Subject: lamer mtu question.
To: None <>
From: Herb Peyerl <>
List: tech-net
Date: 07/21/2001 08:27:15
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 > P 1:52(51) ack 26 wi
n 16384 <nop,nop,timestamp 4 1013822>
08:10:09.684592 > . ack 52 win 17520 <
nop,nop,timestamp 1013826 2>
08:10:11.649212 > P 26:302(276) ack 52
 win 17520 <nop,nop,timestamp 1013829 2>
08:10:15.649177 > 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?