Candidate Number
G5115
THE UNIVERSITY OF SUSSEX
BSc SECOND YEAR EXAMINATION
May/June 2016 (A2)
COMPUTER NETWORKS
Assessment Period: May/June 2016 (A2)
DO NOT TURN OVER UNTIL INSTRUCTED
TO BY THE CHIEF 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 ONE AND A HALF hours.
Each question is worth 50 marks.
At the end of the examination the question paper and/or answer book,
used or unused, will be collected from you before you leave the
examination room.
G5115 COMPUTER NETWORKS
1. (a) Consider sending a packet from a source host to a destination host
over a fixed route. List the delay components in the end-to-end delay.
Which of these delays are constant and which are variable? Explain.
[10 marks]
(b) Do HTTP, SMTP and POP3 run on top of TCP or UDP? Explain why.
[5 marks]
(c) Consider an e-commerce site that wants to keep a purchase record for
each of its customers. Describe how this can be done with cookies.
[5 marks]
(d) Describe how Web caching can reduce the delay in receiving a
requested object. Will Web caching reduce the delay for all objects
requested by a user or for only some of the objects? Why? [5 marks]
(e) True or false?
i. A user requests a Web page that consists of some text and three
images. For this page, the client will send one request message
and receive four response messages. [5 marks]
ii. Two distinct Web pages (http://www.sussex.ac.uk/aboutus/ and
http://www.sussex.ac.uk/alumni/) can be sent over the same
persistent connection. [5 marks]
iii. With non-persistent connections between browser and origin server,
it is possible for a single TCP segment to carry two distinct HTTP
request messages. [5 marks]
(f) Suppose that the University of Sussex has a single local DNS server
for all computers in the university. You are an ordinary user (i.e., not a
network/system administrator). Can you determine if an external Web
site was likely accessed from a computer in your department a couple
of seconds ago? Explain. [10 marks]
2
COMPUTER NETWORKS G5115
2. (a) Consider a TCP connection between Host A and Host B. Suppose that
the TCP segments travelling from Host A to Host B have source port
number 3457 and destination port number 80. What are the source and
destination port numbers for the segments travelling from Host B to Host
A? [5 marks]
(b) Suppose a process in Host C has a UDP socket with port number 6789.
Suppose both Host A and Host B each send a UDP segment to Host
C with destination port number 6789. Will both of these segments be
directed to the same socket at Host C? If so, how will the process at
C know that these two segments originated from two different hosts?
[5 marks]
(c) Explain the mechanisms used by TCP to detect and deal with duplicate
packets. Explain how UDP deals with duplicate packets. [10 marks]
(d) Briefly describe flow control in TCP and how the receive window and
receive buffer are involved in it. [15 marks]
(e) Suppose Host A sends two TCP segments back to back to Host B over
a TCP connection (assume that either the connection has just been
established or that all previously sent segments have been received and
acknowledged by the receiver). The first segment has sequence number
1000; the second has sequence number 1100.
i. How much data is in the first segment? Explain. [5 marks]
ii. Suppose that the first segment is lost but the second segment
arrives at B. In the acknowledgement that Host B sends to Host
A, what will be the acknowledgement number? Explain. [10 marks]
3 Turn over/
G5115 COMPUTER NETWORKS
3. (a) Describe how packet loss can occur at output ports. Can this loss be
prevented by increasing the switch fabric speed? Explain. [10 marks]
(b) 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 195.251.252/24 (i.e., a /24 CIDR
portion of the IP space is provided). Also suppose that Subnet 1 is
required to support 60 interfaces, Subnet 2 is to support 100 interfaces,
and Subnet 3 is to support 12 interfaces. What would be the network
and broadcast addresses (of the form a.b.c.d/x) of these three subnets?
[10 marks]
(c) How large are the MAC and IPv4 address spaces? [5 marks]
(d) What does the ARP protocol do? Why is an ARP query sent within a
broadcast frame? [5 marks]
(e) What are the differences between message confidentiality and message
integrity? Can you have confidentiality without integrity? Can you have
integrity without confidentiality? Justify your answer. [10 marks]
(f) Can you “decrypt” a hash of a message to get the original message?
Explain your answer. [5 marks]
(g) Explain the usage of digital certificates in digital signatures. [5 marks]
4 End of Paper