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Versions: 00 01 02 03 04 RFC 3467

INTERNET-DRAFT                                John C. Klensin
February 25, 2002
Expires July 2002

                  Role of the Domain Name System

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups.  Note that
other groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

This document represents a summary of the personal opinions of the
author on the subject covered and is not intended to evolve into a
standard of any kind.

Copyright Notice

Copyright (C) The Internet Society (2000).  All Rights Reserved.

0. Abstract

The original function and purpose of the DNS is reviewed, and
contrasted with some of the functions into which it is being forced
today and some of the newer demands being placed upon it or suggested
for it.  A framework for an alternative to placing these additional
stresses on the DNS is then outlined.  This document and that
framework are not a proposed solution, only a strong suggestion that
the time has come to begin thinking more broadly about the problems
we are encountering and possible approaches to solving them.

A mailing list has been initiated for discussion of this draft, its
successors, and closely-related issues at ietf-irnss@lists.elistx.com.
See http://lists.elistx.com/archives/ for subscription and archival

Table of Contents

0. Abstract
1. History
1.1  Context for DNS development
1.2 Review of the DNS and its role as designed
1.3 The web and user-visible domain names
1.4 A pessimistic history of the evolution of Internet applications
2. Signs of DNS overloading
3.  The search system story.
3.1 Overview
3.2 Some details and comments.
4.  Examining internationalization
4.1. ASCII isn't just because of English
4.2.  The "ASCII Encoding" approaches
4.3.  "Nameprep" and its complexities
4.4 The UCS Stability Problem
4.5. Audiences, end users, and the UI problem
4.6 Business cards and other natural uses of natural languages
4.7 ASCII encodings and the Roman keyboard assumption
4.8 A pessimistic summary of IDN WG directions
5.  The Key Controversies
5.1. One directory or many
5.2 Why not a proposal?
6.  Security Considerations
7.  References
7.1. Normative References
7.2. Explanatory and Informative References
8. Acknowledgements
10. Culprit address

1. History

Several of the comments that follow are somewhat revisionist.  Good
design and engineering often requires a level of intuition by the
designers about things that will be necessary in the future; the
reasons for some of these design decisions are not made explicit at
the time because no one is able to articulate them.  The discussion
below reconstructs some of the decisions about the Internet's primary
namespace (the "Class=IN" DNS) in the light of subsequent development
and experience.  In addition, the historical reasons for particular
decisions about the Internet were often severely underdocumented
contemporaneously and, not surprisingly, different participants have
different recollections about what happened and what was considered
important.  Consequently, the quasi-historical story below is just
one story.  There may be (indeed, almost certainly are) other stories
about how we got to where we are today, but they probably don't, of
themselves, invalidate the inferences and conclusions.

1.1  Context for DNS development

During the entire post-startup-period life of the ARPANET and nearly
the first decade or so of operation of the Internet, the list of host
names and their mapping to and from addresses was maintained in a
frequently-updated "host table" [RFC625, 811, 952].  The names
themselves were restricted to a subset of ASCII chosen to avoid
ambiguities in printed form, to permit interoperation with systems
using other character codings (notably EBCDIC), and to avoid the
"national use" code positions of ISO 646 [IS646]. This table was
just a list with a common format that was eventually agreed-upon;
sites were expected to frequently obtain copies of, and install, new
versions.  The host tables themselves were introduced to

  * Eliminate the requirement for people to remember host numbers
  (addresses).  Despite apparent experience to the contrary in the
  conventional telephone system, numeric numbering systems, including
  the numeric host number strategy, did not (and do not) work well for
  more than a (large) handful of hosts.

  * Provide stability when addresses changed.  Since addresses --to
  some degree in the ARPANET and more importantly in the contemporary
  Internet-- are a function of network topology and routing, they
  often had to be changed when connectivity or topology changed.  The
  names could be kept stable even as addresses changed.

  * Some hosts (so-called "multihomed" ones) needed multiple
  addresses to reflect different types of connectivity and topology.
  Again, the names were very useful for avoiding the requirement that
  would otherwise exist for users and other hosts to track these
  multiple host numbers and addresses and the topological
  considerations for selecting one over others.

Toward the end of that long (in network time) period, the community
concluded that the host table model did not scale adequately and that
it would not adequately support new service variations.  A working
group was created, and the DNS was the result of that effort.  The
role of the DNS was to preserve the capabilities of the host table
arrangements (especially unique, unambiguous, host names), provide for
addition of additional services (e.g., the special record types for
electronic mail routing which quickly followed introduction of the
DNS), and to do so on the base of a robust, hierarchical, distributed,
name lookup system.  That system also permitted distribution of name
administration, rather than requiring that each host be entered into a
single, central, table by a central administration.

