[RFC Home] [TEXT|PDF|HTML] [Tracker] [IPR] [Errata] [Info page]

Errata Exist
Internet Engineering Task Force (IETF)                        J. Klensin
Request for Comments: 5891                                   August 2010
Obsoletes: 3490, 3491
Updates: 3492
Category: Standards Track
ISSN: 2070-1721

    Internationalized Domain Names in Applications (IDNA): Protocol


   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 Internationalized 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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Klensin                      Standards Track                    [Page 1]

RFC 5891                    IDNA2008 Protocol                August 2010

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Klensin                      Standards Track                    [Page 2]

RFC 5891                    IDNA2008 Protocol                August 2010

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  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.  Input to IDNA Registration . . . . . . . . . . . . . . . .  7
     4.2.  Permitted Character and Label Validation . . . . . . . . .  7
       4.2.1.  Input Format . . . . . . . . . . . . . . . . . . . . .  7
       4.2.2.  Rejection of Characters That Are Not Permitted . . . .  8
       4.2.3.  Label Validation . . . . . . . . . . . . . . . . . . .  8
       4.2.4.  Registration Validation Requirements . . . . . . . . .  9
     4.3.  Registry Restrictions  . . . . . . . . . . . . . . . . . .  9
     4.4.  Punycode Conversion  . . . . . . . . . . . . . . . . . . .  9
     4.5.  Insertion in the Zone  . . . . . . . . . . . . . . . . . . 10
   5.  Domain Name Lookup Protocol  . . . . . . . . . . . . . . . . . 10
     5.1.  Label String Input . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Conversion to Unicode  . . . . . . . . . . . . . . . . . . 10
     5.3.  A-label Input  . . . . . . . . . . . . . . . . . . . . . . 10
     5.4.  Validation and Character List Testing  . . . . . . . . . . 11
     5.5.  Punycode Conversion  . . . . . . . . . . . . . . . . . . . 13
     5.6.  DNS Name Resolution  . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2. Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Summary of Major Changes from IDNA2003  . . . . . . . 17

Klensin                      Standards Track                    [Page 3]

RFC 5891                    IDNA2008 Protocol                August 2010

1.  Introduction

   This document supplies the protocol definition for Internationalized
   Domain Names in Applications (IDNA), with the version specified here
   known as IDNA2008.  Essential definitions and terminology for
   understanding this document and a road map of the collection of
   documents that make up IDNA2008 appear in a separate Definitions
   document [RFC5890].  Appendix A discusses the relationship between
   this specification and the earlier version of IDNA (referred to here
   as "IDNA2003").  The rationale for these changes, along with
   considerable explanatory material and advice to zone administrators
   who support IDNs, is provided in another document, known informally
   in this series as the "Rationale document" [RFC5894].

   IDNA works by allowing applications to use certain ASCII [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 change any infrastructure.  In
   particular, IDNA does not depend on any changes to DNS servers,
   resolvers, or DNS protocol elements, because the ASCII name service
   provided by the existing DNS can be used for IDNA.

   IDNA applies only to a specific subset of 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 names.

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

2.  Terminology

   As mentioned above, terminology used as part of the definition of
   IDNA appears in the Definitions document [RFC5890].  It is worth
   noting that some of this terminology overlaps with, and is consistent
   with, that used in Unicode or other character set standards and the
   DNS.  Readers of this document are assumed to be familiar with the
   associated Definitions document and with the DNS-specific terminology
   in RFC 1034 [RFC1034].

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

Klensin                      Standards Track                    [Page 4]

RFC 5891                    IDNA2008 Protocol                August 2010

3.  Requirements and Applicability

3.1.  Requirements

   IDNA makes the following requirements:

   1.  Whenever a domain name is put into a domain name slot that is not
       IDNA-aware (see Section of the Definitions document
       [RFC5890]), it MUST contain only ASCII characters (i.e., its
       labels must be either A-labels or NR-LDH labels), unless the DNS
       application is not subject to historical recommendations for
       "hostname"-style names (see RFC 1034 [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 (however, in
       practice, comparison of U-labels requires first verifying that
       they actually are U-labels and not just Unicode strings).  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.  While
       it is not necessary to validate labels in order to compare them,
       successful comparison does not imply validity.  In many cases,
       not limited to comparison, 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 that do not use the LDH syntax rules as
   described in the Definitions document [RFC5890].

