RE: Alternatives to source address rewriting (was RE: Preserving established communications (was RE: about draft-nordmark-multi6-noid-00)

[Erik Nordmark <Erik.Nordmark@sun.com>]

> So i guess that there is no need for special routing protocols to support
> source address based routing

So you would envision manually installing the rules in each internal router
that says "packets with source address prefix X gravitate towards exit
router Rx"?
Or do you envision internal routers infering this from information already
present in the IGP?
Or do you envision tunnels between the exit routers to get the packets
to the correct exit based on the source address prefix?

> I think that the host should be able to decide if the packet will be routed
> according the destiantion address (in which case rewriting is probably
> needed to deal with ingress filtering) or it will be routed based on the
> source address (in which case no rewriting occurs)

In the latter case, if the packet arrives at an exit router and
the ISP matching the source address is known to be down, should the router
just drop the packet?

What if there never was a matching or preferred route for the destination
address through that ISP?

I think when source address based routing is used it is benefitial
to also look at the destination address when chosing routes.
One question is the details of the relationship between the source based
lookup and the destination based lookup.

> I see source address based routing as a mechanism to let the host source
> address selection act as ISP selection mechanism that overrules the isp
> selection performed by the routing system.
> The host then would force the ISP selection in cases when it knows better
> (ULP hints) or when it knowns it can't use source address rewriting (the
> other end in not M6 enabled)

But the host will never know better than the routing system because it
is operating on aggregated information.
What the host might know better than the routing system is that the
last N packets sent using a given source and destination didn't result
in packets being returned from the peer, thus something might be
broken.

Thus for the host to tell the routing system "please try sending this packet 
over a different path than the default one" might be useful. But that is quite
different than having the routing system blindly honor the source address
for routing lookups.

  Erik