--On Monday, 07 April, 2008 22:51 -0400 "Robert A. Rosenberg"
At 23:51 +0200 on 04/07/2008, Frank Ellermann wrote about Re:
IPV6: Where are we? (SMTP):
Hector argues that it's the job of IPv6 domains to be
reachable, for that they must either also offer A, or use an
explicit MX with an IPv4 SMTP at a third party in addition to
their AAAA. Assuming that 2821bis will be as long a standard
as STD 10 (about 30 years) adding this pragmatical idea could
backfire when IPv4 is remote history.
You are assuming that IPv6 domains will not initially be Dual
Stack. So long as they are dual stack (or have someone who is
dual stack act as a secondary MX to forward to the IPv6-Only
MTA) it will work. You start with:
FQDN MX 10 IPv4
MX 20 IPv6
with both the IPv4 and IPv6 MTAs being able to save to the
IMAP/POP mailboxes or both set to hand off to an internal MTA
for final delivery.
As IPv6 becomes more widespread make the IPv4 MX a 30 not a 10
and the IPv6 will become the preferred MTA.
Eventually you can remove the IPv4 (if there is a flag-day for
depreciation of IPv4) or keep running the IPv4 or farm it out
to a dual stack who will hand off to the IPv6 (as above).
This is certainly a model. However, if I'm a potential SMTP
receiver, I'm dual-stack, I have exactly one MTA serving both
protocols, and my IPv4 and IPv6 connectivity is equally good, I
may not have a preference. Instead, I'd prefer that the sender
figure out the best path to me using whatever mechanisms it uses
to deal with multihomed hosts, find routes, etc. For that case,
using your notation, I want
FQDN MX 10 IPv4-IPv6
IPv4-IPv6 A address...
Once you use a IPv4/IPv6 MX, the only use of the A will be by
a antiquated resolver/MTA that does not understand MX or has
its MX support turned off for some reason. All others will
locate and talk to the IPv4 MTA via the MX.
Now, I think most of your argument still applies for the above
case, except, under either the language of 2821bis or the
language of 974, the MX record is optional and does not change