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How to Change the Default TCP Window Size in Your Network
These instructions provide a starting point for setting the systemwide default for the TCP receive window. For more information on specific platforms, refer to the vendor documentation.
Windows NT 4.0
Using REGEDIT, navigate to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Paramaters key and add a new DWORD value called TcpWindowSize if one does not already exist. The system default is 8192 rounded up to a multiple of the MSS (maximum segment size)--this is 8760 for Ethernet--, or four times the MSS, whichever is larger. To set your own value, enter the results from your calculations (be sure to click the "decimal" radio button first) and reboot the system.
Solaris 7
System parameters are tweaked using NDD. To view the current TCP window size, execute the "ndd /dev/tcp tcp_recv_hiwat" command, which will display the value currently assigned with the tcp_recv_hiwat variable. To set the window size, execute the "ndd -set /dev/tcp tcp_recv_hiwat" command. Note that Solaris has a minimum value of six times the MSS, so you cannot set the window size to a multiple of four.
Digital Unix 4.0d
To view the current TCP window size, execute the "sysconfig -q inet tcp_recvspace" command. The default setting is 32 kilobytes. To temporarily change the default value, use the "sysconfig -r inet tcp_recvspace=XXX" command, where "XXX" is the value you want to use. This will affect all new processes, but will not be permanent across reboots. If you want to change this setting on a permanent basis, you will need to rebuild the system kernel.
Linux 2.0 kernel
TCP parameters can be set a number of different ways with Linux, though the most common method is to tweak the system's routing table. The system default is 32 kilobytes, so hard-core users will want to recompile the kernel to use a more appropriate value. To view the current setting, execute the "netstat -r" command, which will display the current routing table, including the MTU (maximum transmission unit) and window sizes associated with each destination. To set the value, modify the system startup scripts so that the default route (and/or internal routes) have more appropriate levels.
NetWare
The TCP window size cannot be modified in NetWare.
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