Xref: utzoo comp.protocols.tcp-ip:12013 comp.protocols.tcp-ip.domains:177
Path: utzoo!attcan!uunet!mcsun!hp4nl!star.cs.vu.nl!sater
From: sater@cs.vu.nl (Staveren van Hans)
Newsgroups: comp.protocols.tcp-ip,comp.protocols.tcp-ip.domains
Subject: parallel networks problem
Message-ID: <7050@star.cs.vu.nl>
Date: 3 Jul 90 08:36:20 GMT
Sender: news@cs.vu.nl
Lines: 52

Recently I posted a question to these groups to which, to my surprise,
I received no reactions at all. This seems strange since it addresses a
problem that many of us are going to face. Now there is of course the
possibility that this is so obvious that you were all embarrassed by
the question, but I doubt it.

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?

Using the DNS address lookup you get two addresses for each machine,
but most software doesn't understand that currently and only uses the
first one. Now the documentation suggests that you can order the
addresses so that certain preferred networks come in front, but looking
at our bind source code shows that that is not implemented.

So host foo looks up host bar, and gets the address set (A.2, B.7).
Both addresses are on directly connected networks. How do you make
sure that either it uses B.7, or it uses A.2 but still routes it over
network B.

Ideally this should be done by the routing layer of course, but current
routing software will probably send a packet for A.2 onto network A, unless
a host route for A.2 points to B.7. You can of course set up all these
routes by hand, but if the network size increases this gets to be too
much trouble.

You could imagine that host bar propagates a host route for A.2 on
network B, and all hosts are persuaded that network A is expensive
by messing around with metrics, but that would lead to a proliferation
of host routes floating around the network, and maybe even propagating to
the rest of the Internet, which would be extremely undesirable.

How do people solve this? Do people actually have situations like this
already? Inquiring minds want to know.

	Hans van Staveren
	Vrije Universiteit
	Amsterdam, Holland