Subject: Re: lamer mtu question.
To: Herb Peyerl <firstname.lastname@example.org>
From: Manuel Bouyer <email@example.com>
Date: 07/22/2001 17:44:20
On Sat, Jul 21, 2001 at 08:27:15AM -0600, Herb Peyerl wrote:
> so I have an HP Jornada 720 with a novatelwireless CDPD card that
> is basically a 16550 look-alike that presents a 'ppp' interface to
> the host.
> Once establishing the ppp connection, I try to ssh to my 1.4.3 box
> (or other hosts on the net, including nbftp, with same results) but
> it wedges at 'waiting for server public key'. A tcpdump of both ends
> shows the 1.4.3 box sends the key:
> 08:10:09.684564 18.104.22.168.65530 > lager.beer.org.ssh: P 1:52(51) ack 26 wi
> n 16384 <nop,nop,timestamp 4 1013822>
> 08:10:09.684592 lager.beer.org.ssh > 22.214.171.124.65530: . ack 52 win 17520 <
> nop,nop,timestamp 1013826 2>
> 08:10:11.649212 lager.beer.org.ssh > 126.96.36.199.65530: P 26:302(276) ack 52
> win 17520 <nop,nop,timestamp 1013829 2>
> 08:10:15.649177 lager.beer.org.ssh > 188.8.131.52.65530: P 26:302(276) ack 52
> win 17520 <nop,nop,timestamp 1013837 2>
> but at the Jornada, the 26:302 packets never arrive, says the tcpdump
> on that host (running 1.5V netbsd/hpcarm).
> My /etc/ppp/options file says:
> connect 'chat -v -f /etc/ppp/chatscript'
> mru 250
> mtu 250
> Initially I used an mtu of '296' and started seeing the problem. So I
> bumped it down to 250 out of curiosity ...
> Later I bumped the mtu up to '502' (some random number higher than 250)
> and it all works fine.
> So what am I missing? Shouldn't this all be independant of the mtu/mru?
> Or is there some sysctl I'm missing?
Maybe it's just that the other (ppp) end doesn't propely fragment big packets ?
Did you try to turn on path MTU discovery on both host ?
sysctl -w net.inet.ip.mtudisc=1
Manuel Bouyer, LIP6, Universite Paris VI. Manuel.Bouyer@lip6.fr