Internet Draft M. Duerst
<draft-duerst-dns-i18n-02.txt> Keio University
Expires in six months July 1998
Internationalization of Domain Names
Status of this Memo
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Abstract
Internet domain names are currently limited to a very restricted
character set. This document proposes the introduction of a new
"zero-level" domain (ZLD) to allow the use of arbitrary characters
from the Universal Character Set (ISO 10646/Unicode) in domain names.
The proposal is fully backwards compatible and does not need any
changes to DNS. Version 02 is reissued without changes just to
keep this draft available.
Table of contents
0. Change History ................................................. 2
0.8 Changes Made from Version 01 to Version 02 .................. 2
0.9 Changes Made from Version 00 to Version 01 .................. 2
1. Introduction ................................................... 3
1.1 Motivation .................................................. 3
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1.2 Notational Conventions ...................................... 4
2. The Hidden Zero Level Domain ................................... 4
3. Encoding International Characters .............................. 5
3.1 Encoding Requirements ....................................... 5
3.2 Encoding Definition ......................................... 5
3.3 Encoding Example ............................................ 7
3.4 Length Considerations ....................................... 8
4. Usage Considerations ........................................... 8
4.1 General Usage ............................................... 8
4.2 Usage Restrictions .......................................... 9
4.3 Domain Name Creation ....................................... 10
4.4 Usage in URLs .............................................. 12
5. Alternate Proposals ........................................... 13
5.1 The Dillon Proposal ........................................ 13
5.2 Using a Separate Lookup Service ............................ 13
6. Generic Considerations ........................................ 14
5.1 Security Considerations .................................... 14
5.2 Internationalization Considerations ........................ 14
Acknowledgements ................................................. 14
Bibliography ..................................................... 15
Author's Address .................................................=
16
0. Change History
0.8 Changes Made from Version 01 to Version 02
No significant changes; reissued to make it available officially.
Changed author's address.
Changes deferred to future versions (if ever):
- Decide on ZLD name (.i or .i18n.int or something else)
- Decide on casing solution
- Decide on exact syntax
- Proposals for experimental setup
0.9 Changes Made from Version 00 to Version 01
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- Minor rewrites and clarifications
- Added the following references: [RFC1730], [Kle96], [ISO3166],
[iNORM]
- Slightly expanded discussion about casing
- Added some variant proposals for syntax
- Added some explanations about different kinds of name parallelism
- Added some explanation about independent addition of internation-
alized names in subdomains without bothering higher-level domains
- Added some explanations about tools needed for support, and the
MX/CNAME problem
- Change to RFC1123 (numbers allowed at beginning of labels)
1. Introduction
1.1 Motivation
The lower layers of the Internet do not discriminate any language or
script. On the application level, however, the historical dominance
of the US and the ASCII character set [ASCII] as a lowest common
denominator have led to limitations. The process of removing these
limitations is called internationalization (abbreviated i18n). One
example of the abovementioned limitations are domain names [RFC1034,
RFC1035], where only the letters of the basic Latin alphabet (case-
insensitive), the decimal digits, and the hyphen are allowed.
While such restrictions are convenient if a domain name is intended
to be used by arbitrary people around the globe, there may be very
good reasons for using aliases that are more easy to remember or type
in a local context. This is similar to traditional mail addresses,
where both local scripts and conventions and the Latin script can be
used.
There are many good reasons for domain name i18n, and some arguments
that are brought forward against such an extension. This document,
however, does not discuss the pros and cons of domain name i18n. It
proposes and discusses a solution and therefore eliminates one of the
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most often heard arguments agains, namely "it cannot be done".
The solution proposed in this document consists of the introduction
of a new "zero-level" domain building the root of a new domain
branch, and an encoding of the Universal Character Set (UCS)
[ISO10646] into the limited character set of domain names.
1.2 Notational Conventions
In the domain name examples in this document, characters of the basic
Latin alphabet (expressible in ASCII) are denoted with lower case
letters. Upper case letters are used to represent characters outside
ASCII, such as accented characters of the Latin alphabet, characters
of other alphabets and syllabaries, ideographic characters, and vari-
ous signs.
