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Unformatted text preview: or 104 ms. This corresponds to the IIO-ms
value measured on svr4.
The source port number in Figure 8.1 (42804) seems high. traceroute sets the source
port number of the UDP datagrams that it sends to the logical-OR of its Unix process ID
with 32768. In case traceroute is being run multiple times on the same host, each
process looks at the source port number in the UDP header that's returned by ICMP, and
only handles those messages that are replies to probes that it sent.
There are several points to note with traceroute. First, there is no guarantee that the
route today will be in use tomorrow, or even that two consecutive IP datagrams follow the
same route. If a route changes while the program is running you'll see it occur because
traceroute prints the new IP address for the given TTL if it changes. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (5 of 17) [12/09/2001 14.46.47] Traceroute Program Second, there is no guarantee that the path taken by the returned ICMP message retraces the
path of the UDP datagram sent by traceroute. This implies that the round-trip times
printed may not be a true indication of the outgoing and returning datagram times. (If it
takes 1 second for the UDP datagram to travel from the source to a router, but 3 seconds for
the ICMP message to travel a different path back to the source, the printed round-trip time
is 4 seconds.)
Third, the source IP address in the returned ICMP message is the IP address of the interface
on the router on which the UDP datagram arrived. This differs from the IP record route
option (Section 7.3), where the IP address recorded was the outgoing interface's address.
Since every router by definition has two or more interfaces, running traceroute from
host A to host B can generate different output than from host B to host A. Indeed, if we run
traceroute from host slip to svr4 the output becomes:
slip % traceroute svr4
traceroute to svr4 (18.104.22.168), 30 hops max, 40 byte
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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