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rfc:rfc4185

Network Working Group J. Klensin Request for Comments: 4185 October 2005 Category: Informational

 National and Local Characters for DNS Top Level Domain (TLD) Names

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).

IESG Note

 This RFC is not a candidate for any level of Internet Standard.  The
 IETF disclaims any knowledge of the fitness of this RFC for any
 purpose and notes that the decision to publish is not based on IETF
 review apart from IESG review for conflict with IETF work.  The RFC
 Editor has chosen to publish this document at its discretion.  See
 RFC 3932 [RFC3932] for more information.

Abstract

 In the context of work on internationalizing the Domain Name System
 (DNS), there have been extensive discussions about "multilingual" or
 "internationalized" top level domain names (TLDs), especially for
 countries whose predominant language is not written in a Roman-based
 script.  This document reviews some of the motivations for such
 domains, several suggestions that have been made to provide needed
 functionality, and the constraints that the DNS imposes.  It then
 suggests an alternative, local translation, that may solve a superset
 of the problem while avoiding protocol changes, serious deployment
 delays, and other difficulties.  The suggestion utilizes a
 localization technique in applications to permit any TLD to be
 accessed using the vocabulary and characters of any language.  It is
 not restricted to language- or country-specific "multilingual" TLDs
 in the language(s) and script(s) of that country.

Klensin Informational [Page 1] RFC 4185 Characters for DNS TLD Names October 2005

Table of Contents

 1. Introduction ....................................................3
    1.1. Terminology ................................................3
    1.2. Background on the "Multilingual Name" Problem ..............3
         1.2.1. Approaches to the Requirement .......................3
         1.2.2. Writing the Name of One's Country in its Own
                Characters ..........................................4
         1.2.3. Countries with Multiple Languages and
                Countries with Multiple .............................5
         1.2.4. Availability of Non-ASCII Characters in Programs ....5
    1.3. Domain Name System Constraints .............................6
         1.3.1. Administrative Hierarchy ............................6
         1.3.2. Aliases .............................................6
    1.4. Internationalization and Localization ......................7
 2. Client-Side Solutions ...........................................7
    2.1. IDNA and the Client ........................................8
    2.2. Local Translation Tables for TLD Names .....................8
 3. Advantages and Disadvantages of Local Translation ...............9
    3.1. Every TLD Appears in the Local Language and Character Set ..9
    3.2. Unification of Country Code Domains .......................10
    3.3. User Understanding of Local and Global References .........11
    3.4. Limits on Expansion of the Number of TLDs .................11
    3.5. Standardization of the Translations .......................12
    3.6. Implications for Future New Domain Names ..................13
    3.7. Mapping for TLDs, Not Domain Names or Keywords ............13
 4. Information Interchange, IDNs, Comparisons, and Translations ...13
 5. Internationalization Considerations ............................15
 6. Security Considerations ........................................15
 7. Acknowledgements ...............................................16
 8. Informative References .........................................17

Klensin Informational [Page 2] RFC 4185 Characters for DNS TLD Names October 2005

1. Introduction

1.1. Terminology

 This document assumes the conventional terminology used to discuss
 the domain name system (DNS) and its hierarchical arrangements.
 Terms such as "top level domain" (or just "TLD"), "subdomain",
 "subtree", and "zone file" are used without further explanation.  In
 addition, the term "ccTLD" is used to denote a "country code top
 level domain" and "gTLD" is used to denote a "generic top level
 domain" as described in [RFC1591] and in common usage.

1.2. Background on the "Multilingual Name" Problem

 People who share a language usually prefer to communicate in it,
 using whatever characters are normally used to write that language,
 rather than in some "foreign" one.  There have been standards for
 using mutually-agreed characters and languages in electronic mail
 message bodies and selected headers since the introduction of MIME in
 1992 [MIME] and the Web has permitted multilingual text since its
 inception, also using MIME.  Actual use of non-Roman-character
 content came even earlier, using private conventions.  However,
 domain names are exposed to users in email addresses and URLs.
 Corresponding arrangements, typically also exposing domain names, are
 made for other application protocols.  The combination of exposed
 domain names with internationalization requirements led rapidly to
 demands to permit domain names in applications that used characters
 other than those of the very restrictive, ASCII-subset, "hostname"
 (or "letter-digit-hyphen" ("LDH")) conventions recommended in the DNS
 specifications [RFC1035].  The effort to do this soon became known as
 "multilingual domain names".  That was actually a misnomer, since the
 DNS deals only with characters and identifier strings, and not,
 except by accident or local registration conventions, with what
 people usually think of as "names".  There has also been little
 interest in what would actually be a "multilingual name", i.e., a
 name that contains components from more than one language.  Instead,
 interest has focused on the use, in the context of the DNS, of
 strings that conform to specific individual languages.

