Network Working Group J. Klensin
Request for Comments: 4185 October 2005
Category: Informational
National and Local Characters for DNS Top Level Domain (TLD) Names
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
IESG Note
This RFC is not a candidate for any level of Internet Standard. The
IETF disclaims any knowledge of the fitness of this RFC for any
purpose and notes that the decision to publish is not based on IETF
review apart from IESG review for conflict with IETF work. The RFC
Editor has chosen to publish this document at its discretion. See
RFC 3932 [RFC3932] for more information.
Abstract
In the context of work on internationalizing the Domain Name System
(DNS), there have been extensive discussions about "multilingual" or
"internationalized" top level domain names (TLDs), especially for
countries whose predominant language is not written in a Roman-based
script. This document reviews some of the motivations for such
domains, several suggestions that have been made to provide needed
functionality, and the constraints that the DNS imposes. It then
suggests an alternative, local translation, that may solve a superset
of the problem while avoiding protocol changes, serious deployment
delays, and other difficulties. The suggestion utilizes a
localization technique in applications to permit any TLD to be
accessed using the vocabulary and characters of any language. It is
not restricted to language- or country-specific "multilingual" TLDs
in the language(s) and script(s) of that country.
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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
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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:
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1. Perform processing in client software that recodes a user-visible
string into an ASCII-compatible form that can safely be passed
through the DNS protocols and stored in the DNS. This is the
approach used, for example, in the IETF's "IDNA" protocol
[RFC3490].
2. Modify the DNS to be more hospitable to non-ASCII names and
strings. There have been a variety of proposals to do this,
using several different techniques. Some of these have been
implemented on a proprietary basis by various vendors. None of
them have gained acceptance in the IETF community, primarily
because they would take a long time to deploy, would leave many
problems unsolved, and have been shown to cause problems with
deployed approaches that had not yet been upgraded.
3. Move the problem out of the DNS entirely, relying instead on a
"directory" or "presentation" layer to handle
internationalization. The rationale for this approach is
discussed in [RFC3467].
This document proposes a fourth approach, applicable to the top level
domains (TLDs) only (see Section 1.3.1 for a discussion of the
special issues that make TLDs both problematic and a special
opportunity). That approach involves having the user interface of
applications map non-ASCII names for TLDs to existing TLDs and could
be used as an alternate or supplement to the strategies summarized
above.
1.2.2. Writing the Name of One's Country in its Own Characters
An early focus of the "multilingual domain name" efforts was
expressed in statements such as "users in my country, in which ASCII
is rarely used, should be able to write an entire domain name in
their own character set". In particular, since all top-level domain
names, at present, follow the LDH rules, the modified naming rules
discussed in [RFC1123], and the coding conventions specified in
[RFC1591], all fully-qualified DNS names were effectively required to
contain at least one ASCII label (the TLD name). Some advocates for
internationalized names have considered the presence of any ASCII
labels inappropriate. One should, instead, be able to write the name
of the ccTLD for China in Chinese, the name of the ccTLD for Saudi
Arabia in Arabic, the name for Spain in Spanish, and so on.
That much could be accomplished, given updated applications, by using
a new TLD name with IDNA encoding. Of course, adding such a TLD
would raise new questions: what to do about gTLDs, how to handle
countries with several official languages (perhaps even using
different scripts), how should name strings be chosen, and whether
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there should be an attempt to coordinate the contents of the local-
language TLD zone and the traditional ISO 3166-coded one. A few of
these issues are addressed below. But, if one examines (or even
thinks about) user behavior and preferences, it is almost as
important that one be able to write the name of the ccTLD for China
in Arabic and that of Saudi Arabia in Chinese: true
internationalization implies that, at least to the extent to which
ambiguity and conflicts can be avoided, people should be able to use
the languages and character sets they prefer. For the same reasons
that one would like to have all-Chinese domain names available in
China, it is important to have the capability to have an apparent
Chinese-language TLD for a domain whose second level and beyond are
Chinese characters, even when the TLD itself serves predominantly
non-Chinese-speaking registrants and users.
