Microsoft KB Archive/126967

= New TCP/IP Registry Parameter Ignores Push Bit on Receives =

Article ID: 126967

Article Last Modified on 10/31/2006

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APPLIES TO


 * Microsoft Windows NT Workstation 3.51
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q126967



SUMMARY
Microsoft has added a registry parameter for Afd.sys, in the Windows NT 3.51 Service Pack 3, that allows TCP/IP to ignore the push bit on receives. However, TCP still waits until the receive buffer is full if the push bit is not set.



MORE INFORMATION
To allow TCP/IP to ignore the push bit on receives, do the following:

WARNING: Using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall Windows NT to correct them. Microsoft cannot guarantee that any problems resulting from the use of Registry Editor can be solved. Use this tool at your own risk.

 Start Registry Editor (REGEDT32.EXE) and locate the following Registry subkey in the HKEY_LOCAL_MACHINE subtree:

\SYSTEM\CurrentControlSet\Services\Afd\Parameters

 Select the following value name: IgnorePushBitOnReceives From the Edit menu, select DWORD. Change the Data to 1. Click OK and quit the Registry Editor. Shut down and restart Windows NT.</ol>

In Windows NT version 3.1, TCP/IP behaved as if the PUSH bit was always set by the sender. In Windows NT 3.5, the receive logic was optimized to use the PUSH bit. This reduces the number of times an application has to wake up for incoming data. However, when Windows NT is communicating with a TCP/IP implementation (or application) that does not set the PUSH bit at the appropriate times, performance can suffer.

Performance degrades because, when data arrives without the PUSH bit set, TCP holds onto the application's recv while waiting for the rest of the data. Windows NT TCP/IP completes a recv call when:


 * Data arrives with the PUSH bit set.

-or-
 * The user recv buffer is full.

-or-
 * 0.5 seconds have elapsed since any data arrived.

When the third test above is required to complete a recv call, performance degrades.

<div class="resolution_section">

RESOLUTION
Microsoft has modified the file, AFD.SYS, to allow TCP/IP to ignore push bit on receives.

<div class="status_section">

STATUS
Microsoft has confirmed this to be a problem in Windows NT version 3.51. This problem was corrected in the latest Windows NT 3.51 U.S. Service Pack. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

<pre class="fixed_text">  S E R V P A C K

Additional query words: prodnt

Keywords: kbnetwork KB126967

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