Disabling TCP/IP Nagle Algorithm Improves Speed on Slow Nets

• Microsoft SNA Server for Windows NT, versions 2.1 and 2.11

SUMMARY

This article explains the Nagle TCP/IP algorithm and how to manually disable it on different the SNA Server client and server releases for different operating systems. It also mentions how to obtain an update that automatically disables this algorithm. Microsoft has updated the file <snaroot>\SYSTEM\SNAIP.DLL to correct this problem.

MORE INFORMATION

The Nagle TCP/IP Algorithm

The Nagle TCP/IP algorithm was designed to avoid problems with small packets, called tinygrams, on slow networks. The algorithm says that a TCP/IP connection can have only one outstanding small segment that has not yet been acknowledged. The definition of "small" varies but usually it is defined as "less than the segment size" which on ethernet is about 1500 bytes.

Manually Disabling the Nagle Algorithm

You can disable the Nagle algorithm by performing the SNA Server client and server modifications mentioned below and following the instructions in the Activating the Modifications section.

SNA Server Modifications

1. Under the subtree HKEY_LOCAL_MACHINE add the subkey SNATCP under the following key:

         SYSTEM\CurrentControlSet\Services\Snabase\Parameters\
   

2. Choose Add Key from the Edit menu and use the following information:

         Value Name: NoDelay
         Data Type:  REG_SZ
         String:     yes
   
   

Windows 95 SNA Client Modifications

Add the same registry entry as SNA Server above.

Windows 3.x Modifications

Windows 3.x client always disables the Nagle algorithm and there is no way to enable it.

Activating the Modifications

When the NoDelay is set to yes, the TCP/IP transport DLL uses the TCP_NODELAY option when it opens a socket.

For this to take effect, the client server interface must be stopped and restarted on both the server and client after the registry changes have been made. The following table shows the name of the SNA client server interface program and the corresponding operating system platform:

   Client Server Interface      Platform
   ----------------------------------------
   SNABASE Service              Windows NT
   SNABASE Service              Windows 95
   WNAP.EXE                     Windows 3.x

Obtaining the Update to Automatically Disable the Nagle Algorithm

This feature is included in the latest U.S. Service Pack for SNA Server for Windows NT, version 2.11. 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