fast retransmit and fast recovery mechanism

Question

Consider the following plot of TCP window size as a function of time. Assume TCP is operating with slow start, congestion avoidance, fast retransmit and fast recovery mechanism.

What is the time interval where the slow start runs again because of a time-out and because of 3 ACKs respectively ?

A 16, 22

B 23, 17

C 17, 23

D 17, 22

Answer given 
Solution :

(b)

If a time out occurs there is a stronger possibility of congestion, hence TCP reacts strongly. It sets the value of threshold to one-half of current window size. Set cwnd to size of one segment and starts slow start phase again.
If three ACKs are received, there is a weaker possibility of congestion, a segment may have been dropped. Hence TCP has a weaker reaction. It sets value of threshold to half of current window size. It sets cwnd to the value of threshold. It starts congestion avoidance phase again.

Please explain in more details about the concept underlying?

Comments

Can someone explain why do three Acks mean, the weaker possibility of congestion? And when are these generated?

Are these acks for the same packet or are these consecutive acks?

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Since acks are still being received thus congestion is less severe. While in timeout the acks are getting lost thus more severe congestion.

the acks are for the packet that was lost in the network when being sent to receiver. Since receiver is expecting it.

Answer 1

tcp follow selective repeat protocal and go back n protocol.

so when  receiver does not get packet according to ack  , which is sent by receiver to sender . then he sent three dublicate ack packet for same packet and in this time whatever sender send receiver accept it because it can accept out of order packet

Answer 2

according to me its 23,17  i.e. option B