Congestion Control:

It is used for the send to decide what is the packet size it send out due to the limitation of network bandwith.  It use slow start and fast re-transmission

• slow start, in the beginning of tcp start, it send small number of MSS (max segment size) in size, once the ack is received, it double it, the size called  congestion window size 
• once it size reach to threshold , or a loss detected, or receive advertised window limited; it enter the congestion avoidance mode where it only increase 1 MSS per RSS until there is a timeout
• once it reach to timeout, it will reset congestion windown to 1MSS, threshold setto half of the window size before timeout, slow start re-start.

Fast retransmission