freebsd stable - Nov 2009 - em0: watchdog timeout when communicating to windows using 9K MTU

Hi,

My em0 interface repeatedly hangs up with watchdog timeout when 
communicating to the windows host at MTU 9K.

[sobomax@pioneer ~]$ grep em0 /var/run/dmesg.boot
em0: <Intel(R) PRO/1000 Network Connection 6.9.6> port 0xecc0-0xecdf mem 
0xfe6e0000-0xfe6fffff,0xfe6d9000-0xfe6d9fff irq 21 at device 25.0 on pci0
em0: Using MSI interrupt
em0: [FILTER]
em0: Ethernet address: 00:22:19:32:87:2f
[sobomax@pioneer ~]$ uname -a
FreeBSD pioneer.sippysoft.com 7.2-RELEASE-p4 FreeBSD 7.2-RELEASE-p4 #0: 
Sun Oct  4 03:08:04 PDT 2009 
root@pioneer.sippysoft.com:/usr/obj/usr/src/sys/PIONEER  amd64
[sobomax@pioneer ~]$ ifconfig em0
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 9000
         options=98<VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM>
         ether 00:22:19:32:87:2f
         inet 192.168.1.1 netmask 0xffffff00 broadcast 192.168.1.255
         inet 192.168.2.1 netmask 0xffffff00 broadcast 192.168.2.255
         inet6 fec0::1 prefixlen 64
         media: Ethernet autoselect (1000baseTX <full-duplex>)
         status: active
[sobomax@pioneer ~]$ dmesg | grep watchd
em0: watchdog timeout -- resetting
em0: watchdog timeout -- resetting
em0: watchdog timeout -- resetting
em0: watchdog timeout -- resetting
em0: watchdog timeout -- resetting

I have managed to make a packet capture right at the time when hang 
happens. It appears to be that either "MAC Pause" or "TCP Segment
of
reassembled PDU" is the last packet that goes through before the 
interface hangs.

Here is the screenshot, if somebody wants to take closer look at the 
actual packets please let me know.

http://sobomax.sippysoft.com/~sobomax/ScreenShot527.png

Turning off TSO and TXCSUM/RXCSUM has not helped. Bringing MTU down to 
1,500 resolved the problem immediately.

I have had the same problem happening several times in the past 
(although I initially attributed it to the bad cable or something like 
that), so it's definitely not on-off issue.

Given popularity of intel/pro chips in today's computers it look like 
quite serious issue to me. Any help is greatly appreciated.

Regards,
-- 
Maksym Sobolyev
Sippy Software, Inc.
Internet Telephony (VoIP) Experts
T/F: +1-646-651-1110
Web: http://www.sippysoft.com
MSN: sales@sippysoft.com
Skype: SippySoft