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Network Working Group                                         J. KlensinRequest for Comments: 4185                                  October 2005Category: InformationalNational and Local Characters for DNS Top Level Domain (TLD) NamesStatus 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.  SeeRFC 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 2005Table of Contents1. Introduction ....................................................31.1. Terminology ................................................31.2. Background on the "Multilingual Name" Problem ..............31.2.1. 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 .............................51.2.4. Availability of Non-ASCII Characters in Programs ....51.3. Domain Name System Constraints .............................61.3.1. Administrative Hierarchy ............................61.3.2. Aliases .............................................61.4. Internationalization and Localization ......................72. Client-Side Solutions ...........................................72.1. IDNA and the Client ........................................82.2. Local Translation Tables for TLD Names .....................83. Advantages and Disadvantages of Local Translation ...............93.1. Every TLD Appears in the Local Language and Character Set ..93.2. Unification of Country Code Domains .......................103.3. User Understanding of Local and Global References .........113.4. Limits on Expansion of the Number of TLDs .................113.5. Standardization of the Translations .......................123.6. Implications for Future New Domain Names ..................133.7. Mapping for TLDs, Not Domain Names or Keywords ............134. Information Interchange, IDNs, Comparisons, and Translations ...135. Internationalization Considerations ............................156. Security Considerations ........................................157. Acknowledgements ...............................................168. Informative References .........................................17Klensin                      Informational                      [Page 2]

RFC 4185              Characters for DNS TLD Names          October 20051.  Introduction1.1.  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 (seeSection 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 whetherKlensin                      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 inSection 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 20051.3.  Domain Name System Constraints1.3.1.  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.RFC1591 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 referenceableKlensin                      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 keepKlensin                      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 20053.  Advantages and Disadvantages of Local Translation3.1.  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 (seeSection 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 (seeSection 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 (seeSection 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.Klensin                      Informational                     [Page 10]

RFC 4185              Characters for DNS TLD Names          October 2005   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   (seeSection 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 updateKlensin                      Informational                     [Page 11]

RFC 4185              Characters for DNS TLD Names          October 2005   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.Klensin                      Informational                     [Page 12]

RFC 4185              Characters for DNS TLD Names          October 20053.6.  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 (seeSection 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 inSection 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 fromKlensin                      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 theKlensin                      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) formKlensin                      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 20058.  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",RFC2672, 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)",RFC3467, 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)",RFC3491, 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",RFC4033, 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.comKlensin                      Informational                     [Page 18]

RFC 4185              Characters for DNS TLD Names          October 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Klensin                      Informational                     [Page 19]