UNIVERSITY OF TORONTO
FACULTY OF APPLIED SCIENCE AND ENGINEERING ECE361 Midterm– Computer Networks
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 separately, i.e. do not mix answers from different
questions.
Show details of your solutions for all questions clearly and NEATLY, otherwise marks will
be deducted.
Use the last 10 minutes to submit your answers.
Make sure you do time management and not spending too much time on a single question.
MARKS
ECE361 Page 1 of 4
Question 1: [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.
ECE361 Page 2 of 4
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?
ECE361 Page 3 of 4
Question 2: [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.
ECE361
Page 4 of 4
a.
b.
what will be the congestion window size at time T3, T4, and T5?
Find the value of Y, Z, and U in the figure.