【免费】《计算机网络自顶向下（第七版）》课后题最全答案-含题目_计算机网络自顶向下课后题答案,计算机网络第七版课后题答案

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第一章

R1. What is the difference between a host and an end system? List several different types of end

systems. Is a Web server an end system?

没有什么不同。PC、服务器、手机等。Web 服务器是端系统。

R2. The word protocol is often used to describe diplomatic relations. How does Wikipedia

describe diplomatic protocol?

Protocol is commonly described as a set of international courtesy rules. These well-established and

time-honored rules have made it easier for nations and people to live and work together. Part of

protocol has always been the acknowledgment of the hierarchical standing of all present. Protocol rules

are based on the principles of civility. — Dr. P.M. Forni on behalf of the International Association of

Protocol Consultants and Officers.

R3. Why are standards important for protocols?

协议如果没有标准就无法创建一个可以互动的网络系统

R4. List six access technologies. Classify each one as home access, enterprise access, or wide

area wireless access.

住宅接入：DSL、电缆、FTTH、卫星、拨号接入、以太网、WiFi 等

公司接入：以太网、WiFi 等

广域无线接入：3G、4G、LTE 等

R5. Is HFC transmission rate dedicated or shared among users? Are collisions possible in a

downstream HFC channel? Why or why not?

HFC 是用户间共享的。在下行 HFC 信道中，所有的数据包都是从一个来源发出的，即头端，因此

没有碰撞

R6. List the available residential access technologies in your city. For each type of access,

provide the advertised downstream rate, upstream rate, and monthly price.

DSL、电缆、FTTH、拨号接入、以太网、WiFi 等

电信的光纤到户：

R7. What is the transmission rate of Ethernet LANs?

从 10Mbps，100Mbps，1Gbps 到 10Gbps

R8. What are some of the physical media that Ethernet can run over?

双绞铜线，光纤

R9. Dial-up modems, HFC, DSL and FTTH are all used for residential access. For each of these

access technologies, provide a range of transmission rates and comment on whether the

transmission rate is shared or dedicated.

拨号调制解调器：最高速度为 56 Kbps，专用的

HFC：高达 42.8 Mbps 下行速率和高达 30.7 Mbps 的上行速率，共享的

DSL：最高 24 Mbps 下行速率和最高 2.5 Mbps 上行速率，专用的

FTTH: 最高 4Mbps 上行，100Mbps 下行速率，专用的

R10. Describe the most popular wireless Internet access technologies today. Compare and

contrast them

WiFi 和 4G。

WIFI 是短距离的高速无线网络传输，可不用手机卡。它的使用有一定的距离范围，比如距离发射

地点不能超过几十米等，距离过远信号就会不稳，甚至连接不上。

4G 可以认为是远距离的高速网络，它是 2G、3G 后的新一代产物，但手机必须插上手机卡(此卡

需支持 4G)且无欠费才能正常使用。4G 的网络传输速度很快，所以使用时最好包月，否则话费较

高。

R11. Suppose there is exactly one packet switch between a sending host and a receiving host.

The transmission rates between the sending host and the switch and between the switch and the

receiving host are R1 and R2, respectively. Assuming that the switch uses store-and-forward

packet switching, what is the total end-to-end delay to send a packet of length L? (Ignore

queuing, propagation delay, and processing delay.)

R12. What advantage does a circuit-switched network have over a packet-switched network?

What advantages does TDM have over FDM in a circuit-switched network?

电路交换网络为连接方预留了宽带，能确保维持一个恒定的传输速率。而目前的大多数分组交换

网络不能做任何的端到端的带宽保证。

传统模拟 FDM 信号，如果要分离出每个不同的单独信号，需要多个滤波器，才能实现。

R13. Suppose users share a 2 Mbps link. Also suppose each user transmits continuously at 1

Mbps when transmitting, but each user transmits only 20 percent of the time. (See the

discussion of statistical multiplexing in Section 1.3 .)

a. When circuit switching is used, how many users can be supported?

b. For the remainder of this problem, suppose packet switching is used. Why will there be

essentially no queuing delay before the link if two or fewer users transmit at the same

time? Why will there be a queuing delay if three users transmit at the same time?

c. Find the probability that a given user is transmitting.

d. Suppose now there are three users. Find the probability that at any given time, all three

users are transmitting simultaneously. Find the fraction of time during which the queue

grows.

a.很明显两个用户

b.因为可用的共享带宽为 2Mbps，而当两个或更少用户同时传输时，所需要的最高传输速度为

2Mbps，因此没有排队时延。三个用户时需要的带宽为 3Mbps，比可用带宽大，因此会有排队时

延

c.20%

d.0.2^3 = 0.008

因此时间比率为 0.008

R14. Why will two ISPs at the same level of the hierarchy often peer with each other? How does

an IXP earn money?

对等可以减少费用。

IXP 通过向每一个连接到 IXP 的 ISP 收取相对较少的费用来获得它的收入，这可能取决于从 IXP

发送或接收的流量的数量。

R15. Some content providers have created their own networks. Describe Google’s network.

What motivates content providers to create these networks?

谷歌的专用网络将它自己的数据中心连接到一起。在其数据中心之间的流量通过其专用网络传输，

而不是公共网络。许多数据中心位于或者靠近低层的 ISP。因此当谷歌向用户传输数据时，它总

是会绕开高层 ISP。

动机是什么呢？第一，因为它使用了较少的中间 ISP，内容提供商可以更好地控制用户体验。第

二，通过传输更少的流量来省钱。

R16. Consider sending a packet from a source host to a destination host over a fixed route. List

the delay components in the end-to-end delay. Which of these delays are constant and which

are variable?

