Network Working Group                                         J. Klensin
Internet-Draft                                             July 12, 2009
Obsoletes: 3490, 3491
(if approved)
Updates: 3492 (if approved)
Intended status: Standards Track
Expires: January 13, 2010

    Internationalized Domain Names in Applications (IDNA): Protocol

Status of this Memo

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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (
   Please review these documents carefully, as they describe your rights
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   This document is the revised protocol definition for
   internationalized domain names (IDNs).  The rationale for 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 . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Requirements and Applicability . . . . . . . . . . . . . . . .  6
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  6
       3.2.1.  DNS Resource Records . . . . . . . . . . . . . . . . .  7
       3.2.2.  Non-domain-name Data Types Stored in the DNS . . . . .  7
   4.  Registration Protocol  . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Input to IDNA Registration Process . . . . . . . . . . . .  8
     4.2.  Permitted Character and Label Validation . . . . . . . . .  8
       4.2.1.  Input Format . . . . . . . . . . . . . . . . . . . . .  8
       4.2.2.  Rejection of Characters that are not Permitted . . . .  8
       4.2.3.  Label Validation . . . . . . . . . . . . . . . . . . .  8
       4.2.4.  Registration Validation Summary  . . . . . . . . . . .  9
     4.3.  Registry Restrictions  . . . . . . . . . . . . . . . . . . 10
     4.4.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 10
     4.5.  Insertion in the Zone  . . . . . . . . . . . . . . . . . . 10
   5.  Domain Name Lookup Protocol  . . . . . . . . . . . . . . . . . 10
     5.1.  Label String Input . . . . . . . . . . . . . . . . . . . . 11
     5.2.  Conversion to Unicode  . . . . . . . . . . . . . . . . . . 11
     5.3.  Character Changes in Preprocessing or the User
           Interface  . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.4.  A-label Input  . . . . . . . . . . . . . . . . . . . . . . 12
     5.5.  Validation and Character List Testing  . . . . . . . . . . 13
     5.6.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 14
     5.7.  DNS Name Resolution  . . . . . . . . . . . . . . . . . . . 15
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Local Mapping Alternatives  . . . . . . . . . . . . . 18
     A.1.  Transitional Mapping Model . . . . . . . . . . . . . . . . 19
       A.1.1.  Fallback Lookup  . . . . . . . . . . . . . . . . . . . 19
       A.1.2.  Two-step Lookup  . . . . . . . . . . . . . . . . . . . 20
     A.2.  Internationalized Resource Identifier (IRI) Mapping
           Model  . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Appendix B.  Summary of Major Changes from IDNA2003  . . . . . . . 21
   Appendix C.  Change Log  . . . . . . . . . . . . . . . . . . . . . 22
     C.1.  Changes between Version -00 and -01 of
           draft-ietf-idnabis-protocol  . . . . . . . . . . . . . . . 22
     C.2.  Version -02  . . . . . . . . . . . . . . . . . . . . . . . 22
     C.3.  Version -03  . . . . . . . . . . . . . . . . . . . . . . . 22

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     C.4.  Version -04  . . . . . . . . . . . . . . . . . . . . . . . 23
     C.5.  Version -05  . . . . . . . . . . . . . . . . . . . . . . . 23
     C.6.  Version -06  . . . . . . . . . . . . . . . . . . . . . . . 23
     C.7.  Version -07  . . . . . . . . . . . . . . . . . . . . . . . 23
     C.8.  Version -08  . . . . . . . . . . . . . . . . . . . . . . . 24
     C.9.  Version -09  . . . . . . . . . . . . . . . . . . . . . . . 24
     C.10. Version -10  . . . . . . . . . . . . . . . . . . . . . . . 24
     C.11. Version -11  . . . . . . . . . . . . . . . . . . . . . . . 25
     C.12. Version -12  . . . . . . . . . . . . . . . . . . . . . . . 25
     C.13. Version -13  . . . . . . . . . . . . . . . . . . . . . . . 25
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26

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1.  Introduction

   This document supplies the protocol definition for internationalized
   domain names.  Essential definitions and terminology for
   understanding this document and a road map of the collection of
   documents that make up IDNA2008 appear in [IDNA2008-Defs].
   Appendix B discusses the relationship between this specification and
   the earlier version of IDNA (referred to here as "IDNA2003") and the
   rationale for these changes, along with considerable explanatory
   material and advice to zone administrators who support IDNs is
   provided in another 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 changes 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
   can be used for IDNA.

