Unformatted text preview: d sends back
the time exceeded. This means the second line of output (for the TTL of 2) identifies R3,
not R2. The third line of output correctly identifies R3. The clue that this bug is present is
two consecutive lines of output that identify the same router.
8.4 In this case the TTL of 1 identifies R1, the TTL of 2 identifies R2, and the TTL of 3
identifies R3; but when the TTL is 4 the UDP datagram gets to the destination with an
incoming TTL of 1. The ICMP port unreachable is generated, but its TTL is 1
(incorrectly copied from the incoming TTL). This ICMP message goes to R3 where the
TTL is decremented and the message discarded. An ICMP time exceeded is not
generated, since the datagram that was discarded was an ICMP error message (port
unreachable). A similar scenario occurs for the probe with a TTL of 5, but this time the
outgoing port unreachable starts with a TTL of 2 (the incoming TTL) and makes it back
to R2, where it's discarded. The port unreachable corresponding to the probe with a TTL
of 6 makes it back to R1, where it's discarded. Finally the port unreachable for the probe
with a TTL of 7 makes it all the way back, where it arrives with an incoming TTL of 1.
(traceroute considers an arriving ICMP message with a TTL of 0 or 1 to be
suspicious, so it prints an exclamation point after the RTT.) In summary, the lines for a
TTL of 1, 2, and 3 correctly identify R1, R2, and R3, followed by three lines each
containing three timeouts, followed by the line for a TTL of 7 that identifies the
8.5 It appears that all these routers initialize the outgoing TTL of an ICMP message to
255. This is common. The incoming value of 255 from netb is what we expect, but the
value of 253 from butch means there is probably a missing router between it and netb.
Otherwise we would expect an incoming TTL of 254 at this point. Similarly, from
enss142.UT.westnet.net we expect a value of 252, not 249. It appears these
missing routers are not handling the outbound UDP datagram correctly, but they...
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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