1.2.1. Approaches to the Requirement

 When the requirement was seen, not as "modifying the DNS", but as
 "providing users with access to the DNS from a variety of languages
 and character sets", three sets of proposals emerged in the IETF and
 elsewhere.  They were:

Klensin Informational [Page 3] RFC 4185 Characters for DNS TLD Names October 2005

 1.  Perform processing in client software that recodes a user-visible
     string into an ASCII-compatible form that can safely be passed
     through the DNS protocols and stored in the DNS.  This is the
     approach used, for example, in the IETF's "IDNA" protocol
     [RFC3490].
 2.  Modify the DNS to be more hospitable to non-ASCII names and
     strings.  There have been a variety of proposals to do this,
     using several different techniques.  Some of these have been
     implemented on a proprietary basis by various vendors.  None of
     them have gained acceptance in the IETF community, primarily
     because they would take a long time to deploy, would leave many
     problems unsolved, and have been shown to cause problems with
     deployed approaches that had not yet been upgraded.
 3.  Move the problem out of the DNS entirely, relying instead on a
     "directory" or "presentation" layer to handle
     internationalization.  The rationale for this approach is
     discussed in [RFC3467].
 This document proposes a fourth approach, applicable to the top level
 domains (TLDs) only (see Section 1.3.1 for a discussion of the
 special issues that make TLDs both problematic and a special
 opportunity).  That approach involves having the user interface of
 applications map non-ASCII names for TLDs to existing TLDs and could
 be used as an alternate or supplement to the strategies summarized
 above.

1.2.2. Writing the Name of One's Country in its Own Characters

 An early focus of the "multilingual domain name" efforts was
 expressed in statements such as "users in my country, in which ASCII
 is rarely used, should be able to write an entire domain name in
 their own character set".  In particular, since all top-level domain
 names, at present, follow the LDH rules, the modified naming rules
 discussed in [RFC1123], and the coding conventions specified in
 [RFC1591], all fully-qualified DNS names were effectively required to
 contain at least one ASCII label (the TLD name).  Some advocates for
 internationalized names have considered the presence of any ASCII
 labels inappropriate.  One should, instead, be able to write the name
 of the ccTLD for China in Chinese, the name of the ccTLD for Saudi
 Arabia in Arabic, the name for Spain in Spanish, and so on.
 That much could be accomplished, given updated applications, by using
 a new TLD name with IDNA encoding.  Of course, adding such a TLD
 would raise new questions: what to do about gTLDs, how to handle
 countries with several official languages (perhaps even using
 different scripts), how should name strings be chosen, and whether

Klensin Informational [Page 4] RFC 4185 Characters for DNS TLD Names October 2005

 there should be an attempt to coordinate the contents of the local-
 language TLD zone and the traditional ISO 3166-coded one.  A few of
 these issues are addressed below.  But, if one examines (or even
 thinks about) user behavior and preferences, it is almost as
 important that one be able to write the name of the ccTLD for China
 in Arabic and that of Saudi Arabia in Chinese: true
 internationalization implies that, at least to the extent to which
 ambiguity and conflicts can be avoided, people should be able to use
 the languages and character sets they prefer.  For the same reasons
 that one would like to have all-Chinese domain names available in
 China, it is important to have the capability to have an apparent
 Chinese-language TLD for a domain whose second level and beyond are
 Chinese characters, even when the TLD itself serves predominantly
 non-Chinese-speaking registrants and users.

1.2.3. Countries with Multiple Languages and Countries with Multiple

      Names
 From a user interface standpoint, writing ccTLD names in local
 characters is a problem.  As discussed below in Section 1.3.2, the
 DNS itself does not easily permit a domain to be referred to by more
 than one name (or spelling or translation of a name).  Countries with
 more than one official language would require that the country name
 be represented in each of those languages.  And, just as it is
 important that a user in China be able to represent the name of the
 Chinese ccTLD in Chinese characters, she should be able to access a
 Chinese-language site in France using Chinese characters.  That would
 require that she be able to write the name of the French ccTLD in
 Chinese characters rather than in a form based on a Roman character
 set.

