Internet Engineering Task Force (IETF)                        J. Klensin
Request for Comments: 5890                                   August 2010
Obsoletes: 3490
Category: Standards Track
ISSN: 2070-1721

        Internationalized Domain Names for Applications (IDNA):
                   Definitions and Document Framework


   This document is one of a collection that, together, describe the
   protocol and usage context for a revision of Internationalized Domain
   Names for Applications (IDNA), superseding the earlier version.  It
   describes the document collection and provides definitions and other
   material that are common to the set.

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 5890                    IDNA Definitions                 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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  IDNA2008 . . . . . . . . . . . . . . . . . . . . . . . . .  4
       1.1.1.  Audiences  . . . . . . . . . . . . . . . . . . . . . .  4
       1.1.2.  Normative Language . . . . . . . . . . . . . . . . . .  5
     1.2.  Road Map of IDNA2008 Documents . . . . . . . . . . . . . .  5
   2.  Definitions and Terminology  . . . . . . . . . . . . . . . . .  6
     2.1.  Characters and Character Sets  . . . . . . . . . . . . . .  6
     2.2.  DNS-Related Terminology  . . . . . . . . . . . . . . . . .  6
     2.3.  Terminology Specific to IDNA . . . . . . . . . . . . . . .  7
       2.3.1.  LDH Label  . . . . . . . . . . . . . . . . . . . . . .  7
       2.3.2.  Terms for IDN Label Codings  . . . . . . . . . . . . . 11  IDNA-valid strings, A-label, and U-label . . . . . 11  NR-LDH Label . . . . . . . . . . . . . . . . . . . 13  Internationalized Domain Name and
                   Internationalized Label  . . . . . . . . . . . . . 13  Label Equivalence  . . . . . . . . . . . . . . . . 14  ACE Prefix . . . . . . . . . . . . . . . . . . . . 14  Domain Name Slot . . . . . . . . . . . . . . . . . 14
       2.3.3.  Order of Characters in Labels  . . . . . . . . . . . . 15
       2.3.4.  Punycode is an Algorithm, Not a Name or Adjective  . . 15
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     4.1.  General Issues . . . . . . . . . . . . . . . . . . . . . . 16
     4.2.  U-label Lengths  . . . . . . . . . . . . . . . . . . . . . 16
     4.3.  Local Character Set Issues . . . . . . . . . . . . . . . . 17
     4.4.  Visually Similar Characters  . . . . . . . . . . . . . . . 17
     4.5.  IDNA Lookup, Registration, and the Base DNS
           Specifications . . . . . . . . . . . . . . . . . . . . . . 18
     4.6.  Legacy IDN Label Strings . . . . . . . . . . . . . . . . . 18
     4.7.  Security Differences from IDNA2003 . . . . . . . . . . . . 19
     4.8.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . 20
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 21

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1.  Introduction

1.1.  IDNA2008

   This document is one of a collection that, together, describe the
   protocol and usage context for a revision of Internationalized Domain
   Names for Applications (IDNA) that was largely completed in 2008,
   known within the series and elsewhere as "IDNA2008".  The series
   replaces an earlier version of IDNA [RFC3490] [RFC3491].  For
   convenience, that version of IDNA is referred to in these documents
   as "IDNA2003".  The newer version continues to use the Punycode
   algorithm [RFC3492] and ACE (ASCII-compatible encoding) prefix from
   that earlier version.  The document collection is described in
   Section 1.2.  As indicated there, this document provides definitions
   and other material that are common to the set.

1.1.1.  Audiences

   While many IETF specifications are directed exclusively to protocol
   implementers, the character of IDNA requires that it be understood
   and properly used by those whose responsibilities include making
   decisions about:

   o  what names are permitted in DNS zone files,

   o  policies related to names and naming, and

   o  the handling of domain name strings in files and systems, even
      with no immediate intention of looking them up.

   This document and those documents concerned with the protocol
   definition, rules for handling strings that include characters
   written right to left, and the actual list of characters and
   categories will be of primary interest to protocol implementers.
   This document and the one containing explanatory material will be of
   primary interest to others, although they may have to fill in some
   details by reference to other documents in the set.

