1102/159.334 CP ALB Internal - Page 1 of 7 - MASSEY UNIVERSITY ALBANY CAMPUS EXAMINATION FOR 159.334 Computer Networks Semester Two - 2011 Time allowed: THREE (3) hours Attempt ALL 6 (six) questions. All questions are of equal value. This examination contributes 60% to the final assessment. Calculators permitted – no restrictions. 1102/159.334 CP ALB Internal - Page 2 of 7 - Question 1: a) Name four socket functions (C functions) used to create client/server applications. [4 marks] b) Name the five layers in a typical TCP/IP computer network. [2 marks] c) What are the services provided by TCP? [2 marks] d) The source and destination port numbers are included in the packet header. Which layer needs to be able to read these port numbers? [2 marks] 1102/159.334 CP ALB Internal - Page 3 of 7 - Question 2: (The Application Layer) a) If an FTP client is in active mode, what are the two FTP commands (protocol commands) issued by the client when the user issues: ftp> ls [2 marks] b) If an FTP client is in passive mode, what are the two FTP commands (protocol commands) issued by the client when the user issues: ftp> get [2 marks] c) The command netstat is used by a system administrator. Explain the difference between LISTEN and ESTABLISHED states. Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 localhost:1234 *:* LISTEN tcp 0 0 localhost:43000 it046500:20 ESTABLISHED [2 marks] d) How many simultaneous connections does HTTP needs in order to download a single file? How many simultaneous connections does FTP needs in order to download a single file? [2 marks] e) Briefly explain what is a DNS (Domain Name Service) iterated query. Explain why this method is important to keep the scalability of the DNS. [2 marks] 1102/159.334 CP ALB Internal - Page 4 of 7 - Question 3: (The Transport Layer) a) A distributed system uses UDP to transfer files between nodes. You are asked to design a simple reliable protocol to replace it. What are the essential elements that you need to include in such a protocol? Your solution should focus on possible lost and damaged packets, not performance. [4 marks] b) Explain the difference between Go-Back-N and Selective Repeat. Given a choice, which one is better for the performance of the network? [3 marks] c) Explain the difference between “Congestion Control” and “Flow Control” from the view point of a transport protocol. [3 marks] Question 4. (Network Layer) a) What are the two important functions provided by the network layer protocols? Emphasize the importance of the difference between the two functions. [3 marks] b) The network administrator checks an Ethernet card configuration using the command ipconfig. He extracts the values for the IP address and for the Mask: IP address 192.123.

1. 1102/159.334 CP
ALB
Internal
- Page 1 of 7 -
MASSEY UNIVERSITY
ALBANY CAMPUS
EXAMINATION FOR
159.334 Computer Networks
Semester Two - 2011
Time allowed: THREE (3) hours
Attempt ALL 6 (six) questions.
All questions are of equal value.
This examination contributes 60% to the final assessment.
Calculators permitted – no restrictions.

2. 1102/159.334 CP
ALB
Internal
- Page 2 of 7 -
Question 1:
a) Name four socket functions (C functions) used to create
client/server applications.
[4 marks]
b) Name the five layers in a typical TCP/IP computer network.
[2 marks]
c) What are the services provided by TCP?
[2 marks]
d) The source and destination port numbers are included in the
packet header. Which
layer needs to be able to read these port numbers?
[2 marks]

3. 1102/159.334 CP
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- Page 3 of 7 -
Question 2: (The Application Layer)
a) If an FTP client is in active mode, what are the two FTP
commands (protocol
commands) issued by the client when the user issues:
ftp> ls
[2 marks]
b) If an FTP client is in passive mode, what are the two FTP
commands (protocol
commands) issued by the client when the user issues:
ftp> get
[2 marks]
c) The command netstat is used by a system administrator.
Explain the difference
between LISTEN and ESTABLISHED states.
Proto Recv-Q Send-Q Local Address Foreign Address State

