rfc3467
Network Working Group J. Klensin
Request for Comments: 3467 February 2003
Category: Informational
Role of the Domain Name System (DNS)
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 (2003). All Rights Reserved.
Abstract
This document reviews the original function and purpose of the domain
name system (DNS). It contrasts that history with some of the
purposes for which the DNS has recently been applied 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.
Table of Contents
1. Introduction and History ..................................... 2
1.1 Context for DNS Development ............................... 3
1.2 Review of the DNS and Its Role as Designed ................ 4
1.3 The Web and User-visible Domain Names ..................... 6
1.4 Internet Applications Protocols and Their Evolution ....... 7
2. Signs of DNS Overloading ..................................... 8
3. Searching, Directories, and the DNS .......................... 12
3.1 Overview ................................................. 12
3.2 Some Details and Comments ................................. 14
4. Internationalization ......................................... 15
4.1 ASCII Isn't Just Because of English ....................... 16
4.2 The "ASCII Encoding" Approaches ........................... 17
4.3 "Stringprep" and Its Complexities ......................... 17
4.4 The Unicode Stability Problem ............................. 19
4.5 Audiences, End Users, and the User Interface Problem ...... 20
4.6 Business Cards and Other Natural Uses of Natural Languages. 22
4.7 ASCII Encodings and the Roman Keyboard Assumption ......... 22
Klensin Informational [Page 1]
RFC 3467 Role of the Domain Name System (DNS) February 2003
4.8 Intra-DNS Approaches for "Multilingual Names" ............. 23
5. Search-based Systems: The Key Controversies .................. 23
6. Security Considerations ...................................... 24
7. References ................................................... 25
7.1 Normative References ...................................... 25
7.2 Explanatory and Informative References .................... 25
8. Acknowledgements ............................................. 30
9. Author's Address ............................................. 30
10. Full Copyright Statement ..................................... 31
1. Introduction and History
The DNS was designed as a replacement for the older "host table"
system. Both were intended to provide names for network resources at
a more abstract level than network (IP) addresses (see, e.g.,
[RFC625], [RFC811], [RFC819], [RFC830], [RFC882]). In recent years,
the DNS has become a database of convenience for the Internet, with
many proposals to add new features. Only some of these proposals
have been successful. Often the main (or only) motivation for using
the DNS is because it exists and is widely deployed, not because its
existing structure, facilities, and content are appropriate for the
particular application of data involved. This document reviews the
history of the DNS, including examination of some of those newer
applications. It then argues that the overloading process is often
inappropriate. Instead, it suggests that the DNS should be
supplemented by systems better matched to the intended applications
and outlines a framework and rationale for one such system.
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 the DNS evolved to its present state, but those variants do
not invalidate the inferences and conclusions.
This document presumes a general understanding of the terminology of
RFC 1034 [RFC1034] or of any good DNS tutorial (see, e.g., [Albitz]).
Klensin Informational [Page 2]
RFC 3467 Role of the Domain Name System (DNS) February 2003
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], [RFC811], [RFC952]. The
names themselves were restricted to a subset of ASCII [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]. These
restrictions later became collectively known as the "LDH" rules for
"letter-digit-hyphen", the permitted characters. The 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:
o 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.
o 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.
o Provide the capability to have multiple addresses associated with
a given host to reflect different types of connectivity and
topology. Use of names, rather than explicit addresses, avoided
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.
After several years of using the host table approach, the community
concluded that model did not scale adequately and that it would not
adequately support new service variations. A number of discussions
and meetings were held which drew several ideas and incomplete
proposals together. The DNS was the result of that effort. It
continued to evolve during the design and initial implementation
period, with a number of documents recording the changes (see
[RFC819], [RFC830], and [RFC1034]).
Klensin Informational [Page 3]
RFC 3467 Role of the Domain Name System (DNS) February 2003
The goals for the DNS included:
o Preservation of the capabilities of the host table arrangements
(especially unique, unambiguous, host names),
o Provision for addition of additional services (e.g., the special
record types for electronic mail routing which quickly followed
introduction of the DNS), and
o Creation of a robust, hierarchical, distributed, name lookup
system to accomplish the other goals.
The DNS design 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 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 accommodate 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 it is actually 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 accommodate any binary representation, DNS
names as used were historically not even unrestricted ASCII, but a
very restricted subset of it, a subset that derives 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]).
Klensin Informational [Page 4]
RFC 3467 Role of the Domain Name System (DNS) February 2003
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 very large percentage of the total names
representing physical hosts. There are domains that follow this
model: many university and corporate domains use fairly deep
hierarchies, as do a few country-oriented top level domains
("ccTLDs"). Historically, the "US." domain has been an excellent
example of the deeply hierarchical approach. However, by 1998,
comparison of several efforts to survey the DNS showed a count of SOA
records that approached (and may have passed) the number of distinct
hosts. Looked at differently, we appear to be moving toward a
situation in which the number of delegated domains on the Internet is
approaching or exceeding the number of hosts, or at least the number
of hosts able to provide services to others on the network. This
presumably results from synonyms or aliases that map a great many
names onto a smaller number of hosts. While experience up to this
time 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 flat, structure under ".COM" containing
well over ten million delegated subdomains [COMSIZE]), 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 (e.g., see
[RFC1034]), electronic mail addresses in the Internet have preserved
the original, pre-DNS, "user (or mailbox) at location" conceptual
format rather than a flatter or strictly dot-separated one.
