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Final
Exam
of

Computer Networks

(ICE
1230
)

2008.
5. 27.

Answer in English and Total
15

points


1.

Why is it said that packet switching employs statistical
multiplexing? Contrast statistical multiplexing with the
multiplexing that
takes

place in TDM. (1 point)

Answer:
In a packet switched network, the packets from different sources
flowing on a link do

not follow any fixed, pre
-
defined pattern. In TDM
circuit switching, each host gets

the same slot in a revolving TDM frame.


2.

In BitTorrent, suppose Alice provide
s chunks to Bob
throughout a 30
-
second interval. Will Bob necessarily
return the favor and provide chunks to Alice in the same
inte
rval? Why or why not? (1 point)

Answer: It is not necessary that Bob will also provide chunks to
Alice. Alice has to be in th
e top 4 neighbors of Bob for Bob to
send out chunks to her; this might not occur even if Alice
provides chunks to Bob throughout a 30 second interval
.


3.

What is a way using
Last
-
Modified:

header line in HTTP? (1
point)

Answer: The header line is used by Con
ditional GET to check
whether the object asked has been modified.


4.

We saw that there is no network
-
layer protocol that can be
used to identify the hosts participating in a multicast group.
Given this, how can multicast applications learn the
identities

of
the hosts that are participating in a multicast
group?
(1 point)

Answer:
The protocol must be built at the application layer. For
example, an application may

periodically multicast its identity to all other group members in
an application
-
layer

message.

5

In these days, the Internet needs to include the wireless
communications network in addition to the wired networks.
What is a way to extend the congestion control of TCP in
this case? Why? (2 point)

Answer:
Packet losses occur from network congestion and
w
ireless communication. Since the wired Internet does not
consider the packet loss due to errors in wireless
communication, the
congestion

control of TCP should be
revised. A way is



6.
Describe the overall steps of Link State Routing algorithm.
And show
the technical issues (at least two) and their
solutions of its implementation as Open Shortest Path First.

(2 points)

1.

discover its neighbors and learn their network address.

2.

measure the delay or cost to each of its neighbors.

3.

construct a packet telling al
l it has just learned.

4.

send this packet to all other routers.

5.

compute the shortest path to every other router.


For example, OSPF has an architecture consisting of areas where the link
state packets are flooding inside. Sequence number is with age number i
n
order to solve



7.
Suppose two nodes start to transmit at the same time a
packet of length L over a broadcast channel of rate R.
Denote the propagation delay between the two nodes as
d
prop
. Will there be a collision d
prop

< L/R ? Why or why not?
(
1

poi
nts)

There will be a collision in the sense that while a node is transmitting it will start
to

receive a packet from the other node.



8.
What are the purposes of the HLR and VLR in GSM
networks? What elements of mobile IP are similar to the
HLR and VLR?

(
1

point)

The home network in GSM maintains a database called the home location
register

(HLR), which contains the permanent cell phone number and
subscriber profile

information about each of its subscribers. The HLR also
contains information

about the curr
ent locations of these subscribers. The
visited network maintains a

database known as the visitor location register
(VLR) that contains an entry for

each mobile user that is currently in the
portion of the network served by the

VLR. VLR entries thus come a
nd go as
mobile users enter and leave the network.

The edge router in home network in mobile IP is similar to the HLR in
GSM and

the edge router in foreign network is similar to the VLR in
GSM.


9.
What are some of the difficulties associated with the Ints
erv
model and per
-
flow reservation of resources? (1 point)

Scalability: Per
-
flow resource reservation implies the need for a router to
process

resource reservations and maintain per
-
flow state for each flow
passing through the

router. Flexibly service: Th
e Intserv framework
provides for a small number of prespecified

service classes.

10. What are the differences between message confidentiality
and message integrity? Can you have one
without

the
other? Justify your answer.

(2 points)

Confidentiality is the
property that the original plaintext message can not be determined

by an attacker who intercepts the ciphertext
-
encryption of the original plaintext message.

Message integrity is the property that the receiver can detect whether the message sent

(whether e
ncrypted or not) was altered in transit. The two are thus different concepts,

and one can have one without the other. An encrypted message that is altered in transmit

may still be confidential (the attacker can not determine the original plaintext) but wil
l

not have message integrity if the error is undetected. Similarly, a message that is altered

in transit (and detected) could have been sent in plaintext and thus would not be

confidential.

11.
What is the purpose of a nonce in an end
-
point
authentication
protocol?

(1 point)

A nonce is used to ensure that the person being authenticated is “live.” Nonces thus are

used to combat playback attacks.

12. What is the role of ASN.1 in the ISO/OSI reference model

s
presentation

layer?

(1 point)

The role of the prese
ntation layer is to allow the sending and receiving of data in a

machine
-
independent format (i.e., without regard to the particular storage and

architectural conventions of the sender and receiver).