In Section 3.5.4, we saw that TCP waits until it has received three duplicate ACKs before performing a fast retransmit. Why do you think the TCP designers chose not to perform a fast retransmit after the first duplicate ACK for a segment is received?
Packets can arrive out of order from the Internet Protocal layer. So, whenever an out of order packet would be received, it would generate a duplicate acknowledgement(ACK).If we perform retransmission after the first duplicate ACK, it would lead the sender to introduce too many redundant packets in the network.
Hence, the TCP designers chose not to perform a fast retransmit after the first duplicate ACK for a segment is received.