1.2.4. Availability of Non-ASCII Characters in Programs

 Over the years, computer users have gotten used to the fact that not
 every computer has a full set of characters available to every
 program.  An extreme example is an Arabic speaker using a public
 kiosk computer in an airport in the United States: there is only a
 small chance that the web browser there will be able to input and
 render Arabic correctly.  This has a direct effect on the
 multilingual TLD problem in that it is not possible to simply change
 a name of the ccTLDs in the DNS to be one of a given country's non-
 ASCII names without possibly preventing people from entering those
 names throughout the world.

Klensin Informational [Page 5] RFC 4185 Characters for DNS TLD Names October 2005

1.3. Domain Name System Constraints

1.3.1. Administrative Hierarchy

 The domain name system is firmly rooted in the idea of an
 "administrative hierarchy", with the entity responsible for a given
 node of the hierarchy responsible for policies applicable to its
 subhierarchies (Cf. [RFC1034], [RFC1035], and [RFC1591]).  The model
 works quite well for the domain and subdomains of a particular
 enterprise.  In an enterprise situation, the hierarchy can be
 organized to match the organizational structure; there are
 established ways to set policies; and there are, at least presumably,
 shared assumptions about overall goals and objectives among all
 registrants in the domain.  It is more problematic when a domain is
 shared by unrelated entities that lack common policy assumptions
 because it is difficult to reach agreement on rules that should apply
 to all of the entities and subdomains of such a domain.  In general,
 the unrelated entities situation always prevails for the labels
 registered in a TLD (second-level names).  Exceptions occur in those
 TLDs for which the second level is structural (e.g., the .CO, .AC,
 .GOV conventions in many ccTLDs or in the historical geographical
 organization of .US [RFC1480]).  In those cases, it exists for the
 labels within that structural level.
 TLDs may, but need not, have consistent registration policies for
 those second (or third) level names.  Countries (or ccTLD
 administrators) have often adopted rules about what entities may
 register in their ccTLDs, and what forms the names may take.  RFC
 1591 outlined registration norms for most of the then-extant gTLDs;
 however, those norms have been largely ignored in recent years.  Some
 recent "sponsored" and purpose-specific domains are based on quite
 specific rules about appropriate registrations.  Homogeneous
 registration rules for the root are, by contrast, impossible: almost
 by definition, the subdomains registered in the root (TLDs) are
 diverse, and no single policy about types and formats of names
 applying to all root subdomains is feasible.

1.3.2. Aliases

 In an environment different from the DNS, a rational way to permit
 assigning local-language names to a country code (or other) domain
 would be to set up an alias for the name, or to use some sort of "see
 instead" reference.  But the DNS does not have facilities for either.
 Instead, it supports a "CNAME" record, whose label can refer only to
 a particular label and not to a subtree.  For example, if A.B.C is a
 fully-qualified name, then a CNAME reference in B.C from X to A would
 make X.B.C appear to have the same values as A.B.C. However, a CNAME
 reference from Y to C in the root would not make A.B.Y referenceable

Klensin Informational [Page 6] RFC 4185 Characters for DNS TLD Names October 2005

 (or even defined) at all.  A second record type, DNAME [RFC2672], can
 provide an alias for a portion of the tree.  But many believe that it
 is problematic technically.  At a minimum, it can cause
 synchronization issues when references across zones occur, and its
 use has been discouraged within the IETF, except as a means of
 enabling a transition from one domain to another.  Even if the design
 of yet another alias-type record type were contemplated, DNS
 technical constraints of query-response integrity and DNSSec zone
 signing (cf. [RFC4033], [RFC4034], and [RFC4035]) make it extremely
 unlikely that one could be defined that would meet the desired
 requirements for "see instead" or true synonym references.

1.4. Internationalization and Localization

 It has often been observed that, while many people talk about
 "internationalization", they often really mean, and want,
 "localization".  "Internationalization", in this context, suggests
 making something globally accessible while incorporating a broad-
 range "universal" character set and conventions appropriate to all
 languages and cultures.  "Localization", by contrast, involves having
 things work well in a particular locality or for a broad range of
 localities, although aspects of the style of operation might differ
 for each locality.  Anything that actually involves the DNS must be
 global, and hence internationalized, since the DNS cannot
 meaningfully support different responses or query and matching models
 based, e.g., on the location of the user making a query.  While the
 DNS cannot support localization internally, many of the features
 discussed earlier in this section are much more easily thought about
 in local terms -- whether localized to a geographical area, users of
 a language, or using some other criteria -- than in global ones.

