计算机网络模拟题

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《Computer network》Midterm Examination

Name:

I.

Student number: 100min

Choice (30*1).

1．The ( ) is a worldwide computer network, that is, a network that interconnects millions of

computing devices throughout the world. A. Internet B. Intranet C. switch net D. television net 2．The units of data exchanged by a link-layer protocol are called ( ).

A. Frames B. Segments C. Datagrams D. bit streams

3．( ) means that the switch must receive the entire packet before it can begin to transmit the first

bit of the packet onto the outbound link.

A. Store-and-forward transmission B. FDM C. End-to-end connection D. TDM 4．In the following options, which one is not a guided(有线) media? ( )

A. twisted-pair wire B. fiber optics C. coaxial cable D. satellite 5．Delay does not include ( ).

A. Process delay B. propagation delay C. transmission delay D. control delay 6. In the following options, which belongs to the network core? ( )

A.end systems B.routers C.clients D.servers 7．The Internet’s connection-oriented service has a name, it is ( ).

A.TCP B. UDP C.TCP/IP D.IP

8．There are two fundamental approaches to building a network core, ( ) and packet switching.

A.electrical current switching B.circuit switching

C.data switching D.message switching 9．A ( ) protocol is used to move a datagram over an individual link.

A. application-layer B.transport-layer C. network-layer D. link-layer 10．Which of the following protocols is not a link-layer protocol? ( )

A. Ethernet B.PPP C.HDLC D.IP 11．The ARP protocol can translate ( ) into ( ). ( )

A. host name, IP address B. host name, MAC address C. IP address, MAC address D. broadcast address, IP address

12．In CSMA/CD, the adapter waits some time and then returns to sensing the channel. In the following four times, which one is impossible? ( )

A.0 bit times B. 512 bit times C. 1024 bit times D. 1028 bit times 13．The principal components of PPP include but not( ).

A.framing B. physical-control protocol C. link-layer protocol D. network-layer protocol

14．MAC address is ( ) bits long.

A.32 B. 48 C.128 D.

15．Which of the following is not belonging to channel partitioning protocols? ( )

A.CSMA B.FDM C.CDMA D. TDM 16．Which of the following is correct? ( )

A.No two adapters have the same MAC address. B.MAC broadcast address is FF-FF-FF-FF-FF-FF.

Computer network instructor: Wang Xia 2015-6-15

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C.A portable computer with an Ethernet card always has the same MAC address, no matter where the computer goes.

D.All of the above

17．In the Ethernet frame structure, the Data field carries the ( ).

A.IP datagram B.segment C.frame D.message 18．A switch is a( )device that acts on frame.

A.physical-layer B.link-layer C.network-layer D.transport-layer 19．A router forwards a packet by examining the value of a field in the arriving packet’s header, and then using this value to index into the router’s forwarding table, this value is? （ ）

A. destination IP address B. source IP address C. destination MAC address D. source MAC address 20．Which layer provides host-to-host services? （ ）

A. transport B. network C. data link D. physical 21．IPV4 has a ___ bit address? （ ）

A. 48 B. 16 C. 32 D. 22．Data-grams are rarely larger than ____?（ ）

A. 1500 bytes B. 65535 bytes C.500 bytes D.1024 bytes 23．ICMP is used for____?（ ）

A. Reliable data transfer B. Error reporting C. Flow control D.Congestion control

24．IP addressing assigns an address to 223.10.198.250/29, the network address for this network is____.（ ）

A. 223.10.198.248 B. 223.10.198.250 C. 223.10.198.0 D. 223.10.0.0 25．A switch works in ____ layer? （ ）

A. Networking B. Data link C. Application D. Physical

26．The internet’s network layer provides a single service----that is ____.（ ）

A. Reliable data transfer B. Flow control C. Congestion control D. Best-effort-service 27．Which of the following is not a routing protocol? （ ）

A. TCP B. RIP C. OSPF D.BGP 28．IP address is identifier for ____. （ ）

A. Host B. Router interface C. Both of above D. None of above 29．“233.1.1.0/24”, where the /24 notation, sometimes known as a____. （ ）

A. Subnet mask B. Networking address C. Host address D. None of above 30．Which of the following is not intra-AS routing protocol? （ ）

A. RIP B. OSPF C. IGRP D.BGP

31. Which layer-function is mostly implemented in an adapter? ( )

A.physical layer and link layer B. network layer and transport layer C.physical layer and network layer D. transport layer and application layer 32．In the following applications, which one is a loss-tolerant application?

