Consider a TCP connection operating at a point of time with the congestion window of size 12 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 _________
Answer
4
Explanation
On a timeout, TCP sets the slow-start threshold to half the current window, \(12 / 2 = 6\) MSS, and restarts slow start from a window of 1 MSS. In slow start the window doubles every RTT while all segments are acknowledged:
- first RTT after the timeout: 1 MSS;
- second RTT: 2 MSS;
- third RTT: 4 MSS.
The window is still below the threshold of 6, so slow start has not yet given way to linear growth. Continuing from 6 MSS would be the fast-recovery response to triple duplicate ACKs, not to a timeout.
Answer: 4