Internet DRAFT - draft-klensin-idnabis-protocol


Network Working Group                                    J. Klensin, Ed.
Internet-Draft                                          February 6, 2008
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: August 9, 2008

    Internationalizing Domain Names in Applications (IDNA): Protocol

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   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

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   changes to the DNS itself.  IDNA is only meant for processing domain
   names, not free text.

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 . . . . . . . . . . . . . . . . .  5
       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  . . . . . . . . .  6
     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  . . . . . . . . . . . . . . . . . .  8
     4.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . .  8
     4.6.  Insertion in the Zone  . . . . . . . . . . . . . . . . . .  8
   5.  Domain Name Resolution (Lookup) Protocol . . . . . . . . . . .  8
     5.1.  Label String Input . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Conversion to Unicode  . . . . . . . . . . . . . . . . . .  9
     5.3.  Character Changes in Preprocessing or the User
           Interface  . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.4.  Validation and Character List Testing  . . . . . . . . . . 10
     5.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 11
     5.6.  DNS Name Resolution  . . . . . . . . . . . . . . . . . . . 11
   6.  Name server Considerations . . . . . . . . . . . . . . . . . . 11
     6.1.  Processing Non-ASCII Strings . . . . . . . . . . . . . . . 11
     6.2.  DNSSEC Authentication of IDN Domain Names  . . . . . . . . 12
     6.3.  Root Server Considerations . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   9.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Version -00  . . . . . . . . . . . . . . . . . . . . . . . 13
     9.2.  Versions -01 and -02 . . . . . . . . . . . . . . . . . . . 13
     9.3.  Version -03  . . . . . . . . . . . . . . . . . . . . . . . 14
     9.4.  Version -04  . . . . . . . . . . . . . . . . . . . . . . . 14
   10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     12.2. Informative References . . . . . . . . . . . . . . . . . . 16

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   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 18

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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 [IDNA200X-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

   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 [IDNA200X-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 on the mailing list

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 [IDNA200X-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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

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 [IDNA200X-Rationale]), it MUST contain
       only ASCII characters (i.e., must be either an A-label or an LDH-
       label), or must 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

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 protocols
   MUST be in A-label form.

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.  This will remain true, even as new types

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   are defined, unless there is a compelling reason for a new type that
   requires type-specific rules.  It is worth noting that the special
   naming conventions applicable to SRV records are precisely such type-
   specific rules and that the SRV requirement for a leading underscore
   ("_") in some labels is incompatible with IDNA coding.

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 specifically does not
   update the existing email standards, which allow only ASCII
   characters in local parts.  Other work is under development to define
   internationalization for email addresses [RFC4952], but changes to
   the email address part of the SOA RDATA would require action in other
   standards.  Such standards could also specify IDNA interpretation of
   labels that follow the local part such as by permitting them to be
   A-labels or even U-labels.

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

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

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.  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

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   is strongly recommended that, to avoid any possible ambiguity,
   entities responsible for zone files ("registries") accept
   registrations only for A-labels 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 examined to prohibit characters that IDNA does
   not permit in input.  Those characters are identified in the
   "DISALLOWED" and "UNASSIGNED" lists that are discussed in
   [IDNA200X-Rationale].  The normative rules for producing that list
   and the initial version of it are specified in [IDNA200X-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 is then examined, performing tests that require
   examination of more than one character.  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.  Contextual Rules

   Each code point is checked for its identification as characters for
   registration (the list of characters appears as the combination of
   CONTEXTJ and CONTEXTO in [IDNA200X-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
   [IDNA200X-Rationale].)  Labels Containing Characters Written Right to Left

   Additional special tests for right-to-left strings are applied (See
   [IDNA200X-BIDI].  Strings that contain right to left characters that
   do not conform to the rule identified there MUST NOT be inserted in
   zone files.

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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
   may be rejected.

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 IDNA200X 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 [IDNA200X-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
   prefix included, i.e., the "xn--..." form).

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

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   Section 5.4 ff.), the resolution-side tests are more permissive and
   rely heavily on the assumption that names that are present in the DNS
   are valid.  Among other things, this distinction, applied carefully,
   facilitates expansion of the permitted character lists to include new
   scripts and accommodate new versions of Unicode without introducing
   ambiguity into domain name processing.

