Filtering Round Trip Time Measurements
The TCP header has no way to distinguish a retransmitted segment from the original
If the sender gets an ACK for a retransmitted packet, there’s no way to know if it’s for the original transmission or a retransmission
- I.e., the RTT measurement is unreliable
Therefore, only RTT measurements on segments that were ACKed the first time are used
Also, the RTO backoff is “clamped” for the next packet after a retransmitted one
- avoids stable collapse state