《计算机网络：自顶向下方法》(第四版)英文课后答案解析

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"这是一份关于《计算机网络：自顶向下方法》第四版的英文课后习题答案，由Jim Kurose和Keith Ross编写。这份解答仅提供给教师，禁止复制或分发给他人，也不应发布在公开网站上。" 在《计算机网络：自顶向下方法》这本书中，作者通过一种自顶向下的方式来讲解计算机网络的原理和应用，这种方法将网络视为一个整体，从应用层开始，逐层深入到物理层，使读者能够更好地理解网络的运作机制。这本书特别强调了互联网的相关知识，适合学习计算机网络的初学者和专业人士。课后习题解答涵盖了书中的各个章节，帮助学生和教师检查对课程内容的理解程度。例如，第一章的复习问题包括： 1. 这个问题解释了"主机"和"端系统"这两个术语在本书中是等价的。端系统是指与网络连接的各种设备，如个人电脑、工作站、Web服务器、邮件服务器、联网的个人数字助手（PDA）、WebTV等。 2. 这个问题以一种寓言的方式提出，假设Alice是A国的大使，她想邀请B国的大使Bob共进晚餐。这用来比喻网络通信的过程，即Alice（发送方）需要通过某种方式（如网络）有效地联系Bob（接收方），而这个过程涉及到了网络通信的基本原理，如数据的封装、传输和解封装。这些习题解答旨在加深对计算机网络基本概念的理解，如OSI模型、TCP/IP协议栈、网络层次结构、数据传输、错误检测与纠正、路由选择、拥塞控制等。此外，还涵盖了互联网的关键服务，如HTTP、FTP、DNS等，以及网络安全和隐私问题。通过解答这些问题，读者可以巩固对网络协议、网络层次模型、网络设备工作原理等知识的理解，进一步提升分析和解决实际网络问题的能力。这些习题和答案对于教师评估学生学习进度和准备考试都是非常有价值的资源。

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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