   IDNA applies only to DNS labels.  The base DNS standards [RFC1034]
   [RFC1035] and their various updates specify how to combine labels
   into fully-qualified domain names and parse labels out of those

   This document describes two separate protocols, one for IDN
   registration (Section 4) and one for IDN lookup (Section 5), that
   share some terminology, reference data and operations. [[anchor2:
   Note in draft: See the note in the introduction to.]]Section 5

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

2.  Terminology

   Terminology used in IDNA, but also in Unicode or other character set
   standards and the DNS, appears in [IDNA2008-Defs].  Terminology that
   is required as part of the IDNA definition, including the definitions
   of "ACE", appears in that document as well.  Readers of this document
   are assumed to be familiar with [IDNA2008-Defs] and with the DNS-
   specific terminology in RFC 1034 [RFC1034].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in BCP 14, RFC 2119

3.  Requirements and Applicability

3.1.  Requirements

   IDNA makes the following requirements:

   1.  Whenever a domain name is put into an IDN-unaware domain name
       slot (see Section 2 and [IDNA2008-Defs]), it MUST contain only
       ASCII characters (i.e., must be either an A-label or an NR-LDH-
       label), unless the DNS application is not subject to historical
       recommendations for "hostname"-style names (see [RFC1034] and
       Section 3.2.1).

   2.  Labels MUST be compared using equivalent forms: either both
       A-Label forms or both U-Label forms.  Because A-labels and
       U-labels can be transformed into each other without loss of
       information, these comparisons are equivalent.  A pair of
       A-labels MUST be compared as case-insensitive ASCII (as with all
       comparisons of ASCII DNS labels).  U-labels must be compared
       as-is, without case-folding or other intermediate steps.  Note
       that it is not necessary to validate labels in order to compare
       them.  In many cases, validation may be important for other
       reasons and SHOULD be performed.

   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 applies to all domain names in all domain name slots in
   protocols except where it is explicitly excluded.  It does not apply
   to domain name slots which do not use the Letter/Digit/Hyphen (LDH)
   syntax rules.

   Because it uses the DNS, IDNA applies to many protocols that were
   specified before it was designed.  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 explicitly upgraded
   to be aware of IDNs in Unicode.  IDNs actually appearing in DNS
   queries or responses MUST be A-labels.

   IDNA is not defined for extended label types (see RFC 2671, Section 3

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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.  See RFC 1034 [RFC1034] for
   precise definitions of these terms.

   The application of IDNA to DNS resource records depends entirely on
   the CLASS of the record, and not on the TYPE except as noted below.
   This will remain true, even as new types are defined, unless a new
   type defines type-specific rules.  Special naming conventions for SRV
   records (and "underscore names" more generally) are incompatible with
   IDNA coding.  The first two labels on a SRV type record (the ones
   required to start in "_") MUST NOT be A-labels or U-labels, because
   conversion to an A-label would lose information (since the underscore
   is not a letter, digit, or hyphen and is consequently DISALLOWED in
   IDNs).  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 ( would be
   represented as  IDNA 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, or
   updates to, 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

   Note that, while the registration and lookup protocols (Section 5)
   are very similar in most respects, they are different and
   implementers should carefully follow the appropriate steps.

