Security update 893066 returns the default size of the TCP receive window in Windows 2000 to the pre-SP3 default size of 17,520 bytes (890345)

Important This article contains information about how to modify the registry. Make sure to back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows registry

CAUSE

The default size of the TCP receive window is typically a low-level optimization that does not affect general networking performance. However, specific applications may benefit from different values of the TCP receive window size.

In the original release versions of Microsoft Windows 2000 Service Pack 1 (SP1) and Windows 2000 Service Pack 2 (SP2), the default size of the TCP receive window was 17,520 bytes. Windows 2000 Service Pack 3 (SP3) and Windows 2000 Service Pack 4 (SP4) included a change that increased the default size of the TCP receive window to 64 KB.

In Microsoft Windows 2000 Service Pack 3 (SP3), the size of the TCP receive window is set to 64 KB on 100 megabits per second (Mbps) networks. This setting may cause frequent retransmissions. Consistent customer and product support feedback suggested that the default size should be reverted to the pre-SP3 default of 17 KB. Therefore, this change was released as a hotfix for affected customers. Security update 893066 decreases the default size of the TCP receive window to 17,520 bytes.

However, depending on the network configuration and typical functionality, the decrease in default size of the TCP receive window may limit average throughput for specific applications that use significant network resources. The reduced value may be better for slower networks, while the larger value may work better for high-performance networks.

This performance optimization depends on network bandwidth, load and use factors, and specific applications that use TCP/IP, and varies by customer and networking infrastructure. Generally, most customers will not notice significant performance variations with either setting. We recommend that you use the default setting unless there are specific and measurable benefits to changing the default in your environment.

Note You can pre-deploy the registry setting before you apply the security update. This will prevent any changes in behavior when you apply the security update.

RESOLUTION

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

To resolve this problem, use one of the following methods to set the receive window size to another value.

Method 1: Add the TcpWindowSize registry entry

Add the TcpWindowSize registry entry. To do this, follow these steps:

Click Start, click Run, type regedit, and then click OK.
Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
On the Edit menu, point to New, and then click DWORD Value.
In the New Value box, type TcpWindowSize, and then press ENTER.
On the Edit menu, click Modify.
In the Value data box, type the window size that you want.

Method 2: Use the Setsockopt function

Use the Setsockopt Windows Sockets function to set the receive window size on a per-socket basis. For more information about the Setsockopt function, visit the following Microsoft Web site:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winsock/winsock/setsockopt_2.asp