EKT335 PRINCIPLES OF COMPUTER NETWORK

curvyrawrNetworking and Communications

Oct 23, 2013 (3 years and 5 months ago)

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1






EKT335

PRINCIPLES OF
COMPUTER
NETWORK

TEST
I


-

ANSWER

DATE


:
17

OCT

20
1
2

















2


1.

a)

Draw and compare
the 7
-
layer defined in the ISO OSI Reference Model and
5
-
layer
TCP/IP layer architecture.


[3 marks]


Application

Presentation

Session

Transport

Network

Data Link

Physical

7
-
Layer ISO OSI Reference Model

5
-
Layer TCP/IP Model


The key difference i
s that the 5
-
layer Internet model does not have the presentation and
session layers found in the OSI model. Otherwise the models are essentially the same
.


b)

Which TCP/IP

layer is responsible for the following?


[3 marks]

i)

Determining the best path to route packet.

Network Layer


ii)

Providing end
-
to
-
end communications with reliable service
.

Transport Layer


iii)

Providing node
-
to
-
node communications with reliable service.

Data Link Layer


c)

Briefly explain the process of en
capsulation happening in layered architecture.

[4 marks]


-

The
Application layer sends
message
to the Transport layer.


-

The T
ransport layer converts the
Application layer
message

to segments
by
appending
transport
-
layer header information
to the Application

layer message
and
sends it down to the Network layer.


-

The N
etwork layer converts the segments to packets

by
adding

the Network layer
header
information
to the segments
and sends them to the Data Link layer.


-

The Data Link layer converts the packets to fr
ames
by adding the Data Link layer
header
and sends them to the Physical layer.


-

The Physical layer converts the frames to 1's and 0's (electrical signals) and sends
them across the network.












Application

Transport

Network

Link

Physical

3


2.

The following question is based on Figure 1 below:


T
racing route to webserver.unimap.edu.my [58.27.57.229]

over a maximum of 30 hops:


1

2 ms

1 ms

1 ms

10.208.129.27

2

3 ms

3 ms

3 ms

vl162
-
61 [202.179.118.122]

3

4 ms

3 ms

3 ms

218
.1
00
.
45
.1
27

4

8 ms

4 ms

3 ms

10.55.64.39

5

9

ms

7 ms

6 ms

210.187.75.23

6

7 ms

6 ms

5

ms

mcl
-
getaway.gw.unimap.edu.my [58.27.57.1]

7

7 ms

9 ms

9 ms

www.unimap.edu.my

[58.27.57.229]


Trace complete.


Figure 1


‘traceroute’ command is used to determine the path from your home PC to your uni
versity’s
main web page

as shown in Figure 1.


a)

Using the output from

traceroute

in Figure 1,
identify
how many different
routers

are
t
raversed.


[2 marks]


6

different networks and service providers. The university
getaway router
is reached at hop
6.


b)

Briefly explain how this command

can be used as a debugging tool by network managers.



[3 marks]

The

traceroute

command discovers the routes packets follow when traveling to their
destinations. The

traceroute

command works by using the err
or message generated by switch
routers when a datagram exceeds its time
-
to
-
live (TTL) value. First, probe datagrams are sent
with a TTL value of one. This causes the first switch router to discard the probe datagrams and
send back "time exceeded" error mes
sages. The

traceroute

command then sends several
probes, and displays the round
-
trip time for each. After every third probe, the TTL increases by
one.

Each outgoing packet can result in one of two error messages. A "time exceeded" error message
indicates t
hat an intermediate switch router has seen and discarded the probe. A "port
unreachable" error message indicates that the destination node has received the probe and
discarded it because it could not deliver the packet to an application. If the timer goes
off before
a response comes in, the

traceroute

command displays an asterisk (*).


3.

Consider a packet of length
L

which begins at end system
A

and travels over 3 links to a
destination end system
B
. These 3 links are connected by 2 packet switches. Let
d
i

,
s
i

, and
R
i

denote the length, propagation speed, and the transmission rate of link
i
, for
i
= 1, 2, 3
. The
packet switch
also
delays each packet by
d
proc
.

