Data transfer stops for five seconds in a Windows Socket-based application in Windows 7 and Windows Server 2008 R2

Applies to: Windows 7 EnterpriseWindows 7 ProfessionalWindows 7 Ultimate


Assume that you use a Windows Socket-based application to send data locally between a client and a server that are running Windows 7 or Windows Server 2008 R2. In this situation, the data transfer stops intermittently or is delayed for five seconds.


This problem occurs because of the Silly Window Syndrome (SWS) avoidance timer. The SWS timer is scheduled to send the remaining data bytes, and this represents less than one maximum segment size (MSS) or 1,460 bytes. Additionally, the receiver is supposed to send an acknowledgement (ACK) message that advertises the increased receive window and indicates that the remaining data bytes can be sent. However, if the receiver sends an ACK within 5 seconds when it has a sufficient buffer, the SWS timer cannot recover the five-second delay status because of a race condition. The race condition exists when packets reach the destination in reverse order.


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 the problem described in this article. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

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, contact 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.


To apply this hotfix, you must be running Service Pack 1 for Windows 7 or Windows Server 2008 R2.

Restart requirement

You must restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any previously released hotfix.


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.


For information about SWS, see Internet RFC 813 and Internet RFC 1122.

For information about MSS, See Internet RFC 879.

See the terminology Microsoft uses to describe software updates .