Subject: Re: lamer mtu question.
To: Manuel Bouyer <>
From: Herb Peyerl <>
List: tech-net
Date: 07/23/2001 17:31:39
Manuel Bouyer <>  wrote:
 > > The other end is a DMS-500 phone switch that I have no control of. 
 > > 
 > >  > Did you try to turn on path MTU discovery on both host ?
 > >  > sysctl -w net.inet.ip.mtudisc=1
 > > 
 > > you mean my jornada running pppd and the 20 hop away machine with
 > > sshd?  or you mean both ends of the ppp connection?  
 > No, both end of the ssh connection. If the DMS-500 fails to fragment
 > pakets, maybe at last it properly returns an ICMP need-frag for such
 > packets ...

So I turned on net.inet.ip.mtudisc on both the jornada and my 1.4.3
host, then set the MTU on the PPP link to 296, and saw the same problem
as before.  basically, the sshd is sending the server public key, but
a tcpdump on the client (jornada, ppp endpoint) doesn't show the packets
arriving at all.  As soon as I set the mtu back up to 502, it all works

I don't mind leaving it at something high like 500, it's just that it's a
high latency and low reliability/throughput link.  

I guess I just wanted to understand the problem.  It could very well be
the DMS-500 just doesn't know how to fragment packets correctly.