课程名称︰计算机网路 课程性质︰系必修 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）︰2009.4.15 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 : 九十七学年度第二学期(April 15, 2009)计算机网路 期中考 1.(20%) Circuit-switching vs. packet-switching. a.(4%) Why is it said that packet switching employs statistical multiplexing? b.(8%) Suppose users share a 2Mbps link. Also suppose each user requires 1Mbps when transmitting, but each user transmits only 20 percent of the time. i. When circuit-switching is used, how many users can be supported? ii. Suppose packet switching is used. Suppose now there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows. c.(8%) Consider an application that transmits data at a steady rate (for example, the sender generates an N-bit unit of data every k times units, where k is small and fixed). Also, when such an application starts, it will continue running for a relatively long period of time. Answer the following questions, briefly justifying your answer. i. Would a packet-switched network or a circuit-switched network be more appropriate for this application? Why? ii. Suppose that a packet-switched network is used and the only traffic in this network comes from such applications as described aboved. Furthermore, assume that the sum of the application data rates is less than the capacities of each and every link. Is some form of congestion control needed? Why? 2.(10%) Packet Loss and Delay a.(2%) How do packet loss and delay occur in the Internet? b.(4%) How long does it take a packet of length 2,000 bytes to propagete over a link of distance 2,000 km, propagation speed 2x10^8 m/s, and transmission rate 2 Mbps? c.(4%) Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rates R1=250 kbps, R2=500kbps, R3=1Mbps. Suppose that the file is 2 million bytes. Roughly, how long will it take to transfer the file to Host B? 3.(10%) Layering of Internet a.(2%) Why do we need the layering concept for Internet? b.(4%) Please elaborate on the pros and cons of the Layering. c.(4%) A system has an n-layer protocol hierarchy. Applications generate messages of the M-byte length. At each of the layers, a K-byte header is added. What fraction of the headers is for a packet being transmitted? 4.(10%) HTTP and DNS a.(3%) What is web caching? b.(3%) Why HTTP/1.1 uses persistent TCP connection for web accessing? c.(4%) What are Top-level domain (TLD) servers and Authoritative DNS servers? 5.(20%) P2P Applications a.(10%) Consider a new peer Alice who joins BitTorrent without possessing any chunks. Without any chunks, she connot become a top-four uploader for any of the other peers, since she has nothing to upload. How then will Alice get her first chunk? b.(10%) Consider distributing a file of F=5Gbits to N peers. The server has an upload rate of us=20Mbps, and each peer has download rate of di=1Mbps and an upload rate of u. For N=10,100 and 1,000 and u=100Kbps, 250 Kbps, and 500Kbps, prepare a chart giving the minimum distribution time for each of the combinations of N and u for both client-server distribution and P2P distribution. 6.(10%) Answer true or false to the following questions and briefly justify your answer: a. With the SR protocol, it is possible for the sender to receive an ACK for a packet that falls outside of its current window. b. With GBN, it is possible for the sender to receive an ACK for packet that falls outside of its current window. c. The alternating-bit protocol is the same as the SR protocol with a sender and receiver window size of 1. d. The alternating-bit protocol is the same as the GBN protocol with a sender and receiver window size or 1. 7.(20%) Consider a scenario in which Host A wants to simultaneously send packets to Hosts B and C. A is connected to B and C via a broadcast channel - a packet sent by A is carried by the channel to both B and C. Suppose that the broadcast channel connecting A, B and C can independently lose and corrupt packets(and so, for example, a packet sent from A might be correctly received by B, but not by C). Design a stop-and-wait-like error-control protocol for reliably transferring packets from A to B and C, such that A will not get new data from the upper layer until it knows that both B and C have correctly received the current packet. Give FSM descriptions of A and C. (Hint: the FSM for B should be essentially the same as for C). Also, give a description of the packet format(s) used. --

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