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4.1.  Input to IDNA Registration Process

   Registration processes, especially processing by entities, such as
   "registrars" who deal with registrants before the request actually
   reaches the zone manager ("registry") are outside the scope of these
   protocols and may differ significantly depending on local needs.  By
   the time a string enters the IDNA registration process as described
   in this specification, it is expected to be in Unicode and MUST be in
   Unicode Normalization Form C (NFC [Unicode-UAX15]).  Entities
   responsible for zone files ("registries") are expected to accept only
   the exact string for which registration is requested, free of any
   mappings or local adjustments.  They SHOULD avoid any possible
   ambiguity by accepting registrations only for A-labels, possibly
   paired with the relevant U-labels so that they can verify the

4.2.  Permitted Character and Label Validation

4.2.1.  Input Format

   The registry SHOULD permit submission of labels in A-label form and
   is encouraged to accept both the A-label form and the U-label one.
   If it does so, it MUST 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 4.4 matches the one provided as input.  In
   addition, if a U-label was provided, that U-label and the one
   obtained by conversion of the A-label MUST match exactly.  If, for
   some reason, these tests fail, the registration MUST be rejected.  If
   the conversion to a U-label is not performed, the registry MUST still
   verify that the A-label is superficially valid, i.e., that it does
   not violate any of the rules of Punycode [RFC3492] encoding such as
   the prohibition on trailing hyphen-minus, appearance of non-basic
   characters before the delimiter, and so on.  Fake A-labels, i.e.,
   invalid strings that appear to be A-labels but are not, MUST NOT be
   placed in DNS zones that support IDNA.

4.2.2.  Rejection of Characters that are not Permitted

   The candidate Unicode string MUST NOT contain characters in the
   "DISALLOWED" and "UNASSIGNED" lists specified in [IDNA2008-Tables].

4.2.3.  Label Validation

   The proposed label (in the form of a Unicode string, i.e., a string
   that at least superficially appears to be a U-label) is then
   examined, performing tests that require examination of more than one
   character.  Character order is considered to be the on-the-wire
   order, not the display order.

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

   The Unicode string MUST NOT begin with a combining mark or combining
   character (see The Unicode Standard, Section 2.11 [Unicode] for an
   exact definition).  Contextual Rules

   The Unicode string MUST NOT contain any characters whose validity is
   context-dependent, unless the validity is positively confirmed by a
   contextual rule.  To check this, each code-point marked as CONTEXTJ
   and CONTEXTO in [IDNA2008-Tables] MUST have a non-null rule.  If such
   a code-point is missing a rule, it is invalid.  If the rule exists
   but the result of applying the rule is negative or inconclusive, the
   proposed label is invalid.

   NOTE: These contextual rules are required to support characters that
   could be used, under some conditions, to produce misleading labels or
   to cause unacceptable ambiguity in label matching and interpretation.
   For example, labels containing zero-width characters may be permitted
   in context with characters whose presentation forms are significantly
   changed by the zero-width characters, while other labels in which
   zero-width characters appear may be rejected.
   [[anchor11: Note in draft: Should this note be moved to Rationale???
   It has no normative consequences here.]]  Labels Containing Characters Written Right to Left

   If the proposed label contains any characters that are written from
   right to left it MUST meet the "bidi" criteria [IDNA2008-BIDI].

4.2.4.  Registration Validation Summary

   Strings that contain at least one non-ASCII character, have been
   produced by the steps above, whose contents pass all of the tests in
   Section 4.2, and are 63 or fewer characters long in ACE form (see
   Section 4.4), are U-labels.

   To summarize, tests are made in Section 4.2 for invalid characters,
   invalid combinations of characters, for labels that are invalid even
   if the characters they contain are valid individually, and for labels
   that do not conform to the restrictions for strings containing right
   to left characters.

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4.3.  Registry Restrictions

   In addition to the rules and tests above, there are many reasons why
   a registry could reject a label.  Registries at all levels of the
   DNS, not just the top level, establish policies about label
   registrations.  Policies are likely to be informed by the local
   languages and may depend on many factors including what characters
   are in the label (for example, a label may be rejected based on other
   labels already registered).  See [IDNA2008-Rationale] for a
   discussion and recommendations about registry policies.

   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.

