Motivation for this modification is that especially on some wireless
network technologies there are delay spikes that trigger RTO even though
no packets are lost. F-RTO sender continues by sending new data after RTO
retransmission in order to avoid unnecessary retransmissions in that case.  
If the sender sees any duplicate acks after the RTO retransmission, it
reverts to traditional slow start retransmissions. If new acks arrive
after forward transmissions, they very likely indicate that the RTO was
indeed spurious and the sender can continue sending new data (because 
only one segment was retransmitted).