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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.

Fast Retransmit:

The sender can now retransmit the missing packets before the timer expires. This is said to be fast reransmit.

Fast Recovery:

• The receipt of acknowledgements shows that there is no congestion to justify a slow start.
• The sender can continue with the current congestion window and performs a fast recovery from the packet loss.

     2). The other reason for activating slow start is a time-out due to missing acknowledgment.

See more:

• searchnetworking.techtarget.com/sDefinition/0,,sid7_gci542023,00.html