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 and www.mit.edu/students. html) can be sent over the same persistent connection.

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?

通知

第二次试验

时间：11月8日，上午10:00 ~ 12:00 地点：九教北401 试验内容：《实验指南》实验四、实验五、实验六； 参考GBN源代码资料存储在：

? 邮箱：net_class_test@sina.cn ? 密码：123456abcd

Chapter 3

1. （教材R3）Describe why an application developer might choose to run an application over

UDP rather than TCP.

A: An application developer may not want its application to use TCP’s congestion control, which can throttle the application’s sending rate at times of congestion. Often, designers of IP telephony and IP videoconference applications choose to run their applications over UDP because they want to avoid TCP’s congestion control. Also, some applications do not need the reliable data transfer provided by TCP.

2. （教材R4）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.)

A: Since most firewalls are configured to block UDP traffic, using TCP for video and voice traffic lets the traffic though the firewalls.

3. （教材R5）Is it possible for an application to enjoy reliable data transfer even when the

application runs over UDP? If so, how?

A: Yes. The application developer can put reliable data transfer into the application layer protocol. This would require a significant amount of work and debugging, however.

4. （教材R6）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?

A: Source port number y and destination port number x.

5. （教材R7）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?

A: Yes, both segments will be directed to the same socket. For each received segment, at the socket interface, the operating system will provide the process with the IP addresses to determine the origins of the individual segments.

6. （教材R9）In our rdt protocols, why did we need to introduce sequence numbers?

A: Sequence numbers are required for a receiver to find out whether an arriving packet contains new data or is a retransmission.

7. （教材10）In our rdt protocols, why did we need to introduce timers?

A: To handle losses in the channel. If the ACK for a transmitted packet is not received within the duration of the timer for the packet, the packet (or its ACK or NACK) is assumed to have been lost. Hence, the packet is retransmitted.

8. （教材R14）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?

A: a) 20 bytes b) ack number = 90

9. （教材R15）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