TCP IP Illustrated

This programming interface normally requires the user

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Unformatted text preview: Traceroute. This programming interface normally requires the user to have superuser privilege, meaning it may take special privilege to run it on your host. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (2 of 17) [12/09/2001 14.46.47] Traceroute Program 8.3 LAN Output We're now ready to run traceroute and see the output. We'll use our simple internet (see the figure on the inside front cover) going from svr4 to slip, through the router bsdi. The hardwired SLIP link between bsdi and slip is 9600 bits/sec. svr4 % traceroute slip traceroute to slip (, 30 hops max. 40 byte packets 1 bsdi ( 20 ms 10 ms 10 ms 2 slip ( 120 ms 120 ms 120 ms The first unnumbered line of output gives the name and IP address of the destination and indicates that traceroute won't increase the TTL beyond 30. The datagram size of 40 bytes allows for the 20-byte IP header, the 8-byte UDP header, and 12 bytes of user data. (The 12 bytes of user data contain a sequence number that is incremented each time a datagram is sent, a copy of the outgoing TTL, and the time at which the datagram was sent.) The next two lines in the output begin with the TTL, followed by the name of the host or router, and its IP address. For each TTL value three datagrams are sent. For each returned ICMP message the round-trip time is calculated and printed. If no response is received within 5 seconds for any of the three datagrams, an asterisk is printed instead and the next datagram is sent. In this output the first three datagrams had a TTL of 1 and the ICMP messages were returned in 20, 10, and 10 ms. The next three datagrams were sent with a TTL of 2 and the ICMP messages were returned 120 ms later. Since the TTL of 2 reached the final destination, the program then stopped. The round-trip times are calculated by the traceroute program on the sending host. They are the total RTTs from the traceroute program to that router. If we're interested in the per-hop time we have to subtract the value printed for TTL N from the value printed for...
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