4.4.  Punycode Conversion

   The resulting U-label is converted to an A-label.  The A-label, more
   precisely defined elsewhere, is the encoding of the U-label according
   to the Punycode algorithm [RFC3492] with the ACE prefix "xn--" added
   at the beginning of the string.  The resulting string must, of
   course, conform to the length limits imposed by the DNS.  This
   document updates RFC 3492 only to the extent of replacing the
   reference to the discussion of the ACE prefix.  The ACE prefix is now
   specified in this document rather than as part of RFC 3490 or
   Nameprep [RFC3491] but is the same in both sets of documents.

   The failure conditions identified in the Punycode encoding procedure
   cannot occur if the input is a U-label as determined by the steps

4.5.  Insertion in the Zone

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

5.  Domain Name Lookup Protocol

   Lookup is different from registration and different tests are applied
   on the client.  Although some validity checks are necessary to avoid
   serious problems with the protocol, the lookup-side tests are more
   permissive and rely on the assumption that names that are present in
   the DNS are valid.  That assumption is, however, a weak one because
   the presence of wild cards in the DNS might cause a string that is
   not actually registered in the DNS to be successfully looked up.

   The two steps in Section 5.2 and Section 5.3 are required.

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   [[anchor14: Note in Draft: Try to reorganize and renumber Section 5
   (Lookup) so that it exactly parallels Section 4 (Registration).  This
   has not been done in drafts -10 through -13 because the task will be
   much easier if the local mapping material is pulled from here (and
   there is no point trying to align the section numbers twice).]]

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.  Alternately, 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.

5.2.  Conversion to Unicode

   The string is converted from the local character set into Unicode, if
   it is not already Unicode.  A Unicode string may require
   normalization as discussed in Section 4.1.  The result MUST be a
   Unicode string in NFC form.

5.3.  Character Changes in Preprocessing or the User Interface

   [[anchor15: Note in Draft -13.  This entire section is likely to need
   significant revision when we make final decisions about mapping.  The
   changes from -12 are intended simply to illustrate one of the ways in
   which the mapping material might be incorporated if we keep it... and
   to fix a possible ambiguity about the NFC requirement for mapping

   The Unicode string MAY then be processed to prevent confounding of
   user expectations.  For instance, it might be reasonable, at this
   step, to convert all upper case characters to lower case, if this
   makes sense in the user's environment, but even this should be
   approached with caution due to some edge cases: in the long term, it
   is probably better for users to understand IDNs strictly in lower-
   case, U-label, form.  More generally, preprocessing may be useful to
   smooth the transition from IDNA2003, especially for direct user
   input, but with similar cautions.  In general, IDNs appearing in
   files and those transmitted across the network as part of protocols
   are expected to be in either ASCII form (including A-labels) or to
   contain U-labels, rather than being in forms requiring mapping or
   other conversions.

   The mapping issue and some suggestions and tradeoffs are discussed in
   [IDNA2008-Mapping].  Note that this specification does not require

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   that the processing into Unicode (See Section 5.2 above) be applied
   as a separate step if it incorporated into some mapping process as
   described in [IDNA2008-Mapping].

   Other examples of processing for localization might be applied,
   especially to direct user input, 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 halfwidth or
   fullwidth East Asian characters to the common form permitted in
   labels, 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

   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].  It
   is important to note that the intent of these specifications is that
   labels in application protocols, files, or links are intended to be
   in U-label or A-label form.  Preprocessing MUST NOT map a character
   that is valid in a label as specified elsewhere in this document or
   in [IDNA2008-Tables] into another character.  Excessively liberal use
   of preprocessing, especially to strings stored in files, poses a
   threat to consistent and predictable behavior for the user even if
   not to actual 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.

   If this step is applied, the results still MUST be in NFC form as
   above.  The step must not denormalize the characters.

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 the conversion of
   Section 5.6 to that form.  If the label is converted to Unicode
   (i.e., to U-label form) using the Punycode decoding algorithm, then
   the processing specified in those two sections MUST be performed, and
   the label MUST be rejected if the resulting label is not identical to
   the original.  See the Name Server Considerations section of
   [IDNA2008-Rationale] for additional discussion on this topic.

