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From: mrc@Tomobiki-Cho.CAC.Washington.EDU (Mark Crispin)
Newsgroups: comp.protocols.tcp-ip,comp.protocols.tcp-ip.domains
Subject: Re: parallel networks problem
Message-ID: <4718@milton.u.washington.edu>
Date: 3 Jul 90 20:10:00 GMT
References: <7050@star.cs.vu.nl>
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In article <7050@star.cs.vu.nl> sater@cs.vu.nl (Staveren van Hans) writes:
>Suppose part of a network looks like this
>
>  ---------------------------------------------------------  network A
>		|			|		|
>	-----------------	-----------------   ----------
>	|      A.1      |	|      A.2	|   |  A.3   |
>	|    Host foo	|	|    Host bar	|   |Host zot|
>	|      B.6	|	|      B.7      |   |        |
>	-----------------	-----------------   ----------
>		|			|
>  --------------------------------------------------------- network B
>
>so two parallel networks with some multihomed and some singlehomed hosts.
>Further suppose that network B is preferable to network A, because of
>load, politics or because it is 10x as fast (hint: FDDI vs Ethernet).
>How would one set up addressing and routing for such a configuration?

TOPS-20 had a concept (which I invented) called "preferred IP address"
and "preferred network" which answered this question.  There was, from
the beginning, a "default IP address" which was used in IP datagrams
where it was not obvious which local address of a multi-homed host was
best to use as the source IP address.

The problem was that very often network A would be a lower-speed
backbone and network B a higher-speed stub network.  The default
mechanism would ensure that host foo would use its A.1 address in
talking to host zot and host zowie on a network C (since the only way
to reach the B.6 address from outside of network B would be to go
through an A->B gateway, so you might as well use the A.1 address and
save a hop).  In talking to host bink which only had a B.8 address,
host foo would use its B.6 address.  The problem was which address to
use for bar.

Hence the "preferred" address and network, which would take precedence
in any choice between a "preferred" and any other (including the
default) address.  Foo and bar would make network A be "default" and
network B be "preferred."

I'm surprised Unix hasn't picked up on this yet.

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