4



a)

Assuming no queuing delays,
express the total end
-
to
-
end delay for the packet
in terms of

d
i

,
s
i

,
R
i

, (
i

= 1, 2, 3
), and
L
.


[5 marks]


d
end
-
end

= L/R
1

+ L/R
2

+ L/R
3

+ d
1
/s
1

+ d
2
/s
2

+ d
3
/s
3
+ d
proc
+ d
proc


b)

Suppose now the packet is 1,500 bytes, the prop
agation speed on all 3 links is 2.5 x 10
8

m/s,
the transmission rates for all 3 links are 2 Mbps, the packet switch processing delay is 3
msec, the length of the first link is 5000 km, the length of the second link is 4000 km, and
the length of the last li
nk is 1000 km. Calculate the end
-
to
-
end delay for this packet to be
transmitted from
A

to
B
.

[5 marks]


d
end
-
end

= 6 + 6 + 6 + 20+16 + 4 + 3 + 3 = 64 m
sec

4.

Suppose two hosts,
A

and
B
, are separated by 20,000 kilometers and are connected by a direct
link of
R

= 2 Mbps. Suppose the propagation speed over the link is 2.5 x 10
8

m/s.


a)

Calculate the propagation delay,
d
prop
.


[2 marks]


d
prop

= d/s = 20000 x 10
3
/(2.5 x 10
8
) = 0.08 s


b)

Calculate the
maximum
number of bits that can be in the link

(i.e the

bandwidth
-
delay
product,
R
x

d
prop
.

).



[2 marks]


BDP = R x d
prop

= (2 x 10
6
) x 0.08 = 160, 000 bits


c)

Consider sending a file of 800,000 bits from Host
A

to Host
B
. Suppose the file is sent
continuously as one large message.
Calculate the total time taken to send the file.



[4 marks]



t
trans

= L/R = 800
,000 / (2 x 10
6
) = 0.4 s



Time = t
prop

+ t
tran
s

= 0.08 +
0.4 = 0.48 s


5.

a)

Define

the usage of
a web cache

(i.e., what benefits are derived from using a web cache)?



W
here can a web cache be located or placed?


[3 marks]


A web cache may be used to reduce response time as experienced by a user, reduce load on
a link, and/or reduce load on a web server. A web cache may be placed in the user client
(e.g., within the browser applic
ation), at “my” network edge (we call this a proxy server),
and/or at the edge of the network containing the server (we call this a transparent cache).


b)

Define w
hat is DNS and what is it used for?
Explain what would happen to the Internet i
f all
DNS se
rver
s

“crashed” (taken offline)
.

[3 marks]


DNS is Domain Name Service and is used to associate host names (as in say,
www.yahoo.com

or chr
isten@csee.usf.edu) with IP addresses. If all DNS servers were to be
crashed one would be unable to use hostnames and could only use IP addresses when using
Internet services.


5


c)

Consider what happens when a browser (that is, an HTTP client), running on some

user’s
host, requests the URL
www.somesite.com/index.html
. In order for the user’s host to be
able to send an HTTP request message to the Web server
www.somesite.com
, t
he user’s
host must first obtain the IP address of
www.somesites.com
. Explain the steps through
which the IP address for such a hostname is obtained by the client.

[4 marks]

Steps:

a)

The brow
ser extract hostname,
www.somesites.com
, from the URL and passes the
hostname to the client side of the DNS application.

b)

The DNS client sends a query containing the hostname to a DNS server.

c)

The DNS client receives a
reply, which includes the IP address for the hostname.

d)

Once the browser receives the IP address from DNS, it can initiate a TCP connection
to the HTTP server process located at port 80 at the IP address.


d)

For the client
-
server application over TCP, why mus
t the server program be executed before
the client program? For the client
-
server application over UDP, why may the client program
be executed before the server program?

[2 marks]


TCP:

Client

needs to establish a TCP
connection first with the server
.

If you run client program

first, then the client will attempt to make a TCP connection with a non
-
existent server
process. A TCP connection will not be made.


UDP:

No connection establi
shment is needed prior to communicating. Client may send request
even when the server is not ready.

UDP

c
lient doesn't establish a TCP connection with the
server. Thus, everythin
g should work fine if you
run UDP

c
lient

program first.



-
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o
Oooo
-