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计算机网络自顶向下第六版英文答案.pdf
Computer Networking: A Top-Down Approach,
6th 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.
All material © copyright 1996-2012 by J.F. Kurose and K.W. Ross. All rights reserved
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
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.
these
11. At time t0 the sending host begins to transmit. At time t1 = L/R1, 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 t1, it can begin to transmit the
packet to the receiving host at time t1. At time t2 = t1 + L/R2, 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/R1 + L/R2.
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)3 = 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
IXP.
amount
or
received
from
the
of
traffic
sent
to
the
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
333133pp
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
--------
HELO
purpose
-------
Let server know that there is a card in the
ATM machine
ATM card transmits user ID to Server
User enters PIN, which is sent to server
User requests balance
PASSWD
BALANCE
WITHDRAWL User asks to withdraw money
user all done
BYE
Messages from Server to ATM machine (display)
Msg name
--------
PASSWD
OK
ERR
purpose
-------
Ask user for PIN (password)
last requested operation (PASSWD, WITHDRAWL)
OK
last requested operation (PASSWD, WITHDRAWL)
in ERROR
sent in response to BALANCE request
user done, display welcome screen at ATM
<------------- ERR (not enough funds)
<------------- OK
withdrawl
--------------> (check if valid userid)
<------------- PASSWD
<------------- OK (password is OK)
-------------->
<------------- AMOUNT
AMOUNT
BYE
Correct operation:
client server
HELO (userid)
PASSWD --------------> (check password)
BALANCE
WITHDRAWL --------------> check if enough $ to cover
ATM dispenses $
BYE
In situation when there's not enough money:
HELO (userid)
PASSWD --------------> (check password)
BALANCE
WITHDRAWL --------------> check if enough $ to cover
withdrawl
error msg displayed
no $ given out
BYE
<------------- OK (password is OK)
-------------->
<------------- AMOUNT
--------------> (check if valid userid)
<------------- PASSWD
-------------->
<------------- BYE
-------------->
<------------- BYE
Problem 2
At time N*(L/R) the first packet has reached the destination, the second packet is stored
in the last router, the third packet is stored in the next-to-last router, etc. At time N*(L/R)
+ L/R, the second packet has reached the destination, the third packet is stored in the last
router, etc. Continuing with this logic, we see that at time N*(L/R) + (P-1)*(L/R) =
(N+P-1)*(L/R) all packets have reached the destination.
Problem 3
a) A circuit-switched network would be well suited to the application, because the
application involves long sessions with predictable smooth bandwidth requirements.
Since the transmission rate is known and not bursty, bandwidth can be reserved for
each application session without significant waste. In addition, the overhead costs of
setting up and tearing down connections are amortized over the lengthy duration of a
typical application session.
b) In the worst case, all the applications simultaneously transmit over one or more
network links. However, since each link has sufficient bandwidth to handle the sum
of all of the applications' data rates, no congestion (very little queuing) will occur.
Given such generous link capacities, the network does not need congestion control
mechanisms.
Problem 4
a) Between the switch in the upper left and the switch in the upper right we can have 4
connections. Similarly we can have four connections between each of the 3 other
pairs of adjacent switches. Thus, this network can support up to 16 connections.
b) We can 4 connections passing through the switch in the upper-right-hand corner and
another 4 connections passing through the switch in the lower-left-hand corner,
giving a total of 8 connections.
c) Yes. For the connections between A and C, we route two connections through B and
two connections through D. For the connections between B and D, we route two
connections through A and two connections through C. In this manner, there are at
most 4 connections passing through any link.
Problem 5
Tollbooths are 75 km apart, and the cars propagate at 100km/hr. A tollbooth services a
car at a rate of one car every 12 seconds.
a) There are ten cars. It takes 120 seconds, or 2 minutes, for the first tollbooth to service
the 10 cars. Each of these cars has a propagation delay of 45 minutes (travel 75 km)
before arriving at the second tollbooth. Thus, all the cars are lined up before the
second tollbooth after 47 minutes. The whole process repeats itself for traveling
between the second and third tollbooths. It also takes 2 minutes for the third tollbooth
to service the 10 cars. Thus the total delay is 96 minutes.
b) Delay between tollbooths is 8*12 seconds plus 45 minutes, i.e., 46 minutes and 36
seconds. The total delay is twice this amount plus 8*12 seconds, i.e., 94 minutes and
48 seconds.
Problem 6
a)
b)
c)
seconds.
seconds.
seconds.
d) The bit is just leaving Host A.
e) The first bit is in the link and has not reached Host B.
f) The first bit has reached Host B.
g) Want
km.
Problem 7
Consider the first bit in a packet. Before this bit can be transmitted, all of the bits in the
packet must be generated. This requires
sec=7msec.
The time required to transmit the packet is
sec=
sec.
Propagation delay = 10 msec.
smdprop/RLdtrans/)//(RLsmdendtoend536105.2105612083sRLm310648566102856224
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