TCP IP Illustrated

The reason for the timeouts must be that the router

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Unformatted text preview: ctly from gemini to netb, and don't go back the way they came? We use our version of traceroute with loose source routing from Section 8.5: sun % traceroute -g 140.252.3.54 sun traceroute to sun (140.252.13.33), 30 hops max, 40 byte packets 1 netb (140.252.1.183) 244 ms 256 ms 234 ms 2 * * * 3 gemini (140.252.3.54) 285 ms 227 ms 234 ms 4 netb (140.252.1.183) 263 ms 259 ms 294 ms 5 sun (140.252.13.33) 534 ms 498 ms 504 ms When we specify loose source routing, the router swnrt never responds. If we look at the earlier output from traceroute, without source routing, we see that swnrt is indeed the second hop. The reason for the timeouts must be that the router does not generate the ICMP time exceeded errors when the datagram specifies loose source routing. What we are looking for in this traceroute output is that the return path from gemini (TTLs 3,4, and 5) goes directly to netb, and not through the router swnrt. The question that we need SNMP to answer is what does the routing table entry on netb look like file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/snmp_sim.htm (28 of 33) [12/09/2001 14.47.40] Chapter 25. SNMP: Simple Network Management Protocol for the destination network 140.252.3? It is netb that sends the packets to swnrt and not directly to gemini. We use the get command to fetch the value of the next-hop router for this destination: sun % snmpi -a netb -c secret get ipRouteNextHop.140.252.3.0 ipRouteNextHop.l40.252.3.0=140.252.1.6 This routing table entry tells netb to send the packets to swnrt, which is what we see happen. Why does gemini send the packets directly back through netb? Because on gemini the destination address of the return packets is 140.252.1.29, and that network (140.252.1) is a directly connected interface. What we're seeing in this example is a policy routing decision. The default route to network 140.252.3 is through the router swnrt because gemini is intended to be a multihomed host, not a router. This is an example of a multihomed host that does not want t...
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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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