Location, in that instance, is a reference to a host. The sole
exception, at least in the "IN" class, has been one field of the SOA
record.
Both the DNS architecture itself and the two-level (host name and
mailbox name) provisions for email and similar functions (e.g., see
the finger protocol [FINGER]), 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.
Just as was the case for the host table before it, the DNS provided
critical uniqueness for names, and universal accessibility to them,
as part of overall "single internet" and "end to end" models (cf.
Klensin Informational [Page 5]
RFC 3467 Role of the Domain Name System (DNS) February 2003
[RFC2826]). However, there are many signs that, as new uses evolved
and original assumptions were abused (if not violated outright), the
system was 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 design
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 directly in
URLs, 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 [REGISTRAR] who have been anxious to "sell" additional
names. And, of course, the perception that one needed a second-level
(or even top-level) domain per product, rather than having names
associated with a (usually organizational) collection of network
resources, has led to a rapid acceleration in the number of names
being registered. That acceleration has, in turn, clearly benefited
registrars charging on a per-name basis, "cybersquatters", and others
in the business of "selling" names, but it has not obviously
benefited the Internet as a whole.
This 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. These might include significant changes to the legislation
and conventions that govern disputes over "names" and "marks". Or
Klensin Informational [Page 6]
RFC 3467 Role of the Domain Name System (DNS) February 2003
they might result in 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.
Instead they have depended largely on political or economic power,
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 also 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.
Additional implications are still being discovered and evaluated.
Approaches that involve interception of DNS queries and rewriting of
DNS names (or otherwise altering the resolution process based on the
topological location of the user) seem, however, to risk disrupting
end-to-end applications in the general case and raise many of the
issues discussed by the IAB in [IAB-OPES]. These problems occur even
if the rewriting machinery is accompanied by additional workarounds
for particular applications. For example, security associations and
applications that need to identify "the same host" often run into
problems if DNS names or other references are changed in the network
without participation of the applications that are trying to invoke
the associated services.
1.4 Internet Applications Protocols and Their Evolution
At the applications level, few of the protocols in active,
widespread, use on the Internet reflect 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 permits 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
Klensin Informational [Page 7]
RFC 3467 Role of the Domain Name System (DNS) February 2003
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.
The DNS is both an illustration of, and an exception to, parts of
this pessimistic interpretation. It was a second-generation
development, with the host table system being seen as at the end of
its useful life. There was a serious attempt made to reflect the
computing state of the art at the time. However, deployment was much
slower than expected (and very painful for many sites) and some fixed
(although relaxed several times) deadlines from a central network
administration were necessary for deployment to occur at all.
Replacing it now, in order to add functionality, while it continues
to perform its core functions at least reasonably well, would
presumably be extremely difficult.
There are many, perhaps obvious, examples of this. Despite many
known deficiencies and weaknesses of definition, the "finger" and
"whois" [WHOIS] protocols have not been replaced (despite many
efforts to update or replace the latter [WHOIS-UPDATE]). The Telnet
protocol and its many options drove out the SUPDUP [RFC734] one,
which was arguably much better designed for a diverse collection of
network hosts. A number of efforts to replace the email or file
transfer protocols with models which their advocates considered much
better have failed. And, more recently and below the applications
level, there is some reason to believe that this resistance to change
has been one of the factors impeding IPv6 deployment.
2. Signs of DNS Overloading
Parts of the historical discussion above identify areas in which the
DNS has become overloaded (semantically if not in the mechanical
ability to resolve names). Despite this overloading, it appears that
DNS performance and reliability are still within an acceptable range:
there is little evidence of serious performance degradation. Recent
proposals and mechanisms to better respond to overloading and scaling
issues have all focused on patching or working around limitations
that develop when the DNS is utilized for out-of-design functions,
rather than on dramatic rethinking of either DNS design or those
uses. The number of these issues that have arisen at much the same
time may argue for just that type of rethinking, and not just for
adding complexity and attempting to incrementally alter the design
(see, for example, the discussion of simplicity in section 2 of
[RFC3439]).
Klensin Informational [Page 8]
RFC 3467 Role of the Domain Name System (DNS) February 2003
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 resources.
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
does not deal especially well with many names associated with a
given host (e.g., 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 accommodate longer names, they impose their own restrictions
and cause their own problems.
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
differentiation 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 also 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 occur 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. If the name resolution
Klensin Informational [Page 9]
RFC 3467 Role of the Domain Name System (DNS) February 2003
mechanism is expected to provide this functionality, there are
three possible models (which might be combined):
- supply information about multiple sites (or locations or
references). Those sites would, in turn, provide information
associated with the name and sufficient site-specific
attributes to permit the application to make a sensible choice
of destination, or
- accept client-site attributes and utilize them in the search
process, or
- return different answers based on the location or identity of
the requestor.
While there are some tricks that can provide partial simulations of
these types of function, DNS responses cannot be reliably conditioned
in this way.
