Re: trying multiple addresses

Clive Dawson (AI.CLIVE@MCC.COM)
Mon 15 Feb 88 17:27:07-CST

Re: Date: Sun, 14 Feb 88 18:18:09 PST
        From: Mark Crispin <MRC@PANDA.PANDA.COM>
        Subject: trying multiple addresses


Your message implies that trying an alternate IP address after the
best one has failed will only win 1% of the time. I disagree. As
somebody who still has the job of dealing with the day-to-day hassles
of "keeping the mail flowing", I cannot afford to sit back and
consider what the "correct solution" will ultimately be once the whole
world arrives at some standard. I will gladly accept any "kludgy
algorithms" that get the job done with no adverse effects.

Not counting the host-table-related problems (I know, you don't want
to hear this!) such as SU's Sierra, whose 10 address still appears
over a year after it died, or the more recent HI-MULTICS/CIM-VAX
address problem at Honeywell, where both addresses work, but SMTP
listens only on one, I have also had to deal with a multitude of other
problems in just the last week which were all solved by using
non-"best" IP addresses. The following scenario comes up A LOT, at
least at this site:
        Q: Why isn't my mail getting through to host X?
        A: The host must be down.
        Q: Then why can I Telnet to it?
        A: Ah, your workstation is not directly connected to net 10,
           and is so "dumb" that it doesn't realize that net 10 is the
           only way out of this place anyway, so it chooses to use host
           X's 128 address. Meanwhile, our "smart" mail relay host (and
           gateway) is directly connected to net 10 and therefore
           insists on sticking to host X's 10 address regardless.

How am I supposed to tell users that "correct failure" is better
than "kludgy success"? They could care less about these details,
and rightfully so.

             If I understand correctly, he's saying that Xerox.COM
        will use SUMEX-AIM's net 36 address just because they couldn't connect
        to the net 10 address the last time. If this is common behavior it's
        no wonder those of us who must use the net 10/36 gateway find it so
        unusable. Will I have to instruct the servers on multi-homed net
        10/36 hosts to refuse connections on net 36 from non-net 36 hosts to
        get them to stop?

Mark, that's not how I understood JLarson at all. He said that
this procedure was followed on RETRIES, and then ONLY at the
next retry interval. This is quite different from adopting the
alternative IP address "permanently" thereafter for all deliveries.

It seems to me that this procedure is almost a complete win. There is
no extra load on the sending mailer, and non-"best" IP addresses are
used only when they have to be, on a message-by-message basis. It
might possibly be improved by a heuristic which says that if, on the
first retry, the second address is found to be down too, then 4 out
of every 5 (say) future retries should go back to using the "best" IP
address. This way, if SCORE is really down, the poor 10/36 gateway
won't get so much of a pounding with doomed connection attempts from



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