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Unformatted text preview: TTL N+1.
Figure 8.1 shows the tcpdump output for this run. As we might have guessed, the reason
that the first probe packet to bsdi had an RTT of 20 ms and the next two had an RTT of 10
ms was because of an ARP exchange, tcpdump shows this is indeed the case.
The destination UDP port starts at 33435 and is incremented by one each time a datagram is
sent. This starting port number can be changed with a command-line option. The UDP
datagram contains 12 bytes of user data, which we calculated earlier when traceroute
output that it was sending 40-byte datagrams.
Next, tcpdump prints the comment [ttl 1] when the IP datagram has a TTL of 1. It
prints a message like this when the TTL is 0 or 1, to warn us that something looks funny in
the datagram. Here we expect to see the TTL of 1, but with some other application it could file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (3 of 17) [12/09/2001 14.46.47] Traceroute Program be a warning that the datagram might not get to its final destination. We should never see a
datagram passing by with a TTL of 0, unless the router that put it on the wire is broken.
arp who-has bsdi tell svr4
arp reply bsdi is-at 0:0:c0:6f:2d:40
svr4.42804 > slip.33435; udp 12 [ttl 1]
bsdi > svr4: icmp: time exceeded in0.004325
svr4.42804 > slip.33436: udp 12 [ttl 1]
bsdi > svr4: icmp: time exceeded in0.071149
svr4.42804 > slip.33437: udp 12 [ttl 1]
bsdi > svr4: icmp: time exceeded in0.086375
svr4.42804 > slip.33438: udp 12
slip > svr4: icmp: slip udp port 33438
svr4.42804 > slip.33439: udp 12
slip > svr4: icmp: slip udp port 33439
svr4.42804 > slip.33440: udp 12
slip > svr4: icmp: slip udp port 33440...
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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.
- Spring '12