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Network Working Group                                         J. Klensin
Internet-Draft                                         December 11, 2008
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: June 14, 2009

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

Status of this Memo

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   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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  IDNA2008 . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.1.  Audiences  . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.2.  Normative Language . . . . . . . . . . . . . . . . . .  3
     1.2.  Discussion Forum . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Roadmap of IDNA2008 Documents  . . . . . . . . . . . . . .  4
   2.  Definitions and Terminology  . . . . . . . . . . . . . . . . .  4
     2.1.  Characters and Character Sets  . . . . . . . . . . . . . .  4
     2.2.  DNS-related Terminology  . . . . . . . . . . . . . . . . .  5
     2.3.  Terminology Specific to IDNA . . . . . . . . . . . . . . .  6
       2.3.1.  Terms for IDN Label Codings  . . . . . . . . . . . . .  6  IDNA-valid strings, A-label, and U-label . . . . .  6  LDH-label and Internationalized Label  . . . . . .  8  Internationalized Domain Name  . . . . . . . . . .  8  Label Equivalence  . . . . . . . . . . . . . . . .  8  ACE Prefix . . . . . . . . . . . . . . . . . . . .  9  Domain Name Slot . . . . . . . . . . . . . . . . .  9
       2.3.2.  Strings Proposed to be Used or Looked Up as Labels . . 11
       2.3.3.  Order of Characters in Labels  . . . . . . . . . . . . 11
       2.3.4.  Punycode is an Algorithm, not a Name or Adjective  . . 11
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
     4.1.  General Issues . . . . . . . . . . . . . . . . . . . . . . 12
     4.2.  Local Character Set Issues . . . . . . . . . . . . . . . . 12
     4.3.  Visually Similar Characters  . . . . . . . . . . . . . . . 12
     4.4.  IDNA Lookup, Registration, and the Base DNS
           Specifications . . . . . . . . . . . . . . . . . . . . . . 13
     4.5.  Security Differences from IDNA2003 . . . . . . . . . . . . 13
     4.6.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 17
     A.1.  Version -00  . . . . . . . . . . . . . . . . . . . . . . . 17
     A.2.  Version -01  . . . . . . . . . . . . . . . . . . . . . . . 17
     A.3.  Version -02  . . . . . . . . . . . . . . . . . . . . . . . 17
     A.4.  Version -03  . . . . . . . . . . . . . . . . . . . . . . . 18
     A.5.  Version -04  . . . . . . . . . . . . . . . . . . . . . . . 18
     A.6.  Version -05  . . . . . . . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19

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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, described in [RFC3490] and
   [RFC3491].  It continues to use the Punycode algorithm [RFC3492] and
   ACE (ASCII-compatible encoding) prefix from that earlier version.
   The document collection is described in Section 1.3.  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

   o  Making decisions about what names are permitted in DNS zone files

   o  About policies related to names and naming, and

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

   This document and those 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.

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.  Discussion Forum

   [[ RFC Editor: please remove this section. ]]

   IDNA2008 is being discussed in the IETF "idnabis" Working Group and
   on the mailing list idna-update@alvestrand.no

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1.3.  Roadmap 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 [IDNA2008-Rationale] 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.  It is referred to
      informally in other documents in the set as "Rationale".  It is
      not normative.

   o  A document [IDNA2008-Protocol] 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 [IDNA2008-Bidi] that specifies special rules ("Bidi")
      for labels that contain characters that are written from right to

   o  A specification [IDNA2008-Tables] 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 5.1 [Unicode51] 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
      specifications 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".

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 [Unicode51] is a coded character set with about 100,000
   characters assigned to code points as of version 5.1.  A single

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   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 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 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 [Unicode51]).

2.2.  DNS-related Terminology

   When discussing the DNS, this document generally assumes the
   terminology used in the DNS specifications [RFC1034] [RFC1035].  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", similar to common contemporary usage
   in other contexts.  Consequently, any DNS zone administration is
   described as a "registry", regardless of the actual administrative
   arrangements or level in the DNS tree.  More detail about that
   relationship is included in the "Rationale" document.

   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".

   The base DNS specifications [RFC1034] [RFC1035] discuss "domain
   names" and "host names", 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., host name 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

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   usually shown separated by dots; for example, the domain name
   "www.example.com" is composed of three labels: "www", "example", and
   "com".  (The zero-length root label 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 the bitstring ones described in RFC 2673
   [RFC2673]).  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".

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.

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

   To improve clarity, this subsection of the document introduces three
   new terms.  In the next subsection, it defines a historical term to
   be slightly more precise for IDNA contexts.  The relationship among
   these terms and some others is illustrated in Figure 1.

   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, trivially, be ASCII strings that conform to the traditional
      "hostname" (or "LDH") rule and that do not contain "--" as the
      third and fourth character.  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 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 DNS
      including conformance to the rules for the preferred form
      described in RFC 1034, RFC 1035, and RFC 1123.  A string meeting
      that above requirements is still not an A-label unless it can be
      decoded into a U-label.