1.2.3. Countries with Multiple Languages and Countries with Multiple
Names
From a user interface standpoint, writing ccTLD names in local
characters is a problem. As discussed below in Section 1.3.2, the
DNS itself does not easily permit a domain to be referred to by more
than one name (or spelling or translation of a name). Countries with
more than one official language would require that the country name
be represented in each of those languages. And, just as it is
important that a user in China be able to represent the name of the
Chinese ccTLD in Chinese characters, she should be able to access a
Chinese-language site in France using Chinese characters. That would
require that she be able to write the name of the French ccTLD in
Chinese characters rather than in a form based on a Roman character
set.
1.2.4. Availability of Non-ASCII Characters in Programs
Over the years, computer users have gotten used to the fact that not
every computer has a full set of characters available to every
program. An extreme example is an Arabic speaker using a public
kiosk computer in an airport in the United States: there is only a
small chance that the web browser there will be able to input and
render Arabic correctly. This has a direct effect on the
multilingual TLD problem in that it is not possible to simply change
a name of the ccTLDs in the DNS to be one of a given country's non-
ASCII names without possibly preventing people from entering those
names throughout the world.
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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
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(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
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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.
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3. Advantages and Disadvantages of Local Translation
3.1. Every TLD Appears in the Local Language and Character Set
The notion of a top-level domain whose name matches, e.g., the name
that is used for a country in that country or the name of a language
in that language as, as mentioned above, is immediately appealing.
But most of the reasons for it argue equally strongly for other TLDs
being accessible from that language. A user in Korea who can access
the national ccTLD in the Korean language and character set has every
reason to expect that both generic top level domains and domains
associated with other countries would be similarly accessible,
especially if the second-level domains bear Korean names. A user
native to Spain or Portugal, or in Latin America, would presumably
have similar expectations, but would expect to use Spanish or
Portuguese names, not Korean ones.
That level of local optimization is not realistic -- some would argue
not possible -- with the DNS since it would ultimately require that
every top level domain be replicated for each of the world's
languages. That replication process would involve not just the top
level domain itself; in principle, all of its subtrees would need to
be completely replicated as well. Perhaps in practice, not all
subtrees would require replication, but only those for which a
language variation or translation was significant. But, while that
restriction would change the scale of the problem, it would not alter
its basic nature. The administrative hierarchy characteristics of
the DNS (see Section 1.3.1) turn the replication process into an
administrative nightmare: every administrator of a second-level
domain in the world would be forced to maintain dozens, probably
hundreds, of similar zone files for the replicates of the domain.
Even if only the zones relevant to a particular country or language
were replicated, the administrative and tracking problems to bind
these to the appropriate top-level domain and keep all of the
replicas synchronized would be extremely difficult at best. And many
administrators of third- and fourth-level domains, and beyond, would
be faced with similar problems.
By contrast, dealing with the names of TLDs as a localization
problem, using local translation, is fairly simple, although it
places some burden of understanding on the user (see Section 4).
Each function represented by a TLD -- a country, generic
registrations, or purpose-specific registrations -- could be
represented in the local language and character set as needed. And,
for countries with many languages -- or users living, working in, or
visiting countries where their language is not dominant -- "local"
could be defined in terms of the needs or wishes of each particular
user.
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An additional benefit is that, if two countries called themselves by
the same name in their local languages -- if, e.g., Western Slobbovia
and Eastern Slobbovia both called themselves "Slobland" -- local
conventions could be followed as long as users understood that only
internal forms (in this case, the ISO 3166-based ccTLD name) could be
exported outside the country (see Section 3.3).
Note that this proposal is to allow mapping of native-language
strings to existing TLDs. It would almost certainly be ill-advised
to stretch this idea too far and try to map strings that local users
would be unlikely to guess into TLDs. For example, there are
probably no languages in which the country known in English as
"Finland" is called "FI". Thus, one would not want to create a
mapping from two characters that look or sound like a Roman "F" and a
Roman "I" to the ccTLD ".fi".
3.2. Unification of Country Code Domains
It follows from some of the comments above that, while there appears
to be some immediate appeal from having (at least) two domains for
each country, one using the ISO 3166-1 code [ISO3166] and another one
using a name based on the national name in the national language,
such a situation would create considerable problems for registrants
in both domains. For registrants maintaining enterprise or
organizational subdomains, ease of administration of a single family
of zone files will usually make a registration in a single top-level
domain preferable to replicated sets of them, at least as long as
their functional requirements (such a local-language access) are met
by the unified structure. For those registrants with no interest in
any Internet function or protocols other than use of the HTTP/HTTPS-
based web, this problem can be dealt with at the applications level
by the use of redirects but, in the general case, that is not a
feasible solution.
