[e2e] TCP's RTT measurements

[Sebastian Zimmermann]

Hello,

I have a question regarding TCP's RTT measurement in case of packet
loss.

Please assume that segment 14 is the one being timed. However, the
previous segment 13 is dropped on the way to the receiver. This is
detected by a timeout. Segment 13 is then retransmitted. Karn's
algorithm says that retransmitted segments may not be timed. In this
case, however, segment 14 is not retransmitted. So it will still be used
for RTT measurement. Since the acknowledgement 14 is only generated
after the second segment 13 is received by the receiver, the RTT
measurement will be much too large.

How does TCP avoid this problem?

Thanks,
 Sebastian