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   o  A "U-label" is an IDNA-valid string of Unicode characters, in
      normalization form NFC and including at least one non-ASCII
      character, expressed in a standard Unicode Encoding Form -- in an
      Internet transmission context this will normally be UTF-8 -- and
      subject to the constraints about permitted characters that are
      specified in the Protocol and Tables documents as well as the
      symmetry constraint described.  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 must be capable of being produced by conversion from
   a U-label and a U-label must be capable of being produced by
   conversion from an A-label.  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, such as
   rules about lengths of strings, apply to whichever of the U-label or
   A-label would be more restrictive.  For the U-label, constraints
   imposed by existing protocols and their presentation forms make the
   length restriction apply to the length in octets of the UTF-8 form of
   those labels (which will always be greater than or equal to the
   length in code points).  The exception to this, of course, is that
   the restriction to ASCII characters does not apply to the U-label.

   A different way to look at these terms, which may be more clear to
   some readers, is that U-labels, A-labels, and LDH-labels (see the
   next subsection) are disjoint categories that, together, make up the
   forms of legitimate strings for use in domain names that describe
   hosts.  Of the three, only A-labels and LDH-labels can actually
   appear in DNS zone files or queries; U-labels can appear, along with
   the other two, in presentation and user interface forms and in
   selected protocols other than those of the DNS itself.  Strings that
   do not conform to the rules for one of these three categories and, in
   particular, strings that contain "--" in the third and fourth
   character position but are:

   o  not A-labels or

   o  cannot be processed as U-labels or A-labels as described in these

   are invalid in IDNA-conformant applications as labels in domain names
   that identify Internet hosts or similar resources.

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   These specifications use the term "LDH-label" strictly to refer to an
   all-ASCII label that obeys the preferred syntax (often known as
   "hostname" (from RFC 952 [RFC0952]) or "LDH") conventions and that is
   not an IDN.  It should be stressed that an A-label obeys the
   "hostname" rules and is sometimes described as "LDH-conformant", or
   in similar language, but it is not an LDH-label as that term is
   defined in these specifications.  Internationalized Domain Name

   An "internationalized domain name" (IDN) is a domain name that may
   contain any mixture of LDH-labels, A-labels, or U-labels.  This
   implies that every conventional domain name is an IDN (which implies
   that it is possible for a domain name to be an IDN without it
   containing any non-ASCII characters).  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.  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 assumed to have any knowledge of zone-specific
   restrictions or conventions.  See the section on registration policy
   in [IDNA2008-Rationale] 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 LDH-label,
   A-label, or U-label.  There are some standardized DNS label formats,
   such as those 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 [IDNA2008-Protocol] for
   details.  Traditional LDH labels already have a notion of

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   equivalence: within that list of characters, upper case and lower
   case are considered equivalent.  The IDNA notion of equivalence is an
   extension of that older notion.  Equivalent labels in IDNA are
   treated as alternate forms of the same label, just as "foo" and "Foo"
   are treated as alternate forms of the same label.  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;
   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 "IDN-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).

   An "IDN-unaware domain name slot" is defined for this set of
   documents to be any domain name slot that is not an IDN-aware domain
   name slot.  Obviously, this includes any domain name slot whose
   specification predates IDNA.

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   The figure on this page illustrates the relationships among some of
   the terms defined above.  The parenthesized numbers refer to the
   notes below the figure.

              _______________________          _______________________
              | ASCII Labels        |          |  Non-ASCII           |
              |                     |          |                      |
              |  ___________________|          |    __________________|
              |  |LDH-conforming (1)|          |    | U-label (2)     |
              |  |                  |          |    |_________________|
              |  |  ________________|          |    |                 |
              |  |  | LDH-label     |          |    |  Binary Label   |
              |  |  |_______________|          |    | (including      |
              |  |  | A-label       |          |    |  high bit on)   |
              |  |  |_______________|          |    |_________________|
              |  |  |               |          |    |                 |
              |  |  | Broken IDN    |          |    | Bit String      |
              |  |  |   e.g., xn--?,|          |    |   Label         |
              |  |  |      abc--def |          |    |_________________|
              |  |  |_______________|          |______________________|
              |  |__________________|
              |  ___________________|
              |  |Not-LDH-Conforming|
              |  |                  |
              |  |  ________________|
              |  |  |SRV & SRV-like |
              |  |  | e.g., _tcp    |
              |  |  |_______________|
              |  |  | Leading or    |
              |  |  |  trailing     |
              |  |  |  hyphens      |
              |  |  |_______________|
              |  |  | Other non-LDH |
              |  |  |  ASCII chars  |
              |  |  | e.g., #$%&_   |
              |  |  |_______________|
              |  |__________________|

        (1) These subtypes are indistinguishable to IDNA-unaware
        (2) To IDNA-unaware applications, U-labels are
                indistinguishable from Binary ones.

             Figure 1: IDNA and Related DNS Terminology Space

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2.3.2.  Strings Proposed to be Used or Looked Up as Labels

   Strings are encountered at many places in these specifications that
   are expected to be processed as labels of particular types but that
   are not yet fully validated to conform to the requirements for the
   particular type of label in question.  If XYZ is a type of label
   (e.g., "A" for A-label or "U" for a U-label), then the term "putative
   XYZ-label" is used to refer to such a string before it is fully
   validated or tested.

