Jade Cheng March 4th, 2009 (late) ICS 451 Exercise 2 [Question] In Section 3.5.3 we discussed TCP's estimation of RTT. Why do you think TCP avoids measuring the SampleRTT for retransmitted segments? Answer: The retransmitted segments contain the same data and the same sequence numbers as the previously sent packets. Therefore, for the retransmitted segments, we would expect the same ACK numbers as the previously sent TCP segments. When we receive these paticular ACKs from the receiver, we don't really know or care whether they were sent as responses to the retransmitted segments or to the previously sent TCP segment. It is possible that after a very short time, we receive the ACK for the retransmitted data segment. But that could actually be a delayed ACK for the original TCP segment. Either the data transmission or the ACK reply was probably delayed. Both of them could result in a late ACK. If we count this time in to compute the RTT estimation, we would mistakenly drop the RTT average by a big percentage. This RTT would trigger a faster retransmission, which would worsen the already jammed network traffic.