   This document and the associated ones are written from the
   perspective of an IDNA-aware user, application, or implementation.
   While they may reiterate fundamental DNS rules and requirements for
   the convenience of the reader, they make no attempt to be
   comprehensive about DNS principles and should not be considered as a
   substitute for a thorough understanding of the DNS protocols and

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1.1.2.  Normative Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Road Map of IDNA2008 Documents

   IDNA2008 consists of the following documents:

   o  This document, containing definitions and other material that are
      needed for understanding other documents in the set.  It is
      referred to informally in other documents in the set as "Defs" or

   o  A document, RFC 5894 [RFC5894], that provides an overview of the
      protocol and associated tables together with explanatory material
      and some rationale for the decisions that led to IDNA2008.  That
      document also contains advice for registry operations and those
      who use Internationalized Domain Names (IDNs).  It is referred to
      informally in other documents in the set as "Rationale".  It is
      not normative.

   o  A document, RFC 5891 [RFC5891], that describes the core IDNA2008
      protocol and its operations.  In combination with the Bidi
      document, described immediately below, it explicitly updates and
      replaces RFC 3490.  It is referred to informally in other
      documents in the set as "Protocol".

   o  A document, RFC 5893 [RFC5893], that specifies special rules
      (Bidi) for labels that contain characters that are written from
      right to left.

   o  A specification, RFC 5892 [RFC5892], of the categories and rules
      that identify the code points allowed in a label written in native
      character form (defined more specifically as a "U-label" in
      Section below), based on Unicode 5.2 [Unicode52] code
      point assignments and additional rules unique to IDNA2008.  The
      Unicode-based rules are expected to be stable across Unicode
      updates and hence independent of Unicode versions.  That
      specification obsoletes RFC 3941 and IDN use of the tables to
      which it refers.  It is referred to informally in other documents
      in the set as "Tables".

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   o  A document [IDNA2008-Mapping] that discusses the issue of mapping
      characters into other characters and that provides guidance for
      doing so when that is appropriate.  That document, referred to
      informally as "Mapping", provides advice; it is not a required
      part of IDNA.

2.  Definitions and Terminology

2.1.  Characters and Character Sets

   A code point is an integer value in the codespace of a coded
   character set.  In Unicode, these are integers from 0 to 0x10FFFF.

   Unicode [Unicode52] is a coded character set containing somewhat over
   100,000 characters assigned to code points as of version 5.2.  A
   single Unicode code point is denoted in these documents by "U+"
   followed by four to six hexadecimal digits, while a range of Unicode
   code points is denoted by two four to six digit hexadecimal numbers
   separated by "..", with no prefixes.

   ASCII means US-ASCII [ASCII], a coded character set containing 128
   characters associated with code points in the range 0000..007F.
   Unicode is a superset of ASCII and may be thought of as a
   generalization of it; it includes all the ASCII characters and
   associates them with the equivalent code points.

   "Letters" are, informally, generalizations from the ASCII and
   common-sense understanding of that term, i.e., characters that are
   used to write text and that are not digits, symbols, or punctuation.
   Formally, they are characters with a Unicode General Category value
   starting in "L" (see Section 4.5 of The Unicode Standard

2.2.  DNS-Related Terminology

   When discussing the DNS, this document generally assumes the
   terminology used in the DNS specifications [RFC1034] [RFC1035] as
   subsequently modified [RFC1123] [RFC2181].  The term "lookup" is used
   to describe the combination of operations performed by the IDNA2008
   protocol and those actually performed by a DNS resolver.  The process
   of placing an entry into the DNS is referred to as "registration".
   This is similar to common contemporary usage of that term in other
   contexts.  Consequently, any DNS zone administration is described as
   a "registry", and the terms "registry" and "zone administrator" are
   used interchangeably, regardless of the actual administrative
   arrangements or level in the DNS tree.  More details about that
   relationship are included in the Rationale document.

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   The term "LDH code point" is defined in this document to refer to the
   code points associated with ASCII letters (Unicode code points
   0041..005A and 0061..007A), digits (0030..0039), and the hyphen-minus
   (U+002D).  "LDH" is an abbreviation for "letters, digits, hyphen" but
   is used specifically in this document to refer to the set of naming
   rules described in Section 2.3.1 below.

   The base DNS specifications [RFC1034] [RFC1035] discuss "domain
   names" and "hostnames", but many people use the terms
   interchangeably, as do sections of these specifications.  Lack of
   clarity about that terminology has contributed to confusion about
   intent in some cases.  These documents generally use the term "domain
   name".  When they refer to, e.g., hostname syntax restrictions, they
   explicitly cite the relevant defining documents.  The remaining
   definitions in this subsection are essentially a review: if there is
   any perceived difference between those definitions and the
   definitions in the base DNS documents or those cited below, the
   definitions in the other documents take precedence.

