Microsoft KB Archive/171351

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Loopback Send/Receive Over IPX/SPX May Not Work Under Stress


The information in this article applies to:

  • Microsoft Windows 95


In Windows 95, the IPX/SPX-compatible protocol (NWLINK) may not successfully send and receive data under the following conditions:

  • The same computer is both the sender and receiver of the data.
  • A large amount of data is being sent in a continuous stream.
  • The network adapter (NDIS) driver does not provide loopback functionality.

Symptoms of this problem may include:

  • The data transfer never finishes. This is the most common symptom.

  • The computer may restart. This occurs rarely, and only if a large block of data is being transferred.

  • The computer may stop responding (hang), requiring a cold boot (power- off or reset). This occurs rarely, and only if a large block of data is being transferred.


Loopback packets (in which the source and destination addresses are the same) are not recognized and handled internally by NWLINK. Instead, they are passed to the network adapter driver. If the network adapter driver does not recognize and handle loopback packets, the packet is sent out on the network wire. Under continuous stress of this type, and under certain timing conditions, the symptoms listed above may occur.


This condition may occur when you are running a program designed to test data throughput over IPX/SPX (including WinSock over IPX/SPX). We recommend that such tests be conducted between two separate computers, instead of on one computer in loopback mode. This not only avoids the problems mentioned here, but also returns more meaningful results.

You can also work around this problem by using a network adapter driver that handles loopback packets internally without transmitting them on the network medium. For information about whether a particular network adapter driver handles loopback packets internally, contact the vendor of your network adapter.


Microsoft has confirmed this to be a problem in Windows 95. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.


For additional information about issues related to loopback operations with NWLINK, please see the following articles in the Microsoft Knowledge Base:

Q127054 Loopback Functionality Does Not Work with Miniport Drivers

Q124648 DC21040 Causes Windows NT 3.5 to Stop Responding (Hang)

Keywords : kbnetwork win95 kbAPI kbSDKPlatform kbWinsock kbGrpDSNet
Issue type :
Technology : kbWin95search kbWin95

Last Reviewed: December 15, 2000
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