When a Windows-based computer is initialized, the route table normally contains only a few entries. One of those entries specifies a default gateway. Datagrams that have a destination IP address with no better match in the route table are sent to the default gateway. However, because routers share information about network topology, the default gateway may know a better route to a given address. When this is the case, then upon receiving a datagram that could take the better path, the router forwards the datagram normally. It then advises the sender of the better route, using an ICMP Redirect message. These messages typically specify redirection for a specific destination address. When a Windows-based computer receives an ICMP redirect, a validity check is performed to be sure that it came from the first-hop gateway in the current route, and that the gateway is on a directly connected network. If so, a host route with a 10-minute lifetime is added to the route table for that destination IP address. If the ICMP redirect did not come from the first-hop gateway in the current route, or if that gateway is not on a directly connected network, the ICMP redirect is ignored.
In Windows Server 2003 Service Pack 1, the new MaxICMPHostRoutes registry value defines the maximum number of host routes that can be added through the receipt of ICMP Redirect messages. For more information, see Appendix A.
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