Explanation of ICMP Redirect Behavior

The assumption on IP internetworks is that the IP hosts (nonrouters) only need minimal routing information and can rely on IP routers having knowledge of the topology of the internetwork and all of the best routes. Therefore, IP hosts are typically only configured with an IP address of a default router (also called a default gateway). Any remote traffic from the IP host is forwarded to the default IP router. While this makes it easier to configure the IP hosts, in IP internetworks where there are multiple routers on a given network, the behavior of sending all remote traffic to the same router can produce non-optimal host routing. To prevent the perpetuation of non-optimal host routing, IP routers can update the routing tables of hosts using an ICMP Redirect message.

The ICMP Redirect message is only sent in the following situation:

1. Host A is sending a packet to Host B. Host A's default IP router is router R1. Because Host B is a remote host, Host A forwards the packet destined for Host B to its default router R1.
2. R1 checks its routing table and finds that the next hop for the route to the network for Host B is router R2.
3. If Host A and R2 are on the same network that is also directly attached to R1, an ICMP Redirect message is sent to Host A informing it that R2 is the better route when sending to Host B.
4. Router R1 then forwards the IP datagram to R2.
5. Host A adds a host route to its routing table for Host B's IP address with router R2's IP address as the forwarding address. Subsequent datagrams from Host A to Host B are forwarded by means of router R2.

An ICMP Redirect message is not sent for IP datagrams with IP source route options even if there is a better route to the ultimate destination than the next address in the source route.

In the ICMP Redirect message:



Router IP Address - The IP address of the router corresponding to the better route as determined by the router originally sent the IP datagram. The sending host uses this information to update its routing table to include this new route.

IP Header and First 64 Bits of Original IP Payload - To identify the forwarded packet for the source, the IP header and 64 bits of the IP payload are encapsulated and sent back to the source. Included in the encapsulated IP header is the IP Identification field uniquely identifying a specific IP packet as sent by the source and the destination IP address. Between the destination IP address stored here and the router IP address field in the ICMP header, a routing table entry is made. Windows NT Specifics:

A host route learned by means of an ICMP Redirect will be added to the route table for 10 minutes, after which time it is removed and must be relearned through another ICMP Redirect.