.  IANA Repository of IDN Practices of TLDs

   This registry, historically described as the "IANA Language Character
   Set Registry" or "IANA Script Registry" (both somewhat misleading
   terms), is maintained by IANA at the request of ICANN.  It is used to
   provide a central documentation repository of the IDN policies used
   by top level domain (TLD) registries who volunteer to contribute to
   it and is used in conjunction with ICANN Guidelines for IDN use.

   It is not an IETF-managed registry and, while the protocol changes
   specified here may call for some revisions to the tables, IDNA2008
   has no direct effect on that registry and no IANA action is required
   as a result.

11.  Security Considerations

11.1.  General Security Issues with IDNA

   This document is purely explanatory and informational and
   consequently introduces no new security issues.  It would, of course,
   be a poor idea for someone to try to implement from it; such an
   attempt would almost certainly lead to interoperability problems and
   might lead to security ones.  A discussion of security issues with
   IDNA, including some relevant history, appears in the Definitions
   document [RFC5890].

12.  Acknowledgments

   The editor and contributors would like to express their thanks to
   those who contributed significant early (pre-working group) review
   comments, sometimes accompanied by text, Paul Hoffman, Simon
   Josefsson, and Sam Weiler.  In addition, some specific ideas were
   incorporated from suggestions, text, or comments about sections that
   were unclear supplied by Vint Cerf, Frank Ellerman, Michael Everson,
   Asmus Freytag, Erik van der Poel, Michel Suignard, and Ken Whistler.
   Thanks are also due to Vint Cerf, Lisa Dusseault, Debbie Garside, and
   Jefsey Morfin for conversations that led to considerable improvements
   in the content of this document and to several others, including Ben

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   Campbell, Martin Duerst, Subramanian Moonesamy, Peter Saint-Andre,
   and Dan Winship, for catching specific errors and recommending
   corrections.

   A meeting was held on 30 January 2008 to attempt to reconcile
   differences in perspective and terminology about this set of
   specifications between the design team and members of the Unicode
   Technical Consortium.  The discussions at and subsequent to that
   meeting were very helpful in focusing the issues and in refining the
   specifications.  The active participants at that meeting were (in
   alphabetic order, as usual) Harald Alvestrand, Vint Cerf, Tina Dam,
   Mark Davis, Lisa Dusseault, Patrik Faltstrom (by telephone), Cary
   Karp, John Klensin, Warren Kumari, Lisa Moore, Erik van der Poel,
   Michel Suignard, and Ken Whistler.  We express our thanks to Google
   for support of that meeting and to the participants for their
   contributions.

   Useful comments and text on the working group versions of the working
   draft were received from many participants in the IETF "IDNABIS"
   working group and a number of document changes resulted from mailing
   list discussions made by that group.  Marcos Sanz provided specific
   analysis and suggestions that were exceptionally helpful in refining
   the text, as did Vint Cerf, Martin Duerst, Andrew Sullivan, and Ken
   Whistler.  Lisa Dusseault provided extensive editorial suggestions
   during the spring of 2009, most of which were incorporated.

13.  Contributors

   While the listed editor held the pen, the core of this document and
   the initial working group version represents the joint work and
   conclusions of an ad hoc design team consisting of the editor and, in
   alphabetic order, Harald Alvestrand, Tina Dam, Patrik Faltstrom, and
   Cary Karp.  Considerable material describing mapping principles has
   been incorporated from a draft of the Mapping document
   [IDNA2008-Mapping] by Pete Resnick and Paul Hoffman.  In addition,
   there were many specific contributions and helpful comments from
   those listed in the Acknowledgments section and others who have
   contributed to the development and use of the IDNA protocols.

14.  References

14.1.  Normative References

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

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   [RFC3492]    Costello, A., "Punycode: A Bootstring encoding of
                Unicode for Internationalized Domain Names in
                Applications (IDNA)", RFC 3492, March 2003.

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

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

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

   [RFC5893]    Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
                Internationalized Domain Names for Applications (IDNA)",
                RFC 5893, August 2010.

   [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/>.

14.2.  Informative References

   [IDNA2008-Mapping]
                Resnick, P. and P. Hoffman, "Mapping Characters in
                Internationalized Domain Names for Applications (IDNA)",
                Work in Progress, April 2010.

   [RFC0952]    Harrenstien, K., Stahl, M., and E. Feinler, "DoD
                Internet host table specification", RFC 952,
                October 1985.

   [RFC1034]    Mockapetris, P., "Domain names - concepts and
                facilities", STD 13, RFC 1034, November 1987.

   [RFC1035]    Mockapetris, P., "Domain names - implementation and
                specification", STD 13, RFC 1035, November 1987.

   [RFC1123]    Braden, R., "Requirements for Internet Hosts -
                Application and Support", STD 3, RFC 1123, October 1989.

   [RFC2136]    Vixie, P., Thomson, S., Rekhter, Y., and J.  Bound,
                "Dynamic Updates in the Domain Name System (DNS
                UPDATE)", RFC 2136, April 1997.

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   [RFC2181]    Elz, R. and R. Bush, "Clarifications to the DNS
                Specification", RFC 2181, July 1997.

   [RFC2277]    Alvestrand, H., "IETF Policy on Character Sets and
                Languages", BCP 18, RFC 2277, January 1998.

   [RFC2671]    Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
                RFC 2671, August 1999.

   [RFC2782]    Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
                specifying the location of services (DNS SRV)",
                RFC 2782, February 2000.

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

   [RFC3491]    Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                Profile for Internationalized Domain Names (IDN)",
                RFC 3491, March 2003.

   [RFC3743]    Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
                Engineering Team (JET) Guidelines for Internationalized
                Domain Names (IDN) Registration and Administration for
                Chinese, Japanese, and Korean", RFC 3743, April 2004.

   [RFC3987]    Duerst, M. and M. Suignard, "Internationalized Resource
                Identifiers (IRIs)", RFC 3987, January 2005.

   [RFC4290]    Klensin, J., "Suggested Practices for Registration of
                Internationalized Domain Names (IDN)", RFC 4290,
                December 2005.

   [RFC4343]    Eastlake, D., "Domain Name System (DNS) Case
                Insensitivity Clarification", RFC 4343, January 2006.

   [RFC4690]    Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
                and Recommendations for Internationalized Domain Names
                (IDNs)", RFC 4690, September 2006.

   [RFC4713]    Lee, X., Mao, W., Chen, E., Hsu, N., and J.  Klensin,
                "Registration and Administration Recommendations for
                Chinese Domain Names", RFC 4713, October 2006.

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   [Unicode-UAX31]
                The Unicode Consortium, "Unicode Standard Annex #31:
                Unicode Identifier and Pattern Syntax, Revision 11",
                September 2009,
                <http://www.unicode.org/reports/tr31/tr31-11.html>.

   [Unicode-UTS39]
                The Unicode Consortium, "Unicode Technical Standard #39:
                Unicode Security Mechanisms, Revision 2", August 2006,
                <http://www.unicode.org/reports/tr39/tr39-2.html>.

Author's Address

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

   Phone: +1 617 245 1457
   EMail: john+ietf@jck.com