Path: utzoo!utgpu!watserv1!watmath!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!samsung!uunet!munnari.oz.au!cs.mu.OZ.AU!kre
From: kre@cs.mu.OZ.AU (Robert Elz)
Newsgroups: comp.protocols.tcp-ip
Subject: Redirects, and multiple subnets on a cable
Message-ID: <6490@munnari.oz.au>
Date: 21 Jan 91 15:34:18 GMT
Sender: news@cs.mu.oz.au
Lines: 56

I sent a comment to Paul Tsuchiya wrt his internet draft
(draft-tsuchiya-subnetnos-00.txt) on subnet number allocation
policies.

This doesn't touch on the main subject of his draft, but a
side issue he mentions.

Paul says (in mail to me) ...

	Could you put your message on all of tcp-ip, so we can see what
	other people have to say about it?

So, here it is ...

	In there you say (wrt two subnets on one cable) ...

	   In fact, this is not such a bad solution,
	   because assuming that the gateway is capable of recognizing multiple
	   subnet numbers on the same subnet, the gateway will simply send the
	   host an ICMP Redirect [4], and subsequent packets will go directly to
	   the host [1].

	I don't think that can be true can it?  That would require the ICMP
	redirect to contain an ethernet address.  The sending host is in no
	doubt of the destination's IP address, sending a redirect that contains
	that address can do no more than confuse it, if its routing table has
	it believe that to reach that address it must route through a gateway.

	Oh - do you mean send the host a redirect, containing its own address
	as the gateway?  I guess that might work, assuming that the host's
	software understands the BSD type convention "if I am the gateway,
	I send directly out on my ethernet", if not, almost anything might
	happen.

	If that's it, I think I'd be more explicit about it.

To provide a little context - this is related to a cable being used
to carry two subnets, with a router somewhere on the cable configured
to forward between the two - and perhaps send redirects to hosts sending
through the router, when, with a little more intelligence, the host would
be able to send directly to the destination.

In a reply to that message, Paul indicated that the first of my two
scenarios was the one he intended, with the idea that on receiving a
redirect, (containing the hosts IP address as the gateway to send
through) the host receiving the redirect would not look inside it (much),
but would simply ARP for the "gateway" address drom the ICMP, and
end up sending directly to the destination.

Personally, I would expect that a host receiving a redirect like that
(if any gateway sent one) would simply ignore it - as a defence against
bogus redirects.

Anyway - comments?

kre