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Figure 8.1 tcpdump output for traceroute example from svr4 to slip. The ICMP message "time exceeded in transit" is what we expect to see from the router
bsdi, since it will decrement the TTL to 0. The ICMP message comes from the router even
though the IP datagram that was thrown away was going to slip.
There are two different ICMP "time exceeded" messages (Figure 6.3), each with a different
code field in the ICMP message. Figure 8.2 shows the format of this ICMP error message. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (4 of 17) [12/09/2001 14.46.47] Traceroute Program Figure 8.2 ICMP time exceeded message.
The one we've been describing is generated when the TTL reaches 0, and is specified by a
code of 0.
It's also possible for a host to send an ICMP "time exceeded during reassembly" when it
times out during the reassembly of a fragmented datagram. (We talk about fragmentation
and reassembly in Section 11.5.) This error is specified by a code of 1.
Lines 9-14 in Figure 8.1 correspond to the three datagrams sent with a TTL of 2. These
reach the final destination and generate an ICMP port unreachable message.
It is worthwhile to calculate what the round-trip times should be for the SLIP link, similar to
what we did in Section 7.2 when we set the link to 1200 bits/sec for the Ping example. The
outgoing UDP datagram contains 12 bytes of data, 8 bytes of UDP header, 20 bytes of IP
header, and 2 bytes (at least) of SLIP framing (Section 2.4) for a total of 42 bytes. Unlike
Ping, however, the size of the return datagrams changes. Recall from Figure 6.9 that the
returned ICMP message contains the IP header of the datagram that caused the error and the
first 8 bytes of data that followed that IP header (which is a UDP header in the case of
traceroute). This gives us a total of 20+8+20+8+2, or 58 bytes. With a data rate of 960
bytes/sec the expected RTT is (42 + 58/960)...
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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