1.2 Review of the DNS and its role as designed

The DNS was designed primarily to identify network resources.
Although there was speculation about including, e.g., personal names
and email addresses, it was not designed primarily to identify people,
brands, etc.  At the same time, the system was designed with the
flexibility to accomodate new data types and structures, both through
the addition of new record types to the initial "INternet" class, and,
potentially, through the introduction of new classes.  Since the
appropriate identifiers and content of those future extensions could
not be anticipated, the design provided that these fields could
contain any (binary) information, not just the` restricted text forms
of the host table.

However, the DNS as-used is intimately tied to the applications and
application protocols that utilize it, often at a fairly low level.

In particular, despite the ability of the protocols and data
structures themselves to accomodate any binary representation, DNS
names as used are historically not [even] ASCII, but a very
restricted subset of it, a subset that derives primarily from the
original host table naming rules.  Selection of that subset was
driven in part by human factors considerations, including a desire to
eliminate possible ambiguities in an international context.  Hence
character codes that had international variations in interpretation
were excluded, the underscore character and case distinctions were
eliminated as being confusing (in the underscore's case, with the
hyphen character) when written or read by people, and so on.  These
considerations appear to be very similar to those that resulted in
similarly restricted character sets being used as protocol elements
in many ITU and ISO protocols (cf. [X29]).

Another assumption was that there would be a high ratio of physical
hosts to second level domains and, more generally, that the system
would be deeply hierarchical, with most systems (and names) at the
third level or below and a large ratio of names representing physical
hosts to total names.  There are domains that follow this model: many
university and corporate domains use fairly deep hierarchies, as do a
few country code TLDs (".US" is an excellent example).  However, the
RIPE hostcount list is now showing a count of SOA records that is
approaching (and may have passed) the number of distinct hosts.
While recent experience has shown that the DNS is robust enough
--given contemporary machines as servers and current bandwidth
norms-- to be able to continue to operate reasonably well when those
historical assumptions are not met (e.g., with a huge, flat,
structure under ".COM"), it is still useful to remember that the
system could have been designed to work optimally with a flat
structure (and very large zones) rather than a deeply hierarchical
one, and was not.

Similarly, despite some early speculation about entering people's
names and email addresses into the DNS directly, with the sole
exception (at least in the "IN" class) of one field of the SOA
record, electronic mail addresses in the Internet have preserved the
original, pre-DNS, "user at location" conceptual format rather than a
flatter or strictly faceted one.  Location, in that instance, is a
reference to a host.

Both the DNS architecture itself and the two-level provisions for
email and similar functions (e.g., see the finger protocol), also
anticipated a relatively high ratio of users to actual hosts.  Despite
the observation in RFC 1034 that the DNS was expected to grow to be
proportional to the number of users (section 2.3), it has never been
clear that the DNS was seriously designed for, or could, scale to the
order of magnitude of number of users (or, more recently, products or
document objects), rather than that of physical hosts.

Like the host table before it, the DNS has provided criticial
uniqueness for names and universal accessibility to them as part of
overall "single internet" and "end to end" models (cf [RFC2826]).
However, there are many signs that, as new uses evolve and original
assmumptions are abused, the system is being stretched to, or beyond,
its practical limits.

The original design effort that led to the DNS included examination
of the directory technologies available at the time.  The working
group concluded that the DNS design, with its simplifying assumptions
and restricted capabilities, would be feasible to deploy and make
adequately robust, which the more comprehensive directory approaches
were not.  At the same time, some of the participants feared that the
limitations might cause future problems; this document essentially
takes the position that they were probably correct.  On the other
hand, directory technology and implementations have evolved
significantly in the ensuing years: it may be time to revisit the
assumptions, either in the context of the two- (or more) level
mechanism contemplated by the rest of this document or, even more
radically, as a path toward a DNS replacement.

1.3 The web and user-visible domain names

From the standpoint of the integrity of the domain name system --and
scaling of the Internet, including optimal accessibility to content--
the web design decision to use "A record" domain names, rather than
some system of indirection, has proven to be a serious mistake in
several respects.  Convenience of typing, and the desire to make
domain names out of easily-remembered product names, has led to a
flattening of the DNS, with many people now perceiving that
second-level names under COM (or in some countries, second- or
third-level names under the relevant ccTLD) are all that is
meaningful (this perception has been reinforced by some domain name
registrars who have been anxious to "sell" additional names).  And,
of course, the perception that one needs a top-level domain per
product, rather than a (usually organizational) collection of network
resources has led to a rapid acceleration in the number of names
being registered, a phenonenum that has clearly benefited registrars
charging on a per-name basis, "cybersquatters", and others in the
business of "selling" names, but has not obviously benefitted the
Internet as a whole.