For countries with multiple national languages that are considered
equal and legally equivalent, the advantages of a translation-based
approach, rather than multiple registrations and replicated trees,
would be even more significant. Actually installing and maintaining
a separate TLD for each language would be an administrative
nightmare, especially if it was intended that the associated zones be
kept synchronized. The oft-suggested proposal to adopt an "exactly
one extra domain for each country" rule would essentially require
some of the multiple-official-language countries to violate their own
constitutions. Conversely, having multiple domains for a given
country, based on the number of official languages and without any
expectation of synchronization, would give some countries an
additional allocation of TLDs that others would certainly consider
unfair.
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Of course, having replicated domains might be popular with some
registries and registrars, since replication would almost inevitably
increase the total number of domains to be registered. Helping that
group of registries and registrars, while hurting Internet users by
adding administrative overhead and confusion, is not a goal of this
document.
3.3. User Understanding of Local and Global References
While the IDNA tables (actually Nameprep [RFC3491] and Stringprep
[RFC3454]) must be identical globally for IDNA to work reliably, the
tables for mapping between local names and TLD names could be locally
determined, and differ from one locale to another, as long as users
understood that international interchange of names required using the
standard forms. That understanding puts some additional burden of
learning on users, although part of it could be assisted by software
(see Section 4).
In any event, at least in the foreseeable future, it is likely that
DNS names being passed among users in different countries, or using
different languages, will be forced to be in punycode form to
guarantee compatibility, since those users would not, in general,
have the ability to read each other's scripts or have appropriate
input facilities (keyboards, etc.) for then. So the marginal
knowledge or effort needed to put TLD names into standard form and
transmit them in that way would actually be fairly small.
3.4. Limits on Expansion of the Number of TLDs
The concept of using local translation does have one side effect that
some portions of the Internet community might consider undesirable.
The size and complexity of translation tables, and maintaining those
tables, will be, to a considerable extent, a function of the number
of top-level domains of interest, the frequency with which new
domains are added, and the number of domains added at a time. A
country or other locale that wished to maintain a complete set of
translations (i.e., so that every TLD had a representation in the
local language) would presumably find setting up a table for the
current collection of a few hundred domains to be a task that would
take some days. If the number of TLDs were relatively stable, with a
relatively small number being added at infrequent intervals, the
updates could probably be dealt with on an ad hoc basis. But, if
large numbers of domains were added frequently, or if the total
number of TLDs became very large, maintaining the table might require
dedicated staff if each new TLD is to be accommodated. Worse,
updating the tables stored on client machines might require update
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and synchronization protocols and all of the complexities that tend
to go with them (see [RFC3696] for a discussion of some related
issues in applications).
In practice, there will be little requirement to translate every TLD
into a local language. There are already existing TLDs for which
there is no obvious translations in many languages (most notably,
".arpa") or where the translation will be far from obvious to typical
users (for example, ".int" and ".aero"). Of course, these could be
translated by function: ".arpa" to the local term for
"infrastructure", ".int" with "international" or "international
organization", ".aero" with "aeronautical" or "airlines", and so on;
but it is not clear whether doing so would have significant value.
For almost every language, there are dozens of ccTLDs for which there
are no translations of the country names into the local language that
would be known by anyone other than geographers. If new TLDs are
added, there might not be a strong need (or even capability) to have
language-specific equivalents for each.
3.5. Standardization of the Translations
An immediate question when proposals such as this one are considered
is whether the names for the various TLDs that do not match the
strings that are actually in the DNS should be standardized and, if
so, by what mechanism. Standardization would promote communication
within a country or among people sharing a language. However, it is
likely to be very difficult to reach appropriate international
agreements to which wide conformance could be expected. Exceptions
might arise within particular countries or language groups but, even
then, there might be advantages to users being able to specify
additional synonymous names that are easy for them to remember. As
with IDNA-based IDNs, users who wish to transmit information about
domain names to people whose exact capabilities and software are
unknown, and to do so with minimal risk of confusion, will probably
confine themselves to the names that actually appear in the DNS,
i.e., the "punycode" representations.
