Microsoft KB Archive/154579

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Slow TCP/IP Performance When Resuming Large Data Transfer

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Q154579

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The information in this article applies to:


 * Microsoft Windows 95

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SYMPTOMS
Windows 95 clients using the TCP/IP protocol may experience a considerable delay resuming data transfer to client applications that take a long time to clear the receive window.

Observing a Protocol Analyzer trace of the slow performance shows the following behavior:


 * 1) The server sends a large data transfer.
 * 2) The client runs out of receive window space and indicates a zero-byte window.
 * 3) The server enters exponential backoff until it hits the maximum limit of 240 seconds between window probe packets. Because the time between probes is longer than the 120-second arp cache life, each new window probe requires that the server arp for the client's Media Access Control (MAC) address.
 * 4) The client clears its receive window and advertises additional window space.
 * 5) The server begins transmitting data with an incorrect sequence number.
 * 6) The client acknowledges a previous data send.
 * 7) The server takes up to four minutes to send the correct sequence number.

Sample Protocol Analyzer Trace
1 0.000   Client -> Server TCP .A...., len:   0, seq:2192824888, ack: 578989364, win:    0 2 153.452 Client -> Server TCP .AP..., len: 512, seq:2192824888, ack: 578989364, win:    0 3 0.008   Client -> Server TCP .A...., len:   0, seq:2192825400, ack: 578989364, win:14336 4 0.002   Client -> Server ARP Reply 5 0.001   Server -> Client TCP .A...., len: 1460, seq: 578992284, ack:2192825400, win:48640 6 0.005   Client -> Server TCP .A...., len:   0, seq:2192825400, ack: 578989364, win:14336 7 0.195   Client -> Server TCP .A...., len:   0, seq:2192825400, ack: 578989364, win:14336 8 4.785   Server -> Client TCP .A...., len: 357, seq: 578993744, ack:2192825400, win:48640 9 0.003   Client -> Server TCP .A...., len:   0, seq:2192825400, ack: 578989364, win:14336 10 0.193  Client -> Server TCP .A...., len:   0, seq:2192825400, ack: 578989364, win:14336 11 235.180 Client -> Server ARP Reply 12 0.000  Server -> Client TCP .A...., len: 1460, seq: 578989364, ack:2192825400, win:48640

CAUSE
The server falls out of sequence because its window probes are 240 seconds apart. Therefore, the server has to arp for the client's MAC address when the client advertises its new window space.

The reason this causes the server to fall out of sequence is that the server begins sending data immediately after discovering that the client has additional window space. Because the arp table entry for the client is in the resolving state when the server starts indicating data, TCP/IP cannot send the data to the client. Per RFC, the arp cache only buffers one packet when the destination IP address is in the resolving state. Windows 95 buffers the last packet only, so all sends, except for the last, are dropped until the arp entry has been resolved. When the arp entry for the client is resolved, TCP/IP sends the last packet that was cached to the client, which is out of sequence because the prior sends were dropped while the arp entry was resolving.

The delay that occurs is caused by the server taking 240 seconds to send the correct sequence number.

The reason the server takes 240 seconds to send the correct sequence number is that a retransmit timer started running. The retransmit timer takes 240 seconds to finish before the packet with the correct sequence number can be sent.

RESOLUTION
This issue is resolved by the following updated file for Windows 95 and OEM Service Release 2 (OSR2), and later versions of this file:

  VTCP.386  version 4.00.951  dated 8/6/96  47,401 bytes An updated version of this file is included in the Windows Sockets 2.0 update for Windows 95. For information on obtaining this update, see the following article in the Microsoft Knowledge Base:

"Q182108 Availability of Windows Sockets 2.0 for Windows 95"

TCP/IP was modified to reset the running timer value so that the packet with the correct sequence number is sent immediately.

STATUS
Microsoft has confirmed this to be a problem in Microsoft Windows 95 and OEM Service Release 2 (OSR2). An update to address this problem is now available, but is not fully regression tested and should be applied only to computers experiencing this specific problem. Unless you are severely impacted by this specific problem, Microsoft does not recommend implementing this update at this time. Contact Microsoft Technical Support for additional information about the availability of this update.

This issue is resolved in Microsoft Windows 98.

MORE INFORMATION
For additional information about the ARP cache, please refer to RFC 1122.

For additional information about issues resolved by updates to this component, please see the following articles in the Microsoft Knowledge Base:

Q178022 NetBIOS SEND WAIT Over TCP/IP Completes Without NetBIOS RECEIVE

Q177539 Windows 95 Stops Responding Because of Land Attack

Q170791 Windows 95 TCP Clients Run Out of Ports

Q168747 Update to Windows 95 TCP/IP to Address Out-of-Band Issue For additional information about Windows 95 updates, please see the following article in the Microsoft Knowledge Base:

"Q161020 Implementing Windows 95 Updates" Keywords : kbnetwork kbpolicy win95

Issue type : kbprb

Technology : kbWin95search kbWin95