   A label is an individual component of a domain name.  Labels are
   usually shown separated by dots; for example, the domain name
   "www.example.com" is composed of three labels: "www", "example", and
   "com".  (The complete name convention using a trailing dot described
   in RFC 1123 [RFC1123], which can be explicit as in "www.example.com."
   or implicit as in "www.example.com", is not considered in this
   specification.)  IDNA extends the set of usable characters in labels
   that are treated as text (as distinct from the binary string labels
   discussed in RFC 1035 and RFC 2181 [RFC2181] and bitstring ones
   [RFC2673]), but only in certain contexts.  The different contexts for
   different sets of usable characters are outlined in the next section.
   For the rest of this document and in the related ones, the term
   "label" is shorthand for "text label", and "every label" means "every
   text label", including the expanded context.

2.3.  Terminology Specific to IDNA

   This section defines some terminology to reduce dependence on terms
   and definitions that have been problematic in the past.  The
   relationships among these definitions are illustrated in Figure 1 and
   Figure 2.  In the first of those figures, the parenthesized numbers
   refer to the notes below the figure.

2.3.1.  LDH Label

   This is the classical label form used, albeit with some additional
   restrictions, in hostnames [RFC0952].  Its syntax is identical to
   that described as the "preferred name syntax" in Section 3.5 of RFC
   1034 [RFC1034] as modified by RFC 1123 [RFC1123].  Briefly, it is a

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   string consisting of ASCII letters, digits, and the hyphen with the
   further restriction that the hyphen cannot appear at the beginning or
   end of the string.  Like all DNS labels, its total length must not
   exceed 63 octets.

   LDH labels include the specialized labels used by IDNA (described as
   "A-labels" below) and some additional restricted forms (also
   described below).

   To facilitate clear description, two new subsets of LDH labels are
   created by the introduction of IDNA.  These are called Reserved LDH
   labels (R-LDH labels) and Non-Reserved LDH labels (NR-LDH labels).
   Reserved LDH labels, known as "tagged domain names" in some other
   contexts, have the property that they contain "--" in the third and
   fourth characters but which otherwise conform to LDH label rules.
   Only a subset of the R-LDH labels can be used in IDNA-aware
   applications.  That subset consists of the class of labels that begin
   with the prefix "xn--" (case independent), but otherwise conform to
   the rules for LDH labels.  That subset is called "XN-labels" in this
   set of documents.  XN-labels are further divided into those whose
   remaining characters (after the "xn--") are valid output of the
   Punycode algorithm [RFC3492] and those that are not (see below).  The
   XN-labels that are valid Punycode output are known as "A-labels" if
   they also meet the other criteria for IDNA-validity described below.
   Because LDH labels (and, indeed, any DNS label) must not be more than
   63 octets in length, the portion of an XN-label derived from the
   Punycode algorithm is limited to no more than 59 ASCII characters.
   Non-Reserved LDH labels are the set of valid LDH labels that do not
   have "--" in the third and fourth positions.

   A consequence of the restrictions on valid characters in the native
   Unicode character form (see U-labels) turns out to be that mixed-case
   annotation, of the sort outlined in Appendix A of RFC 3492 [RFC3492],
   is never useful.  Therefore, since a valid A-label is the result of
   Punycode encoding of a U-label, A-labels should be produced only in
   lowercase, despite matching other (mixed-case or uppercase) potential
   labels in the DNS.

   Some strings that are prefixed with "xn--" to form labels may not be
   the output of the Punycode algorithm, may fail the other tests
   outlined below, or may violate other IDNA restrictions and thus are
   also not valid IDNA labels.  They are called "Fake A-labels" for

   Labels within the class of R-LDH labels that are not prefixed with
   "xn--" are also not valid IDNA labels.  To allow for future use of
   mechanisms similar to IDNA, those labels MUST NOT be processed as

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   ordinary LDH labels by IDNA-conforming programs and SHOULD NOT be
   mixed with IDNA labels in the same zone.

   These distinctions among possible LDH labels are only of significance
   for software that is IDNA-aware or for future extensions that use
   extensions based on the same "prefix and encoding" model.  For
   IDNA-aware systems, the valid label types are: A-labels, U-labels,
   and NR-LDH labels.

