COMPSCI 742 THE UNIVERSITY OF AUCKLAND SECOND SEMESTER, 2008 Campus: City COMPUTER SCIENCE Data Communications and Networks (Time allowed: TWO hours) NOTE: Attempt all questions. Calculators are NOT permitted. Write your answers in the space provided (extra space for answers is available on pages 16 and 17). No marks will be awarded if you merely state a correct answer. To obtain full credit, your script must clearly explain why your answer is correct. If you require additional information in order to answer a question, you should make a reasonable assumption as required for your answer, and you should explain your assumption on your script. Surname:.................................... Forenames:.................................... Departmental Use Only Question Marks allocated Marks gained Question Marks allocated Marks gained 1 10 6 6 2 12 7 10 3 10 8 12 4 10 9 10 5 10 10 10 Total Total 100
2 COMPSCI 742 1. Domain Name System [10 marks] (a) i. A site caching nameserver sends DNS Queries to a remote nameserver, then waits for Responses. How does the site nameserver match a Response to an outstanding Query? [1 mark] ii. How is a Domain Name represented in a DNS Request packet? [1 mark] iii. What are DNS Resource Records (RR)? Where do they appear in a DNS Response message? [1 mark] iv. A site caching nameserver requests an Address (A) RR for a single domain. How many answers would you expect to see in the DNS Response to this query? Explain your answer.
3 COMPSCI 742 (b) The time between sending a DNS Query and receiving its Response is usually described as a Round Trip Time (RTT) for the remote nameserver. i. What factors contribute to a DNS RTT? ii. Explain briefly why RTTs for many nameservers belonging to a Top-Level Domain (TLD) can have widely differing RTTs. iii. Why might the RTT for a single remote nameserver, e.g. one in the.uk domain, change over a period of time? [1 mark]
4 COMPSCI 742 2. Queueing Theory [12 marks] (a) Explain the difference between a queue described as M/D/1 and a queue described as D/M/2 in Kendall notation. (b) What is the Kendall notation for a motorway on-ramp with two lanes, controlled by a traffic light marked one vehicle per green each lane that changes every five seconds? Explain your answer and state any assumptions made. (c) What other factors can be included in the queue notation in addition to the three used above?
5 COMPSCI 742 ρ (d) For an M/M/1 queue, the formula W = (1 ρ) λ shows how average waiting time W depends on the utilisation rate ρ and the average arrival rate λ. Consider an IP router where the average arrival rate is one packet per microsecond. What utilisation will give an average waiting time of two microseconds? (Use the formula, or estimate from the graph below.) 1 (e) For the same queue, what will happen if the utilisation is 90% (0.9)? (Use the formula, or estimate from the graph below.) Waiting time 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Utilisation
6 COMPSCI 742 (f) What else would you need to know to calculate the average rate at which the router will drop (discard) packets for a given utilisation rate? 3. Quality of Service [10 marks] (a) Describe best effort IP service in one or two sentences. (b) What queue management mechanism might be used in a router to make sure that its output does not exceed a given average rate of packets? What controls the average rate? (c) Which QoS metrics are important for the quality of a voice over IP call?
