COMP30023_2022_SM1 Modules Exam prepara!on Prac!ce exam!
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Prac!ce exam
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Submi”ed May 31 at 11:05
COMP30023 Computer Systems
Semester 1 2022
Prac!ce Exam
Total !me: 135 minutes.
Disclaimer: This is a sample exam to give you an idea of how the final exam will look like so you can prac!ce with the interface
of Canvas quizzes.
Authorised Materials:
– Kurose, J.F and Ross, K.W. (2017). Computer Networking: A Top-Down Approach (Seven Edi!on) .
– Tanenbaum, A.S. & (2015). Modern opera!ng systems (Fourth Edi!on) .
– any of the material linked from the weekly outline on the subject’s home page including lecture notes, prac!cal
instruc!ons, tutes and their answers as well as annota!ons you have made, but excluding external links.
It is advisable to have paper and pen ready by your side.
For Part B, please keep your answers brief, at most 100 words. Dot point answers are acceptable. You can see the
number of words below your answer.
Instruc!ons to Students:
This paper counts for 50% of your final grade. (Actually, this prac!ce counts for 0%…)
There are three parts to the exam:
Part A – 8 x 1 mark mul!ple choice ques!ons
Part B – 8 x 2 mark short answer ques!ons.
Part C – 6 short answer ques!ons worth total of 26 marks.
A”empt all ques!ons.
Type your answers in text boxes provided . Only typed answers will be marked.
Do not upload images or files. Uploaded images, files, etc will not be marked.
By submi#ng this exam, you cer!fy that you complied with “Declara!on of Academic Honesty”:
The answers I am submi#ng for this assessment are my own unassisted work; and
I have not made any use of communica!ons devices or channels such as mobile phones, text messages, WeChat or
WhatsApp, email, or other messaging technologies, while undertaking this assessment;
I have not made use of any material outside of what is specified under Authorised Material of this assessment;
I have not made use of any world-wide web or internet based resources, including google and other search services,
Wikipedia, and StackOverflow;
I have not taken any ac!ons that would encourage, permit, or support other enrolled students to violate the
Academic Honesty expecta!ons that apply to this assessment.
Take the Quiz Again
Part A (8 marks).
Pick the most appropriate answer to each of the following ques!ons. 1 pts per
0 / 1 ptsQues!on 1UnansweredUnanswered
Which of the following is NOT a valid IPv6 address?
All four are true
::cafe:beef
None of these is true
123:456:789:abc:def Correct AnswerCorrect Answer
1234:5678:9abc:def0:1234:4568:9abc:def0
0 / 1 ptsQues!on 2UnansweredUnanswered
Which of the following is true of the handshake se#ng up a TCP connec!on?
If there is no loss, it involves two packets that have the SYN flag set, and two packets
with the ACK flag set
All four are true Correct AnswerCorrect Answer
If a packet gets lost, it is safe to re-send it without risking crea!ng two connec!ons
None of these is true
It is used to agree upon the ini!al sequence numbers
It can take less than two round-trip !mes
0 / 1 ptsQues!on 3UnansweredUnanswered
Consider a TCP connec!on between hosts A and B.
Host A sends segments with (seq, ACK) pairs (0, 0), (1000, 500), (1000, 1000), (1500,
Which of the following is a *possible* (seq, ACK) sequence that B may have sent, if
no packets are lost?
(Remember that the ACK sent by B is determined by the seq sent by A and vice versa.)
None of these is true
(0, 0), (500, 0), (1000, 1000), (1000, 1500)
(500, 1000), (1000, 1000), (1000, 1500)
All four are true
(0, 1000), (500, 1000), (1000, 1500) Correct AnswerCorrect Answer
(1000, 0), (1000, 500), (1500, 1000)
0 / 1 ptsQues!on 4UnansweredUnanswered
Which of the following is true of IP mul!cast?
