计算机网络--自顶向下方法（第四版）习题答案

Computer Networking: A Top-Down Approach

Featuring the Internet, 4

th

Edition

Solutions to Review Questions and Problems

Version Date: October 29, 2007

This document contains the solutions to review questions and problems for the 4th

edition of Computer Networking: A Top-Down Approach Featuring the Internet by Jim

Kurose and Keith Ross. These solutions are being made available to instructors ONLY.

used interchangeably. End systems include PCs, workstations, Web servers, mail

servers, Internet-connected PDAs, WebTVs, etc.

2. Suppose Alice, an ambassador of country A wants to invite Bob, an ambassador of

country B, over for dinner. Alice doesn’t simply just call Bob on the phone and say,

“come to our dinner table now”. Instead, she calls Bob and suggests a date and time.

Bob may respond by saying he’s not available that particular date, but he is available

another date. Alice and Bob continue to send “messages” back and forth until they

agree on a date and time. Bob then shows up at the embassy on the agreed date,

hopefully not more than 15 minutes before or after the agreed time. Diplomatic

protocols also allow for either Alice or Bob to politely cancel the engagement if they

residential or small office; 3. Cable to HFC: residential; 4. 100 Mbps switched

Etherent: company; 5. Wireless LAN: mobile; 6. Cellular mobile access (for example,

WAP): mobile

5. HFC bandwidth is shared among the users. On the downstream channel, all packets

emanate from a single source, namely, the head end. Thus, there are no collisions in

the downstream channel.

6. Current possibilities include: dial-up; DSL; cable modem; fiber-to-the-home.

7. Ethernet LANs have transmission rates of 10 Mbps, 100 Mbps, 1 Gbps and 10 Gbps.

For an X Mbps Ethernet (where X = 10, 100, 1,000 or 10,000), a user can

continuously transmit at the rate X Mbps if that user is the only person sending data.

is up to 1 Mbps, bandwidth is dedicated; HFC, downstream channel is 10-30 Mbps

and upstream channel is usually less than a few Mbps, bandwidth is shared.

10. There are two most popular wireless Internet access technologies today:

In a wireless LAN, wireless users transmit/receive packets to/from a base station

(wireless access point) within a radius of few tens of meters. The base station is

typically connected to the wired Internet and thus serves to connect wireless users

to the wired network.

b) Wide-area wireless access network

In these systems, packets are transmitted over the same wireless infrastructure

used for cellular telephony, with the base station thus being managed by a

telecommunications provider. This provides wireless access to users within a

radius of tens of kilometers of the base station.

for the duration of a call. Most packet-switched networks today (including the

the same slot in a revolving TDM frame.

13. At time t

0

the sending host begins to transmit. At time t

completes transmission and the entire packet is received at the router (no propagation

delay). Because the router has the entire packet at time t1, it can begin to transmit the

packet to the receiving host at time t

. At time t

, the router completes

transmission and the entire packet is received at the receiving host (again, no

propagation delay). Thus, the end-to-end delay is L/R

.

the tier-1 ISPs. Also, a tier-2 ISP is a customer of one or more tier-1.

15. a) 2 users can be supported because each user requires half of the link bandwidth.

b) Since each user requires 1Mbps when transmitting, if two or fewer users transmit

that all three users are transmitting simultaneously) is 0.008.

16. The delay components are processing delays, transmission delays, propagation

delays, and queuing delays. All of these delays are fixed, except for the queuing

delays, which are variable.

17. Java Applet

18. 10msec; d/s; no; no

19. a) 500 kbps

b) 64 seconds

c) 100kbps; 320 seconds

20. End system A breaks the large file into chunks. To each chunk, it adds header

generating multiple packets from the file. The header in each packet includes the

address of the destination: end system B. The packet switch uses the destination

layers. For example, error control is often provided at more than one layer.

23. The five layers in the Internet protocol stack are – from top to bottom – the

application layer, the transport layer, the network layer, the link layer, and the

physical layer. The principal responsibilities are outlined in Section 1.5.1.

24. Application-layer message: data which an application wants to send and passed onto

the transport layer; transport-layer segment: generated by the transport layer and

encapsulates application-layer message with transport layer header; network-layer

datagram: encapsulates transport-layer segment with a network-layer header; link-

layer frame: encapsulates network-layer datagram with a link-layer header.

Requires some form of human interaction to spread. Classic example: E-mail

viruses.

b)Worms

numbers, looking for vulnerable processes to infect.

c) Trojan horse

Hidden, devious part of some otherwise useful software.

27. Creation of a botnet requires an attacker to find vulnerability in some application or

system (e.g. exploiting the buffer overflow vulnerability that might exist in an

application). After finding the vulnerability, the attacker needs to scan for hosts that

are vulnerable. The target is basically to compromise a series of systems by

exploiting that particular vulnerability. Any system that is part of the botnet can

automatically scan its environment and propagate by exploiting the vulnerability. An

control and issue commands to all the nodes in the botnet. Hence, it becomes

modify the message(s) being sent from Bob to Alice. For example, she can easily

change the phrase “Alice, I owe you $1000” to “Alice, I owe you $10,000”.

Furthermore, Trudy can even drop the packets that are being sent by Bob to Alice

(and vise-versa), even if the packets from Bob to Alice are encrypted.

Here's a simple answer below:

Messages from ATM machine to Server