   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 their implementations are explicitly upgraded to
   be aware of IDNs and to accept the U-label form.  IDNs actually
   appearing in DNS queries or responses MUST be A-labels.

Klensin                      Standards Track                    [Page 5]

RFC 5891                    IDNA2008 Protocol                August 2010

   IDNA-aware protocols and implementations MAY accept U-labels,
   A-labels, or both as those particular protocols specify.  IDNA is not
   defined for extended label types (see RFC 2671 [RFC2671], Section 3).

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 the DNS specification
   [RFC1035] 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 labels" more generally) are incompatible
   with IDNA coding as discussed in the Definitions document [RFC5890],
   especially Section  Of course, underscore 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 (e.g., hostmaster@example.com would be
   represented as hostmaster.example.com).  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 model for registering an IDN.  The model 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 (this section) and lookup protocols
   (Section 5) are very similar in most respects, they are not
   identical, and implementers should carefully follow the steps
   described in this specification.

Klensin                      Standards Track                    [Page 6]

RFC 5891                    IDNA2008 Protocol                August 2010

4.1.  Input to IDNA Registration

   Registration processes, especially processing by entities (often
   called "registrars") who deal with registrants before the request
   actually reaches the zone manager ("registry") are outside the scope
   of this definition and may differ significantly depending on local
   needs.  By the time a string enters the IDNA registration process as
   described in this specification, it MUST be in Unicode and in
   Normalization Form C (NFC [Unicode-UAX15]).  Entities responsible for
   zone files ("registries") MUST accept only the exact string for which
   registration is requested, free of any mappings or local adjustments.
   They MAY accept that input in any of three forms:

   1.  As a pair of A-label and U-label.

   2.  As an A-label only.

   3.  As a U-label only.

   The first two of these forms are RECOMMENDED because the use of
   A-labels avoids any possibility of ambiguity.  The first is normally
   preferred over the second because it permits further verification of
   user intent (see Section 4.2.1).

4.2.  Permitted Character and Label Validation

4.2.1.  Input Format

   If both the U-label and A-label forms are available, the registry
   MUST ensure that the A-label form is in lowercase, perform a
   conversion to a U-label, perform the steps and tests described below
   on that U-label, and then verify that the A-label produced by the
   step in Section 4.4 matches the one provided as input.  In addition,
   the U-label that was provided as input 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 only an A-label was provided and 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 encoding [RFC3492] such as the prohibition on trailing
   hyphen-minus, the requirement that all characters be ASCII, and so
   on.  Strings that appear to be A-labels (e.g., they start with
   "xn--") and strings that are supplied to the registry in a context
   reserved for A-labels (such as a field in a form to be filled out),
   but that are not valid A-labels as described in this paragraph, MUST
   NOT be placed in DNS zones that support IDNA.

Klensin                      Standards Track                    [Page 7]

RFC 5891                    IDNA2008 Protocol                August 2010

   If only an A-label is provided, the conversion to a U-label is not
   performed, but the superficial tests described in the previous
   paragraph are performed, registration procedures MAY, and usually
   will, bypass the tests and actions in the balance of Section 4.2 and
   in Sections 4.3 and 4.4.

4.2.2.  Rejection of Characters That Are Not Permitted

   The candidate Unicode string MUST NOT contain characters that appear
   in the "DISALLOWED" and "UNASSIGNED" lists specified in the Tables
   document [RFC5892].

