RFC 3364 (rfc3364) - Page 2 of 11

Tradeoffs in Domain Name System (DNS) Support for Internet Protocol version 6 (IPv6)

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
   [RFC 1886] 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 [RFC 2874] are
   more general than the mechanisms proposed in [RFC 1886], 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
   deployment.

   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 [RFC 2874] is very general and provides a
   superset of the functionality provided by the AAAA RR in [RFC 1886],
   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
   DNS.

   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