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Why do I get connection refused/reset under heavy loads (Windows OOB server)?

Last Reviewed:
28th October 2008

The main OOB Server thread listens on the specified port (defaults to 8888) for incoming connections from OOB clients. Once a connection is accepted the main OOB server thread creates a new thread/process which handles the ODBC connection. Whilst the main server thread is starting up a new connection thread/process it is not accepting other connections. The time required to start a new connection handler is usually very short but to avoid newly incoming connections being dropped the server attempts to set the listen backlog queue to 20. The listen backlog queue allows the TCP/IP stack to queue the connection until the server gets around to accepting the connection.

If the listen backlog queue fills the TCP/IP stack will reject the connection and the OOB Server never sees it.

Unfortunately, in Windows 9X and NT Workstation the maximum listen backlog is 5. Since the OOB Server is usually pretty quick at accepting connections this low backlog value is not usually a problem. However, it does mean that concerted attempts to make lots of connections in a short period of time can fill the backlog queue. This makes Windows NT Workstation and Win9X in particular a bad choice for a high-load server.

The OOB Server could use SOMAXCONN in Winsock2 to set the listen backlog queue to its maximum value (this has no affect on NT workstation and Win9x). However, large backlogs make SYN flood attacks much more effective since the queue is non-pageable system memory.

The OOB client can help with connection refused problems due to the socket backlog filling in Win9X and NT Workstation by making multiple connection attempts. See the question "Does the OOB client support connection timeouts and connections retries?"

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