=head1 Improving Round-Trip Time Estimates in Reliable Transport Protocol
Phil Karn (Bell Communications Research) and Craig Partridge (Harvard), 1991

B<Summary.> The authors give a new method for improving RTT
estimations that converges quickly to accurate estimated RTTs and,
therefore, RTOs when RTTs change but does not modify the SRTT
(smoothed round-trip time) due to retransmission ambiguity or congestion.

=head1 More Detail

The performance of TCP depends (among other things) on the successful
estimation of the round-trip time (RTT). A I<smoothed round-trip time> (SRTT)
is calculated as a function of the current SRTT and an I<alpha> term The
I<alpha> term determines how quickly the SRTT adapts to changes in the
most recently measured RTT. The SRTT is multiplied by a I<beta>
factor (>1) to determine the retransmission time-out (RTO). Typical
values for I<alpha> and I<beta> are .0875 & 2, respectively.

If the RTO is too large, bandwidth is wasted because time-outs are not
detected quickly. It the RTO is too small, packets are erroneously
retransmitted and can lead to congestion and, again, wasted
bandwidth. It is especially critical to converge to accurate RTOs when
the RTT has changed (perhaps the shortest link has gone down & an
alternate route is now being used).

I<Retransmission ambiguity> arises when retransmissions have been sent 
out and, when the ACK is received, it is not clear which packet (original
or retransmission) is being acked. Retransmission ambiguity causes
the inaccuracies in RTT estimation.

Previous algorithms to estimate RTT (and their drawbacks) include:

=over 4

=item *

Measuring RTT from the last transmission: leads to underestimation of
the RTT -> RTO too small -> unnecessary retransmissions.

=item *

Measure RTT from the first transmission: leads to overestimation of RTT
-> RTO too big -> wasted bandwidth while waiting for ACKS that never come 

=item *

Measure only RTTs of packets that aren't retransmitted: if the RTO is 
smaller than a new, larger RTT, the algorithm never has a chance to catch
up: all the samples are discarded because they are retransmissions. 

=back

The I<Karn Algorithm> measures only those packets that aren't 
retransmitted I<plus> more aggressive RTO backoff to enable the collection
of accurate RTT measurements "uncontaminated by retransmission ambiguity".
The algorithm is:

B<When an acknowledgement arrives for a datagram that has been sent more>
B<than once ... , ignore any RTT measurement based on this datagram thus>
B<avoiding the retransmission ambiguity problem. In addition, the >
B<backed-off RTO for this datagram is kept for the next datagram. Only >
B<when it (or a succeeding datagram) is acknowledged without an >
B<intervening retransmission will the RTO be recalculated ...>

An implementation in a very lossy network (not congested -- so the RTT
should *not* go up just because a datagram was lost -- only if the RTT
time really goes up), extimated RTT times remained very stable changing
in response only to true RTT changes.