Subject: (IPng 7036) Re: route distribution via ND

From: Richard Draves <[email-masked]>
Date: Fri, 8 Jan 1999 18:14:54 -0800

> Correct (at least in principle).
> I didn't understand what you meant by "virtual".
> What I was trying to point out that that while the definition for
> multihomed is >1 IPv6 address it is really the number of interfaces
> that creates an issue.

The definition of multihomed in Appendix A of the ND spec is >1 interface.

> But in your specific case (an ethernet and a 6-over-4 
> interface) it would make
> most sense to only use one of those at a time.
> One way of implementing this would be:
> 	When the host finds a default router on the ethernet it would 
> 	effectively turn off (or at least stop transmitting on) 
> the 6-over-4
> 	interface.
> 	When there are no default routers on the ethernet (or 
> no non-link-local 
> 	addresses assigned to the ethernet interface) it would 
> switch back
> 	to transmitting on the 6-over-4 interface.
> 	[I personally think preferring the ethernet over 
> 6-over-4 since it
> 	avoids encapsulating overhead as well as reduces IPv4 multicast
> 	traffic.]

I think there are scenarios (not too contrived I hope) where that heuristic
would fail. For example:

A site first starts experimenting with v6. It installs a router R1 on an
ethernet that speaks both v4 and v6. The router R1 has a tunnel to the
6bone. Inside the site, the router R1 advertises on its ethernet interface
and on a 6-over-4 interface. Over time dual-stack hosts H are installed in
the site. The hosts H hear R1's router advertisements via their 6-over-4
interface. If a host H is on the same ethernet as R1, then it also hears
R1's router advertisements on its ethernet interface. So far everything
works with your heuristic.

Now the site tries experimenting with v6-only machines. It installs a router
R2 that only speaks v6. Routers R1 and R2 are not directly connected via a
link. Router R2 connects some hosts H1 that only speak v6 and some hosts H2
that are dual-stack.

Consider a host H2 that is directly connected via ethernet to router R2, but
not directly connected to router R1. On its ethernet interface H2 hears
advertisements from R2. On its 6-over-4 interface H2 hears advertisements
from R1. Now host H2 wants to send a packet to host H1, a v6-only node that
is reachable via R2.

How does H2 know to send the packet to R2 instead of R1? Note that R1 can
not forward the packet to R2 - there is no v4 or v6 connectivity between R1
and R2.

> But that doesn't change my basic argument: there will be pressure to
> ensure that your new ND option has all the features and capabilities
> of RIPng. Why invent a new packet/option format when RIPng can be used
> for this today?

1. Architectural simplicity. Why have two mechanisms for communicating
information from routers to hosts, with the resulting increases in code
complexity & packet overhead?

2. RIPng is missing lifetimes on routes.

Rich
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