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Date: 14 Nov 89 04:37:58 GMT
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tcp-ip-relay
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VAX01.AMS.COM
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tcp-sig
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Received: from MATHOM.CISCO.COM by NIC.DDN.MIL with TCP; Mon, 13 Nov 89 13:34:44 PST
Date: Mon 13 Nov 89 13:33:51-PST
From: William Westfield <BILLW@MATHOM.CISCO.COM>
Subject: Re: ARP+
To: cs.utexas.edu!wuarchive!brutus.cs.uiuc.edu!zweig@tut.cis.ohio-state.edu
cc: tcp-ip@NIC.DDN.MIL
In-Reply-To: <1989Nov7.210118.13557@brutus.cs.uiuc.edu>
Message-ID: <12541990931.21.BILLW@MATHOM.CISCO.COM>

	ARP will give you a good answer if W.X.Y.Z is on the same physical
	subnet as you. Also works if there is only one gateway on that net
	and it decides to do proxy-ARP for everybody else.

I don't think you understand Proxy-ARP.  A router should only issue a
proxy ARP reply to an ARP request if it is the BEST route to the host.
The routers, of course, know such things.  If routes are equivilent,
then multiple routers can answer and it doesn't matter.  If the routers
are stupid, and issue proxy ARP replies for anything not on the local
network (and I don't know of any routers that would do such a thing),
then at worst, they will get get a packet that should have gone to a
different router, and issue an ICMP redirect, which the host should
listen to (it then knows that there is a router involved, and knows
which router to use).

In general, you should NOT use ARP to make routing decisions at all.
"proxy ARP" is a (clever and useful) HACK, not a feature of ARP.  The
IETF is trying to come up with a standard "Router discovery protocl"
so that relatively dumb hosts can reasonably pick a "deafult router"
(and get re-directed to other routers as appropriate).  This is quite
separate from ARP, which is used only to resolve IP addresses to
physical addresses.


    QUESTIONS: Would it be hip just to use RIP as though it were
    ARP+? That is, would it be bad to send out a RIP-request every
    time I send out an ARP packet?

No.  I would be a bad idea.  RIP is on its way out as a routing protocol.
The "Host requirements" RFCs explicitly state that HOSTS should not muck
with routing protocols.  And this still confuses ARP and Router selection.

Bill Westfield
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