Question

Consider a TCP connection operating at a point of time with the congestion window of size \(12 \, \text{MSS}\) (Maximum Segment Size), when a timeout occurs due to packet loss. Assuming that all the segments transmitted in the next two RTTs (Round Trip Time) are acknowledged correctly, the congestion window size (in MSS) during the third RTT will be ............

Hint:

After a timeout in TCP, the congestion window grows exponentially in the slow-start phase.

Answer 1

When a timeout occurs, TCP congestion control reduces the congestion window to \(1 \, \text{MSS}\) and enters the slow-start phase. In slow start:
During the first RTT, the window size doubles: \(1 \to 2 \, \text{MSS}\).
During the second RTT, the window size doubles again: \(2 \to 4 \, \text{MSS}\).
Thus, at the start of the third RTT, the congestion window size will be \(4 \, \text{MSS}\). Final Answer: \[ \boxed{4 \, \text{MSS}} \]