The emphasis on second-level domain names has also created a problem
for the trademark community.  Since the Internet is international,
and names are being populated in a flat and unqualified space,
similarly-named entities are in conflict even if there would
ordinarily be no chance of confusing them in the marketplace.  The
problem appears to be unsolvable except by a choice between draconian
measures --possibly including significant changes to the underlying
legislation and conventions-- and a situation in which the "rights"
to a name are typically not settled using the subtle and traditional
product (or industry) type and geopolitical scope rules of the
trademark system but by depending largely on main force, e.g., the
organization with the greatest resources to invest in defending (or
attacking) names will ultimately win out.  The latter raises not only
important issues of equity, but the risk of backlash as the numerous
small players are forced to relinquish names they find attractive and
to adopt less-desirable naming conventions.

Independent of these sociopolitical problems, content distribution
issues have made it clear that it should be possible for an
organization to have copies of data it wishes to make available
distributed around the network, with a user who asks for the
information by name getting the topologically-closest copy.  This is
not possible with simple, as-designed, use of the DNS: DNS names
identify target resources or, in the case of email "MX" records, a
preferentially-ordered list of resources "closest" to a target (not
to the source/user).  Several technologies (and, in some cases,
corresponding business models) have arisen to work around these
problems, including intercepting and altering DNS requests so as to
point to other locations,

While additional implications are still being discovered and
seriously evaluated, it appears, not surprisingly, that rewriting DNS
names in the middle of the network, or trying to give them different
values or interpretations depending on the topological location of
the user trying to resolve the name interferes, in the general case,
with end-to-end applications.  These problems occur even if the
rewriting machinery is accompanied by additional workarounds for
particular applications: security associations and applications that
need to identify "the same host" as the applications for which these
tools have been designed often run into one problem or another.

1.4 A pessimistic history of the evolution of Internet applications

At the applications level, few of the protocols in active, widespread
use on the Internet reflect the either contemporary knowledge in
computer science or human factors or experience accumulated through
deployment and use.  Instead, protocols tend to be deployed at a
just-past-prototype level, typically including the types of expedient
compromises typical with prototypes.  If they prove useful, the
nature of the network permit very rapid dissemination (i.e., they
fill a vacuum, even if a vacuum that no one previously knew existed).
But, once the vacuum is filled, the installed base provides its own
inertia: unless the design is so seriously faulty as to prevent
effective use (or there is a widely-perceived sense of impending
disaster unless the protocol is replaced), future developments must
maintain backward compatibility and workarounds for problematic
characteristics rather than benefiting from redesign in the light of
experience.  Applications that are "almost good enough" prevent
development and deployment of high-quality replacements.

2. Signs of DNS overloading

Parts of the historical discussion above identify areas in which it
is becoming clear that the DNS is becoming overloaded (semantically
if not in the mechanical ability to resolve names).  While we seem to
still be well within the "just about good enough" range -- current
mechanisms and proposals to deal with these problems are all focused
on patching or working around limitations within the DNS rather than
dramatic rethinking -- the number of these issues that are arising
at the same time may argue for rethinging mechanisms and
relationships, not just more patches and kludges.  For example:

o While technical approaches such as larger and higher-powered
servers and more bandwidth, and legal/political mechanisms such as
dispute resolution policies, have arguably kept the problems from
becoming critical, the DNS has not proven adequately responsive to
business and individual needs to describe or identify things (such as
product names and names of individuals) other than strict network

o While stacks have been modified to better handle multiple addresses
on a physical interface and some protocols have been extended to
include DNS names for determining context, the DNS doesn't deal
especially well with high-multiple names per host (needed for web
hosting facilities with multiple domains on a server).

o Efforts to add names deriving from languages or character sets
based on other than simple ASCII and English-like names (see below),
or even to utilize complex company or product names without the use
of hierarchy have created apparent requirements for names (labels)
that are over 63 octets long.  This requirement will undoubtedly
increase over time; while there are workarounds to accomodate longer
names, they impose their own restrictions and cause their own

o Increasing commercialization of the Internet, and visibility of
domain names that are assumed to match names of companies or
products, has turned the DNS and DNS names into a trademark
battleground.  The traditional trademark system in (at least) most
countries makes careful distinctions about fields of applicability.
When the space is flattened, without differentiators by either
geography or industry sector, not only are there likely conflicts
between "Joe's Pizza" (of Boston) and "Joe's Pizza" (of San
Francisco) but between both and "Joe's Auto Repair" (of Los Angeles):
all three would like to control "Joes.com" (and would prefer, if it
were permitted by DNS naming rules, to spell it as "Joe's.com" and
have both resolve the same way) and may claim trademark rights to do
so, even though conflict or confusion would not occcur with
traditional trademark principles.

