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Versions: 00 01 02 03                                                   
Network Working Group                                          M. Duerst
Internet-Draft                                                       W3C
Expires: May 4, 2003                                    November 3, 2002

                  Internationalized Domain Names in URIs

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
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    This Internet-Draft will expire on May 4, 2003.

Copyright Notice

    Copyright (C) The Internet Society (2002).  All Rights Reserved.


    This document proposes to upgrade the definition of URIs (RFC 2396)
    [RFC2396] to work consistently with internationalized domain names.

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Table of Contents

    1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
    2.  URI syntax changes . . . . . . . . . . . . . . . . . . . . . .  3
    3.  Security considerations  . . . . . . . . . . . . . . . . . . .  5
    4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
    5.  Change Log . . . . . . . . . . . . . . . . . . . . . . . . . .  5
    5.1 Changes from draft-ietf-idn-uri-02 to draft-ietf-idn-uri-03  .  5
    5.2 Changes from draft-ietf-idn-uri-01 to draft-ietf-idn-uri-02  .  5
    5.3 Changes from draft-ietf-idn-uri-00 to draft-ietf-idn-uri-01  .  5
        References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
        Author's Address . . . . . . . . . . . . . . . . . . . . . . .  7
        Full Copyright Statement . . . . . . . . . . . . . . . . . . .  8

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1. Introduction

    Internet domain names serve to identify hosts and services on the
    Internet in a convenient way.  The IETF IDN working group [IDNWG] has
    been working on extending the character repertoire usable in domain
    names beyond a subset of US-ASCII.

    One of the most important places where domain names appear are
    Uniform Resource Identifiers (URIs, [RFC2396], as modified by
    [RFC2732]).  However, in the current definition of the generic URI
    syntax, the restrictions on domain names are 'hard-coded'.  In
    Section 2, this document relaxes these restrictions by updating the
    syntax, and defines how internationalized domain names are encoded in

    The syntax in this document has been chosen to further increase the
    uniformity of URI syntax, which is a very important principle of

    In practice, escaped domain names should be used as rarely as
    possible.  Wherever possible, the actual characters in
    Internationalized Domain Names should be preserved as long as
    possible by using IRIs [IRI] rather than URIs, and only converting to
    URIs and then to ACE-encoded [IDNA] domain names (or ideally directly
    to ACE-encoding without even using URIs) when resolving the IRI.
    Also, this document does not exclude the use of ACE encoding directly
    in an URI domain name part.  ACE encoding may be used directly in an
    URI domain name part if this is considered necessary for

    Please note that even with the definition of URIs in [RFC2396], some
    URIs can already contain host names with escaped characters.  For
    example, mailto:example@w%33.org is legal per [RFC2396] because the
    mailto: URI scheme does not follow the generic syntax of [RFC2396].

2. URI syntax changes

    The syntax of URIs [RFC2396] currently contains the following rules
    relevant to domain names:

           hostname      = *( domainlabel "." ) toplabel [ "." ]
           domainlabel   = alphanum | alphanum *( alphanum | "-" ) alphanum
           toplabel      = alpha | alpha *( alphanum | "-" ) alphanum

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    The later two rules are changed as follows:

           domainlabel   = anchar | anchar *( anchar | "-" ) anchar
           toplabel      = achar | achar *( anchar | "-" ) anchar

    and the following rules are added:

                anchar        = alphanum | escaped
                achar         = alpha | escaped

    Characters outside the repertoire (alphanum) are encoded by first
    encoding the characters in UTF-8 [RFC 2279], resulting in a sequence
    of octets, and then escaping these octets according to the rules
    defined in [RFC2396].

    Using UTF-8 assures that this encoding interoperates with IRIs [IRI].
    It is also aligned with the recommendations in [RFC2277] and
    [RFC2718], and is consistent with the URN syntax [RFC2141] as well as
    recent URL scheme definitions that define encodings of non-ASCII
    characters based on UTF-8 (e.g., IMAP URLs [RFC2192] and POP URLs

    The above syntax rules permit for domain names that are neither
    permitted as US-ASCII only domain names nor as internationalized
    domain names.  However, such domain names should never be used, and
    will never be resolved because no such domains will be registered.
    For US-ASCII only domain names, the syntax rules in [RFC2396] are
    relevant.  For example, http://www.w%33.org is legal, because the
    corresponding 'w3' is a legal 'domainlabel' according to [RFC2396].
    However, http://%2a.example.org is illegal because the corresponding
    '*' is not a legal 'domainlabel' according to [RFC2396].

