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Versions: (draft-klensin-idnabis-protocol) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 5891

Network Working Group                                         J. Klensin
Internet-Draft                                             July 27, 2008
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: January 28, 2009


    Internationalized Domain Names in Applications (IDNA): Protocol
                  draft-ietf-idnabis-protocol-03.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on January 28, 2009.

Abstract

   This document supplies the protocol definition for a revised and
   updated specification for internationalized domain names (IDNs).  The
   rationale for these changes, the relationship to the older
   specification, and important terminology are provided in other
   documents.  This document specifies the protocol mechanism, called
   Internationalizing Domain Names in Applications (IDNA), for
   registering and looking up IDNs in a way that does not require
   changes to the DNS itself.  IDNA is only meant for processing domain
   names, not free text.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Requirements and Applicability . . . . . . . . . . . . . . . .  5
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  DNS Resource Records . . . . . . . . . . . . . . . . .  6
       3.2.2.  Non-domain-name Data Types Stored in the DNS . . . . .  6
   4.  Registration Protocol  . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Proposed label . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Conversion to Unicode and Normalization  . . . . . . . . .  7
     4.3.  Permitted Character and Label Validation . . . . . . . . .  7
       4.3.1.  Rejection of Characters that are not Permitted . . . .  7
       4.3.2.  Label Validation . . . . . . . . . . . . . . . . . . .  7
       4.3.3.  Registration Validation Summary  . . . . . . . . . . .  8
     4.4.  Registry Restrictions  . . . . . . . . . . . . . . . . . .  9
     4.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . .  9
     4.6.  Insertion in the Zone  . . . . . . . . . . . . . . . . . .  9
   5.  Domain Name Resolution (Lookup) Protocol . . . . . . . . . . .  9
     5.1.  Label String Input . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Conversion to Unicode  . . . . . . . . . . . . . . . . . . 10
     5.3.  Character Changes in Preprocessing or the User
           Interface  . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  A-label Input  . . . . . . . . . . . . . . . . . . . . . . 11
     5.5.  Validation and Character List Testing  . . . . . . . . . . 11
     5.6.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 13
     5.7.  DNS Name Resolution  . . . . . . . . . . . . . . . . . . . 13
   6.  Name Server Considerations . . . . . . . . . . . . . . . . . . 13
     6.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 13
     6.2.  DNSSEC Authentication of IDN Domain Names  . . . . . . . . 13
     6.3.  Root and other DNS Server Considerations . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Changes between Version -00 and -01 of
           draft-ietf-idnabis-protocol  . . . . . . . . . . . . . . . 15
     9.2.  Version -02  . . . . . . . . . . . . . . . . . . . . . . . 15
     9.3.  Version -03  . . . . . . . . . . . . . . . . . . . . . . . 16
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 16
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     12.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  The Contextual Rules Registry . . . . . . . . . . . . 19
   Appendix B.  Contextual Rules Registry - Alternate Syntax  . . . . 22
     B.1.  HYPHEN-MINUS . . . . . . . . . . . . . . . . . . . . . . . 23



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     B.2.  ZERO WIDTH NON-JOINER  . . . . . . . . . . . . . . . . . . 23
     B.3.  ZERO WIDTH JOINER  . . . . . . . . . . . . . . . . . . . . 24
     B.4.  MIDDLE DOT . . . . . . . . . . . . . . . . . . . . . . . . 24
     B.5.  GREEK LOWER NUMERAL SIGN (KERAIA)  . . . . . . . . . . . . 25
     B.6.  MODIFIER LETTER PRIME  . . . . . . . . . . . . . . . . . . 25
     B.7.  COMBINING CYRILLIC TITLO . . . . . . . . . . . . . . . . . 26
     B.8.  HEBREW PUNCTUATION GERESH  . . . . . . . . . . . . . . . . 26
     B.9.  HEBREW PUNCTUATION GERSHAYIM . . . . . . . . . . . . . . . 26
     B.10. IDEOGRAPHIC ITERATION MARK;  . . . . . . . . . . . . . . . 27
     B.11. VERTICAL IDEOGRAPHIC ITERATION MARK  . . . . . . . . . . . 27
     B.12. KATAKANA MIDDLE DOT  . . . . . . . . . . . . . . . . . . . 27
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 28
   Intellectual Property and Copyright Statements . . . . . . . . . . 29






































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1.  Introduction

   This document supplies the protocol definition for a revised and
   updated specification for internationalized domain names.  The
   rationale for these changes and relationship to the older
   specification and some new terminology is provided in other
   documents, notably [IDNA2008-Rationale].

   IDNA works by allowing applications to use certain ASCII string
   labels (beginning with a special prefix) to represent non-ASCII name
   labels.  Lower-layer protocols need not be aware of this; therefore
   IDNA does not depend on changes to any infrastructure.  In
   particular, IDNA does not depend on any changes to DNS servers,
   resolvers, or protocol elements, because the ASCII name service
   provided by the existing DNS is entirely sufficient for IDNA.

   IDNA is applied only to DNS labels.  Standards for combining labels
   into fully-qualified domain names and parsing labels out of those
   names are covered in the base DNS standards [RFC1035].  An
   application may, of course, apply locally-appropriate conventions to
   the presentation forms of domain names as discussed in
   [IDNA2008-Rationale].

   While they share terminology, reference data, and some operations,
   this document describes two separate protocols, one for IDN
   registration (Section 4) and one for IDN lookup (Section 5).