4. tcp 0 0 localhost:1234 *:* LISTEN
tcp 0 0 localhost:43000 it046500:20 ESTABLISHED
[2 marks]
d) How many simultaneous connections does HTTP needs in
order to download a single
file?
How many simultaneous connections does FTP needs in order to
download a single file?
[2 marks]
e) Briefly explain what is a DNS (Domain Name Service)
iterated query. Explain why this
method is important to keep the scalability of the DNS.
[2 marks]
1102/159.334 CP
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- Page 4 of 7 -
Question 3: (The Transport Layer)

5. a) A distributed system uses UDP to transfer files between
nodes. You are asked to design
a simple reliable protocol to replace it. What are the essential
elements that you need to
include in such a protocol? Your solution should focus on
possible lost and damaged
packets, not performance.
[4 marks]
b) Explain the difference between Go-Back-N and Selective
Repeat. Given a choice, which
one is better for the performance of the network?
[3 marks]
c) Explain the difference between “Congestion Control” and
“Flow Control” from the view
point of a transport protocol.
[3 marks]
Question 4. (Network Layer)
a) What are the two important functions provided by the
network layer protocols?
Emphasize the importance of the difference between the two
functions.

6. [3 marks]
b) The network administrator checks an Ethernet card
configuration using the command
ipconfig. He extracts the values for the IP address and for the
Mask:
IP address 192.123.246.12
Mask 255.255.255.128
Write two examples of IP addresses that this machine can
access:
1) one IP address that is accessible directly (no routing) by this
machine.
2) one IP address that is only accessible via the gateway
(router).
[2 marks]
Q4 continued on next page...
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7. Q4 continued...
c)
Figure 1
There are four routers connected via certain links (Figure 1).
Suppose that the following table
is used for the initialisation of D’s routing table:
D VIA
Destination
A B C
A 3 ∞ ∞
B ∞ 70 ∞
C ∞ ∞ 2
And the minimum cost is an array of 4 integers in the order A,
B, C, D:
A advertises its minimum costs as: 0, 12, 7, 3
B advertises its minimum costs as: 12, 0, ∞, 70
C advertises its minimum costs as: 7, ∞, 0, 2
Show the first update on D’s table if node D simultaneously
receives packets with the

8. minimum cost information from the routers A, B and D.
[3 marks]
d) There are possibly hundreds of millions of IP addresses in
the Internet. Explain why the
routing tables in TCP/IP routers do not need to contain lots of
entries, and are usually
rather small.
[2 marks]
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Question 5. (Data-link and Physical Layers)
a) What is the purpose of CRC (Cyclic Redundancy Check)?
[2 marks]
b) Compute the CRC value for the character “1110 0001”
considering:
- the algorithm uses a standard polynomial method with 16 bits

9. - the generator is 0x8005
- no reverse bytes, no reverse results
Write the final result of the CRC in hexadecimal.
[3 marks]
c) Collisions are a challenging problem for data link protocols.
Explain the main differences
between data-link layer protocols for wired networks and
protocols for wireless networks.
[3 marks]
d) Explain brifly how the ARP protocol keeps a table with the
IP addresses and
correspondent MAC (hardware) addresses.
[2 marks]
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Question 6. (Network Security)

10. a) MD5 is a well known method for message digests. Message
digests find a fixed-length
number (say, 128 bits) that correlates to a certain file.
Give two reasons why CRC is not a suitable method to be used
as a message digest, even
though CRC also finds a fixed-length number that correlated to
a file.
[2 marks]
b) Considering the RSA algorithm, find suitable e and d for the
following pair of prime
numbers, explaining how you chose them:
p = 5 q = 11
n = ?
z = ?
e = ? d = ?
[3 marks]
c) Discuss the difference between asymetric encryption and
symmetric encryption.
Complete the table stating advantages and disadvantages of
these two encryption methods

11. (copy it to the answer book).
Advantages Disadvantages
asymmetric
symmetric
[2 marks]
d) Explain briefly what is a “packet sniffing” attack. Discuss
and compare the implications
of such atacks for wired networks and wireless networks.
[3 marks]
+ + + + + + + +