Path: utzoo!attcan!uunet!husc6!mailrus!ames!pasteur!ucbvax!oliver.CRAY.COM!dab
From: dab@oliver.CRAY.COM (Dave Borman)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Dumb vs. smart host routing
Message-ID: <8805161548.AA15786@oliver.cray.com>
Date: 16 May 88 15:48:29 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 29

Bill Westfield of cisco Systems while talking about "Proxy ARP" writes:

> Of course, this only works with hosts on networks where ARP is used for
> address resolution (broadcasts are possible).  But if a host has only a
> point-to-point link to the network, it might as well use whatever gateway
> is at the other end of that link as its default gateway.  This leaves only
> networks like X.25 and the ARPANet (non-broadcast, but not really point-to-
> point either), and it might not be such a bad idea to have those hosts know
> about routing, at least to the extent of having multiple default gateways.

There's more.  For instance, the A series of HYPERchannel adaptors does not
support broadcast, and is not point-to-point.

As far as I know, "Proxy ARP" only works with subnets, for nodes that
don't know about subnets.   In order to send out the ARP request, the
host has to already belive that he can get to the remote host in one
hop.  If my machine is on net 128.62, and I want to get to a machine on
net 128.63, I don't send out an ARP request for the machine on net 128.63.
I have to have a route to 128.63 which points to a machine on net 128.62,
and then I'll be ARPing for the machine on 128.62 which is the next hop to
128.63.

"Proxy ARP" is a wonderful fix in the real world to keep your subnetted
network running when you have hosts that don't know about subnets.  I
don't think that it should be used for anything else, or promoted as a
standard.

			-Dave Borman
			CRAY Research, Inc.