Re: About dest MAC@

[Dr G Fairhurst <gorry@erg.abdn.ac.uk>]

alain.ritoux@6wind.com wrote:
> 
> Dr G Fairhurst wrote:
> 
> >
> >>- If the IRD is itself a router, there is still the case (that may be
> >>of (most?) common usage ??) that the network behind it is a
> >>leaf-network, and by no mean a transit network.
> >
> >
> > OK, so specifically you mean a leaf network where routing protocols are
> > not used to determine forwarding to the "leaf" network. One example is a
> > network with one external receive interface (via the MPEG-2 port).
> 
> Not exactly, in fact, the "leaf" network I had in mind could have
> several point of access (i.e CPE), but does NOT provide transit, hence
> only packets destined to the "leaf" network should be accepted.
> 

OK, but what happens whene there are several places (networks)
via which the "leaf: network may receive an Ip packet?
- routers normally advertise that a site is reachable 
via an interface. Do routers do this on each network to which they
are attached?

If so, how do the routers connected to the MPEG-2 feed know whether 
to forward the packet from the MPEG-2 feed via the other network(s)
to the destination end host, or to discard this packet because
someone else will be forwarding? - This is usually the function of the 
IP routing protocol in combination with the link MAC address.

In the case of just one receive interface, the above point is moot,
and we don't have the same routing issues.

> >
> > That is, theer are no alternative delivery paths, and therefore no
> > reachability via other routers that may also receive the packet. Specifically
> > you must also require all other routers to silently drop packets with
> > an unreachable network address. If you do all this, I agree - but
> > although this would work, it seems a "tweak", and I'm not sure the
> > latter
> I admit this is tweaky, and I'm not definitly proud of it ;-)
> 
> > is a robust recommendation (if one router returns an ICMP message to
> > the source, what happens??)
> Well, the source is informed that dest is unreachable, and may stop.
> So indeed, it works only when ALL routers perform the same trick, which
> is somehow fragile I admit.
> 

Ok, so, you can do it this way if you have an operational need, but
it's probably not a recommended solution.  

> >
> > My thoughts are that we have some cases here that can use IP packets
> > without MAC addresses, providing:
> >
> > (a) they can efficiently filter on IP level addresses
> >
> > AND
> >
> > (b) If they are routers they MUST also differentiate packets with
> > a MAC dest address from those without a MAC address, and MUST discard
> > packets with no MAC address that do not correspond to their own IP address
> > (or with all the rules above to the prefix used for hosts on a leaf IP network.)

> Definitely agree, the trick was only for the case where there's no MAC
> addr. Of course, if there is a MAC addr, is MUST be used to reject or
> accept packets whatever their dst IP addr is.
> 
> Alain.
> --
> Alain RITOUX
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