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 (ii) 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 swnd 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. Give or Mark the times at which TCP is in slow start, congestion avoidance and fast recovery at the start of a time slot, when the flight of packets is sent. Give or Mark the times at which the first packet in the sent flight of packets is lost, and indicate whether that packet loss is detected via timeout, or by triple duplicate ACKs. 15 Give or Mark the times at which the value of ssthresh changes, and give the new value of ssthresh.