7 COMPSCI 742 (d) What will happen if Voice over IP is run through a very busy network that only provides best effort IP service? (e) A large office is using Voice over IP for internal telephone calls, but users are unhappy with the sound quality. Which of the following solutions would you choose? Explain your answer. (1) Replace the 100 Mbit/s office Ethernets with 1 Gbit/s Ethernets. (2) Reconfigure the IP telephones to run over TCP. (3) Reconfigure the IP telephones and the office routers to use Differentiated Services. 4. Transport Control Protocol [10 marks] (a) The TCP Header has fields called Sequence Number, Acknowledgement Number, and ACK Flag. Explain briefly how these fields are used by the TCP protocol. [3 marks]
8 COMPSCI 742 (b) A TCP sender transmits a segment containing 1460 payload bytes, with Sequence Number 12005. What Acknowledgement Number would indicate that the packet has been safely received? [1 mark] (c) What does a TCP sender do when it receives an Acknowledgement Number less than the Sequence Number of the last segment it transmitted? (d) How does a TCP sender measure and maintain its estimate of a TCP connection s Round Trip Time (RTT)? Why does TCP need to do this? [4 marks]
9 COMPSCI 742 5. TCP Operating Modes [10 marks] (a) Explain what is meant by a TCP Congestion Window. [1 mark] (b) TCP Tahoe begins a connection in Slow Start mode, then switches to Congestion Avoidance mode. Explain this sequence; you should sketch a plot of Congestion Window size vs time, and use it to illustrate your answer. [4 marks] (c) What is meant by TCP Self Clocking? What conditions allow a TCP connection to operate in Self-Clocking mode? [3 marks]
10 COMPSCI 742 (d) Sketch a plot of TCP Congestion Window vs time for a self-clocking connection. 6. Wireless Media Access: MACA Consider three wireless nodes A, B, and C that implement MACA. A and C can both communicate with B, but A cannot communicate with C. Considering the RTS/CTS/ACK handshake involved if A wants to send a data packet to B, name three different timeouts that are involved. Name the node at which the timeouts are used in this context and state when are they started and which purpose they serve. [6 marks]
11 COMPSCI 742 7. Link budgets [10 marks] (a) Consider a geostationary digital TV satellite positioned 36,000 km above the equator. The satellite is transmitting a TV signal to a receiver directly beneath it on the equator (i.e., also at a distance of 36,000 km) using a wavelength of 3.6 cm. The signal has a bandwidth of 4 MHz, which corresponds to a natural noise floor of roughly -96 dbm. The transmitter on the satellite has an output power of +20 dbm, and the signal is fed into an antenna with a gain of 16.85 dbd. Assume that the decoding of our modulation scheme requires at least a signal-to-noise ratio of 10 db. We want a fade margin of at least 15 db. What is the minimal antenna gain required at the receiver? [8 marks] Note: You may wish to use the following approximations: Path loss = (4πd)2 λ 2 π = 3.14..., π 2 10 P 1 P 2 (db) = 10 log 10 P 1 P 2 0 dbd = 2.15 dbi
12 COMPSCI 742 (b) Consider a link budget in which a parabolic dish antenna with 20 cm diameter on the receiver produces a fade margin of 0 db. All other things left unchanged, how large would the diameter of such an antenna have to be to produce a face margin of 12 db, and why? 8. Mobile Ad Hoc Networks [12 marks] (a) What is meant by the term Mobile Ad Hoc Network (MANET)? (b) How does a MANET differ from the more usual wired network? [4 marks]
13 COMPSCI 742 (c) Why are the usual routing protocols (e.g. RIP2, OSPF) not normally used in MANETs? (d) What are the two main routing strategies that are used in MANETs? (e) What is meant by Geographical Routing? Could IPv4 or IPv6 addresses be used for the nodes in a MANET that relies on geographical routing?
14 COMPSCI 742 9. IPv6 and IPv4 [10 marks] Consider a computer X that contains an IPv4/IPv6 dual stack. X wishes to contact an HTTP server S that only supports IPv4. (a) What method will X use to contact S if they are both connected to the existing Internet? (b) Assume that X is moved to a new IPv6-only network that cannot route IPv4 packets. Give two methods that could be used by X to contact S. Which would you choose, and why? [4 marks] (c) Assume that you are in charge of an IPv4-only network containing S. You have run out of IPv4 addresses for new customers, and the address registry tells you that no more new IPv4 addresses are available. What choices do you have?
15 COMPSCI 742 (d) Which of your choices above would you select, and why? 10. Global Routing, IPv4 [10 marks] (a) What is an Autonomous System? What is a Network Prefix? Why might an Autonomous System have more than one Network Prefix? [3 marks] (b) The BGP Protocol is used to maintain routing tables in the global Internet. Explain how BGP prevents routing loops.
16 COMPSCI 742 (c) Why might an Enterprise Network, e.g. one that serves a large University, want to multihome, i.e. connect to more than one Internet Service Provider? (d) Outline one approach to multihoming using BGP to handle the Internet connections for an IPv4 network. Comment on how suitable your approach would be for IPv6. [3 marks]
17 COMPSCI 742 SPARE PAGE FOR EXTRA ANSWERS Cross out rough working that you do not want marked. Specify the question number for work that you do want marked.
18 COMPSCI 742 SPARE PAGE FOR EXTRA ANSWERS Cross out rough working that you do not want marked. Specify the question number for work that you do want marked.