2. Client-Side Solutions

 Traditionally, the IETF avoided becoming involved in standardization
 for actions that take place strictly on individual hosts on the
 network, instead confining itself to behavior that is observable "on
 the wire", i.e., in protocols between network hosts.  Exceptions to
 this general principle have been made when different clients were
 required to utilize data or interpret values in compatible ways to
 preserve interoperability: the standards for email and web body
 formats, and IDNA itself, are examples of these exceptions.
 Regardless of what is required to be standardized, it is almost never
 required, and often unwise, that a user interface present "on the
 wire" formats to the user, at least by default (debugging options
 that show the wire formats are common and often quite useful).
 However, in most cases when the presentation format and the wire
 format differ, the client program must take precautions to ensure
 that the wire format can be reconstructed from user input, or to keep

Klensin Informational [Page 7] RFC 4185 Characters for DNS TLD Names October 2005

 the wire format, while hidden, bound to the presentation mechanism so
 that it can be reconstructed.  While it is rarely a goal in itself,
 it is often necessary that the user be at least vaguely aware that
 the wire ("real") format is different from the presentation one and
 that the wire format be available for debugging.
 In fact, the DNS itself is an excellent example of the difference
 between the wire format and the user presentation format.  Most
 Internet users do not realize that the wire format for DNS queries
 and responses does not include the "." character.  Instead, each
 label is represented by a length in bytes of the label, followed by
 the label itself.

2.1. IDNA and the Client

 As mentioned above, IDNA itself is entirely a client-side protocol.
 It works by performing some mappings and then encoding labels to be
 placed into the DNS in a special format called "punycode" [RFC3492].
 When labels in that format are encountered, they are transformed, by
 the client, back into internationalized (normally Unicode [ISO10646])
 characters.  In the context of this document, the important
 observation about IDNA is that any application program that supports
 it is already doing considerable transformation work in the client;
 it is not simply presenting the "on the wire" formats to the user.
 It is also the case that, if an application implementation makes
 different mappings than those called for by IDNA, it is likely to be
 detected only when, and if, users complain about unexpected behavior.
 As long as the punycode strings sent to it are valid, the server
 cannot tell what mappings were applied to develop those strings.

2.2. Local Translation Tables for TLD Names

 We suggest that, in addition to maintaining the code and tables
 required to support IDNA, authors of application programs may want to
 maintain a table that contains a list of TLDs and locally-desirable
 names for each one.  For ccTLDs, these might be the names (or
 locally-standard abbreviations) by which the relevant countries are
 known locally (whether in ASCII characters or others).  With some
 care on the part of the application designer (e.g., to ensure that
 local forms do not conflict with the actual TLD names), a particular
 TLD name input from the user could be either in local or standard
 form without special tagging or problems.  When DNS names are
 received by these client programs, the TLD labels would be mapped to
 local form before IDNA is applied to the rest of the name; when names
 are received from users, local TLD names would be mapped to the
 global ones before applying IDNA or being used in other DNS
 processing.

Klensin Informational [Page 8] RFC 4185 Characters for DNS TLD Names October 2005

3. Advantages and Disadvantages of Local Translation

3.1. Every TLD Appears in the Local Language and Character Set

 The notion of a top-level domain whose name matches, e.g., the name
 that is used for a country in that country or the name of a language
 in that language as, as mentioned above, is immediately appealing.
 But most of the reasons for it argue equally strongly for other TLDs
 being accessible from that language.  A user in Korea who can access
 the national ccTLD in the Korean language and character set has every
 reason to expect that both generic top level domains and domains
 associated with other countries would be similarly accessible,
 especially if the second-level domains bear Korean names.  A user
 native to Spain or Portugal, or in Latin America, would presumably
 have similar expectations, but would expect to use Spanish or
 Portuguese names, not Korean ones.
 That level of local optimization is not realistic -- some would argue
 not possible -- with the DNS since it would ultimately require that
 every top level domain be replicated for each of the world's
 languages.  That replication process would involve not just the top
 level domain itself; in principle, all of its subtrees would need to
 be completely replicated as well.  Perhaps in practice, not all
 subtrees would require replication, but only those for which a
 language variation or translation was significant.  But, while that
 restriction would change the scale of the problem, it would not alter
 its basic nature.  The administrative hierarchy characteristics of
 the DNS (see Section 1.3.1) turn the replication process into an
 administrative nightmare: every administrator of a second-level
 domain in the world would be forced to maintain dozens, probably
 hundreds, of similar zone files for the replicates of the domain.
 Even if only the zones relevant to a particular country or language
 were replicated, the administrative and tracking problems to bind
 these to the appropriate top-level domain and keep all of the
 replicas synchronized would be extremely difficult at best.  And many
 administrators of third- and fourth-level domains, and beyond, would
 be faced with similar problems.
 By contrast, dealing with the names of TLDs as a localization
 problem, using local translation, is fairly simple, although it
 places some burden of understanding on the user (see Section 4).
 Each function represented by a TLD -- a country, generic
 registrations, or purpose-specific registrations -- could be
 represented in the local language and character set as needed.  And,
 for countries with many languages -- or users living, working in, or
 visiting countries where their language is not dominant -- "local"
 could be defined in terms of the needs or wishes of each particular
 user.

