THE UNIVERSITY OF SUSSEX
BSc and MComp SECOND YEAR EXAMINATION May/June 2019 (A2)
Computer Networks
Assessment Period: May/June 2019 (A2)
DO NOT TURN OVER UNTIL INSTRUCTED TO BY THE LEAD INVIGILATOR
Candidates should answer TWO questions out of THREE. If all three questions are attempted only the first two answers will be marked.
The time allowed is TWO hours.
Each question is worth 50 marks.
At the end of the examination the question paper and any answer books/answer sheets, used or unused, will be collected from you before you leave the examination room.
Candidate Number
G5115
G5115
1. (a)
Computer Networks
The following output is produced by running ¡®traceroute www.google.com¡¯ in the command line.
1 139.184.0.14 (139.184.0.14) 0.854 ms
2 146.97.146.181 (146.97.146.181) 2.455 ms
3 ae1-0.aldess-rbr1.ja.net (146.97.68.46) 3.540 ms
4 ae23.londpg-sbr2.ja.net (146.97.37.249) 4.878 ms
5 ae29.londhx-sbr1.ja.net (146.97.33.1) 5.599 ms
6 193.62.157.22 (193.62.157.22) 5.579 ms
7 108.170.246.144 (108.170.246.144) 5.991 ms
8 …
i. What is traceroute used for? [5 marks]
ii. Briefly explain the various components of the output. [10 marks]
iii. Which protocol is traceroute based on and how is the tracing of a route made possible using this protocol? [10 marks]
(b) Suppose there is exactly one packet switch between a sending host and a receiving host. The transmission rates between the sending host and the switch and between the switch and the receiving host are R1 and R2, respectively. Assuming that the switch uses store-and-forward packet switching, how much time does it take for the receiver to receive a packet of length L (ignore queuing, propagation delay, and processing delay)? Explain. [10 marks]
(c) Are the following statements true or false? Briefly explain your answer.
i. A user requests a Web page that consists of the base HTML code, two images, one CSS file and one JavaScript file. For this page, the client will send one request message and receive five response messages. [5 marks]
ii. The Web pages http://www.sussex.ac.uk/aboutus/ and http:// www.sussex.ac.uk/alumni/ can be sent over the same persistent HTTP connection. [5 marks]
iii. Developing a new client-server application requires developing a new underlying transport-layer protocol. [5 marks]
2
2. (a)
(b) (c) (d)
(e)
Computer Networks G5115
Suppose that an SMTP server runs in Host C on port 25. Suppose that two SMTP clients (A and B) are currently connected to the SMTP server. Do all of the messages sent by A and B go through the same socket at Host C? If they go through two different sockets, are both of these sockets bound to port 25? Discuss and explain. [10 marks]
In the ¡®stop-and-wait¡¯ protocol, how does the receiver react upon receiving a corrupted segment and why? [10 marks]
Explain the mechanism (if any exists) used by TCP to detect and deal with duplicate packets. [10 marks]
At time t, the congestion window of a TCP sender is 1000 bytes long. The maximum segment size (MSS) used by the connection is 500 bytes. What is the size of the congestion window after it sends out 2 packets (of MSS each) and receives acknowledgements for all of them if the connection is in slow-start mode and why? Suppose the receiver sends one acknowledgement per packet and that the sender has always data available to send. [10 marks]
Hosts A and B are communicating over a TCP connection. Host B has already received from A and acknowledged all previously sent data. Suppose Host A then sends two segments to Host B back-to-back. The first and second segments contain 100 and 200 bytes of data, respectively. In the first segment, the sequence number is 10100, the source port number is 1000, and the destination port number is 25. Host B sends an acknowledgement whenever it receives a segment from Host A.
i. In the second segment sent from Host A to B, what is the sequence number, source port number, and destination port number and why? [5 marks]
ii. If the second segment arrives before the first segment, in the acknowledgement of this segment, what is the acknowledgement number, the source port number, and the destination port number and why? [5 marks]
3 Turn over/
G5115
3. (a)
Computer Networks
Consider a router that interconnects three subnets: Subnet 1, Subnet 2, and Subnet 3. Suppose that the available address space for allocating IP addresses to network interfaces in all these subnets consists of the prefixes 145.10.0/24, 145.10.1/24 and 145.10.2/24 (i.e., three /24 CIDR portions of the IP space are provided). Also suppose that Subnet 1 is required to support 400 interfaces, Subnet 2 is required to support 100 interfaces, and Subnet 3 is required to support 100 interfaces. What would be the network and broadcast addresses (of the form a.b.c.d/x) of these three subnets, assuming the most economical allocation of IP addresses to the given subnets? What would be an example address (of the form a.b.c.d/x) for three hosts belonging to subnets 1, 2 and 3,
respectively?
(b) You are given the IP routing table below.
[20 marks]
IP Subnet
0.0.0.0 (default gateway) 134.0.0.0/8 134.64.0.0/16 134.64.32.0/24 134.64.32.128/25
Outgoing interface if1
if2
if3
if4 if5
Which interfaces are used for packets with the following destination IP addresses and why?
i. 134.64.32.110 ii. 134.64.32.150
iii. 134.64.32.43 iv. 134.65.32.16 v. 134.64.35.17
[2 marks] [2 marks] [2 marks] [2 marks] [2 marks]
(c) What is the Network Address Translation protocol and why was it introduced? [10 marks]
(d) Why is the Address Resolution Protocol (ARP) protocol crucial for Local Area Networks? Why is an ARP query sent within a broadcast frame? [10 marks]
4 End of Paper