Cover sheet for Assignment 5

Transcription

1 Faculty of Arts and Science University of Toronto CSC Introduction to Computer Networks, LEC0101 Cover sheet for Assignment 5 Due Monday April 2, 10:00am. Complete this page and attach it to the front of your assignment. Name: (Underline your last name) Student number: I declare that this assignment is solely my own work, and is in accordance with the University of Toronto Code of Behavior on Academic Matters. Signature: 1

2 Question 1 (5 points) Suppose packets can get dropped or arbitrarily delayed inside a packet network. Suppose that two peer processes are exchanging packets in a session and want to terminate the session. We would like a protocol that exchanges packets in such a way that both peer processes know that they can terminate with the knowledge that no further packets will arrive from the other peer process. Can such a protocol be designed? Answer yes or no, if your answer is yes provide a protocol that achieves the task, otherwise provide an explanation why it is not possible. 2

3 Question 2 (5 points) In class we discussed that in order to safely establish a TCP connection and avoid packets from different sessions being confused, we need the following mechanism: (1) Time-stamps to ensure that packets die after some fixed time T. (2) Clock to determine sequence numbers. (3) Connection setup through three way handshake Show by means of a counter example that we cannot safely establish a TCP connection if we use a two way handshake instead of a three way handshake. 3

4 Question 3 (15 points) Consider the figure below, where we use the leaky bucket mechanism to regulate/police the sessions a, b, and c. Assume that all packets have the same length L = 20 bits. Furthermore, assume that the transmission capacity C 0 of the link after the leaky-bucketcontrolled buffers are equal to ( very fast ) The three sessions access a single link with transmission capacity C = 300 bits per second and space (in queue or in service) for B = 180 bits. However, before accessing the link, the session a and b go through the buffer 1 which is again controlled by a leaky bucket. Buffer 1 has space (in queue or in service) for B 1 = 200 bits and can again transmit data at a speed of C 0 =. The parameters for the system are as follows, β a = 3 permits, r b = 5 permits per second, β 1 = 6 permits, r c = 4 permits per second What are the maximal rates r a, r 1, and the maximal number of saved permits β b, β c, that we can allocate to the corresponding leaky buckets so that we can guarantee that there will never be a buffer overflow (packet loss) at either of the two buffers? 4

5 Question 4 (10 points) Consider two sessions, 1 and 2, which share a single link with capacity C. Assume that both sessions use a congestion window (as in TCP) to control the transmission rate. Let w 1, and w 2, be the window size of session 1, and 2, respectively; and let x 1, and x 2, be the corresponding transmission rate of sessions 1, and 2, respectively. For this situation, we used in class a simplified model to analyze the AIMD (additiveincrease, multiplicative-decrease) algorithm of TCP congestion control, where we ignored the slow start phase and assumed that the threshold value w/2 is constant (steadystate). Using the same model, we can analyze the MIAD (multiplicative-increase, additivedecrease) algorithm for adjusting the congestion window size w 1 and w 2, where we set for n = 1, 2 w n = a I w n, when (x 1 + x 2 ) < C w n = w n 1, when (x 1 + x 2 ) C where a I > 1. Show the dynamic behavior of the MIAD algorithm using the diagrams on the next two pages, where X indicates the initial values for x 1 and x 2. 5

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8 Question 5 (10 points) Consider three node network given in the figure below. There are 6 sessions currently using the network: one using the route (1,2,3), one using the route (2,3,1), two sessions using the link (2,3), one using the route (3,1,2), and one using the link (3,1). What is the max-min fair rate allocation in this case? Is the whole network capacity used? 8