Default TCP Window Size Is Still Used After You Specify a Different TCPWindowSize Value

In Windows 2000, when you set the

SynAttackProtect

registry value to 2, you may not be able to configure the TCP receive window size globally for all interfaces. For example, if you set the

SynAttackProtect

value in the

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

key to 2, and then set the

TCPWindowSize

value in the

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

key to a value other than the default value, the new window size setting may not apply. The default TCP receive window size is still used.

This problem occurs because the

TCPWindowSize

value is not processed correctly at the parameters (global) level when the

SynAttackProtect

value is set to 2.

Service Pack Information

To resolve this problem, obtain the latest service pack for Microsoft Windows 2000. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

Hotfix Information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, submit a request to Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language. The Global version of this fix has the file attributes (or later) that are listed in the following table. The dates and times for these files are listed in coordinated universal time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article. This problem was first corrected in Microsoft Windows 2000 Service Pack 4.

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk. To work around this problem, use one of the following methods.

Method 1: Set the SynAttackProtect Registry Value to Either 0 (Zero) or 1

To set the

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcip\Parameters\SynAttackProtect

value to either 0 (zero) or 1:

1. Click Start, and then click Run.
2. In the Open box, type regedit, and then click OK.
3. Locate, and then click the following registry key:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcip\Parameters
4. In the right pane, double-click
   SynAttackProtect
   .
5. In the Value data box, type either 0 or 1, and then click OK.
6. Quit Registry Editor.

Method 2: Configure the TCPWindowSize Registry Value at the Interfaces Level

To configure the

TCPWindowSize

registry value at the interfaces level:

1. Click Start, and then click Run.
2. In the Open box, type regedit, and then click OK.
3. Locate, and then click the following registry key:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcip\Interfaces
4. On the Edit menu, point to New, and then click DWORD Value.
5. Type TCPWindowSize, and then press ENTER.
6. In the right pane, double-click TCPWindowSize.
7. In the Value data box, type the value that you want, and then click OK.
   
   Note You must use a value that is less than 64240.
8. Quit Registry Editor.

The

SynAttackProtect

registry value is used to harden the TCP/IP stack against denial of service attacks. You configure this value according to the level of protection that you want to set. For additional information about TCP/IP-related registry values that you can configure to harden the TCP/IP stack on computers that are directly connected to the Internet, click the following article number to view the article in the Microsoft Knowledge Base:
The

TCPWindowSize

registry value is used to determine the maximum TCP receive window size that is offered by a computer when it establishes a TCP connection. The receive window specifies the number of bytes that a sender can transmit without receiving an acknowledgment. For additional information about TCP features in Windows 2000, click the following article number to view the article in the Microsoft Knowledge Base: