计算机网络期中考试题 A卷_Ans.docx

计算机网络期中考试题 A卷基础概念题(10 points)1. What are the five layers in the Internet protocol stack?Ans: Application layer, transport layer, network layer, link layer and physical layer.2. What transport service does an app need? Ans: Data loss, Timing, Throughput, Security.3. We say that FTP has out-of-band control. What do we mean by that? Ans: We mean that commands and data flow across different TCP connections.4. Why not centralize DNS?Ans: Single point of failure 单点故障，该DNS服务器崩溃，整个因特网随之瘫痪 Traffic volume 通信容量，单个DNS服务器处理上亿请求 Distant centralized 远距离集中式数据库，导致严重时延 Maintenance 维护：单个DNS服务器将不得不为所有的因特网主机保留记录，这个中央数据库十分庞大，且需为每个新添加的主机频繁更新5. What is cookie?Ans: Cookie：某些网站为了辨别用户身份、进行session跟踪而储存在用户本地终端上的数据 。用户首次访问站点时，可能需要提供一个用户标识。在后继访问中，浏览器向服务器传递一个cookie首部，供服务器识别该用户。Delay: (10points)Consider two hosts A and B connected by a single link of rate r bits/sec. The two hosts are separated by d meters. Signal propagation is p meters per second. Host A is sending to host B a packet of size S bits. a) (2pts)What is the propagation delay, tpr? tpr = d / p b) (2pts)What is the transmission delay, ttr, of the packet?ttr=S / r c) (2pts)Ignoring any possible processing and queueing delays, what is the end-to-end delay? (d /p) + (S/r ) d) (3pts)If Host A starts to transmit the packet at time t = 0, where is the packet at time t = ttr? The last bit is just leaving Host A.e) (3pts)Suppose that tpr tfr. At time t = tfr where is the first bit of the packet? The first bit is on the link and has not reached Host B.f) (3pts)Suppose that tpr ttr. At time t = tfr where is the first bit of the packet? The first bit has reached (and likely received by) Host B.Application layer (15 points)Answer the following questions regarding the Application Layer. Http:1.a) What is HTTP? 1 each and round-up (5pts)Ans:The HyperText Transfer Protocol is a stateless application layer protocol. HTTP is used to transfer web content between a browser application (client) and an HTTP server. All web content is identified by a URL. HTTP is a request response protocol that uses TCP for assured delivery. HTTP uses ASCII encoded headers. The HTTP GET command retrieves HTML files and other objects. The GET header includes the URL of the object and other optional fields such as capability, language, and so on. The response includes a response header with a response code (code 200 is OK and 404 is page not found). Other commands include POST and HEAD. b) HTTP supports both non-persistent and persistent connections. Describe each type of connection and state which HTTP protocol version supports each type. How is the type of connection specified in the HTTP protocol? (5pts)Ans:In a non-persistent connection each HTTP request/response pair has its own TCP connection. A persistent connection supports multiple request/response pairs over a single TCP connection. HTTP 1.0 supports only nonpersistent connection, HTTP 1.1 supports both types. The HTTP GET header specifies the type of connection to be used.c) Consider an HTTP client that wants to retrieve a Web document at a given URL. The IP address of the HTTP server is initially unknown. What transport and application-layer protocols besides HTTP are needed in this scenario? Ans: Application layer protocols: DNS and HTTPTransport layer protocols: UDP for DNS; TCP for HTTPTransportation layer(15 points)TCP&UDP1. Answer the questions about TCP&UDPShow the packet flows for TCP connection establishment and termination.Ans:2. Internet transport-layer protocols does NOT provide these services:Select one or more:（ ）a. reliable, in-order deliveryb. delay guaranteesc. bandwidth guaranteesd. unreliable, unordered deliveryThe correct answer is:bandwidth guarantees,delay guarantees3. UDP and TCP use 1s complement for their checksums. Suppose you have the following three 8-bit words: 01110101, 01110100, 11001100. What is the 1s complement of the sum of these words? Show all work. Why is it that UDP take the 1s complement of the sum, i.e., why not just use the sum? With the 1s complement scheme, how does the receiver detect errors. Is it possible that a 1-bit error will go undetected? How about a 2-bit error?01110101 +01110100 _1110100111101001 +11001100 _101101011s complement:01001010检测错误，接收方把4个字，包括三个原本的字和一个checksum相加。如果和包含一个0，接收方就能意识到有一个错误。所有one-bit错误都能被检测，但无法检测two-bit错误。SR & GBN（10 points）Whats the different between Go-back-N and Selective Repeat protocols.a) GBN：累积ACK，如果有个gap，不会发ACK可以一次发最多N个包有一个最远未回复的包的计时器，维护一个发送窗口；如果超时，重发之前所有的包。报文信息简单，占用带宽多。b) SR：每发一个包回复一个ACK每一个未ACK的包都有计时器，超时之后发还没回复ACK的包，会重新传输DNSThe Internets Directory Service (10 points)1. please refer to various DNS servers below and explain the mechanism of iterated query.(8 points)Ans:iterated query:v contacted server replies with name of server to contactv “I dont know this name, but ask this server”能正确写出具体流程即可。2. Which server has the heaviest burden during iterated querying? (2 points)Ans: local DNS server流量控制与拥塞控制(20 points)Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer.a. Give two reasons why slow start is used, and explain why it does a better job than congestion avoidance for that function. Ans: 1 TCP链接刚建立的时候需要发送大量的报文，慢启动可以防止建立连接初期单个TCP链接发送太多报文占用太大带宽；2慢启动有利于TCP从故障中恢复，并快速回到高效稳定的传输阶段慢启动比阻塞控制好的原因是慢启动在带宽空闲的时候可以快速增长TCP的窗口大小，TCP传输可以快速达到高效。b. Identify the intervals of time when TCP fast retransmission is used. Please explain what fast retransmission does and how it is triggered.Ans: 16-17。快速重传发生在收到三个冗余的ACK之后，采取的动作为：1.把ssthresh设置为cwnd的一半2.把cwnd再设置为ssthresh的值(具体实现有些为ssthresh+3)3.重新进入快速恢复阶段。c. After the 16th transmission round, is segment loss detected by triple duplicate ACK or by a timeout？Ans: After the 16th transmission round, packet loss is recognized by a triple duplicate ACK. If there was a timeout, the congestion window size would have dropped to 1. d. What is the initial value of Threshold at the first transmission round?Ans: The threshold is initially 32, since it is at this window size that slow start stops and congestion avoidance begins. e. During what transmission round is the 70th segment sent?Ans: During the 1st transmission round, packet 1 is sent; packet 2-3 are sent in the 2nd transmission round; packets 4-7 are sent in the 3rd transmission round; packets 8-15 are sent in the 4th transmission round; packets 16-31 are sent in the 5th transmission round; packets 32-63 are sent in the 6th transmission round; packets 64 96 are sent in the 7th transmission round. Thus packet 70 is sent in the 7th transmission round. f. Suppose TCP Tahoe is used (instead of TCP Reno), and assume that triple duplicate ACKs are received at the 16th round. What are the threshold and the congestion window size at the 18th round?Ans: Threshold is 21, and congestion window size is 2. (10pts)Appendix A contains the RDT sender and receive