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Re: About dest MAC@

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.
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.



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.
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Alain RITOUX
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