2. The Hidden Zero Level Domain
The domain name system uses the domain "in-addr.arpa" to convert
internet addresses back to domain names. One way to view this is to
say that in-addr.arpa forms the root of a separate hierarchy. This
hierarchy has been made part of the main domain name hierarchy just
for implementation convenience. While syntactically, in-addr.arpa is
a second level domain (SLD), functionally it is a zero level domain
(ZLD) in the same way as "." is a ZLD. A similar example of a ZLD is
the domain tpc.int, which provides a hierarchy of the global phone
numbering system [RFC1530] for services such as paging and printing
to fax machines.
For domain name i18n to work inside the tight restrictions of domain
name syntax, one has to define an encoding that maps strings of UCS
characters to strings of characters allowable in domain names, and a
means to distinguish domain names that are the result of such an
encoding from ordinary domain names.
This document proposes to create a new ZLD to distinguish encoded
i18n domain names from traditional domain names. This domain would
be hidden from the user in the same way as a user does not see in-
addr.arpa. This domain could be called "i18n.arpa" (although the use
of arpa in this context is definitely not appropriate), simply
"i18n", or even just "i". Below, we are using "i" for shortness,
while we leave the decision on the actual name to further=
discussion.
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3. Encoding International Characters
3.1 Encoding Requirements
Until quite recently, the thought of going beyond ASCII for something
such as domain names failed because of the lack of a single encom-
passing character set for the scripts and languages of the world.
Tagging techniques such as those used in MIME headers [RFC1522] would
be much too clumsy for domain names.
The definition of ISO 10646 [ISO10646], codepoint by codepoint iden-
tical with Unicode [Unicode], provides a single Universal Character
Set (UCS). A recent report [RFCIAB] clearly recommends to base the
i18n of the Internet on these standards.
An encoding for i18n domain names therefore has to take the charac-
ters of ISO 10646/Unicode as a starting point. The full four-byte
(31 bit) form of UCS, called UCS4, should be used. A limitation to
the two-byte form (UCS2), which allows only for the encoding of the
Base Multilingual Plane, is too restricting.
For the mapping between UCS4 and the strongly limited character set
of domain names, the following constraints have to be considered:
- The structure of domain names, and therefore the "dot", have to be
conserved. Encoding is done for individual labels.
- Individual labels in domain names allow the basic Latin alphabet
(monocase, 26 letters), decimal digits, and the "-" inside the
label. The capacity per octet is therefore limited to somewhat
above 5 bits.
- There is no need nor possibility to preserve any characters.
- Frequent characters (i.e. ASCII, alphabetic, UCS2, in that order)
should be encoded relatively compactly. A variable-length encoding
(similar to UTF-8) seems desirable.
3.2 Encoding Definition
Several encodings for UCS, so called UCS Transform Formats, exist
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already, namely UTF-8 [RFC2044], UTF-7 [RFC1642], and UTF-16 [Uni-
code]. Unfortunately, none of them is suitable for our purposes. We
therefore use the following encoding:
- To accommodate the slanted probability distribution of characters
in UCS4, a variable-length encoding is used.
- Each target letter encodes 5 bits of information. Four bits of
information encode character data, the fifth bit is used to indi-
cate continuation of the variable-length encoding.
- Continuation is indicated by distinguishing the initial letter
from the subsequent letter.
- Leading four-bit groups of binary value 0000 of UCS4 characters
are discarded, except for the last TWO groups (i.e. the last
octet). This means that ASCII and Latin-1 characters need two
target letters, the main alphabets up to and including Tibetan
need three target letters, the rest of the characters in the BMP
need four target letters, all except the last (private) plane in
the UTF-16/Surrogates area [Unicode] need five target letters, and
so on.
