Update of smoothed RTT in case of retransmitted segments

[Sebastian Zimmermann <[e-mail]>]

Hello,

I need some help with the implementation of the smoothed RTT calculation
in TCP in case of retransmitted segments:

Assume that segment 9 is the one being timed. However, segment 8 is
lost, so an acknowledgement for segment 9 cannot be generated by the
receiver.
The sender detects a timeout for segment 8 and retransmits it. Karn's
algorithm only says that retransmitted segments may not be timed, but -
in this case - segment 9 never was retransmitted, so the timer is not
stopped. After the receiver receives the retransmitted segment 8, it
will ack segment 9.

The problem: the measured RTT is completely wrong, because it still
contains the retransmission of segment 8.

Since Karn's algorithm doesn't avoid this problem, which one does?

Thanks,
 Sebastian