Klensin Informational [Page 9] RFC 4185 Characters for DNS TLD Names October 2005

 An additional benefit is that, if two countries called themselves by
 the same name in their local languages -- if, e.g., Western Slobbovia
 and Eastern Slobbovia both called themselves "Slobland" -- local
 conventions could be followed as long as users understood that only
 internal forms (in this case, the ISO 3166-based ccTLD name) could be
 exported outside the country (see Section 3.3).
 Note that this proposal is to allow mapping of native-language
 strings to existing TLDs.  It would almost certainly be ill-advised
 to stretch this idea too far and try to map strings that local users
 would be unlikely to guess into TLDs.  For example, there are
 probably no languages in which the country known in English as
 "Finland" is called "FI".  Thus, one would not want to create a
 mapping from two characters that look or sound like a Roman "F" and a
 Roman "I" to the ccTLD ".fi".

3.2. Unification of Country Code Domains

 It follows from some of the comments above that, while there appears
 to be some immediate appeal from having (at least) two domains for
 each country, one using the ISO 3166-1 code [ISO3166] and another one
 using a name based on the national name in the national language,
 such a situation would create considerable problems for registrants
 in both domains.  For registrants maintaining enterprise or
 organizational subdomains, ease of administration of a single family
 of zone files will usually make a registration in a single top-level
 domain preferable to replicated sets of them, at least as long as
 their functional requirements (such a local-language access) are met
 by the unified structure.  For those registrants with no interest in
 any Internet function or protocols other than use of the HTTP/HTTPS-
 based web, this problem can be dealt with at the applications level
 by the use of redirects but, in the general case, that is not a
 feasible solution.
 For countries with multiple national languages that are considered
 equal and legally equivalent, the advantages of a translation-based
 approach, rather than multiple registrations and replicated trees,
 would be even more significant.  Actually installing and maintaining
 a separate TLD for each language would be an administrative
 nightmare, especially if it was intended that the associated zones be
 kept synchronized.  The oft-suggested proposal to adopt an "exactly
 one extra domain for each country" rule would essentially require
 some of the multiple-official-language countries to violate their own
 constitutions.  Conversely, having multiple domains for a given
 country, based on the number of official languages and without any
 expectation of synchronization, would give some countries an
 additional allocation of TLDs that others would certainly consider
 unfair.

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 Of course, having replicated domains might be popular with some
 registries and registrars, since replication would almost inevitably
 increase the total number of domains to be registered.  Helping that
 group of registries and registrars, while hurting Internet users by
 adding administrative overhead and confusion, is not a goal of this
 document.

3.3. User Understanding of Local and Global References

 While the IDNA tables (actually Nameprep [RFC3491] and Stringprep
 [RFC3454]) must be identical globally for IDNA to work reliably, the
 tables for mapping between local names and TLD names could be locally
 determined, and differ from one locale to another, as long as users
 understood that international interchange of names required using the
 standard forms.  That understanding puts some additional burden of
 learning on users, although part of it could be assisted by software
 (see Section 4).
 In any event, at least in the foreseeable future, it is likely that
 DNS names being passed among users in different countries, or using
 different languages, will be forced to be in punycode form to
 guarantee compatibility, since those users would not, in general,
 have the ability to read each other's scripts or have appropriate
 input facilities (keyboards, etc.) for then.  So the marginal
 knowledge or effort needed to put TLD names into standard form and
 transmit them in that way would actually be fairly small.

