TCP IP Illustrated

The g flag means its an indirect route to a gateway

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Unformatted text preview: Interface lo0 le0 le0 This is the first time we've seen the D flag, which means the route was installed by an ICMP redirect. The G flag means it's an indirect route to a gateway (netb), and the H flag means it's a host route (as we expect), not a network route. Since this is a host route, added by a host redirect, it handles only the host bsdi. If we then access the host svr4, another redirect is generated, creating another host route. Similarly, accessing the host slip creates another host route. The point here is that each redirect is for a single host, causing a host route to be added. All three hosts on the author's subnet file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/ip_rout.htm (12 of 17) [12/09/2001 14.46.51] Chapter 9. IP Routing (bsdi, svr4, and slip) could also be handled by a single network route pointing to the router sun. But ICMP redirects create host routes, not network routes, because the router generating the redirect in this example (gateway) has no knowledge of the subnet structure on the 140.252.13 network. More Details Figure 9.4 shows the format of the ICMP redirect message. Figure 9.4 ICMP redirect message. There are four different redirect messages, with different code values, as shown in Figure 9.5. code 0 1 2 3 Description redirect for network redirect for host redirect for type-of-service and network redirect for type-of-service and host Figure 9.5 Different code values for ICMP redirect. There are three IP addresses that the receiver of an ICMP redirect must look at: (1) the IP address that caused the redirect (which is in the IP header returned as the data portion of the ICMP redirect), (2) the IP address of the router that sent the redirect (which is the source IP address of the IP datagram containing the redirect), and (3) the IP address of the router that should be used (which is in bytes 4-7 of the ICMP message). There are numerous rules about ICMP redirects. First, redirects are generated only by routers, not by hosts. Also, redirects are intended to be used by hosts, not routers. It is assumed that router...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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