TCP/IP Dead Gateway Detection Algorithm Updated for Windows NT

You may experience a loss of connectivity to computers across the router when your primary gateway fails, even though your computer running Windows NT is configured with multiple default gateways for redundancy.

Windows NT 4.0 supports dead gateway detection using the Triggered Reselection method described in RFC 816. This implementation can cause the current default gateway to switch to the second or third configured gateway very easily; however, it does not test the gateway being switched to prior to making the change.

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

Microsoft has confirmed that this is a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4.

The description and explanation of the Windows NT 4.0 dead gateway detection algorithm follows:

• When any TCP connection that is routed through the default gateway has attempted to send a TCP packet to the destination a number of times equal to one-half of the registry value TcpMaxDataRetransmissions, but receives no response, the algorithm advises IP to switch to the next gateway in the list.
  
  The new algorithm, however, is more cautious about advising IP to switch gateways:
  
  
• When any TCP connection that is routed through the default gateway has attempted to send a TCP packet to the destination a number of times equal to one-half of the registry value TcpMaxDataRetransmissions, but receives no response, the algorithm changes the Route Cache Entry (RCE) for that one remote IP address to use the next default gateway in the list. When 25 percent of the TCP connections have moved to the next default gateway, the algorithm advises IP to change the default gateway for the whole computer to the one that the connections are now using.

Example of the New Algorithm

Assume that there are currently TCP connections to 11 different IP addresses that are being routed through the default gateway. Now assume that the default gateway fails, that there is a second default gateway configured, and that the value for TcpMaxDataRetransmissions is at the default of 5.

When the first TCP connection tries to send data, it will not receive any acknowledgments. After the third retransmission, the RCE for that remote IP address will be switched to use the next default gateway in the list. At this point, any TCP connections to that one remote IP address will have switched over, but the remaining connections will still try to use the original default gateway.

When the second TCP connection tries to send data, the same thing will happen. Now, two of the 11 RCEs will point to the new gateway.

When the third TCP connection tries to send data, after the third retransmission, three of 11 RCEs will have been switched to the second default gateway. Since, at this point, over 25 percent of the RCEs have been moved, the default gateway for the whole computer will be moved to the new one.

At this point, that default gateway remains the primary one for the computer until it experiences problems (causing the dead gateway algorithm to try the next one in the list again) or until the computer is restarted.

When the end of the list of default gateways is reached, the search returns to the beginning again.