Kurose/Ross《计算机网络：自顶向下方法》第五版习题答案（互联网特色）

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《计算机网络：自顶向下方法》（ComputerNetworking:ATop-DownApproach）第五版，由Jim Kurose和Keith Ross合著，是国际软件学院网络和分布式计算课程的经典教材。该教材以互联网为核心，提供了一套详尽的复习问题和问题解答，适用于教学使用。版本日期为2009年9月1日，这些解决方案仅供授课教师参考，不得随意复制或分发给学生或其他非授权人员。书中强调，术语“主机”和“端系统”在文本中可以互换使用，包括个人电脑、工作站、Web服务器、邮件服务器、互联网连接的掌上电脑、WebTV等各类设备都属于端系统范畴。作者特别感谢多年来帮助编撰解决方案的学生和同事，如Honggang Zhang、Rakesh Kumar和Prithula Dunghel。所有内容受版权保护，由Kurose和Ross共同享有，未经许可，禁止任何形式的复制或公开发布。第1章的复习问题涵盖了基本概念，旨在引导读者从整体视角理解计算机网络。其中，第一题可能涉及网络体系结构的基础，即理解“主机”和“端系统”的通用定义，以及它们在网络中的作用。这一章可能还会探讨网络层次模型，例如OSI七层模型或TCP/IP四层模型，以及各层的主要功能和交互。后续章节可能会深入到具体的网络技术，如数据链路层的帧传输、网络层的路由选择算法、传输层的协议（如TCP和UDP）、应用层的协议和服务等。每一章的练习题目旨在检验学生对理论知识的理解和实际应用能力，如配置网络设备、解析网络故障、设计网络架构等。《Computer Networking: A Top-Down Approach》第五版通过解答详实的问题，不仅提供了理论知识的巩固，还培养了学生的实践技能，是学习和教授计算机网络不可或缺的参考资料。对于正在寻求提升网络知识的专业人士或学生来说，这是一个宝贵的学习工具。

Chapter 2 Review Questions

1. The Web: HTTP; file transfer: FTP; remote login: Telnet; Network News: NNTP;

e-mail: SMTP.

2. Network architecture refers to the organization of the communication process into

layers (e.g., the five-layer Internet architecture). Application architecture, on the

other hand, is designed by an application developer and dictates the broad

structure of the application (e.g., client-server or P2P)

3. The process which initiates the communication is the client; the process that waits

to be contacted is the server.

4. No. As stated in the text, all communication sessions have a client side and a

server side. In a P2P file-sharing application, the peer that is receiving a file is

typically the client and the peer that is sending the file is typically the server.

5. The IP address of the destination host and the port number of the destination

socket.

6. You would use UDP. With UDP, the transaction can be completed in one

roundtrip time (RTT) - the client sends the transaction request into a UDP socket,

and the server sends the reply back to the client's UDP socket. With TCP, a

minimum of two RTTs are needed - one to set-up the TCP connection, and

another for the client to send the request, and for the server to send back the reply.

7. There are no good examples of an application that requires no data loss and

timing. If you know of one, send an e-mail to the authors.

8. a) Reliable data transfer

TCP provides a reliable byte-stream between client and server but UDP does

not.

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

same connection that the file is sent over, FTP sends control information out of

band.

16. Message is sent from Alice’s host to her mail server over HTTP. Alice’s mail

server then sends the message to Bob’s mail server over SMTP. Bob then

transfers the message from his mail server to his host over POP3.

17.

Received:

from 65.54.246.203 (EHLO bay0-omc3-s3.bay0.hotmail.com)

(65.54.246.203) by mta419.mail.mud.yahoo.com with SMTP; Sat, 19

May 2007 16:53:51 -0700

Received:

from hotmail.com ([65.55.135.106]) by bay0-omc3-s3.bay0.hotmail.com

with Microsoft SMTPSVC(6.0.3790.2668); Sat, 19 May 2007 16:52:42 -

0700

Received:

from mail pickup service by hotmail.com with Microsoft SMTPSVC; Sat,

19 May 2007 16:52:41 -0700

Message-ID: <BAY130-F26D9E35BF59E0D18A819AFB9310@phx.gbl>

Received:

from 65.55.135.123 by by130fd.bay130.hotmail.msn.com with HTTP;

Sat, 19 May 2007 23:52:36 GMT

From: "prithula dhungel" <prithuladhungel@hotmail.com>

To:

rithula@yahoo.co

Bcc:

Subject: Test mail

Date: Sat, 19 May 2007 23:52:36 +0000

Mime-Version:1.0

Content-Type: Text/html; format=flowed

Return-Path:

rithuladhungel@hotmail.co

Figure: A sample mail message header

Received: This header field indicates the sequence in which the SMTP servers

send and receive the mail message including the respective timestamps.

In this example there are 4 “Received:” header lines. This means the mail

message passed through 5 different SMTP servers before being delivered to the

receiver’s mail box. The last (forth) “Received:” header indicates the mail

message flow from the SMTP server of the sender to the second SMTP server in

the chain of servers. The sender’s SMTP server is at address 65.55.135.123 and

the second SMTP server in the chain is by130fd.bay130.hotmail.msn.com.

The third “Received:” header indicates the mail message flow from the second

SMTP server in the chain to the third server, and so on.

Finally, the first “Received:” header indicates the flow of the mail message from

the forth SMTP server to the last SMTP server (i.e. the receiver’s mail server) in

the chain.

Message-id: The message has been given this number BAY130-

F26D9E35BF59E0D18A819AFB9310@phx.gbl (by bay0-omc3-

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. Mesh DHT: The advantage is to a route a message to the peer closest to the key,

only one hop is required; the disadvantage is that each peer must track all other

peers in the in the DHT. Circular DHT: the advantage is that each peer needs to

track only a few other peers; the disadvantage is that O(N) hops are needed to

route a message to a peer responsible for the key.

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

will fail to make a connection. For the UDP application, the client does not

initiate connections (or attempt to communicate with the UDP server)

immediately upon execution

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