These, and similar, issues of performance or content choices can, of
course, be thought of as not involving the DNS at all. For example,
the commonly-cited alternate approach of coupling these issues to
HTTP content negotiation (cf. [RFC2295]), requires that an HTTP
connection first be opened to some "common" or "primary" host so that
preferences can be negotiated and then the client redirected or sent
alternate data. At least from the standpoint of improving
performance by accessing a "closer" location, both initially and
thereafter, this approach sacrifices the desired result before the
client initiates any action. It could even be argued that some of
the characteristics of common content negotiation approaches are
workarounds for the non-optimal use of the DNS in web URLs.
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
appropriate rule-set) and to which queries may be ambiguous or
fuzzy. The DNS, by contrast, can accommodate only one set of
(quite rigid) matching rules. Proposals to permit different rules
in different localities (e.g., matching rules that are TLD- or
zone-specific) help to identify the problem. But they cannot be
applied directly to the DNS without either abandoning the desired
level of flexibility or isolating different parts of the Internet
from each other (or both). Fuzzy or ambiguous searches are
desirable for resolution of names that might have spelling
variations and for names that can be resolved into different sets
of glyphs depending on context. Especially when
internationalization is considered, variant name problems go
beyond simple differences in representation of a character or
Klensin Informational [Page 10]
RFC 3467 Role of the Domain Name System (DNS) February 2003
ordering of a string. Instead, avoiding user astonishment and
confusion requires consideration of relationships such as
languages that can be written with different alphabets, Kanji-
Hiragana relationships, Simplified and Traditional Chinese, etc.
See [Seng] for a discussion and suggestions for addressing a
subset of these issues in the context of characters based on
Chinese ones. But that document essentially illustrates the
difficulty of providing the type of flexible matching that would
be anticipated by users; instead, it tries to protect against the
worst types of confusion (and opportunities for fraud).
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. See the IAB's
discussion of some of these issues [RFC2825] for more information.
o In order to provide proper functionality to the Internet, the DNS
must have a single unique root (the IAB provides more discussion
of this issue [RFC2826]). There are many desires for local
treatment of names or character sets that cannot be accommodated
without either multiple roots (e.g., a separate root for
multilingual names, proposed at various times by MINC [MINC] and
others), 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 not only
an index entry (labels or fully-qualified names in the DNS case),
but their values or targets (DNS data). 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 (see the discussion in
[IQUERY]) and it can be simulated only by extracting all of the
relevant records (perhaps by zone transfer if the source permits
doing so, but that permission is becoming less frequently
available) 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 arise with protection of
that information. There are increasing calls to make different
information available based on the credentials and authorization
of the source of the inquiry. As with information keyed to site
locations or proximity (as discussed above), the DNS protocols
make providing these differentiated services quite difficult if
not impossible.
Klensin Informational [Page 11]
RFC 3467 Role of the Domain Name System (DNS) February 2003
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. But the price of each of these
changes is added complexity and, with it, added risk of unexpected
and destabilizing problems.
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 become bad enough as usage grows, that new
solutions are needed and can be deployed.
3. Searching, Directories, and the DNS
3.1 Overview
The constraints of the DNS and the discussion above suggest the
introduction of an intermediate protocol mechanism, referred to below
as a "search layer" or "searchable system". The terms "directory"
and "directory system" are used interchangeably 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
involve searching other systems, rather than looking up identifiers
in the DNS itself. As explained below, this would permit relaxation
of several constraints, leading to a more capable and comprehensive
overall system.
Ultimately, many of the issues with domain names arise as the result
of efforts to use the DNS as a directory. While, at the time this
document was written, sufficient pressure or demand had not occurred
to justify a change, it was already 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.
Klensin Informational [Page 12]
RFC 3467 Role of the Domain Name System (DNS) February 2003
The following points illustrate particular aspects of this
conclusion.
o A directory system would not require imposition of particular
length limits on names.
o A directory system could permit explicit association of
attributes, 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 that a system provides,
the greater the possibility of real or imagined trademark
conflicts. But the opportunity would exist to design a directory
structure that dealt with those issues in an intelligent way,
while DNS constraints almost certainly 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
even if mapping of addresses to DNS names continues to be, 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
recipients trying to contact a person can access romanized
spellings and numbers if the original language is not
comprehensible to them) can be easily handled in a directory
system by inserting both sets of entries.
o A directory system could be designed 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"
Klensin Informational [Page 13]
RFC 3467 Role of the Domain Name System (DNS) February 2003
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, as discussed in section 1.3 above, use
of the DNS in trademark-significant contexts tends to require
worldwide "flattening" of the trademark system.
Many of these issues are a consequence of another property of the
DNS: names must be unique across the Internet. The need to have a
system of unique identifiers is fairly obvious (see [RFC2826]).
However, if that requirement were to be eliminated in a search or
directory system that was visible to users instead of the DNS, many
difficult problems -- of both an engineering and a policy nature --
would be likely to vanish.
3.2 Some Details and Comments
Almost any internationalization 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 differentiate 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 historical argument for a single directory
structure (implying a need for mechanisms for registration,
delegation, etc.). But a single structure 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 5). If a single structure is not
needed, then, unlike the DNS, there would be no requirement for a
global organization to authorize or delegate operation of portions of
the structure.
