计算机网络大题

第一章1. Consider sending a packet of F bits over a path of Q links. Each link transmits at R bps. The network is lightly loaded so that there are no queuing delays. Propagation delay is negligible.a. Suppose the network is a packet-switched virtual-circuit network. Denote the VC setup time by t s seconds. Suppose the sending layers add a total of h bits of header to the packet. How long does it take to send the file from source to destination?b. 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?c. Finally, suppose that the network is a circuit-switched network. Further suppose that the transmission rate of the circuit between source and destination is R bps. Assuming t s setup time and h bits of header appended to the packet, how long does it take to send the packet? 1、a. t s +(F+h)/R*Qb. (F+2h)/R*Qc. C.t s +(F+h)/R2. This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meter, and suppose the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B. a. Express the propagation delay, d prop , in terms of m and s .b. Determine the transmission time of the packet, d trans , in terms of L and R .c. Ignoring processing and queuing delays, obtain an expression for the end-to-end dealy.d. Suppose Host A begins to transmit the packet at time t=0. At time t=d trans , where is the last bit of the packet?e. Suppose d prop is greater than d trans . At time t= d trans , where is the first bit of the packet?f. Suppose d prop is less than d trans . At time t=d trans , where is the first bit of the packet?g. Suppose s=2.5·108, L =100bits, and R=28kbps. Find the distance m so that d prop equals d trans . 2、a) m/s b) L/Rc) m/s+ L/Rd) Interface of host A e) s*d trans =s*L/R f) Host Bg) m/s= L/R=> m=8.928*105第二章1. In the following figure, Alice sends an e-mail to Bob, and Bob gives a reply to Alice. If Alice and Bob both use mail user agent, please write what protocol is used in procedur e ①～⑥? If both Alice and Bob use Web browser, what protocol is used in procedure ①～⑥?（1）①SMTP; ②POP3 or IMAP; ③SMTP; ④SMTP; ⑤POP3 or IMAP; ⑥SMTP （2）①HTTP; ②HTTP; ③SMTP; ④SMTP; ⑤HTTP; ⑥HTTPAlice BobAlice ’s mail Bob ’s mail ① ② ③ ④⑤ ⑥2. A client requests a Web page /jwzx/index.do from a server through IE browser. The version of HTTP is 1.1 and default connection is persistent connection. The page is supported in Chinese. Please write the request message.If the server gives a reply to the client, the response message is created at 8:00:00 on 27-09-2011, and the object is modified at 00:00:00 on 18-09-2011. Please write the response message.The known-conditions are: server is apache/2.0.40; the object is a text file and consists 1796 bytes. If there is a proxy server between the client and the Web server, how does the proxy server check the page it caches is newer or older?（1）GET /jwzx/index.do HTTP/1.1Host: Connection: Keep-AliveUser-agent: Mozilla/4.0Accept-language: zh-cn（2）HTTP/1.1 200 OKDate: Tue, 27 Sep 2011 08:00:00 GMTServer: Apache/2.0.40Content-Length: 1796Content-Type: text/htmlLast-Modified: Sun, 18 Sep 2011 00:00:00 GMT（3）the proxy server uses conditional GETGET /jwzx/index.do HTTP/1.1Host: If-modified-since: Sun, 18 Sep 2011 00:00:00 GMT第三章1. Consider a TCP connection between Host A and Host B. Suppose that the TCP segmentstraveling from Host A to Host B have source port number x and destination port number y.What are the source and destination port numbers for the segments traveling from Host B to Host A?Source port number is y, destination port number is x.2. True of False?(1) Host A is sending Host B a large file over a TCP connection. Assume Host B has no data tosend Host A. Host B will not send acknowledgments to Host A because Host B can not piggyback the acknowledgments on data.×(2) The size of the TCP RcvWindow never changes throughout the duration of the connection.×(3) Suppose Host A is sending Host B a large file over a TCP connection. The number ofunacknowledged bytes that A sends cannot exceed the size of the receive buffer. ×(4) Suppose Host A is sending a large file to Host B over a TCP connection. If the sequencenumber for a segment of this connection is m, then the sequence number for the subsequent segment will necessarily be m+1. ×(5) The TCP segment has a field in its header for RcvWindow.√(6) Suppose that the last SampleRTT in a TCP connection is equal to 1 sec. Then the currentvalue of TimeoutInterval for the connection will necessarily be≥1 sec. ×(7) Suppose Host A sends over a TCP connection to Host B one segment with sequence number 38and 4 bytes of data. In this same segment the acknowledgment number is 42. ×(8) Consider congestion control in TCP. When the timer expires at the sender, the threshold is set to one half of its previous value. ×3. Suppose Host A sends two TCP segments back to back to Host B over a TCP connection. The first segment has sequence number 90; the second has sequence number 110. (1) How much data is in the first segment? 