Tradeoffs in Domain Name System (DNS) Support for Internet Protocol version 6 (IPv6)
Network Working Group R. Austein
Request for Comments: 3364 Bourgeois Dilettant
Updates: 2673, 2874 August 2002
Tradeoffs in Domain Name System (DNS) Support
for Internet Protocol version 6 (IPv6)
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 (C) The Internet Society (2002). All Rights Reserved.
The IETF has two different proposals on the table for how to do DNS
support for IPv6, and has thus far failed to reach a clear consensus
on which approach is better. This note attempts to examine the pros
and cons of each approach, in the hope of clarifying the debate so
that we can reach closure and move on.
RFC 1886 [RFC1886] specified straightforward mechanisms to support
IPv6 addresses in the DNS. These mechanisms closely resemble the
mechanisms used to support IPv4, with a minor improvement to the
reverse mapping mechanism based on experience with CIDR. RFC 1886 is
currently listed as a Proposed Standard.
RFC 2874 [RFC2874] specified enhanced mechanisms to support IPv6
addresses in the DNS. These mechanisms provide new features that
make it possible for an IPv6 address stored in the DNS to be broken
up into multiple DNS resource records in ways that can reflect the
network topology underlying the address, thus making it possible for
the data stored in the DNS to reflect certain kinds of network
topology changes or routing architectures that are either impossible
or more difficult to represent without these mechanisms. RFC 2874 is
also currently listed as a Proposed Standard.
Austein Informational [Page 1]
RFC 3364 Tradeoffs in DNS Support for IPv6 August 2002
Both of these Proposed Standards were the output of the IPNG Working
Group. Both have been implemented, although implementation of
[RFC1886] is more widespread, both because it was specified earlier
and because it's simpler to implement.
There's little question that the mechanisms proposed in [RFC2874] are
more general than the mechanisms proposed in [RFC1886], and that
these enhanced mechanisms might be valuable if IPv6's evolution goes
in certain directions. The questions are whether we really need the
more general mechanism, what new usage problems might come along with
the enhanced mechanisms, and what effect all this will have on IPv6
The one thing on which there does seem to be widespread agreement is
that we should make up our minds about all this Real Soon Now.
Main Advantages of Going with A6
While the A6 RR proposed in [RFC2874] is very general and provides a
superset of the functionality provided by the AAAA RR in [RFC1886],
many of the features of A6 can also be implemented with AAAA RRs via
preprocessing during zone file generation.
There is one specific area where A6 RRs provide something that cannot
be provided using AAAA RRs: A6 RRs can represent addresses in which a
prefix portion of the address can change without any action (or
perhaps even knowledge) by the parties controlling the DNS zone
containing the terminal portion (least significant bits) of the
address. This includes both so-called "rapid renumbering" scenarios
(where an entire network's prefix may change very quickly) and
routing architectures such as the former "GSE" proposal [GSE] (where
the "routing goop" portion of an address may be subject to change
without warning). A6 RRs do not completely remove the need to update
leaf zones during all renumbering events (for example, changing ISPs
would usually require a change to the upward delegation pointer), but
careful use of A6 RRs could keep the number of RRs that need to
change during such an event to a minimum.
Note that constructing AAAA RRs via preprocessing during zone file
generation requires exactly the sort of information that A6 RRs store
in the DNS. This begs the question of where the hypothetical
preprocessor obtains that information if it's not getting it from the
Note also that the A6 RR, when restricted to its zero-length-prefix
form ("A6 0"), is semantically equivalent to an AAAA RR (with one
"wasted" octet in the wire representation), so anything that can be
done with an AAAA RR can also be done with an A6 RR.
Austein Informational [Page 2]
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