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Re: History of fallback to A

2008-03-30 09:43:00

On Sun, Mar 30, 2008 at 07:42:39AM -0700, ned+ietf-smtp(_at_)mrochek(_dot_)com 
wrote:

So this means that SMTP had been in use for at least a year using
HOSTS.TXT, and then another couple of years using A, MD, and MF,
before MX came along, and I get the impression that MD and MF were
clunky enough that a most people just used the A record.  Under the
circumstances, MX without fallback to A wouldn't have worked because
of the substantial installed base of mail servers using A records.

Yes, that's more or less the case.

Then: people didn't know MX records and thus didn't use them. Or maybe
they did already know about MX records but did not yet support them.
Some fallback mechanism was necessary to bridge the gap between pre-MX
and post-MX time, or else MX didn't stand a chance.

Now: people do know MX records and do use them, except for a couple
of spammers maybe. There is no need anymore to design a fallback.

This also ignores the issue of what is the right thing to do in the
transitional case where there's no MX record but a mixture of A and AAAA
records. Only falling back to the A record subset seems, well, wrong.

Why?

You present a gut feeling, with no more and no less value than the
opposite gut feeling.

One could also argue that if such a mix is present, this indicates
mail delivery is not possible for that name.

Here's why: the fallback mechanism was designed for receivers not
yet aware of MX records. If receivers know about AAAA records, they
most certainly will also be aware of MX records.


So I believe the correct course going forward is to have the same fallback
rules for AAAA that we do for A. Having said all this, it certainly isn't a
showstopper for me if this goes the other way, if for no other reason than the
fact that we'll have plenty of time to fix this if whatever decision we make
turns out to be wrong. The only outcome that is a showstopper for me is for
this issue to remain unresolved.

I believe adding those fallback rules is wrong.  Apart from the two
arguments I presented above, I have more:

Adding a fallback mechanism for 2^32 minus a bit possible addresses,
in a time when the number of actually used addresses was very low, is
one thing.

Adding the same fallback now, for many more possible addresses, has
the potential of ruining the net.

Last but not least: not having a fallback for AAAA is not a change.
Having a fallback for AAAA is a change.

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