Subject: RE: dual stack & IPv6 on by default

From: "Bound, Jim" <[email-masked]>
Date: Sun, 9 Mar 2003 21:04:42 -0500

>However, this rule *is* somewhat problematic with multihomed 
>hosts as well as in cases where network interfaces are set up 
>on-demand. In the former case, the packets apparently have to 
>be replicated to all links that have no default routers. In 
>the latter case, an application might trigger an on-demand 
>network interface setup. However, the packets from the 
>application would still be directed on-link, since it takes a 
>little while to receive and process a router advertisement. An 
>implementation that caches the routing decision will have 
>problems with this.

Good point.  But isn't it up to the implementation for links that had to
deal with on-demand.  As long as I have route for a link then I should
apply it?  The problem would be if the link information was not applied
before the on-demand was accessed in a multi-home manner.  But then I
would assume destination unreachable would be the response to the
application, though it should say link not known IMO.  So isn't the
multi-home scenario a two step problem.  1) the on-demand NIC has to see
the link before used by the app, and 2) the implementation must be able
to propogate attributes for multiple interfaces?

/jim

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From: Mika Liljeberg <[email-masked]>
Date: 10 Mar 2003 21:15:54 +0200

> to propogate attributes for multiple interfaces?

The question also is: what triggers the on-demand link setup? Two
potential cases:

a) there is an on-link default route to the on-demand interface -> first
packet routed to the interface starts link setup

b) on-demand link setup starts on no-route, according to preset policy

Suppose we have a wireless terminal with two interfaces: a dialup one
(e.g. cellular circuit switched data) for Internet connectivity [DOWN],
and 802.11 for ad-hoc or cheap Internet access via an access point [UP].

If one takes the ND spec strictly, case a) might or might not trigger
dialup, depending on which default route we hit first. Case b) would
never trigger the dialup [although an implementation might choose to do
so after NUD].

On the other hand, as soon as the host receives the first RA from the
dialup interface and configures a default router, the other interface
stops working for ad-hoc.

All this just goes to show that the default on-link route implied by the
ND specification perhaps isn't fully thought out, at least with
multihoming. Besides, such a route could always be configured explicitly
if the user wants to use a particular interface for ad-hoc.

[I'm not sure if this actually has any additional implications to the
dual stack issue raised by the original poster.]

	MikaL