Network Working Group                                         J. Klensin
Internet-Draft                                          October 14, 2008
Obsoletes: 3490 (if approved)
Intended status: Standards Track
Expires: April 17, 2009

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

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 17, 2009.


   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  . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Terminology about 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
       2.3.2.  Punycode is an Algorithm, not a Name . . . . . . . . .  9
   3.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 13
     A.1.  Version -00  . . . . . . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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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 making
   decisions about what names are permitted in DNS zone files and about
   policies related to names and naming.  This document and those
   concerned with the protocol definition, rules for 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 details of interest 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

1.3.  Roadmap of IDNA2008 Documents

   IDNA2008 consists of the following documents:

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   o  This document, containing definitions and other material that are
      needed for understanding other documents in the set.

   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 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.

   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.  That
      specifications obsoletes RFC 3941 and IDN use of the tables to
      which it refers.

2.  Definitions

2.1.  Terminology about Characters and Character Sets

   [[anchor8: Formerly Section 1.5.2 of Rationale-03]]

   A code point is an integer value associated with a character in a
   coded character set.

   Unicode [Unicode51] is a coded character set containing about 100,000
   characters as of the current version.  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 may be thought of as an extension of ASCII; it includes all
   the ASCII characters and associates them with equivalent code points.

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   "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

   [[anchor10: Formerly Section 1.5.3 of Rationale-03.]]

   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, digits, and the hyphen-
   minus; that is, U+002D, 0030..0039, 0041..005A, and 0061..007A. "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 cites 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
   "" 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 "" or implicit as in
   "", 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,

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   the term "label" is shorthand for "text label", and "every label"
   means "every text label".

2.3.  Terminology Specific to IDNA

   [[anchor11: Formerly Section 1.5.4 of Rationale-03 with some material
   removed and left in that document.]]

   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.

   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.

   o  A "U-label" is an IDNA-valid string of Unicode characters,
      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
      constraint below.  Conversions between U-labels and A-labels are
      performed according to the "Punycode" specification [RFC3492],
      adding or removing the ACE prefix as needed.

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   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.  LDH-label and Internationalized Label

   In the hope of further clarifying discussions about IDNs, 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

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   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.  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.
   Traditional LDH labels already have a notion of 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.

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   "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 in this document 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 in this document 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.

2.3.2.  Punycode is an Algorithm, not a Name

   [[anchor17: Formerly Section 1.5.5 of Rationale-03.]]

   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 terms such as "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

   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.

   When systems use local character sets other than ASCII and Unicode,
   these specifications leave the problem of transcoding 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 transcoding rules, they could
   interpret the same name differently and contact different servers.
   This problem is not solved by security protocols, such as TLS, that
   do not take local character sets into account.

   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.  Such
   mechanisms can also be used to show when a name contains a mixture of
   simplified and traditional Chinese characters, or to distinguish zero
   and one from O and 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

   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.

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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

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.

              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

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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", September 2008, <https://

              Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", October 2008, <

              Faltstrom, P., "The Unicode Code Points and IDNA",
              July 2008, <

              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.

   [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.

              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.

Author's Address

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

   Phone: +1 617 245 1457

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

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