When troubleshooting a problem, it is helpful to be able to eliminate variables. The primary variable that Alacritech introduces into the network environment is the TCP offload driver. Furthermore, Server 2008 ships with chimney disabled, and KB948496 for Server 2003 disables all SNP features. For these reasons, it is useful to know how to enable or disable TCP offload both globally and on a per interface basis.

For an overview of TCP chimney in Windows Server 2008, see Microsoft KB 951037

Our ATCP driver (ATCP.sys, included in Release 7 and earlier) is essentially a separate TCP stack from the Microsoft stack that enables us to offload the TCP connection to our Accelerator card (both network and iSCSI accelerators). If you disable this driver on an Alacritech interface and then reboot the system, that interface will henceforth behave as an ordinary NIC, using the Microsoft TCP stack.

Our SNP driver (slic.sys, sxg.sys, etc., included in Release 9 and later) hooks into the Microsoft Chimney API. Changing the global TCP Chimney setting with netsh or changing the per interface setting via the network properties pane should take effect immediately. Changing the per interface setting via the registry requires the interface to be disabled and re-enabled for the change to take effect.

 
Globally Enable/Disable TCP Offload for Microsoft SNP

The easiest way to globally enable or disable TCP Offload in chimney is to use the netsh command.

For Windows Vista and Server 2008, use:
  Enable:  netsh int tcp set global chimney=enabled
  Disable: netsh int tcp set global chimney=disabled

	C:\Users\csl>netsh int tcp show global
	Querying active state...

	TCP Global Parameters
	----------------------------------------------
	Receive-Side Scaling State          : enabled
	Chimney Offload State               : enabled
	Receive Window Auto-Tuning Level    : disabled
	Add-On Congestion Control Provider  : ctcp
	ECN Capability                      : disabled
	RFC 1323 Timestamps                 : disabled
		

For Server 2003 and XP x64, use:
  Enable:  netsh int ip set chimney enabled
  Disable: netsh int ip set chimney disabled

Unfortunately, you can't query chimney status in Server 2003. Instead you need to query the registry either with regedit or with the reg command. Here is an example of the latter starting with TCP Offload disabled and then enabled:

	C:\Temp>reg query HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters /v EnableTCPChimney

	HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
	    EnableTCPChimney    REG_DWORD    0x0

	C:\Temp>netsh int ip set chimney enabled
	Ok.

	C:\Temp>reg query HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters /v EnableTCPChimney

	HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
	    EnableTCPChimney    REG_DWORD    0x1

		

The change should take effect immediately, but it does not always, especially on Server 2003. Sometimes it is necessary to disable and re-enable an interface, or to reboot.

You can verify offload status by checking the "Offload State" with netstat -nt

	C:\Temp>netstat -nt

	Active Connections

	  Proto  Local Address          Foreign Address        State           Offload State

	  TCP    172.16.50.108:1104     172.16.50.1:53         TIME_WAIT       InHost

	  TCP    192.168.50.108:1027    192.168.51.3:3260      ESTABLISHED     Offloaded

	  TCP    192.168.50.108:1028    192.168.51.3:3260      ESTABLISHED     Offloaded

		

 
Interface Enable/Disable TCP Offload for Driver Release 10.x.

With the 10.x release TCP offload can once again be be enabled/disabled on a per interface basis through the driver advanced properties pane. Furthermore, on SEN3000 series products, IPv6 TCP connection offload can be enabled/disabled separately from IPv4 TCP connection offload. This example shows TCP offload for IPv4, but IPv6 is handled exactly the same way.

First just bring up the network interface properties and click Configure.


Then go to the Advanced tab and set TCP Connection Offload (IPv4) to Disabled.

The change will take place as soon as you exit network connection properties.

 
Interface Enable/Disable TCP Offload for Driver Version 9.x.

Some versions of the release 9 driver do not have a way to enable/disable TCP offload via the driver advanced properties pane. For those drivers, TCP Offload can only be enabled or disabled on a per interface basis by editing the registry.

Anyway, the relevant key is:
HKLM\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}\(instance)
Where (instance) is a four digit zero padded number, e.g. 0001,0002,... that coresponds to the network interface in question.

 
The value that controls TCP Offload is:
 

This is what it looks like in regedit on server 2008

Note that this view does not actually display the interface name. To get that, you need to find the NetCfgInstanceID. In our case, as seen in the above image, it's {B644F143-FB5A-4F57-8D56-AA39A8371225}, and look it up under the key: HKLM\SYSTEM\CurrentControlSet\Control\Network\{4D36E972-E325-11CE-BFC1-08002BE10318}
.

Remember the change will not take effect until the interface is disabled then re-enabled.

To disable TCP offload on an interface, bring up properties pane for that network interface and uncheck the box for the "Alacritech TCP Fast-path driver". Then close the properties pane and reboot the system.

Disabling TCP Offload will result in a performance hit, so we recommend that you do this only as a troubleshooting step, or as a work around for a bug.