- The letters representing the various bit groups in the various
positions are chosen according to the following table:
Nibble Value Initial Subsequent
Hex Binary
0 0000 G 0
1 0001 H 1
2 0010 I 2
3 0011 J 3
4 0100 K 4
5 0101 L 5
6 0110 M 6
7 0111 N 7
8 1000 O 8
9 1001 P 9
A 1010 Q A
B 1011 R B
C 1100 S C
D 1101 T D
E 1110 U E
F 1111 V F
[Should we try to eliminate "I" and "O" from initial? "I" might be
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eliminated because then an algorithm can more easily detect ".i". "O"
could lead to some confusion with "0". What other protocols are
there that might be able to use a similar solution, but that might
have other restrictions for the initial letters? Proposal to run ini-
tial range from H to X. Extracting the initial bits then becomes ^
'H'. Proposal to have a special convention for all-ASCII labels
(start label with one of the letters not used above).]
Please note that this solution has the following interesting proper-
ties:
- For subsequent positions, there is an equivalence between the hex-
adecimal value of the character code and the target letter used.
This assures easy conversion and checking.
- The absence of digits from the "initial" column, and the fact that
the hyphen is not used, assures that the resulting string conforms
to domain name syntax.
- Raw sorting of encoded and unencoded domain names is equivalent.
- The boundaries of characters can always be detected easily.
(While this is important for representations that are used inter-
nally for text editing, it is actually not very important here,
because tools for editing can be assumed to use a more straight-
forward representation internally.)
- Unless control characters are allowed, the target string will
never actually contain a G.
3.3 Encoding Example
As an example, the current domain
with the components standing for information science, science, the
University of Tokyo, academic, and Japan, might in future be repre-
sented by
(a transliteration of the kanji that might probably be chosen to rep-
resent the same domain). Writing each character in U+HHHH notation as
in [Unicode], this results in the following (given for reference
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only, not the actual encoding or something being typed in by the
user):
The software handling internationalized domain names will translate
this, according to the above specifications, before submitting it to
the DNS resolver, to:
3.4 Length Considerations
DNS allows for a maximum of 63 positions in each part, and for 255
positions for the overall domain name including dots. This allows up
to 15 ideographs, or up to 21 letters e.g. from the Hebrew or Arabic
alphabet, in a label. While this does not allow for the same margin
as in the case of ASCII domain names, it should still be quite suffi-
cient. [Problems could only surface for languages that use very long
words or terms and don't know any kind of abbreviations or similar
shortening devices. Do these exist? Islandic expert asserted
Islandic is not a problem.] DNS contains a compression scheme that
avoids sending the same trailing portion of a domain name twice in
the same transmission. Long domain names are therefore not that much
of a concern.
4. Usage Considerations
4.1 General Usage
To implement this proposal, neither DNS servers nor resolvers need
changes. These programs will only deal with the encoded form of the
domain name with the .i suffix. Software that wants to offer an
internationalized user interface (for example a web browser) is
responsible for the necessary conversions. It will analyze the domain
name, call the resolver directly if the domain name conforms to the
domain name syntax restrictions, and otherwise encode the name
according to the specifications of Section 3.2 and append the .i suf-
fix before calling the resolver. New implementations of resolvers
will of course offer a companion function to gethostbyname accepting
a ISO10646/Unicode string as input.
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For domain name administrators, them main tool that will be needed is
a program to compile files configuring zones from an UTF-8 notation
(or any other suitable encoding) to the encoding described in Section
3.3. Utility tools will include a corresponding decompiler, checkers
for various kinds of internationalization-related errors, and tools
for managing syntactic parallelism (see Section 4.3).
4.2 Usage Restrictions
While this proposal in theory allows to have control characters such
as BEL or NUL or symbols such as arrows and smilies in domain names,
such characters should clearly be excluded from domain names. Whether
this has to be explicitly specified or whether the difficulty to type
these characters on any keyboard of the world will limit their use
has to be discussed. One approach is to start with a very restricted
subset and gradually relax it; the other is to allow almost anything
and to rely on common sense. Anyway, such specifications should go
into a separate document to allow easy updates.
A related point is the question of equivalence. For historical rea-
sons, ISO 10646/Unicode contain considerable number of compatibility
characters and allow more than one representation for characters with
diacritics. To guarantee smooth interoperability in these and related
cases, additional restrictions or the definition of some form of nor-
malization seem necessary. However, this is a general problem
affecting all areas where ISO 10646/Unicode is used in identifiers,
and should therefore be addressed in a generic way. See [iNORM] for
an initial proposal.
