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BBN810S - BROADBAND NETWORKS - 1ST OPP - JUNE 2023 |
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nAmlBIA UnlVERSITY
OF SCIEn CE Ano TECHn OLOGY
FACULTY OF COMPUTING AND INFORMATICS
DEPARTMENT OF COMPUTER SCIENCE
QUALIFICATION: BACHELOR OF COMPUTER SCIENCE HONOURS: COMMUNICATION
NETWORKS
QUALIFICATION CODE: 08BCCH
LEVEL: 8
COURSE: BROADBAND NETWORKS
COURSECODE: BBN810S
DATE: JUNE 2023
SESSION:THEORY
DURATION: 3 HOURS
MARKS: 100
EXAMINER{S)
FIRST OPPORTUNITY EXAMINATION QUESTION PAPER
PROF GUY-ALAIN LUSILAO ZODI
MODERATOR: MS ESNA MANGUNDU
INSTRUCTIONS
1. Answer ALL the questions.
2. Write clearly and neatly.
3. Number the answers clearly.
THIS QUESTION PAPER CONSISTS OF 13 PAGES (Including this front page)
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QUESTION 1 [15 marks)
a) Give an example of an application-level requirement that might take
advantage of the drop priority field in an ATM cell? List three reasons
explaining your answer.
[2 marks]
b) Why will two ISPsat the same level of the hierarchy often peer with each [2 marks]
other?
c) How does an IXP earn money?
[2 marks]
d) Name four features of Multi-Protocol Label Switching explaining your [4 marks]
answer.
e) Many Internet companies today provide streaming video, including
YouTube, Netflix, and Hulu. What are the three features that characterized
streaming video applications?
[3 marks]
f) Since 2009, AS numbers are coded using the X.Yformat, where X and Y are [2 marks]
integer's number coded with 16-bits each. With this representation, how
many AS numbers are possible?
QUESTION 2 [8 marks)
We consider a set of autonomous systems as shown in the figure 2. The
relationships between these autonomous systems are of the peering or transit
type (customer to provider). The letters a to q represent any IP equipment on the
networks of each of these autonomous systems. We assume that only the type of
service agreement between autonomous systems governs routing: no other
decision criteria are taken into account in the routing selection. Looking at the
topology in figure 2, answer the questions below. Note that a question may have
more than one answer.
2
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KoreaTelecom
/ Twiner
...
Pccfing
---
Client-> Provider
1) Looking at the network topology, and the service agreement between Korea
Telecom and Sprint, what can deduct?
Korea Telecom is billed for the volume of traffic sent and
received from Sprint.
[2 mark]
Sprint is billed for the volume of traffic sent and received
from Korea Telecom.
Traffic exchanged between Korea Telecom and Sprint is not
billed.
Sprint routes traffic from Korea Telecom to the rest of the
Internet.
2) According to the proposed network topology, and the service agreement between
Sprint and TeliaSonera, what can you deduct
[2 marks]
Sprint is billed for the volume of traffic exchanged with
TeliaSonera.
TeliaSonera routes traffic from Sprint to Cogent.
TeliaSonera routes traffic from Sprint to Twitter.
The exchange of traffic between Sprint and TeliaSonera is not
invoiced.
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3) For a communication from Korea Telecom to Twitter, what are the paths that [2 mark]
respect the service agreements between the autonomous systems?
Korea Telecom -> Sprint -> TeliaSonera-> Twitter
Cogent-> Sprint->Cogent->TeliaSonera->Twitter
Cogent-> Sprint->Cogent->SFR->TeliaSonera->Twitter
Korea Telecom->Sprint-> TeliaSonera-> Cogent-> Twitter
4) For a communication from Twitter to Cogent, what is the path that respects the [2 marks]
service agreements between the autonomous systems?
Twitter-> TeliaSonera-> Cogent
Twitter-> TeliaSonera -> Sprint -> Cogent
Twitter-> TeliaSonera-> SFR -> Cogent
Twitter-> TeliaSonera-> SFR-> SFR ->Sprint-> Cogent
QUESTION 3 (6 marks)
The measurements available on the CAIDA AS Rank ranking site reveal the
properties of three autonomous systems (ASs), Level3, Renater and lrisa. On the
CAIDA web page corresponding to the three ASs (see Table 1), 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 following questions.
