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BBN810S - BROADBAND NETWORKS - 1ST OPP - JUNE 2024 |
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nAmlBIA unlVERSITY
OF SCIEn CE Ano TECHn OLOGY
FACULTYOF COMPUTING AND INFORMATICS
DEPARTMENTOF COMPUTERSCIENCE
QUALIFICATION:BACHELOROF COMPUTERSCIENCEHONOURS: COMMUNICATION
NETWORKS
QUALIFICATIONCODE: 0SBCCH
LEVEL:8
COURSE:BROADBANDNETWORKS
COURSECODE: BBN810S
DATE: JUNE 2024
SESSION:THEORY
DURATION: 3 HOURS
MARKS: 100
FIRSTOPPORTUNITYEXAMINATION QUESTIONPAPER
EXAMINER(S) PROFGUY-ALAINLUSILAO2001
MODERATOR: MS HELENAHAINANA
INSTRUCTIONS
1. Answer ALLthe questions.
2. Write clearly and neatly.
3. Number the answers clearly.
THIS QUESTIONPAPERCONSISTSOF 9 PAGES(Including this front page)
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Question 1 [18 marks]
a) In TCPflow control, what is the relationship between segment sequence 4 Marks
numbers and acknowledgement numbers? Use a random sequence
number for your answer.
b) Many Internet companies today provide streaming video, including 2 Marks
YouTube, Netflix, and Hulu. Name any two distinguished features that
characterize streaming stored video?
c) List two advantages of using optical fiber in contrast to the conventional
copper cable links.
2 Marks
d) What are the two advantages of using small, fixed-length cells in ATM
2 Marks
e) Explain the main difference between ADSLand VDSLused for broadband
communication. Which of these two technologies give better performance
in terms of speed?
3 Marks
f) Why is round time an appropriate timescale for frame retransmission?
3 marks
g) Name two factors that can cause variance in the network round-trip time
2 marks
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Question 2 [ 16 marks]
Consider two hosts A and B that communicate via a router R. The linl<"(A, R) has infinite
bandwidth: packets experience no delay on this link. The link (R, B) has finite ban.dwidth:
the packet transmission rate on the link (R,B) is 1 packet per second. However,
acknowledgements from B to Rare sent instantaneously, as are ACKs from R to A. Host A
sends data to host B over a TCP connection using slow start but with an arbitrarily large
window size. Router R has a queue size of 1 packet in addition to the packet that router R
is transmitting on the outbound link (R, B). Consider the timeline illustrated in Figure 1.
Time advances in units of 1 second. At each second the sender first processes any arriving
ACl(s and then responds to any timeouts. Assume that the timeout period Tis 2 seconds.
Figure 1 shows the items sent and received during the period T = 1, 2... 7 seconds.
Note that in Figure 1, it is assumed that when TCP encounters a timeout it reverts to stop-
and-wait as the outstanding lost packets in the existing window are re-transmitted one at
a time, and that the slow start phase will begin again only when the existing window is fully
acknowledged. Note also that once a timeout and re-transmission is pending, subsequent
timeouts of later packets are ignored until their earlier acknowledgment is received.
Answer the following questions concerning the timeline illustrated in Figure 1.
Time A receives
T
A sends
R
sends
cwnd
0
dataO (slow start)
dataO 1
1
ackO
2
ackl
3
ack2
4
ack3 (timeout data4)
datal, data2
datal 2
data3, data4 (data4 dropped) data2 3
dataS, data6 (data6 dropped) data3 4
data4
dataS 1
5
ack3 (timeout dataS, data6)
data4 1
6
ackS
data6
data6 1
7
ack6
data 7, data8 (slow start)
data7 2
Figure 1. TCPTimeline
a)
At time T = 2: why is packet data4 dropped?
2 Marks
b)
At time T = 2: why are only 2 packets data3 and data4 sent even though 2 Marks
cwnd=3?
c)
At time T = 3: why are only 2 packets data5 and data6 sent even though 2 Marks
cwnd=4?
d)
At time T = 3: why is data6 dropped?
2 Marks
e)
At time T = 4: why does a timeout occur?
2 Marks
f)
At time T = 5: why does A receive ack3 even though data5 was received 2 Marks
at Bat the previous step?
g)
At time T = 5: why do 2 timeouts occur?
2 Marks
h)
At time T = 6: why does A receive ack5 even though data4 was received 2 Marks
at Bat the previous step?
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Question 3 [ 8 marks]
Suppose that a router has three input flows and one output port. It receives
packets continuously as per Table 1, with all flows beginning at the same time
and queues being empty before the arrival of the first packet. Packet order in
each separate flow is listed in the table (packets 1, 5 and 7 are the first to arrive).
