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To things lead to a reduction of the congestion threshold are,
1). One is a sender receiving continuous acknowledgments for the same packet. This explains 2 things, One is that the receiver got all packets upto the acknowledged packet in sequence. In TCP, a receiver sends acknowledgments only if it receives any packets from the sender. Hence, the gap in the packet stream is not only due to congestion, it may also due to a transmission error.
The sender can now retransmit the missing packets before the timer expires. This is said to be fast reransmit.
2). The other reason for activating slow start is a time-out due to missing acknowledgment.