   A good deal of the background material that appeared in RFC 3490 has
   been removed from this update.  That material is either of historical
   interest only or has been covered from a more recent perspective in
   RFC 4690 [RFC4690] and [IDNA2008-Rationale].

   [[anchor2: Note in Draft: This document still needs more specifics
   about how to perform some of the tests in the Registration and Lookup
   protocols described below.  Those details will be supplied in a later
   revision, but the intent should be clear from the existing text.]]

1.1.  Discussion Forum

   [[anchor4: RFC Editor: please remove this section.]]

   This work is being discussed in the IETF IDNABIS WG and on the
   mailing list idna-update@alvestrand.no


2.  Terminology

   General terminology applicable to IDNA, but with meanings familiar to



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   those who have worked with Unicode or other character set standards
   and the DNS, appears in [IDNA2008-Rationale].  Terminology that is an
   integral, normative, part of the IDNA definition, including the
   definitions of "ACE", appears in that document as well.  Familiarity
   with the terminology materials in that document is assumed for
   reading this one.  The reader of this document is assumed to be
   familiar with DNS-specific terminology as defined in RFC 1034
   [RFC1034].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in BCP 14, RFC 2119
   [RFC2119].


3.  Requirements and Applicability

3.1.  Requirements

   IDNA conformance means adherence to the following requirements:

   1.  Whenever a domain name is put into an IDN-unaware domain name
       slot (see Section 2 and [IDNA2008-Rationale]), it MUST contain
       only ASCII characters (i.e., must be either an A-label or an LDH-
       label), or must be a label associated with a DNS application that
       is not subject to either IDNA or the historical recommendations
       for "hostname"-style names [RFC1034].

   2.  Comparison of labels MUST be done on the A-label form, using an
       ASCII case-insensitive comparison as with all comparisons of DNS
       labels.

   3.  Labels being registered MUST conform to the requirements of
       Section 4.  Labels being looked up and the lookup process MUST
       conform to the requirements of Section 5.

3.2.  Applicability

   IDNA is applicable to all domain names in all domain name slots
   except where it is explicitly excluded.  It is not applicable to
   domain name slots which do not use the LDH syntax rules.

   This implies that IDNA is applicable to many protocols that predate
   IDNA.  Note that IDNs occupying domain name slots in those older
   protocols MUST be in A-label form until and unless those protocols
   and implementations of them are upgraded.





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3.2.1.  DNS Resource Records

   IDNA applies only to domain names in the NAME and RDATA fields of DNS
   resource records whose CLASS is IN.

   There are currently no other exclusions on the applicability of IDNA
   to DNS resource records.  Applicability depends entirely on the
   CLASS, and not on the TYPE except as noted below.  This will remain
   true, even as new types are defined, unless there is a compelling
   reason for a new type that requires type-specific rules.  The special
   naming conventions applicable to SRV records are examples of type-
   specific rules that are incompatible with IDNA coding.  Hence the
   first two labels (the ones required to start in "_") on a record with
   TYPE SRV MUST NOT be A-labels or U-labels (while it would be possible
   to write a non-ASCII string with a leading underscore, conversion to
   an A-label would be impossible without loss of information because
   the underscore is not a letter, digit, or hyphen).  Of course, those
   labels may be part of a domain that uses IDN labels at higher levels
   in the tree.

3.2.2.  Non-domain-name Data Types Stored in the DNS

   Although IDNA enables the representation of non-ASCII characters in
   domain names, that does not imply that IDNA enables the
   representation of non-ASCII characters in other data types that are
   stored in domain names, specifically in the RDATA field for types
   that have structured RDATA format.  For example, an email address
   local part is stored in a domain name in the RNAME field as part of
   the RDATA of an SOA record (hostmaster@example.com would be
   represented as hostmaster.example.com).  IDNA specifically does not
   update the existing email standards, which allow only ASCII
   characters in local parts.  Even though work is in progress to define
   internationalization for email addresses [RFC4952], changes to the
   email address part of the SOA RDATA would require action in other
   standards, specifically those that specify the format of the SOA RR.


4.  Registration Protocol

   This section defines the procedure for registering an IDN.  The
   procedure is implementation independent; any sequence of steps that
   produces exactly the same result for all labels is considered a valid
   implementation.

4.1.  Proposed label

   The registrant submits a request for an IDN.  The user typically
   produces the request string by the keyboard entry of a character



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   sequence in the local native character set (which might, of course,
   be Unicode).  The registry MAY permit submission of labels in A-label
   form.  If it does so, it SHOULD perform a conversion to a U-label,
   perform the steps and tests described below, and verify that the
   A-label produced by the step in Section 4.5 matches the one provided
   as input.  If, for some reason, it does not, the registration MUST be
   rejected.
   [[anchor9: Editorial: Should the sentences starting with "The
   registry" be moved to 4.3?  I.e., would they be more in sequence
   there?]]

4.2.  Conversion to Unicode and Normalization

   Some system routine, or a localized front-end to the IDNA process,
   ensures that the proposed label is a Unicode string or converts it to
   one as appropriate.  That string MUST be in Unicode Normalization
   Form C (NFC [Unicode-UAX15]).

   As a local implementation choice, the implementation MAY choose to
   map some forbidden characters to permitted characters (for instance
   mapping uppercase characters to lowercase ones), displaying the
   result to the user, and allowing processing to continue.  However, it
   is strongly recommended that, to avoid any possible ambiguity,
   entities responsible for zone files ("registries") accept
   registrations only for A-labels (to be converted to U-labels by the
   registry) or U-labels actually produced from A-labels, not forms
   expected to be converted by some other process.

