前言
本文包含了 Computer Networking A Top-Down Approach, 7th Edition 中部分回顧性習題的問題與解答，主要參考了英文第7版和中文第7版的正文內容，歡迎各位的交流與指正！
CHAPTER 1
SECTION 1.1
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?
- 在計算機網絡中，二者的含義是相同的。
- 端系統列舉：手機、平板電腦、環境傳感器等
- 網絡服務器是端系統
R2. The word protocol is often used to describe diplomatic relations. How does Wikipedia describe diplomatic protocol?
- There are two meanings of the word "protocol". In the legal sense, it is defined as an international agreement that supplements or amends a treaty. In the diplomatic sense, the term refers to the set of rules, procedures, conventions and ceremonies that relate to relations between states. In general, protocol represents the recognized and generally accepted system of international courtesy.
- “協議”一詞有兩種含義。從法律意義上講，它被定義為補充或修正條約的國際協議。在外交意義上，該術語是指與國家之間的關系有關的一組規則，程序，公約和儀式。通常，協議代表國際公認和普遍接受的禮節制度。
R3. Why are standards important for protocols?
- 無標准，則無統一的數據收發規則，那么通信實體之間將難以交換信息。
SECTION 1.2
R4. List six access technologies. Classify each one as home access, enterprise access, or wide area wireless access.
-
home access
- FTTH，光纖到戶
- cable
-
enterprise access
- 以太網
- WIFI
-
wide area wireless access
- 3G/4G
- LTE
R5. Is HFC transmission rate dedicated or shared among users? Are collisions possible in a downstream HFC channel? Why or why not?
- 混合光纖同軸（Hybrid Fiber Coax, HFC)的傳輸率是共享的。
- 網絡下行線路中會發生碰撞，因為多個家庭用戶共享一個光纖節點和一條光纜。
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.
-
中國·武漢
類型 上行速率(Mbit/s) 下行速率(Mbit/s) 價格(元/年) FTTH 20 100 1000 FTTH 30 200 1200 FTTH 4 20 618 FTTH 10 50 1099 FTTH 20 100 1299
R7. What is the transmission rate of Ethernet LANs?
- 用戶通常以100 Mbps或1 Gbps的速度接入太網交換機，而服務器可能具有1 Gbps甚至10 Gbps的接入速率。
R8. What are some of the physical media that Ethernet can run over?
- 雙絞銅線
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.
| access technology | upstream rate | downstream rate | shared/dedicated |
|---|---|---|---|
| Dial-up modems | 56 kbps | 56 kbps | dedicated |
| HFC | 2 Mbps | 30 Mbps | shared |
| DSL | <2.5 Mbps, typically < 1 Mbps | <24 Mbps, typically < 10 Mbps | dedicated |
| FTTH | <30 Mbps, in 2020 | <200 Mbps, in 2020 | shared |
R10. Describe the most popular wireless Internet access technologies today. Compare and contrast them.
- WiFi LANS
- 建築內部
- 傳輸速率：54, 450 Mbps
- 3G/4G， LTE
- 由電信（蜂窩）運營商提供，10公里
- 傳輸速率：1-10 Mbps
SECTION 1.3
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 \(R_1\) and \(R_2\) , 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?
- 電路交換的延時低，連接穩定。
- TDM為時分復用，FDM為頻分復用，TDM擁有更大的帶寬
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.
\[\frac{2 Mbps}{1 Mbps/user} = 2 \ users \] -
b.
\[traffic \ \ intensity = \frac{aL}{R} = \frac {n · 1 Mbps}{2 Mbps} = \frac{n}{2} \]當流量強度大於1，即\(n \geq 2\)時，出現排隊現象，否則不會。
-
c.
\[\frac{1}{5} \] -
d.
\[p =\frac{1}{5} \times \frac{1}{5} \times \frac{1}{5} = \frac{1}{125} \]\[v = 3 Mbps \]
R14. Why will two ISPs at the same level of the hierarchy often peer with each other? How does an IXP earn money?
- 兩個同級的網絡服務提供商被稱為對等。此時，二者間的的數據不流經上級 ISP，從而節省了這一部分數流量的費用。
- IXP 以低於上級ISP的價格提供集中的對等連接，從而得到了ISP顧客的青睞
R15. Some content providers have created their own networks. Describe Google’s network. What motivates content providers to create these networks?
- 谷歌專用網絡僅承載出入谷歌服務器主機的流量。谷歌專用網絡通過與較低層ISP對等(無結算)嘗試“繞過”因特網的較高層，采用的方式可以是直接與它們連接，或者在IXP處與它們連接。然而，因為許多接入ISP通過第一層網絡的承載仍能到達，所以谷歌網絡也與第一層ISP連接，並就與它們交換的流量向這些ISP付費。
- 通過創建自己的網絡，內容提供商不僅減少了向頂層ISP 支付的費用，而且對其服務最終如何交付給端用戶有了更多的控制。
SECTION 1.4
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?
- 處理時延
- 常量
- 排隊時延
- 變量，取決於流量強度和性質
- 傳輸時延
- 變量，取決於包長度與傳輸速率
- 傳播時延
- 變量，取決於物理媒介和路由器距離
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.
- 傳輸vs傳播
- 傳輸快於傳播：10 km, 100 Mbps, 100 Bytes
- 傳播快於傳輸：10 km, 512 kbps, 100 Bytes
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?
- 文件首先被分割成長度一定的片段，形成報文。報文加上運輸層首部信息，構成報文段。這樣每次加上首部信息，依次得到網絡層數據包、鏈路層幀。
- 網絡層首部信息包含了包的目的地信息。
- 轉發表和路由選擇機制的作用是：解析地址並確定傳輸路線，故作此類比。
SECTION 1.5
R23. What are the five layers in the Internet protocol stack? What are the principal responsibilities of each of these layers?
-
應用層
- 在端系統之間使用協議交換信息
-
傳輸層
- 在應用程序端點之間傳送應用層報文
-
網絡層
- 將數據報(datagram)從一台主機移動到另一台主機
-
鏈路層
- 將包從一個結點(主機或路由器）移動到路徑上的下一個結點
-
物理層
- 將幀中的每個比特從一個結點移動到下一個結點
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?
- 路由器對應傳輸層
- 鏈路層交換機對應鏈路層
- 主機對應網絡層
SECTION 1.6
R26. What is the difference between a virus and a worm?
- 病毒需要被用戶接受且執行（比如執行郵件附件），蠕蟲不需要明顯的用戶交互就能進入設備（比如網絡）
- 個人理解：蠕蟲更具有“自主性”
R27. Describe how a botnet can be created and how it can be used for a DDoS attack.
- 當病毒或蠕蟲感染了一個網絡的多台設備，並形成了一個被它控制的局部網絡時，就得到了一個僵屍網絡。
- 在分布式拒絕服務攻擊中，僵屍網絡利用多台主機同時發起攻擊。
- 其基本原理是：單台主機的攻擊容易被識破並屏蔽，但分布式攻擊使得攻擊源難以被甄別。
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.
- 竊真
- 獲得兩人互相發送的信息，竊取密碼、口令、個人隱私等內容
- 造假
- 偽造成某一方給另一方發送信息，達到詐騙錢財、挑撥關系等目的