Date: Wed, 29 Oct 2008 10:39:07 -0700 From: John-Mark Gurney <jmg@funkthat.com> To: Eitan Shefi <eitans@mellanox.co.il> Cc: freebsd-net@freebsd.org Subject: Re: It seems that FreeBSD-7.0 does not use the available MTU Message-ID: <20081029173907.GG51033@funkthat.com> In-Reply-To: <5D49E7A8952DC44FB38C38FA0D758EADD00926@mtlexch01.mtl.com> References: <5D49E7A8952DC44FB38C38FA0D758EADC72E72@mtlexch01.mtl.com> <5D49E7A8952DC44FB38C38FA0D758EADCC5FB7@mtlexch01.mtl.com> <20081028210215.GE51033@funkthat.com> <5D49E7A8952DC44FB38C38FA0D758EADD00926@mtlexch01.mtl.com>
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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@funkthat.com]
> Sent: Tuesday, October 28, 2008 11:02 PM
> To: Eitan Shefi
> Cc: freebsd-net@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."
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