RFC 3364 (rfc3364) - Page 3 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


Main Advantages of Going with AAAA

   The AAAA RR proposed in [RFC 1886], while providing only a subset of
   the functionality provided by the A6 RR proposed in [RFC 2874], has
   two main points to recommend it:

   - AAAA RRs are essentially identical (other than their length) to
     IPv4's A RRs, so we have more than 15 years of experience to help
     us predict the usage patterns, failure scenarios and so forth
     associated with AAAA RRs.

   - The AAAA RR is "optimized for read", in the sense that, by storing
     a complete address rather than making the resolver fetch the
     address in pieces, it minimizes the effort involved in fetching
     addresses from the DNS (at the expense of increasing the effort
     involved in injecting new data into the DNS).

Less Compelling Arguments in Favor of A6

   Since the A6 RR allows a zone administrator to write zone files whose
   description of addresses maps to the underlying network topology, A6
   RRs can be construed as a "better" way of representing addresses than
   AAAA.  This may well be a useful capability, but in and of itself
   it's more of an argument for better tools for zone administrators to
   use when constructing zone files than a justification for changing
   the resolution protocol used on the wire.

Less Compelling Arguments in Favor of AAAA

   Some of the pressure to go with AAAA instead of A6 appears to be
   based on the wider deployment of AAAA.  Since it is possible to
   construct transition tools (see discussion of AAAA synthesis, later
   in this note), this does not appear to be a compelling argument if A6
   provides features that we really need.

   Another argument in favor of AAAA RRs over A6 RRs appears to be that
   the A6 RR's advanced capabilities increase the number of ways in
   which a zone administrator could build a non-working configuration.
   While operational issues are certainly important, this is more of
   argument that we need better tools for zone administrators than it is
   a justification for turning away from A6 if A6 provides features that
   we really need.









Austein                      Informational