o Many organizations wish to have different web sites under the same
URL and domain name.  Sometimes this is to create local variations
--the Widget Company might want to present different material to a UK
user relative to a US one-- and sometimes it is to provide higher
performance by supplying information from the server topologically
closest to the user.  Arguably, the name resolution mechanism should
provide information about multiple sites that can provide information
associated with the same name and sufficient attributes associated
with each of those sites to permit applications to make sensible
choices, or should accept client-site attributes and utilize them in
the search process.    Or, it should be able to return different
answers based on the location or identity of the requestor.  While
there are some tricks that can provide partial simulations of this
type of function, DNS responses cannot be reliably conditioned in
this way.

o Many existing and proposed systems for "finding things on the
Internet" require a true search capability in which near matches can
be reported to the user, or to some user agent with an apppropriate
rule-set, and queries may be slightly ambiguous or fuzzy.  The DNS
can accomodate only one set of (quite rigid) matching rules.  Current
proposals to permit different rules in different localities help to
identify the problem, but, if applied directly to the DNS, either
don't provide the level of flexibility that would be desirable or
tend to isolate different parts of the Internet from each other (or
both).  Fuzzy or ambiguous searches are desirable for (at least)
resolution of business names that might have spelling variations and
for names that can be resolved into different sets of glyphs
depending on context.  This goes beyond "mere" canonicalization
differences (different ways of representing the same character or
ordering the same string) and into such relationships as the use of
different alphabets for the same language, Kanji-Hiragana
relationships, Simplified and Traditional Chinese, etc.

o The historical DNS and applications that make assumptions about how
it works impose significant risk (or forces technical kludges and
consequent odd restrictions), when one considers adding mechanisms
for use with various multi-character-set and multilingual
"internationalization" systems.  Cf RFC 2825.

o In order to provide proper functionality to the Internet, the DNS
must have a single unique root (see RFC 2826 for a discussion of this
issue).  There are many desires for local treatment of names or
character sets that cannot be accomodated without either multiple
roots (e.g., a separate root for multilingual names) or mechanisms
that would have similar effects in terms of Internet fragmentation
and isolation.

o For some purposes, it is desirable to be able to search targets
(i.e., by value, not just by name (label)).  One might, for example,
want to locate all of the host (and virtual host) names which cause
mail to be directed to a given server via MX records.  The DNS does
not support this capability and it can be simulated only by
extracting all of the relevant records (perhaps by zone transfer if
the source doesn't prohibit that through access lists) and then
searching a file built from those records.

o Finally, as additional types of personal or identifying information
are added to the DNS, issues of protection of that information and
making different information available based on the credentials and
authorization of the source of the inquiry.  As with site locational
and proximity information (as discussed above), the DNS protocols
make the mechanisms needed to do this quite difficult if not

In each of these cases, it is, or might be, possible to devise ways
to trick the DNS system into supporting mechanisms that were not
designed into it.  Several ingenious solutions have been proposed in
many of these areas already, and some have been deployed into the
marketplace with some success.

Several of the above problems are addressed well by a good directory
system (supported by the LDAP protocol or some protocol more
precisely suited to these specific applications) or searching
environment (such as common web search engines) although not by the
DNS.  Given the difficulty of deploying new applications discussed
above, an important question is whether the tricks and kludges are
bad enough, or will scale up to bad enough, that new solutions are
needed and can be deployed.

3.  The search system story.

3.1 Overview

The constraints of the DNS argue for introducing an intermediate
protocol mechanism, referred to here as a "search layer".  The
terms "directory" and "directory system" are used interchangably with
"searchable system" in this document although the latter is far more
precise.  Search layer proposals would use a two (or more) -stage
lookup, not unlike several of the proposals for internationalized
names in the DNS (see section 4), but all operations but the final
one would involving searching other systems, rather than looking up
identifiers in the DNS itself.  This would permit us to relax several
constraints and produce a more comprehensive system.

Ultimately, many of the issues with domain names arise as the result
of people attempting to use the DNS as a directory.  While there has
not been enough pressure/demand to justify a change to date, it has
already been quite clear that, as a directory system, the DNS is a
good deal less than ideal.  This document suggests that there
actually is a requirement for a directory system, and that the right
solution to a searchable system requirement is a searchable system,
not a series of DNS patches, kludges, or workarounds.

In particular...

o A directory system would not require imposition of particular
length limits on names.

o A directory system could permit explicit association of attributes
of, e.g., language and country, with a name, without having to
utilize trick encodings to incorporate that information in DNS labels
(or creating artificial hierarchy for doing so).

o There is considerable experience (albeit not much of it very
successful) in doing fuzzy and "sonex" (similar-sounding) matching in
directory systems.  Moreover, it is plausible to think about
different matching rules for different areas and sets of names so
that these can be adapted to local cultural requirements.
Specifically, it might be possible to have a single form of a name in
a directory, but to have great flexibility about what queries matched
that name (and even have different variations in different areas).
Of course, the more flexibility one provides, the greater the
possibility of real or imagined trademark conflicts, but we would
have the opportunity to design a directory structure that dealt with
those issues in an intelligent way, while DNS constraints arguably
make a general and equitable DNS-only solution impossible.

