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From: shj@ultra.com (Steve Jay)
Newsgroups: comp.protocols.tcp-ip
Subject: Multi-homed hosts and parallel networks
Keywords: multi-homed hosts, host naming, efficient routing
Message-ID: <1990Jun13.213400.299@ultra.com>
Date: 13 Jun 90 21:34:00 GMT
Organization: Ultra Network Technologies
Lines: 71

Fast networks (FDDI or UltraNet) are being installed in parallel with
existing ethernet networks.  Hosts are becoming multi-homed (more than
one IP address) for reasons other than IP routing.  Have there been any
discussions on how IP can cope with multi-homed hosts and (semi-)parallel
networks, such that traffic automatically takes the "best" path between
hosts?

Consider the following combination of networks and hosts.  Network 1 is
ethernet, Network 2 is something much faster.  Hosts B and C are connected
to both networks, host A is on network 1 only, and host D is on 2 only.

                         1                        2

        +-------+        |                        |
	|       |	 |                        |
	|   A   +--------+                        |
	|       |	 |       +------+         |
        +-------+        |       |      |         |
			 +-------+  B   +---------+
			 |       |      |         |
                         |       +------+         |
	         	 |       +------+         |
                         |       |      |         |
			 +-------+  C   +---------+
			 |       |      |         |        +------+
                         |       +------+         |        |      |
			 |                        +--------+   D  |
                         |                        |        |      |
			 |                        |        +------+


How can it be arranged so that IP traffic between any two hosts travels
over the "best" path?

I put "best" in quotes for a reason.  It may be considered better to have
some traffic between B & C go over the ethernet rather than the faster
network.  You may not want to clog up the fast network with the tiny
packets generated by rlogin, but ftp data connections should be on the
fast network.

One of the reasons that this isn't easy is that names and IP addresses
refer to specific interfaces, not hosts.  In the above configuration,
host A has one interface (call it A-1), host B whas B-1 and B-2, host C
has C-1 and C-2, and host D has D-2.  Normally, in this situation, the
system administrator would alias the name B to B-1 or B-2 in /etc/hosts
(or equivalent). This leads to very inefficient routing.  If B is aliased
to B-2, then IP traffic between A and B could end up routing through host
C, rather than using the direct A-1 <-> B-1 path.  If B is aliased to B-1,
traffic between D and B could go through C.

The issue is not how to route an IP packet once an IP addresse has been
selected.  The issue is, how to decide what interface (and which network)
to use when there is more than one choice.  It's possible that the "best"
choice will vary during the life of one connection, based on packet sizes
and network loading.

I'm not looking for "clever" ways to assign names to IP addresses.  I'm
looking for ideas which might challenge some of the basic IP concepts,
like names being associated with interfaces instead of hosts, and routing
decisions based on a name specified by the user/application.  Have these
issues already been discussed anywhere?  A quick look at the RFC index
didn't help me.
 
I'm looking forward to benefiting from the accumulated wisdom on the net.
Thanks in advance.

Steve Jay
shj@ultra.com  ...ames!ultra!shj
Ultra Network Technologies / 101 Dagget Drive / San Jose, CA 95134 / USA
(408) 922-0100 x130	"Home of the 1 Gigabit/Second network"