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RFC 4185 - National and Local Characters for DNS Top Level Domai

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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).


   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.


   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.

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

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:

   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

   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

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

   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

   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

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.

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

   (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

   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

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

   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

   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

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

   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.

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

   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

   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

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

   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

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.

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

              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

   [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.

   [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

   Phone: +1 617 491 5735
   EMail: john-ietf@jck.com

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