o If a directory system is used to translate to DNS names, and then
DNS names are looked up in the normal fashion, it may be possible to
relax several of the constraints that have been traditional (and
perhaps necessary) with the DNS.  For example, reverse-mapping of
addresses to directory names may not be a requirement, since the DNS
name(s) would (continue to) uniquely identify the host.

o Solutions to multilingual transcription problems that are common in
"normal life" (e.g., two-sided business cards to be sure that a
recipient trying to contact a person can access romanized spellings
and numbers when the original language may not be comprehensible to
that recipient) can be easily handled in a directory system by
inserting both sets of entries.

o One can easily imagine a directory system that would return, not a
single name, but a set of names paired with network-locational
information or other context-establishing attributes.  This type of
information might be of considerable use in resolving the "nearest
(or best) server for a particular named resource" problems that are a
significant concern for organizations hosting web and other sites
that are accessed from a wide range of locations and subnets.

o Names bound to countries and languages might help to manage
trademark realities, while use of the DNS in trademark-significant
areas tends to require worldwide "flattening" of the trademark

3.2 Some details and comments.

Almost any internationalization (i18n) proposal for names that are
in, or map into, the DNS will require changing DNS resolver API calls
("gethostbyname" or equivalent or adding some pre-resolution
preparation mechanism) in almost all Internet applications -- whether
to cause the API to take a different character set (no matter how it
is then mapped into the bits used in the DNS or another system), to
accept or return more arguments with qualifying or identifying
information, or otherwise.  Once applications must be opened to make
such changes, it is a relatively small matter to switch from calling
into the DNS to calling a directory service and then the DNS (in many
situations, both actions could be accomplished in a single API call).

A directory approach can be consistent both with "flat" models and
multi-attribute ones.  The DNS requires strict hierarchies, limiting
its ability to handle differentiation among names by their
properties.  By contrast, modern directories can utilize
independently-searched attributes and other structured schema to
provide flexibilities not present in a strictly hierarchical system.

There is a strong argument for a single directory structure (implying
a need for mechanisms for registration, delegation, etc.).  But it is
not a strict requirement, especially if in-depth case analysis and
design work leads to the conclusion that reverse-mapping to directory
names is not a requirement (see section 4).

Conversely, there is a case to be made for, e.g., faceted systems in
which most of the facets use restricted vocabularies.  Such systems
could be designed to avoid the need for procedures to ensure
uniqueness across, or even within, providers and databases of the
faceted entities being searched for.  (Cf. [DNS-Search] for further

While the discussion above includes very general comments about
attributes, it appears that only a very small number of attributes
would be needed.  The list would almost certainly include country and
language for IDN purposes and might require "charset" if we cannot
agree on a character set and encoding.  Trademark issues might
motivate "commercial" and "non-commercial" (or other) attributes if
they would be helpful in bypassing trademark problems.  And
applications to resource location might argue for a few other
attributes (as outlined above).

4.  Examining internationalization

Much of the thinking underlying this document has been driven by
considerations of internationalizing the DNS or, more specifically,
providing access to the functions of the DNS from languages and
naming systems that cannot be accurately expressed in ASCII (or in
the traditional DNS subset of ASCII).  Much of this work has been
done in the "IETF Internationalized Access to Domain Names" (IDN)
Working Group.  This section contains an evaluation of what that
group has learned and how that learning might reasonably impact
IETF's next steps.  It assumes familiarity with the work and
terminology of that working group.

When the IDN effort started, several of us made the observation that
the first important task for the WG was an undocumented one: to
increase the understanding of the complexities of the problem
sufficiently that naive solutions could be rejected and people could
go to work on the harder problems.  That has clearly been
accomplished.  With the exception of some continuing background noise,
the simplistic approaches, with promises of one-year deployment, have
just disappeared and almost no one thinks the issues are simple any

But some of the lessons learned are quite painful and should give us
pause, both generally and in the context of the remarks above:

4.1. ASCII isn't just because of English

The hostname rules chosen in the mid-70s weren't just "ASCII because
English uses ASCII", although that was a starting point.  We have
discovered that almost every other script (and, I think, even ASCII if
we permit the rest of the characters specified in to ISO 646
International Reference Version) is more complex than hostname-
restricted-ASCII.  In some cases, with a broader selection of scripts,
case mapping works from one case to the other, but is not reversible.
In others, there are conventions about alternate ways to represent
characters (in the language, not [just] in character coding) that work
most of the time, but not always.  And there are issues in coding,
with Unicode/10646 [UNICODE, IS10646] providing different ways to
represent the same character (I am using that word, rather than
"glyph", deliberately here).  And, in still others, there are
questions as to whether two glphs "match", which may be a
distance-function question, not one with a binary answer.  We have
tried to solve this set of problems with "nameprep" (see below).

