Dead gateway detection allows Transmission Control Protocol (TCP) to detect failure of the default gateway and to adjust the Internet Protocol (IP) routing table to use another default gateway.

Dead Gateway Detection in Windows Server 2003

Windows Server 2003 TCP/IP uses the triggered reselection method of dead gateway detection described in RFC 816. In this method, when a TCP connection routed through the default gateway attempts to send a TCP packet to the destination IP address a certain number of times without receiving a response, the triggered reselection algorithm changes the route cache entry (RCE) for that remote IP address to use the next default gateway in the list.

The number of times that the connection attempts to send the packet while not receiving a response equals one-half of the registry value TcpMaxDataRetransmissions. When 25 percent of the TCP connections have moved to the next default gateway, the algorithm advises IP to change the computer's default gateway to the one that the connections are now using. For more information about the implementation of TCP retransmission in Windows, see TCP Retransmission Behavior.

To enable dead gateway detection, set the registry value EnableDeadGWDetect to 1. To disable dead gateway detection, set it to 0.

Dead Gateway Detection Example

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 does not receive any acknowledgments. After the third retransmission, the RCE for that remote IP address is switched to the next default gateway in the list. At this point, any current TCP connections to that remote IP address are switched over, but the connections to come still try to use the original default gateway.

When the second TCP connection tries to send data, the same thing happens. Now, 2 of the 11 RCEs point to the new gateway.

When the third TCP connection tries to send data, after the third retransmission 3 of 11 RCEs have switched to the second default gateway. Because at this point more than 25 percent of the RCEs have moved, the default gateway for the whole computer is moved to the new one.

That new default gateway remains the primary one for the computer until it experiences problems, causing the dead gateway algorithm to try the next gateway in the list, or until the computer is restarted.

When the search reaches the last default gateway, it returns to the beginning of the list.

For more information on dead gateway detection, see at RFC 816: FAULT ISOLATION AND RECOVERY.

See Also

Windows TCP Implementation Features