CAIDA DATA FOR Level 3
AS number 3356
AS Name
Level3
Organisation Level 3 Parent, LLC
Country
USA
AS rank
I
Customer
cone
48548
asn
740857
prefix
191 I 19864
address
AS degree
6322
~lobal
63220
transit
0
orovider
67
I 6255
I
peer
custo~1cr
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CAIDA DATA FOR RENATER
AS number 2200
AS name
FR-RENATER
Ogamisation RENATER
Country
European Union
AS rank
683
Customer 51
cone
asn
296
prefix
1074176
address
AS degree
81
global
80
transit
4
provider
CAIDA DATA FOR IRISA
AS number 1938
AS Name FR-RENATER-IRISA
Ogamisation RENATER
Country
European Union
AS rank
13236
Customer I
cone
asn
I
prefix
65536
address
AS degree
I
ulobal
0
transit
I
orovider
I I 38
3
peer
:Ustomcr
I I 0
oeer
0
customer
a) In which Tier will you categorize AS Level37
b) In which AStier will you categorize ASRENATER?
c) In which ASTier will you categorize AS!RISA?
[2 marks]
[2marks]
[2 marks]
QUESTION 4 [8 marks)
Consider the figure below. 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
QJ
..a B
E:::, 7
C6
-ti
0
5
..c 4
0 3-
<l.l
"O
2
5
Constant bit
rate video
transmission
by server-----.____
Video
reception
at client"'
""-
111 1 1 1
to
t,
Time
Figure 1. Video streaming transmission
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a) Suppose that the client begins playout as soon as the first block arrives 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.
[2 marks]
b) Suppose that the client begins playout now at tl+l. 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.
[2 marks]
c) In the same scenario at (b) above, what is the largest number of b.locks
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
block has arrived in time for its playout? Explain how you arrived at your
answer.
[2 marks]
[2 marks]
QUESTION 5 (5 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. Packets in each separate
flow are listed in the order they are received at the router. For example, packets
1, 3 and 6 are the first to arrive .. Length represents the number of clock ticks it
takes to transmit a packet.
Table 1. Queuing Router
Packet id Length Flow
1
200
1
2
200
1
3
160
2
4
120
2
5
160
2
6
210
3
7
150
3
8
90
3
6
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Determine the order in which packets are transmitted by the router if Weighted [Smarks]
fair queuing is used, with flow 2 having weight 2, and the other two with weight 1:
QUESTION 6 [8 marks]
a) Assume that an application transmits a 100-byte IP to an ATM network over
a local loop using the ADSL protocol stack. At the ATM network, if the edge
switch that process the packet uses the ATM Adaptation Layer 3/4, how
many ATM cells are transmitted?
[4 marks]
b) What percentage of an ATM link's total bandwidth, the payload bits [4 marks]
consumes in ATM networks?
QUESTION 7 [5 marks]
You are hired to design a reliable byte-stream protocol that uses a sliding window
(like TCP). This protocol will run over a 100-Mbps network. The RTTof the network
is 100 ms, and the maximum segment lifetime is 60 seconds.
(a) How many bits would you include in the AdvertisedWindow of your protocol
header?
(b) How many bits would you include in SequenceNum fields, assuming a
minimum packet size of 40 bytes?
[3 marks]
[2 marks]
QUESTION 8 [ 11 marks]
The Google network operator implements an IP/ MPLS / Ethernet network whose
topology is given in figure 2. Networks A, B, C, D and E are IP's networks.
Google Network
Figure2. MPLSNetwork
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The routing tables of the routers Rl to R7 are given in the Table 2.
Table 2. Routing Table
RI Routin Table
Dest.
NH Cost
R2
RS
I
R3
R7
2
R4
R7
2
RS
RS
I
R6
RS
2
R7
R7
I
Network RA
I
A
Network RS
3
B
Network R7
4
C
Network R7
3
D
Network R7
3
E
R4 Routing Table
Dest.
NH Cost
RI
R7
20
R2
R6
10
R3
R6
10
R4
R7
-
RS
R6
JO
R6
R7
10
Network R7
20
A
Net\\vork R6
20
B
Network R6
20
C
Network RD 20
D
Network RE
JO
E
R7 Routin Table
Dest.
NH
Cost
RI
RI
I
R2
RS
2
R3
R3
I
R4
R4
I
RS
RS
I
R6
R6
I
Network RI
2
A
Network RS
3
B
Nehvork R3
2
C
Network R4
2
D
Net\\vork R4
2
E
R2 Routin Table
Dest.
NH 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
D
Network R6
3
E
RS Routin, Table
Dest.
NH Cost
RI
RI
30
R2
R2
20
R3
R6
20
R4
R6
JO
R6
R6
-
R7
R7
20
Network RI
20
A
Net\\'lork R2
10
B
Network R6
30
C
Network R6
40
D
Network R6
10
E
R3 Routin Table
Dest.