4.2.  The "ASCII Encoding" approaches

While the DNS can handle arbitrary binary strings without known
internal problems (see [RFC2181]), some restrictions are imposed by
the requirement that text be interpreted in a case-independent way
([RFC1034], [RFC1035]).  More important, most internet applications
assume the hostname-restricted (so-called "LDH) syntax specified in
the hosttable RFCs and as "prudent" in RFC 1035.  Many conforming
implementations of those applications may exhibit unpredicted behavior
if those assumptions are not met.  To avoid these potential problems,
the work of the IDN WG has focused on "ASCII-Compatible Encodings"
(ACE), which preserve the LDH conventions in the DNS itself (and for
implementations of applications that have not been upgraded) while
permitting newer implementations to recognize the special codings and
map them into non-ASCII characters.  These approaches are, however,
not problem-free.  Among other issues, they rely on what is ultimately
a heuristic to determine whetehr a DNS lablel is to be considered as
an IDN or interpreted as an actual LDH name in its own right.  And,
while all determination of whether a particular query matches a stored
object are traditionally made by DNS servers, the ACE systems, when
combined with the complexities of international scripts and names,
require that much of the matching work be abstracted into a separate,
client-side, "preparation" process.

4.3.  "Nameprep" and its complexities

The model for getting around the various problems described above and
elsewhere has evolved into a notion that all strings are to be placed
into the DNS only after being passed through a string preparation
function that eliminates or rejects spurious character codes, maps
some characters onto others, performs some sequence canonicalization,
and generally creates forms that can be accurately compared.  The
impact of this process on host-table-subset ASCII is trivial and
essentially adds only overhead.  For other scripts, the impact is, of
necessity, quite significant.

Defining that process was quite complex and, as of the time of this
writing, remains controversial.  Although the general notion
was simple, the devil is often in the details, and there are many
details.  A design team worked on it for months, with considerable
effort placed into clarifying and fine-tuning the protocol.  Despite
general agreement that the IETF would avoid getting into the business
of defining character sets, character codings, and the associated
conventions, the group several times took excursions into
special treatment of code positions to more nearly match the
distinctions of Unicode with user-perceptions about similarities and
differences between characters.  The IETF-specific code position work
has been removed from the protocol draft, but the fact that the
temptation has been strong may indicate problems we haven't solved to
everyone's satisfaction.

There have also been controversies about how far one should go in
these processes of preparation and transformation and,
ultimately, about the validity of various analogies.  Is stripping of
vowels in Arabic or Hebrew analogous to case-mapping?  Matching of
characters that appear to be the same but that are assigned to
different code points?  Mapping of Traditional and Simplified Chinese
characters?  Matching of Serbo-Croatian words whether written in
Roman-derived or Cyrillic characters?

At the same time, the nameprep work has been extremely useful, both
in identifying many of the problem code points and issues and
providing a reasonable set of rules.  The problem is arguably not
with nameprep, but with the DNS-imposed requirement that nameprep, as
with all other parts of the matching and comparison process, yield a
binary "match or no match" answer, rather than, e.g., a value on a
similarity scale that can be evaluated by the user or by user-driven
heuristic functions.

4.4 The UCS Stability Problem

ISO 10646 basically defines only code points, and not rules for using
or comparing the characters.  This is a long- standing issue with
standards coming out of ISO/IEC JTC1/SC2; internationalization issues,
as contrasted with character-listing and code point assignment issues,
have just not been effectively dealt with in that group.  The Unicode
Technical Committee has defined some rules for canonicalization and
comparision, many of which have been factored into the "nameprep"
work, but it is not straightforward to make or define those rules in a
sufficiently precise and permanent fashion that the DNS can depend on
them.  Perhaps more important, our nameprep efforts have identified
several areas in which the UTC rules do not adequately define things
to make matching precise and unambiguous.  For example, it is tempting
to define some rules on the basis of membership in particular scripts,
or for punctuation characters, but there is not precise definition of
what characters belong to which script or which ones are, or are not,
punctuation.  That raises two issues: whether trying to do precise
matching at the character set level is actually possible (addressed
below) and whether driving toward more precision could create issues
that cause instability in the implementation and resolution models.

In addition, JTC1 has recently assigned some (most?) of these issues
to JTC1/SC22/WG20 (the Internationalization WG within the
subcommittee that deals with programming languages, systems, and
environments).  WG20 is historically strong and deals with
internationalization issues thoughtfully and in depth.  Whether or
not they get it right, assignment of these matters to WG20
significantly increases the risk of an eventual ISO standard that
specifies different behavior from the UTC specification.

4.5. Audiences, end users, and the UI problem

Part of what has "caused" the DNS i18n problem, as well as the DNS
trademark problem and several others, is that we have stopped thinking
about "identifiers for objects", which normal people are not expected
to see, and started thinking about "names" -- strings that are
expected not only to be readable, but to have linguistically-sensible
and culturally-dependent meaning to non-specialist users.

