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Computer Networking: A Top-Down Approach,

Edition

Solutions to Review Questions and Problems

Version Date: May 2012

This document contains the solutions to review questions and problems for the 5th

edition of Computer Networking: A Top-Down Approach by Jim Kurose and Keith Ross.

These solutions are being made available to instructors ONLY. Please do NOT copy or

distribute this document to others (even other instructors). Please do not post any

solutions on a publicly-available Web site. We’ll be happy to provide a copy (up-to-date)

of this solution manual ourselves to anyone who asks.

Acknowledgments: Over the years, several students and colleagues have helped us

prepare this solutions manual. Special thanks goes to HongGang Zhang, Rakesh Kumar,

Prithula Dhungel, and Vijay Annapureddy. Also thanks to all the readers who have made

suggestions and corrected errors.

Chapter 1 Review Questions

1. There is no difference. Throughout this text, the words “host” and “end system” are

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

servers, PDAs, Internet-connected game consoles, etc.

2. From Wikipedia: Diplomatic protocol is commonly described as a set of international

courtesy rules. These well-established and time-honored rules have made it easier for

nations and people to live and work together. Part of protocol has always been the

acknowledgment of the hierarchical standing of all present. Protocol rules are based

on the principles of civility.

3. Standards are important for protocols so that people can create networking systems

and products that interoperate.

4. 1. Dial-up modem over telephone line: home; 2. DSL over telephone line: home or

small office; 3. Cable to HFC: home; 4. 100 Mbps switched Ethernet: enterprise; 5.

Wifi (802.11): home and enterprise: 6. 3G and 4G: wide-area wireless.

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. In most American cities, the 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.

8. Today, Ethernet most commonly runs over twisted-pair copper wire. It also can run

over fibers optic links.

9. Dial up modems: up to 56 Kbps, bandwidth is dedicated; ADSL: up to 24 Mbps

downstream and 2.5 Mbps upstream, bandwidth is dedicated; HFC, rates up to 42.8

Mbps and upstream rates of up to 30.7 Mbps, bandwidth is shared. FTTH: 2-10Mbps

upload; 10-20 Mbps download; bandwidth is not shared.

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

a) Wifi (802.11) In a wireless LAN, wireless users transmit/receive packets to/from an

base station (i.e., 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) 3G and 4G wide-area wireless access networks. 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.

11. At time t

the sending host begins to transmit. At time t

= L/R

, the sending host

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

delay). Because the router has the entire packet at time t

, it can begin to transmit the

packet to the receiving host at time t

. At time t

= t

+ L/R

, 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

+ L/R

12. A circuit-switched network can guarantee a certain amount of end-to-end bandwidth

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

Internet) cannot make any end-to-end guarantees for bandwidth. FDM requires

sophisticated analog hardware to shift signal into appropriate frequency bands.

13. 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

simultaneously, a maximum of 2Mbps will be required. Since the available

bandwidth of the shared link is 2Mbps, there will be no queuing delay before the

link. Whereas, if three users transmit simultaneously, the bandwidth required

will be 3Mbps which is more than the available bandwidth of the shared link. In

this case, there will be queuing delay before the link.

c) Probability that a given user is transmitting = 0.2

d) Probability that all three users are transmitting simultaneously =

 















= (0.2)

= 0.008. Since the queue grows when all the users are transmitting, the

fraction of time during which the queue grows (which is equal to the probability

that all three users are transmitting simultaneously) is 0.008.

14. If the two ISPs do not peer with each other, then when they send traffic to each other

they have to send the traffic through a provider ISP (intermediary), to which they

have to pay for carrying the traffic. By peering with each other directly, the two ISPs

can reduce their payments to their provider ISPs. An Internet Exchange Points (IXP)

(typically in a standalone building with its own switches) is a meeting point where

multiple ISPs can connect and/or peer together. An ISP earns its money by charging

each of the the ISPs that connect to the IXP a relatively small fee, which may depend

on the amount of traffic sent to or received from the IXP.

15. Google's private network connects together all its data centers, big and small. Traffic

between the Google data centers passes over its private network rather than over the

public Internet. Many of these data centers are located in, or close to, lower tier ISPs.

Therefore, when Google delivers content to a user, it often can bypass higher tier

ISPs. What motivates content providers to create these networks? First, the content

provider has more control over the user experience, since it has to use few

intermediary ISPs. Second, it can save money by sending less traffic into provider

networks. Third, if ISPs decide to charge more money to highly profitable content

providers (in countries where net neutrality doesn't apply), the content providers can

avoid these extra payments.

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. a) 1000 km, 1 Mbps, 100 bytes

b) 100 km, 1 Mbps, 100 bytes

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. It adds header to each chunk, thereby

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

address of the destination (end system B). The packet switch uses the destination IP

address in the packet to determine the outgoing link. Asking which road to take is

analogous to a packet asking which outgoing link it should be forwarded on, given

the packet’s destination address.

21. The maximum emission rate is 500 packets/sec and the maximum transmission rate is

350 packets/sec. The corresponding traffic intensity is 500/350 =1.43 > 1. Loss will

eventually occur for each experiment; but the time when loss first occurs will be

different from one experiment to the next due to the randomness in the emission

process.

22. Five generic tasks are error control, flow control, segmentation and reassembly,

multiplexing, and connection setup. Yes, these tasks can be duplicated at different

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.

25. Routers process network, link and physical layers (layers 1 through 3). (This is a little

bit of a white lie, as modern routers sometimes act as firewalls or caching

components, and process Transport layer as well.) Link layer switches process link

and physical layers (layers 1 through2). Hosts process all five layers.

26. a) Virus

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

viruses.

b) Worms

No user replication needed. Worm in infected host scans IP addresses and port

numbers, looking for vulnerable processes to infect.

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

important property of such botnets is that the originator of the botnet can remotely

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

possible for the attacker to issue a command to all the nodes, that target a single

node (for example, all nodes in the botnet might be commanded by the attacker to

send a TCP SYN message to the target, which might result in a TCP SYN flood

attack at the target).

28. Trudy can pretend to be Bob to Alice (and vice-versa) and partially or completely

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.

Chapter 1 Problems

Problem 1

There is no single right answer to this question. Many protocols would do the trick.

Here's a simple answer below:

Messages from ATM machine to Server

Msg name purpose

-------- -------

HELO <userid> Let server know that there is a card in the

ATM machine

ATM card transmits user ID to Server

PASSWD <passwd> User enters PIN, which is sent to server

BALANCE User requests balance

WITHDRAWL <amount> User asks to withdraw money

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