A E-mail B file transfer C instant messaging D real-time audio 33.What is the main function of the network layer? ( )

A. node-to-node delivery B. process-to-process message delivery C. synchronization D.updating and maintenance of routing tables 34.As a data packet moves from the upper to the lower layers, headers are( ) .

A.Added B.subtracted C.rearranged D. modified 35.．In Address：http://www.uzz.edu.cn,（ ） indicates “host name”.

A. www B. uzz C. edu D. Cn

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36．The FIN segment is used for ( ).

A. TCP connection setup B. TCP flow control C. TCP congestion control D. Closing a TCP connection

37． If a user brings his computer from zaozhuang to beijing, and accesses Internet again. Now, ( )of his computer needs to be changed.

A.MAC address B.IP address C. e-mail address D. user address

II．Fill in the Blanks（20*1）。

1. As a packet travels from one node to the subsequent node along the path, the packet suffers from several types of delays at each node along the path. The most important of these delays are the transmission_delay, _propagation_delay, _processing_ delay, and _queuing_delay.

2. The Internet mail system has three major components: _User Agent_, _Mail Server_, and _SMTP_.

3. Historically, two routing protocols have been used extensively for routing within an autonomous system in the Internet: RIP protocol and _OSPF_ protocol. And RIP is a distance-vector protocol. And OSPF is a link-state protocol.

4. A hub is a _physical_-layer device, and switches are link_-layer devices.

III．True or false.

1．IP address is associated with the host or router. SO one device only have one IP address. F 2．In C/S architecture, client has a fixed well-known address. F 3．DNS resolves hostnames for hosts anywhere in the Internet.T 4．An ARP query sent to all over the Internet. F

5．UDP is a kind of unreliable transmission layer protocol, so there is not any checksum field in UDP datagram header. F

6．IP is a kind of reliable transmission protocol. F

7. The transmission delay is a function of the distance between the two routers. F 8．RIP and OSPF are Intra-AS routing protocols.T 9．125.35.36.37 is a C-class IP address. F

10．Packet switching is suitable for real-time services, and offers better sharing of bandwidth than circuit switching. F 11.

Any protocol that performs handshaking between the communication entities before transferring data is a connection-oriented service.T

IV. Translation.（共10题，每空1分，共 10分） 1. IP(Internet Protocol) ：___________________________ 2. Flow-control ：___________________________

3. FTP(File Transfer Protocol) ：___________________________ 4. HTTP( Hyper-Text Transfer Protocol )：___________________________ 5. ISP(Internet Service Provider)：___________________________

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6. DNS(Domain Name System) ：___________________________ 7. BGP(Border Gateway Protocol) ：___________________________ 8. ARP (Address Resolution Protocol) ：___________________________ 9. URL (Uniform Resource Locator) ：___________________________ 10. CSMA/CD ：___________________________ 11. Congestion-control ：___________________________

12. SMTP(Simple Mail Transfer Protocol) ：___________________________

V．Answer （4*5）；

1．简述CSMA/CD的工作原理。

答：（1）当一个站点想要发送数据的时候，先侦听信道是否空闲。（1分）

（2）如果信道忙，则等待，直到信道空闲；如果信道空闲，站点就准备好要发送的数据。（1分） （3）发送数据的同时，继续侦听网络，若无冲突则继续发送，直到发完全部数据。

