TCP IP Illustrated

This indicates that traceroute received an icmp

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Unformatted text preview: te that fails. The key here is the notation ! s following the RTTs for the TTL of 3. This indicates that traceroute received an ICMP "source route failed" error message: a type of 3 and a code of 5 from Figure 6.3. The asterisk for the second RTT for the TTL of 3 indicates no response was received for that probe. This is what we expect, since it's impossible for gateway to send the datagram directly to gabby, because they're not directly connected. The reason that both TTLs 2 and 3 are from gateway is that the values for the TTL of 2 are from gateway when it receives the datagram with an incoming TTL of 1. It discovers that the TTL has expired before it looks at the (invalid) strict source route, and sends back the ICMP time exceeded. The line with a TTL of 3 is received by gateway with an incoming TTL of 2, so it looks at the strict source route, discovers that it's invalid, and sends back the ICMP source route failed error. Figure 8.10 shows the tcpdump output corresponding to this example. This output was collected on the SLIP link between sun and netb. We had to specify the -v option for tcpdump to display the source route information. This produces other output that we don't need, such as the datagram ID, which we've deleted. Also, the notation SSRR stands for file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (13 of 17) [12/09/2001 14.46.48] Traceroute Program "strict source and record route." 1 0.0 2 0.270278 (0.2703) 3 0.284784 (0.0145) 4 0.540338 (0.2556) 5 0.550062 (0.0097) 6 0.810310 (0.2602) 7 0.818030 (0.0077) 8 1.080337 (0.2623) 9 1.092564 (0.0122) 10 1.350322 (0.2578) 11 1.357382 (0.0071) 12 1.590586 (0.2332) 13 1.598926 (0.0083) 14 1.860341 (0.2614) 15 1.875230 (0.0149) sun.33593 > netb.33435: udp 12 [ttl 1] (optlen=16 SSRR{#gateway gabby westgate} EOL) netb > sun: icmp: time exceeded intransit sun.33593 > netb.33436: udp 12 [ttl 1] (optlen=16 SSRR{#gateway gabby westgate} EOL) netb > sun: icmp: time exceeded intransit sun.33593 > netb.33437: udp 12 [ttl 1] (optlen...
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