The Babel RTT extension employs metric smoothing to dampen route
oscillations in the face of varying RTT values between two peers[0].
This patch implements such dampening in Bird, roughly following the
implementation in babeld (i.e., using the same exponential function
definition). The main difference is that we calculate everything in the
native Bird microsecond time unit (and increase constants accordingly), and
that we split out the smoothed metric calculation in two function variants,
one that has no side effects and one that does.
[0] https://arxiv.org/pdf/1403.3488.pdf
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
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