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   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).  If those steps
   are not performed, the lookup process SHOULD at least make tests to
   determine that the string is actually an A-label, examining it for
   the invalid formats specified in the Punycode decoding specification.
   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 described in Section 4, the
   Unicode string is checked to verify that all characters that appear
   in it are valid as input to IDNA lookup processing.  As discussed
   above and in [IDNA2008-Rationale], the lookup check is more liberal
   than the registration one.  Labels that have not been fully evaluated
   for conformance to the applicable rules are referred to as "putative"
   labels as discussed in [IDNA2008-Defs][[anchor16: ???  Insert section
   number -- 2.2.3 as of Defs-09]].  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
      category of [IDNA2008-Tables].

   o  Labels that are not in NFC form as defined in [Unicode-UAX15].

   o  Labels containing prohibited code points, i.e., those that are
      assigned to the "DISALLOWED" category in the permitted character
      table [IDNA2008-Tables].

   o  Labels containing code points that are identified in
      [IDNA2008-Tables] as "CONTEXTJ", i.e., requiring exceptional
      contextual rule processing on lookup, but that do not conform to
      that rule.  Note that this implies that a rule much be defined,
      not null: a character that requires a contextual rule but for
      which the rule is null is treated in this step as having failed to
      conform to the rule.

   o  Labels containing code points that are identified in
      [IDNA2008-Tables] as "CONTEXTO", but for which no such rule
      appears in the table of rules.  Applications resolving DNS names
      or carrying out equivalent operations are not required to test
      contextual rules for "CONTEXTO" characters, only to verify that a
      rule is defined (although they MAY make such tests to provide
      better protection or give better information to the user).

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   o  Labels whose first character is a combining mark (see

   In addition, the application SHOULD apply the following test.  The
   test may be omitted in special circumstances, such as when the lookup
   application knows that the conditions are enforced elsewhere, because
   an attempt to look up and resolve such strings will almost certainly
   lead to a DNS lookup failure except when wildcards are present in the
   zone.  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 -- than DNS
   resolution failure information alone.  In any event, lookup
   applications should avoid attempting to resolve labels that are
   invalid under that test.

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

   For all other strings, the lookup application 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.  While a lookup application
   may reasonably issue warnings about strings it believes may be
   problematic, applications that decline to process a string that
   conforms to the rules above (i.e., does not look it up in the DNS)
   are not in conformance with this protocol.

5.6.  Punycode Conversion

   The string that has now been validated for lookup is converted to ACE
   form using the Punycode algorithm (with the ACE prefix added).  With
   the understanding that this summary is not normative (the steps above
   are), the string is either

   o  in Unicode NFC form that contains no leading combining marks,
      contains no DISALLOWED or UNASSIGNED code points, has rules
      associated with any code points in CONTEXTJ or CONTEXTO, and, for
      those in CONTEXTJ, to satisfies the conditions of the rules; or

   o  in A-label form, was supplied under circumstances in which the
      U-label conversions and tests have not been performed (see
      Section 5.4).

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5.7.  DNS Name Resolution

   That resulting validated string is looked up in the DNS, using normal
   DNS resolver procedures.  That lookup can obviously either succeed
   (returning information) or fail.

6.  Security Considerations

   Security Considerations for this version of IDNA, except for the
   special issues associated with right to left scripts and characters,
   are described in [IDNA2008-Defs].  Specific issues for labels
   containing characters associated with scripts written right to left
   appear in [IDNA2008-BIDI].

7.  IANA Considerations

   IANA actions for this version of IDNA are specified in
   [IDNA2008-Tables] and discussed informally in [IDNA2008-Rationale].
   The components of IDNA described in this document do not require any
   IANA actions.

8.  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.

9.  Acknowledgments

   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

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   Specific textual changes were incorporated into this document after
   suggestions from the other contributors, Stephane Bortzmeyer, Vint
   Cerf, Lisa Dusseault, Mark Davis, Paul Hoffman, Kent Karlsson, Erik
   van der Poel, Marcos Sanz, Andrew Sullivan, Ken Whistler, and other
   WG participants.  Special thanks are due to Paul Hoffman for
   permission to extract material from his Internet-Draft to form the
   basis for Appendix B.

