Re: multi-homing vs multi-connecting

["J. Noel Chiappa" <jnc@ginger.lcs.mit.edu>]

    > From: "Michael H. Lambert" <lambert@psc.edu>

    >> two fairly different things are with so-called "site multi-homing" and
    >> "host multi-homing" - the latter being a single host with multiple
    >> physical interfaces.

    > Is there a fundamental difference in these two "types" of multihoming?

Yes, I think - although the two can have an area of overlap where blend into
each other, as you scale up.

If you have a host which is host multi-homed to widely different places in the
topology, it's crystal clear that you can't support this in the routing. (No
global host routes.) Therefore, if you want to do many of the more powerful
robustness things that multi-homing gives you, you have to separate location
and identity, and have that host manage multiple addresses.

(Of course, if you're willing to always have a particular connection associated
with a particular interface, and always use only that interface, then this is
no longer true - but at that point you're doing less powerful robustness
things. This may still be enough, though, in a world in which the connection
is no longer so important, and things like cookies are used to keep track of
the identity of the partner across multiple connections from different
[usually dial-up, but the mechanism works for multiple interfaces] addresses.)

At the other end of the scale, some of the more complex site multi-homing
things can really only be done through the routing (e.g. if you have N local
ISP's which are in turn connected to M global ISP's) - although it often needs
a more powerful routing architecture than the lame one we have now.

And then in the middle there's this region where it looks like the way to
handle small-site multi-homing is to use multiple addresses, so the mechanism
looks like host-multihoming even though no single host has multiple interfaces.


    > In the IPv4 world, I would argue that there is not

The IPv4 world is a second-rate beginner's quick hack in many ways, including
mobility, routing and multi-homing, and I suggest you delete all knowledge of
if from your mind. I refuse to discuss it.

    > Other than adding the non-trivial problem of source address selection to
    > the mix, I don't see that IPv6 changes things.

Classical IPv6 dosn't. That's a big part of its problems.

	Noel