   IDNA labels come in two flavors: an ACE-encoded form and a Unicode
   (native character) form.  These are referred to as A-labels and
   U-labels, respectively, and are described in detail in the next

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                                    ASCII Label
      |                                                                |
      |     ____________________ LDH Label (1) (4) ________________    |
      |    |  ___________________________________                  |   |
      |    |  |IDN Reserved LDH Labels          |                  |   |
      |    |  | ("??--") or R-LDH Labels        | _______________  |   |
      |    |  | _______________________________ | |NON-RESERVED |  |   |
      |    |  | |       XN-labels             | | | LDH Labels  |  |   |
      |    |  | | _____________   ___________ | | | (NR-LDH     |  |   |
      |    |  | | | A-labels  |   | Fake (3) || | |   labels)   |  |   |
      |    |  | | | "xn--"(2) |   | A-labels || | |_____________|  |   |
      |    |  | | |___________|   |__________|| |                  |   |
      |    |  | |_____________________________| |                  |   |
      |    |  |_________________________________|                  |   |
      |    |_______________________________________________________|   |
      |                                                                |
      |       _____________NON-LDH label________                       |
      |       |      ______________________    |                       |
      |       |      | Underscore labels  |    |                       |
      |       |      |  e.g., _tcp        |    |                       |
      |       |      |____________________|    |                       |
      |       |      | Labels with leading|    |                       |
      |       |      | or trailing        |    |                       |
      |       |      | hyphens "-abcd"    |    |                       |
      |       |      | or "xyz-"          |    |                       |
      |       |      | or "-uvw-"         |    |                       |
      |       |      |____________________|    |                       |
      |       |      | Labels with other  |    |                       |
      |       |      | non-LDH ASCII chars|    |                       |
      |       |      | e.g., #$%_         |    |                       |
      |       |      |____________________|    |                       |
      |       |________________________________|                       |

             (1) ASCII letters (uppercase and lowercase), digits,
                    hyphen.  Hyphen may not appear in first or last
                    position.  No more than 63 octets.
             (2) Note that the string following "xn--" must
                    be the valid output of the Punycode algorithm
                    and must be convertible into valid U-label form.
             (3) Note that a Fake A-label has a prefix "xn--"
                    but the remainder of the label is NOT the valid
                    output of the Punycode algorithm.
             (4) LDH label subtypes are indistinguishable to
                    applications that are not IDNA-aware.

    Figure 1: IDNA and Related DNS Terminology Space -- ASCII Labels

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                        |  Non-ASCII             |
                        |                        |
                        |    ___________________ |
                        |    | U-label (5)     | |
                        |    |_________________| |
                        |    |                 | |
                        |    |  Binary Label   | |
                        |    | (including      | |
                        |    |  high bit on)   | |
                        |    |_________________| |
                        |    |                 | |
                        |    | Bit String      | |
                        |    |   Label         | |
                        |    |_________________| |

             (5) To applications that are not IDNA-aware, U-labels
                    are indistinguishable from Binary ones.

                        Figure 2: Non-ASCII Labels

2.3.2.  Terms for IDN Label Codings  IDNA-valid strings, A-label, and U-label

   For IDNA-aware applications, the three types of valid labels are
   "A-labels", "U-labels", and "NR-LDH labels", each of which is defined
   below.  The relationships among them are illustrated in Figure 1 and
   Figure 2.

   o  A string is "IDNA-valid" if it meets all of the requirements of
      these specifications for an IDNA label.  IDNA-valid strings may
      appear in either of the two forms defined immediately below, or
      may be drawn from the NR-LDH label subset.  IDNA-valid strings
      must also conform to all basic DNS requirements for labels.  These
      documents make specific reference to the form appropriate to any
      context in which the distinction is important.

   o  An "A-label" is the ASCII-Compatible Encoding (ACE, see
      Section form of an IDNA-valid string.  It must be a
      complete label: IDNA is defined for labels, not for parts of them
      and not for complete domain names.  This means, by definition,
      that every A-label will begin with the IDNA ACE prefix, "xn--"
      (see Section, followed by a string that is a valid output
      of the Punycode algorithm [RFC3492] and hence a maximum of 59
      ASCII characters in length.  The prefix and string together must
      conform to all requirements for a label that can be stored in the

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      DNS including conformance to the rules for LDH labels
      (Section 2.3.1).  If and only if a string meeting the above
      requirements can be decoded into a U-label is it an A-label.

   o  A "U-label" is an IDNA-valid string of Unicode characters, in
      Normalization Form C (NFC) and including at least one non-ASCII
      character, expressed in a standard Unicode Encoding Form (such as
      UTF-8).  It is also subject to the constraints about permitted
      characters that are specified in Section 4.2 of the Protocol
      document and the rules in the Sections 2 and 3 of the Tables
      document, the Bidi constraints in that document if it contains any
      character from scripts that are written right to left, and the
      symmetry constraint described immediately below.  Conversions
      between U-labels and A-labels are performed according to the
      "Punycode" specification [RFC3492], adding or removing the ACE
      prefix as needed.

