习题课(2)

这个连接。在数据传输过程中,各数据报地址不需要携带目的地址,而是使用连接号。接收到的数据与发送方的数据在内容和顺序上是一致的。~

无连接服务的特点,每个报文带有完整的目的地址,每个报文在系统中独立传送。无连接服务不能保证报文到达的先后顺序,先发送煦报文不一定先到。无连接服务不保证报文传输的可靠性。

Chapter 2

1. For a communication session between a pair of processes, which process is the client and which is the server?

2. What is the difference between network architecture and application architecture?

3. What information is used by a process running on one host to identify a process running on

another host? 4. Suppose you wanted to do a transaction from a remote client to a server as fast as possible.

Would you use UDP or TCP? Why?

5. List the four broad classes of services that a transport protocol can provide. For each of the service classes, indicate if either UDP or TCP (or both) provides such a service. 6. Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?

7. What is meant by a handshaking protocol?

8. Suppose Alice, with a Web-based e-mail account (such as Hotmail or gmail), sends a message

to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the message between the two hosts.

9. From a user's perspective, what is the difference between the download-and-delete mode

and the download-and-keep mode in POP3? 10. Is it possible for an organization's Web server and mail server to have exactly the same alias

for a hostname (for example, foo. com)? What would be the type for the RR that contains the hostname of the mail server?

11. Why is it said that FTP sends control information \

12. True or false?

a. A user requests a Web page that consists of some text and three images. For this page, the client will send one request message and receive four response messages.

b. Two distinct Web pages (for example, www.mit.edu/research.Html www.mit.edu/students. html) can be sent over the same persistent connection.

and

c. With nonpersistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages.

d. The Date: header in the HTTP response message indicates when the object in the response was last modified.

13. Suppose within your Web browser you click on a link to obtain a Web page. The IP address

for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain

the IP address. Suppose that n DNS servers are visited before your host receives the IP address from DNS; the successive visits incur an RTT of RTT1,...,RTTn. Further suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Let RTT0 denote the RTT between the local host and the server containing the object. Assuming zero transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?

14. Referring to Problem P7, suppose the HTML file references three very small objects on the same server. Neglecting transmission times, how much time elapses with a. Non-persistent HTTP with no parallel TCP connections? b. Non-persistent HTTP with parallel connections? c. Persistent HTIP?

15. 在TCP协议中,为了使通信不致发生混乱,引入了所谓套接字的概念,这里,套接字由( )

和IP地址两部分组成。 A。端口号 B.域名 C。接口 D。物理地址

解析:端口号和IP地址合起来,称为套接字,套接字可以在全网范围内唯一标识一个端口。在TCP协议中,一条连接两端的套接字就可以唯一标识该连接了。所以选项A为正确答案。 答案:A

16. 面向连接的传输有三个过程:连接建立、( )和连接释放。

A．连接请求 B。连接应答 C。数据传输 D。数据共享

解析:面向连接服务具有连接建立、数据传输和连接释放这三个阶段。所以选项C为正确答案。 答案:C

17. 试述UDP和TCP协议的主要特点及它们的使用场合。(华中科技大学2003年试题)

解析:用户数据报协议是对IP协议组的扩充,它增加了一种机制,发送方使用这种机制可以区分一台计算机上的多个接收者。每个UDP报文除了包含某用户进程发送数据外,还有报文目的端口的编号和报文源端口的编号,从而使UDP的这种扩充,在两个用户进程之间的递送数据报成为可能。

TCP提供的是一种可靠的数据流服务。当传送受差错干扰的数据,或基础网络故障,或网络负荷太重而使网络基本传输系统(无连接报文递交系统)不能正常工作时,就需要通过TCP这样的协议来保证通信的可靠。

答案:UDP是一个简单的面向数据报的传输层协议。应用进程的每个输出操作都产生一个UDP数据报,并组装成一份待发送的IP数据报中发送。UDP提供不可靠、无连接的数据报服务,它把应用程序传给IP层的数据发送出去,但是并不保证它们能到达目的地。因此,uDP通常用于不要求可靠传输的场合,另外也常用于客户机/服务器模式中。

TCP协议被用来在一个不可靠的互联网中为应用程序提供可靠的端点间的字节流服务。所有TCP连接都是全双工和点对点的,因而TCP不支持广播和组播的功能。TCP实体间以“段”为单位进行数据交换。为实现可靠的数据传输服务,TCP提供了对段的检错、应答、重传和排序的功能,提供了可靠地建立连接和拆除连接的方法,还提供了流量控制和阻塞控制的机制。TCP适用于传输大量重要数据的场合。

18. 在使用TCP协议传送数据时,如果有一个确认报文段丢失了,也不一定会引起对方数据的重传。试说明为什么?

