Flow control is used to avoid that the sender is overflowing the receiver’s buffer.
Many protocols provide flow control by a so-called sliding window scheme.
In sliding window protocols the sender maintains a set of sequence numbers corresponding to packets it is permitted to send –
the sending window. Similarly the receiver maintains a receiving window corresponding to the set of packets it is permitted to accept.
In acknowledgements the receiver informs the sender about the correctly received packets and how many packets it is able to receive (credit).
The sender adjusts the sending window according to this information.
In the animation you are able to set the credit variable at the receiver. The sequence numbers are numerated from 0 to 7 and start again at 0.