计算机网络第四版自顶向下方法课后习题解答

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《计算机网络：自顶向下方法（第四版）》是一本经典的IT教材，由Jim Kurose和Keith Ross合著，专为学习者提供全面的计算机网络理论知识。该书采用自顶向下的方法，从网络应用的全局视角出发，逐步剖析底层原理，将互联网作为核心主题。本书的课后习题是检验和巩固理解的重要环节，对于理解和掌握网络概念至关重要。第一章的复习问题探讨了术语的统一性，"host"和"end system"在书中被互换使用，指的是任何接入网络并能发起或接收数据传输的设备，如个人电脑、工作站、服务器、移动设备等。例如，当大使Alice邀请另一个国家的大使Bob共进晚餐时，她不会直接通过电话传达邀请，而是会安排一个具体的日期和时间，这个过程隐含了网络通信的步骤和层次结构。课后习题的设计旨在帮助学生理解网络通信的基本流程，如数据交换、路由选择、协议栈的运作以及网络安全等内容。这些问题可能涉及TCP/IP协议、IP地址与子网掩码、域名系统（DNS）、网络层次模型（如OSI七层模型或TCP/IP四层模型）等基础知识。解答这些习题不仅要求学生熟悉理论知识，还要具备实际操作的能力，比如配置路由器、理解网络拓扑结构、处理网络故障等。正确理解和解决这些问题，能够帮助读者构建一个完整的网络通信框架，从而更好地应对现实生活中的网络应用场景。阅读这本书并完成课后习题，不仅能够提升对计算机网络技术的理解，还能锻炼解决问题和逻辑思维的能力。对于那些希望在IT领域深造或者从事网络工程师、系统管理员等相关职业的人来说，这是一本不可或缺的学习资料。同时，为了保护知识产权，这些习题答案仅供教师使用，个人或非授权机构不得复制或公开传播。

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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