There are standards for it, but it cannot be implemented because rou!ng is NP complete
The source address of a mul!cast packet must be a mul!cast IP address
Specific IP addresses are reserved for mul!cast des!na!ons Correct AnswerCorrect Answer
Mul!cast is used for DNS, so that the packet can go to any one of mul!ple DNS
None of these is true
All four are true
0 / 1 ptsQues!on 5UnansweredUnanswered
Consider the following pseudocode.
print_hello:
print “Hello”
pthread_t tid1, tid2, tid3;
pthread_create(&tid1, …, print_hello, …)
pthread_create(&tid2, …, print_hello, …)
pthread_create(&tid3, …, print_hello, …)
pthread_t tid1, tid2;
pthread_create(&tid1, NULL, spawn, NULL)
pthread_create(&tid2, NULL, spawn, NULL)
How many !mes can “Hello” message be printed out by an invoca!on of this program?
Anywhere between 0 and 6 Correct AnswerCorrect Answer
Anywhere between 1 and 6
0 / 1 ptsQues!on 6UnansweredUnanswered
Which of the following statements best describes an asymmetric encryp!on scheme?
As opposed to a symmetric encryp!on scheme, it can be used for confiden!ality and
integrity.
It is more secure than symmetric encryp!on as it has two keys.
It involves two keys, a public and a secret key, where the public key is used to verify
integrity of communicated messages.
It involves two keys, a public and a secret key, where only the secret key can be used to
encrypt messages.
It involves two keys, a public and a secret key, where the public key is used to encrypt
Correct AnswerCorrect Answer
0 / 1 ptsQues!on 7UnansweredUnanswered
A process has been allocated 4 pages of physical RAM while its virtual address space
consists of 8 pages. It makes accesses to (virtual) pages in the following order: 0, 1, 2,
3, 4, 5, 2, 1, 3.
Assuming there are no pages in RAM when the process starts, how many page faults
would the above access pa”ern result in if an op!mal algorithm was used (i.e., the one
that knows all future accesses).
6 Correct AnswerCorrect Answer
0 / 1 ptsQues!on 8UnansweredUnanswered
When using the Advanced Encryp!on Standard (AES), why should you not use the
Electronic Codebook (ECB) mode?
It uses a weak source of randomness. As a result, this mode allows key recovery a”acks.
It is an old standard that uses key sizes that are too short.
It does not use randomness. As a result, this mode allows key recovery a”acks.
The codebook is predictable and therefore keys can be guessed.
Repeated pa”erns in the plaintext will be evident in the ciphertext. Correct AnswerCorrect Answer
Part B (16 marks)
Short answers. 2 pts per ques!on.
Please keep your answers brief, at most 100 words. Dot point answers are acceptable.
You can see the number of words below your answer.
Not yet graded / 2 ptsQues!on 9
Your Answer:
UnansweredUnanswered
Consider an applica!on opening a TCP socket to a host on a different subnet.
Describe, in one line each, the ac!ons performed by hosts and routers as part of the
protocols at layers 3 and 4 to achieve this.
Not yet graded / 2 ptsQues!on 10
Your Answer:
UnansweredUnanswered
A set of routers all support both IPv4 and IPv6 on all interfaces. Can the path a packet
takes from a router A to a router B be different for IPv4 and IPv6?
Why or why not?
Not yet graded / 2 ptsQues!on 11
Your Answer:
UnansweredUnanswered
If a physical link gets cut, which OSI layers will definitely be affected?
Which other layers may be affected, and how?
Not yet graded / 2 ptsQues!on 12
Your Answer:
UnansweredUnanswered
What is the rela!onship between rou!ng and packet forwarding?
Not yet graded / 2 ptsQues!on 13
Your Answer:
UnansweredUnanswered
What does it mean for a hash func!on to be collision resistant?
Not yet graded / 2 ptsQues!on 14
Your Answer:
UnansweredUnanswered
Does virtual address transla!on require a TLB lookup? Explain your answer.
Not yet graded / 2 ptsQues!on 15
Your Answer:
UnansweredUnanswered
Explain why encryp!on alone is not sufficient to establish a secure communica!on
channel between two par!es.
Not yet graded / 2 ptsQues!on 16
Your Answer:
UnansweredUnanswered
Consider the following programs whose execu!on can lead to a deadlock, where M1
and M2 are two files.
