《计算机网络：自顶向下方法》第四版英文答案解析

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"计算机网络(自顶向下方法)第四版答案(英文版)" 是一本专为教学设计的解决方案文档，包含了Jim Kurose和Keith Ross合著的《计算机网络：自顶向下方法》第四版的复习问题和习题解答。此文档仅供教师使用，禁止复制或分发给他人，也不应发布在公开网站上。最新版本的解答手册可直接向作者请求获取。本资源的核心知识点涵盖了计算机网络的基础理论和实践应用，采用自顶向下的教学方法，即从应用层开始，逐层深入到网络的底层机制。以下是部分关键知识点的详细说明： 1. **主机与端系统**：在计算机网络中，"主机"和"端系统"是同义词，指的是能够发起或接收网络通信的设备，如个人电脑、工作站、Web服务器、邮件服务器、连接互联网的掌上电脑、WebTV等。 2. **网络通信的基本概念**：文档中以Alice邀请Bob用餐的场景举例，说明在网络通信中，数据传输通常涉及多个步骤，而非简单的直接通讯。这涉及到网络协议的应用，如TCP/IP模型中的应用层、传输层、网络层和数据链路层。 3. **TCP/IP模型**：这是一个四层模型，包括应用层（HTTP、FTP等）、传输层（TCP、UDP）、网络层（IP）和链路层（以太网、PPP等）。每一层都有特定的任务，例如应用层负责处理应用程序的通信需求，网络层则负责数据包的路由选择。 4. **网络协议**：如TCP（传输控制协议）确保数据的可靠传输，通过确认、重传和拥塞控制机制；而UDP（用户数据报协议）则更轻量级，不保证数据的顺序或可靠性，适用于实时应用。 5. **IP地址和子网掩码**：每个端系统在网络中都有一个唯一的IP地址，用于标识其在网络中的位置。子网掩码用于划分网络地址和主机地址，有助于网络的管理和路由。 6. **域名系统(DNS)**：将人类可读的域名转换为IP地址的系统，使得用户可以方便地访问网络资源。 7. **网络层次结构**：大型网络通常采用层次化设计，包括核心层、分布层和接入层，以优化性能、安全性和管理效率。 8. **网络设备**：路由器、交换机、集线器等在网络中起着关键作用，分别负责数据包的路由、交换和广播。 9. **网络安全**：包括加密、认证、授权等，以保护网络中的数据和资源免受未经授权的访问或攻击。 10. **网络性能指标**：如带宽、延迟、吞吐量、丢包率等，用于衡量网络的效率和质量。以上仅是部分内容概述，完整的解决方案文档会涵盖更多细节和具体问题的解答，包括网络模型的深入探讨、网络协议的工作原理、网络安全策略以及各种网络问题的解决方法。

s3.bay0.hotmail.com. Message-id is a unique string assigned by the mail system

when the message is first created.

From: This indicates the email address of the sender of the mail. In the given

example, the sender is “prithuladhungel@hotmail.com”

To: This field indicates the email address of the receiver of the mail. In the

example, the receiver is “prithula@yahoo.com”

Subject: This gives the subject of the mail (if any specified by the sender). In the

example, the subject specified by the sender is “Test mail”

Date: The date and time when the mail was sent by the sender. In the example,

the sender sent the mail on 19

May 2007, at time 23:52:36 GMT.

Mime-version: MIME version used for the mail. In the example, it is 1.0.

Content-type: The type of content in the body of the mail message. In the example,

it is “text/html”.

Return-Path: This specifies the email address to which the mail will be sent if the

receiver of this mail wants to reply to the sender. This is also used by the sender’s

mail server for bouncing back undeliverable mail messages of mailer-daemon

error messages. In the example, the return path is

“prithuladhungel@hotmail.com”.

18. With download and delete, after a user retrieves its messages from a POP server,

the messages are deleted. This poses a problem for the nomadic user, who may

want to access the messages from many different machines (office PC, home PC,

etc.). In the download and keep configuration, messages are not deleted after the

user retrieves the messages. This can also be inconvenient, as each time the user

retrieves the stored messages from a new machine, all of non-deleted messages

will be transferred to the new machine (including very old messages).

19. Yes an organization’s mail server and Web server can have the same alias for a

host name. The MX record is used to map the mail server’s host name to its IP

address.

20. It is not necessary that Bob will also provide chunks to Alice. Alice has to be in

the top 4 neighbors of Bob for Bob to send out chunks to her; this might not occur

even if Alice is provides chunks to Bob throughout a 30-second interval.

21. Alice will get her first chunk as a result of she being selected by one of her

neighbors as a result of an “optimistic unchoke,” for sending out chunks to her.

22. The overlay network in a P2P file sharing system consists of the nodes

participating in the file sharing system and the logical links between the nodes.

There is a logical link (an “edge” in graph theory terms) from node A to node B if

there is a semi-permanent TCP connection between A and B. An overlay network

does not include routers. With Gnutella, when a node wants to join the Gnutella

network, it first discovers (“out of band”) the IP address of one or more nodes

already in the network. It then sends join messages to these nodes. When the node

receives confirmations, it becomes a member of the of Gnutella network. Nodes

maintain their logical links with periodic refresh messages.

23. It is a hybrid of client server and P2P architectures:

a) There is a centralized component (the index) like in the case of a client

server system.

b) Other functions (except the indexing) do not use any kind of central server.

This is similar to what exists in a P2P system.

24. Whenever a user, say Alice wants to locate another user in the instant messaging

system, say Bob, her node would send out a query message asking for the (unique)

username (Bob) into the overlay network, which would be flooded into the

network in the same manner a query requesting the location of a resource (eg: a

file in Gnutella network) is flooded. If the user with the username (Bob) is online,

as the query reaches the node where the user is online from, the node sends back

the query response to the original query source (Alice). In this way, Alice locates

the IP address of the node Bob is currently at in the instant messaging system.

The advantage of such a design is that there is no centralized component in the

system and hence no single point of failure. The disadvantage however, is that the

flooding of each such request into the overlay network results in huge traffic in

the network. One option is to have limited scope query as in Gnutella. However,

with this approach, the user might not be located even if she is online.

25. a) User location

b) NAT traversal

26. a) File Distribution

b) Instant Messaging

c) Video Streaming

d) Distributed Computing

27. With the UDP server, there is no welcoming socket, and all data from different

clients enters the server through this one socket. With the TCP server, there is a

welcoming socket, and each time a client initiates a connection to the server, a

new socket is created. Thus, to support n simultaneous connections, the server

would need n+1 sockets.

28. For the TCP application, as soon as the client is executed, it attempts to initiate a

TCP connection with the server. If the TCP server is not running, then the client

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《计算机网络：自顶向下方法》第四版英文答案解析

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