Re: iperf -M in FreeBSD
I haven't tried -M on FreeBSD, but it wouldn't suprise me if
it doesn't allow it to go up above the interface MTU setting
(minus packet overheads). On Free, I've played with
large MSS sizes on both syskonnect and 3com cards by using
the mtu option to ifconfig, and then letting TCP just do it's
thing. If you're interface is set to 9000 bytes, Free's TCP
will use an 8192 MSS (assuming the path supports it).
By the way, if you are setting the interface MTU size dynamically,
Free seems to cache it in routing entries, so it may not actually
go up if you raise it. It does seem to lower just fine. So,
I've set the interface MTU large, and then lowered it when testing
various MTU sizes. [Caveat: My detailed experimentation was
done on aFreeBSD 3.6-RELEASE kernel, so details may have changed.]
--Matt
--On Thursday, June 12, 2003 10:44 AM -0500 Kevin Gibbs <kgibbs --at-- ncsa.uiuc.edu> wrote:
> What MSS size are you using? You may need to jack up the kernel level
> default to support a larger size then it is currently using. At the bottom
> of that man page is shows that this is accomplshed with the sysctl
> utility.
>
> Kevin
>
> On Thu, 12 Jun 2003, Venkatesh S Obanaik wrote:
>
>> Hi,
>>
>> I am using iperf 1.7.0 on FreeBSD 5.0
>> <Also tried the same on older versions of FreeBSD>
>>
>> There appears to be some problem in using iperf -M <MSS> option
>> in FreeBSD.
>>
>> Throws a warning saying :
>> Attempt to set TCP maximum segment size failed.
>> Setting the MSS maynot be implemented on this OS.
>>
>> However, FreeBSD seems to have support for
>> setting the MSS.
>> http://www.gsp.com/cgi-bin/man.cgi?section=4&topic=tcp
>>
>> Could you let me know how to resolve this problem.
>>
>> <However, there is no such problem in Linux >