4.3.  Permitted Character and Label Validation

4.3.1.  Rejection of Characters that are not Permitted

   The Unicode string is checked to verify that no characters that IDNA
   does not permit in input appear in it.  Those characters are
   identified in the "DISALLOWED" and "UNASSIGNED" lists that are
   discussed in [IDNA2008-Rationale].  The normative rules for producing
   that list and the initial version of it are specified in
   [IDNA2008-Tables].  Characters that are either DISALLOWED or
   UNASSIGNED MUST NOT be part of labels being processed for
   registration in the DNS.

4.3.2.  Label Validation

   The proposed label (in the form of a Unicode string, i.e., a putative
   U-label) is then examined, performing tests that require examination
   of more than one character.





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4.3.2.1.  Rejection of Confusing or Hostile Sequences in U-labels

   The Unicode string MUST NOT contain "--" (two consecutive hyphens) in
   the third and fourth character positions.

4.3.2.2.  Leading Combining Marks

   The first character of the string is examined to verify that it is
   not a combining mark.  If it is a combining mark, the string MUST NOT
   be registered.

4.3.2.3.  Contextual Rules

   Each code point is checked for its identification as characters
   requiring contextual processing for registration (the list of
   characters appears as the combination of CONTEXTJ and CONTEXTO in
   [IDNA2008-Tables]).  If that indication appears, the table of
   contextual rules is checked for a rule for that character.  If no
   rule is found, the proposed label is rejected and MUST NOT be
   installed in a zone file.  If one is found, it is applied (typically
   as a test on the entire label or on adjacent characters).  If the
   application of the rule does not conclude that the character is valid
   in context, the proposed label MUST BE rejected.  (See the IANA
   Considerations: IDNA Context Registry section of [IDNA2008-Rationale]
   and Appendix A of this document.)

4.3.2.4.  Labels Containing Characters Written Right to Left

   Additional special tests for right-to-left strings are applied (See
   [IDNA2008-BIDI].  Strings that contain right to left characters that
   do not conform to the rule(s) identified there MUST NOT be inserted
   in zone files.
   [[anchor15: If the bidi specification continues to specify checking
   more than one label, this subsection will need to be revised and/or
   moved to a separate "FQDN validation" section.]]

4.3.3.  Registration Validation Summary

   Strings that have been produced by the steps above, and whose
   contents pass the above tests, are U-labels.

   To summarize, tests are made here for invalid characters, invalid
   combinations of characters, and for labels that are invalid even if
   the characters they contain are valid individually.  For example,
   labels containing invisible ("zero-width") characters may be
   permitted in context with characters whose presentation forms are
   significantly changed by the presence or absence of the zero-width
   characters, while other labels in which zero-width characters appear



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   may be rejected.
   [[anchor17: Should the example text be removed or moved?  Note that
   I've been strongly encouraged to supply specific examples to reduce
   abstraction and questions about the appropriateness of the text.
   -JcK]]

4.4.  Registry Restrictions

   Registries at all levels of the DNS, not just the top level, are
   expected to establish policies about the labels that may be
   registered, and for the processes associated with that action.  While
   exact policies are not specified as part of IDNA2008 and it is
   expected that different registries may specify different policies,
   there SHOULD be policies.  These per-registry policies and
   restrictions are an essential element of the IDNA registration
   protocol even for registries (and corresponding zone files) deep in
   the DNS hierarchy.  As discussed in [IDNA2008-Rationale], such
   restrictions have always existed in the DNS.

   The string produced by the above steps is checked and processed as
   appropriate to local registry restrictions.  Application of those
   registry restrictions may result in the rejection of some labels or
   the application of special restrictions to others.

4.5.  Punycode Conversion

   The resulting U-label is converted to an A-label (i.e., the encoding
   of that label according to the Punycode algorithm [RFC3492] with the
   ACE prefix added, i.e., the "xn--..." form).
   [[anchor18: Explain why 3492 failures cannot occur or explain what to
   do if they do.]]

4.6.  Insertion in the Zone

   The A-label is registered in the DNS by insertion into a zone.


5.  Domain Name Resolution (Lookup) Protocol

   Resolution is conceptually different from registration and different
   tests are applied on the client.  Although some validity checks are
   necessary to avoid serious problems with the protocol (see
   Section 5.5 ff.), the resolution-side tests are more permissive and
   rely heavily on the assumption that names that are present in the DNS
   are valid.






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5.1.  Label String Input

   The user supplies a string in the local character set, typically by
   typing it or clicking on, or copying and pasting, a resource
   identifier, e.g., a URI [RFC3986] or IRI [RFC3987] from which the
   domain name is extracted.  Or some process not directly involving the
   user may read the string from a file or obtain it in some other way.
   Processing in this step and the next two are local matters, to be
   accomplished prior to actual invocation of IDNA, but at least these
   two steps must be accomplished in some way.

5.2.  Conversion to Unicode

   The string is converted from the local character set into Unicode, if
   it is not already Unicode.  The exact nature of this conversion is
   beyond the scope of this document, but may involve normalization, as
   described in Section 4.2.

