COMPSCI1511, 2020 Midterm Exam
Total marks 60. Partial marks will be awarded whenever reasonable.
Question 1 [8pts]: Specify packet switching and circuit switching? Which one does the Internet adopt? Why does the Internet make such a choice?
Question 2 [12pts]: Assume there is one router and two links between the ftp server and client (see picture below). FTP is on top of TCP (thus uses pipelining). The first link has transmission rate R1 and the second link has transmission rate R2. Assume the file gets broken into three packets each of size L. We will ignore all propagation and processing delays. How long does it take from when the server starts sending the file until the client has received the whole file if R2 ≥ R1? What if R2 < R1? In the second case, how long does the second packet spend in the router’s queue? EMBED Visio.Drawing.11 Question 3 [12 pts]: What is the difference between persistent HTTP and non-persistent HTTP? What is the difference between pipelining and parallel connection? Assume that the Web page consists of: 1 base HTML page (of size A bits), and M images (each of size B bits). Let the transmission rate of the connection be R. What is the response time for persistent HTTP without pipeline? We ignore TCP congestion control. Question 4 [10pts]: Read the receiver side of rdt2.1 as follows. Consider STATE “wait for 0 from below” (the STATE on the left). Please explain the following two event/actions in detail, i.e., what happens, why happens, what are the reactions and why are the reactions. 1) EMBED Equation.3 2) EMBED Equation.3 Question 5 [6pts]: Explain the difference between flow control and congestion control. Question 6 [12pts]: Consider sending a large file from a host to another host over a TCP connection (using TCP Reno). The threshold for TCP congestion control in the first round is 16. a. If no loss occurs, in which round does the congestion window to increase from 1 MSS (maximum segment size) to 20 MSS? b. If no loss occurs, what is the throughput up to time 10th round (in terms of MSS and RTT)? c. If the sender receives three duplicate ACK at the 11th round, what is the congestion window size and threshold in the 14th round (in terms of MSS)?