TCP IP Illustrated


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Unformatted text preview: rived (the LAN to which the host and the two routers are attached). This is the clue to a router that a redirect can be sent to the original sender. file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/ip_rout.htm (10 of 17) [12/09/2001 14.46.51] Chapter 9. IP Routing 3. Rl sends an ICMP redirect to the host, telling it to send future datagrams to that destination to R2, instead of Rl. A common use for redirects is to let a host with minimal routing knowledge build up a better routing table over time. The host can start with only a default route (either Rl or R2 from our example in Figure 9.3) and anytime this default turns out to be wrong, it'll be informed by that default router with a redirect, allowing the host to update its routing table accordingly. ICMP redirects allow TCP/IP hosts to be dumb when it comes to routing, with all the intelligence in the routers. Obviously Rl and R2 in our example have to know more about the topology of the attached networks, but all the hosts attached to the LAN can start with a default route and learn more as they receive redirects. An Example We can see ICMP redirects in action on our network (inside front cover). Although we show only three hosts (aix, solaris, and gemini) and two routers (gateway and netb) on the top network, there are more than 150 hosts and 10 other routers on this network. Most of the hosts specify gateway as the default router, since it provides access to the Internet. How is the author's subnet (the bottom four hosts in the figure) accessed from the hosts on the 140.252.1 subnet? First recall that if only a single host is at the end of the SLIP link, proxy ARP is used (Section 4.6). This means nothing special is required for hosts on the top network (140.252.1) to access the host sun ( The proxy ARP software in netb handles this. When a network is at the other end of the SLIP link, however, routing becomes involved. One solution is for every host and router to know that the router netb is the gateway f...
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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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