Subject: Update of smoothed RTT in case of retransmitted segments

From: Sebastian Zimmermann <[email-masked]>
Date: 24 Apr 2002 15:09:22 +0200

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

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From: Craig Partridge <[email-masked]>
Date: Tue, 30 Apr 2002 09:59:46 -0400

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

I haven't gone back to re-read our paper, but clearly Phil and I intended
that the ack be ignored, as it may implicitly contain the RTT feedback.

Craig