10.  References

10.1.  Normative References

              Alvestrand, H. and C. Karp, "An updated IDNA criterion for
              right-to-left scripts", July 2008, <https://

              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              February 2009, <

              Faltstrom, P., "The Unicode Codepoints and IDNA",
              July 2008, <

              A version of this document is available in HTML format at

   [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

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              for Internationalized Domain Names in Applications
              (IDNA)", RFC 3492, March 2003.

              The Unicode Consortium, "Unicode Character Database:
              PropertyValueAliases", March 2008, <http://

              The Unicode Consortium, "Unicode Technical Standard #18:
              Unicode Regular Expressions", May 2005,

              The Unicode Consortium, "Unicode Standard Annex #24:
              Unicode Script Property", February 2008,

              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", 2006,

10.2.  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.

              Resnick, P., "Mapping Characters in IDNA", July 2009, <htt

              Klensin, J., Ed., "Internationalizing Domain Names for
              Applications (IDNA): Issues, Explanation, and Rationale",
              February 2009, <

   [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.

   [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.

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

   [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.  Local Mapping Alternatives

   The subsections of this appendix are temporary and represent
   different sketches of possible replacements for Section 5.3.  They do
   not represent an assertion of WG consensus or any assertion about the
   possibility of including one of them as part of the WG's work
   program.  Instead, they are supplied only for purposes of comparison,
   discussion, and, should it be relevant, refinement.

   The first paragraph of each subsection describes how the material
   would be placed relative to the existing main document text.

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   Subsequent paragraphs are the actual suggestions, although in
   incomplete sketch form.

A.1.  Transitional Mapping Model

   If this subsection were adopted, Section 5.3 would be deleted and
   this one would be inserted after, or integrated with, Section 5.7.

   This specification does not support the extensive mappings from one
   character to another, including Unicode Case Folding and
   Compatibility Character mapping, of IDNA2003.  It also changes the
   interpretations of a small number of characters relative to IDNA2003.
   Most applications, especially those with which IDNs have been used
   for some time, will need to maintain reasonable compatibility with
   files created under IDNA2003 and user interfaces designed for it.
   This section specifies additional steps to be taken to provide
   maximum IDNA2003 compatibility.

   If an application requires IDNA2003 backward compatibility, it MUST
   execute the steps in one of the two subsections that immediately

A.1.1.  Fallback Lookup

   If the string validates and the resolution attempt in Section 5.7
   successfully returns a result, the lookup process terminates with
   that result.  If validation succeeds but resolution fails, the
   validated string is proceeded through the ToASCII operation specified
   in IDNA2003 [RFC3490].  Assuming it produces a valid result, the
   resulting string is compared to the previous validated one.  If they
   are not identical, a resolution attempt is made with the ToASCII
   output and the result of that attempt is returned as the result of
   the lookup operation.

   Should IDNA2008 validation fail, the string is processed through
   ToASCII and, assuming the result is valid, the resulting string is
   resolved and the result of that attempt returned as the result of the
   lookup operation.

   If ToASCII (IDNA2003) conversion is attempted and fails, the lookup
   operation behaves as if no name was found in the DNS.

   Note that this procedure involves, at most, one DNS lookup
   (resolution attempt).  If IDNA2008 string validation, conversion, and
   resolution succeed, no attempt is made to use IDNA2003 mechanisms.
   The procedure does, however, require that lookup applications fully
   support both IDNA2008 and IDNA2003 lookup operations so that the
   fallback can occur.

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A.1.2.  Two-step Lookup

   Prior to the resolution attempt in Section 5.7, ACE strings are
   computed using both IDNA2003 (ToASCII) and IDNA2008 methods (as
   specified here).  Assuming both validate, those strings are compared.
   If they are identical, or only one was valid, then a single DNS
   resolution is performed and its result is the result of the lookup
   operation.  If both are valid but they are not identical, one
   resolution attempt is made with each of the two ACE strings.

