Flow control - is a mechanism to prevent the server from overwhelming the receiver with data it may not be able to process. This is by advertising rwnd (receive window) by each side. rwnd - the size of available buffer to receive data.

Congestion control - is a set of mechanisms aimed at preventing the network from being overwhelmed. Example mechanisms are - Slow Start, Congestion Avoidance

Slow Start - allows TCP to slowly probe the network to determine the available capacity. It's used at the beginning of the transfer or after repairing loss, detected by retransmission timer. To start, a server initialises a new congestion window - sender side limit on the amount of data it can have in flight before receiving an ACK.

Congestion Avoidance - when TCP detects a packet loss, it adjusts a cwnd and after that Congestion Avoidance algorithm kicks in (e.g. BBR, CUBIC etc…)

BDP (also called “the optimal window size”) - the maximum amount of unacknowledged data that can be in the flight at any point in time.

Example calculating the max BW for rwnd = 16KB and RTT = 100ms.

BW bits/s = RWND bits / RTT sec

BDP (RWND bits) = BW bits/s * RTT sec

(16*1024*8)/0.1 = 131072/0.1 = 1,310,720 bits/s (1,31 Mbps)