课程名称︰计算机网路 课程性质︰必修 课程教师︰周承复 开课学院：资讯工程系 开课系所︰电机资讯学院 考试日期（年月日）︰2013年11月7日 考试时限（分钟）：180 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : Computer Network Midterm 2013 Question 1:"Quickies" (40%)Answer each of the following questions briefly, i.e. in at most a few sentences. a) (10%)What does it mean for HTTP1.1 to be "persistent"? What is the value of HTTP1.1 being persistent, as opposed to being non-persistent, as in HTTP1.0? b) (10%)What is the difference between a recursive query and an iterated query in the DNS? c) (10%)Consider an http client that wants to retrieve a WWW document at a given URL. The IP address of the http server is initially unknown. The WWW object at the URL has one embedded GIF image that resides at the same server as the original object. a. What transport and application layer protocol besides http are needed in this scenario? b. Suppose that the time needed to contact and receive a reply from any server(for any protocol) is RTT, and the time to transmit the WWW object and GIF image is T. How much time(in RTT and T) is needed from when the user first enters the URL until the complete document is displayed? Assume that non-persistent http is used. Consider the delays of all protocols in your answer, not just those of http. Question 2:"DHT" (35%)Distributed Hash Table(DHT) (a)(5%)What are two fundamental advantages of peer-to-peer(P2P) systems over client-server systems? (b)(20%)Consider a circular Distributed Hash Table(DHT) with node identifiers in the range[0:15]. Suppose there are seven peers with identifiers 1,2,4,6,8,11 and 13. (i) Suppose that the following(key,value) pairs should be stored in the DHT: (2,1),(6,4),(7,15)and(15,4). Which peers will store which (key,value) pairs? Fill in the table below. (key,value) Identifier of the responsible peer ( 2, 1) ( 6, 4) ( 7,15) (15, 4) (ii)Suppose that peer 4 learns that peer 6 has left the DHT. How does peer 4 updates its successor state information? Which peer is now its first successor? Its second successor? (c)(10%)Explain how DHT improves search performance from O(n) to O(log n), where n is the total number hosts in DHT method. Question 3:"DNS" (10%)For a new startup company "Network Utopia" and we would like to register name networkuptopia.com at DNS registrar. Suppose that this new startup company domain is "networkutopia.com", the dns server is "dns1.networkutopia. com", and its IP address is "212.212.212.1", the registrar needs to insert what DNS comments are in com TLD server. Question 4:"Transmission Delay" (20%)Consider sending a large file of F bits from Host A to Host B. As shown in the following diagram, there are four links and three switches between A and B, and the links are uncongested(that is, no queuing delays). Host A segments the file into segments of S bits each and adds 90 bits of header to each segment forming packets of L = 90 + S bits. Each links has a transmission rate of R bps . Find the value of S that minimizes the delay of moving the file from Host A to Host B. Disregard propagation delay. Question 5:"Homework" (15%)Suppose 100 users share a single link, and packet-switching is used. At any given time, the probability if a user is transmitting is p. Find the probability that exactly n users is transmitting simultaneously. --

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