   If neither string is valid as an IDN, then the lookup operation

   When two resolutions are attempted, if one of the two is successful
   and the other is not, the successful value is used as the result of
   the lookup.  If both are successful, the user or calling application
   must be presented with a choice in some way.

   This procedure will require two DNS lookups (resolution attempts) in
   all cases except those in which the label string fails IDNA2008
   validation, neither IDNA2003 or IDNA2008 can validate the string and
   translate it to ACE form, or the strings obtained from the two
   conversions are identical.  As with the prior option, IDNA
   implementations will need to support both the IDNA2003 algorithm and
   tables and the IDNA2008 one.  The question of how multiple results
   from different interpretations of the same input string should be
   handled by applications is a difficult one, with potential false
   positive and security attack vector implications as well as the
   possibility of general confusion.

   In particular, if both interpretations of the name return values, the
   lookup application has no practical way to tell whether the relevant
   registry has applied "variant" or "bundling" techniques to ensure
   that both domain names are under the same control or not.  From that
   perspective, the approach in the previous subsection assumes that has
   been done (if the IDNA2003-interpretation label is present at all)
   while this one assumes that such bundling is unlikely to have

   [[anchor26: Note in Draft: If this appendix is used, RFC3490 must be
   moved from Informative to Normative.]]

A.2.  Internationalized Resource Identifier (IRI) Mapping Model

   This subsection is intended to be descriptive of an approach that
   lies outside IDNA, rather than a normative component of it.  If it
   were adopted, Section 5.3 would be deleted and the material below
   would be referenced, either as a non-normative Appendix in Protocol

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   or, more reasonably, as a section of Rationale.

   IDNA2003 supported extensive mappings from one character to another,
   including Unicode Case Folding and Compatibility Character mapping.
   Those mappings are no longer supported on registration and are
   inconsistent with the "exact match" lookups that people expect from
   the DNS.  Some mapping should still be supported, both for
   compatibility with applications that assume IDNA2003 and to avoid
   confounding user expectations.  The specific mappings involved are
   not part of IDNA, but are expected to be specified as part of a
   revision to the IRI specification [RFC3987] and the conversion from
   IRI form to URI form.  That change leaves mapping unspecified and
   prohibited for actual domain names, however, in practice, most domain
   names, especially in the web applications that appear to have been
   most important for IDNs between the publication of IDNA2003 and the
   release of this specification, are not interpreted as themselves but
   as abbreviated form of URIs or IRIs and hence subject to the
   transformation rules of the latter.

Appendix B.  Summary of Major Changes from IDNA2003

   1.   Update base character set from Unicode 3.2 to Unicode version-

   2.   Separate the definitions for the "registration" and "lookup"

   3.   Disallow symbol and punctuation characters except where special
        exceptions are necessary.

   4.   Remove the mapping and normalization steps from the protocol and
        have them instead done by the applications themselves, possibly
        in a local fashion, before invoking the protocol.

   5.   Change the way that the protocol specifies which characters are
        allowed in labels from "humans decide what the table of
        codepoints contains" to "decision about codepoints are based on
        Unicode properties plus a small exclusion list created by

   6.   Introduce the new concept of characters that can be used only in
        specific contexts.

   7.   Allow typical words and names in languages such as Dhivehi and
        Yiddish to be expressed.

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   8.   Make bidirectional domain names (delimited strings of labels,
        not just labels standing on their own) display in a less
        surprising fashion whether they appear in obvious domain name
        contexts or as part of running text in paragraphs.

   9.   Remove the dot separator from the mandatory part of the

   10.  Make some currently-valid labels that are not actually IDNA
        labels invalid.

Appendix C.  Change Log

   [[anchor29: RFC Editor: Please remove this appendix.]]

C.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.

C.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.

C.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.

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

C.4.  Version -04

   o  Removed Contextual Rule appendices for transfer to Tables.

   o  Several changes, including removal of discussion anchors, based on
      discussions at IETF 72 (Dublin)

   o  Rewrote the preprocessing material (Section 5.3) somewhat.