3.4. Limits on Expansion of the Number of TLDs

 The concept of using local translation does have one side effect that
 some portions of the Internet community might consider undesirable.
 The size and complexity of translation tables, and maintaining those
 tables, will be, to a considerable extent, a function of the number
 of top-level domains of interest, the frequency with which new
 domains are added, and the number of domains added at a time.  A
 country or other locale that wished to maintain a complete set of
 translations (i.e., so that every TLD had a representation in the
 local language) would presumably find setting up a table for the
 current collection of a few hundred domains to be a task that would
 take some days.  If the number of TLDs were relatively stable, with a
 relatively small number being added at infrequent intervals, the
 updates could probably be dealt with on an ad hoc basis.  But, if
 large numbers of domains were added frequently, or if the total
 number of TLDs became very large, maintaining the table might require
 dedicated staff if each new TLD is to be accommodated.  Worse,
 updating the tables stored on client machines might require update

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 and synchronization protocols and all of the complexities that tend
 to go with them (see [RFC3696] for a discussion of some related
 issues in applications).
 In practice, there will be little requirement to translate every TLD
 into a local language.  There are already existing TLDs for which
 there is no obvious translations in many languages (most notably,
 ".arpa") or where the translation will be far from obvious to typical
 users (for example, ".int" and ".aero").  Of course, these could be
 translated by function: ".arpa" to the local term for
 "infrastructure", ".int" with "international" or "international
 organization", ".aero" with "aeronautical" or "airlines", and so on;
 but it is not clear whether doing so would have significant value.
 For almost every language, there are dozens of ccTLDs for which there
 are no translations of the country names into the local language that
 would be known by anyone other than geographers.  If new TLDs are
 added, there might not be a strong need (or even capability) to have
 language-specific equivalents for each.

3.5. Standardization of the Translations

 An immediate question when proposals such as this one are considered
 is whether the names for the various TLDs that do not match the
 strings that are actually in the DNS should be standardized and, if
 so, by what mechanism.  Standardization would promote communication
 within a country or among people sharing a language.  However, it is
 likely to be very difficult to reach appropriate international
 agreements to which wide conformance could be expected.  Exceptions
 might arise within particular countries or language groups but, even
 then, there might be advantages to users being able to specify
 additional synonymous names that are easy for them to remember.  As
 with IDNA-based IDNs, users who wish to transmit information about
 domain names to people whose exact capabilities and software are
 unknown, and to do so with minimal risk of confusion, will probably
 confine themselves to the names that actually appear in the DNS,
 i.e., the "punycode" representations.
 In any event, neither standardization nor uniform use of either the
 system outlined here or of a specific collection of names is required
 to make the system work for those who would find it useful.
 Similarly, mechanisms for country-wide coordination, and examination
 of the appropriateness or inappropriateness of such mechanisms, is
 beyond the scope of this document.

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3.6. Implications for Future New Domain Names

 Applications that implement the proposal in this document are likely
 to make the subsequent creation and acceptance of new IDNA-based TLDs
 significantly more difficult.  If this proposal becomes widely
 adopted, local language names mapped as it suggests will be generally
 expected by users of those languages to mean the same as a current
 TLD.  Creating a new, stand-alone IDNA-based TLD will then require
 more deliberation and care to avoid conflicts and, when executed,
 will require all the application software that maps the name to the
 existing TLD to change the mapping tables.
 For several reasons, this problem may not be as serious in practice
 as it might first appear.  For ccTLDs allocated according to the ISO
 3166-1 list, there will presumably be no problem at all: not only are
 the 3166-1 alpha-2 codes strictly in ASCII, but general trends, such
 as those embodied in ICANN's "GAC Recommendations" against using
 country names or codes for any purpose not associated with those
 specific countries, make conflicts with internationalized names
 extremely unlikely.  Because the DNS does not currently have a usable
 aliasing function (see Section 1.3.2), it is likely that new IDNA-
 based TLDs will be allocated only after there is considerable
 opportunity for countries and other individual entities to identify
 any problems they see with proposed new names.

3.7. Mapping for TLDs, Not Domain Names or Keywords

 It should be clear to anyone who has read this far that the mapping
 described in this document is limited to TLDs, not full domain names
 or keywords.  In particular, nothing here should be construed as
 applying to anything other than TLDs, due at least in part to the
 limitations described in Section 3.1.  Further, this document is only
 about the domain name system (DNS), not about any keyword system.
 The interactions between particular keyword systems and the proposals
 here are left as a (possibly very difficult) exercise for the reader
 or implementer of such systems.  However, for the subset of such
 systems whose intent is to entirely hide DNS names or URIs from the
 user, their output would presumably be the LDH names that actually
 appeared in the DNS, i.e., in punycode form for IDNA names and
 without any application processing of the type contemplated here.