NH Cost
RI
R7
2
R2
R6
2
R4
R6
2
RS
R6
2
R6
R6
I
R7
R7
I
Network RI
3
Net\\'lork R4
3
B
Network R4
I
C
Net\\'lork RI
3
D
Network R4
3
E
R6 Routing Table
Dest.
NH Cost
RI
RS
30
R2
R2
20
R3
R3
20
R4
R4
10
RS
RS
20
R6
R7
-
Network RI
10
A
Network R2
30
B
Network R3
30
C
Network R4
40
D
Network R4
10
E
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It is assumed that the Google network administrator has enabled MPLS on his
network. Equipment Rl to R7 are Label Switch Routers (LSP). LSPsare built on
demand; that is, we wait until the path is necessary to build it. The Google 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 Google network router Rl with 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.
a) Which router will decide the label to use when a packet exits the LSPlink
Rl -R7 towards Network D?
[2 marks]
b) 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 Google network, which router pops
the MPLS header?
c) Which LSRsent the packets to router R4 for the communication from Rl
to D?
[2 marks]
d) Which next hop is associated with these packets in the switching table of
R4?
[2 marks]
e) If packets leaving router R7 are labelled 9, draw the switching table of
router R7.
[3 marks]
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QUESTION 9 [ 12 marks]
a) In the network topology in figure 3, A, B, C and D are different hosts that
all want to send traffic at the maximum possible rate to host E.The arrows
are links, and the numbers on the arrows represent the capacities of the
links. How should the network administrator allocate the transmission rate
to nodes A, B, C and D to satisfy the max-min fairness principle? [Hint: 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.
[B marks]
B
C
D
b) Assume a link of capacity 10 Mbps that is traversed by four flows with
arrival rates of 6, 4, 2, and 1 Mbps, respectively. How much bandwidth will
each flow get if the max-min principle is applied? (Show all your
calculations.)
[4 marks]
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QUESTION 10 [10 marks)
Using the network in Figure 2, give the virtual circuit tables for all the switches
after each of the following connections is established. Assume that the sequence
of connections is cumulative; that is the first connection is still up when the second
connection is being established and so on. Also assume that the VCI assignment
always picks the lowest unused VCI on each link, starting with 0.
1
Switch 1
0
0
Ho:.t H
Host I
Figure 2. Virtual Circuit Network
1) Host D connects to host H (2 marks)
2) Host B connects to host G (2 marks)
3) Host F connects to host A (2 marks)
4) Host H connects to host C (2 marks)
5) Host I connects to host E (2 marks)
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QUESTION 11 [16 marks]
Consider two hosts A and B that communicate via a router R. The link (A, R) has
infinite bandwidth: packets experience no delay on this link. The link (R, B) has
finite bandwidth: 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 the Table 1. Time advances in units of 1 second.
At each second the sender first processes any arriving ACKsand then responds to
any timeouts. Assume that the timeout period T is 2 seconds. Figure 1 shows the
items sent and received during the period T = 1, 2... 7 seconds.
Note that in this table, it is assumed that when TCPencounters 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 too 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 Table 1. [16
marks].
Time
T
0
A receives
Table 1. TCPTimeline
A sends
dataO (slow start)
R
sends
dataO
cwnd
I
I
ackO
2
ackl
3
ack2
4
ack3 (timeout data4)
data I, data2
data I 2
data3, data4 (data4 dropped) data2 3
data5, data6 (data6 dropped) data3 4
data4
data5 I
5
ack3 (timeout data5, data6)
data4 I
6
ack5
data6
data6 I
7
ack6
data7, data8 (slow start)
data7 2
a) At time T = 2: why is packet data4 dropped?
b) At time T = 2: why are only 2 packets data3 and data4 sent even though
cwnd=3?
[2 marks]
[2 marks]
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c) At time T = 3: why are only 2 packets data5 and data6 sent even though
cwnd=4?
d) At time T = 3: why is data6 dropped?
e) At time T = 4: why does a timeout occur?
f) At time T = 5: why does A receive ack3 even though data5 was received at
B at the previous step?
g) At time T = 5: why do 2 timeouts occur?
h) At time T = 6: why does A receive ack5 even though data4 was received at
Bat the previous step?
[2 marks]
[2 marks]
[2 marks]
[2 marks]
[2 marks]
[2 marks]
Question 12 (6 marks)
a) A client initiates a TCP connection setup with a server by sending a SYN
message, and immediately dies (crashes). A while later, the SYN message
reaches the server, and it sends a SYNACK in response. Is the TCP
connection now established from the server's point of view? Explain.
[2 marks]
[2 marks]
b) What is the difference between the flow control and congestion control
mechanisms provided by TCP?
c) Why does the TCP congestion control mechanism have a "slow-start" and a [2 marks]
"congestion avoidance" phase?
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