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(CSIT561)midterm-sol fall 2008.pdf
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CSIT 561 Computer Networks
Fall Semester 2008
Midterm Examination
Date: 11 Oct. 2008 Time: 3:00 pm - 5:00 pm
Name:___SOLUTION_______ Student ID: ________________Email:____________________
Instructions
1. Please type in your name, student ID and email address on this page.

2. Please answer all the questions within the space provided on the examination paper.

3. This paper consists of 6 questions and 8 pages.

4. Please read each question very carefully and answer the questions carefully and clearly to the point. Make sure your answers are neatly written, legible and readable.

5. Show all the steps used in deriving your answer, wherever appropriate.

6. Please submit your solutions electronically either in MS Word or PDF format using the LMES system http://lmes2.ust.hk.

7. Please label your answer file as <Your Student ID>-mid. doc or <Your Student ID>-mid.pdf, for example 12345678-mid.doc. Submissions not labelled in the right format cannot be accepted.



Question
Points
Score

1
10


2
10


3
15


4
15


5
25


6
25


Total
100





1. (10 points) Suppose a 100-Mbps point-to-point link is being set up between the earth and a new lunar colony. The distance from the moon to the earth is approximately 385,000 km, and data travels over the link at the speed of light C 3 108 m/s.

a) Calculate the minimum RTT for the link. (5 points)


The minimum RTT is 2 385000000 / 3 108 = 2.57 sec.


b) A camera on the lunar base takes pictures of the earth and saves them in digital format to a disk. Suppose Mission Control on earth wishes to download the most current image, which is 25 Mbytes. What is the minimum amount of time that will elapse between when the request for the data goes out and the transfer is finished? (5 points)



The transmission delay for the image is 25 8 / 100 = 2.0 sec.
So the total delay is RTT + Transmission delay = 2.57 + 2.0 = 4.57 sec.


2. (10 points) The following questions are about peer-to-peer networks:

a) In a P2P file-sharing system, what constitutes an overlay network?



The overlay network in a P2P file sharing system consists of the nodes participating in the file sharing system and the logical links between the nodes. (2 pts)

b) Does the overlay network include routers? What are the edges in the overlay network?


An overlay network does not include routers. (2 pts)
There is a logical directed link (a directed edge in graph theory terms) from node A to node B if node A sends datagrams that are explicitly addressed to node Bs IP address. (2 pts)

c) In BitTorrent, how does a newly joining peer in BitTorrent locate its neighbours? Be brief in your answer.


A joining peer contacts the tracker and obtains a list of candidate peers who can form its neighbours. (2 pts)
d) What mechanism does BitTorrent use to ensure that peers