Question

We discussed TCP’s estimation of RTT. Why do you think TCP avoids measuring the SampleRTT for retransmitted segments?

Answer 1

This is because the same data is contained in the retransmitted segment. This also means that sequence numbers would also be the same as the ones sent before with previous packets.

This makes one expect the similar ACK number. When ACK is received, there is no way of knowing if it is for the previous packet or retransmitted packet.

Usually delayed ACKs are received for retransmitted packets. But it could also be related to the previous packet. Which means there has been some delay either in the ACK for the previous packet or retransmitted one.

If the delayed time is also counted in RTT estimation, RTT average would drop by a big number. It would in turn trigger retransmission again, and that would cause jammed traffic in the network.