How to Determine Loss of Client/Server Connection

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Article ID: 140325 - View products that this article applies to.
This article was previously published under Q140325
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This article explains how to use keepalive probes to determine loss of stream connection.


In the context of this article, the "client" is the peer computer requesting a connection, and the "server" is the peer computer accepting a connection. This notation does not reflect Client/Server relationships as an architectural principal.

Consider the following scenario (half-open connection):

The client has established stream connection with the server. If client crashes or a network error occurs (for example, intermediate router goes down), the server has no way of knowing this, unless it tries to use send(). In this case, send fails with the WSAECONNRESET error. If the server does not try to send data to a client, the connection is kept open on the server side for infinite amount of time.

Even if WSAAsyncSelect() with the FD_CLOSE parameter is used to determine loss of connection, event notification is not triggered. No events get posted because virtual circuit never go to a FIN WAIT or CLOSE WAIT state (crashed client never called shutdown or closesocket).

This situation can be avoided if keepalive probe packets are used. Note that keepalive probe packets are not a required part of the TCP specification. Keepalive packets are used to probe a connection that has been inactive for a long time. The peer initiates a disconnect when the probes do not get through. Keepalive packets for TCP connection can be turned on by using setsockopt() call with SO_KEEPALIVE option.

The following registry entries control keepalive probe packet parameters on computers running Windows NT or Windows 95 (note that not all of the entries can be found in the registry by default). Changing registry parameters affects all TCP/IP stream connections on the system.

KeepAliveInterval - This parameter determines the interval separating keep alive retransmissions until a response is received.

KeepAliveTime - This parameter controls how often TCP attempts to verify that an idle connection is still intact by sending a keep alive packet.

TcpMaxDataRetransmissions - This parameter controls the number of times TCP will retransmit an individual data segment (non-connect segment) before aborting the connection.

NOTE: The TcpMaxDataRetransmission parameter is NOT a valid setting under Windows 95. For additional information on valid TCP/IP registry parameters in Windows 95, please see the following article in the Microsoft Knowledge Base:
158474 Windows 95 TCP/IP Registry Entries
The above entries are found in following registry locations:

Windows 95

NOTE: To make the appropriate changes to the value above in the registry under Windows 95, go to the above location, click the Edit menu, point to New, and click String Value.

Windows NT

NOTE: To make the appropriate changes to the values above in the registry under Windows NT, go to the above location, click Add Value on the Edit menu. The Data Type should be REG_DWORD.

For information on how to configure above entries, please see the following article in the Microsoft Knowledge Base:
120642 TCP/IP and NBT Configuration Parameters for Windows 2000 or Windows NT
IMPORTANT: After you alter the registry, you must restart your computer.


Article ID: 140325 - Last Review: December 5, 2003 - Revision: 3.0
  • Microsoft Windows NT Workstation 3.5
  • Microsoft Windows NT Workstation 3.51
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 3.5
  • Microsoft Windows NT Server 3.51
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows 95
kbinfo kbnetwork KB140325

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