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rfc:rfc5891

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

Abstract

 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
 http://www.rfc-editor.org/info/rfc5891.

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

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 2.3.2.6 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 2.3.2.3.  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
 implementation.
 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.

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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.

4.2.3.1. 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).

4.2.3.2. 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).

4.2.3.3. 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.

4.2.3.4. 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].

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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
 2.3.2.1 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
 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 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

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 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 2.3.2.1 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
    [RFC5892].
 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

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    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 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
    [RFC5893].
 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.

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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
 [RFC5894].

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.
 [Unicode-UAX15]
              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", September 2009,
              <http://www.unicode.org/reports/tr15/>.

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.
 [IDNA2008-Mapping]
              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
      agnostic.
 2.   Separate the definitions for the "registration" and "lookup"
      activities.
 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
      humans".
 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
      protocol.
 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
 USA
 Phone: +1 617 245 1457
 EMail: john+ietf@jck.com

Klensin Standards Track [Page 17]

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