TCP IP Illustrated

Since traceroute identifies the incoming interface

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Unformatted text preview: utput for TTLs 2 and 19: both are for the router gateway.tuc.noao.edu, but the two IP addresses are different. Since traceroute identifies the incoming interface, and since we're passing through the router in two different directions, once on the outbound path (TTL 2) and then on the return path (TTL 19), we expect this. We see the same effect comparing TTLs 3 and 18, and TTLs 4 and 17. 8.6 Summary Traceroute is an indispensable tool when working with a TCP/IP network. Its operation is simple: send UDP datagrams starting with a TTL of 1, increasing the TTL by 1, to locate each router in the path. An ICMP time exceeded is returned by each router when it discards the UDP datagram, and an ICMP port unreachable is generated by the final destination. We ran examples of traceroute on both LANs and WANs, and used it to examine IP source routing. We used loose source routing to see if the route to a destination is the same as the return route from that destination. Exercises file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (16 of 17) [12/09/2001 14.46.48] Traceroute Program 8.1 What can happen if an IP implementation decrements the incoming TTL and then tests for equal to 0? 8.2 How does traceroute calculate the RTT? Compare this to the RTT calculation done by ping. 8.3 (This exercise and the next one are based on actual problems determined when traceroute was being developed, and are from comments in the traceroute source code.) Assume there are three routers (Rl, R2, and R3) between the source and destination and that the middle router (R2) decrements the TTL but incorrectly forwards the IP datagram when the incoming TTL was 1. Describe what happens. How can you see this occur when running traceroute? 8.4 Again assume there are three routers between the source and destination. This time the destination host has a bug whereby it always uses the incoming TTL as the outgoing TTL of an ICMP message. Describe what happens and how you would see...
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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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