Maintaining Route Tables for IPv4
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4/8/2010
When a Windows Embedded CE-based device initializes, the route table normally contains only a handful of 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 with each other, the default gateway may know of a better route to a given address. When this is the case, upon receiving a datagram that could be taking the better path, the router forwards the datagram normally, then advises the sender of the better route using an ICMP Redirect message. These messages can specify redirection for one host, a subnet, or for an entire network. When a Windows Embedded CE-based device receives an ICMP redirect, a 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, then the ICMP redirect is ignored.