CENExam1v2Answer

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CEN 4500C Exam 1 (Spring 2007)



Last Name _____
Key
____________

Time: 50 mins.





First Name _____Answer
_________






For numerical problems, y
ou can leave your answer as
mathematical
expressions.

Explain
your answers when asked to ge
t full credits. Be as brief as possible in your explanations.


1.

(4

points)
If an application uses TCP as the transport layer, what information is used to
identify the application process for which an application segment is destined?


IP address + port numbe
r can be used to
uniquely

identify the destination.



2.

(5 points)
Does
a

TCP
segment with
ACK number of 1000 always means that 999 bytes has
been succe
ssfully received?

Explain

briefly

in one or two sentences
.


No, it means that TCP is expecting the packet
numbered 1000 packet should be
delivered next. TCP looks at the packet with largest sequence number among
received packets and add 1 as the ACK number, even if there are packets that were
missed from the previous sliding window.


3.

(5 points)
Is it possible
for an application to enjoy reliable data transfer even when the
application runs over UDP. Explain why, in one or two sentences.


Yes, although UDP is not designed to be reliable transport layer protocol, we can
always try to make data transmission reliab
le from the layers above. For instance,
applications can label chunk of data to be transmitted with sequence number and the
size of data chunk, the
recipients

can then check if all the data transmitted has been
received, even without the help from lower la
yers.



4.

Suppose several users share a 1 Mbps

link. Suppose each user r
equires 2
00 Kbps when
transmitting, but each user

only transmits 20
% of the time.

a.

(4

points)
If circuit switching is used, what is the maximum number of users that

can
be supported?


1 M
bps/200 Kbps = 5


b.

(6

points)
Assume there are 12 users, and packet switching is used. At any time

instant, what is the probability that the traffic introduced by users on the

link exceeds
the capacity of the link?

Just provide the expression.


if there are

more than 5 users decide to send packets at the same time, we have a
congestion. The total probability of congestion can be expressed as:



5.

(15 points)
Host A wants to send a file that is 7.5 x 10
6

bits long to Host B over a path wi
th
three links as shown in figure below. The sender host divides this file into 5000 packets, each
packet being 1500 bits long, and sends these packets over the path. The receiver on getting
all these packets reassembles them. Suppose each link is a 1Mbps
link. Say the length of each
link is 10,000 m and the propagation speed over each link is 2.5 x 10
8

m/sec. Say the queuing
delay at each intermediate router is 15 msec.
What is the

end
-
to
-
end delay faced by the
first packet
?

Ignore the processing delay.






d end
-
to
-
end = d proc + d queue + d trans + d propagation



There is no processing delay, d proc = 0


There are 2 queuing delays,
occurred

at router 1 and router 2, each cost 15 x 10
-
3 s

The transmission delay
is the time ne
eded to push all the bits into the wire, and because of
store
-
and
-
forward property, would have to be repeated three times, each cost (7.5 x 10 6 / 1 x
10 6) s


The propogation delay consists of the time needed to travel through all three wires,
each cost


(10000/2.5 x 10 8) s



d end
-
to
-
end = d proc + d queue + d trans + d propagation




= 0 + 2 x 15 x 10
-
3 + 3 x 7.5 + 12 x 10
-
3




= 22.542 s





6.

(15 points)
Suppose an ISP owns the block of addresses of the form 101.101.128/17.
Suppose it wants to c
reate 6 subnets of equal number of hosts from this block. What prefixes
can be used for these 6 subnets?


T
o create 6 subnets, we need at least 3 extra bits in the mask for subnetting (23 = 8 > 6).
Hence the prefixes to be used should be 20 bits, which are

listed as following,


1000 0000


101.101.128/20



1001 0000


101.101.144/20

101.101.

1010 0000


101.101.160/20

101.101.

1011 0000


101.101.176/20

101.101.

1100 0000


101.101.192/20

101.101.

1101 0000


101.101.208/20

101.101.

1110 0000


101.101.224
/20

101.101.

1111 0000


101.101.240/20 (any six will do)

Host A

Rout
er 1

Router 2

Host B

7.

Suppose an Internet router uses longest prefix matching and has the following forwarding
table.


Prefix Match




Link Interface

11100000 0

111000
01 00000000




1

11100001





2

otherwise




3


a.

(15 points)
For each of the four interfaces, give the associated address range of
destination addresses.


Prefix Match




Link Interface

0



1110 0000.0000 0000.0000 000
0.0000 0000
--

1110 0000.1111 1111.1111 1111.1111 1111

1



1110 0001.0000 0000.0000 0000.0000 0000
--

1110 0001.0000 0000.1111 1111.1111 1111

2



1110 0001.0000 0001.0000 0000.0000 0000
--

1110 0001.1111 1111.1111 1111.1111 1111



3




otherwise



b.

(5 point
s)
Which interface will a packet with the following destination address be
forw
arded to? Explain
as briefly as possible,
how the forwarding link is identified
.


11100001 10000000 00010001 01110111


Interface 2. Longest prefix match tells us that this addre
ss does not belong to interface
1 (16 bits). When matching against 8
-
bit prefixes, it is found that it matches with
11100001, which means the packet will be forwarded to interface 2.




8.

(5 points)
Suppose you have a wireless router connected to a cable mod
em. Also suppose that
your ISP dynamically assigns your wireless router one IP address. But you have 5 PCs that
wirelessly connect to your wireless router.
Explain briefly how (if at all)
can
any of the
PCs
in your network connect to web servers

outside yo
ur home network

and download
information from them
?


The wireless router will serve as NAT. When the web request arrives at the wireless router,
the NAT creates a one
-
to
-
one mapping between internal IP and port number to an external IP
(belongs to the wir
eless router) and port number. The use of external IP allows the response
from the web servers can be routed using this external IP, and once the response arrives at the
wireless router, NAT performs a table lookup and route the packet using the mapping st
ored
in the NAT table.



9.

(15 points)
Consider the network shown below. Using Dijkstra’s Algorithm, and
showing
your work in the table provided below
, compute the shortest path from
x

to all the network
nodes.
D
raw the graph showing the shortest paths
.





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㄰N

(6 points) Name the 5 layers in the Internet protocol stack. Name a protocol that exists at
each of the 4 top layers. Also name one functionality supported at each of the 3 top layer.

LAYER


PROTOCOL


FUNCTIONALITY

Application Layer

HTTP/TCP/SMTP

Appli
cation/User Interface/Presentation

Tran
sport Layer

TCP



Flow/Congestion Control/Reliability

Network Layer

IP



Routing
/Forwarding

Link Layer


802.3, 802.11

Physical Layer

u

z

v

y

w

x

2

2

1

3

1

1

2

5

3

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X

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y

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w

u

2

1
2

1

2

1