Klensin Informational [Page 14]
RFC 3467 Role of the Domain Name System (DNS) February 2003
The "no single structure" concept could be taken further by moving
away from simple "names" in favor of, e.g., multiattribute,
multihierarchical, faceted systems in which most of the facets use
restricted vocabularies. (These terms are fairly standard in the
information retrieval and classification system literature, see,
e.g., [IS5127].) 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 for which the search is to be
performed. (See [DNS-Search] for further discussion.)
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 internationalization purposes. It might require
"charset" if we cannot agree on a character set and encoding,
although there are strong arguments for simply using ISO 10646 (also
known as Unicode or "UCS" (for Universal Character Set) [UNICODE],
[IS10646] coding in interchange. 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, such as those contemplated for Uniform Resource
Identifiers (URIs) [RFC2396, RFC3305] or the Service Location
Protocol [RFC2608], might argue for a few other attributes (as
outlined above).
4. Internationalization
Much of the thinking underlying this document was 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 the traditional
DNS subset of ASCII. Much of the relevant work was done in the
IETF's "Internationalized Domain Names" Working Group (IDN-WG),
although this document also draws on extensive parallel discussions
in other forums. This section contains an evaluation of what was
learned as an "internationalized DNS" or "multilingual DNS" was
explored and suggests future steps based on that evaluation.
When the IDN-WG was initiated, it was obvious to several of the
participants that its first important task 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. The IDN-WG clearly accomplished
that task. The beliefs that the problems were simple, and in the
corresponding simplistic approaches and their promises of quick and
painless deployment, effectively disappeared as the WG's efforts
matured.
Klensin Informational [Page 15]
RFC 3467 Role of the Domain Name System (DNS) February 2003
Some of the lessons learned from increased understanding and the
dissipation of naive beliefs should be taken as cautions by the wider
community: the problems are not simple. Specifically, extracting
small elements for solution rather than looking at whole systems, may
result in obscuring the problems but not solving any problem that is
worth the trouble.
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 even ASCII if we
permit the rest of the characters specified in the ISO 646
International Reference Version) is more complex than hostname-
restricted-ASCII (the "LDH" form, see section 1.1). And ASCII isn't
sufficient to completely represent English -- there are several words
in the language that are correctly spelled only with characters or
diacritical marks that do not appear in ASCII. With a broader
selection of scripts, in some examples, 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 [only] in character coding) that work most of the time,
but not always. And there are issues in coding, with Unicode/10646
providing different ways to represent the same character
("character", rather than "glyph", is used deliberately here). And,
in still others, there are questions as to whether two glyphs
"match", which may be a distance-function question, not one with a
binary answer. The IETF approach to these problems is to require
pre-matching canonicalization (see the "stringprep" discussion
below).
The IETF has resisted the temptations to either try to specify an
entirely new coded character set, or to pick and choose Unicode/10646
characters on a per-character basis rather than by using well-defined
blocks. While it may appear that a character set designed to meet
Internet-specific needs would be very attractive, the IETF has never
had the expertise, resources, and representation from critically-
important communities to actually take on that job. Perhaps more
important, a new effort might have chosen to make some of the many
complex tradeoffs differently than the Unicode committee did,
producing a code with somewhat different characteristics. But there
is no evidence that doing so would produce a code with fewer problems
and side-effects. It is much more likely that making tradeoffs
differently would simply result in a different set of problems, which
would be equally or more difficult.
Klensin Informational [Page 16]
RFC 3467 Role of the Domain Name System (DNS) February 2003
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 "LDH" syntax that is specified in the
host table RFCs and as "prudent" in RFC 1035. If those assumptions
are not met, many conforming implementations of those applications
may exhibit behavior that would surprise implementors and users. To
avoid these potential problems, IETF internationalization work has
focused on "ASCII-Compatible Encodings" (ACE). These encodings
preserve the LDH conventions in the DNS itself. Implementations of
applications that have not been upgraded utilize the encoded forms,
while newer ones can be written to recognize the special codings and
map them into non-ASCII characters. These approaches are, however,
not problem-free even if human interface issues are ignored. Among
other issues, they rely on what is ultimately a heuristic to
determine whether a DNS label is to be considered as an
internationalized name (i.e., encoded Unicode) or interpreted as an
actual LDH name in its own right. And, while all determinations 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 separated into a separate, client-side,
canonicalization or "preparation" process before the DNS matching
mechanisms are invoked [STRINGPREP].
4.3 "Stringprep" and Its Complexities
As outlined above, the model for avoiding problems associated with
putting non-ASCII names in the DNS and elsewhere evolved into the
principle that 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
hostname-restricted ASCII (i.e., "LDH") strings is trivial and
essentially adds only overhead. For other scripts, the impact is, of
necessity, quite significant.