5.3.  Character Changes in Preprocessing or the User Interface

   The Unicode string MAY then be processed, in a way specific to the
   local environment, to make the result of the IDNA processing match
   user expectations.  For instance, it would be reasonable, at this
   step, to convert all upper case characters to lower case, if this
   makes sense in the user's environment.

   Other examples of processing for localization might be applied, if
   appropriate, at this point.  They include interpreting various
   characters as separating domain name components from each other
   (label separators) because they either look like periods or are used
   to separate sentences, mapping different "width" forms of the same
   character into the one form permitted in labels[[anchor20: This needs
   clarification]], or giving special treatment to characters whose
   presentation forms are dependent only on placement in the label.
   Such localization changes are also outside the scope of this
   specification.

   Recommendations for preprocessing for global contexts (i.e., when
   local considerations do not apply or cannot be used) and for maximum
   interoperability with labels that might have been specified under
   liberal readings of IDNA2003 are given in [IDNA2008-Rationale].

   [[anchor21: The question of preprocessing remains controversial in
   the WG.  One school of thought is that, for compatibility with
   IDNA2003, preprocessing should be standardized and required, with
   only one form permitted.  Another sees important advantages in having
   the mappings between U-labels and A-labels be symmetric, unambiguous,
   and information-preserving.  And a third believes that local mappings



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   will occur regardless of what we specify and that it is better to
   specify the protocol on that basis than to indirectly encourage local
   inventions.  The first group (and perhaps others) believe that local
   mappings will be, to put it mildly, "very bad... for
   interoperability.]]

   Because these transformations are local, it is important that domain
   names that might be passed between systems (e.g., in IRIs) be
   U-labels or A-labels and not forms that might be accepted locally as
   a consequence of this step.  This step is not standardized as part of
   IDNA, and is not further specified here.

5.4.  A-label Input

   If the input to this procedure appears to be an A-label (i.e., it
   starts in "xn--"), the lookup application MAY attempt to convert it
   to a U-label and apply the tests of Section 5.5 and, of course, the
   conversion of Section 5.6 to that form.  If the A-label is converted
   to a U-label then the processing specified in those two sections MUST
   yield an A-label identical to the original one.  See also
   Section 6.1.

   In general, that conversion and testing should be performed if the
   domain name will later be presented to the user in native character
   form (this requires that the lookup application be IDNA-aware).
   Applications that are not IDNA-aware will obviously omit that
   testing; others may treat the string as opaque to avoid the
   additional processing at the expense of providing less protection and
   information to users.

5.5.  Validation and Character List Testing

   As with the registration procedure, the Unicode string is checked to
   verify that all characters that appear in it are valid for IDNA
   resolution input.  As discussed above and in [IDNA2008-Rationale],
   the resolution check is more liberal than the registration one.
   Putative labels with any of the following characteristics MUST BE
   rejected prior to DNS lookup:

   o  Labels containing code points that are unassigned in the version
      of Unicode being used by the application, i.e., in the
      "Unassigned" Unicode category or the UNASSIGNED category of
      [IDNA2008-Tables].

   o  Labels that are not in NFC form.

   o  Labels containing prohibited code points, i.e., those that are
      assigned to the "DISALLOWED" category in the permitted character



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      table [IDNA2008-Tables].

   o  Labels containing code points that are shown in the permitted
      character table as requiring a contextual rule and that are
      flagged as requiring exceptional special processing on lookup
      ("CONTEXTJ" in the Tables) MUST conform to the rule, which MUST be
      present.

   o  Labels containing other code points that are shown in the
      permitted character table as requiring a contextual rule
      ("CONTEXTO" in the tables), but for which no such rule appears in
      the table of rules.  With the exception in the rule immediately
      above, applications resolving DNS names or carrying out equivalent
      operations are not required to test contextual rules, only to
      verify that a rule exists.

   o  Labels whose first character is a combining mark. [[anchor23: Note
      in Draft: this definition may need to be further tightened.]]

   In addition, the application SHOULD apply the following test.  The
   test may be omitted in special circumstances, such as when the
   resolver application knows that the conditions are enforced
   elsewhere, because an attempt to resolve such strings will almost
   certainly lead to a DNS lookup failure.  However, applying the test
   is likely to give much better information about the reason for a
   lookup failure -- information that may be usefully passed to the user
   when that is feasible -- then DNS resolution failure alone.  In any
   event, resolvers should avoid looking up labels that are invalid
   under that test.
   [[anchor24: Should this be a MUST?  Pro: this is the only remaining
   SHOULD (true?), the test is relatively straightforward, and it helps
   avoid visual ambiguity.  Con: the "special circumstances" that might
   justify doing something different are explained above.]]

   o  Verification that the string is compliant with the requirements
      for right to left characters, specified in [IDNA2008-BIDI].

   For all other strings, the resolver MUST rely on the presence or
   absence of labels in the DNS to determine the validity of those
   labels and the validity of the characters they contain.  If they are
   registered, they are presumed to be valid; if they are not, their
   possible validity is not relevant.  A resolver that declines to look
   up a string that conforms to the above rules is not in conformance
   with this protocol.







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5.6.  Punycode Conversion

   The validated string, a U-label, is converted to an A-label using the
   Punycode algorithm with the ACE prefix added.

5.7.  DNS Name Resolution

   The A-label is looked up in the DNS, using normal DNS procedures.


6.  Name Server Considerations

6.1.  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]) are IDN-unaware 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 design of the algorithms in Section 4 and Section 5 (a
   domain name containing only ASCII codepoints can not be converted to
   an A-label), there can not be more than one A-label form for any
   given U-label.

