Standard TCP
When fast retransmit is invoked because of receipt of a third duplicate ACK (or other signal, if conventional fast retransmit initiation is not used), the follow- ing actions are performed:
1. ssthresh is updated to no more than the value given in equation [1].
2. The fast retransmit algorithm is performed, and cwnd is set to (ssthresh + 3*SMSS).
3. cwnd is temporarily increased by SMSS for each duplicate ACK received.
4. When a good ACK is received, cwnd is reset back to ssthresh.
The actions in steps 2 and 3 constitute fast recovery.
Step 2 first adjusts cwnd, which usually causes it to be reduced to half of its former value, and then tempo- rarily inflates it to take into account the fact that the receipt of each duplicate ACK indicates that some packet has left the network (and thus should permit another to be inserted).