Network Working Group M. Duerst
Internet-Draft W3C
Expires: May 4, 2003 November 3, 2002
Internationalized Domain Names in URIs
draft-ietf-idn-uri-03
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
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
URIs.
The syntax in this document has been chosen to further increase the
uniformity of URI syntax, which is a very important principle of
URIs.
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
interoperability.
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
[RFC2384]).
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
escaped.
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
draft.
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
[IRI])
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Various wording changes
References
[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-
idn-idna-14.txt>.
[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-
drafts/draft-duerst-iri-02.txt>.
[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,
<http://www.ietf.org/internet-drafts/draft-ietf-idn-
nameprep-11.txt>.
[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
1999.
[RFC2732] Hinden, R., Carpenter, B. and L. Masinter, "Format for
Literal IPv6 Addresses in URL's", RFC 2732, December
1999.
Author's Address
Martin Duerst
World Wide Web Consortium
200 Technology Square
Cambridge, MA 02139
U.S.A.
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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