In any event, neither standardization nor uniform use of either the
system outlined here or of a specific collection of names is required
to make the system work for those who would find it useful.
Similarly, mechanisms for country-wide coordination, and examination
of the appropriateness or inappropriateness of such mechanisms, is
beyond the scope of this document.
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3.6. Implications for Future New Domain Names
Applications that implement the proposal in this document are likely
to make the subsequent creation and acceptance of new IDNA-based TLDs
significantly more difficult. If this proposal becomes widely
adopted, local language names mapped as it suggests will be generally
expected by users of those languages to mean the same as a current
TLD. Creating a new, stand-alone IDNA-based TLD will then require
more deliberation and care to avoid conflicts and, when executed,
will require all the application software that maps the name to the
existing TLD to change the mapping tables.
For several reasons, this problem may not be as serious in practice
as it might first appear. For ccTLDs allocated according to the ISO
3166-1 list, there will presumably be no problem at all: not only are
the 3166-1 alpha-2 codes strictly in ASCII, but general trends, such
as those embodied in ICANN's "GAC Recommendations" against using
country names or codes for any purpose not associated with those
specific countries, make conflicts with internationalized names
extremely unlikely. Because the DNS does not currently have a usable
aliasing function (see Section 1.3.2), it is likely that new IDNA-
based TLDs will be allocated only after there is considerable
opportunity for countries and other individual entities to identify
any problems they see with proposed new names.
3.7. Mapping for TLDs, Not Domain Names or Keywords
It should be clear to anyone who has read this far that the mapping
described in this document is limited to TLDs, not full domain names
or keywords. In particular, nothing here should be construed as
applying to anything other than TLDs, due at least in part to the
limitations described in Section 3.1. Further, this document is only
about the domain name system (DNS), not about any keyword system.
The interactions between particular keyword systems and the proposals
here are left as a (possibly very difficult) exercise for the reader
or implementer of such systems. However, for the subset of such
systems whose intent is to entirely hide DNS names or URIs from the
user, their output would presumably be the LDH names that actually
appeared in the DNS, i.e., in punycode form for IDNA names and
without any application processing of the type contemplated here.
4. Information Interchange, IDNs, Comparisons, and Translations
This specification is based on a pair of fairly explicit assumptions.
The first is that the greatest and most important impact and value of
any internationalization or localization technique is to permit users
who share a language or culture to communicate with others who also
share that language or culture. Communication among users from
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different cultures, using different languages or different scripts is
inherently more difficult, and still more difficult if they cannot
easily identify languages and scripts in common. The reason for
those difficulties are age-old issues in language translation and
differences among languages and scripts, not problems associated with
the DNS or IDNs, however they are represented. That is the second
assumption: when communication across language or cultural groups is
required, the users who need to do it -- typically a much smaller
number than those communicating within the same language and culture
-- are going to need to rely on commonly-understood languages and
scripts and will need to exert somewhat more care and effort than
within their own groups.
As outlined in the sections above, the suggestions made in this
document could clearly be turned into major problems by misuse or
misunderstanding. For example, if two applications on the same host
used different translation tables, a situation could easily result
that would be very confusing to the user. However, in some cases,
this would be only slightly worse than some of the alternatives. For
example, if, on a given system, IDNs are expressed in native script,
but ASCII TLD names are used, cutting and pasting from one
application to another may not work as expected, unless both
applications and the underlying operating system are all Unicode-
based and use the same encoding model for Unicode. Some applications
writers have already discovered, even without significant use of
IDNs, that they need to support separate "copy string" and "copy link
location", and the corresponding "paste" operations. Any use of IDNs
or Internationalized Resource Identifiers (IRIs, see [RFC3987]) may
require similar operations, or extensions to those operations, to
force strings into internal ("punycode" or URI) form on the copy
operation and to translate them back on paste. Were that done, the
appropriate translations could be performed as part of the same
process. If this author's hypothesis is correct -- that these
operations are likely to be required on many systems whether this
proposal is adopted or not -- then the additional translation
operations are likely to be invisible to the user.
In particular, precisely because the translated names proposed here
are part of a presentation form, rather than the internal form names,
they are inappropriate in a number of circumstances in which a
globally-unique, internal-form name is actually required. It would
be a poor, indeed dangerous, idea to use these names in security
contexts such as names in certificates, access lists, or other
contexts in which accurate comparisons are necessary.
