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htlcswitch/link: correct bias in fee update backoff

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This commit corrects the distribution used to
schedule a link's randomized backoff for fee
updates. Currently, our algorithm biases the
lowest value in the range, with probability
equal to lower/upper, or the ratio of the lower
bound to the upper. This distribution is skewed
more heavily as lower approaches upper.

The solution is to sample a random value in the
range upper-lower, then add this to our lower
bound. The effect is a uniformly distributed
timeout in [lower, upper).
This commit is contained in:
Conner Fromknecht 2018-06-29 18:22:08 -07:00
parent 9205720bea
commit 5fa8124aa1
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GPG Key ID: 39DE78FBE6ACB0EF
1 changed files with 1 additions and 6 deletions
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7
htlcswitch/link.go
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@ -995,12 +995,7 @@ out:
func (l *channelLink) randomFeeUpdateTimeout() time.Duration {
lower := int64(l.cfg.MinFeeUpdateTimeout)
upper := int64(l.cfg.MaxFeeUpdateTimeout)
rand := prand.Int63n(upper)
if rand < lower {
rand = lower
}
return time.Duration(rand)
return time.Duration(prand.Int63n(upper-lower) + lower)
}
// handleDownStreamPkt processes an HTLC packet sent from the downstream HTLC