The IDN WG, and others, have attempted to avoid addressing the
implications of that transition by taking "someone else's problem"
approaches or by suggesting that we can adopt conventions to which
people will just become accustomed.  I suggest that neither will work

  * If we want to make it a problem in a different part of the
  UI structure, we need to figure out where it goes in order
  to have proof of concept of our solution.  Unlike those
  whose sole [business] model is the selling or registering of
  names, any solution IETF produces actually needs to work, in
  applications context, as seen by the end user.

  * The "they will get used to our conventions and adapt"
  principle is fine if we are writing rules for programming
  languages or an API.  But the conventions we are talking
  about aren't part of a semi-mathematical system, they are
  deeply ingrained in culture.  No matter how often we tell an
  English-speaking American that the Internet requires that the
  correct spelling of "colour" be used, he or she isn't going to be
  convinced. Getting a French-speaker in Lyon to use exactly
  the same lexical conventions as a French-speaker in Quebec
  in order to accomodate the decisions of the IETF or of a
  registrar or registry is just not likely.  "Montreal" is
  either a misspelling or an anglicization (anglicisation?) of
  Montrëal (with an acute accent mark over the "e"), but we
  are as unlikely to get global agreement on a rule that will
  determine whether the two forms should match --and that
  won't astonish end users and speakers of one language or the
  other-- as we are to get agreement on whether "misspelling"
  or "anglicization" is the greater travesty.

More generally, it is not clear that the outcome of any conceivable
nameprep-like process is going to be good enough.  In the use of
human languages by humans, we have many cases in which things that do
not match are nonetheless interpreted as matching.  The
Norwegian/Danish glyph "—" (lower case 'o' with forward slash) and
the German glyph "§" (lower case 'o' with umlaut) are clearly
different and no matching program should yield an "equal" comparison.
But they are more similar than either of them is to, e.g., "e", and
humans are able to mentally make the correction in context and can be
surprised if computers cannot do so.

This text uses examples in Roman scripts because it is being written
in English and those examples are relatively easy to render.  But one
of the important lessons of the IDN discussions of the recent years is
that problems like this exist in almost every language and script.
Each one has its idiosyncracies, and each set of idiosyncracies is
tied to common usage and cultural issues that are deeply embedded.  As
long as a schoolchild in the US can get a bad grade on a spelling test
for using a perfectly valid British spelling, or one in France or
Germany can get a poor grade for leaving off a diacritical mark, or
one in Egypt or Israel will find it acceptable to write a word with or
without vowels or stress marks, but, if they are included, that they
must be the correct ones, there are issues with the relevant language.
We are dealing with culture, not identifier symbol-strings for geeks
or computers, and the recent efforts have made it ever more clear
that, if we ignore that distinction, we are solving an insufficient

4.6 Business cards and other natural uses of natural languages

We have some established local conventions in the world for dealing
with multilingual situations.  Looking at them may be helpful.  If
one visits a country where the language is different from ones own,
business cards are often printed on two sides, one side in each
language.  This is usually a high-tolerance situation: exact
translations are often not possible, and people typically smile at
errors, appreciate the effort, and move on.  The DNS situation
differs from this in at least two ways: since we need a global
solution, the business card would need a number of sides
approximating the number of languages in the world, which is probably
impossible without violating laws of physics.  And the opportunities
for tolerance don't exist: the DNS requires a exact match or the
lookup fails.

4.7 ASCII encodings and the Roman keyboard assumption

Part of the argument for ACE-based solutions is that they provide an
escape for multilingual environments when applications have not been
upgraded.  When an older application encounters an ACE-based name,
the assumption is that the (admittedly ugly) ASCII string will be
displayed and can be typed in.  This argument is reasonable from the
standpoint of mixtures of Latin-based alphabets, but may not be
relevant if user-level systems and devices are involved that do not
support the entry of Roman-based characters or which cannot
conveniently render such characters.

4.8 A pessimistic summary of IDN WG directions

It appears, from the cases above and others, that none of the
intra-DNS-based solutions for "multilingual names" are workable.  They
just rest on too many assumptions that do not appear to be feasible --
that people will adapt deeply-entrenched language habits to
conventions laid down to make the lives of computers easy; that we can
make "freeze it now, no need for changes in these areas" decisions
about Unicode and nameprep; that ACE will smooth over applications
problems, even in environments without the ability to key or render
roman-based glyphs (or where user experience is such that they cannot
easily be told apart); that the Unicode Consortium will never decide
to repair an error in a way that creates a risk of DNS
incompatibility; that we can either deploy EDNS or that long names
aren't really important; that Chinese computer users (and others) will
either give up their local or IS 2022-based character coding solutions
(for which UTC adding large fractions of a million new code points is
almost certainly a necessary, but probably not sufficient condition)
or build leakproof boundary conversion mechanisms; that out of band or
contextual information will always be sufficient for the "map glyph
onto script" problem; and so on.  In each case, we can get about 80%
or 90%, but it is not clear that is going to be good enough.  For
example, suppose someone can spell her name 90% correctly: is that
likely to be considered adequate?