   The current update to the definition of the DNS protocol [RFC2181]
   explicitly allows domain labels to contain octets beyond the ASCII
   range (0000..007F), and this document does not change that.  Note,
   however, that there is no defined interpretation of octets 0080..00FF
   as characters.  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 current DNS
   protocol.

6.2.  DNSSEC Authentication of IDN Domain Names

   DNS Security [RFC2535] is a method for supplying cryptographic
   verification information along with DNS messages.  Public Key
   Cryptography is used in conjunction with digital signatures to
   provide a means for a requester of domain information to authenticate
   the source of the data.  This ensures that it can be traced back to a
   trusted source, either directly or via a chain of trust linking the
   source of the information to the top of the DNS hierarchy.



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   IDNA specifies that all internationalized domain names served by DNS
   servers that cannot be represented directly in ASCII must use the
   A-label form.  Conversion to A-labels must be performed prior to a
   zone being signed by the private key for that zone.  Because of this
   ordering, it is important to recognize that DNSSEC authenticates a
   domain name containing A-labels or conventional LDH-labels, not
   U-labels.  In the presence of DNSSEC, no form of a zone file or query
   response that contains a U-label may be signed or the signature
   validated.

   One consequence of this for sites deploying IDNA in the presence of
   DNSSEC is that any special purpose proxies or forwarders used to
   transform user input into IDNs must be earlier in the resolution flow
   than DNSSEC authenticating nameservers for DNSSEC to work.

6.3.  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,
   so EDNS0 [RFC2671] support may be more important (otherwise, queries
   and responses may be forced to go to TCP instead of UDP).


7.  Security Considerations

   The general security principles and issues for IDNA appear in
   [IDNA2008-Rationale].  The comments below are specific to this pair
   of protocols, but should be read in the context of that material and
   the definitions and specifications, identified there, on which this
   one depends.

   This memo describes procedures for registering and looking up labels
   that are not compatible with the preferred syntax described in the
   base DNS specifications (STD13 [RFC1034] [RFC1035] and Host
   Requirements [RFC1123]) because they contain non-ASCII characters.
   These procedures depend on the use of a special ASCII-compatible
   encoding form that contains only characters permitted in host names
   by those earlier specifications.  The encoding is specified in
   [RFC3492].  No security issues such as string length increases or new
   allowed values are introduced by the encoding process or the use of
   these encoded values, apart from those introduced by the ACE encoding
   itself.

   Domain names (or portions of them) are sometimes compared against a
   set domains to be given special treatment if a match occurs, e.g.,
   treated as more privileged than others or blocked in some way.  In



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   such situations it is especially important that the comparisons be
   done properly, as specified in requirement 2 of Section 3.1.  For
   labels already in ASCII form (i.e., are LDH-labels or A-labels), the
   proper comparison reduces to the same case-insensitive ASCII
   comparison that has always been used for ASCII labels.

   The introduction of IDNA means that any existing labels that start
   with the ACE prefix would be construed as A-labels, at least until
   they failed one of the relevant tests, whether or not that was the
   intent of the zone administrator or registrant.  There is no evidence
   that this has caused any practical problems since RFC 3490 was
   adopted, but the risk still exists in principle.


8.  IANA Considerations

   IANA actions for this version of IDNA are specified in
   [IDNA2008-Rationale].


9.  Change Log

   [[anchor30: RFC Editor: Please remove this section.]]

9.1.  Changes between Version -00 and -01 of draft-ietf-idnabis-protocol

   o  Corrected discussion of SRV records.

   o  Several small corrections for clarity.

   o  Inserted more "open issue" placeholders.

9.2.  Version -02

   o  Rewrote the "conversion to Unicode" text in Section 5.2 as
      requested on-list.

   o  Added a comment (and reference) about EDNS0 to the "DNS Server
      Conventions" section, which was also retitled.

   o  Made several editorial corrections and improvements in response to
      various comments.

   o  Added several new discussion placeholder anchors and updated some
      older ones.






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9.3.  Version -03

   o  Trimmed change log, removing information about pre-WG drafts.

   o  Incorporated a number of changes suggested by Marcos Sanz in his
      note of 2008.07.17 and added several more placeholder anchors.

   o  Several minor editorial corrections and improvements.

   o  "Editor" designation temporarily removed because the automatic
      posting machinery does not accept it.


10.  Contributors

   While the listed editor held the pen, the original versions of this
   document represent 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.  This document
   draws significantly on the original version of IDNA [RFC3490] both
   conceptually and for specific text.  This second-generation version
   would not have been possible without the work that went into that
   first version and its authors, Patrik Faltstrom, Paul Hoffman, and
   Adam Costello.  While Faltstrom was actively involved in the creation
   of this version, Hoffman and Costello were not and should not be held
   responsible for any errors or omissions.


11.  Acknowledgements

   This revision to IDNA would have been impossible without the
   accumulated experience since RFC 3490 was published and resulting
   comments and complaints of many people in the IETF, ICANN, and other
   communities, too many people to list here.  Nor would it have been
   possible without RFC 3490 itself and the efforts of the Working Group
   that defined it.  Those people whose contributions are acknowledged
   in RFC 3490, [RFC4690], and [IDNA2008-Rationale] were particularly
   important.

   Specific textual changes were incorporated into this document after
   suggestions from Stephane Bortzmeyer, Mark Davis, and others.