    For domain names containing non-ASCII characters, the legal domain
    names are those for which the ToASCII operation ([IDNA], [Nameprep];
    using the unescaped UTF-8 values as input), with the flags
    "UseSTD3ASCIIRules" and "AllowUnassigned" set, is successful.  The
    URI resolver MUST apply any steps required as part of domain name
    resolution by [IDNA], in particular the ToASCII operation, with the
    above-mentioned flags set.  URIs where the ToASCII operation results
    in an error should be treated as unresolvable.

    For domain names containing non-ASCII characters, the Nameprep
    specification ([Nameprep]) defines some mappings, which mainly
    include normalization to NFKC and folding to lower case.  When
    encoding an internationalized domain name in an URI, these mappings
    SHOULD NOT be applied.  It should be assumed that the domain name is
    already normalized as far as appropriate.

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    For consistency in comparison operations and for interoperability
    with older software, the following should be noted: 1) US-ASCII
    characters in domain names should not be escaped.  2) Because of the
    principle of syntax uniformity for URIs, it is always more prudent to
    take into account the possibility that US-ASCII characters are

3. Security considerations

    The security considerations of [RFC2396] and those applying to
    internationalized domain names apply.  There may be an increased
    potential to smuggle escaped US-ASCII-based domain names across
    firewalls, although because of the uniform syntax principle for URIs,
    such a potential is already existing.

4. Acknowledgements

    Erik Nordmark

5. Change Log

5.1 Changes from draft-ietf-idn-uri-02 to draft-ietf-idn-uri-03

    Clarified expectations on name checking.

5.2 Changes from draft-ietf-idn-uri-01 to draft-ietf-idn-uri-02

    Moved change log to back

    Changed to only change URIs; IRI syntax updated directly in IRI

    Removed syntax restriction on %hh in the US-ASCII part, but made
    clear that restrictions to domain names apply.

    Made clear that escaped domain names in URIs should only be an
    intermediate representation.

    Gave example of mailto: as already allowing escaped host names.

    Corrected some typos.

5.3 Changes from draft-ietf-idn-uri-00 to draft-ietf-idn-uri-01

    Changed requirement for URI/IRI resolvers from MUST to SHOULD

    Changed IRI syntax slightly (ichar -> idchar, based on changes in

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    Various wording changes


    [IDNA]      Faltstrom, P., Hoffman, P. and A. Costello,
                "Internationalizing Domain Names in Applications (IDNA)",
                draft-ietf-idn-idna-14.txt (work in progress), October
                2002, <http://www.ietf.org/internet-drafts/draft-ietf-

    [IDNWG]     "IETF Internationalized Domain Name (idn) Working Group".

    [IRI]       Duerst, M. and M. Suignard, "Internationalized Resource
                Identifiers (IRI)", draft-duerst-iri-02.txt (work in
                progress), November 2002, <http://www.ietf.org/internet-

    [ISO10646]  International Organization for Standardization,
                "Information Technology - Universal Multiple-Octet Coded
                Character Set (UCS) - Part 1: Architecture and Basic
                Multilingual Plane", ISO Standard 10646-1, October 2000.

    [Nameprep]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
                Profile for Internationalized Domain Names", draft-ietf-
                idn-nameprep-11.txt (work in progress), June 2002,

    [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

    [RFC2141]   Moats, R., "URN Syntax", RFC 2141, May 1997.

    [RFC2192]   Newman, C., "IMAP URL Scheme", RFC 2192, September 1997.

    [RFC2277]   Alvestrand, H., "IETF Policy on Character Sets and
                Languages", BCP 18, RFC 2277, January 1998.

    [RFC2279]   Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 2279, January 1998.

    [RFC2384]   Gellens, R., "POP URL Scheme", RFC 2384, August 1998.

    [RFC2396]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
                Resource Identifiers (URI): Generic Syntax", RFC 2396,
                August 1998.

    [RFC2640]   Curtin, B., "Internationalization of the File Transfer

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                Protocol", RFC 2640, July 1999.

    [RFC2718]   Masinter, L., Alvestrand, H., Zigmond, D. and R. Petke,
                "Guidelines for new URL Schemes", RFC 2718, November

    [RFC2732]   Hinden, R., Carpenter, B. and L. Masinter, "Format for
                Literal IPv6 Addresses in URL's", RFC 2732, December

Author's Address

    Martin Duerst
    World Wide Web Consortium
    200 Technology Square
    Cambridge, MA  02139

    Phone: +1 617 253 5509
    Fax:   +1 617 258 5999
    EMail: duerst@w3.org
    URI:   http://www.w3.org/People/D%C3%BCrst/

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Full Copyright Statement

    Copyright (C) The Internet Society (2002).  All Rights Reserved.

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    This document and the information contained herein is provided on an


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