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COMP3331/9331 — Computer Networks and Applications Term 2, 2022 Mid-term Examination
发布时间:2022-07-07
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COMP3331/9331 — Computer Networks and Applications
Term 2, 2022
Mid-term Examination
1 DNS Q1
1. Which of the provided Resource Records are Type A entries?
Select one or more alternatives:
(b)
(f)
None of the provided records
(e)
(c)
(a)
(d)
Maximum marks: 0.5
Type A records provide a mapping from hostname to IP address. Thus the correct answer is (b), (e) and (f). 0.167 mark for each correct answer and -0. 167 mark for each incorrect answer. 0 if "none of the provided records" is chosen. Minimum possible mark is 0.
2 DNS Q2
Which of the provided Resource Records are Type MX entries?
Select one or more alternatives:
(b)
(a)
(c)
(f)
(e)
(d)
None of the provided records
Maximum marks: 0.5
MX refers to the mail server record, thus the correct answer is (a). 0.5 mark for the correct answer and -0. 1 for each incorrect answer. 0 if "none of the provided records" is chosen. Minimum possible mark is 0.
3 DNS Q3
Which of the provided Resource Records are Type NS entries?
Select one or more alternatives:
(f)
None of the provided records
(a)
(b)
(d)
(e)
(c)
Maximum marks: 0.5
NS refers to a name server record which provides the name of the nameserver responsible for the hostname/domain.
Thus the correct answer is (c) and (d) , which are the primary and secondary nameserver for google.com. 0.25 for each correct answer and -0. 125 answer for each incorrect answer. 0 if "none of the provided records" is chosen. Minimum possible mark is 0.
4 DNS Q4
Which of the provided Resource Records are type CNAME entries?
Select one or more alternatives:
None of the provided records
(c)
(b)
(a)
(f)
(d)
(e)
Maximum marks: 0.5
CNAME refers to a canonical name record for the hostname. None of the provided records are of this type. 0.5 mark for the correct answer. -0.5 mark if any of the other answers is chosen. Minimum possible mark is 0.
5 DNS Q5
Which of the provided Resource Records are stored in the .com TLD servers?
Select one or more alternatives:
(b)
(f)
None of the provided records
(a)
(e)
(c)
(d)
Maximum marks: 0.5
Information of the authoritative name servers of a hostname is stored in the TLD servers so that when a local DNS server queries for the mapping for the hostname, they can be directed to the authoritative name servers which would contain the actual answer (i.e. the mapping requested). Thus, the NS records for the Google name servers (primary and secondary) would be stored in the .com TLD server. In addition, the corresponding IP addresses of these name servers (i.e. the A records) would also be stored in the .com TLD server.
Thus, the correct answer is (c), (d), (e) and (f). 0.125 for each correct answer and -0.25 for each incorrect answer. 0 if "none of the provided records" is chosen. Minimum possible mark is 0.
6 DNS Q6
Which of the provided Resource Records are stored at an authoritative name server?
Select one or more alternatives:
(a)
(d)
(f)
(e)
(c)
None of the provided records
(b)
Maximum marks: 0.5
The MX record for the gmail mail server and the corresponding A record that holds the IP address for this server would be stored at the authoritative name servers (i.e. ns1.google.com and ns2.google.com). These are the "final" answers that are provided in response to a MX query for the gmail mail server.
Thus, the correct answer is (a) and (b). 0.25 for each correct answer and -0. 125 mark for each incorrect answer. 0 if "none of the provided records" is chosen. Minimum possible mark is 0.
Assume a webpage comprised of 10 objects which includes the index.html file, 8 embedded images and one embedded audio clip. The 10 objects are so small that: (i) their transmission time is negligible and (ii) each object can be completely transmitted in one TCP segment. Consider a client wishing to download the webpage.
You are asked to make the following assumptions:
the round trip time between the client and all servers is T
the time to set up and tear down a TCP connection is S and F, respectively.
You must account for both these times in your computations . Note
that, S includes the 3-way handshake (SYN, SYN-ACK, ACK) and F includes the time for sending FINs and ACKs from both endpoints.
there are no packet losses .
the client knows the IP address of all servers (i .e . neglect DNS resolution
delay) .
neither the client nor any of the servers support parallel TCP connections.
Answer the following 5 questions. No explanations are required. Simply write the expression for each answer which should ONLY contain the
variables T, S and F (e.g., 20T+100S+50F) in the space provided.
7 HTTP Q1
Assume that the client uses non-persistent HTTP for downloading the web page. What is the time required to complete the transfer of the web page (including the time for setting up and tearing down each TCP connection involved)?
Fill in your answer here
Maximum marks: 0.6
In non-persistent HTTP, every object is downloaded over a fresh TCP connection. Since parallel connections are not supported, this would mean the ten objects are fetched serially.