Although the general notion underlying stringprep is simple, the many
details are quite subtle and the associated tradeoffs are complex. A
design team worked on it for months, with considerable effort placed
into clarifying and fine-tuning the protocol and tables. 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 considered and rejected special
Klensin Informational [Page 17]
RFC 3467 Role of the Domain Name System (DNS) February 2003
treatment of code positions to more nearly match the distinctions
made by Unicode with user perceptions about similarities and
differences between characters. But there were intense temptations
(and pressures) to incorporate language-specific or country-specific
rules. Those temptations, even when resisted, were indicative of
parts of the ongoing controversy or of the basic unsuitability of the
DNS for fully internationalized names that are visible,
comprehensible, and predictable for end users.
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. For example, each of the
following operations has been claimed to be similar to case-mapping
in ASCII:
o stripping of vowels in Arabic or Hebrew
o matching of "look-alike" characters such as upper-case Alpha in
Greek and upper-case A in Roman-based alphabets
o matching of Traditional and Simplified Chinese characters that
represent the same words,
o matching of Serbo-Croatian words whether written in Roman-derived
or Cyrillic characters
A decision to support any of these operations would have implications
for other scripts or languages and would increase the overall
complexity of the process. For example, unless language-specific
information is somehow available, performing matching between
Traditional and Simplified Chinese has impacts on Japanese and Korean
uses of the same "traditional" characters (e.g., it would not be
appropriate to map Kanji into Simplified Chinese).
Even were the IDN-WG's other work to have been abandoned completely
or if it were to fail in the marketplace, the stringprep and nameprep
work will continue to be extremely useful, both in identifying issues
and problem code points and in providing a reasonable set of basic
rules. Where problems remain, they are arguably not with nameprep,
but with the DNS-imposed requirement that its results, 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.
Klensin Informational [Page 18]
RFC 3467 Role of the Domain Name System (DNS) February 2003
4.4 The Unicode Stability Problem
ISO 10646 basically defines only code points, and not rules for using
or comparing the characters. This is part of a long-standing
tradition with the work of what is now ISO/IEC JTC1/SC2: they have
performed code point assignments and have typically treated the ways
in which characters are used as beyond their scope. Consequently,
they have not dealt effectively with the broader range of
internationalization issues. By contrast, the Unicode Technical
Committee (UTC) has defined, in annexes and technical reports (see,
e.g., [UTR15]), some additional rules for canonicalization and
comparison. Many of those rules and conventions have been factored
into the "stringprep" and "nameprep" work, but it is not
straightforward to make or define them in a fashion that is
sufficiently precise and permanent to be relied on by the DNS.
Perhaps more important, the discussions leading to nameprep also
identified several areas in which the UTC definitions are inadequate,
at least without additional information, to make matching precise and
unambiguous. In some of these cases, the Unicode Standard permits
several alternate approaches, none of which are an exact and obvious
match to DNS needs. That has left these sensitive choices up to
IETF, which lacks sufficient in-depth expertise, much less any
mechanism for deciding to optimize one language at the expense of
another.
For example, it is tempting to define some rules on the basis of
membership in particular scripts, or for punctuation characters, but
there is no precise definition of what characters belong to which
script or which ones are, or are not, punctuation. The existence of
these areas of vagueness 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 for the DNS.
The Unicode definition also evolves. Version 3.2 appeared shortly
after work on this document was initiated. It added some characters
and functionality and included a few minor incompatible code point
changes. IETF has secured an agreement about constraints on future
changes, but it remains to be seen how that agreement will work out
in practice. The prognosis actually appears poor at this stage,
since UTC chose to ballot a recent possible change which should have
been prohibited by the agreement (the outcome of the ballot is not
relevant, only that the ballot was issued rather than having the
result be a foregone conclusion). However, some members of the
community consider some of the changes between Unicode 3.0 and 3.1
and between 3.1 and 3.2, as well as this recent ballot, to be
Klensin Informational [Page 19]
RFC 3467 Role of the Domain Name System (DNS) February 2003
evidence of instability and that these instabilities are better
handled in a system that can be more flexible about handling of
characters, scripts, and ancillary information than the DNS.
In addition, because the systems implications of internationalization
are considered out of scope in SC2, ISO/IEC JTC1 has assigned some of
those issues to its SC22/WG20 (the Internationalization working group
within the subcommittee that deals with programming languages,
systems, and environments). WG20 has historically dealt with
internationalization issues thoughtfully and in depth, but its status
has several times been in doubt in recent years. However, assignment
of these matters to WG20 increases the risk of eventual ISO
internationalization standards that specify different behavior than
the UTC specifications.
4.5 Audiences, End Users, and the User Interface Problem
Part of what has "caused" the DNS internationalization 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.
Within the IETF, the IDN-WG, and sometimes other groups, avoided
addressing the implications of that transition by taking "outside our
scope -- someone else's problem" approaches or by suggesting that
people will just become accustomed to whatever conventions are
adopted. The realities of user and vendor behavior suggest that
these approaches will not serve the Internet community well in the
long term:
o If we want to make it a problem in a different part of the user
interface structure, we need to figure out where it goes in order
to have proof of concept of our solution. Unlike vendors whose
sole [business] model is the selling or registering of names, the
IETF must produce solutions that actually work, in the
applications context as seen by the end user.
o The principle that "they will get used to our conventions and
adapt" is fine if we are writing rules for programming languages
or an API. But the conventions under discussion are not part of a
semi-mathematical system, they are deeply ingrained in culture.
