It seems that FreeBSD-7.0 does not use the available MTU

[John-Mark Gurney]

Eitan Shefi wrote this message on Wed, Oct 29, 2008 at 11:34 +0200:
> This is indeed the problem, thanks.
> 
> Is this a known issue with FreeBSD-7.0 ?
> I do not want to hide a potential bug in the NIC driver.

This is a known issue since I fixed the transmit code not to munge the
route's MTU so that the route's mtu field would have meaning and allow
you to have hosts w/ different MTU's on the network (I've done this in
production)...  

This is definately not a bug in the NIC driver...

> -----Original Message-----
> From: John-Mark Gurney [mailto:jmg at funkthat.com] 
> Sent: Tuesday, October 28, 2008 11:02 PM
> To: Eitan Shefi
> Cc: freebsd-net at freebsd.org
> Subject: Re: It seems that FreeBSD-7.0 does not use the available MTU
> 
> Eitan Shefi wrote this message on Mon, Oct 27, 2008 at 23:53 +0200:
> > I am using 2 hosts with FreeBSD-7.0 connected directly.
> > When I change the MTU to a value greater then 1500, for example 3000, 
> > and then send "ping" with message size 2500, from one host to the 
> > other, the other host gets more then one ICMP packet, even thaw the 
> > message that was send is match smaller then the MTU.
> >  
> > I tried to run this test using a different NIC, but I got the same 
> > behavior.
> >  
> > I run:
> > 1.  On both hosts:
> >      ifconfig mtnic0 mtu 3000
> > 2.  Than on one host I run:
> >      tcpdump -i mtnic0 icmp
> > 3.  And on the other host I run:
> >     ping -s 2500 -c 1 OTHER_HOST_IP   (ping to "mtnic0" on the other
> > host)
> 
> run netstat -rnW to see what the route's MTU is.  Most likely you need
> to set the mtu before you configure an ip address on the interface so
> that the network's route is created w/ the correct MTU...
> 
> either readd the network route, or change the mtu of the route for the
> host:
> route change -mtu 3000 OTHER_HOST_IP
> 
> also change it on the network route so that when the host route gets
> flushed and recreated it will be created w/ the correct MTU...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."