   To be valid, U-labels and A-labels must obey an important symmetry
   constraint.  While that constraint may be tested in any of several
   ways, an A-label A1 must be capable of being produced by conversion
   from a U-label U1, and that U-label U1 must be capable of being
   produced by conversion from A-label A1.  Among other things, this
   implies that both U-labels and A-labels must be strings in Unicode
   NFC [Unicode-UAX15] normalized form.  These strings MUST contain only
   characters specified elsewhere in this document series, and only in
   the contexts indicated as appropriate.

   Any rules or conventions that apply to DNS labels in general apply to
   whichever of the U-label or A-label would be more restrictive.  There
   are two exceptions to this principle.  First, the restriction to
   ASCII characters does not apply to the U-label.  Second, expansion of
   the A-label form to a U-label may produce strings that are much
   longer than the normal 63 octet DNS limit (potentially up to 252
   characters) due to the compression efficiency of the Punycode
   algorithm.  Such extended-length U-labels are valid from the
   standpoint of IDNA, but caution should be exercised as shorter limits
   may be imposed by some applications.

   For context, applications that are not IDNA-aware treat all LDH
   labels as valid for appearance in DNS zone files and queries and some
   of them may permit additional types of labels (i.e., not impose the
   LDH restriction).  IDNA-aware applications permit only A-labels and
   NR-LDH labels to appear in zone files and queries.  U-labels can
   appear, along with the other two, in presentation and user interface
   forms, and in protocols that use IDNA forms but that do not involve
   the DNS itself.

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   Specifically, for IDNA-aware applications and contexts, the three
   allowed categories are A-label, U-label, and NR-LDH label.  Of the
   Reserved LDH labels (R-LDH labels) only A-labels are valid for IDNA

   Strings that appear to be A-labels or U-labels are processed in
   various operations of the Protocol document [RFC5891].  Those strings
   are not yet demonstrably conformant with the conditions outlined
   above because they are in the process of validation.  Such strings
   may be referred to as "unvalidated", "putative", or "apparent", or as
   being "in the form of" one of the label types to indicate that they
   have not been verified to meet the specified conformance

   Unvalidated A-labels are known only to be XN-labels, while Fake
   A-labels have been demonstrated to fail some of the A-label tests.
   Similarly, unvalidated U-labels are simply non-ASCII labels that may
   or may not meet the requirements for U-labels.  NR-LDH Label

   These specifications use the term "NR-LDH label" strictly to refer to
   an all-ASCII label that obeys the LDH label syntax discussed in
   Section 2.3.1 and that is neither an IDN nor a label form reserved by
   IDNA (R-LDH label).  It should be stressed that all A-labels obey the
   "hostname" [RFC0952] rules other than the length restriction in those
   rules.  Internationalized Domain Name and Internationalized Label

   An "internationalized domain name" (IDN) is a domain name that
   contains at least one A-label or U-label, but that otherwise may
   contain any mixture of NR-LDH labels, A-labels, or U-labels.  Just as
   has been the case with ASCII names, some DNS zone administrators may
   impose restrictions, beyond those imposed by DNS or IDNA, on the
   characters or strings that may be registered as labels in their
   zones.  Because of the diversity of characters that can be used in a
   U-label and the confusion they might cause, such restrictions are
   mandatory for IDN registries and zones even though the particular
   restrictions are not part of these specifications (the issue is
   discussed in more detail in Section 4.3 of the Protocol document
   [RFC5891].  Because these restrictions, commonly known as "registry
   restrictions", only affect what can be registered and not lookup
   processing, they have no effect on the syntax or semantics of DNS
   protocol messages; a query for a name that matches no records will
   yield the same response regardless of the reason why it is not in the
   zone.  Clients issuing queries or interpreting responses cannot be

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   assumed to have any knowledge of zone-specific restrictions or
   conventions.  See the section on registration policy in the Rationale
   document [RFC5894] for additional discussion.

   "Internationalized label" is used when a term is needed to refer to a
   single label of an IDN, i.e., one that might be any of an NR-LDH
   label, A-label, or U-label.  There are some standardized DNS label
   formats, such as the "underscore labels" used for service location
   (SRV) records [RFC2782], that do not fall into any of the three
   categories and hence are not internationalized labels.  Label Equivalence

   In IDNA, equivalence of labels is defined in terms of the A-labels.
   If the A-labels are equal in a case-independent comparison, then the
   labels are considered equivalent, no matter how they are represented.
   Because of the isomorphism of A-labels and U-labels in IDNA2008, it
   is possible to compare U-labels directly; see the Protocol document
   [RFC5891] for details.  Traditional LDH labels already have a notion
   of equivalence: within that list of characters, uppercase and
   lowercase are considered equivalent.  The IDNA notion of equivalence
   is an extension of that older notion but, because the protocol does
   not specify any mandatory mapping and only those isomorphic forms are
   considered, the only equivalents are:

   o  Exact (bit-string identity) matches between a pair of U-labels.

