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Date: Sat, 17 Nov 2012 18:54:05 -0500
From: Adam McDougall <mcdouga9@egr.msu.edu>
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To: Zaphod Beeblebrox <zbeeble@gmail.com>, hackers@freebsd.org
Subject: Re: Jumbo Packet fail.
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On 11/17/2012 5:32 PM, Zaphod Beeblebrox wrote:
> I recently started using an iSCSI disk on my ZFS array seriously from
> a windows 7 host on the network.  The performance is acceptable, but I
> was led to believe that using Jumbo packets is a win here.  My win7
> motherboard adapter did not support jumbo frames, so I got one that
> did... configured it, etc.  Just in case anyone cares, the motherboard
> had an 82567V-2 (does not support jumbo frames) and I added in an
> intel 82574L based card.
>
> Similarly, I configured em0 on my FreeBSD host to have an MTU of 9014
> bytes (I also tried 9000).  The hardware on the FreeBSD 9.1RC2 side
> is:
>
> em0: <Intel(R) PRO/1000 Network Connection 7.3.2> port 0xdc00-0xdc1f
> mem 0xfcfe0000-0xfcffffff,0xfcfc0000-0xfcfdffff irq 16 at device 0.0
> on pci3
>
> pciconf -lv identifies the chipset as 82572EI
>
> Now... my problem is that the windows machine correctly advertises an
> MSS of 8960 bytes in it's SYN packet while FreeBSD advertises 1460 in
> the syn-ack.
>
> [1:42:342]root@vr:/usr/local/etc/istgt> ifconfig em0
> em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 9014
>          options=4019b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,TSO4,VLAN_HWTSO>
>          ether 00:15:17:0d:04:a8
>          inet 66.96.20.52 netmask 0xffffffe0 broadcast 66.96.20.63
>          inet6 fe80::215:17ff:fe0d:4a8%em0 prefixlen 64 scopeid 0x5
>          inet6 2001:1928:1::52 prefixlen 64
>          inet 192.168.221.2 netmask 0xffffff00 broadcast 192.168.221.255
>          nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
>          media: Ethernet autoselect (1000baseT <full-duplex>)
>          status: active
>
> I have tested this with both ipv4 and ipv6 connections between the
> win7 host and the FreeBSD server.  win7 always requests the larger
> mss, and FreeBSD the smaller.
Did you reboot or alter the existing route so it also uses the higher 
MTU? I realize that need is not obvious.  Check netstat -rni.