Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!adm!pvm@venera.isi.EDU
From: pvm@venera.isi.EDU (Paul Mockapetris)
Newsgroups: comp.mail.headers
Subject: Re: Braden on secondary IP addresses
Message-ID: <11902@brl-adm.ARPA>
Date: 19 Feb 88 04:09:32 GMT
Sender: news@brl-adm.ARPA
Lines: 43

Mark, Bob,

There are real issues behind this debate, and they are 1) what are the
right policies with regard to hosts with multiple addresses and 2) where
should the policies be implemented.  It would be nice if everyone agreed
what the rules are, but the waters are pretty muddy.

There can be a large difference in performance depending on which
address you use.  My experience with domain and mail services suggests
that sometimes a host offers different services on different addresses,
and sometimes only a subset of the addresses are reachable due to
gateway, etc factors.  The bottom line is that sometimes whether you win
or lose depends on the source and/or destination address you choose.

There may always be a best address, but it isn't always possible to know
which one it is without empirical information.  I can always prefer
destination addresses which share a common network, but sometimes the
problem is "Given N non-local class C addresses, which one should I
use?"  Since the answer would seem to depend on both the identity of the
source and the identity of the destination, it doesn't seem that either
can solve the problem alone.

Its not really just a mail issue.  For example, domain resolvers
and mailers have similar options.  [A resolver has to decide (1) which
sending address to use, (2) which name server to ask, and (3) which
destination address to use when it targets a query.  A mailer has to
decide (1) which sending address to use (2) which MX RR (i.e. mail
server) to use, and (3) which destination address to use.  The parallels
continue when you think about what to do in case of errors, how you
tradeoff network resources vs local resources vs time to finish a
request, etc.  Admittedly, there are also large differences, but dealing
with multiple addresses is an issue which comes up whenever you have
such network services.]

The domain system won't solve this problem.  In fact, it will make it
worse by spreading multiple addresses in more contexts.

What we need is a consensus on policies regarding the use of multiple
addresses and single module that can be used by all applications to
implement the policies.  (Maybe we have discovered one of the missing
layers in the Internet 7 layer cake.)

paul