Poor Performance May Occur During FTP File Transfers

 Last reviewed: March 24, 1997
Article ID: Q149525

The information in this article applies to:
  • Microsoft Windows NT Workstation version 3.51
  • Microsoft Windows NT Server version 3.51

SYMPTOMS

When you use the Microsoft FTP client you may notice delays of up to 5 seconds between 4096-byte data transmissions to an FTP server.

CAUSE

The retail Windows NT FTP client has a hard-coded send size of 4096 bytes. Because 4096 may not be evenly divisible by the maximum segment size (MSS) for that network interface, the last transmit in every FTP send will not fill the space available in the TCP packet. For example, if the TCP/IP MSS is 536 bytes, FTP sends are not evenly divisible by the MSS, ensuring that the last segment sent will always be less than one MSS.

If a send request occurs that is less than one segment, Silly Window Syndrome (SWS) can occur. To avoid SWS, the TCP/IP stack will hold the send for 5 seconds, or until there is enough data queued to send a full segment.

Note: For more information on SWS, please refer to Internet RFC 813. For more information on MSS, please refer to Internet RFC 879.

RESOLUTION

To resolve this problem, install the fix mentioned below.

To correct this problem, apply the fix mentioned below. The following new switch can now be used with the fix: 

   -w:buffersize
This switch overrides the default transfer buffer size of 4096. The switch has been added to FTP to allow the send size to be configurable. This will allow FTP users to ensure that the send size is always evenly divisible by the MSS on the network interface being used, thus preventing SWS.

STATUS

Microsoft has confirmed this to be a problem in Windows NT Workstation version 3.51 and Windows NT Server version 3.51. This problem was corrected in the latest U.S. Service Pack for Windows NT Workstation or Server version 3.51. For information on obtaining this update, 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 : kbbug3.51 kbnetwork NTSrvWkst nttcp
Version : 3.51
Platform : WinNT

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Last reviewed: March 24, 1997
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