   o  Matches between a pair of A-labels, using normal DNS
      case-insensitive matching rules.

   o  Equivalence between a U-label and an A-label determined by
      translating the U-label form into an A-label form and then testing
      for a match between the A-labels using normal DNS case-insensitive
      matching rules.  ACE Prefix

   The "ACE prefix" is defined in this document to be a string of ASCII
   characters, "xn--", that appears at the beginning of every A-label.
   "ACE" stands for "ASCII-Compatible Encoding".  Domain Name Slot

   A "domain name slot" is defined in this document to be a protocol
   element or a function argument or a return value (and so on)
   explicitly designated for carrying a domain name.  Examples of domain
   name slots include the QNAME field of a DNS query; the name argument
   of the gethostbyname() or getaddrinfo() standard C library functions;

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   the part of an email address following the at sign ("@") in the
   parameter to the SMTP MAIL or RCPT commands or the "From:" field of
   an email message header; and the host portion of the URI in the "src"
   attribute of an HTML "<IMG>" tag.  A string that has the syntax of a
   domain name but that appears in general text is not in a domain name
   slot.  For example, a domain name appearing in the plain text body of
   an email message is not occupying a domain name slot.

   An "IDNA-aware domain name slot" is defined for this set of documents
   to be a domain name slot explicitly designated for carrying an
   internationalized domain name as defined in this document.  The
   designation may be static (for example, in the specification of the
   protocol or interface) or dynamic (for example, as a result of
   negotiation in an interactive session).

   Name slots that are not IDNA-aware obviously include any domain name
   slot whose specification predates IDNA.  Note that the requirements
   of some protocols that use the DNS for data storage prevent the use
   of IDNs.  For example, the format required for the underscore labels
   used by the service location protocol [RFC2782] precludes
   representation of a non-ASCII label in the DNS using A-labels because
   those SRV-related labels must start with underscores.  Of course,
   non-ASCII IDN labels may be part of a domain name that also includes
   underscore labels.

2.3.3.  Order of Characters in Labels

   Because IDN labels may contain characters that are read, and
   preferentially displayed, from right to left, there is a potential
   ambiguity about which character in a label is "first".  For the
   purposes of these specifications, labels are considered, and
   characters numbered, strictly in the order in which they appear "on
   the wire".  That order is equivalent to the leftmost character being
   treated as first in a label that is read left to right and to the
   rightmost character being first in a label that is read right to
   left.  The Bidi specification contains additional discussion of the
   conditions that influence reading order.

2.3.4.  Punycode is an Algorithm, Not a Name or Adjective

   There has been some confusion about whether a "Punycode string" does
   or does not include the ACE prefix and about whether it is required
   that such strings could have been the output of the ToASCII operation
   (see RFC 3490, Section 4 [RFC3490]).  This specification discourages
   the use of the term "Punycode" to describe anything but the encoding
   method and algorithm of RFC 3492 [RFC3492].  The terms defined above
   are preferred as much more clear than the term "Punycode string".

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3.  IANA Considerations

   IANA actions for this version of IDNA (IDNA2008) are specified in the
   Tables document [RFC5892].  An overview of the relationships among
   the various IANA registries appears in the Rationale document
   [RFC5894].  This document does not specify any actions for IANA.

4.  Security Considerations

4.1.  General Issues

   Security on the Internet partly relies on the DNS.  Thus, any change
   to the characteristics of the DNS can change the security of much of
   the Internet.

   Domain names are used by users to identify and connect to Internet
   hosts and other network resources.  The security of the Internet is
   compromised if a user entering a single internationalized name is
   connected to different servers based on different interpretations of
   the internationalized domain name.  In addition to characters that
   are permitted by IDNA2003 and its mapping conventions (see
   Section 4.6), the current specification changes the interpretation of
   a few characters that were mapped to others in the earlier version;
   zone administrators should be aware of the problems that this might
   raise and take appropriate measures.  The context for this issue is
   discussed in more detail in the Rationale document [RFC5894].

   In addition to the Security Considerations material that appears in
   this document, the Bidi document [RFC5893] contains a discussion of
   security issues specific to labels containing characters from scripts
   that are normally written right to left.