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 this
   one and the next one must be accomplished in some way.

5.2.  Conversion to Unicode

   The local character set, character coding conventions, and, as
   necessary, display and presentation conventions, are converted to
   Unicode (without surrogates), paralleling the process described above
   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, at this step, it would be
   reasonable to convert all upper case characters to lower case, if
   this makes sense in the user's environment.

   Other examples of processing for localization that might be applied,
   if appropriate, at this point (but even further outside the scope of
   this specification) include interpreting the KANA MIDDLE DOT as
   separating domain name components from each other, mapping different
   "width" forms of the same character into the one form permitted in
   labels, or giving special treatment to characters whose presentation
   forms are dependent only on placement in the label.

   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 [IDNA200X-Rationale].

   Because these transformations are local, it is important that domain
   names that might be passed between systems (e.g., in IRIs) be

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   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.  Validation and Character List Testing

   In parallel 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 in [IDNA200X-Rationale], the
   resolution check is more liberal than that of 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

   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
      table [IDNA200X-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

   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. [[anchor15: 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

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   when that is feasible -- then DNS resolution failure alone.

   o  Verification that the string is compliant with the requirements
      for right to left characters, specified in [IDNA200X-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.

5.5.  Punycode Conversion

   The validated string, a U-label, is converted to an A-label using the
   punycode algorithm.

5.6.  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 label for each domain

   The current 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,

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   which cannot contain those characters, is the only standard
   representation for internationalized labels in the current DNS

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.

   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 validated against.

   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 Server Considerations

   IDNs are likely to 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 today, so more 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
   [IDNA200X-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 valid according to the base DNS specifications (STD13
   [RFC1034] [RFC1035] and Host Requirements [RFC1123]) because they

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   contain non-ASCII characters.  Those procedures depends on the use of
   a special ACE encoded form, with the encoding specified in [RFC3492],
   that contains only characters permitted in host names by those
   specifications.  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 of privileged or anti-privileged domains.  In 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 U-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

9.  Change Log

   [[anchor22: RFC Editor: Please remove this section.]]

9.1.  Version -00

   Version -00 of this draft was produced in November 2007 by moving
   text from draft-klensin-idnabis-issues and by copy considerable text
   from RFC 3490.  The result was then extensively edited.

9.2.  Versions -01 and -02

   These versions reflected a number of editorial changes, some of them
   significant, and alignment of terminology with

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9.3.  Version -03

   o  Abstract rewritten to bring its length within RFC Editor

   o  Corrections and revisions in response to extensive comments by
      Mark Davis and others.

   o  Small modifications to several operations, including moving the
      Normalization steps to a different place in the sequence.

   o  Many editorial changes.

9.4.  Version -04

   o  Revised terminology and removed the MAYBE category as a
      consequence of design discussions on 30 January 2003 and followup
      conversations.  Also restructed the various operations to treat
      CONTEXTUAL RULE REQUIRED as a validation step (paralleling bidi),
      rather than a category.  Those changes required changes elsewhere
      in the document for consistency.

   o  Changed the requirements for normalization, making this a
      requirement on the calling application rather than an action of
      this protocol.  This is consistent with the general "mappings
      belong somewhere else" principle.

   o  Updated references.

   o  More editorial work, some independent of the changes, described
      immediately above.

10.  Contributors

   While the listed editor held the pen, this document represents the
   joint work and conclusions of an ad hoc design team consisting of the
   editor and, in alphabetic order, Harald Alvestrand, Tina Dam, Patrik
   Faltstrom, and Cary Karp.  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.

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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 [IDNA200X-Rationale] were particularly

12.  References

12.1.  Normative References

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

              Klensin, J., Ed., "Internationalizing Domain Names for
              Applications (IDNA): Issues, Explanation, and Rationale",
              February 2008, <

              Faltstrom, P., "The Unicode Codepoints and IDN",
              February 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.

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

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

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.

   [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.

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

   [Unicode]  The Unicode Consortium, "The Unicode Standard, Version

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              5.0", 2007.

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

Author's Address

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

   Phone: +1 617 245 1457

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