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Practice exam

web | Network代写 | 代写network | Algorithm | quiz | Operating Systems代做 – 本题是一个利用Algorithm进行练习的代做, 对Algorithm的流程进行训练解析, 是比较典型的web/Network/network/Algorithm/Operating Systems等代写方向, 该题目是值得借鉴的quiz代写的题目

Started: Jun 1 at 15:

quiz Instructions

COMP 30023 Computer Systems

Semester 1 2022

Practice Exam

Total time: 135 minutes.

Disclaimer: This is a sample exam to give you an idea of how the final exam will look like so you can practice with the interface of Canvas quizzes.

Authorised Materials:

• Kurose, J.F and Ross, K.W. (2017). Computer Networking: A Top-Down Approach (Seven Edition).
• Tanenbaum, A.S. & Herbert Bos (2015). Modern Operating Systems (Fourth Edition).
• any of the material linked from the weekly outline on the subjects home page including lecture notes, practical instructions, tutes and their answers as well as annotations 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.

Instructions to Students:

This paper counts for 50% of your final grade. (Actually, this practice counts for 0%...)
There are three parts to the exam:
Part A - 8 x 1 mark multiple choice questions
Part B - 8 x 2 mark short answer questions.
Part C - 6 short answer questions worth total of 26 marks.
Attempt all questions.
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 submitting this exam, you certify that you complied with "Declaration of Academic Honesty":

The answers I am submitting for this assessment are my own unassisted work; and

I have not made any use of communications 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 actions that would encourage, permit, or support other enrolled students to violate the Academic Honesty expectations that apply to this assessment.

Part A (8 marks).

Pick the most appropriate answer to each of the following questions. 1 pts per
question.

Question 1 1 pts

None of these is true

All four are true

123:456:789:abc:def

::cafe:beef

1234:5678:9abc:def0:1234:4568:9abc:def

face::0ff

Which of the following is NOT a valid IPv6 address?

Question 2 1 pts

It is used to agree upon the initial sequence numbers

Which of the following is true of the handshake setting up a TCP connection?

All four are true

If there is no loss, it involves two packets that have the SYN flag set, and two packets
with the ACK flag set

None of these is true

It can take less than two round-trip times

If a packet gets lost, it is safe to re-send it without risking creating two connections

Question 3 1 pts

All four are true

(0, 1000), (500, 1000), (1000, 1500)

(1000, 0), (1000, 500), (1500, 1000)

(500, 1000), (1000, 1000), (1000, 1500)

None of these is true

(0, 0), (500, 0), (1000, 1000), (1000, 1500)

Consider a TCP connection between hosts A and B. Host A sends segments with (seq, ACK) pairs (0, 0), (1000, 500), (1000, 1000), (1500, 1000).

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.)

Question 4 1 pts

Specific IP addresses are reserved for multicast destinations

Which of the following is true of IP multicast?

There are standards for it, but it cannot be implemented because routing is NP complete

None of these is true

Multicast is used for DNS, so that the packet can go to any one of multiple DNS
databases

All four are true

The source address of a multicast packet must be a multicast IP address

Question 5 1 pts

2

0,2,4,

Anywhere between 1 and 6

Anywhere between 0 and 6

6

Consider the following pseudocode.

print_hello:
print "Hello"
spawn:
pthread_t tid1, tid2, tid3;
pthread_create(&tid1, ..., print_hello, ...)
pthread_create(&tid2, ..., print_hello, ...)
pthread_create(&tid3, ..., print_hello, ...)
main:
pthread_t tid1, tid2;
pthread_create(&tid1, NULL, spawn, NULL)
pthread_create(&tid2, NULL, spawn, NULL)

How many times can "Hello" message be printed out by an invocation of this program?

Question 6 1 pts

It is more secure than symmetric encryption as it has two keys.

As opposed to a symmetric encryption scheme, it can be used for confidentiality and
integrity.

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 the public key is used to encrypt
messages.

It involves two keys, a public and a secret key, where only the secret key can be used to
encrypt messages.

Which of the following statements best describes an asymmetric encryption scheme?

Question 7 1 pts

1

6

2

4

0

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 pattern result in if an optimal Algorithm was used (i.e., the one that knows all future accesses).

Question 8 1 pts

It uses a weak source of randomness. As a result, this mode allows key recovery attacks.

Repeated patterns in the plaintext will be evident in the ciphertext.

The codebook is predictable and therefore keys can be guessed.

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 attacks.

When using the Advanced Encryption Standard (AES), why should you not use the Electronic Codebook (ECB) mode?

Part B (16 marks)

Short answers. 2 pts per question.

Please keep your answers brief, at most 100 words. Dot point answers are acceptable. You can see the number of words below your answer.

Question 9 2 pts

Consider an application opening a TCP socket to a host on a different subnet.

Describe, in one line each, the actions performed by hosts and routers as part of the protocols at layers 3 and 4 to achieve this.

Edit View Insert Format Tools Table

12pt Paragraph

p 0 words </>

Question 10 2 pts

p 0 words

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?

</>

Question 11 2 pts

Edit View Insert Format Tools Table

12pt Paragraph

p 0 words

If a physical link gets cut, which OSI layers will definitely be affected? Which other layers may be affected, and how?

</>

Question 12 2 pts

What is the relationship between routing and packet forwarding?

Edit View Insert Format Tools Table

12pt Paragraph

Edit View Insert Format Tools Table

12pt Paragraph

p 0 words </>

Question 13 2 pts

p 0 words

What does it mean for a hash function to be collision resistant?

</>

Question 14 2 pts

Does virtual address translation require a TLB lookup? Explain your answer.

Edit View Insert Format Tools Table

12pt Paragraph

Question 15 2 pts

Explain why encryption alone is not sufficient to establish a secure communication channel between two parties.

Question 16 2 pts

Consider the following programs whose execution 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 solution.

Part C (26 marks)

Variable points per question.

Question 17 5 pts

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) identification (D) DF (E) MF (F) Fragment Offset (G) Time to live (H) header checksum (I) source address (J) destination address

Question 18 4 pts

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 after each iteration. 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) - - - -

Question 19 4 pts

Some applications 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.

Question 20 4 pts

Consider the trust model between 4 parties with IDs A , B , C , D as depicted in the image below, where each party:

has a signing and a verification key (i.e., secret and public key of a signature
scheme)
stores a table where each row contains an ID of the party that it trusts along
with the verification key of this party.

For example, party A ‘s signing key is SK and verification 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 communication messages and verification 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 parties over an untrusted network.

A A
B.

Z
Z. Z

Question 21 4 pts

Suppose that, as one way to optimise the overall performance of an operating system, a software developer could give hints about their program to a process scheduler and a memory manager.

What type of information about a program would be useful for the OS to know and why?

List one example for a process scheduler and one for a memory manager. For each one, explain how this information can be used and what it would improve.

Question 22 5 pts

Honest parties 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 encryption scheme: Alice has PK , SK and Bob has PK , SK. Alice begins the communication 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 communication 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 communication transcript of key and message exchange that Eve could use to achieve this and encryption/decryption 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 )).

A A B B
A

A
A

A
A
A A A

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