Equally related is the problem of case equivalence. Users can very
well distinguish between upper case and lower case. Also, casing in
an i18n context is not as straightforward as for ASCII, so that case
equivalence is best avoided. Problems therefore result not from the
fact that case is distinguished for i18n domain names, but from the
fact that existing domain names do not distinguish case. Where it is
subdomains would easily be distinguishable if subordinate to a i18n
domain. There are several possible solutions. One is to try to grad-
ually migrate from a case-insensitive solution to a case-sensitive
solution even for ASCII. Another is to allow case-sensitivity only
beyond ASCII. Another is to restrict anything beyond ASCII to lower-
case only (lowercase distinguishes better than uppercase, and is also
generally used for ASCII domain names).
A problem that also has to be discussed and solved is bidirectional-
ity. Arabic and Hebrew characters are written right-to-left, and the
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mixture with other characters results in a divergence between logical
and graphical sequence. See [HTML-I18N] for more explanations. The
proposal of [Yer96] for dealing with bidirectionality in URLs could
probably be applied to domain names. Anyway, there should be a gen-
eral solution for identifiers, not a DNS-specific solution.
4.3 Domain Name Creation
The ".i" ZLD should be created as such to allow the internationaliza-
tion of domain names. Rules for creating subdomains inside ".i"
should follow the established rules for the creation of functionally
equivalent domains in the existing domain hierarchy, and should
evolve in parallel.
For the actual domain hierarchy, the amount of parallelism between
the current ASCII-oriented hierarchy and some internationalized hier-
archy depends on various factors. In some cases, two fully parallel
hierarchies may emerge. In other cases, if more than one script or
language is used locally, more than two parallel hierarchies may
emerge. Some nodes, e.g. in intranets, may only appear in an i18n
hierarchy, whereas others may only appear in the current hierarchy.
In some cases, the pecularities of scripts, languages, cultures, and
the local marketplace may lead to completely different hierarchies.
Also, one has to be aware that there may be several kinds of paral-
lelisms. The first one is called syntactic parallelism. If there is
will have to exist both in the traditional DNS hierarchy as well as
within the hierarchy starting at the .i ZLD, with appropriate encod-
ing.
The second type of parallelism is called transcription parallelism.
It results by transcribing or transliterating relations between ASCII
domain names and domain names in other scripts.
The third type of parallelism is called semantic parallelism. It
results from translating elements of a domain name from one language
to another, possibly also changing the script or set of used charac-
ters.
On the host level, parallelism means that there are two names for the
same host. Conventions should exist to decide whether the parallel
names should have separate IP addresses or not (A record or CNAME
record). With separate IP addresses, address to name lookup is easy,
otherwise it needs special precautions to be able to find all names
corresponding to a given host address. Another detail entering this
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consideration is that MX records only work for hostnames/domains,
not for CNAME aliases. This at least has the consequence that alias
resolution for internationalized mail addresses has to occur before
MX record lookup.
When discussing and applying the rules for creating domain names,
some peculiarities of i18n domain names should be carefully consid-
ered:
- Depending on the script, reasonable lengths for domain name parts
may differ greatly. For ideographic scripts, a part may often be
only a one-letter code. Established rules for lengths may need
adaptation. For example, a rule for country TLDs could read: one
ideographic character or two other characters.
- If the number of generic TLDs (.com, .edu, .org, .net) is kept
low, then it may be feasible to restrict i18n TLDs to country
TLDs.
- There are no ISO 3166 [ISO3166] two-letter codes in scripts other
than Latin. I18n domain names for countries will have to be
designed from scratch.
- The names of some countries or regions may pose greater political
problems when expressed in the native script than when expressed
in 2-letter ISO 3166 codes.
- I18n country domain names should in principle only be created in
those scripts that are used locally. There is probably little use
in creating an Arabic domain name for China, for example.
