Subject: Re: fragmentation by NetBSD routers vs. reassembly on other systems....
To: NetBSD Networking Technical Discussion List <>
From: Robert Elz <kre@munnari.OZ.AU>
List: tech-net
Date: 09/02/2000 18:07:08
    Date:        Sat,  2 Sep 2000 02:46:30 -0400 (EDT)
    From: (Greg A. Woods)
    Message-ID:  <>

  | The proof was clearly visible in the
  | evidence collected by actual experimentation.

I have no idea what you have been experimenting with, but...

lavender$ sysctl net.inet.tcp.mssdflt
net.inet.tcp.mssdflt = 512
lavender$ telnet
telnet: Unable to connect to remote host: Connection refused

17:58:50.397583 > S 845201410:845201410(0) win 16384 <mss 1460,[|tcp]>
17:58:50.917854 > R 0:0(0) ack 845201411 win 0 (DF)

Note the advertised MSS, and its relationship with the net.inet.tcp.mssdflt
( is lavender).   Clearly it is not on the same net as   (That the telnet attempt was refused is no surprise).

I know - I also acted without thinking, and tried fiddling with that when
I was experiencing a problem with a host that had broken PMTU discovery
(from home, I use a very low MTU) - I thought if I set the MSS real low,
I could cause the remote host to send small packets, and its broken PMTU
discovery would stop bothering me.   And I expect it would have, except
that net.inet.tcp.mssdflt doesn't do that.   It does just what jhawk said
it does.   I read the code...