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From: tmallory@CCV.BBN.COM
Newsgroups: comp.protocols.tcp-ip
Subject: Re: ICMP in ISO
Message-ID: <8704162138.AA16073@ucbvax.Berkeley.EDU>
Date: Thu, 16-Apr-87 16:16:51 EST
Article-I.D.: ucbvax.8704162138.AA16073
Posted: Thu Apr 16 16:16:51 1987
Date-Received: Sun, 19-Apr-87 09:29:35 EST
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I just wanted to add to the debugging scenarios presented by Rudy.

I have on many occasions used the message generators in our gateways to ping
hosts in order to determine just where a routing problem may lie.  The receipt
of an echo reply packet by the gateway generates a trap message which is sent
to the monitoring center.

Using a gateway on a network to which the host is directly attached
can show whether the host is able to communicate at all.  By merely changing
the source address in the ping to be the other side of the gateway, one can
determine whether the host has the necessary routing information.  Depending
on the number of gateways under my control along the path, I can learn much
about the type of problem.

We often get complaints from users who cannot connect to some other host using
tcp.  It is very frequently case that one of the hosts does not have a route
to the other network, but occasionally the problem turns out to be protocol
related, as when it turned out that Sun's telnet could not use Class C
internet addresses for opening connections in one direction.   I don't
remember whether it was client or server mode now.

Such a simple low level tool can be implemented in virtually any network
device, though even 4.2 bsd got it wrong for a while, so that a minimum of 48
bytes needed to be sent to get a reply where only 28 should be necessary.
The more complex your debugging aids, the more likely they won't work when you
need them.

Tracy Mallory
BBN Communciations
617-497-3193