The sender maintains cwnd as a completely separate parameter that does not go in any packets. It doesn't "know" anything; the value of this parameter is arrive

The sender maintains cwnd as a completely separate parameter that does not go in any packets. It doesn't "know" anything; the value of this parameter is arrived at by some set of heuristics. See for instance https://en.wikipedia.org/wiki/TCP_congestion-avoidance_algorithm https://en.wikipedia.org/wiki/TCP_congestion-avoidance_algor...