New TCP/IP Registry Parameter Ignores Push Bit on Receives

ID: Q126967

The information in this article applies to:

Microsoft Windows NT Workstation versions 3.51, 4.0
Microsoft Windows NT Server versions 3.51, 4.0

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.

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.

RESOLUTION

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

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):


   S E R V P A C K

Additional query words: prodnt

Keywords : kbnetwork nttcp kbbug3.51.sp3 NTSrvWkst
Version : 3.51 4.0
Platform : winnt
Issue type :