New registry entry for controlling the TCP Acknowledgment (ACK) behavior in Windows XP and in Windows Server 2003 (328890)


The information in this article applies to:

  • Microsoft Windows Server 2003, Standard Edition
  • Microsoft Windows Server 2003, Enterprise Edition
  • Microsoft Windows Server 2003, Datacenter Edition
  • Microsoft Windows Server 2003, Web Edition
  • Microsoft Windows Server 2003, 64-Bit Enterprise Edition
  • Microsoft Windows Server 2003, 64-Bit Datacenter Edition
  • Microsoft Windows XP 64-Bit Edition Version 2002
  • Microsoft Windows XP 64-Bit Edition Version 2003
  • Microsoft Windows XP Home Edition
  • Microsoft Windows XP Professional
  • Microsoft Windows XP Media Center Edition
  • Microsoft Windows XP Tablet PC Edition
This article was previously published under Q328890
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 more 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

SUMMARY

TcpAckFrequency is a new registry entry in Microsoft Windows XP and Microsoft Windows Server 2003 that determines the number of TCP acknowledgments (ACKs) that will be outstanding before the delayed ACK timer is ignored.

MORE INFORMATION

As specified in RFC 1122, TCP uses delayed acknowledgments to reduce the number of packets that are sent on the media. Instead of sending an acknowledgment for each TCP segment received, TCP in Windows 2000 and later takes a common approach to implementing delayed acknowledgments. As data is received by TCP on a particular connection, it sends an acknowledgment back only if one of the following conditions is true:
  • No acknowledgment was sent for the previous segment received.
  • A segment is received, but no other segment arrives within 200 milliseconds for that connection.
Typically, an acknowledgment is sent for every other TCP segment that is received on a connection unless the delayed ACK timer (200 milliseconds) expires. You can adjust the delayed ACK timer by editing the following registry entry.

Important Before you edit this registry entry, you must first install the hotfix that is described in the following article:

815230 Changing the TcpAckFrequency value to 1 does not have any effect



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.

Subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\<Interface GUID>
Entry: TcpAckFrequency
Value Type: REG_DWORD, number
Valid Range: 0-255
Default: 2
Description: Specifies the number of ACKs that will be outstanding before the delayed ACK timer is ignored. Microsoft does not recommend changing the default value without careful study of the environment. For more information, visit the following Microsoft Web page:

http://technet2.microsoft.com/WindowsServer/en/library/823ca085-8b46-4870-a83e-8032637a87c81033.mspx?mfr=true

If you set the value to 1, every packet is acknowledged immediately because there is only one outstanding TCP ACK as a segment is just received. The value of 0 (zero) is not valid and is treated as the default, 2. The only time the ACK number is 0 is when a segment is not received and the host is not going to acknowledge the data.
Modification Type:MinorLast Reviewed:8/2/2006
Keywords:kbinfo KB328890
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