12.  References







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12.1.  Normative References

   [IDNA2008-BIDI]
              Alvestrand, H. and C. Karp, "An updated IDNA criterion for
              right-to-left scripts", July 2008, <https://
              datatracker.ietf.org/drafts/draft-ietf-idnabis-bidi/>.

   [IDNA2008-Rationale]
              Klensin, J., Ed., "Internationalizing Domain Names for
              Applications (IDNA): Issues, Explanation, and Rationale",
              July 2008, <https://datatracker.ietf.org/drafts/
              draft-ietf-idnabis-rationale>.

   [IDNA2008-Tables]
              Faltstrom, P., "The Unicode Codepoints and IDNA",
              July 2008, <https://datatracker.ietf.org/drafts/
              draft-ietf-idnabis-tables/>.

              A version of this document is available in HTML format at
              http://stupid.domain.name/idnabis/
              draft-ietf-idnabis-tables-02.html

   [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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [Unicode-PropertyValueAliases]
              The Unicode Consortium, "Unicode Character Database:
              PropertyValueAliases", March 2008, <http://
              www.unicode.org/Public/UNIDATA/PropertyValueAliases.txt>.

   [Unicode-RegEx]
              The Unicode Consortium, "Unicode Technical Standard #18:
              Unicode Regular Expressions", May 2005,
              <http://www.unicode.org/reports/tr18/>.




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   [Unicode-Scripts]
              The Unicode Consortium, "Unicode Standard Annex #24:
              Unicode Script Property", February 2008,
              <http://www.unicode.org/reports/tr24/>.

   [Unicode-UAX15]
              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", 2006,
              <http://www.unicode.org/reports/tr15/>.

12.2.  Informative References

   [ASCII]    American National Standards Institute (formerly United
              States of America Standards Institute), "USA Code for
              Information Interchange", ANSI X3.4-1968, 1968.

              ANSI X3.4-1968 has been replaced by newer versions with
              slight modifications, but the 1968 version remains
              definitive for the Internet.

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

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

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

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

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

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

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




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   [RFC4952]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 4952, July 2007.

   [Unicode]  The Unicode Consortium, "The Unicode Standard, Version
              5.0", 2007.

              Boston, MA, USA: Addison-Wesley.  ISBN 0-321-48091-0


Appendix A.  The Contextual Rules Registry

   [[anchor38: Note in Draft: The WG seems to be concluding that this
   material should actually be in the Tables document, possibly with
   some additional material added from Rationale.  Unless there are
   objections and consensus on some other plan, that move will be made
   with -03 of this document.  Regardless of where they are placed, the
   WG will still need to review the specific content of the rules.  In
   this version of the document, the table remains something of a
   illustrative placeholder, not a final specification.]]

   [[anchor39: The next appendix sketches out an alternate way to
   present this information.  See the notes there.]]

   As discussed in the IANA Considerations section of
   [IDNA2008-Rationale], a registry of rules that define the contexts in
   which particular PROTOCOL-VALID characters, characters associated
   with a requirement for Contextual Information, are permitted.  These
   rules are expressed as tests on the label in which the characters
   appear (all, or any part of, the label may be tested). [[anchor40:
   Probably the IANA registry spec should be moved directly from
   Rationale to Tables -- see above.]]

   For each character specified as requiring a contextual rule, a rule
   MAY be established with the following data elements:

   1.  The code point associated with the character.

   2.  The name of the character.

   3.  An indication as to whether the code point requires the rule be
       processed at lookup time (this indication is equivalent to the
       difference between "CONTEXTJ" and "CONTEXTO" in the tables
       document [IDNA2008-Tables]).

   4.  A prose description of the contextual rule.

   5.  A description of the contextual rule using Unicode Regular
       Expression notation [Unicode-RegEx].  Only a Level 1



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       implementation is needed for the expressions below, which also
       make reference to the Unicode Script definition [Unicode-Scripts]
       and the Unicode Property Value Aliases list
       [Unicode-PropertyValueAliases].  Note that in these regular
       expressions, the label is taken to be an entire line, i.e., "^"
       refers to the beginning of the label and "$" refers to the end of
       the label.

       These regular expressions are used as tests.  The contextual
       requirement is met if there is a match for the regular expression
       and not met if there is no match.

       [[anchor41: Patrik and I (JcK) would like to find a way to state
       these rules that does not require the reader and implementer to
       understand what we believe to be a fairly exotic element of the
       Unicode specification.  See the second Appendix for a possible
       alternative.  Suggestions welcome.]]

   6.  An optional comment preceded by "#"

   Should there be any conflict between the two statements of a rule,
   the regular expression form MUST be considered normative until the
   registry can be corrected.

   The rules for the characters listed in the Tables document as
   exception cases or Join_Controls and for which rules are being
   defined at this time appear below.

   [[anchor42: Note in draft: This table is not complete and the rule
   entries below are temporarily only examples.]]

   002D; HYPHEN-MINUS; F;
      Must not appear at the beginning or end of a label;
      Regular expression:
      [^^]\u002D|\u002D[^$] ;
      # Note that there are some additional prohibitions in the
      specification on consecutive hyphens in anything but a valid
      A-label.