4.2.  U-label Lengths

   Labels associated with the DNS have traditionally been limited to 63
   octets by the general restrictions in RFC 1035 and by the need to
   treat them as a six-bit string length followed by the string in
   actual calls to the DNS.  That format is used in some other
   applications and, in general, that representations of domain names as
   dot-separated labels and as length-string pairs have been treated as
   interchangeable.  Because A-labels (the form actually used in the
   DNS) are potentially much more compressed than UTF-8 (and UTF-8 is,
   in general, more compressed that UTF-16 or UTF-32), U-labels that
   obey all of the relevant symmetry (and other) constraints of these
   documents may be quite a bit longer, potentially up to 252 characters
   (Unicode code points).  A fully-qualified domain name containing
   several such labels can obviously also exceed the nominal 255 octet

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   limit for such names.  Application authors using U-labels must exert
   due caution to avoid buffer overflow and truncation errors and
   attacks in contexts where shorter strings are expected.

4.3.  Local Character Set Issues

   When systems use local character sets other than ASCII and Unicode,
   these specifications leave the problem of converting between the
   local character set and Unicode up to the application or local
   system.  If different applications (or different versions of one
   application) implement different rules for conversions among coded
   character sets, they could interpret the same name differently and
   contact different servers.  This problem is not solved by security
   protocols, such as Transport Layer Security (TLS) [RFC5246], that do
   not take local character sets into account.

4.4.  Visually Similar Characters

   To help prevent confusion between characters that are visually
   similar (sometimes called "confusables"), it is suggested that
   implementations provide visual indications where a domain name
   contains multiple scripts, especially when the scripts contain
   characters that are easily confused visually, such as an omicron in
   Greek mixed with Latin text.  Such mechanisms can also be used to
   show when a name contains a mixture of Simplified Chinese characters
   with Traditional ones that have Simplified forms, or to distinguish
   zero and one from uppercase "O" and lowercase "L".  DNS zone
   administrators may impose restrictions (subject to the limitations
   identified elsewhere in these documents) that try to minimize
   characters that have similar appearance or similar interpretations.

   If multiple characters appear in a label and the label consists only
   of characters in one script, individual characters that might be
   confused with others if compared separately may be unambiguous and
   non-confusing.  On the other hand, that observation makes labels
   containing characters from more than one script (often called "mixed-
   script labels") even more risky -- users will tend to see what they
   expect to see and context is a powerful reinforcement to perception.
   At the same time, while the risks associated with mixed-script labels
   are clear, simply prohibiting them will not eliminate problems,
   especially where closely related scripts are involved.  For example,
   there are many strings that are entirely in Greek or Cyrillic scripts
   that can be confused with each other or with Latin script strings.

   It is worth noting that there are no comprehensive technical
   solutions to the problems of confusable characters.  One can reduce
   the extent of the problems in various ways, but probably never

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   eliminate it.  Some specific suggestions about identification and
   handling of confusable characters appear in a Unicode Consortium
   publication [Unicode-UTR36].

4.5.  IDNA Lookup, Registration, and the Base DNS Specifications

   The Protocol specification [RFC5891] describes procedures for
   registering and looking up labels that are not compatible with the
   preferred syntax described in the base DNS specifications (see
   Section 2.3.1) because they contain non-ASCII characters.  These
   procedures depend on the use of a special ASCII-compatible encoding
   form that contains only characters permitted in hostnames by those
   earlier specifications.  The encoding used is Punycode [RFC3492].  No
   security issues such as string length increases or new allowed values
   are introduced by the encoding process or the use of these encoded
   values, apart from those introduced by the ACE encoding itself.

   Domain names (or portions of them) are sometimes compared against a
   set of domains to be given special treatment if a match occurs, e.g.,
   treated as more privileged than others or blocked in some way.  In
   such situations, it is especially important that the comparisons be
   done properly, as specified in the "Requirements" section of the
   Protocol document [RFC5891].  For labels already in ASCII form, the
   proper comparison reduces to the same case-insensitive ASCII
   comparison that has always been used for ASCII labels although
   IDNA-aware applications are expected to look up only A-labels and
   NR-LDH labels, i.e., to avoid looking up R-LDH labels that are not

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

4.6.  Legacy IDN Label Strings

   The URI Standard [RFC3986] and a number of application specifications
   (e.g., SMTP [RFC5321] and HTTP [RFC2616]) do not permit non-ASCII
   labels in DNS names used with those protocols, i.e., only the A-label
   form of IDNs is permitted in those contexts.  If only A-labels are
   used, differences in interpretation between IDNA2003 and this version
   arise only for characters whose interpretation have actually changed
   (e.g., characters, such as ZWJ and ZWNJ, that were mapped to nothing
   in IDNA2003 and that are considered legitimate in some contexts by
   these specifications).  Despite that prohibition, there are a
   significant number of files and databases on the Internet in which

