Computer Networks

Tutorial #9

1. What do you think is the origin of the term datagram?

2. Why must a router always have at least two different IP addresses? What do you think would be the most basic thing you would have to do if you were asked to configure a router?

3. An IP datagram encapsulates a TCP segment. The TCP segment, in turn, encapsulates portion of an HTTP GET request. Draw a diagram showing the boundaries between the various headers, and the location of the application data in the datagram. Assume minimum-sized headers.

4. The three characteristics of datagram delivery in the Internet are:
   1. Unreliable
   2. Connectionless
   3. Best Effort
   Explain how each of these terms is interpreted in the context of datagram delivery.

5. What is UDP? In the lecture, some examples where UDP is useful were given. However, one of the appropriate uses of UDP occurs in multi-player (networked) games. Why is it likely to be particularly appropriate in this application?

6. Consider^[1] a machine with two physical network connections, and two IP addresses, I(1) and I(2). Is it possible for that machine to receive a datagram destined for I(2) over the network with address I(1)? Explain. Hint: consider the algorithm for datagram delivery given in the lecture.

7. Is^[2] it possible to address a datagram to a router's (IP gateway's) IP address? Does it make sense to do so?

8. The following is the output of a run of the traceroute command, "looking from" outside La Trobe back towards Bendigo. It was, in fact, run on a Unix system called morinda in the School of IT at NTU in Darwin.

   morinda> traceroute ironbark.bendigo.latrobe.edu.au
       [....six lines deleted]
    6  nsw-vic.atm.net.aarnet.edu.au (192.12.76.2)  79 ms  79 ms  79 ms
    7  vic-gw.vrn.EDU.AU (203.21.130.162)  80 ms  80 ms  87 ms
    8  latrobe-gw.vrn.EDU.AU (203.21.130.133)  81 ms  83 ms  80 ms
    9  r-elt-fddi.latrobe.edu.au (131.172.20.8)  80 ms  81 ms  80 ms
   10  r-bgoatm34-atm.latrobe.edu.au (131.172.239.5)  81 ms  328 ms  84 ms
   11  busfddi0.bendigo.latrobe.edu.au (149.144.10.1)  81 ms  81 ms  81 ms
   12  ironbark.bendigo.latrobe.edu.au (149.144.21.60)  81 ms  81 ms  81 ms
   morinda>
   

   Use the information contained in this traceroute output to fill in the missing IP addresses in the "Internet Structure" diagram in the lecture.

Practical Exercises

• Try running traceroute for yourself. It's installed on all of our Unix systems, and is also available (in various versions) for PCs and Macs.

• Out on the Internet, there are many traceroute servers, which will show you the route from wherever they are located "back to" La Trobe Uni, Bendigo. See if you can locate one or more, and give them a try.