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ECE361 Midterm fall 2020 Page 1 of 8

UNIVERSITY OF TORONTO

FACULTY OF APPLIED SCIENCE AND ENGINEERING

ECE361 Midterm– Computer Networks October 22, 2020

DURATION: 90 Minutes

Textbook and lecture slides are allowed.

Calculator Type: 2 (non-programmable calculators)

No other electronics devices are allowed except using a computer for communication purposes.

Examiner: Hamid S. Timorabadi

INSTRUCTIONS:

▪ Submission:

o Provide answers to each question on separate pages, i.e. do not mix answer to different questions on the same page.

o Submit answers to all the question ONLY in PDF format and in ONE file.

o Use any of the following methods for submission:

▪ Write answers to all the questions on papers and then

• Scan the pages and submit them as one PDF file OR

• Take pictures of pages and copy paste on a word document then submit. (remember to “save as” or convert your document to PDF)

▪ Write answers in any electronic software and then build and submit them as one PDF file.

▪ Show details of your solutions for all questions clearly and NEATLY, otherwise marks will be deducted.

M

ARKS

Q1 /8

Q2 /8

Q3 /8

Q4 /6

Total /30

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ECE361 Midterm fall 2020 Page 2 of 8 Question 1: [8 Marks] In the following Figure, a network is shown with three clients named C1, C2, and C3.

Both C1 and C3 intend to fetch the “www.utoronto.ca/courses/ece361” webpage. The webpage includes 10 images of size 5 KB each. Assume that the webpage is zero bytes itself. For this, each client must first contact the local cache server to check for the content. If the content is available on the local cache, the client will receive the content directly from the cache server. Otherwise, the client must request the content from the origin server directly. Note that, for this, the client must create a separate connection to the origin server. Answer the following questions if the speed of the light is 2.5x108 m/s.

Note: the effective transmission rate for an end-to-end connection is the minimum rate of all the links included in the transmission path.

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ECE361 Midterm fall 2020 Page 3 of 8 a. Assume the local cache is empty and C3 wants to fetch the webpage for the first time. How long does

it take for C3 to download the webpage and all the objects if C3 uses a non-persistent HTTP connection with parallel downloads?

Note: The transmission rate will be divided by the number of active connections, if the client downloads content in parallel mode.

b. Assume the cache hit rate is 40 percent, how long does it take for C1 to fetch the webpage including its objects, if the client uses a persistent HTTP connection.

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ECE361 Midterm fall 2020 Page 4 of 8 Question 2: [8 Marks] In the following diagram, a network topology is demonstrated with one-way propagation delay shown on each network link. In this network both C1 and C2 intend to download some contents from the two web servers (server1, and server 2). C1 and C2 do not have the IP address of the web servers and the Local DNS repository is empty initially. Assume for every communication, the path with the least number of links will be selected and any DNS resolution happens in iterated mode. Answer the following questions if the size of each object (including the Web Page) is 10 KB and the transmission rate on all links is 1 Mb/s. Note: the size of control messages is negligible.

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ECE361 Midterm fall 2020 Page 5 of 8 a. At the same time as C2 contacts server2, C1 contacts server1 to fetch the

“server1.utoronto.ca/exam/ece361.html” webpage. Once the webpage is received, C1 must contact server2 to download the webpage objects. Assume the webpage has 5 objects. How long does it take for C1 to fetch the webpage and download the page objects if non-persistent HTTP (without parallel connection) is used?

b. At the same time as C1 contacts server1, C2 contacts server2 to fetch the

“server2.utoronto.ca/exam/ece361.html” webpage. Once the webpage is received, C2 must contact the server1 to download the webpage objects. Assume the webpage has 5 objects. How long does it take for C2 to fetch the webpage and download the page objects if persistent HTTP is used?

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ECE361 Midterm fall 2020 Page 6 of 8 Question 3: [8 marks] Consider the following graph which illustrates the TCP Reno window size versus time. Assume at time 0 sender attempts to open a connection. Moreover, assume that the sender can write data instantaneously. Hence, the only latency factor is the propagation delay of the network. Assume that the network has an MSS of 1000 bytes.

1. Name the events at A, B, and C.

2. At point A, sender sets its window size to w1 and sends the appropriate number of packets accordingly. Right after the sender was supposed to increase its window size, the sender realizes that it must decrease its window size to w2 instead.

a. Find the value of w1 if the sender has written 31000 bytes to the network before decreasing its window size.

b. Find the value of the round-trip-time (RTT) between sender and receiver if sender reduces its window size at T1= 1.5sec .

3. At point B, sender sets its window size to w4 and sends the appropriate number of packets accordingly. Right after the sender was supposed to increase its window size, the sender realizes that it must decrease its window size to w1 instead.

a. Find the value of w4.

b. Find the value of T2, i.e., the time that sender reduces its window size.

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ECE361 Midterm fall 2020 Page 7 of 8 4. Assume that at point D, the sender's window size is w3=25,000 bytes. Let T4 be the time when

sender increases its window size to w3+1.

a. Complete the diagram by plotting T3 to T4.

b. Find the value of (T 4 − T 3).

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ECE361 Midterm fall 2020 Page 8 of 8 Question 4: [6 marks] In the figure below, TCP transmits packet with the given sequence numbers on a 3.75Mb/sec link. At time T1, EstimateRTT = 8msec, and DevRTT = 2.25msec. All values on the figure are in msec. Given the following computation:

EstimateRTT(new) = 0.85*ExtimatedRTT(old) + 0.15*SampleRTT

DevRTT(new)=0.8*DevRTT(old) + 0.2* | SampleRTT – EstimateRTT(old) |

TimeoutInterval = EstimatedRTT(new) + 4.5 * DevRTT(new)

1. Compute the value of X.

2. Assume that the TCP congestion window size at time T2 is W.

a. what will be the congestion window size at time T3, T4, and T5?

b. Find the value of Y, Z, and U in the figure.

References

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