（4）若有冲突，则立即停止发送数据，并发送干扰信号；等待一个预定的随机时间，且在总线为空闲时，再重新发送未发完的数据。（1分）

2．试述五层协议的网络体系结构的要点，包括各层的主要功能。 3．简述传输层两种主要协议的相同点和不同点。

答：相同点：TCP和UDP都是传输层协议；都具有分用和复用的功能； （1分）

不同点： TCP是面向连接的可靠协议，UDP面向无连接的不可靠协议；TCP具有流量控制，拥塞控制功能，UDP协议没有；TCP协议处理的数据单元是数据段，而UDP的是用户数据报。（1分）

4. 一个应用程序用UDP，到了IP层把数据报再划分为4个数据报片发送出去。结果前两个数据报片丢失，后两个到达目的站。过了一段时间应用程序重传UDP，而IP层仍然划分为4个数据报片来传送。结果这次前两个到达目的站而后两个丢失。试问：在目的站能否将这两次传输的4个数据报片组装成为完整的数据报？为什么？（假定目的站第一次收到的后两个数据报片仍然保存在目的站的缓存中。）

答：不行。（1分）

5．画出TCP的三次握手协议示意图。

6.用表格表述Ehternet帧格式。

VI．Applications: (3*10)

1．一个自治系统有5个局域网，其连接图如图所示，LAN2~LAN5的主机数为：91，151，

23，15。该自治系统分配到的IP地址快为134.138.118/23。试给出每一个局域网的地址

LAN2：91个主机LAN3：151个主机LAN4：23个主机LAN5：15个主机LAN1Computer network instructor: Wang Xia 2015-6-15

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块（包括前缀）。（10分）

2．假定要传送的报文共x（bit），从源站到目的站共经过k段链路，每段链路的传播时延为d（s），数据率为C（bit/s）。在电路交换时电路的建立时间为s（s）。在分组交换时分组长度为p（bit），且各结点的排队等待时间可忽略不计。问在怎样的条件下，分组交换的时延比电路交换的要小？（8分）

3． 一个UDP用户数据的数据字段为4992字节。在数据链路层要使用以太网来传送。应当划分为几个IP数据报片？说明每一个IP数据报字段长度和片偏移字段的值。（8分）

4．有如下的四个/24地址块，试进行最大可能的聚合。

212.56.132.0/24，212.56.133.0/24，212.56.134.0/24，212.56.135.0/24。（6分

5．数据传输过程中，若接收方收到发送方送来的信息为101100110101，生成多项式为G（x）＝x4＋x3＋1，接收方收到的数据是否是正确的？（请写出判断依据及推演过程）（5分

6．设TCP使用的最大窗口为65535字节，而传输信道不产生差错，带宽也不受。若报文段的平均往返时延为20ms，问所能得到的最大吞吐量是多少?（5分）

7. 现有五个站分别连接在三个局域网上，并且用两个透明网桥连接起来，如下图所示。每一

Computer network instructor: Wang Xia 2015-6-15

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个网桥的两个端口号都标明在图上。在一开始，两个网桥中的转发表都是空的。以后有以下各站向其他的站发送了数据帧，即H1发送给H5，H3发送给H2，H4发送给H3，H2发送给H1。

B11H1H2B22H312H4H5MAC1MAC2MAC3MAC4MAC5

试将有关数据填写在下表中。（8分）

网桥1的转发表 发送的帧 站地址 H1H5 H4H3 端口 网桥2的转发表 网桥1的处理 （转发？丢弃？站地址 端口 登记？） 1 1 转发 转发 网桥2的处理 （转发？丢弃？登记？） 8．主机A向主机B连续发送了两个TCP报文段，其序号分别为70和100。请画出滑动窗口示意图并回答如下问题：

（1）第一个报文段携带了多少个字节的数据？

（2）主机B收到第一个报文段后发回的确认中的确认号应当是多少？

（3）如果主机B收到第二个报文段后发回的确认中的确认号是180，试问A发送的第二个报文段中的数据有多少字节？

（4）如果A发送的第一个报文段丢失了，但第二个报文段到达了B。B在第二个报文段到达后向A发送确认。试问这个确认号应为多少？（共8分）