A more general issue exists when DNS or IRI references are
transferred among users whose systems may be localized for different
languages or conventions. In general, a user in one part of the
Klensin Informational [Page 14]RFC 4185 Characters for DNS TLD Names October 2005
world will not actually know how another user's systems are set up,
precisely what software is being used, etc., nor should users be
expected or forced to learn that information. But, if the user
transmitting an internationalized reference doesn't know that the
receiving system supports the same characters and fonts, and that the
receiving user is prepared to deal with them, the prudent user will
transmit the internal form of the reference in addition to, or even
instead of, the native-character form. And, of course, if the
reference is transmitted on paper, on a sign, in some coded character
set other than Unicode, or even as an image, rather than as a Unicode
string, the importance of supplementing it with the internal form
becomes even more important. The addition of a translation
requirement for TLD labels makes availability of internal forms in
interchange significantly more important, but does not actually
change the requirement to do so.
It may be helpful to note that, in a different networking model than
that used in the Internet, both this proposal and IDNA itself are
essentially "presentation layer" approaches rather than constructions
that can be expected to work well in interchange.
5. Internationalization Considerations
This entire specification addresses issues in internationalization
and especially the boundaries between internationalization and
localization and between network protocols and client/user interface
actions.
6. Security Considerations
IDNA provides a client-based mechanism for presenting Unicode names
in applications while passing only ASCII-based names on the wire. As
such, it constitutes a major step along the path of introducing a
client-based presentation layer into the Internet. Client-based
presentation layer transformations introduce risks from non-
conforming tables that can change meaning without external
protection. For example, if a mapping table normally maps A onto C,
and that table is altered by an attacker so that A maps onto D
instead, much mischief can be committed. On the other hand, these
are not the usual sort of network attacks: they may be thought of as
falling into the "users can always cause harm to themselves"
category. The local translation model outlined here does not
significantly increase the risks over those associated with IDNA, but
may provide some new avenues for exploiting them.
Both this approach and IDNA rely on having updated programs present
information to the user in a very different form than the one in
which it is transmitted on the wire. Unless the internal (wire) form
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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.
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RFC 4185 Characters for DNS TLD Names October 2005
8. Informative References
[ISO10646] International Organization for Standardization,
"Information Technology - Universal Multiple-octet coded
Character Set (UCS) - Part 1: Architecture and Basic
Multilingual Plane", ISO Standard 10646-1, May 1993.
[ISO3166] International Organization for Standardization, "Codes for
the representation of names of countries and their
subdivisions -- Part 1: Country codes", ISO Standard
3166-1:1977, 1997.
[MIME] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet
Mail Extensions): Mechanisms for Specifying and Describing
the Format of Internet Message Bodies", RFC 1341, June
1992.
Updated and replaced by Freed, N. and N. Borenstein,
"Multipurpose Internet Mail Extensions (MIME) Part One:
Format of Internet Message Bodies", RFC2045, November
1996. Also, Moore, K., "Representation of Non-ASCII Text
in Internet Message Headers", RFC 1342, June 1992.
Updated and replaced by Moore, K., "MIME (Multipurpose
Internet Mail Extensions) Part Three: Message Header
Extensions for Non-ASCII Text", RFC 2047, November 1996.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1480] Cooper, A. and J. Postel, "The US Domain", RFC 1480, June
1993.
[RFC1591] Postel, J., "Domain Name System Structure and Delegation",
RFC 1591, March 1994.
[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection", RFC
2672, August 1999.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
Klensin Informational [Page 17]RFC 4185 Characters for DNS TLD Names October 2005
[RFC3467] Klensin, J., "Role of the Domain Name System (DNS)", RFC
3467, February 2003.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)", RFC
3491, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3696] Klensin, J., "Application Techniques for Checking and
Transformation of Names", RFC 3696, February 2004.
[RFC3932] Alvestrand, H., "The IESG and RFC Editor Documents:
Procedures", BCP 92, RFC 3932, October 2004.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
Author's Address
John C Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
USA
Phone: +1 617 491 5735
EMail: john-ietf@jck.com
Klensin Informational [Page 18]RFC 4185 Characters for DNS TLD Names October 2005
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