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BBN810S - BROADBAND NETWORKS - 2ND OPP - JULY 2024 |
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n Am I BI A u n IVER s ITY
OF SCIEnCE Ano TECHnOLOGY
FACULTYOF COMPUTING AND INFORMATICS
DEPARTMENTOF COMPUTERSCIENCE
QUALIFICATION:BACHELOROF COMPUTERSCIENCEHONOURS:COMMUNICATION
NETWORKS
QUALIFICATIONCODE: 0SBCCH
LEVEL: 8
COURSE:BROADBANDNETWORKS
COURSECODE: BBN810S
DATE: JULY2024
SESSION:THEORY
DURATION: 3 HOURS
MARl<S:100
SECONDOPPORTUNITY/ SUPPLEMENTARYEXAMINATION QUESTIONPAPER
EXAMINER($) PROFGUY-ALAINLUSILAOZODI
MODERATOR: MS HELENAHAINANA
INSTRUCTIONS
1. Answer ALLthe questions.
2. Write clearly and neatly.
3. Number the answers clearly.
THIS QUESTION PAPERCONSISTSOF 9 PAGES(Including this front page)
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Question 1 [ 10 marks]
The Telecom operator implements an IP/ MPLS / Ethernet network whose topology is given in
figure. 1. Networks A, B, C, D and E are IP's networks
Figure 2. MPLS Network
he routing tables of the routers Rl to R7 are given in the Table 1.
It is assumed that the Telecom network administrator has enabled MPLS on his network.
Equipment Rl to R7 are Label Switch Routers {LSP).(They switch packets using label. LSPsare
built on demand; that is, we wait until the path is necessary to build it. The Delta network does
not implement quality of service.
An IP packet is sent from a computer in Network A to another computer in Network D. The
packet is forwarded by router RA via the Delta Network Router Rlwith a TTL of 40. Router Rl is
the Border Router receiving the packet. It will trigger the creation of the LSPthat will route the
packets to network D. It is assumed that this LSPis the first created in the network.
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Table 1. Routing Table
Rl Roulin1 Table
Dest.
NII
Cost
R2
RS
I
R3
R7
2
R4
R7
2
RS
RS
I
R6
RS
2
R7
R7
I
Network RA
1
A
Network RS
3
13
Network R7
4
('
Network R7
3
D
Network R7
3
E
R2 Routim Table
Dest.
NII
Cost
RI
RS
2
R3
R6
2
R4
R6
2
RS
RS
I
R6
R6
I
R7
RS
2
Network RS
3
A
Network RB
I
B
Network R6
4
C
Network R6
3
I)
Network R6
3
E
R3 Routin~ Table
Dest.
NH
Cost
RI
R7
2
R2
R6
2
R4
R6
2
RS
R6
2
R6
R6
I
R7
R7
1
Network RI
3
Network R4
3
13
Network R4
I
C
Network RI
3
D
Network R4
3
E
R4 Routin1 Table
Dest.
NH
RI
R7
R2
R6
R3
R6
R4
R7
RS
R6
R6
R7
Network R7
A
Network R6
13
Network R6
C
Network RD
D
Network RE
E
R7 Routin1 Table
Dest.
NI-I
RI
RI
R2
RS
R3
R3
R4
R4
RS
RS
R6
R6
Network RI
A
Network RS
B
Network R3
C
Network R4
D
Network R4
E
Cost
20
10
10
-
10
10
20
20
20
20
10
Cost
I
2
I
I
I
I
2
3
2
2
2
RS Routin1 Table
Dest.
NH
Cost
RI
RI
30
R2
R2
20
R3
R6
20
R4
R6
10
R6
R6
-
R7
R7
20
Network RI
20
A
Network R2
10
B
Network R6
30
C
Network R6
40
D
Network R6
10
E
R6 Routim Table
Dest.
NH
Cost
Rl
RS
30
R2
R2
20
lU
R3
20
R4
R4
10
RS
RS
20
R6
R7
-
Network RI
10
A
Network R2
30
n
Network R3
30
C
Network R4
40
D
Network R4
10
E
a)
Which router will choose the label to use on the LSP link at the exit of 2 marks
Rl (LSPRl to R7) towards Network D?
b)
Which LSRsent the packets to router R4 for the communication from Rl 2 marks
to D?