4. Information Interchange, IDNs, Comparisons, and Translations

 This specification is based on a pair of fairly explicit assumptions.
 The first is that the greatest and most important impact and value of
 any internationalization or localization technique is to permit users
 who share a language or culture to communicate with others who also
 share that language or culture.  Communication among users from

Klensin Informational [Page 13] RFC 4185 Characters for DNS TLD Names October 2005

 different cultures, using different languages or different scripts is
 inherently more difficult, and still more difficult if they cannot
 easily identify languages and scripts in common.  The reason for
 those difficulties are age-old issues in language translation and
 differences among languages and scripts, not problems associated with
 the DNS or IDNs, however they are represented.  That is the second
 assumption: when communication across language or cultural groups is
 required, the users who need to do it -- typically a much smaller
 number than those communicating within the same language and culture
 -- are going to need to rely on commonly-understood languages and
 scripts and will need to exert somewhat more care and effort than
 within their own groups.
 As outlined in the sections above, the suggestions made in this
 document could clearly be turned into major problems by misuse or
 misunderstanding.  For example, if two applications on the same host
 used different translation tables, a situation could easily result
 that would be very confusing to the user.  However, in some cases,
 this would be only slightly worse than some of the alternatives.  For
 example, if, on a given system, IDNs are expressed in native script,
 but ASCII TLD names are used, cutting and pasting from one
 application to another may not work as expected, unless both
 applications and the underlying operating system are all Unicode-
 based and use the same encoding model for Unicode.  Some applications
 writers have already discovered, even without significant use of
 IDNs, that they need to support separate "copy string" and "copy link
 location", and the corresponding "paste" operations.  Any use of IDNs
 or Internationalized Resource Identifiers (IRIs, see [RFC3987]) may
 require similar operations, or extensions to those operations, to
 force strings into internal ("punycode" or URI) form on the copy
 operation and to translate them back on paste.  Were that done, the
 appropriate translations could be performed as part of the same
 process.  If this author's hypothesis is correct -- that these
 operations are likely to be required on many systems whether this
 proposal is adopted or not -- then the additional translation
 operations are likely to be invisible to the user.
 In particular, precisely because the translated names proposed here
 are part of a presentation form, rather than the internal form names,
 they are inappropriate in a number of circumstances in which a
 globally-unique, internal-form name is actually required.  It would
 be a poor, indeed dangerous, idea to use these names in security
 contexts such as names in certificates, access lists, or other
 contexts in which accurate comparisons are necessary.
 A more general issue exists when DNS or IRI references are
 transferred among users whose systems may be localized for different
 languages or conventions.  In general, a user in one part of the

Klensin Informational [Page 14] RFC 4185 Characters for DNS TLD Names October 2005

 world will not actually know how another user's systems are set up,
 precisely what software is being used, etc., nor should users be
 expected or forced to learn that information.  But, if the user
 transmitting an internationalized reference doesn't know that the
 receiving system supports the same characters and fonts, and that the
 receiving user is prepared to deal with them, the prudent user will
 transmit the internal form of the reference in addition to, or even
 instead of, the native-character form.  And, of course, if the
 reference is transmitted on paper, on a sign, in some coded character
 set other than Unicode, or even as an image, rather than as a Unicode
 string, the importance of supplementing it with the internal form
 becomes even more important.  The addition of a translation
 requirement for TLD labels makes availability of internal forms in
 interchange significantly more important, but does not actually
 change the requirement to do so.
 It may be helpful to note that, in a different networking model than
 that used in the Internet, both this proposal and IDNA itself are
 essentially "presentation layer" approaches rather than constructions
 that can be expected to work well in interchange.

5. Internationalization Considerations

 This entire specification addresses issues in internationalization
 and especially the boundaries between internationalization and
 localization and between network protocols and client/user interface
 actions.

6. Security Considerations

 IDNA provides a client-based mechanism for presenting Unicode names
 in applications while passing only ASCII-based names on the wire.  As
 such, it constitutes a major step along the path of introducing a
 client-based presentation layer into the Internet.  Client-based
 presentation layer transformations introduce risks from non-
 conforming tables that can change meaning without external
 protection.  For example, if a mapping table normally maps A onto C,
 and that table is altered by an attacker so that A maps onto D
 instead, much mischief can be committed.  On the other hand, these
 are not the usual sort of network attacks: they may be thought of as
 falling into the "users can always cause harm to themselves"
 category.  The local translation model outlined here does not
 significantly increase the risks over those associated with IDNA, but
 may provide some new avenues for exploiting them.
 Both this approach and IDNA rely on having updated programs present
 information to the user in a very different form than the one in
 which it is transmitted on the wire.  Unless the internal (wire) form

Klensin Informational [Page 15] RFC 4185 Characters for DNS TLD Names October 2005

 is always used in interchange, or at least made available when DNS
 names are exchanged, there are possibilities for ambiguity and
 confusion about references.  As with IDNA itself, if only the "wire"
 form is presented, the user will perceive that nothing of value has
 been done, i.e., that no internationalization or localization has
 occurred.  So presentation of the "wire" form to eliminate the
 potential ambiguities is unlikely to be considered an acceptable
 solution, regardless of its security advantages.
 If the translation tables associated with the technique suggested
 here are obtained from a server, or translations are obtained from a
 remote machine using some protocol, the mechanisms used should ensure
 that the values received are authentic, i.e., that neither they, nor
 the query for them, have been intercepted and tampered with in any
 way.