Length represents the number of clock ticks it takes to transmit a packet.
Table 1. Router Input Flows
Packet id
1
2
3
4
5
6
7
8
Length
100
110
50
160
80
240
90
180
Flow
1
1
1
2
2
2
3
3
a)
Fair queuing is used.
4 Marks
b)
Weighted fair queuing with flow 2 having twice as much share as flow 1, 4 Marks
and flow 3 having 1.5 times as much share as flow 1.
Question 4 [ 6 marks]
One difficulty with the round-trip time estimator (sRTT) in TCP is the choice of
an initial value. If the absence of any special knowledge of network conditions,
the typical approach is to pick an arbitrary value, and hope that this will
converge quickly to an accurate value. Assume there is no loss of segments in
the network, and that the initial value of sRTT is 100. 75 ms, the first
acknowledgment for the TCP connection have been returned with RTT for the
first ACK =120 ms with an estimated standard deviation measurement of 5.06.
Determine the value of the timeout after the acknowledgments using RFC793
formula with a= 0.175 and y =0.25 for the standard deviation
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Question 5 [ 8 marks]
a)
Consider 10 flows passing through a Fair Queue (FQ) router with an 4 Marks
outgoing link running at 100Mbps. Five of the flows are part of a file backup
service and can each fill the link if they are allowed to. The other five are
video streams running at 2Mbps. Given that the router is the bottleneck
'or all the flows, how fast do the flows operate?
b)
Assume a link of capacity 10 Mbps that is traversed by four flows with 4 Marks
arrival rates of 6, 4, 2, and 1 Mbps, respectively. What bandwidth will
each flow get? (Show all your work.)
Question 6 [ 12marks]
Consider the IPnetwork shown in Figure 2. Routers Rl to R6belong to a network using MPLS(Rl to R6 are
LSRs).These routers connect networks A, B, C and D. The topology links, whether internal to the MPLS
network or external, have a capacity of 1 Gb/s. We are interested in communications to C and D and have
the following information: Rl has announced to R2 and R3 that it can route packets but only to network
D; R2 has announced to R4 that it can route packets to Network C and Network D; R3 has announced to
R4 that it can route packets to Network D; R4 has announced to RSand R6that it can route packets to C
and D. We know the routing tables of routers Rl to R6.
Rl RoutingTable
Dest.
NH
Rl
-
R2
R2
R3
R3
R4
R2
RS
R2
R6
R2
A
R2
B
R2
C
R2
D
D
Default R2
R4RoutingTable
Dest. NH
Rl
R3
R2
R2
R3
R3
R4
-
RS
RS
R6
R6
A
R6
B
RS
C
R2
D
R3
Default R2
Cost
-
10
10
10
10
10
10
10
10
10
10
Cost
20
10
10
-
10
10
20
20
20
20
10
R2RoutingTable
Dest. NH Cost
Rl
Rl
10
R2
-
-
R3
R4
20
R4
R4
10
RS
R4 20
R6
R4 20
A
R4
30
B
R4 30
C
C
10
D
Rl
20
Default R4
10
RSRoutingTable
Dest. NH Cost
Rl
R4
30
R2
R4 20
R3
R4
20
R4
R4
10
RS
-
-
R6
R4
20
A
R4
20
B
B
10
C
R4 30
D
R4 40
Default R4
10
R3RoutingTable
Dest. NH
Rl
Rl
R2
R4
R3
-
R4
R4
RS
R4
R6
R4
A
Rl
B
R4
C
R4
D
Rl
Default R4
R6RoutingTable
Dest. NH
Rl
R4
R2
R4
R3
R4
R4
R4
RS
R4
R6
-
A
A
B
R4
C
R4
D
R4
Default R4
Cost
10
20
-
10
20
20
30
30
30
20
10
Cost
30
20
20
10
20
-
10
30
30
40
10
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We have several information on the labels used:
R4 switching table:
(FEC)
(FEC A)
(FEC B)
(FEC C)
(FEC D)
label
IN
6
2
7
3
label OUT Next Hop
1
R6
5
RS
8
R2
10
R3
Hypotheses: The network operator is sought to minimize the number of labels used
and to group streams having the same FECas much as possible by assigning.them the
same label when possible.
The edge LSRsdo the POPs.
The packets destined for A received by R2 carry a label equal to 12.
The packets destined for B received by R2 carry a label equal to 5.
The packets destined for C received by R2 carry a label equal to 8.
The packets destined for D received by R2 carry a label equal to 10.
