《计算机网络：自顶向下方法》第四版英文答案解析

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"这是《计算机网络：自顶向下方法》第四版的英文答案文档，由Jim Kurose和Keith Ross编写。这份文档包含了书中的复习问题和习题解答，仅供教师使用，禁止复制或分发给他人，也不应上传至公共网站。如果需要最新的解答手册，可以直接向作者请求。" 《计算机网络：自顶向下方法》是计算机网络领域的一本经典教材，它以一种易于理解的方式介绍了网络的基本原理和概念。第四版中，作者通过将复杂的网络技术分解成更小、更易消化的部分，帮助读者逐步理解网络的工作机制。 1. 计算机网络基本概念 - "主机"和"端系统"：在本书中，这两个术语是互换使用的，都包括个人电脑、工作站、Web服务器、邮件服务器、连接到互联网的PDA、WebTV等设备。这些设备在网络中扮演着发送和接收数据的角色。 2. 网络通信的比喻 - 以Alice邀请Bob吃饭为例，说明网络通信的过程。Alice不会直接告诉Bob立刻来参加晚餐，而是会提出一个具体的日期和时间，这与网络中的数据传输类似，需要约定好时间和顺序，也就是网络协议。 3. 问题与习题解答 - 书中提供了丰富的复习问题和习题，旨在帮助学生深入理解每个章节的内容，涵盖网络层次结构、TCP/IP协议栈、网络传输、路由选择、网络安全等多个方面。解答部分详细解析了这些问题，有助于读者巩固学习成果。 4. 教师资源 - 这份解答文档专为教师设计，帮助他们准备课程和评估学生理解程度。教师可以参考这些解答来指导课堂讨论或批改作业。 5. 知识产权保护 - 作者强调，此解答手册的使用权仅限于教师，不应被非法复制或公开传播，以维护作者的知识产权。通过这个资源，教师可以更好地支持他们的教学活动，而学生则可以通过解答来深化对网络概念的理解，提高学习效率。同时，这也体现了学术界对知识分享和版权保护的尊重。

Problem 22

a) 150 msec

b) 1,500,000 bits

c) 600,000,000 bits

Problem 23

Let’s suppose the passenger and his/her bags correspond to the data unit arriving to the

top of the protocol stack. When the passenger checks in, his/her bags are checked, and a

tag is attached to the bags and ticket. This is additional information added in the

Baggage layer if Figure 1.20 that allows the Baggage layer to implement the service or

separating the passengers and baggage on the sending side, and then reuniting them

(hopefully!) on the destination side. When a passenger then passes through security, and

additional stamp is often added to his/her ticket, indicating that the passenger has passed

through a security check. This information is used to ensure (e.g., by later checks for the

security information) secure transfer of people.

Problem 24

a) Time to send message from source host to first packet switch =

. With store-and-forward switching, the total time to move

message from source host to destination host =

b) Time to send 1

packet from source host to first packet switch = .

. Time at which 2

packet is received at the first switch =

time at which 1

packet is received at the second switch =

c) Time at which 1

packet is received at the destination host = .

. After this, every 1msec one packet will be received;

thus time at which last (5000

) packet is received =

. It can be seen that delay in using message

segmentation is significantly less (almost 1/3

d) Drawbacks:

i. Packets have to be put in sequence at the destination.

ii. Message segmentation results in many smaller packets. Since

header size is usually the same for all packets regardless of their

size, with message segmentation the total amount of header bytes

is more.

Problem 25

Java Applet

Problem 26

b) A guarantee that a certain value for throughput will be maintained

Neither

c) A guarantee that data will be delivered within a specified amount of time

Neither

d) Security

Neither

9. SSL operates at the application layer. The SSL socket takes unencrypted data from

the application layer, encrypts it and then passes it to the TCP socket. If the

application developer wants TCP to be enhanced with SSL, she has to include the

SSL code in the application.

10. A protocol uses handshaking if the two communicating entities first exchange

control packets before sending data to each other. SMTP uses handshaking at the

application layer whereas HTTP does not.

11. The applications associated with those protocols require that all application data

be received in the correct order and without gaps. TCP provides this service

whereas UDP does not.

12. When the user first visits the site, the site returns a cookie number. This cookie

number is stored on the user’s host and is managed by the browser. During each

subsequent visit (and purchase), the browser sends the cookie number back to the

site. Thus the site knows when this user (more precisely, this browser) is visiting

the site.

13. Web caching can bring the desired content “closer” to the user, perhaps to the

same LAN to which the user’s host is connected. Web caching can reduce the

delay for all objects, even objects that are not cached, since caching reduces the

traffic on links.

14. Issued the following command (in Windows command prompt) followed by the

HTTP GET message to the “utopia.poly.edu” web server:

> telnet utopia.poly.edu 80

Since the index.html page in this web server was not modified since Fri, 18 May

2007 09:23:34 GMT, the following output was displayed when the above

commands were issued on Sat, 19 May 2007. Note that the first 4 lines are the

GET message and header lines input by the user and the next 4 lines (starting

from HTTP/1.1 304 Not Modified) is the response from the web server.

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