   Similarly, terms similar to "a string in the form of an XYZ-label"
   are used to refer to a string that appears to obey the syntax for an
   XYZ-label on superficial examination.  Specifically, a string that
   would comply with the LDH syntax except that some characters are non-
   ASCII is considered to be in the form of a U-label and one that
   starts in "xn--" and is otherwise all-ASCII is considered to be in
   the form of an A-label.

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
   righmost 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 [RFC3492].  The terms defined above are
   preferred as much more clear than the term "Punycode string".

3.  IANA Considerations

   Actions for IANA are specified in other documents in this series
   [IDNA2008-Protocol] [IDNA2008-Tables].  An overview of the
   relationships among the various IANA registries appears in
   [IDNA2008-Rationale].  This document does not specify any actions for

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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
   servers.  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, 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 might raise and take appropriate measures.  The
   context for this issue is discussed in more detail in

   In addition to the Security Considerations material that appears in
   this document, [IDNA2008-Bidi] contains a discussion of security
   issues specific to labels containing characters from scripts that are
   normally written right to left.

4.2.  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.

4.3.  Visually Similar Characters

   To help prevent confusion between characters that are visually
   similar, it is suggested that implementations provide visual
   indications where a domain name contains multiple scripts (or what
   are considered multiple scripts in a local environment in which some
   mixed-script use is normal).  Such mechanisms can also be used to
   show when a name contains a mixture of simplified and traditional

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   Chinese characters, or to distinguish zero and one from upper-case
   "O" and lower-case "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.  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 eliminate it.  Some specific
   suggestions about identification and handling of confusable
   characters appear in a Unicode Consortium publication

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

   The Protocol specification [IDNA2008-Protocol] describes procedures
   for registering and looking up labels that are not compatible with
   the preferred syntax described in the base DNS specifications (STD13
   [RFC1034] [RFC1035] and Host Requirements [RFC1123]) 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 host names 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
   [IDNA2008-Protocol].  For labels already in ASCII form (i.e., are
   LDH-labels or A-labels), the proper comparison reduces to the same
   case-insensitive ASCII comparison that has always been used for ASCII

   The introduction of IDNA means 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.

4.5.  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

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   to test is strengthened.  For example, putative labels that contain
   unassigned code points will now be rejected, while IDNA2003 permitted
   them (something that is now recognized as a considerable source of
   risk).  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.

4.6.  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 the "rationale" document [IDNA2008-Rationale].
   See the section of this name, and the one entitled "Contributors", in

   Specific textual suggestions after the extraction process came from
   Vint Cerf and Bill McQuillan.

6.  References

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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.

              The Unicode Consortium, "Unicode Standard Annex #15:
              Unicode Normalization Forms", March 2008,

              The Unicode Consortium, "The Unicode Standard, Version
              5.1.0", 2008.

              defined by: The Unicode Standard, Version 5.0, Boston, MA,
              Addison-Wesley, 2007, ISBN 0-321-48091-0, as amended by
              Unicode 5.1.0

6.2.  Informative References

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

              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", November 2008, <https://

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              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", November 2008, <https://datatracker.ietf.org/

              Faltstrom, P., "The Unicode Code Points and IDNA",
              July 2008, <https://datatracker.ietf.org/drafts/

              A version of this document is available in HTML format at

   [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.

   [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.

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

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

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

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

Appendix A.  Change Log

   [[RFC Editor: Please remove this appendix]]

A.1.  Version -00

   This document was created by pulling selected material out of
   draft-ietf-idnabis-rationale-03 ("Rationale") after a WG consensus
   call indicated that the rearrangement was appropriate.  Mark Davis
   made the major contribution of getting the process started by
   identifying particular sections to be moved, even though this draft
   does not completely reflect his list.

   For Version -00 only, each section is identified with the associated
   former section of Rationale-03.  Those sections were edited after
   incorporation into this document, so "Formerly" should be interpreted
   very loosely.

A.2.  Version -01

   o  Typographical errors corrected and some sections slightly renamed
      for clarity.

   o  Other adjustments made to synchronize with current versions of
      "Rationale" and "Protocol".

A.3.  Version -02

   o  All back pointers to section numbers in Rationale have been

   o  Some definitions clarified.  Added one about string order.

   o  Usual small editorial tuning.

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A.4.  Version -03

   o  Additional fine tuning based on discussions during and immediately
      before IETF 72.

A.5.  Version -04

   o  Corrections of text and improvement of definitions based on
      discussions after -03 was released.

   o  Discussion of label comparisons tightened and made more consistent
      with Protocol.

   o  Definitions of categories of labels supplemented with a picture.

   o  Explicit text added (Section 2.3.2) to define strings that look
      like A-labels or U-labels but are not.

A.6.  Version -05

   o  Consolidated Security Considerations sections, moving material
      from Protocol and Rationale here.

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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