EC 504 End Semester QP

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
End Semester Examination

CODE: EC 504 Reg.No:
DEGREE : B.TECH (ECE) SEMESTER: VI (2020-2021)

SUBJECT : Networks and protocols DATE : 26/05/2021
MAX.MARKS: 25 DURATION: 1.30 hrs
ANSWER ALL THE QUESTIONS

1. (A) Consider a datagram network using 8-bit host addresses. Suppose a router uses longest
prefix matching and has the following forwarding table:
For each of the four interfaces, give the associated range of destination host addresses and the
number of addresses in the range. (2.5 marks)
(B) Suppose datagrams are limited to 1,500 bytes (including header) between source Host A and
destination Host B. Assuming a 20-byte IP header, how many datagrams would be required to
send an MP3 consisting of 5 million bytes? Explain how you computed your answer.
(2.5 marks)
2. (A) Consider a router that interconnects three subnets: Subnet 1, Subnet 2, and Subnet 3.
Suppose all of the interfaces in each of these three subnets are required to have the prefix
223.1.17/24. Also suppose that Subnet 1 is required to support at least 60 interfaces, Subnet 2 is
to support at least 90 interfaces, and Subnet 3 is to support at least 12 interfaces. Provide three
network addresses (of the form a.b.c.d/x) that satisfy these constraints. (3 marks)
(B) Consider the network below.

1. Suppose that this network is a datagram network. Show the forwarding table in router A, such
that all traffic destined to host H3 is forwarded through interface 3.
2. Suppose that this network is a datagram network. Can you write down a forwarding table in
router A, such that all traffic from H1 destined to host H3 is forwarded through interface 3, while
all traffic from H2 destined to host H3 is forwarded through interface 4? (Hint: this is a trick
question.) (2 marks)
3. Consider Figure 1. Assuming TCP Reno is the protocol experiencing the behavior shown
above, answer the following questions. In all cases, you should provide a short discussion
justifying your answer. (Values in figure are approximate)
a. Identify the intervals of time when TCP slow start is operating.

b. Identify the intervals of time when TCP congestion avoidance (Additive Increase) is
operating.
c. After the 16th transmission round, is segment loss detected by a triple duplicate ACK or by a
timeout?
d. After the 22nd transmission round, is segment loss detected by a triple duplicate ACK or by a
timeout?
e. What is the initial value of ssthresh (slow start threshold) at the first transmission round?
f. What is the value of ssthresh at the 24th transmission round?
g. During what transmission round is the 70th segment sent?
h. Assuming a packet loss is detected after the 26th round by the receipt of a triple duplicate
ACK, what will be the values of the congestion window size and of ssthresh?
i.Suppose TCP Tahoe is used (instead of TCP Reno), and assume that triple duplicate ACKs are
received at the 16th round. What are the ssthresh and the congestion window size at the 19th
round?
j. Again suppose TCP Tahoe is used, and there is a timeout event at 22 nd round. How many
packets have been sent out from 17th round till 22 nd round, inclusive? (5 marks)
4. (A) Consider transferring an enormous file of L bytes from Host A to Host B. Assume an
MSS of 536 bytes.
a. What is the maximum value of L such that TCP sequence numbers are not exhausted? Recall
that the TCP sequence number field has 4 bytes.
b. For the L you obtain in (a), find how long it takes to transmit the file. Assume that a total of 66
bytes of transport, network, and data-link header are added to each segment before the resulting
packet is sent out over a 155 Mbps link. Ignore flow control and congestion control so A can
pump out the segments back to back and continuously. (3 marks)
(B) Consider Figure. What are the source and destination port values in the segments flowing
from the server back to the clients’ processes? What are the IP addresses in the network -layer
datagrams carrying the transport-layer segments? What is the four tuple used to uniquely identify
a connection? (2 marks)
5 (A) Suppose you purchase a wireless router and connect it to your cable modem. Also suppose
that your ISP dynamically assigns your connected device (that is, your wireless router) one IP
address. Also suppose that you have five PCs at home that use 802.11 to wirelessly connect to
your wireless router. How are IP addresses assigned to the five PCs? Does the wireless router use
NAT? Why or why not? (2 marks)
(B) State and Prove Little’s Theorem. (3 marks)

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DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

End Semester Examination

DEGREE : B.TECH (ECE) SEMESTER: VI (2020-2021)

ANSWER ALL THE QUESTIONS

(B) Consider the network below.

a. Identify the intervals of time when TCP slow start is operating.

(B) State and Prove Little’s Theorem. (3 marks)

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