computer network

CS420(G), Fall 2020

Assignment 7:

Q1: Consider the figure below, which plots the evolution of TCP's congestion window at the

beginning of each time unit (where the unit of time is equal to the RTT. In the abstract

model for this problem, TCP sends a "flight" of packets of size cwnd at the beginning of

each time unit. The result of sending that flight of packets is that either (i) all packets are

ACKed at the end of the time unit, (ii) there is a timeout for the first packet, or (iii) there is a

triple duplicate ACK for the first packet. In this problem, you are asked to reconstruct the

sequence of events (ACKs, losses) that resulted in the evolution of TCP's cwnd shown

below.

Consider the evolution of TCP's congestion window in the example above and answer the

following questions. The initial value of cwnd is 1 and the initial value of ssthresh (shown as

a red +) is 8.

1. Give the times at which TCP is in slow start. Format your answer like: 1,3,5,9 (If none submit blank)

2. Give the times at which TCP is in congestion avoidance. Format your answer like: 1,3,5,9 (If none submit blank)

3. Give the times at which TCP is in fast recovery. Format your answer like: 1,3,5,9 (If none submit blank)

4. Give the times at which packets are lost via timeout. Format your answer like: 1,3,5,9 (If none submit blank)

5. Give the times at which packets are lost via triple ACK. Format your answer like: 1,3,5,9 (If none submit blank)

6. Give the times at which the value of ssthresh changes (if it changes between t=3 and t=4, use t=4 in your answer)