课程名称︰计算机网路 课程性质︰必修 课程教师：逄爱君 开课学院：电资学院 开课系所︰资工系 考试日期（年月日）︰2014.11.12 考试时限（分钟）： 试题 : 1. (5pts) Suppose we want to send a file of 210,000 bits from Host A to Host B over a circuit-switched network. Suppose that all links in the network use TDM with 8 slots and have a bit rate of 1.2 Mbps. Also suppose that it takes 600 msec to establish an end-to-end circuit before Host A can begin to transmit the file. How long does it take to send the file? 2. (10pts) Consider a packet of length L which begins at end system A and travels over three links to a destination end system. These three links are connected by two packet switches. Let d_i, s_i, and R_i denote the length, propagation speed, and the transmission rate of link i, for i = 1, 2, 3. The packet switch delays each packet by d_proc. Assuming no queuing delays, in terms of d_i, s_i, R_i, (i = 1, 2, 3), and L, what is the total end-to-end delay for the packet? Suppose now the packet is 2,000 bytes, the propagation speed on all three links is 2*10^8 m/s, the transmission rates of all three links are 4 Mbps, the packet switch processing delay is 3 msec, the length of the first link is 7,000 km, the length of the second link is 5,000 km, and the length of the last link is 8,000 km. For these values, what is the end-to-end delay? 3. (10pts) True or false? a. Processes on two different end system communicate with each other by exchanging messages across the computer network. b. A client server architecture achieves perfect security. c. Socket is a hardware interface through which a process sends messages into, and receives messages from the network. d. No data loss is tolerated in multimedia applications such as conversational audio/video. e. Developing a new network application for the Internet often requires one to decide whether to choose UDP or TCP. 4. (10pts) Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that n DNS servers are visited before your host receives the IP address from DNS; visiting k of them incurs an RTT of D_1 per DNS and visiting each of the remaining incurs an RTT of D_2. Further suppose that the Web page associated with the link contains m very small objects. Suppose the HTTP running is non-persistent and let RTT_0 denote the RTT between the local host and the server for each object. Assuming zero transmission time of each object, how much time elapses from when the client clicks on the link until the client receives all the objects? 5. (10pts) Suppose Bob joins a BitTorrent torrent, but he does not want to upload any data to any other peers (so called free-riding). a. Bob claims that he can receive a complete copy of the file that is shared by the swarm. Is Bobs claim possible? Why or why not? b. Bob further claims that he can further make his "free-riding" more efficient by using a collection of multiple computers (with distinct IP addresses) in the computer lab in his department. How can he do that? 6. (8pts) What is the trade-off in designing a DHT with respect to the number of neighbors? 7. (5pts) If you would like to access the Internet at home, what factors do you consider for your access network and physical media? 8. (10pts) Protocol layers a. Please describe the advantages and disadvantages of protocol layering. (5pts) b. Please elaborate on the main functions of each protocol layer of the Internet. (5pts) 9. (5pts) Describe how a botnet can be created, and how it can be used for a DDoS attack. 10. (10pts) By using the following example, please describe how Web caching can significantly reduce the delay in receiving a requested object with low-cost installation. http://i.imgur.com/epQrARR.png 11. (7pts) What is traffic intensity? Please elaborate on the relationship between the traffic intensity and average queuing delay? 12. (10pts) Please describe how demultiplexing works in Transport layer, and compare connectionless and connection-oriented demultiplexing. --

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