Internationalized Domain Names  

M. Duerst University of Zurich Expires 10 June 1996 10 December 1996

Status of this Memo This document is an Internet-Draft. Internet-Drafts are working doc- uments of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute work- ing documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a "working draft" or "work in progress". To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on (US East Coast), (Europe), (US West Coast), or (Pacific Rim). Distribution of this document is unlimited. Please send comments to the author at <>. 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.

Table of contents 1. Introduction ................................................... 2 1.1 Motivation ...................................................2 1.2 Notational Conventions .......................................3 2. The Hidden Zero Level Domain ................................... 3 3. Encoding International Characters .............................. 4

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Internationalization of Domain Names  10 December 1996

3.1 Encoding Requirements ........................................4 3.2 Encoding Definition ..........................................4 3.3 Encoding Example .............................................6 3.4 Length Considerations ........................................7 4. Usage Considerations ........................................... 7 4.1 General Usage ................................................7 4.2 Usage Restrictions ...........................................7 4.3 Domain Name Creation .........................................8 4.4 Usage in URLs ................................................9 5. Alternate Proposals ............................................10 5.1 The Dillon Proposal .........................................10 5.2 Using a Separate Lookup Service .............................11 6. Generic Considerations .........................................11 5.1 Security Considerations .....................................11 5.2 Internationalization Considerations .........................11 Acknowledgements ..................................................11 Bibliography ......................................................12 Author's Address ..................................................13

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 [Page 2]

Internationalization of Domain Names 10 December 1996 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 "" to convert internet addresses back to domain names. One way to view this is to say that 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, is a second level domain (SLD), functionally it is a zero level domain (ZLD) in the same way as "." is a ZLD. 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- This domain could be called "" (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.

3. Encoding International Characters

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 Internationalization of Domain Names  10 December 1996

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), the "-" inside the label, and the ten dec- imal digits in all but the initial position. 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 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:

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Internationalization of Domain Names 10 December 1996 - To accommodate the slanted probability distribution of characters in UCS4, a variable-length encoding is used. - Each target letter encodes 5 bits. Four bits are used as data bits, the fifth bit is used to indicate continuation of the vari- able-length encoding. - Continuation is indicated by distinguishing the initial letter from the subsequent letter [alternative: distinguish leading let- ters from final. Pros? Cons?]. - 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 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 [Page 5]

Internationalization of Domain Names 10 December 1996 have other restrictions for the initial letters?] 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 JOUHOU.RI.TOUDAI.GAKU.NIHON (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 is U+60c5U+5831.U+7406.U+6771U+5927.U+5b66.U+65e5U+672c and will be translated by the software handling internationalized domain names, according to the above specifications, to [Page 6]

Internationalization of Domain Names 10 December 1996 M0C5L831.N406.M771L927.LB66.M5E5M72C.i

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?] 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.

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 [Page 7]

Internationalization of Domain Names 10 December 1996 has to be discussed. 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 affect- ing all areas where ISO 10646/Unicode is used in identifiers, and should therefore be addressed in a generic way. 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 impossible to distinguish between and, the same two subdomains would easily be distinguishable if subordinate to a i18n domain. A problem that also has to be discussed and solved is bidirectional- ity. Arabic and Hebrew characters are written right-to-left, and the 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.

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. However, the peculiarities of i18n domain names should be carefully considered: - 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. - 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. [Page 8]

Internationalization of Domain Names 10 December 1996 - There are no ISO 639 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 639 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. - It may 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, 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 might create a requirement that any domain name registry would be required to register and manage a corresponding .i domain upon request to allow registration of i18n domain names in arbitrary subdomains.

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 [Page 9]

Internationalization of Domain Names 10 December 1996 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: ftp://M0C5L831.N406.M771L927.LB66.M5E5M72C.i (although this will also work) but rather ftp://%e6%83%85%e5%a0%b1.%e7%90%86.%e6%9d%b1%e5%a4%a7. %e5%ad%a6.%e6%97%a5%e6%9c%ac 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. [Page 10]

Internationalization of Domain Names 10 December 1996

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 a solution has various problems. A separate service does not yet exist, whereas DNS is readily usable. Solving the problems of unique- ness, etc., again for this separate service creates a lot of work. On the other side, there are no savings in terms of implementation costs. DNS also does not have a serious capacity problem that might be addressed by using a separate lookup service, nor is such a prob- lem created by 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.

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Internationalization of Domain Names 10 December 1996 Acknowledgements I am grateful in particular to the following persons: Bert Bos, Lori Brownell, Michael Dillon, David Goldsmith, Larry Mas- inter, Keith Moore, and Francois Yergeau 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- [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. [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. [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. [RFC1738] T. Berners-Lee, L. Masinter, and M. McCahill, "Uniform Resource Locators (URL)", CERN, Dec. 1994. [RFC2044] F. Yergeau, "UTF-8, A Transformation Format of Unicode and ISO 10646", Alis Technologies, October 1996. [Page 12]

Internationalization of Domain Names 10 December 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 Multimedia-Laboratory Department of Computer Science University of Zurich Winterthurerstrasse 190 CH-8057 Zurich Switzerland Tel: +41 1 257 43 16 Fax: +41 1 363 00 35 E-mail: NOTE -- Please write the author's name with u-Umlaut wherever possible, e.g. in HTML as D&uuml;rst. [Page 13] Link to this article - Internationalization of Domain Names
History of Internationalised Domain Names
IDN домены - интернационализация доменных имен
Internet Corporation for Assigned Names and Numbers - MULTILINGUAL DOMAIN NAMES
Домен РФ: все хорошо, или только так кажется?
Перепечатано в 2010 году. Кондратюк И. В., проект Регистрация торговой марки.
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