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c)
Which next hop is associated with these packets in the switching table 2 marks
of R4?
d)
The switching table in Rl contains the following line:
2 marks
Entry Label
D
Next host
R7
Release Label
5
In case the IP packet is to leave the Delta network, which router pops the
MPLS header?
e)
If packets leaving router R7 are labelled 9, draw the switching table of 2 marks
router R7.
Question 2 [ 10 marks]
Consider the Figure 2. Suppose that the video is encoded at a fixed bit rate, and thus each video
block contains video frames that are to be played out over the same fixed amount of time of 1
second. The server transmits the first video block at t0=0sec, the second block at t=l sec, the
third block at t=2 sec and so on. Once the client begins playout, each block should be played out
lsecond after the previous block
,_ 9
8
E::, 7
C6
'tJ 5
ll
03
>
2
1
Constant bit
rate video
transmission
by server•-..___
Video
recGption
at client~-
Time
Figure 2. Video streaming transmission
a)
Suppose that the client begins playout as soon as the first block arrives 3 marks
at tl. In the figure below, how many blocks of video {including the first
block) will have arrived at the client in time for their playout? Explain
how you arrived at your answer.
b)
Suppose that the client begins playout now at t1+1. How many blocks 3 marks
of video {including the first block) will have arrived at the client in time
for their playout? Explain how you arrived at your answer.
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c)
In the same scenario at (b) above, what is the largest number of blocks 2 marks
that is ever stored in the client buffer, awaiting playout? Explain how
you arrived at your answer.
d)
What is the smallest playout delay at the client, such that every video 2 marks
block has arrived in time for its playout? Explain how you arrived at your
Question 3 [ 10 marks]
In TCPthe Retransmit Time-Out (RTO)is determined on the basis of RTTobservations for arriving
ACl<s.
a)
Measured RTTvalues are subject to considerable fluctuations. List three 3 marks
reasons that explain this phenomenon.
b)
What is an optimal value of RTOfor a TCPconnection? Explain
3 marks
c)
What are the implications of a RTOthat is too large or too small?
4 marks
Question 4 [ 12 marks]
Compare Go-Back-N (GBN), Selective Repeat, and TCP (no delayed ACK). Assume that the
timeout values for all three protocols are sufficiently long such that 5 consecutive data segments
and their corresponding ACKs can be received (if not lost in the channel) by the receiving host
(Host B) and the sending host (Host A) respectively. Suppose Host A sends 5 data segments to
Host B, and the 2nd segment (sent from A) is lost. In the end, all 5 data segments have been
correctly received by Host B.
a)
How many segments has Host A sent in total and how many ACKshas Host 6 marks
B sent in total? What are their sequence numbers? Answer this question for
all three protocols
b)
If the timeout values for all three protocol are much longer than 5 RTT,then 6 marks
which protocol successfully delivers all five data segments in shortest time
interval?
Question 5 [ 10 marks]
a)
What percentage of an ATM link's total bandwidth is consumed by the ATM 3 marks
cell headers? [1 mark]
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b)
The IP-datagram for a TCPACI<message is 40 bytes long: it contains 20 bytes 3 marks
of TCPheader and 20 bytes of IP header. Assume that this ACI<is traversing
an ATM network that uses AALS to encapsulate IP packets. How many ATM
packets will it take to carry the ACI<? [2 marks]
c)
Cell switching methods essentially always use virtual circuit routing rather 4 marks
than datagram routing. Give two arguments why this is so.
Question 6 [22 marks]
a)
Consider a TCP connection between Host A and Host B. Suppose that the 2 marks
TCP segments travelling from Host A to Host B have source port number x
and destination port number y. What are the source and destination port
numbers for the segments travelling from Host B to Host A?
b)
Suppose two hosts, A and B, are separated by 10,000 kilometers and are 3 marks
connected by a direct link of bandwidth equal to 1 Mbps. Suppose the
propagation speed over the link is 2.5x108 meters/sec.
What is the maximum number of bits that can be in the link at any given
time?
c)
In packet switching, differentiate between networks with virtual circuits 2 marks
(VCs) and networks with datagram
d)
What characteristic of the network would you care most about to get good 6 marks
performance for the following activity: hint (jitter, low delay and high
bandwidth). Explain your answer
(1) Navigating a predominantly text-011/ywebsite.
