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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.
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
redirect for network
redirect for host
redirect for type-of-service and
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.
- Spring '12