This preview shows page 1. Sign up to view the full content.
Unformatted text preview: utput for
TTLs 2 and 19: both are for the router gateway.tuc.noao.edu, but the two IP
addresses are different. Since traceroute identifies the incoming interface, and since
we're passing through the router in two different directions, once on the outbound path
(TTL 2) and then on the return path (TTL 19), we expect this. We see the same effect
comparing TTLs 3 and 18, and TTLs 4 and 17. 8.6 Summary
Traceroute is an indispensable tool when working with a TCP/IP network. Its operation is
simple: send UDP datagrams starting with a TTL of 1, increasing the TTL by 1, to locate
each router in the path. An ICMP time exceeded is returned by each router when it discards
the UDP datagram, and an ICMP port unreachable is generated by the final destination.
We ran examples of traceroute on both LANs and WANs, and used it to examine IP
source routing. We used loose source routing to see if the route to a destination is the same
as the return route from that destination.
Exercises file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tracerou.htm (16 of 17) [12/09/2001 14.46.48] Traceroute Program 8.1 What can happen if an IP implementation decrements the incoming TTL and then tests
for equal to 0?
8.2 How does traceroute calculate the RTT? Compare this to the RTT calculation done
8.3 (This exercise and the next one are based on actual problems determined when
traceroute was being developed, and are from comments in the traceroute source
code.) Assume there are three routers (Rl, R2, and R3) between the source and destination
and that the middle router (R2) decrements the TTL but incorrectly forwards the IP
datagram when the incoming TTL was 1. Describe what happens. How can you see this
occur when running traceroute?
8.4 Again assume there are three routers between the source and destination. This time the
destination host has a bug whereby it always uses the incoming TTL as the outgoing TTL of
an ICMP message. Describe what happens and how you would see...
View Full Document
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