Subject: RE: Comments on VRRP-spec-06 Draft

From: Mike Shand <[email-masked]>
Date: Fri, 3 Apr 1998 08:26:34 +0100

Just a quick comment on this one

	>>8.1 ICMP Redirects
	>>   The IP source address of an ICMP redirect should be the
address the
	>>   end host used when making its next hop routing decision.

	>That's easier said than done,
	True!
	>and may not always be possible.  If a
	Not true
	>particular virtual router serves multiple addresses, and two or
more of
	>the addresses are in the same subnet, then there is no way to
tell which
	>one the host used.  Of course, this problem is not unique to
VRRP routers;
	>any router would have the same limitation.

	The case you describe, although it would result in it being
impossible to determine the correct address, would be very unusual..

	There are two fields you can use to determine the correct return
address.

	1. The source IP address of the original packet

	2. The destination MAC address of the original packet.

	The former is what a normal router would use. So if it has
multiple subnets on an interface, it would choose a return IP address on
its interface which is in the same subnet as the source IP address.

	In the case of a Virtual router, the choice of subnet is made by
the same mechanism, but the choice of address within that subnet is made
by considering the destination MAC address.

	The combination of the two mechanisms will always give the
correct result except in the unusual case of a single (virtual) router
having multiple IP addresses on the interface within the SAME subnet.

		Mike

	I'll look at the rest in more detail later.