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
   using tests that require examination of more than one character.
   Character order is considered to be the on-the-wire order.  That
   order may not be the same as the display order.  Hyphen Restrictions

   The Unicode string MUST NOT contain "--" (two consecutive hyphens) in
   the third and fourth character positions and MUST NOT start or end
   with a "-" (hyphen).  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 identified as
   CONTEXTJ or CONTEXTO in the Tables document [RFC5892] MUST have a
   non-null rule.  If such a code point is missing a rule, the label is
   invalid.  If the rule exists but the result of applying the rule is
   negative or inconclusive, the proposed label is invalid.  Labels Containing Characters Written Right to Left

   If the proposed label contains any characters from scripts that are
   written from right to left, it MUST meet the Bidi criteria [RFC5893].

Klensin                      Standards Track                    [Page 8]

RFC 5891                    IDNA2008 Protocol                August 2010

4.2.4.  Registration Validation Requirements

   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 4.2.3, and are 63 or fewer characters long in
   ASCII-compatible encoding (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.

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, are expected to establish policies about
   label registrations.  Policies are likely to be informed by the local
   languages and the scripts that are used to write them 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 the Rationale document [RFC5894], Section 3.2, for
   further discussion and recommendations about registry policies.

   The string produced by the steps in Section 4.2 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 (defined in Section of the Definitions document [RFC5890]).  The A-label 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 does not update or alter the
   Punycode algorithm specified in RFC 3492 in any way.  RFC 3492 does
   make a non-normative reference to the information about the value and
   construction of the ACE prefix that appears in RFC 3490 or Nameprep
   [RFC3491].  For consistency and reader convenience, IDNA2008
   effectively updates that reference to point to this document.  That
   change does not alter the prefix itself.  The prefix, "xn--", is the
   same in both sets of documents.

Klensin                      Standards Track                    [Page 9]

RFC 5891                    IDNA2008 Protocol                August 2010

   With the exception of the maximum string length test on Punycode
   output, the failure conditions identified in the Punycode encoding
   procedure cannot occur if the input is a U-label as determined by the
   steps in Sections 4.1 through 4.3 above.

4.5.  Insertion in the Zone

   The label is registered in the DNS by inserting the A-label into a

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 wildcards in the DNS might cause a string that is not
   actually registered in the DNS to be successfully looked up.

5.1.  Label String Input

   The user supplies a string in the local character set, for example,
   by typing it, clicking on it, or copying and pasting it from a
   resource identifier, e.g., a Uniform Resource Identifier (URI)
   [RFC3986] or an Internationalized Resource Identifier (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 one specified in Section 5.2 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 in Unicode.  Depending on local needs, this
   conversion may involve mapping some characters into other characters
   as well as coding conversions.  Those issues are discussed in the
   mapping-related sections (Sections 4.2, 4.4, 6, and 7.3) of the
   Rationale document [RFC5894] and in the separate Mapping document
   [IDNA2008-Mapping].  The result MUST be a Unicode string in NFC form.

5.3.  A-label Input

   If the input to this procedure appears to be an A-label (i.e., it
   starts in "xn--", interpreted case-insensitively), the lookup
   application MAY attempt to convert it to a U-label, first ensuring
   that the A-label is entirely in lowercase (converting it to lowercase

Klensin                      Standards Track                   [Page 10]

RFC 5891                    IDNA2008 Protocol                August 2010

   if necessary), and apply the tests of Section 5.4 and the conversion
   of Section 5.5 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 Section 8.1 of the Rationale document [RFC5894]
   for additional discussion on this topic.

   Conversion from the A-label and testing that the result is a U-label
   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 test 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.

5.4.  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 the Rationale document [RFC5894], 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 Section of the
   Definitions document [RFC5890].  Putative U-labels with any of the
   following characteristics MUST be rejected prior to DNS lookup:

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

   o  Labels containing "--" (two consecutive hyphens) in the third and
      fourth character positions.

   o  Labels whose first character is a combining mark (see The Unicode
      Standard, Section 2.11 [Unicode]).

   o  Labels containing prohibited code points, i.e., those that are
      assigned to the "DISALLOWED" category of the Tables document

   o  Labels containing code points that are identified in the Tables
      document as "CONTEXTJ", i.e., requiring exceptional contextual
      rule processing on lookup, but that do not conform to those rules.
      Note that this implies that a rule must be defined, not null: a

Klensin                      Standards Track                   [Page 11]

RFC 5891                    IDNA2008 Protocol                August 2010

      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

   o  Labels containing code points that are identified in the Tables
      document 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).