No matter how often an English-speaking American is told 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-
Klensin Informational [Page 20]
RFC 3467 Role of the Domain Name System (DNS) February 2003
speaker in Quebec in order to accommodate the decisions of the
IETF or of a registrar or registry is just not likely. "Montreal"
is either a misspelling or an anglicization of a similar word with
an acute accent mark over the "e" (i.e., using the Unicode
character U+00E9 or one of its equivalents). But 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 -- is as unlikely as 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 for practical,
user-level, use. In the use of human languages by humans, there are
many cases in which things that do not match are nonetheless
interpreted as matching. The Norwegian/Danish character that appears
in U+00F8 (visually, a lower case 'o' overstruck with a forward
slash) and the "o-umlaut" German character that appears in U+00F6
(visually, a lower case 'o' with diaeresis (or umlaut)) are clearly
different and no matching program should yield an "equal" comparison.
But they are more similar to each other than either of them is to,
e.g., "e". Humans are able to mentally make the correction in
context, and do so easily, and they can be surprised if computers
cannot do so. Worse, there is a Swedish character whose appearance
is identical to the German o-umlaut, and which shares code point
U+00F6, but that, if the languages are known and the sounds of the
letters or meanings of words including the character are considered,
actually should match the Norwegian/Danish use of U+00F8.
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 discussions about domain name
internationalization in recent years is that problems similar to
those described above exist in almost every language and script.
Each one has its idiosyncrasies, and each set of idiosyncracies is
tied to common usage and cultural issues that are very familiar in
the relevant group, and often deeply held as cultural values. 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,
there are issues with the relevant language. Similarly, if children
in Egypt or Israel are taught that it is acceptable to write a word
with or without vowels or stress marks, but that, if those marks are
included, they must be the correct ones, or a user in Korea is
potentially offended or astonished by out-of-order sequences of Jamo,
systems based on character-at-a-time processing and simplistic
matching, with no contextual information, are not going to satisfy
user needs.
Klensin Informational [Page 21]
RFC 3467 Role of the Domain Name System (DNS) February 2003
Users are demanding solutions that deal with language and culture.
Systems of identifier symbol-strings that serve specialists or
computers are, at best, a solution to a rather different (and, at the
time this document was written, somewhat ill-defined), problem. The
recent efforts have made it ever more clear that, if we ignore the
distinction between the user requirements and narrowly-defined
identifiers, we are solving an insufficient problem. And,
conversely, the approaches that have been proposed to approximate
solutions to the user requirement may be far more complex than simple
identifiers require.
4.6 Business Cards and Other Natural Uses of Natural Languages
Over the last few centuries, local conventions have been established
in various parts of the world for dealing with multilingual
situations. It may be helpful to examine some of these. For
example, 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. The conventions are not completely consistent and the
technique assumes that recipients will be tolerant. Translations of
names or places are attempted in some situations and transliterations
in others. Since it is widely understood that exact translations or
transliterations are often not possible, people typically smile at
errors, appreciate the effort, and move on.
The DNS situation differs from these practices in at least two ways.
Since a global solution is required, 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. More
important, 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-coded string will
be displayed and can be typed in. This argument is reasonable from
the standpoint of mixtures of Roman-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. Such systems are few in the
world today, but the number can reasonably be expected to rise as the
Internet is increasingly used by populations whose primary concern is
with local issues, local information, and local languages. It is,
Klensin Informational [Page 22]
RFC 3467 Role of the Domain Name System (DNS) February 2003
for example, fairly easy to imagine populations who use Arabic or
Thai scripts and who do not have routine access to scripts or input
devices based on Roman-derived alphabets.
4.8 Intra-DNS Approaches for "Multilingual Names"
It appears, from the cases above and others, that none of the intra-
DNS-based solutions for "multilingual names" are workable. They 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 such glyphs
cannot easily be distinguished from each other); 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
[RFC2671] or that long names are not really important; that Japanese
and Chinese computer users (and others) will either give up their
local or IS 2022-based character coding solutions (for which addition
of a large fraction of a million new code points to Unicode is almost
certainly a necessary, but probably not sufficient, condition) or
build leakproof and completely accurate 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, it is likely that about 80% or 90% of cases will work
satisfactorily, but it is unlikely that such partial solutions will
be good enough. For example, suppose someone can spell her name 90%
correctly, or a company name is matched correctly 80% of the time but
the other 20% of attempts identify a competitor: are either likely to
be considered adequate?
5. Search-based Systems: The Key Controversies
For many years, a common response to requirements to locate people or
resources on the Internet has been to invoke the term "directory".
While an in-depth analysis of the reasons would require a separate
document, the history of failure of these invocations has given
"directory" efforts a bad reputation. The effort proposed here is
different from those predecessors for several reasons, perhaps the
most important of which is that it focuses on a fairly-well-
understood set of problems and needs, rather than on finding uses for
a particular technology.
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
(even if functionally, and perhaps administratively, distributed)
Klensin Informational [Page 23]
RFC 3467 Role of the Domain Name System (DNS) February 2003
directory with universal applicability, a single directory that
supports 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 more and more names have been associated with many
host addresses and as CIDR [CIDR] has proven problematic for mapping
smaller address blocks to meaningful names.