Your colleague suggests to duplicate M1 and M2 such that process A and B have
access only to their own copy of the file:
Though this may avoid a deadlock, explain why this is not a good solu!on.
Part C (26 marks)
Variable points per ques!on.
Not yet graded / 5 ptsQues!on 17
Your Answer:
UnansweredUnanswered
Consider the IP header fields below.
(a) For those that a router is allowed to change, why does it change?
(b) For those a router is not allowed to change, why should it not?
(c) Some are changed by “middleboxes” (devices on the path that are not routers).
Which ones are changed, by what middle boxes, and why?
(A) version
(B) protocol
(C) iden!fica!on
(F) Fragment Offset
(G) Time to live
(H) header checksum
(I) source address
(J) des!na!on address
Not yet graded / 4 ptsQues!on 18
Your Answer:
UnansweredUnanswered
Execute Dijkstra’s algorithm to find the shortest path from A to E.
Construct a table showing the next hop, path length and state for each node
a&er each itera!on. You can use “C” for closed and “O” for open.
The heading and first line of the table should be
Step A B C D E
1 (-, zero, O) – – – –
Not yet graded / 4 ptsQues!on 19
Your Answer:
UnansweredUnanswered
Some applica!ons send “keep alive” packets every few minutes.
How does that help NAT boxes?
Your answer should explain enough of the behaviour of a NAT box that the answer is
clear to someone who has not heard of NAT.
Not yet graded / 4 ptsQues!on 20
Your Answer:
UnansweredUnanswered
Consider the trust model between 4 par!es with IDs A, B, C, D as depicted in the
image below, where each party:
has a signing and a verifica!on key (i.e., secret and public key of a signature
stores a table where each row contains an ID of the party that it trusts along with
the verifica!on key of this party.
For example, party A’s signing key is SK and verifica!on key is PK . Party A trusts
party B and it knows its public key PK
A receives a message from someone claiming to be D. The message is of the form (m,
s, PK ) where m is the content of the message, s is a digital signature on m that can be
verified with PK That is Verify(PK , m , s) returns true.
Describe the communica!on messages and verifica!on steps using digital signatures
that would allow A to verify whether the message is indeed from D, i.e., that Z is D.
Assume that each party can communicate with any of the other three par!es over an
untrusted network.
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2022 Semester 1
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Not yet graded / 4 ptsQues!on 21
Your Answer:
UnansweredUnanswered
Suppose that, as one way to op!mise the overall performance of an opera!ng system,
a so&ware developer could give hints about their program to a process scheduler and
a memory manager.
What type of informa!on about a program would be useful for the OS to know and
List one example for a process scheduler and one for a memory manager. For each
one, explain how this informa!on can be used and what it would improve.
Not yet graded / 5 ptsQues!on 22
Your Answer:
UnansweredUnanswered
Honest par!es Alice and Bob wish to securely communicate with each other over a
network. Alice and Bob know only their own public/secret key pair for an encryp!on
scheme: Alice has PK , SK and Bob has PK , SK . Alice begins the communica!on
by sending Bob her public key PK .
Eve is a malicious party working together with an accomplice, Charles. Among the
two, only Eve can intercept and tamper with the network communica!on between
Alice and Bob.
Eve wishes to achieve the following: any message sent from Alice to Bob can be
decrypted by Bob as well as Charles and Eve, while any message sent from Bob to
Alice can be decrypted by Alice and Eve but not Charles.
Provide a communica!on transcript of key and message exchange that Eve could use
to achieve this and encryp!on/decryp!on steps that each party would perform. In
your transcript, give an example of a ciphertext c corresponding to a message m
sent from Alice to Bob that Charles can also decrypt. Similarly, give an example of a
ciphertext sent from Bob to Alice that Charles cannot decrypt.
For example, the first line of your transcript will look like: “Alice sends PK to Bob”.
The transcript will also include “Alice sends c to Bob” and your answer will include
how c was obtained (e.g., c = Encrypt(…, m )).
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