Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!julius.cs.uiuc.edu!zweig
From: zweig@cs.uiuc.edu (Johnny Zweig)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: ICMP redirects
Keywords: routing info
Message-ID: <1990Sep24.235052.16616@julius.cs.uiuc.edu>
Date: 24 Sep 90 23:50:52 GMT
References: <6592@ncrcae.Columbia.NCR.COM>
Sender: news@julius.cs.uiuc.edu
Reply-To: zweig@cs.uiuc.edu
Organization: U of Illinois, Dept. of Computer Science, Systems Research Group
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wingo@ncrcae.Columbia.NCR.COM (Dave Wingo) writes:
>
>I have a question about routing and ICMP. Can ICMP redirects be used to update
>routing tables or does it just convey info about a route not being appropriate?
>Example would be to have an X terminal on a network 90. talking to a host A
>also on net 90. Then I introduce yet another host B which has two(2) network
>interface cards that bridge 90 to say net 89. I then put host A on net 89 and 
>provide it with a new 89 internet number. Question then is can the X terminal
>find the gateway to host A via ICMP only or will rip or gated need to be 
>provided on the X terminal to update route tables????
>
>Thanks in advance.... David Wingo  wingo@ncrcae.Columbia.NCR.COM

Finding the gateway (the X terminal sounds like it talks IP) from a given host
(the X terminal is a host in the sense of being an endpoint-machine for IP
traffic) is a problem whose solution is still in the works.  Some fine day
everyone will run Router Discovery Protocol so a host can use ARP to see if
another is on its local net, then punt everything else to the gateway it
selected from RDP.  Right now, there are a number of icky ways the X terminal
might go about finding out that B (in your scenario) is a router.

ICMP redirects are from routers and say "Hey! You're sending IP packets for
host A to me when they should really go directly to A!" if the X terminal uses
ARP to find A in the first place, it will find that A is not around anymore
and have to find the gateway -- the ICMP redirect might occur in the case
where A gets put onto the X terminal's net when it didn't used to be and
somehow the silly thing just had the route stuck in its head.

The netlanders will correct me I'm sure, but I think ICMP redirects are pretty
uncommon on nets that use ARP+whatever ad hoc method to locate gateways (since
nets with more than one gateway are pretty uncommon).

-Johnny ICMP