处理时延，排队时延，传输时延，传播时延。

除了排队时延之外都是固定的。

R17. Visit the Transmission Versus Propagation Delay applet at the companion Web site.

Among the rates, propagation delay, and packet sizes available, find a combination for which the

sender finishes transmitting before the first bit of the packet reaches the receiver. Find another

combination for which the first bit of the packet reaches the receiver before the sender finishes

transmitting.

可能有多种组合

10 km, 100 Nbps, 100 Bytes

10 km, 512 kps, 100 Bytes

R18. How long does it take a packet of length 1,000 bytes to propagate over a link of distance

2,500 km, propagation speed 2.5×10^8 m/s, and transmission rate 2 Mbps? More generally, how

long does it take a packet of length L to propagate over a link of distance d, propagation speed

s, and transmission rate R bps? Does this delay depend on packet length? Does this delay

depend on transmission rate?

0.01s, d/s, 没有关系

R19. Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has

three links, of rates R1=500 kbps, R2=2 Mbps, and R3=1 Mbps.

a. Assuming no other traffic in the network, what is the throughput for the file transfer?

b. Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how

long will it take to transfer the file to Host B?

c. Repeat (a) and (b), but now with R2 reduced to 100 kbps.

min(R1, R2, R3) = R1 = 500 kbps

4MB / 500kbps = 4 * 10^6 * 8 byte / 500kbps = 64s

100 kbps; 320s

R20. Suppose end system A wants to send a large file to end system B. At a very high level,

describe how end system A creates packets from the file. When one of these packets arrives to

a router, what information in the packet does the router use to determine the link onto which the

packet is forwarded? Why is packet switching in the Internet analogous to driving from one city to

another and asking directions along the way?

端系统 A 将文件划分成数据块，并添加首部信息，其中就包括了目的地址的 IP，因此生成

了许多的分组。

分组交换机使用目的地址 IP 来将分组发到特定的链路上。

这就跟行车时问路类似。

R21. Visit the Queuing and Loss applet at the companion Web site. What is the maximum

emission rate and the minimum transmission rate? With those rates, what is the traffic intensity?

Run the applet with these rates and determine how long it takes for packet loss to occur. Then

repeat the experiment a second time and determine again how long it takes for packet loss to

occur. Are the values different? Why or why not?

最大的发送速率 500 packet/s，最小的传输速率 350 packet/s，流量强度 500 / 350 = 1.43 > 1

500/350 = 1.43 > 1500/350=1.43>1。

第一次测试丢包大概 8.6ms，第二次大概 9.7ms，每次都不相同，是因为发送数据包有随机性

R22. List five tasks that a layer can perform. Is it possible that one (or more) of these tasks could

be performed by two (or more) layers?

差错控制 error control，流量控制 flow control，多路复用/分解 multiplexing/demultiplexing，报文

分段 segmentation，重新装配 reassembly，连接建立 connection setup。

这些任务可能由多个层次执行可能，比如差错控制。

R23. What are the five layers in the Internet protocol stack? What are the principal

responsibilities of each of these layers?

应用层，运输层，网络层，链路层，物理层；在 1.5.1 节中有。

R24. What is an application-layer message? A transport-layer segment? A network-layer

datagram? A link-layer frame?

应用层报文是应用想要发送的数据，每经一层都会进行封装。

R25. Which layers in the Internet protocol stack does a router process? Which layers does a

link-layer switch process? Which layers does a host process?

路由器处理网络层、链路层、物理层，链路层（现代的路由器有可能会担当防火墙或缓存，

因此可能会处理运输层）。交换机处理链路层。主机处理所有五个层次。

R26. What is the difference between a virus and a worm?

病毒是需要用户交互来感染用户设备的恶意软件；蠕虫无需用户交互。

R27. Describe how a botnet can be created and how it can be used for a DDoS attack.

攻击者通过各种途径传播僵尸程序感染互联网上的大量主机，而被感染的主机将通过一个控

制信道接收攻击者的指令，在控制者和被感染主机之间所形成的一个可一对多控制的网络，

这被称为僵尸网络。因此控制者可以控制大量主机用来发动 DDos 攻击。

R28. Suppose Alice and Bob are sending packets to each other over a computer network.

Suppose Trudy positions herself in the network so that she can capture all the packets sent by

Alice and send whatever she wants to Bob; she can also capture all the packets sent by Bob and

send whatever she wants to Alice. List some of the malicious things Trudy can do from this

position.

Trudy 可以查看 Alice 与 Bob 之间秘密，还可以扮演其中一个人，甚至两个人。

P1. Design and describe an application-level protocol to be used between an automatic teller

machine and a bank’s centralized computer. Your protocol should allow a user’s card and

password to be verified, the account balance (which is maintained at the centralized computer)

to be queried, and an account withdrawal to be made (that is, money disbursed to the user).Your

protocol entities should be able to handle the all-too-common case in which there is not

enough money in the account to cover the withdrawal. Specify your protocol by listing the

messages exchanged and the action taken by the automatic teller machine or the bank’s

centralized computer on transmission and receipt of messages. Sketch the operation of your

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2022-12-08

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