Round Trip Time Estimation

Useful to know when to retransmit
if have not received ACK within the RTT
(plus a bit)
then assume packet lost

But how to measure the RTT?

Measure delay between packet and its ACK
easy

But
Send packet
wait ... wait ... wait (nothing)
Retransmit packet
ACK arrives

Which packet was acknowledged?
The initial packet
acknowledged slower than expected
Or the retransmit?