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天津理工大学考试试卷 2006 ～ 2007 学年度第 2 学期《计算机网络技术》期末考试试卷课程代码： 0600100 答题时限： 120 得分统计表：大题号总分试卷编号： 1-A 命题日期： 2007 年 5 月 28 日分钟考试形式：闭卷笔试一二三四五 PART Ⅰ: Single Choice (2 point per question – total: 20 points) 得分 1. Which of the following services does the Internet network layer provide for the Internet transport layer? A. In-order delivery of data segments between processes B. Best effort delivery of data segments between communicating hosts C. Multiplexing and demultiplexing of transport layer segments D. Congestion control 2. The main task of the Internet’s Domain Name System (DNS) is to: A. Translate port numbers to IP addresses B. Specify the standards for Internet domain names C. Provide an authority for registering domain names D. Translate mnemonic（记忆的） names to IP addresses 3. The maximum transfer unit(MTU) in Ethernet frame structure is ( A. 1000 B. 1500 C. 800 D. 2000 )byte . 4. Which is not plug and play in the following four items? ( A. NIC B. Hubs C. Routers D. Switches ) 试卷编号： 1-A 第 1 页共 12 页

5. Which of characters is not distance-vector algorithm’s characters？（） A. iterative B. global C. asynchronous D. distributed 6. The header of IPV6 is （）byte. A. 80 B. 60 C. 40 D. 20 7. Two important reasons that the Internet is organized as a hierarchy of networks for the purposes of routing are: A. Least cost and maximum free circuit availability B. Scale and administrative autonomy C. Message complexity and speed of convergence D. Link cost changes and link failure 8. The host component of a CIDR address of the form a.b.c.d/22 can contain addresses for: A. 2(32-22) hosts (minus “special” hosts) B. 512 hosts (minus “special” hosts) C. d times 22 hosts D. 10 hosts (minus “special” hosts) 9. The primary function of the address resolution protocol (ARP) that resides in Internet hosts and routers is: A. To provide LAN router functions B. To translate between LAN addresses and physical interface addresses C. To translate IP addresses to LAN addresses D. To calculate the shortest path between two nodes on a LAN 10. The FTP protocol runs over ____ and uses port ____. A. TCP 21 B. TCP 80 C. UDP 20 D. TCP 110 试卷编号： 1-A 第 2 页共 12 页

PART Ⅱ: Briefly answer the following questions. (Total: 28 points)．得分 1．(1points) Why is there a Web cache? 2 ． (2points) Please briefly describe the function of the five layers in TCP/IP architecture (NOTE: include the protocol data unit) 3. (2points) What are the services of TCP? 4. (2points) Please briefly compare: datagram network versus virtual circuit (VC) network. 5. (2points) What is the difference between routing and forwarding? 6. (3points) How big is MAC address space? The IPv4 address space? The IPv6 address space? 7. (4points) Fill in the blank: 1) RIP advertisements typically announce the number of ( 2) BGP updates announce the ( ) to various destinations 3) For Selective Repeat protocol, sequence number space >=( ) to various )times destinations. window size. 4) The efficiency of Slotted ALOHA =( ) times bigger than Pure ALOHA. 8. (2points)Consider sending M packets over a path of N links. Assume that each packet has L bits. Each link transmits at R bps. The network is lightly loaded so that there are no queuing delays. Propagation delay and node processing delay are negligible. Suppose the network is a packet-switched datagram network and a connectionless service is used. Now suppose each packet has 2h bits of header. How long does it take to send the packet? (all packets go through the same path) 9.(2points)If all the links in the Internet were to provide the reliable delivery service, would the TCP reliable delivery service be redundant? Why or Why not? 10. (Total 4 points) Suppose that four nodes using CDMA (Code Division Multiple Access) protocol to transport their data. The chipping sequences are listed respectively as below: A: (-1,–1,–1,+1,+1,–1,+1,+1) B: (-1,–1,+1,–1,+1,+1,+1,-1) C: (-1,+1,–1,+1,+1,+1,–1,-1) D: (-1, +1,–1, –1,–1,–1, +1,-1) 试卷编号： 1-A 第 3 页共 12 页

Suppose that a CDMA receiver has received an encoded data as (-1,+1,-3,+1,-1,-3,+1,+1) from an aggregate signal channel in one bit-slot time, which node/nodes has/have sent data? What’s the corresponding data bit each of them sent? (3points) 11.(2points) In the figure below, H1 remains in same IP subnet, briefly describe how the switch would know which AP is associated with H1 when H1 is moving within BSS1 and BSS2. 12. (2points) This question is about the “GSM: handoff with common MSC”. According to the figure below, ordering the 8 steps. A. Old BSS informs MSC of impending handoff, provides list of 1+ new BSSs B. Old BSS tells mobile: perform handoff to new BSS C. New BSS allocates radio channel for use by mobile D. New BSS signals MSC, old BSS: ready E. MSC sets up path (allocates resources) to new BSS F. MSC-old BSS resources released G. Mobile signals via new BSS to MSC: handoff complete. MSC reroutes call H. Mobile, new BSS signal to activate new channel 试卷编号： 1-A 第 4 页共 12 页