(2) Downloading <t large video file
(3) A remote shell application (such as telnet or SSH).
e)
Emerging technologies used for better network survivability include ATM. 1 marks
What does ATM stand for?
f)
Why does the TCPcongestion control mechanism have a "slow-start" and a 2 marks
"congestion avoidance" phase?
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g)
In figure 3, A, B, C and D are Autonomous Systems (AS's) that talk to each 2 marks
other using an external routing protocol.
Figure 3. AS Topology
(A,B)and (C,D)are peering relationships, while B provides transit service to
C and D. Which path, traffic from D to B cannot follow to reach autonomous?
h)
Describe the role and significance of Autonomous System {AS) numbers in 4 marks
he context of internet routing. Detail how AS numbers are assigned and
utilized in the operation of Border Gateway Protocol {BGP).
Question 7 [ 8 marks]
Suppose that a router has three input flows and one output port. It receives packets continuously
as per table below, with all flows beginning at the same time and queues being empty before the
arrival of the first packet. Packets order in each separate flow is listed in the table {packets 1 and
2 are the first to arrive). Length represents the number of clock ticks it takes to transmit a packet
Packet JD
Pl
P2
P3
P4
PS
P6
P7
Flow
1
1
2
2
2
3
3
Length
1000
1000
600
400
400
200
200
Arrival
time
0
0
800
800
800
1200
2100
a)
Determine the order in which packets are transmitted by the router if 4 marks
Fair queuing is used.
b)
Determine the order in which packets are transmitted by the router if 4 marks
weighted Fair queuing is used, with flow 1 has a weight of 2, and flow 2
and 3 each a weight of 1.
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Question 8 [ 6 marks]
In the topology in Figure 4, A, B, C and Dare different hosts that all wish to send at the maximum
possible rate to host E.The arrows are links, and the numbers denote link capacities. The network
operator would like to assign to hosts the transmission rates that satisfy the max-min fairness.
Note that an allocation is max-min fair if you cannot increase the rate of one flow without
decreasing the rate of another flow with lower rate.
2Mb/s
B
8Mb/s
C
10Mb/s
D
Figure 4. Network Topology
(a) How should transmission rate be assignedto nodes A, B, C and D so as to satisfy the 4 marks
max-min fairness principle?
(b) You are hired to design a reliable byte-stream protocol that uses sliding 2 marks
window (like TCP). This protocol will run over a 100 Mbps network link. The RTT
of the network is l00ms, and the maximum segment lifetime is 60sec.
Question 9 [ 8 marks]
Suppose you are designing a sliding window protocol for a 1 Mbps p-to-p link to the moon, which
has a one-way latency of 1.25 seconds. Assuming that each frame carries 1 KB of data
(a)
Find the window size of the protocol based on the bandwidth-delay 4 marks
product
(b)
What is the minimum number of bits you need for the sequence number 2 marks
field
(c)
Calculate the round-trip time (RTT) for transmitting a frame from Earth 2 marks
to the moon and back, considering the given one-way latency of 1.25
seconds.
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Question 10 [ 14 marks]
The Transmission Control Protocol uses a method called congestion control to regulate the traffic
entering the network. The behavior of TCP congestion control can be represented as a graph in
which the x-axis indicates the time, and the y-axis indicates congestion window size. Please use
the graph shown below to answer the following questions. Note that the graph does not explicitly
show timeouts, but you should be able to figure out when timeouts happened based on the
events shown.
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TransmissionRound
a)
Give two reasons why slow start is used, and explain why it does a better 2 marks
job than congestion avoidance
b)
Identify the intervals of time when TCP slow start is operating. For each 2 marks
interval, identify which of the above
c)
Identify the intervals of time when TCP congestion avoidance is 2 marks
operating. Why congestion avoidance should be used instead of slow
-
.
d)
Identify the intervals of time when TCP fast retransmission is used. Please 2 marks
explain what fast retransmission does
e)
Identify the intervals of time when TCP fast recovery is operating. What 2 marks
does fast recovery do and explain why is
f)
Identify the interval(s) of time when fast recovery could have happened, 2 marks
but did not. Identify one specific
g)
Which version of TCP is represented in this Figure?
2 marks
End of Examination==========
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