In an earlier post, I described the Large Send Offload (LSO) feature of modern Ethernet adapters and why it can cause havoc with network performance. And since this is enabled by default, you have to manually disable it. For Windows, this can be done in the Ethernet adapter properties (which I prefer) or in the TCP/IP network stack. I'll start with disabling LSO in the TCP/IP network stack since Microsoft uses some confusing terms that you'll want to be familiar with.
Micosoft refers to LSO as "TCP Chimney Offload". For Windows Server 2008 and later, it's described in MS Support Article 951037. LSO was first supported in Windows Server 2003 with the release of the Scalable Networking Pack, which integrated into Service Pack 2, and is described in MS Support Article 912222.
Disabling LSO on Windows Server 2003
This has to be done by editing the registry. Open regedit and locate this key:
Then right-click on EnableTCPChimney and select Modify from the pop-up menu. Change the value to 0 and click on OK. You can also use this REG command (this command is one line but shown here here on two lines for clarity):
reg add HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters /v EnableTCPChimney /t REG_DWORD /d 0
Windows must be restarted before the change will take effect.
Disabling LSO on Windows Server 2008 and higher
This is easily done using a NETSH command:
netsh interface tcp set global chimney=disabled
Disabling LSO on the Ethernet adapter
This works in all versions of versions of Windows Server since it's done at the driver level. Go to where the network adapters are located in the Control Panel. For Windows Server 2003, this will be under Network Connections. For Windows Server 2008, this will be under Network and Sharing Center –> Change Adapter Settings.
Now right-click on the network adapter and choose Properties from the pop-up menu. At the top of this windows will be a "Connect using" text field with the vendor and model of the network adapter. For my example, I'm using an Intel 52575 Gigabit adapter. Just below this text field, click on the Configure button.
Now click on the Advanced tab, which shows the configurable properties for the adapter. Find the entry for Large Send Offload. This is how it's labeled on Intel adapters, but will vary (sometimes wildly) for adapters from other other vendors. If it's modern adapter like this one, there will be a setting for both IPv4 and IPv6. For older adapters, there will only be a setting for IPv4. Change the value for Large Send Offload from "Enabled" (or "On") to "Disabled" (or "Off") and click on OK.
W A R N I N G: Changing any of the adapter policies causes the driver to be restarted. There will be a brief (1 – 2 min) loss of network connectivity.