課程名稱︰計算機網路 課程性質︰資訊大三必修 課程教師︰周承復 開課學院：電資 開課系所︰資訊 考試日期（年月日）︰103/11/13 考試時限（分鐘）：120 是否需發放獎勵金：Y （如未明確表示，則不予發放） 試題 : Question 1: "Quickies" Answer each of the following questions briefly, i.e. in at most a few sentences a) (10%) What does it mean for a protocol to be stateful? What does it mean for a protocol to be stateless? Give an example one stateful protocol and one stateless protocol. b) (10%) What does it mean when we say that control messages are "in-band"? What does it ean when we say that control messages are "out-of-band"? Give an example of a protocol that has in-bond control messages and one example of a protocol that has out-of-bond control messages. c) (10%) List four factors that contribute to the end-to-end delay in a packet-switched network. Which of these are constant and which of these depend on the load in the network? d) (10%) Suppose you would like to urgently deliver 20 terabytes data from Boston to Los Angeles. You have available a 500 Mbps dedicated link for data transfer. Would you prefer to transmit the data via this link or instead use FedEx overnight delivery? Explain. e) (10%) Suppose a web server has 399 ongoing TCP connection. How many server-side sockets are used? How many server-side port number are used? Briefly(two sentences at most each)explain your answer. f) (10%) For each question, please circle a single best answer. (1) What are three common HTTP 1.1 message method fields? (a) GET HTML POST (b) GET PUT HTML (c) GET UPLOAD HTML (d) GET POST PUT (e) none of the above (2) Given that the requested information is not available at any intermediate databases, a purely recursive DNS query from a requesting host would follow the path: (a) Root name server, local name server, authoritative name server (b) authoritative name server, root name server, host name server (c) local name server, root name server, local name server, authoritative name server (d) local name server, root name server, TLD name serve, authoritative name server. (e) none of the above. Question 2: "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 1,3,6,9,12,14 and 15. (i) Suppose that the following(key,value) pairs should be stored in the DHT: (2,0),(6,5),(7,13)and(15,7). Which peers will store which (key,value) pairs? Fill in the table below. (key,value) Identifier of the responsible peer ( 2, 0) ( 6, 5) ( 7,13) (15, 7) (ii) Suppose that peer 6 learns that peer 9 has left the DHT. How does peer 6 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: "Homework" (a) (10%) Suppose users share a 3Mbps link. Also suppose each user requires 150kbps when transmitting,but each user transmits only 20 percent of the time. Suppose there are 100 users. Find the probability that there are 21 or more users transmitting simultaneously.(Write the equation only) (b) (15%) 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 95 bits of header to each segment forming packets of L = 95 + 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. --

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