- In those cases where domain names are open to a wide range of
applicants, a special procedure for accepting applications should
be used so that a reasonable-quality fit between ASCII domain
names and i18n domain names results where desired. This would
probably be done by establishing a period of about a month for
applications inside a i18n domain newly created as a parallel for
an existing domain, and resolving the detected conflicts. For
syntactically parallel domain names, the owners should always be
the same. Administration may be split in some cases to account for
the necessary linguistic knowledge. For domain names with tran-
scription parallelism and semantic parallelism, the question of
owner identity should depend on the real-life situation (trade-
marks,...).
- It will be desirable to have internationalized subdomains in non-
internationalized TLDs. As an example, many companies in France
may want to register an accented version of their company name,
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while remaining under the .fr TLD. For this, .fr would have to be
reregistered as .M6N2.i. Accented and other internationalized sub-
domains would go below .M6N2.i, whereas unaccented ones would go
below .fr in its plain form.
- To generalize the above case, one may need to create a requirement
that any domain name registry would have to register and manage
syntactically parallel domain names below the .i ZLD upon request
to allow registration of i18n domain names in arbitrary subdo-
mains. An alternative to this is to organize domain name search
.i, the name server for .fr is queried for XXXXXX.M6N2.i (with
XXXXXX appropriately encoded). This convention would allow lower-
level domains to introduce internationalized subdomains without
depending on higher-level domains.
4.4 Usage in URLs
According to current definitions, URLs encode sequences of octets
into a sequence of characters from a character set that is almost as
limited as the character set of domain names [RFC1738]. This is
clearly not satisfying for i18n.
Internationalizing URLs, i.e. assigning character semantics to the
encoded octets, can either be done separately for each part and/or
scheme, or in an uniform way. Doing it separately has the serious
disadvantage that software providing user interfaces for URLs in gen-
eral would have to know about all the different i18n solutions of the
different parts and schemes. Many of these solutions may not even be
known yet.
It is therefore definitely more advantageous to decide on a single
and consistent solution for URL internationalization. The most valu-
able candidate [Yer96], for many reasons, is UTF-8 [RFC2044], an
ASCII-compatible encoding of UCS4.
Therefore, an URL containing the domain name of the example of Sec-
tion 3.3 should not be written as:
(although this will also work) but rather
%e5%ad%a6.%e6%97%a5%e6%9c%ac
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In this canonical form, the trailing .i is absent, and the octets can
be reconstructed from the %HH-encoding and interpreted as UTF-8 by
generic URL software. The software part dealing with domain names
will carry out the conversion to the .i form.
5. Alternate Proposals
5.1 The Dillon Proposal
The proposal of Michael Dillon [Dillon96] is also based on encoding
Unicode into the limited character set of domain names. Distinction
is done for each part, using the hyphen in initial position. Because
this does not fully conform to the syntax of existing domain names,
it is questionable whether it is backwards-compatible. On the other
hand, this has the advantage that local i18n domain names can be
installed easily without cooperation by the manager of the superdo-
main.
A variable-length scheme with base 36 is used that can encode up to
1610 characters, absolutely insufficient for Chinese or Japanese.
Characters assumed not to be used in i18n domain names are excluded,
i.e. only one case is allowed for basic Latin characters. This means
that large tables have to be worked out carefully to convert between
ISO 10646/Unicode and the actual number that is encoded with base=
36.
5.2 Using a Separate Lookup Service
Instead of using a special encoding and burdening DNS with i18n, one
could build and use a separate lookup service for i18n domain names.
Instead of converting to UCS4 and encoding according to Section 3.2,
and then calling the DNS resolver, a program would contact this new
service when seeing a domain name with characters outside the allowed
range.
Such solutions have various problems. There are many directory ser-
vices and proposals for how to use them in a way similar to DNS. For
an overview and a specific proposal, see [Kle96]. However, while
there are many proposals, a real service containing the necessary
data and providing the wide installed base and distributed updating
is in DNS does not exist.
Most directory service proposals also do not offer uniqueness.
Defining unique names again for a separate service will duplicate
much of the work done for DNS. If uniqueness is not guaranteed, the
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user is bundened with additional selection steps.
Using a separate lookup service for the internationalization of
domain names also results in more complex implementations than the
proposal made in this draft. Contrary to what some people might
expect, the use of a separate lookup service also does not solve a
capacity problem with DNS, because there is no such problem, nor will
one be created with the introduction of i18n domain names.
