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From: tcp-ip@ucbvax.ARPA
Newsgroups: fa.tcp-ip
Subject: Apparent problem with ICMP Redirects on 4.2 systems
Message-ID: <8448@ucbvax.ARPA>
Date: Mon, 24-Jun-85 19:15:40 EDT
Article-I.D.: ucbvax.8448
Posted: Mon Jun 24 19:15:40 1985
Date-Received: Tue, 25-Jun-85 03:46:06 EDT
Sender: daemon@ucbvax.ARPA
Organization: University of California at Berkeley
Lines: 29

From: "J. Noel Chiappa" 

	We have recently noticed a problem with some 4.2 UNIX systems
when a new gateway (which was a better route to large sections of the
Internet) was installed on a network here. For most destinations, the
old gateways sent ICMP Redirects to the new one. This was fine, except
that apparently the 4.2 system had their routing tables fill up with
the Redirect information as they tried to contact new sites.
Apparently, when the tables filled up, they were unable to accept new
entries, because the machines became unreachable from certain (random)
destinations. I'm not sure why this happened, since the traffic should
still have flowed (albeit generating floods of Redirects by taking a
non-optimal path).
	Does this scenario make sense to any 4.2 network wizards?

	Certainly, it was something to do with routing, because when
we went into the rc.local file and changed the 'default' route (i.e.
in '/etc/route add') to be through the new gateway, and rebooted the
machines, things started working. I'm pretty annoyed that all the 4.2
systems had to be hand tweaked when a new gateway started up.

	I guess this points up a general problem with IP layers,
which is unfortunately not mentioned in Clark's 'IP Implementation
guide'. You should time out old entries in the routing cache, and
if you have a fixed size table and it fills up, you should be prepared
to evict someone.

		Noel
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