Candidate Number
G5115
THE UNIVERSITY OF SUSSEX
BSc and MComp SECOND YEAR EXAMINATION
May/June 2017 (A2)
Computer Networks
Assessment Period: May/June 2017 (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.
G5115 Computer Networks
1. (a) Let us suppose that a number of users share a 4 Mbps link. Each
user transmits continuously at 1 Mbps when transmitting, but each user
transmits only 20 percent of the time.
i. If circuit switching is used, how many users can be supported (if any
upper limit exists) and why? [5 marks]
ii. If packet switching is used, how many users can be supported (if
any upper limit exists) and why? [5 marks]
(b) Consider accessing your e-mail with POP3. Suppose you have
configured your mail client to operate in the ‘download-and-delete’
mode. Complete the following protocol message exchange, which takes
place behind the scenes, when the client (C) fetches all emails from the
server (S). [10 marks]
C: list
S: 1 498
S: 2 912
S: .
C: retr 1
S: blah blah … blah
S: .
?
(c) What transport layer protocol is SMTP build on top of and why?
[5 marks]
(d) Suppose that within your Web browser you click on a link to obtain a
Web page. The IP address for the associated URL is not cached in your
host, so a DNS lookup is necessary to resolve the IP address. Suppose
that n DNS servers are visited before your host receives the IP address
from DNS; the successive visits incur an RTT of RTT1, …, RTTn. Further
suppose that the Web page associated with the link contains the base
HTML file that references a single object on the same server. Let RTT0
be the RTT between the local host and the server containing the page.
Assuming zero transmission time for all files, how much time elapses
from when the client clicks on the link until it receives the object when
non-persistent HTTP is used? [10 marks]
(e) Let us assume that a client sends an HTTP request to a web server
which sends back the response that begins as follows:
HTTP/1.0 200 OK
Date: Thu, 06 Aug 2016 12:00:15 GMT
Content-Length: 1000
Content-Type: text/html
Set-cookie: 1234567
Explain line by line the components of the response and briefly discuss
what purpose they serve. [15 marks]
2
Computer Networks G5115
2. (a) Does UDP support reliable data transfer and why? If so, how? If not,
would it be possible for an application to enjoy reliable data transfer when
it runs over UDP and how? [10 marks]
(b) How does TCP use timers and why are they necessary? When are
timers set and what happens when a timer times out? [10 marks]
(c) Are the following statements true or false? Briefly explain your answer.
i. Host A is sending Host B a large file over a TCP connection.
Assume Host B has no data to send to Host A. Host B will not send
acknowledgements to Host A because Host B cannot piggyback the
acknowledgements on data. [5 marks]
ii. The size of TCP’s receive window (rwnd) never changes throughout
the duration of the connection. [5 marks]
iii. Suppose Host A sends one segment with sequence number 100
and 4 bytes of data over a TCP connection to Host B. In this
same segment the acknowledgement number is necessarily 104.
[5 marks]
iv. Host A sends one segment with sequence number 100 and 4 bytes
of data over a TCP connection to Host B. The sequence number
of the next segment that Host A will send to Host B will be 101.
[5 marks]
(d) Hosts A and B are directly connected with a 1 Gbps link. There is one
TCP connection between the two hosts, and Host A is sending to Host
B an enormous file over this connection. The application at Host A can
send its data through the TCP socket at a rate as high as 200 Mbps.
The application at Host B can read data through its TCP socket from the
respective TCP receive buffer at a maximum rate of 100 Mbps. Describe
how TCP deals with this situation. [10 marks]
3 Turn over/
G5115 Computer Networks
3. (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 30.40.50/24 (i.e., a /24 CIDR
portion of the IP space is provided). Also suppose that Subnet 1 is
required to support 110 interfaces, Subnet 2 is to support 40 interfaces,
and Subnet 3 is to support 40 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? [15 marks]
(b) Given the IP routing table below, how does the router decide on which
output interface to queue an inbound packet? [5 marks]
IP Subnet Outgoing interface
0.0.0.0 (default gateway) if1
195.0.0.0/8 if2
195.32.0.0/16 if3
195.32.111.0/24 if4
195.32.111.128/25 if5
Which interfaces are used for packets with the following destination IP
addresses?
i. 195.32.111.150 [2 marks]
ii. 195.32.111.110 [2 marks]
iii. 195.32.110.17 [2 marks]
iv. 194.32.111.43 [2 marks]
v. 195.30.111.16 [2 marks]
(c) Suppose nodes A and B are attached to the same Ethernet switch
(through their adapters). A needs to send an IP packet to B and
knows B’s IP address. How does the Address Resolution Protocol
(ARP) contribute so that B receives the packet, which needs to be
encapsulated in an Ethernet frame? [10 marks]
(d) Explain the concepts confidentiality, authentication and message
integrity and briefly describe a mechanism that ensures message
confidentiality when sending messages in a network. [10 marks]
4 End of Paper