When R1 receives a packet with an MPLS label equal to 6, it is bound for D.
Complete the forwarding table of R6.
1 mark per
correct
(FEC)
(FEC A)
(FEC B)
(FEC C)
(FEC D)
label
IN
label OUT Next Hop
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Question 7 [ 6 Marks]
The measurements available on the CAIDAAS Rank ranking site reveal the properties
of three autonomous systems (ASs), Level3, CTGNet (China Telecom) and l(DC (Cona
Hosting). On the CAIDA web page corresponding to the three ASs (see Tablel), we
can identify the number of neighbours under the heading "AS degree" and the term
"global". These neighbours can be Internet service providers, AS with a peering
agreement, or customers. Looking at the information providing on the table of each
of the AS, answer the questions below.
CAIDA DATA FOR Level 3
AS number
3356
AS Name
Level3
Organisatio
Level 3 Parent, LLC
n
Country
USA
AS rank
1
Customer
cone
48548
asn
740857
prefix
191119864
address
AS degree
6322 global
63220
transit
0
provider
67
I 6255
peer
customer
I
CAIDA DATA FORCHINA TELECOM GLOBAL LIMITED
AS number
23764
AS name
CTGNet
Ogarnisatio
China Telecom Global limited
n
Country
Hong Kong
AS rank
148
Customer
346
28846
123786317
cone
asn
prefix
address
AS degree
637
global
632
transit
13
provider
546
peer
178
customer
I
CAIDA DATA FORCONA HOSTING SON BHD
AS number
136209
AS Name
KOC-AS-AP
Ogarnisatio
CONA HOSTING SON BHD
n
Country
Malaysia
AS rank
16025
Customer
1
50
12800
cone
asn
prefix
address
AS degree
S global
0
transit
5
provider
0
peer
I ~ustomer
I
a)
By observing the number of neighbours of Leve13, and their types, 2 Marks
indicate whether Level3 is a Tiers 1, Tiers 2 or Tiers 3
b)
By observing the number of CTGNET (China telecom Global Limited) 2 Marks
neighbours and their types on table 1, indicate whether CTGNET is a Tiers
1, Tiers 2 or Tiers 3.
c)
By observing the number of neighbours of CONA and their types, indicate 2 Marks
whether CTGNETis a Tiers 1, Tiers 2 or Tiers 3.
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Question 8 [ 12 marks]
a)
Give an example of an application-level requirement that might take 3 Marks
advantage of the drop priority field in an ATM cell? List two reasons
explaining your answer.
b)
For ATM networks to operate as part of IP networks, ATM needs to 2 marks
provide a form of IP addresses resolution using ARP protocol. However,
the ARP procedure as described in an IP network cannot work because
it depends on the fact that packets can be broadcast to all hosts in a
single network. For this reason there is a different procedure of ARP
that is defined for ATMs network and known as the ATMARP protocol.
Under ATMARP, explain how does the ARP server learn the physical
network addresses of the hosts in its subnet?
c)
What percentage of an ATM link's total bandwidth is consumed by all 3 marks
no payload bits in AAL5 (ATM Adaptation Layer 5} when the user data is
512 bytes long?
d)
The IP-datagram for a TCP ACK message is 40 bytes long: it contains 20 4 marks
bytes of TCP header and 20 bytes of IP header. Assume that this ACK is
traversing an ATM network that uses AAL5 to encapsulate IP packets.
How many ATM packets will it take to carry the ACK? [2 marks]
Question 9 [ 9 marks]
Consider a VC network with a 2-bit filed for the VC number. Suppose that the network wants
to set up a virtual circuit over four links: link A, link B, link C and link D. Suppose that each of
those links is currently carrying two other virtual circuits, and the VC numbers of these other
VCs are as follows:
Link A
Link B
Link C
Link D
00
01
10
11
01
10
11
00
Assume that each of the existing VCs may only traverse one of the four links.
a)
If each VC is required to use the same VC number on all the four links 3 marks
along its path, what VC number could be assigned to the new VC?}
b)
Give one reason why packets do not keep the same VC number on each 2 marks
of the links along its route
c)
If each VC is permitted to have a different VC number in the different 4 marks
links along its path, how many different combinations of four VC
numbers (one for each of the four links} could be used?
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Question 10 [ 4 marks]
..,..._. : Peering
_.
:Transit
IRISA
a)
Looking at the topology in Figure 4 and the agreement between the ISPs. 2 marks
For a communication between IRISA and lnteroute, what are the paths
which respect the agreements between the autonomous systems?
b)
According to the topology in Figure 4, what are the two implications of 2 marks
the agreement between Claranet and RENATER?
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