解析:本题考查的是TCP重传机制。

TCP连接的一个重要的特性就是为上层服务提供了一个可靠的数据流。由于TCP是建立在不可靠的IP层的基础之上的,因此就必然涉及报文丢失的问题,这样,报文的重传就成了保证数据可靠到达的一个重要机制。这方面TCP采取了超时重传的策略,对每个TCP连接都维护一个计时器,每发送一个报文:就设置一次计时器,只要计时器设置的重传时间已到但仍然没有收到相应的确认信息,就重传这一报文。

答案:对方还未来得及重传,就收到了对更高序号的确认,相当于对连同被丢失确认的报文段一并确认。

19. 在连续ARQ协议中,设编号用3位,而发送窗口WT=8,试找出一种情况,使得在此情况下协

议不能正常工作。

解答:发送端:0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0

接收端: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0

当发送方发送0~7数据帧,因发送窗口已满,发送暂停,接收方收到所有数据帧,并对每一个帧都发送确认帧,若所有的确认帧都没有到达发送方,经过发送方计时器控制的超时后,发送方会再次发送之前的8个数据帧,而接收方收到这8个帧却无法判断是新的数据帧或是重传的数据帧。

20. 在选择重传ARQ协议中,设编号用3位。再设发送窗口WT=6,而接收窗口WR=3。试找出

一种情况,使得在此情况下协议不能正常工作。 解答:发送端:0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0

接收端:0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0

Chapter 3

1. Why is it that voice and video traffic is often sent over TCP rather than UDP in today's

Internet. (Hint: The answer we are looking for has nothing to do with TCP's congestion-control mechanism.)

2. Is it possible for an application to enjoy reliable data transfer even when the application runs over UDP? If so, how?

3. Consider a TCP connection between Host A and Host B. Suppose that the TCP segments

traveling 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?

4. Suppose a process in Host C has a UDP socket with port number 6789. Suppose both Host A

and Host B each send a UDP segment to Host C with destination port number 6789. Will both

of these segments be directed to the same socket at Host C? If so, how will the process at Host C know that these two segments originated from two different hosts?

5. In our rdt protocols, why did we need to introduce sequence numbers? 6. In our rdt protocols, why did we need to introduce timers?

7. 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. a. How much data is in the first segment?

b. 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 the acknowledgment number? 8. True or false?

a. The size of the TCP RcvWindow never changes throughout the duration of the connection. b. suppose Host A is sending Host B a large file over a TCP connection. The number of unacknowledged bytes that A sends cannot exceed the size of the receive buffer.

c. Host A is sending Host B a large file over a TCP connection. Assume Host B has no data to send Host A. Host B will not send acknowledgments to Host A because Host B cannot piggyback the acknowledgmens on data.

d. The TCP segment has a field in its header for RcvWindow.

e. Suppose Host A is sending a large file to Host B over a TCP connection. If the sequence number for a segment of this connection is m, then the sequence number for the subsequent segment will necessarily be m + 1.

f. Suppose that the last SampleRTT in a TCP connection is equal to I sec. The current value of Timeoutrnterval for the connection will necessarily be>1 sec.

g. Suppose Host A sends one segment with sequence number 3g and 4 bytes of data over a TCP connection to Host B. In this same segment the acknowledgment number is necessarily 42.

9. Consider congestion control in TCP. when the timer expires at the sender, the threshold is set

to one half of its previous value. 10. Suppose client A initiates a Telnet session with Server S. At about the same time, client B also

initiates a Telnet session with Server S. Provide possible source and destination port numbers for

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