PART Ⅲ: Answer the following questions. (Total: 22 points)．得分 1．(10points)Consider the following network. With the indicated link costs, use Dijkstra’s shortest-path algorithm to compute the shortest path from x to all network node. While selecting the next node, if several nodes have the same min cost, select the one with lowest id, (e.g., if x and y have the same min cost, then select x). z 14 6 x 2 4 y 3 1 1 1 w t 2 1 s u 4 9 v 1 3 D(u),p(u) D(v),p(v) D(w),p(w) D(y),p(y) D(z),p(z) D(s),p(s) D(t),p(t) N’ Step 0 1 2 3 4 5 6 To complete the routing table of node x Cost 6 Destination Next Node w s t u v w y z 试卷编号： 1-A 第 5 页共 12 页

2. (9points)Consider the following plot of TCP window size as a function of time. d. After the 22nd transmission round, is segment loss detected by a triple duplicate ACK or by a timeout? duplicate ACK or by a timeout? Assuming TCP Reno is protocol experiencing the behavior show above, answer the following question. a. b. Identify the intervals of time when TCP slow start is operating. Identify the intervals of time when TCP congestion avoidances is operating. c. After the 16th transmission round, is segment loss detected by a triple e. What is the initial value of Threshold an the first transmission round? f. What is the value of Threshold an the 18th transmission round? g. What is the value of Threshold an the 24th transmission round? h. During what transmission round is the 70th segment sent? i. Assuming a packet loss is detected after the 26th round by the receipt the of a triple duplicate ACK, what will be the values of congestion-window size and of Threshold? 3. (3points)Consider the three-node topology show below. Compute the distance tables of node x after the first iteration of a synchronous version of the distance vector algorithm. 5 x y 2 6 z 试卷编号： 1-A 第 6 页共 12 页

PART Ⅳ: Compositive questions. (Total: 30 points) 得分 The questions are based on the figure below. Each question is worth 1 point except Q17.Total 30 points. A small office network Internet A B C F G R1 R2 D E H Consider the small office network with 3 subnets and 3 gateways. The IP address of the ISP (Internet Service Provider) gateway is 64.28.10.1 and the IP addresses of the corporate gateways are: G1: 10.1.1.1 and 64.28.10.7 G2: 10.1.1.2 and 10.20.0.1 G3: 10.1.1.3 and 10.140.0.1 Q1：According to the small office network, an Intranet must be a part of the Internet mainly used by a specific group of users. A. true B. false Q2. Request For Comments (RFC) are documents used to describe protocols like TCP and IP. A. true B. false Q3：The gateway G1 needs to have a NAT module to give access to the Internet 试卷编号：第 7 页共 12 页 1-A

to users on the small office network. A. True B. False Q4. Because the class C network 192.168.20.0 have only one byte for hosts addresses, you cannot use it to define subnets. A. true B. false Q5 ：The small office replaces its old wired Ethernet network by a brand new wireless Ethernet network. Which of the 4 layers of the TCP/IP stacks of the office computers will need to be upgraded? A. only the application layer B. only the physical layer C. only the internet layer D. both the internet and the physical layer Q6：Consider the following partial (部分的) routing table of G1: Kernel IP routing table Destination Gateway 10.1.0.0 0.0.0.0 10.1.1.1 64.28.10.1 0.0.0.0 Genmask 255.255.0.0 Flags MSS Window irtt Iface U UG 0 0 0 0 0 0 eth0 eth1 Using the above routing table format, write the route for the destination network 10.20.0.0. Q7：In the same context as Q3, write the loopback route. Q8：What happens on an Ethernet shared network if stations A and B both start transmitting at the same instant? A. B will sense the transmission and wait a certain delay before another try B. a collision will be detected in some reasonable time C. A will sense the transmission and wait a certain delay before another try D. each will transmit on its own channel Q9：A TCP port is used A. for multiplexing application data B. to ensure a reliable connection oriented transmission C. to keep track of connections in a NAT module D. all of the above Now we assume that A wants the service from the WEB server E. Q10：A makes a web access by URL. What is the completed format of URL? Q11：HTTP uses TCP rather than UDP. Why? 试卷编号： 1-A 第 8 页共 12 页

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