6. Generic Considerations
6.1 Security Considerations
This proposal is believed not to raise any other security considera-
tions than the current use of the domain name system.
6.2 Internationalization Considerations
This proposal addresses internationalization as such. The main addi-
tional consideration with respect to internationalization may be the
indication of language. However, for concise identifiers such as
domain names, language tagging would be too much of a burden and
would create complex dependencies with semantics.
NOTE -- This section is introduced based on a recommenda-
tion in [RFCIAB]. A similar section addressing internation-
alization should be included in all application level
internet drafts and RFCs.
Acknowledgements
I am grateful in particular to the following persons for their advice
or criticism: Bert Bos, Lori Brownell, Michael Dillon, Donald E.
Eastlake 3rd, David Goldsmith, Larry Masinter, Ryan Moats, Keith
Moore, Thorvardur Kari Olafson, Erik van der Poel, Jurgen Schwertl,
Paul A. Vixie, Francois Yergeau, and others.
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1997
Bibliography
[ASCII] Coded Character Set -- 7-Bit American Standard Code
for Information Interchange, ANSI X3.4-1986.
[Dillon96] M. Dillon, "Multilingual Domain Names", Memra Software
Inc., November 1996 (circulated Dec. 6, 1996 on iahc-
discuss@iahc.org).
[HTML-I18N] F. Yergeau, G. Nicol, G. Adams, and M. Duerst, "Inter-
nationalization of the Hypertext Markup Language",
Work in progress (draft-ietf-html-i18n-05.txt), August
1996.
[iNORM] M. Duerst, "Normalization of Internationalized Identi-
fiers", draft-duerst-i18n-norm-00.txt, July 1997.
[ISO3166] ISO 3166, "Code for the representation of names of
countries", ISO 3166:1993.
[ISO10646] ISO/IEC 10646-1:1993. International standard -- Infor-
mation technology -- Universal multiple-octet coded
character Set (UCS) -- Part 1: Architecture and basic
multilingual plane.
[Kle96] J. Klensin and T. Wolf, Jr., "Domain Names and Company
Name Retrieval", Work in progress (draft-klensin-tld-
whois-01.txt), November 1996.
[RFC1034] P. Mockapetris, "Domain Names - Concepts and Facili-
ties", ISI, Nov. 1987.
[RFC1035] P. Mockapetris, "Domain Names - Implementation and
Specification", ISI, Nov. 1987.
[RFC1522] K. Moore, "MIME (Multipurpose Internet Mail Exten-
sions) Part Two: Message Header Extensions for Non-
ASCII Text", University of Tennessee, September 1993.
[RFC1642] D. Goldsmith, M. Davis, "UTF-7: A Mail-safe Transfor-
mation Format of Unicode", Taligent Inc., July 1994.
[RFC1730] C. Malamud and M. Rose, "Principles of Operation for
the TPC.INT Subdomain: General Principles and Policy",
Internet Multicasting Service, October 1993.
[RFC1738] T. Berners-Lee, L. Masinter, and M. McCahill,
"Uniform Resource Locators (URL)", CERN, Dec. 1994.
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[RFC2044] F. Yergeau, "UTF-8, A Transformation Format of Unicode
and ISO 10646", Alis Technologies, October 1996.
[RFCIAB] C. Weider, C. Preston, K. Simonsen, H. Alvestrand, R.
Atkinson, M. Crispin, P. Svanberg, "Report from the
IAB Character Set Workshop", October 1996 (currently
available as draft-weider-iab-char-wrkshop-00.txt).
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
2.0", Addison-Wesley, Reading, MA, 1996.
[Yer96] F. Yergeau, "Internationalization of URLs", Alis Tech-
nologies,
=
Author's Address
Martin J. Duerst
World Wide Web Consortium
Keio Research Institute at SFC
Keio University
5322 Endo
Fujisawa
252-8520 Japan
Tel: +81 466 49 11 70
E-mail: mduerst@w3.org
NOTE -- Please write the author's name with u-Umlaut wherever
possible, e.g. in HTML as Dürst.
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