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Question 1

a)The following are all sources of delay in IP networks. Briefly describe each (without using equations).

i)Nodal processing

ii)Queueing delay

iii)Transmission delay

iv)Propagation delay

b)A network link has a capacity of 2Gb/s. The delay is 50ms. Packets are 1500bytes. Assuming no packets are ever lost and the major delay is just propagation:

[4 marks]
i)What is the throughput if a protocol sends 1 packet and waits for a ACK before sending the next?

ii)What is the throughput if a protocol sends packets in batches of 100?

(2 marks)

(1 mark)

iii)What is the utilisation of the link in the second case?
(1 mark)

c)Say whether the following are features of TCP/UDP or both:

i)Retransmission of lost packets.

ii)A checksum to enable detection and discard of corrupted packets

iii)Connectionless sockets.

iv)Multiplexing and demultiplexing of connections from and to individual application sockets
(1 mark)

v)A port number in the header.

vi)Reliable in order transmission of packets.

(1 mark)

(1 mark)

d)Draw a diagram that shows how a packet at the application layer is sent via UDP/IP and the MAC layer across a physical network. Show which headers it adds at which layer (no need to add trailers) and how the packet is sent back up again to the application layer at the other end.
[6 marks]

e)Name (not just give the number) the layer of the OSI/ISO model responsible for:

i)Ensuring packets are delivered to the correct address within a sub-network.

ii)Multiplexing packets from different applications into the network.

iii)Ensuring a stream of packets arrive in the correct order if this is required.

iv)Carrier Sense Multiple Access/Collision Avoidance.

v)Translating the HTTP protocol.

Question marking: 4 + 4 + 6 + 6 + 5 = 25

Question 2

a)Compared with rdt2.2, what new issue is rdt 3.0 attempting to handle and what new mechanism is introduced in rdt 3.0?
[2 marks]

b)Assume there is a 500 byte long packet transmitting over a 5Mb/s connection relying on rdt 3.0. The one-way delay over the connection is 10 ms.

i)What are the throughput in bits/sec and utilisation if there are no losses?

[4 marks]

(2 marks)

ii)What are the throughput in bits/sec and utilisation for the same connection using Go-Back- N with a window size 10 with no losses?
(2 marks)

c)What are the purposes of TCP fast retransmission, TCP flow control and TCP congestion control?

[6 marks]

d)The following table represents prefixes and exits in a longest prefix first matching table. The slash after the prefix represent the length of the match

[5 marks]

Which link interface will packets with the following destination IPs be forwarded to (0, 1, 2 or 3)? i) 129.129.7.6
ii) 2.3.4.9
iii) 129.204.126.144
iv) 129.129.129.129
v) 129.130.132.129

e)An organisation is granted a classless range 122.125.192.0/20 IP addresses. The administrator wants to create 4 subnets: subnet #1 with 1500 hosts, #2 with 500 hosts, #3 with 28 hosts and #4 with 12 hosts. Assume that very little future growth of the organisation will take place and that variable- length subnet masking (VLSM) is supported. Allocate the largest subnets earliest in the range.
[8 marks]

i)Calculate the network addresses (in slash notation) of the four subnets in dotted decimal notation.

ii)Find the address ranges in each subnet.

iii)Find the first valid host address and the directed broadcast address for subnet #3.

(4 marks)

(2 marks)

(2 marks)

Question 3
a)List 3 characteristics of the distant vector routing and 3 characteristics of the link state routing.
[6 marks]

b)With reference to the topology shown in Figure 1, use Dijkstra’s algorithm to generate the Shortest Path Tree originating from router U and use it to construct the corresponding routing table. It is important to clearly show how you arrived at your answer.

Figure 1
[9 marks]
c)Software Defined Networking with OpenFlow

i)What is “match-action” in OpenFlow?

[10 marks]

(4 marks)
ii)Briefly describe the differences between the routing using match-action in Open Flow and the traditional routing using a link-state protocol.
(6 marks)
Question 4

a)Host A has host B’s IP address and wishes to know its MAC (layer 2) address.

i)Name the protocol used.

[6 marks]

(2 marks)

ii)Give (in brief) the steps needed for host A to get a reply from B stating which steps are broadcast and which unicast.

iii)How will A avoid having to always ask B for its MAC address?

(4 marks)

(1 mark)

b)These questions are about CSMA/CD

i)What does this stand for?

ii)Describe (briefly) why CSMA/CD requires frames to have a minimum size.

[15 marks]

(2 marks)

(5 marks)

iii)Calculate the size of this minimum frame in bytes, given a maximum one-way propagation delay of 25ms and a bit-rate of 8Mbps.

iv)Briefly, why is CSMA/CD most suitable for wired, not wireless environments.

(6 marks)

(2 marks)

c)What is meant by “stateful” and “stateless” packet filtering in a firewall?

[3 marks]