The time required to fetch one object = time to setup TCP connection + RTT for sending GET request and receiving response + time to tear down TCP connection = S + T + F.
Thus, the total time = 10S +10T + 10F.
8 HTTP Q2
Assume that the client uses persistent HTTP without pipelining for downloading the web page. What is the time required to complete the transfer of the web page (including the time for setting up and tearing down each TCP connection involved)?
Fill in your answer here
Maximum marks: 0.6
In this instance, all objects can be fetched over one single TCP connection but serially (one after the other).
Thus the total time = time to setup TCP connection + 10 x (RTT for sending GET request and receiving the object) + time to tear down TCP connection = S + 10T + F.
9 HTTP Q3
Assume that the client uses persistent HTTP with pipelining for downloading the web page. What is the time required to complete the transfer of the web page (including the time for setting up and tearing down each TCP connection involved)?
Fill in your answer here
Maximum marks: 0.6
In this instance, since pipelining is used once the index page is fetched and the client knows of the 9 embedded objects, these 9 objects can be requested back-to-back (simultaneously) and the corresponding objects would also be received back-to-back.
Thus the total time = time to setup TCP connection + RTT for sending GET request for the index page and receiving that page + RTT for sending 9 GET requests for embedded objects and receiving them + time to tear down TCP connection = S + T + T + F = S + 2T + F.
10 HTTP Q4
Now assume that all 10 objects are located on 10 different servers (one object on each server). The client can only have one active TCP connection at any given time. Assume that the round trip time between the client and each of the 10 servers is T. Neglect DNS queries. Assume that the client uses persistent HTTP with pipelining for downloading the web page. What is the time required to complete the transfer of the web page (including the time for setting up and tearing down each TCP connection involved)?
Fill in your answer here
Maximum marks: 0.6
Now each object is fetched from a different server. Since parallel connections are not supported, this would mean these objects have to be fetched serially (one after the one).
The time to fetch one object from one server = time to setup TCP connection + RTT for sending GET request for the object and receiving that object + time to tear down TCP connection = S + T + F.
Thus the total time = 10 (S + T + F)
11 HTTP Q5
Now assume that the index page and 7 embedded images are on one server, while the remaining image and audio clip are on another server. The client can only have one active TCP connection at any given time. Assume that the round trip time between the client and both servers is T. Neglect DNS queries. Assume that the client uses persistent HTTP with pipelining for downloading the web page. What is the time required to complete the transfer of the web page (including the time for setting up and tearing down each TCP connection involved)?
Fill in your answer here
Maximum marks: 0.6
In this instance, all objects from each server can be fetched over a single TCP connection established with that server.
The client would first fetch the index page and would become aware of the embedded objects. The 7 objects hosted on this same server (as the index page) are then fetched in one go. Next, the client fetches the other two objects from the other server in one go.
Thus the total time = time to setup TCP connection with the first server + RTT for sending GET request for the index page and receiving that page + RTT for sending 7 GET requests for embedded objects and receiving them + time to tear down TCP connection + time to setup TCP connection with the second server + RTT for sending 2 2 GET requests for embedded objects and receiving them = S + T + T + F + S + T + F = 2S + 3T + 2F.
Suppose a number of users share a 4 Mbps link. Also, suppose that each user transmits continuously at 2 Mbps when transmitting, but each user transmits only 20% of the time.
Answer the 3 questions.
12 Packet/Circuit Switching Q1
When circuit switching is used, how many users can be supported? No explanation is required.
Simply enter the numeric value in the space provided: (2)
Maximum marks: 0.25
In circuit switching, the percentage of time a user is active is irrelevant. A circuit needs to be established for each active user. Since each user requires 2Mbps and the link capacity is 4Mbps, 2 users can be supported.
13 Packet/Circuit Switching Q2
Now suppose packet switching is used. Why will there be essentially no queuing delay before the link if two or fewer users transmit at the same time? Why will there be queuing delay if three users transmit at the same time? Be brief (2 sentences at most for each question).
Fill in your answer here
Format
Σ |
Since each user requires 2 Mbps when transmitting, if two or fewer users transmit simultaneously, a maximum of 4 Mbps will be required . Since the available bandwidth of the shared link is 4 Mbps, there will be no queuing delay before the link . (0 .25 marks) Whereas, if three users transmit simultaneously, the bandwidth required will be 6 Mbps which is more than the available bandwidth of the shared link . In this case, there will be queuing delay before the link . (0 . 5 marks) |
Words: 0 |
Maximum marks: 0.75
14 Packet/Circuit Switching Q3
Suppose with packet switching, there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. No explanation is required.Simply enter the
numeric value in the space provided: (0.008)
Maximum marks: 0.5
The probability that all three users are transmitting simultaneously =(0.2)^3 = 0.008