Subject: IAB Technical Comment on the Unique DNS Root

From: Brian Carpenter <[email-masked]>
Date: Mon, 27 Sep 1999 16:33:15 -0400

[The IAB has sent the following note to [email-masked]]

IAB Technical Comment on the Unique DNS Root
============================================
 
SUMMARY

The Internet, to remain a global network, technically requires  the
existence of a globally unique public name space. The DNS name space is a
hierarchical name space derived from a single, globally unique root. 
This is inherent in the design of the DNS system. Therefore it is not
technically meaningful for there to  be more than one root in the public DNS
system. That one root must be supported by a small number of coordinated 
root servers, and administered by a unique naming authority.

Put simply, allowing multiple public DNS roots would raise a very strong 
possibility that users of different ISPs who click on the same link on a 
web page could end up at different destinations, against the will of the 
web page designers.

This does not preclude private networks from operating their own private
name spaces, but if they wish to make use of names uniquely defined for
the global Internet, they have to fetch that information from the global
DNS naming hierarchy, and in particular from the coordinated root servers
of the global DNS naming hierarchy.
 
DETAILED EXPLANATION
 
There are two reasons for a single DNS root:
 
1. For any communications between two parties to be effective there are two
essential preconditions: The existence of a common symbol set, and the 
existence of a common semantic interpretation of these symbols. Failure of
the first condition implies a failure to communicate at all, and failure of
the second implies that the meaning of the communication is lost.
 
In the case of a public communications system this condition of a common
symbol set with a common semantic interpretation must be further
strengthened to that of a unique symbol set with a unique semantic
interpretation. This condition of uniqueness allows any party to initiate a
communication that can be received and understood by any other party. Such
a condition rules out the ability to define a symbol within some bounded
context. In such a case, once the communication moves out of the 
context of interpretation in which it was defined, the meaning of 
the symbol becomes lost.

Within public digital communications networks such as the Internet this
requirement for a uniquely defined symbol set with a uniquely defined
meaning exists at many levels, commencing with the binary encoding
scheme, extending to packet headers and payload formats and the protocol
that an application uses to interact. In each case a variation of the 
symbol set or a difference of interpretation of the symbols being used 
within the interaction causes a protocol failure, and the communication 
fails. The property of uniqueness allows a symbol to be used unambiguously 
in any context, allowing the symbol to be passed on, referred to, and reused,
while still preserving the meaning of the original use.
 
The DNS fulfils an essential role within the Internet protocol environment,
allowing network locations to be referred to using a label other than 
a protocol address. As with any other such symbol set, DNS names are designed to
be globally unique, that is, for any  one DNS name at any one time there
must be a single set of DNS  records uniquely describing protocol
addresses, network resources and services associated  with that DNS name.
All of the applications deployed on the  Internet which use DNS assume
this, and Internet users expect  such behavior from DNS names.  Names are
then constant symbols, whose interpretation does not specifically require
knowledge of the context of any individual party. A DNS name can be passed
from one party to another without altering the semantic intent of the name.

Since the DNS is hierarchically structured into domains, the  uniqueness
requirement for DNS names in their entirety implies  that each of the names
(sub-domains) defined within a domain has a  unique meaning (i.e. set of
DNS records) within that domain. This  is as true for the root domain as
for any other DNS domain.  The requirement for uniqueness within a domain
further implies  that there be some mechanism to prevent name conflicts
within  a domain. In DNS this is accomplished by assigning a single
owner  or maintainer to every domain, including the root domain, who
is  responsible for ensuring that each sub-domain of that domain has  the
proper records associated with it. This is a technical  requirement, not a
policy choice.
 
2. Both the design and implementations of the DNS protocol are  heavily
based on the assumption that there is a single owner or  maintainer for
every domain, and that any set of resources records  associated with a
domain is modified in a single-copy serializable  fashion. That is, even
assuming that a single domain could somehow
be "shared" by uncooperating parties, there is no means within the  DNS
protocol by which a user or client could discover, and choose between,
conflicting definitions of a DNS name made by different  parties. The
client will simply return the first set of resource records that it finds
that matches the requested domain, and assume that these are valid. This
protocol is embedded in the operating software of hundreds of millions of
computer systems, and is not easily updated to support a shared domain
scenario. Morever, even supposing that some other means of resolving
conflicting definitions could be provided in the future, it would have to
be based on objective rules established in advance. (For example, zone A.B
could  declare that naming authority Y had been delegated all subdomains
of  A.B with an odd number of characters, and that naming authority Z had
been delegated authority to define subdomains of A.B with an even  number
of characters.) Thus, a single set of rules would  have to be agreed to
prevent Y and Z from making conflicting  assignments, and with this train
of actions a single unique space has been created in any case. Of course
this would not allow multiple non-cooperating  authorities to assign
arbitrary sub-domains within a single domain; it seems that a degree of
cooperation and agreed technical rules are  required in order to guarantee
the uniqueness of names. In the DNS,  these rules are established
independently for each part of the  naming hierarchy, and the root domain
is no exception. Thus, there  must be a generally agreed single set of
rules for the root.

A PRACTICAL NOTE

There is one specific technical respect in which the root zone is
different from all other DNS zones: the addresses of the name
servers for the root zone come primarily from out-of-band
information (named.root files from the ISC BIND distribution, your
ISP, whatever) rather than via the NS RR delegation chain.  NS RRs
for the root zone, while present, are almost irrelevant.  In
effect, every full-service resolver in the world "delegates" the
root of the public tree to the public root server(s) of its choice.

As a direct consequence, any change to the list of IP addresses
that specify the public root zone is significantly more difficult
than changing any other aspect of the DNS delegation chain.  Thus,
stability of the system calls for extremely conservative and
cautious management of the public root zone (low churn rate,
relatively tight update coupling between the servers, etc), because
it's very difficult to route around a misbehaving root server.

CONCLUSION

The DNS type of unique naming and name-mapping system may 
not be ideal for a number of purposes for which it was 
never designed, such a locating information when the user
doesn't precisely know the correct names. As the 
Internet continues to expand, we would expect directory 
systems to evolve which can assist the user in dealing 
with vague or ambiguous references.  To preserve the 
many important features of the DNS and its multiple 
record types --including the Internet's equivalent of 
number portability-- we would expect the result of 
directory lookups and identification of the correct names
for a particular purpose to be unique DNS names that 
are then resolved normally, rather than having directory 
systems 'replace' the DNS. There is no getting away from
the unique root of the public DNS.

   Brian Carpenter
   IAB Chair