   200C; ZERO WIDTH NON-JOINER; T;
      Between two characters from the same script only.  The script must
      be one in which the use of this character causes significant
      visual transformation of one or both of the adjacent characters;
      Regular expression:
      [\p(Script:Deva)\p(Script:Tamil)]\u200C[\p(Script:Deva)\p(Script:
      Tamil)] ;
      [[anchor43: That script list is _not_ complete and, in particular,
      more Indic scripts certainly need to be listed.  It also does not



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      correctly express the "same script" restriction mentioned in the
      prose, since it only tests adjacent characters.]]  This character
      is also required for Arabic script.  The minimal restriction is
      \p(Joining_Type:L)\p(Joining_Type:T)*\u200C\p(Joining_Type:
      T)*\p(Joining_Type:R) ;
      ; more narrow restrictions may be suggested by the Arabic script
      group.

   200D; ZERO WIDTH JOINER; T;
      Between two characters from the same script only.  The script must
      be one in which the use of this character causes significant
      visual transformation of one or both of the adjacent characters;
      Regular expression:
      [\p(Script:Deva)\p(Script:Tamil)]+
      \u200D[\p(Script:Deva)\p(Script:Tamil)]+ ;
      [[anchor44: That script list is _not_ complete and, in particular,
      more Indic scripts certainly need to be listed.  It also does not
      correctly express the "same script" restriction mentioned in the
      prose, since it only tests adjacent characters.  This character is
      not required for Arabic script.]]

   00B7; MIDDLE DOT; F;
      Between two 'l' (U+006C) characters only, used to permit the
      Catalan character ela geminada to be expressed;
      Regular expression:
      \u006C\u00B7\u006C ;

   0375; GREEK LOWER NUMERAL SIGN (KERAIA); F;
      Greek script only.  Might be further restricted to specific
      following characters;
      Regular expression:
      \u0375\p(Script:Greek) ;

   02B9; MODIFIER LETTER PRIME; F;;;
      # Permitted only in contexts in which GREEK LOWER NUMERAL SIGN,
      U+0375, is permitted.  GREEK NUMERAL SIGN, U+0374, and the Lower
      Numeral Sign (U+0375) are indicators for numeric use of letters in
      older Greek writing systems.  U+02B9 is relevant because
      normalization maps U+0374 into it.;
      Regular expression:
      \p(Script:Greek)\u02B9\p(Script:Greek) ;
      [[anchor45: The test is that the adjacent characters be in the
      Greek script.  It is not clear whether this is sufficient.  The
      requirement for a preceding Greek letter may not be necessary.
      More input needed.]]






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   0483; COMBINING CYRILLIC TITLO; F;
      Cyrillic script only.  Might be further restricted to permit only
      a preceding list of characters.
      Regular expression:
      \p(Script:Cyrillic)\u0483 ;

   05F3; HEBREW PUNCTUATION GERESH; F;
      The script of the preceding character and the subsequent
      character, if any, MUST be Hebrew;
      Regular expression:
      \p(Script:Hebrew)\u05F3\p(Script:Hebrew)? ;

   05F4; HEBREW PUNCTUATION GERSHAYIM; F
      The script of the preceding character and the subsequent
      character, if any, MUST be Hebrew;
      Regular expression:
      \p(Script:Hebrew)\u05F4\p(Script:Hebrew)? ;

   3005; IDEOGRAPHIC ITERATION MARK; F;
      MUST NOT be at the beginning of the label, and the previous
      character MUST be in Han Script;
      Regular expression:
      \p(Script:Hani)\u3005 ;

   303B; VERTICAL IDEOGRAPHIC ITERATION MARK; F;
      MUST NOT be at the beginning of the label, and the previous
      character MUST be in Han Script;
      Regular expression:
      \p(Script:Hani)\u303B ;

   30FB; KATAKANA MIDDLE DOT; F;
      Adjacent characters MUST be Katakana;
      Regular expression:
      \p(Script:Kana)\u30FB\p(Script:Kana) ;

   While the information above is to be used to initialize the registry,
   IANA should treat the table format in this Appendix simply as an
   initial, tentative, suggestion.  Subject to review and comment from
   the IESG and any Expert Reviewers, IANA is responsible for, and
   should develop, a format for that registry, or a copy of it
   maintained in parallel, that is convenient for retrieval and machine
   processing and publish the location of that version.


Appendix B.  Contextual Rules Registry - Alternate Syntax

   [[anchor46: This Appendix is temporary.  It illustrates, for
   discussion, a possible way of presenting the Contextual Rules as a



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   procedural pseudocode rule set rather than as a regular expression or
   property list and also shows a bit of the layout suggested by Mark
   Davis.  Each entry consists of the name for identification, followed
   by an informal description, the code point, and the rule set.  Note
   that the two appendices are alternate forms of the same information;
   only one should be moved to Tablss; the other will be deleted.]]

   [[anchor47: The grammatical rules and operations for the pseudocode
   below are left as an exercise for the reader in this draft.  Note
   however that the "Before" and "After" operations, by themselves,
   match anything including null, i.e., BeforeScript would match any
   script if the character was the first one in the label.  Obviously,
   if something satisfies all of the rules, then it is contextually
   valid.  If any of them yield "False" than it isn't.  If we decide to
   go in this direction, we should form a small ad hoc committee to
   either sort that out or possibly convert it to standard Prolog.]]

B.1.  HYPHEN-MINUS

   Code point:   002D

   Overview:   Must appear at the beginning or end of a label.

   Lookup:   False

   Rule Set:

      If FirstChar .eq.  True Then False;
      If LastChar .eq.  Then False;
      Else True;

   Comment:   Note that there are some additional prohibitions in the
      specification on consecutive hyphens in anything but a valid
      A-label.