   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 the Tables document.

      This requirement means that the application must use a list of
      unassigned characters that is matched to the version of Unicode
      that is being used for the other requirements in this section.  It
      is not required that the application know which version of Unicode
      is being used; that information might be part of the operating
      environment in which the application is running.

   In addition, the application SHOULD apply the following test.

   o  Verification that the string is compliant with the requirements
      for right-to-left characters specified in the Bidi document

   This 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.

   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.

Klensin                      Standards Track                   [Page 12]

RFC 5891                    IDNA2008 Protocol                August 2010

5.5.  Punycode Conversion

   The string that has now been validated for lookup is converted to ACE
   form by applying the Punycode algorithm to the string and then adding
   the ACE prefix ("xn--").

5.6.  DNS Name Resolution

   The A-label resulting from the conversion in Section 5.5 or supplied
   directly (see Section 5.3) is combined with other labels as needed to
   form a fully-qualified domain name that is then looked up in the DNS,
   using normal DNS resolver procedures.  The lookup can obviously
   either succeed (returning information) or fail.

6.  Security Considerations

   Security Considerations for this version of IDNA are described in the
   Definitions document [RFC5890], except for the special issues
   associated with right-to-left scripts and characters.  The latter are
   discussed in the Bidi document [RFC5893].

   In order to avoid intentional or accidental attacks from labels that
   might be confused with others, special problems in rendering, and so
   on, the IDNA model requires that registries exercise care and
   thoughtfulness about what labels they choose to permit.  That issue
   is discussed in Section 4.3 of this document which, in turn, points
   to a somewhat more extensive discussion in the Rationale document

7.  IANA Considerations

   IANA actions for this version of IDNA are specified in the Tables
   document [RFC5892] and discussed informally in the Rationale document
   [RFC5894].  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 especially the contributions of its authors Patrik
   Faltstrom, Paul Hoffman, and Adam Costello.  While Faltstrom was

Klensin                      Standards Track                   [Page 13]

RFC 5891                    IDNA2008 Protocol                August 2010

   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, RFC 4690 [RFC4690], and the Rationale document [RFC5894]
   were particularly important.

   Specific textual changes were incorporated into this document after
   suggestions from the other contributors, Stephane Bortzmeyer, Vint
   Cerf, Lisa Dusseault, Paul Hoffman, Kent Karlsson, James Mitchell,
   Erik van der Poel, Marcos Sanz, Andrew Sullivan, Wil Tan, Ken
   Whistler, Chris Wright, and other WG participants and reviewers
   including Martin Duerst, James Mitchell, Subramanian Moonesamy, Peter
   Saint-Andre, Margaret Wasserman, and Dan Winship who caught specific
   errors and recommended corrections.  Special thanks are due to Paul
   Hoffman for permission to extract material to form the basis for
   Appendix A from a draft document that he prepared.

10.  References

10.1.  Normative References

   [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.

   [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.

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

Klensin                      Standards Track                   [Page 14]

RFC 5891                    IDNA2008 Protocol                August 2010

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

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

                The Unicode Consortium, "Unicode Standard Annex #15:
                Unicode Normalization Forms", September 2009,

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

   [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.

Klensin                      Standards Track                   [Page 15]

RFC 5891                    IDNA2008 Protocol                August 2010

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

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

   [Unicode]    The Unicode Consortium, "The Unicode Standard, Version
                5.0", 2007.  Boston, MA, USA: Addison-Wesley.  ISBN
                0-321-48091-0.  This printed reference has now been
                updated online to reflect additional code points.  For
                code points, the reference at the time this document was
                published is to Unicode 5.2.

Klensin                      Standards Track                   [Page 16]

RFC 5891                    IDNA2008 Protocol                August 2010

Appendix A.  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 code
        points contains" to "decision about code points 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.

   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.

Author's Address

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

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

Klensin                      Standards Track                   [Page 17]