Locally-tailored searches 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. That is not,
in general, a desirable situation, since it implies that users could
not, in the general case, share URLs (or other host references) and
that a particular user might not be able to carry references from one
host or location to another.
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 DNS roots.
Finally, a system of separate directories and databases, if coupled
with removal of the DNS-imposed requirement for unique names, would
largely eliminate the need for a single worldwide authority to manage
the top of the naming hierarchy.
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 locating network resources 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, especially if such a
system were designed without central authority or uniqueness of
names. It is uncertain how much greater those risks would be as
compared to a DNS lookup sequence that involved looking up one name,
Klensin Informational [Page 24]
RFC 3467 Role of the Domain Name System (DNS) February 2003
getting back information, and then doing additional lookups
potentially in different subtrees. That multistage lookup will often
be the case with, e.g., NAPTR records [RFC 2915] unless additional
restrictions are imposed. But additional steps, systems, and
databases almost certainly involve some additional risks of
compromise.
7. References
7.1 Normative References
None
7.2 Explanatory and Informative References
[Albitz] Any of the editions of Albitz, P. and C. Liu, DNS and
BIND, O'Reilly and Associates, 1992, 1997, 1998, 2001.
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), X3.4, 1968,
"USA Code for Information Interchange". ANSI X3.4-1968
has been replaced by newer versions with slight
modifications, but the 1968 version remains definitive
for the Internet. Some time after ASCII was first
formulated as a standard, ISO adopted international
standard 646, which uses ASCII as a base. IS 646
actually contained two code tables: an "International
Reference Version" (often referenced as ISO 646-IRV)
which was essentially identical to the ASCII of the
time, and a "Basic Version" (ISO 646-BV), which
designates a number of character positions for
national use.
[CIDR] Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC 1519, September 1993.
Eidnes, H., de Groot, G. and P. Vixie, "Classless IN-
ADDR.ARPA delegation", RFC 2317, March 1998.
[COM-SIZE] Size information supplied by Verisign Global Registry
Services (the zone administrator, or "registry
operator", for COM, see [REGISTRAR], below) to ICANN,
third quarter 2002.
[DNS-Search] Klensin, J., "A Search-based access model for the
DNS", Work in Progress.
Klensin Informational [Page 25]
RFC 3467 Role of the Domain Name System (DNS) February 2003
[FINGER] Zimmerman, D., "The Finger User Information Protocol",
RFC 1288, December 1991.
Harrenstien, K., "NAME/FINGER Protocol", RFC 742,
December 1977.
[IAB-OPES] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC
3238, January 2002.
[IQUERY] Lawrence, D., "Obsoleting IQUERY", RFC 3425, November
2002.
[IS646] ISO/IEC 646:1991 Information technology -- ISO 7-bit
coded character set for information interchange
[IS10646] ISO/IEC 10646-1:2000 Information technology --
Universal Multiple-Octet Coded Character Set (UCS) --
Part 1: Architecture and Basic Multilingual Plane and
ISO/IEC 10646-2:2001 Information technology --
Universal Multiple-Octet Coded Character Set (UCS) --
Part 2: Supplementary Planes
[MINC] The Multilingual Internet Names Consortium,
http://www.minc.org/ has been an early advocate for
the importance of expansion of DNS names to
accommodate non-ASCII characters. Some of their
specific proposals, while helping people to understand
the problems better, were not compatible with the
design of the DNS.
[NAPTR] Mealling, M. and R. Daniel, "The Naming Authority
Pointer (NAPTR) DNS Resource Record", RFC 2915,
September 2000.
Mealling, M., "Dynamic Delegation Discovery System
(DDDS) Part One: The Comprehensive DDDS", RFC 3401,
October 2002.
Mealling, M., "Dynamic Delegation Discovery System
(DDDS) Part Two: The Algorithm", RFC 3402, October
2002.
Mealling, M., "Dynamic Delegation Discovery System
(DDDS) Part Three: The Domain Name System (DNS)
Database", RFC 3403, October 2002.
Klensin Informational [Page 26]
RFC 3467 Role of the Domain Name System (DNS) February 2003
[REGISTRAR] In an early stage of the process that created the
Internet Corporation for Assigned Names and Numbers
(ICANN), a "Green Paper" was released by the US
Government. That paper introduced new terminology
and some concepts not needed by traditional DNS
operations. The term "registry" was applied to the
actual operator and database holder of a domain
(typically at the top level, since the Green Paper was
little concerned with anything else), while
organizations that marketed names and made them
available to "registrants" were known as "registrars".
In the classic DNS model, the function of "zone
administrator" encompassed both registry and registrar
roles, although that model did not anticipate a
commercial market in names.
[RFC625] Kudlick, M. and E. Feinler, "On-line hostnames
service", RFC 625, March 1974.
[RFC734] Crispin, M., "SUPDUP Protocol", RFC 734, October 1977.
[RFC811] Harrenstien, K., White, V. and E. Feinler, "Hostnames
Server", RFC 811, March 1982.
