Applies To:
  • CitectSCADA x.x

I am using Citect LAN redundancy with 2 network cards. I am running Windows 95 clients talking to a Windows NT file server. My project is installed on the file server. When I disconnect the LAN card that Citect is communicating with, Citect will switch over to the second network card and continue to communicate correctly. However the next time I change pages Citect will pause for 30 seconds. After the pause Citect will run normally. What is causing this pause? 

The pause is being caused by the time out of the Microsoft file redirector. This delay/timeout can be significantly reduced and as a result seamless operation of Citect is achieved.These changes can also be used to reduce the time it takes to change over from the Primary to the Standby IO Server.

This is implemented by changes in the Registry on Windows NT 3.51, Windows 4.0 and Windows 95 as follows;

Windows NT 3.51 and Windows NT 4.0

Run the Registry Editor (RegEdit.ext) and select


BcastNameQueryCount Modify REG_DWord 0x3 to 0x1
BcastQueryTimeout Modify REG_DWord 750 to 300
SessionKeepAlive Modify REG_DWord 3600000 to 3000

To utilise these parameter settings Citect must be using NetBt as its network protocol. This is set in the CITECT file under [LAN] LanA=x, where x is the LanA number that is being used by NetBt. LanA numbers can be checked through the Control Panel Nework settings. See Q2054 LANA settings with Windows NT 4.0.

Also add the following keys to the TCP/IP Parameters. The various TCP/IP keys do not appear in the Registry unless TCP/IP is installed using the Network icon in Control Panel. You must restart your machine for a change in any of these parameters to take effect. Select:


TcpMaxConnectAttempts: REG_DWORD 0x2
Specifies the maximum number of times TCP/IP attempts to establish a connection before reporting failure. The initial delay between connection attempts is 3 seconds. This delay is doubled after each attempt.

TcpMaxConnectRetransmissions: REG_DWORD 0x2
This parameter determines the number of times TCP will retransmit a connect request (SYN) before stoping the attempt. The retransmission timeout is doubled with each successive retransmission in a given connect attempt. The initial timeout value is three seconds.

TcpMaxDataRetransmissions: REG_DWORD 0x1
This parameter controls the number of times TCP will retransmit an individual data segment (non connect segment) before ending the connection. The retransmission timeout is doubled with each successive retransmission on a connection. It is reset when responses resume. The base timeout value is dynamically determined by the measured round-trip time on the connection.

TcpMaxRetransmissionAttempts: REG_DWORD 0x1
Specifies the maximum number of times that TCP/IP attempts to retransmit a piece of data on an established connection before ending the connection. The initial delay before retransmitting is based on the current estimate TCP/IP makes of the round-trip time on the connection. This delay is doubled after each retransmission. Acknowledgment of the data results in a recalculation of the estimate for the round-trip time


  • Run the Registry Editor and select
  • Hkey_Local_Machine\System\CurrentControlSet\Services\VxD\MSTCP
  • Select Add/New.
  • Select DWORD VALUE
  • Add entries:
  • Exit/Close Regedit and restart computer.

MaxConnectRetries specifies the number of times a connection attempt (SYN) will be retransmitted before giving up. The initial retransmission timeout is three seconds and it is doubled each time up to a maximum of two minutes. The default is 3.

MaxDataRetries specifies the maximum number of times a segment carrying data or an FIN will be retransmitted before the connection is aborted. The retransmission timeout itself is adaptive and will vary according to link conditions. The default is 5.

See also Q1884 and Q1893.