5.  The Key Controversies

5.1. One directory or many

As suggested in some of the text above, it is an open question as to
whether the needs of the community would be best served by a single
directory with universal applicability, a single directory but
locally-tailored search (and, most important, matching) functions, or
multiple, locally-determined, directories.  Each has its attractions.
Any but the first would essentially prevent reverse-mapping
(determination of the user-visible name of the host or resource from
target information such as an address or DNS name).  But reverse
mapping has become less useful over the years --at least to users--
as we have assigned more and more names per host address.

Locally-tailored search and mappings would permit national variations
on interpretation of which strings matched which other ones, an
arrangement that is especially important when different localities
apply different rules to, e.g., matching of characters with and
without diacriticals.  But, of course, this implies that a URL may
evaluate properly or not depending on either settings on a client
machine or the network connectivity of the user, which is not, in
general, a desirable situation.

And, of course, completely separate directories would permit
translation and transliteration functions to be embedded in the
directory, giving much of the Internet a different appearance
depending on which directory was chosen.  The attractions of this are
obvious, but, unless things were very carefully designed to preserve
uniqueness and precise identities at the right points (which may or
may not be possible), such a system would have many of the
difficulties associated with multiple roots.

5.2 Why not a proposal?

As this document has gone through various preliminary drafts and
reviews, the question has been raised as to whether it should contain
a specific proposal: a specific directory mechanism, schema, and so
on.  It deliberately does not take that step.  It has been difficult
to get directory systems deployed in significant ways in the Internet
infrastructure, partially because we have a surplus of options.
There are also some approaches that could be used to implement the
general concepts described here, such as the Common Name Resolution
Protocol [RFC2972], which some would not consider directory protocols
at all.  Consequently, it appeared better to present the general
concepts and arguments here and leave the specifics to other sources,
documents, and proposals.

6.  Security Considerations

The set of proposals implied by this document suggests an interesting
set of security issues (i.e., nothing important is ever easy).  A
directory system used for this purpose would presumably need to be as
carefully protected against unauthorized changes as the DNS itself.
There also might be new opportunities for problems in an arrangement
involving two or more [sub]layers; but those problems are not more
severe than a two-stage lookup in the DNS.

7.  References

7.1. Normative References


7.2. Explanatory and Informative References





[DNS-Search] draft-klensin-dns-search-02/03.txt, work in progress.

[RFC625] RFC 625 On-line hostnames service. M.D. Kudlick, E.J.
Feinler.  Mar-07-1974.

[RFC811] RFC 811 Hostnames Server. K. Harrenstien, V. White, E.J.
Feinler.  Mar-01-1982.

[RFC952] RFC 952 DoD Internet host table specification. K.
Harrenstien, M.K.  Stahl, E.J. Feinler. Oct-01-1985.

[RFC882] RFC 882 Domain names: Concepts and facilities. P.V.
Mockapetris.  Nov-01-1983.

[RFC883] RFC 883 Domain names: Implementation specification. P.V.
Mockapetris.  Nov-01-1983.

[RFC1035] RFC 1035 Domain names - implementation and specification.
P.V.  Mockapetris. Nov-01-1987.

[RFC1591] RFC 1591 Domain Name System Structure and Delegation. J.
Postel.  March 1994.

[RFC2181] RFC 2181 Clarifications to the DNS Specification. R. Elz, R.
Bush.  July 1997.

[RFC2825] RFC 2825 A Tangled Web: Issues of I18N, Domain Names, and
the Other Internet protocols. IAB, L. Daigle, ed.. May 2000.

[RFC2826] RFC 2826 IAB Technical Comment on the Unique DNS Root. IAB.
May 2000.

[RFC2972] RFC 2972 Context and Goals for Common Name Resolution. N.
Popp, M.  Mealling, L. Masinter, K. Sollins. October 2000.

[X29] International Telecommuncations Union, "Recommendation X.29:
Procedures for the exchange of control information and user data
between a Packet Assembly/Disassembly (PAD) facility and a packet mode
DTE or another PAD", December 1997.

8. Acknowledgements

Many people have contributed to versions of this document or the
thinking that went into it.  The author would particularly like to
thank Harald Alvestrand, Leslie Daigle, Patrik Faltstrom, Eric A.
Hall, and Paul Hoffman for challenging the assumptions of earlier
versions and suggesting ways to improve them.

10. Culprit address

John C Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140

Expires July 2002

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