[RFC819] Su, Z. and J. Postel, "Domain naming convention for
Internet user applications", RFC 819, August 1982.
[RFC830] Su, Z., "Distributed system for Internet name
service", RFC 830, October 1982.
[RFC882] Mockapetris, P., "Domain names: Concepts and
facilities", RFC 882, November 1983.
[RFC883] Mockapetris, P., "Domain names: Implementation
specification", RFC 883, November 1983.
[RFC952] Harrenstien, K, Stahl, M. and E. Feinler, "DoD
Internet host table specification", RFC 952, October
1985.
[RFC953] Harrenstien, K., Stahl, M. and E. Feinler, "HOSTNAME
SERVER", RFC 953, October 1985.
[RFC1034] Mockapetris, P., "Domain names, Concepts and
facilities", STD 13, RFC 1034, November 1987.
Klensin Informational [Page 27]
RFC 3467 Role of the Domain Name System (DNS) February 2003
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1591] Postel, J., "Domain Name System Structure and
Delegation", RFC 1591, March 1994.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2295] Holtman, K. and A. Mutz, "Transparent Content
Negotiation in HTTP", RFC 2295, March 1998
[RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic Syntax",
RFC 2396, August 1998.
[RFC2608] Guttman, E., Perkins, C., Veizades, J. and M. Day,
"Service Location Protocol, Version 2", RFC 2608, June
1999.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC2825] IAB, Daigle, L., Ed., "A Tangled Web: Issues of I18N,
Domain Names, and the Other Internet protocols", RFC
2825, May 2000.
[RFC2826] IAB, "IAB Technical Comment on the Unique DNS Root",
RFC 2826, May 2000.
[RFC2972] Popp, N., Mealling, M., Masinter, L. and K. Sollins,
"Context and Goals for Common Name Resolution", RFC
2972, October 2000.
[RFC3305] Mealling, M. and R. Denenberg, Eds., "Report from the
Joint W3C/IETF URI Planning Interest Group: Uniform
Resource Identifiers (URIs), URLs, and Uniform
Resource Names (URNs): Clarifications and
Recommendations", RFC 3305, August 2002.
[RFC3439] Bush, R. and D. Meyer, "Some Internet Architectural
Guidelines and Philosophy", RFC 3439, December 2002.
[Seng] Seng, J., et al., Eds., "Internationalized Domain
Names: Registration and Administration Guideline for
Chinese, Japanese, and Korean", Work in Progress.
Klensin Informational [Page 28]
RFC 3467 Role of the Domain Name System (DNS) February 2003
[STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings (stringprep)", RFC 3454,
December 2002.
The particular profile used for placing
internationalized strings in the DNS is called
"nameprep", described in Hoffman, P. and M. Blanchet,
"Nameprep: A Stringprep Profile for Internationalized
Domain Names", Work in Progress.
[TELNET] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983.
Postel, J. and J. Reynolds, "Telnet Option
Specifications", STD 8, RFC 855, May 1983.
[UNICODE] The Unicode Consortium, The Unicode Standard, Version
3.0, Addison-Wesley: Reading, MA, 2000. Update to
version 3.1, 2001. Update to version 3.2, 2002.
[UTR15] Davis, M. and M. Duerst, "Unicode Standard Annex #15:
Unicode Normalization Forms", Unicode Consortium,
March 2002. An integral part of The Unicode Standard,
Version 3.1.1. Available at
(http://www.unicode.org/reports/tr15/tr15-21.html).
[WHOIS] Harrenstien, K, Stahl, M. and E. Feinler,
"NICNAME/WHOIS", RFC 954, October 1985.
[WHOIS-UPDATE] Gargano, J. and K. Weiss, "Whois and Network
Information Lookup Service, Whois++", RFC 1834, August
1995.
Weider, C., Fullton, J. and S. Spero, "Architecture of
the Whois++ Index Service", RFC 1913, February 1996.
Williamson, S., Kosters, M., Blacka, D., Singh, J. and
K. Zeilstra, "Referral Whois (RWhois) Protocol V1.5",
RFC 2167, June 1997;
Daigle, L. and P. Faltstrom, "The
application/whoispp-query Content-Type", RFC 2957,
October 2000.
Daigle, L. and P. Falstrom, "The application/whoispp-
response Content-type", RFC 2958, October 2000.
Klensin Informational [Page 29]
RFC 3467 Role of the Domain Name System (DNS) February 2003
[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, Rob Austein, Bob Braden, Vinton Cerf, Matt
Crawford, Leslie Daigle, Patrik Faltstrom, Eric A. Hall, Ted Hardie,
Paul Hoffman, Erik Nordmark, and Zita Wenzel for making specific
suggestions and/or challenging the assumptions and presentation of
earlier versions and suggesting ways to improve them.
9. Author's Address
John C. Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
EMail: klensin+srch@jck.com
A mailing list has been initiated for discussion of the topics
discussed in this document, and closely-related issues, at
ietf-irnss@lists.elistx.com. See http://lists.elistx.com/archives/
for subscription and archival information.
Klensin Informational [Page 30]
RFC 3467 Role of the Domain Name System (DNS) February 2003
10. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Klensin Informational [Page 31]
ERRATA
