Computer Networks s2

Final Exam Computer Networks CSC522 Spring 2007

Answer only 5 questions from below

Due date: April 27, 2007 (2:00 PM) by email to [email protected]

1. True, or False? Explain your answer.

a. In a point-to-point network, broadcasting is not supported. b. In a point-to-point network, every link needs two addresses to cater to its endpoints. c. A VPN net cannot be supported on a point-to-point network. d. The term “connectivity” refers to a point-to-point physical link. e. DNS uses UDP whereas HTTP uses TCP.

2. (a) As you know, IP provides datagram service in a network. Given this, one asks: Why is it then necessary to provide, on top of it, connection less UDP service?

(b) UDP packets must carry addresses like ( add sender , Portsender , add recev , Port recv ) where add stands for IP_address. Suppose 2 clients are communicating with a common server using UDP packets. Is it possible to multiplex these at the client side and then demult them at the server side? If so, how? If not, why not?

3. A number of consecutive IP addresses are available starting at 197.18.0.0. Four organizations, A, B, C, and D request 4000, 2000, 4000 and 8000 addresses, respectively. For each of these, give the first IP address assigned, the last IP address assigned, and the attendant mask in the form w.x.y.z/s notation.

4. In IPv6 the checksum field of IPv4 is deliberately left out. Comment on the design choice indicated in this.

5. One of the major challenge areas in internetworking is to accommodate real-time packets on end-to-end flows with ordinary data packets without abandoning efficiency and fairness (to an extent). How would you approach such a challenge?

6. Consider the following problem with A and B sending traffic to destination nodes C and D respectively at a rate of 100 kbps and 1000 kbps respectively as shown in the diagram. The link that collects the merged traffic from A and B has a limited capacity of only 110 kbps. The destination node C can only accept traffic at a rate of 100 kbps and D at a rate of 10 kbps, respectively. A C A C

100kbps 100kbps

1000kbps 100kbps 10kbps

B B D D

(a) What would be the total throughput delivered to C and D together? (b) If one wants to increase the throughput what should be done and how?

7. Consider the following network with two routers and 5 LANS with addresses shown:

C: 192.48.36.32/27 C A A : 192.48.36.160/27

D : 192.48.36.64/27

D R1 R2

E : 192.48.36.96/27 E B B : 192.48.36.224/27

What would be the least-cost routing table size at each router?

8. “Traceroute” utility could be used to obtain a listing of the routers an UDP datagram takes en route to a destination. Consult the manual page http://www.rt.com/man/traceroute.8.html to figure out how to use this. For instance, on running

% traceroute peoplepc.com

from a Unix station, I get the following output before I deliberately killed it with a control-C.

1 is16-gw.sunyit.edu (150.156.16.1) 9.943 ms 1.759 ms 4.095 ms 2 at-gw1-utc-6-0-sunyitutica-15M.appliedtheory.net (169.130.52.41) 1.961 ms 3.546 ms 5.738 ms 3 at-gw5-syr-8-1-0-T3.appliedtheory.net (169.130.1.145) 6.578 ms 7.214 ms 14.394 ms 4 at-gw2-syr-4-0-0-OC3.appliedtheory.net (169.130.2.102) 12.210 ms 4.007 ms 3.718 ms 5 at-gsr2-syr-1-3-OC3.appliedtheory.net (169.130.3.53) 5.736 ms 5.466 ms 8.735 ms 6 at-gsr1-syr-6-0-OC12.appliedtheory.net (169.130.3.57) 7.171 ms 3.205 ms 3.048 ms 7 so-4-1-2.nycmny1-hcr3.bbnplanet.net (4.25.133.41) 9.872 ms 11.095 ms 14.956 ms 8 br1nyc4so-0-1-0.xnycmny4-uunet.bbnplanet.net (4.0.2.66) 17.875 ms 13.645 ms 15.997 ms 9 0.so-6-0-0.XL2.NYC4.ALTER.NET (152.63.21.82) 15.438 ms 10.814 ms 11.456 ms 10 0.so-4-0-0.TL2.NYC9.ALTER.NET (152.63.23.129) 11.499 ms 11.546 ms 10.950 ms 11 0.so-6-0-0.TL2.DCA6.ALTER.NET (152.63.13.9) 18.762 ms 16.743 ms 24.218 ms 12 0.so-6-0-0.XL2.DCA6.ALTER.NET (152.63.38.74) 22.629 ms 16.936 ms 18.531 ms 13 0.so-7-0-0.GW6.DCA6.ALTER.NET (152.63.41.225) 17.467 ms 25.545 ms 24.495 ms 14 digex-gw.customer.alter.net (157.130.214.102) 21.962 ms 21.458 ms 23.727 ms 15 vlan48.dca2b-fdisb-sw1-msfc1.netsrv.digex.com (164.109.3.113) 19.283 ms 20.294 ms 25.126 ms 16 ^C

(a) What information can you gather from such a listing? (b) How useful could it be to infer one’s own network neighborhood?

9. Compare and contrast congestion avoidance problem with congestion detection problem. A packet shaper could be used by transmitting hosts (using a leaky bucket or a token bucket algorithm) to induce the latter to spew out traffic at a constant rate. Is it always desirable to use such a device? State your reason.