7. Acknowledgements

 This document was inspired by a number of conversations in ICANN,
 IETF, MINC, and private contexts about the future evolution and
 internationalization of top level domains.  Unknown to the author,
 but unsurprisingly (the general concept should be obvious to anyone
 even slightly skilled in the relevant technologies), the concept has
 been apparently developed independently in other groups but, as far
 as this author knows, not written up for general comment.
 Discussions within, and about, the ICANN IDN Committee were
 particularly helpful, although several of the participants in that
 committee may be surprised about where those discussions led.  Email
 correspondence with several people after the first version of this
 document was posted, notably Richard Hill, Paul Hoffman, Lee
 XiaoDong, and Soobok Lee, led to considerable clarification in the
 subsequent versions.  The author is particularly grateful to Paul
 Hoffman for extensive comments and additional text for the third
 version and to Patrik Faltstrom, Joel Halpern, Sam Hartman, and Russ
 Housley for suggestions incorporated into the final one.
 The first version of this document was posted on October 21, 2002.

Klensin Informational [Page 16] RFC 4185 Characters for DNS TLD Names October 2005

8. Informative References

 [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, May 1993.
 [ISO3166]  International Organization for Standardization, "Codes for
            the representation of names of countries and their
            subdivisions -- Part 1: Country codes", ISO Standard
            3166-1:1977, 1997.
 [MIME]     Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
            Mail Extensions): Mechanisms for Specifying and Describing
            the Format of Internet Message Bodies", RFC 1341, June
            1992.
            Updated and replaced by Freed, N. and N. Borenstein,
            "Multipurpose Internet Mail Extensions (MIME) Part One:
            Format of Internet Message Bodies", RFC2045, November
            1996.  Also, Moore, K., "Representation of Non-ASCII Text
            in Internet Message Headers", RFC 1342, June 1992.
            Updated and replaced by Moore, K., "MIME (Multipurpose
            Internet Mail Extensions) Part Three: Message Header
            Extensions for Non-ASCII Text", RFC 2047, November 1996.
 [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.
 [RFC1480]  Cooper, A. and J. Postel, "The US Domain", RFC 1480, June
            1993.
 [RFC1591]  Postel, J., "Domain Name System Structure and Delegation",
            RFC 1591, March 1994.
 [RFC2672]  Crawford, M., "Non-Terminal DNS Name Redirection", RFC
            2672, August 1999.
 [RFC3454]  Hoffman, P. and M. Blanchet, "Preparation of
            Internationalized Strings ("stringprep")", RFC 3454,
            December 2002.

Klensin Informational [Page 17] RFC 4185 Characters for DNS TLD Names October 2005

 [RFC3467]  Klensin, J., "Role of the Domain Name System (DNS)", RFC
            3467, February 2003.
 [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.
 [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
            for Internationalized Domain Names in Applications
            (IDNA)", RFC 3492, March 2003.
 [RFC3696]  Klensin, J., "Application Techniques for Checking and
            Transformation of Names", RFC 3696, February 2004.
 [RFC3932]  Alvestrand, H., "The IESG and RFC Editor Documents:
            Procedures", BCP 92, RFC 3932, October 2004.
 [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
            Identifiers (IRIs)", RFC 3987, January 2005.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS  Security Introduction and Requirements", RFC
            4033, March 2005.
 [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Resource  Records for the DNS Security Extensions",
            RFC 4034, March 2005.
 [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "Protocol  Modifications for the DNS Security
            Extensions", RFC 4035, March 2005.

Author's Address

 John C Klensin
 1770 Massachusetts Ave, #322
 Cambridge, MA  02140
 USA
 Phone: +1 617 491 5735
 EMail: john-ietf@jck.com

Klensin Informational [Page 18] RFC 4185 Characters for DNS TLD Names October 2005

Full Copyright Statement

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 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.
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Klensin Informational [Page 19]

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