20 bytes(2) Suppose that the first segment is lost but the second segment arrives at B. In the acknowledgment that Host B sends to Host A, what will be acknowledgment number? 904. Suppose you have the following six 16-bit bytes: 01010000 11001101, 00110011 01100110, 0001001001001000, please compute the 16-bit checksum. 0110100110000100第四章1. There is a host A in the Internet. Its IP address is 210.100.89.204/20. Please answer the following questions: (1) Network address. (2)Broadcast address.(3) What is the subnet mask? (4) The fist host IP address. (5) The last host IP address.(6) How many hosts are there in the network? Solution:(1) 210.100.80.0 (2)210.100.95.255 (3) 255.255.240.0 (4) 210.100.80.1 (5) 210.100.95.254 (6) 232-20=212=40942. For the given topolo gy of the network, use Dijstra’s shortest path algorithm to compute the shortest path from A to all network nodes. Give a shortest path tree and node A’s routing table.N ‘ D(B),p(B) D(C),p(C) D(D),p(D) D(E),p(E) D(F),p(F) A5,A2,A1,A2,A ∞1 E A BD C F1 25 2 21532 11 E A BD C F 1 2 2 1AD 2,D 2,A 2,A 4,DADC 2,D 2,A 4,D/4,CADCE 2,D 3,EADCEB 3,EADCEBF第五章1. Please write the typical features of the following popular interconnection devices at trafficisolation, plug and play, optimal routing and cut-through.⑴hubs;⑵routers;⑶link-layer switchesRouters Switches Hubs Traffic isolateion Yes Yes NoPlug-and-play No Yes YesOptimal routing Yes No NoCut-through No Yes Yes2. There are two nodes going to communicate with each other. The sender and the receiver agree on a generator G as 1001. Consider the piece of data, D=10111000, that the sending node wants to send to the receiving node. Suppose the two communicating nodes use CRC method detecting errors.⑴Please write the bit-stream that the sending node transmits to the link.⑵Assume the receiving node receives the value 10111000110, what does the receiving node will do? Is the bit-stream correct or not?Answers:⑴ 1 0 1 0 1 1 0 11 0 0 1)1 0 1 1 1 0 0 0 0 0 01 0 0 11 0 1 01 0 0 11 1 0 01 0 0 11 0 1 01 0 0 11 1 0 01 0 0 11 0 1R＝101,so the bit-stream that the sending node transmits to the link is 10111000101⑵ 1 0 1 0 1 1 0 11 0 0 1)1 0 1 1 1 0 0 0 1 1 01 0 0 11 0 1 01 0 0 11 1 0 01 0 0 11 0 1 11 0 0 11 0 1 01 0 0 11 1R＝011, R≠0, so the bit-stream has bit-error.3. Draw lines.CSMA/CDrandom access protocols FDM point-to-point link channel partitioning protocols slotted ALOHAPPP protocol CDMA broadcast link taking-turns protocols polling protocolTDMToken-passing protocol Answer:TCP connection and tear downIn the following figure, there is a client initiates a TCP connectionto server via packet①. The connection is established after the clientreceives packet③. Suppose the client find it has no data need tosend. The client sends packet④to close the TCP connection.Please write down the Sequence number and Acknowledgmentnumber of each packet. Notice the special bit of each packet.In the following figure, there is a client and a servercommunicating with each other. The connection is established via packet①～③. Reliable data transfer works with Packet④～⑦. Packet⑧is sentfrom server to close the TCP connection.⑴Suppose, packet④carries 10 bytes data, and packet⑤has payload 20 bytes. Packet⑥carries 10 bytes data, and packet⑦has no payload.⑵Suppose, packet④carries 10 bytes data, and packet⑥carries 10 bytes data. Packet⑤and packet⑦ has no payload.The time-sequence diagram is shown on right, and the Sequence number of packet①is 179, the Sequence number of packet②is 531. Please write down the Sequence number and Acknowledgment number of each packet.计算机网络复习大纲CH1 Computer Networks and the Internet1．什么是协议，网络应用有哪几种工作模式，通信的双方如何定义角色2．电路交换网的几种类型，分组交换网的几种类型，什么是带宽3．分组交换的设备都有哪些4．网络接入的方法大致有几种类型，可用的物理媒体有哪些5．分组在网络中所经历的各种时延分别在什么位置发生，每种时延是由什么引起的，分组丢失在哪里发生？6．网络上的流通强度应该满足什么条件？CH2 Application Layer1．应用层进程之间通信，如何对进程进行寻址2．HTTP的工作原理，持久连接和非持久连接是如何工作的；两种工作方式在请求页面时，需要多少个RTT？3．HTTP报文格式：请求报文和响应报文，条件get报文的作用及其响应报文；Web cache 的作用4．FTP的工作原理5．邮件系统的组成部分，SMTP工作原理，邮件格式，MIME，邮件访问协议有哪些？6．DNS的功能，工作原理，DNS层次结构CH3 Transport Layer1．传输层提供的服务2．多路复用、多路分解分别表示什么意思3．TCP、UDP报文段格式中各字段的含义，checksum计算4．TCP的可靠数据传输是如何实现的5．TCP timer的超时间隔如何设置及设置依据6．TCP连接的3次握手过程与TCP连接断开的过程7．TCP流量控制中接收窗口是如何取值的，又是如何通知发送方的？8．TCP拥塞控制算法（Reno），TCP延迟计算（静态）CH4 The Network Layer1．网络层提供的服务2．网络层有哪些主要功能？2．工作在网络层的网络设备是什么，由哪些部分组成，有哪些功能，处理的数据单位及依据3．IPv4数据报格式及编址涉及的计算，获取地址的方法4．ICMP协议的功能5．IPv6数据报格式的变化，编址的变化及地址位数的变化6．LS选路算法7．RIP、OSPF、BGP协议CH5 The Link Layer and Local Area Networks1．链路层提供的服务2．网络链路的几种类型3．适配器的定义、功能4．奇偶校验、CRC检测5．多址访问协议有哪几种类型？6．什么是局域网7．MAC编址、ARP工作原理；DHCP工作原理8．Ethernet链路层帧的结构，CSMA/CD原理9．Ethernet技术，网段的划分；在10BaseT网络中使用hub及双绞线连接两台主机，这两台主机之间的最大距离；在其他网络中，使用hub、switch连接两台主机，这两台主机之间的最大距离。