C.5.  Version -05

   o  Updated part of the A-label input explanation (Section 5.4) per
      note from Erik van der Poel.

C.6.  Version -06

   o  Corrected a few typographical errors.

   o  Incorporated the material (formerly in Rationale) on the
      relationship between IDNA2003 and IDNA2008 as an appendix and
      pointed to the new definitions document.

   o  Text modified in several places to recognize the dangers of
      interaction between DNS wildcards and IDNs.

   o  Text added to be explicit about the handling of edge and failure
      cases in Punycode encoding and decoding.

   o  Revised for consistency with the new Definitions document and to
      make the text read more smoothly.

C.7.  Version -07

   o  Multiple small textual and editorial changes and clarifications.

   o  Requirement for normalization clarified to apply to all cases and
      conditions for preprocessing further clarified.

   o  Substantive change to Section 4.2.1, turning a SHOULD to a MUST
      (see note from Mark Davis, 19 November, 2008 18:14 -0800).

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C.8.  Version -08

   o  Added some references and altered text to improve clarity.

   o  Changed the description of CONTEXTJ/CONTEXTO to conform to that in
      Tables.  In other words, these are now treated as distinction
      categories (again), rather than as specially-flagged subsets of

   o  The discussion of label comparisons has been rewritten to make it
      more precise and to clarify that one does not need to verify that
      a string is a [valid] A-label or U-label in order to test it for
      equality with another string.  The WG should verify that the
      current text is what is desired.

   o  Other changes to reflect post-IETF discussions or editorial

C.9.  Version -09

   o  Removed Security Considerations material to Defs document.

   o  Removed the Name Server Considerations material to Rationale.
      That material is not normative and not needed to implement the
      protocol itself.

   o  Adjusted terminology to match new version of Defs.

   o  Removed all discussion of local mapping and option for it from
      registration protocol.  Such mapping is now completely prohibited
      on Registration.

   o  Removed some old placeholders and inquiries because no comments
      have been received.

   o  Small editorial corrections.

C.10.  Version -10

   o  Rewrote the registration input material slightly to further
      clarify the "no mapping on registration" principle.

   o  Added placeholder notes about several tasks, notably reorganizing
      Section 4 and Section 5 so that subsection numbers are parallel.

   o  Cleaned up an incorrect use of the terms "A-label" and "U-label"
      in the lookup phase that was spotted by Mark Davis.  Inserted a
      note there about alternate ways to deal with the resulting

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      terminology problem.

   o  Added a temporarily appendix (above) to document alternate
      strategies for possible replacements for Section 5.3.

C.11.  Version -11

   o  Removed dangling reference to "C-label" (editing error in prior

   o  Recast the last steps of the Lookup description to eliminate
      "apparent" (previously "putative") terminology.

   o  Rewrote major portions of the temporary appendix that describes
      transitional mappings to improve clarity and add context.

   o  Did some fine-tuning of terminology, notably in Section 3.2.1.

C.12.  Version -12

   o  Extensive editorial improvements, mostly due to suggestions from
      Lisa Dusseault.

   o  Conformance statements have been made consistent, especially in
      Section 4.2.1 and subsequent text, which said "SHOULD" in one
      place and then said "MAY" as the result of incomplete removal of
      registration-time mapping.  Also clarified the definition of
      "registration processes" in Section 4.1 -- the previous text had
      confused several people.

   o  A few new "question to the WG notes have been added about
      appropriateness or placement of text.  If there are no comments on
      the mailing list, the editor will apply his own judgment.

   o  Several of the usual small typos and other editorial errors have
      been corrected.

   o  Section 5 has still not been reorganized to match Section 4 in
      structure and subsection numbering -- will be done as soon as the
      mapping decisions and references are final.

C.13.  Version -13

   o  Modified the "putative label" text to better explain the term and
      explicitly point back to Defs.

   o  Slight rewrite of Section 5.3 to clarify the NFC requirement and
      to start the transition toward having some of the explanation in

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      the Mapping document.  The latter might need to be undone as WG
      consensus evolves.

Author's Address

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

   Phone: +1 617 245 1457

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