B.2.  ZERO WIDTH NON-JOINER

   Code point:   200C

   Overview:   Between two characters from the same script only.  The
      script must be one in which the use of this character causes
      significant visual transformation of one or both of the adjacent
      characters.

   Lookup:   True






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   Rule Set:

      If BeforeScript .eq. ( Deva | Tamil | Arabic ) Then
      If AfterScript .eq. ( Deva | Tamil | Arabic ) Then True;
      Else False;

      [[anchor50: That script list is _not_ complete and, in particular,
      more Indic scripts certainly need to be listed.  It also does not
      correctly express the "same script" restriction mentioned in the
      prose, since it only tests adjacent characters.]]

      This character is also required for Arabic script.  The minimal
      restriction (in regex form) is
      \p(Joining_Type:L)\p(Joining_Type:T)*\u200C\p(Joining_Type:
      T)*\p(Joining_Type:R) ;
      ; more narrow restrictions may be suggested by the Arabic script
      group.

B.3.  ZERO WIDTH JOINER

   Code point:   200D

   Overview:   Between two characters from the same script only.  The
      script must be one in which the use of this character causes
      significant visual transformation of one or both of the adjacent
      characters.

   Lookup:   True

   Rule Set:

      If BeforeScript .eq. ( Deva | Tamil | Arabic ) Then
      If AfterScript .eq. ( Deva | Tamil | Arabic ) Then True;
      Else False;

      [[anchor52: The script list for this character is _not_ complete
      and, in particular, more Indic scripts certainly need to be
      listed.  It also does not correctly express the "same script"
      restriction mentioned in the prose, since it only tests adjacent
      characters.  This character is not required for Arabic script.]]

B.4.  MIDDLE DOT

   Code point:   00B7







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   Overview:   Between 'l' (U+006C) characters only, used to permit the
      Catalan character ela geminada to be expressed

   Lookup:   False

   Rule Set:

      If BeforeChar .eq. \006C Then
      If AfterChar .eq. \006C Then True;
      Else False;

B.5.  GREEK LOWER NUMERAL SIGN (KERAIA)

   Code point:   0375

   Overview:   Greek script only.  Might be further restricted to
      specific following characters

   Lookup:   False

   Rule Set:

      If AfterScript .eq.  Greek Then True;
      Else False;

B.6.  MODIFIER LETTER PRIME

   Code point:   02B9

   Overview:   Permitted only in contexts in which GREEK LOWER NUMERAL
      SIGN, U+0375, is permitted.  GREEK NUMERAL SIGN, U+0374, and the
      Lower Numeral Sign (U+0375) are indicators for numeric use of
      letters in older Greek writing systems.  U+02B9 is relevant
      because normalization maps U+0374 into it.

   Lookup:   False

   Rule Set:

      BeforeScript If .eq.  Greek Then
      If AfterScript .eq.  Greek Then True;
      Else False;

   Comment:   [[anchor56: The test is that the adjacent characters be in
      the Greek script.  It is not clear whether this is sufficient.
      The requirement for a preceding Greek letter may not be necessary.
      More input needed.]]




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B.7.  COMBINING CYRILLIC TITLO

   Code point:   0483

   Overview:   Cyrillic script only.  Might be further restricted to
      permit only a preceding list of characters.

   Lookup:   False

   Rule Set:

      If BeforeScript .eq.  Cyrillic Then
      If AfterScript .eq.  Cyrillic Then True;
      Else False;

B.8.  HEBREW PUNCTUATION GERESH

   Code point:   05F3

   Overview:   The script of the preceding character and the subsequent
      character, if any, MUST be Hebrew.

   Lookup:   False

   Rule Set:

      If FirstChar .eq.  True then False;
      Else If BeforeScript .eq.  Hebrew Then
      If AfterScript .eq.  Hebrew Then True;
      Else False;

B.9.  HEBREW PUNCTUATION GERSHAYIM

   Code point:   05F4

   Overview:   The script of the preceding character and the subsequent
      character, if any, MUST be Hebrew.

   Lookup:   False

   Rule Set:

      If FirstChar .eq.  True then False;
      Else If BeforeScript .eq.  Hebrew Then
      If AfterScript .eq.  Hebrew Then True;
      Else False;





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B.10.  IDEOGRAPHIC ITERATION MARK;

   Code point:   3005

   Overview:   MUST NOT be at the beginning of the label, and the
      previous character MUST be in Han Script.

   Lookup:   False

   Rule Set:

      If FirstChar .eq.  True Then False;
      Else If BeforeScript .eq.  Han Then True;
      Else False;

B.11.  VERTICAL IDEOGRAPHIC ITERATION MARK

   Code point:   303B

   Overview:   MUST NOT be at the beginning of the label, and the
      previous character MUST be in Han Script.

   Lookup:   False

   Rule Set:

      If FirstChar .eq.  True Then False;
      Else If BeforeScript .eq.  Han Then True;
      Else False;

B.12.  KATAKANA MIDDLE DOT

   Code point:   30FB

   Overview:   Adjacent characters MUST be Katakana.

   Lookup:   False

   Rule Set:

      If FirstChar .eq.  True Then False;
      Else If BeforeScript .eq.  Kana Then
      If AfterScript .eq.  Kana Then True;
      Else False;







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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










































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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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Klensin                 Expires January 28, 2009               [Page 29]


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