IMPERIAL COLLEGE OF SCIENCE, TECHNOLOGY AND MEDICINE
EXAMINATIONS 2019
BEng Honours Degree in Electronic and Information Engineering Part II
MEng Honours Degree in Electronic and Information Engineering Part II
BEng Honours Degree in Mathematics and Computer Science Part III
MEng Honours Degree in Mathematics and Computer Science Part III
MSc in Computing Science
for Internal Students of the Imperial College of Science, Technology and Medicine
This paper is also taken for the relevant examinations for the
Associateship of the City and Guilds of London Institute
PAPER C527
COMPUTER NETWORKS AND DISTRIBUTED SYSTEMS
Monday 29th April 2019, 14:00
Duration: 120 minutes
Answer THREE questions
Paper contains 4 questions
Calculators not required
Section A (Use a separate answer book for this Section)
1a Explain the function of client and server stubs in Remote Procedure Calls.
b Briefly explain how Java RMI avoids the use of server stubs (or skeletons).
c A system implemented using Java RMI enables users to share their music and
playlists. Songs and playlists are hosted on music servers and can be accessed
remotely. A music server allows users to upload new songs and to create new
playlists. A playlist has a name and contains several songs. Playing a playlist
returns the audio bytes for all the songs in the playlist. A song has a title and
stores its music as an array of bytes. You can assume that the implementation for
the Song class already exists.
Consider that the objects described above conform to the interfaces given below.
public interface MusicServerI extends Remote {
public SongI uploadSong(String title, byte[] song) throws RemoteException;
public PlayListI createPlayList(String name) throws RemoteException;
}
public interface PlayListI extends Remote {
public String getName() throws RemoteException;
public void addSong(SongI song) throws RemoteException;
public byte[] play() throws RemoteException;
}
public interface SongI extends Remote {
String getTitle() throws RemoteException;
byte[] play() throws RemoteException;
}
i) Briefly discuss whether parameters and return values are passed by value
or by reference in the interfaces given above.
ii) Give a Java RMI implementation for the MusicServer class.
iii) Give a Java RMI implementation for the PlayList class.
iv) Give a Java RMI implementation for an application that adds a song that
exists on one music server to a new playlist on server
rmi://mymusic.com/server. The RMI reference of the song, in its URL
format and the name of the new playlist are parameters of the application.
Strict Java syntax is not required but your answer must include all the
components of the RMI invocations. State explicitly any assumptions that you
make.
The three parts carry, respectively, 15%, 15%, and 70% of the marks.
c Imperial College London 2019 Paper C527 Page 1 of 4
2a i) Explain what a certificate (as used in PGP and X.509) is and list 5 fields
that you would expect to find in a certificate.
ii) Explain what is meant by a certification authority hierarchy in X.509 and
what steps need to be taken to verify a certificate. Use a diagram to
illustrate your answer.
iii) Briefly explain how trust is handled differently in PGP and X.509.
iv) It has been argued that secure group communication i.e., where messages
are sent to a group of recipients, is impractical with PGP. Discuss, using
examples, why this might be the case and how such group communication
could be realised.
b A company riss.com needs to synchronise its computers to the Coordinated
Universal Time (UTC) using the Network Time Protocol (NTP). To this end it
needs to host one or several NTP servers, which synchronise with external NTP
servers and with which the company’s computers can synchronise. riss.com has
several thousand computers. NTP interactions use the UDP protocol and servers
use port 123 to receive queries or to issue queries to other servers. The company
also advertises its activities on a web-server available from the Internet and its
users are allowed to access external web-servers.
i) Assuming riss.com uses a screened subnet firewall architecture discuss
how many time servers the company would need and where they should be
placed with respect to the internal and external firewalls.
ii) Assuming riss.com uses a single time server placed in the DMZ between
the external and the internal firewalls give the configuration for the external
and the internal firewalls in the format below.
Rule
No.
Protocol Dir. Source
Address
Src.
Port
Dest.
Address
Dest.
Port
TCP
Flags
Action
The two parts carry equal marks.
c Imperial College London 2019 Paper C527 Page 2 of 4
Section B (Use a separate answer book for this Section)
3a i) Explain the general idea behind data framing and discuss its advantages
and disadvantages. On which layer of the OSI Reference Model is data
framing defined?
ii) List 2 framing methods that do not require perfect time-synchronisation
between sending and receiving nodes and explain how they work. Discuss
any shortcomings these methods might have and how these shortcomings
might be overcome.
iii) What is the inter-frame gap that is defined in IEEE 802.3 and why is it
used? Explain how burst mode works in this context.
b Explain what ICMP is and what it is used for. Indicate which layer of the OSI
Reference Model it belongs to and explain why. Furthermore, list two different
programs that utilise ICMP. Discuss their use cases and how exactly ICMP is
utilised to achieve their tasks.
The two parts carry, respectively, 65% and 35% of the marks.
c Imperial College London 2018 - 2019 Paper C527 Page 3 of 4
4 Consider the computer network given below, which consists of a router, a switch,
a hub, and 4 workstations (W1, W2, W3, W4). The workstations are connected
to the ports (p1, p2) of the switch or hub, which connect to the network cards of
the router (i.e. eth1 and eth2). The router has a third network card, eth3, which is
connected to the network of an Internet Service Provider and which provides
connectivity to the Internet.
The network uses the Internet Protocol (IP) and the IEEE 802.3 standard with an
MTU size of 1500 bytes. Workstations W1 and W3 are correctly configured
with:
W1 W3
MAC Address 8c:dc:d4:57:61:ab 8c:dc:d4:16:b7:5a
IP Address 146.169.25.10 146.169.28.5
Network Mask 255.255.252.0 255.255.252.0
Default Gateway 146.169.27.254 146.169.28.1
Answer the following questions and explain all your answers:
a All devices are properly configured and all workstations can communicate with
each other as well as the Internet. What are the IP addresses and network masks
of the network cards eth1, eth2, and eth3 of the router? What would a correct
static routing table on the router look like?
b Explain what backwards learning is and how it works in the context of the
operation of a switch.
c Based on the given information above give a detailed explanation of how 2000
bytes of data that is passed to the network layer would be transmitted from
workstation W1 to workstation W3 using the given addresses. Describe the
operation of all involved protocols and communication devices in chronological
order. Your explanation should only relate to the network layer and data link
layer of the OSI Reference Model. Medium access control does not have to be
considered and you can assume that all channels are free of errors.
The three parts carry, respectively, 25%, 15%, and 60% of the marks.
c Imperial College London 2018 - 2019 Paper C527 Page 4 of 4