P36. 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?

The Packets can be arrived out of an order from the Internet Protocol layer.  

So, whenever the out of an order packets would be received then, it would be generated the duplicate ACK's, if we perform the re-transmission after the first duplicate ACK would be lead the senders to introduced too many redundant packets in the networks.

Explanation:

All the bytes, in the Transmission Control Protocol connections, are the numbered, from the beginning at the randomly choose the initial sequence number (ISN).  The SYN packets consumes the one sequence number, so the data will be actual and it begins at the ISN+1.   The receivers ack's sequences the number x acknowledged the receipts of all the data bytes that is less than byte the number x.

Not so sure about this sorry