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   domain name strings appear in native-character form; a subset of
   those strings use native-character labels that require IDNA2003
   mapping to produce valid A-labels.  The treatment of such labels will
   vary by types of applications and application-designer preference: in
   some situations, warnings to the user or outright rejection may be
   appropriate; in others, it may be preferable to attempt to apply the
   earlier mappings if lookup strictly conformant to these
   specifications fails or even to do lookups under both sets of rules.
   This general situation is discussed in more detail in the Rationale
   document [RFC5894].  However, in the absence of care by registries
   about how strings that could have different interpretations under
   IDNA2003 and the current specification are handled, it is possible
   that the differences could be used as a component of name-matching or
   name-confusion attacks.  Such care is therefore appropriate.

4.7.  Security Differences from IDNA2003

   The registration and lookup models described in this set of documents
   change the mechanisms available for lookup applications to determine
   the validity of labels they encounter.  In some respects, the ability
   to test is strengthened.  For example, putative labels that contain
   unassigned code points will now be rejected, while IDNA2003 permitted
   them (see the Rationale document [RFC5894] for a discussion of the
   reasons for this).  On the other hand, the Protocol specification no
   longer assumes that the application that looks up a name will be able
   to determine, and apply, information about the protocol version used
   in registration.  In theory, that may increase risk since the
   application will be able to do less pre-lookup validation.  In
   practice, the protection afforded by that test has been largely
   illusory for reasons explained in RFC 4690 [RFC4690] and elsewhere in
   these documents.

   Any change to the Stringprep [RFC3454] procedure that is profiled and
   used in IDNA2003, or, more broadly, the IETF's model of the use of
   internationalized character strings in different protocols, creates
   some risk of inadvertent changes to those protocols, invalidating
   deployed applications or databases, and so on.  But these
   specifications do not change Stringprep at all; they merely bypass
   it.  Because these documents do not depend on Stringprep, the
   question of upgrading other protocols that do have that dependency
   can be left to experts on those protocols: the IDNA changes and
   possible upgrades to security protocols or conventions are
   independent issues.

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4.8.  Summary

   No mechanism involving names or identifiers alone can protect against
   a wide variety of security threats and attacks that are largely
   independent of the naming or identification system.  These attacks
   include spoofed pages, DNS query trapping and diversion, and so on.

5.  Acknowledgments

   The initial version of this document was created largely by
   extracting text from early draft versions of the Rationale document
   [RFC5894].  See the section of this name and the one entitled
   "Contributors", in it.

   Specific textual suggestions after the extraction process came from
   Vint Cerf, Lisa Dusseault, Bill McQuillan, Andrew Sullivan, and Ken
   Whistler.  Other changes were made in response to more general
   comments, lists of concerns or specific errors from participants in
   the Working Group and other observers, including Lyman Chapin, James
   Mitchell, Subramanian Moonesamy, and Dan Winship.

6.  References

6.1.  Normative 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.

   [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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                The Unicode Consortium, "Unicode Standard Annex #15:
                Unicode Normalization Forms, Revision 31",
                September 2009,

   [Unicode52]  The Unicode Consortium.  The Unicode Standard, Version
                5.2.0, defined by: "The Unicode Standard, Version
                5.2.0", (Mountain View, CA: The Unicode Consortium,
                2009. ISBN 978-1-936213-00-9).

6.2.  Informative References

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

   [RFC0952]    Harrenstien, K., Stahl, M., and E. Feinler, "DoD
                Internet host table specification", RFC 952,
                October 1985.

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

   [RFC2616]    Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2673]    Crawford, M., "Binary Labels in the Domain Name System",
                RFC 2673, August 1999.

   [RFC2782]    Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
                specifying the location of services (DNS SRV)",
                RFC 2782, February 2000.

   [RFC3454]    Hoffman, P. and M. Blanchet, "Preparation of
                Internationalized Strings ("stringprep")", RFC 3454,
                December 2002.

   [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.

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

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

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

   [RFC5246]    Dierks, T. and E. Rescorla, "The Transport Layer
                Security (TLS) Protocol Version 1.2", RFC 5246,
                August 2008.

   [RFC5321]    Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
                October 2008.

   [RFC5891]    Klensin, J., "Internationalized Domain Names in
                Applications (IDNA): Protocol", RFC 5891, August 2010.

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

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

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

                The Unicode Consortium, "Unicode Technical Report #36:
                Unicode Security Considerations, Revision 7", July 2008,

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Author's Address

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

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

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