TCP IP Illustrated

2521365 377 ms 377 ms 377 ms 1492 1006 576 552 544

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Unformatted text preview: d DF set, trying new MTU fragmentation required and DF set, trying new MTU fragmentation required and DF set, trying new MTU fragmentation required and DF set, trying new MTU 2 slip (140.252.13.65) 377 ms 377 ms 377 ms = = = = = = = = 1492 1006 576 552 544 512 508 296 In this example the router bsdi does not return the MTU of the outgoing interface in the ICMP error, so we step through the likely values for the MTU. The first line of output for a TTL of 2 prints a hostname of bsdi, but that's because it's the router returning the ICMP error. The final line of output for a TTL of 2 is what we're looking for. It's not hard to modify the ICMP code on bsdi to return the MTU of the outgoing interface, and if we do that and rerun our program, we get the following output: sun % traceroute.pmtu slip traceroute to slip (140.252.13.65), 30 hops max outgoing MTU = 1500 1 bsdi (140.252.13.35) 53 ms 6 ms 6 ms 2 bsdi (140.252.13.35) 6 ms fragmentation required and DF set, next hop MTU = 296 2 slip (140.252.13.65) 377 ms 378 ms 377 ms Here we don't have to try eight different values for the MTU before finding the right one-the router returns the correct value. The Worldwide Internet file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (12 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol As an experiment, this modified version of traceroute was run numerous times to various hosts around the world. Fifteen countries (including Antarctica) were reached and various transatlantic and transpacific links were used. Before doing this, however, the MTU of the dialup SLIP link between the author's subnet and the router netb (Figure 11.12) was increased to 1500, the same as an Ethernet. Out of 18 runs, only 2 had a path MTU of less than 1500. One of the transatlantic links had an MTU of 572 (a value not even listed as a likely value in RFC 1191) and the router did return the newer format ICMP error. Another link, between two routers in Japan, wouldn't handle a 1500byte frame, and the router did not return the newer form...
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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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