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Automatically exported from code.google.com/p/curvecp
For the additive increase step in congestion.go, the calculation
s.txThrottle*s.txThrottle*s.txThrottle can overflow:
// These maths taken straight from the implementation. I'm
// not sure I get why it adjusts the way it does, but I
// want to make progress.
const timeConstant = 2251799813685248 // 2^51
if s.txThrottle < 16*time.Millisecond {
// N = N - cN^3, avoids 1 division and is close to the next curve.
s.txThrottle -= s.txThrottle * s.txThrottle * s.txThrottle / timeConstant
} else {
// N = N/(1 + cN^2)
s.txThrottle = s.txThrottle / (1 + s.txThrottle*s.txThrottle/timeConstant)
}
Since s.txThrottle here can still be up to 15999999, and 15999999^3 exceeds
2^63, the calculation will overflow and wrap around.
The second expression can actually overflow too, but only if the connection
gets throttled down to less than about one block every three seconds.
Also of note is that since you're using integer arithmetic here, there's going
to be some loss of precision compared to curvecpmessage.c's floating point
calculation for the second case, which will limit how quickly the throughput
grows. It also means once txThrottle is less than about 47ms, it'll stop
reducing further because txThrottle*txThrottle/timeConstant will truncate to 0.
Probably the simplest thing to do is just always use the floating point
calculation strategy:
// N = N/(1 + cN^2)
d := float64(s.txThrottle)
s.txThrottle = time.Duration(d/(1+d*d*(1.0/(1<<51))) + 0.5)
(+0.5 is so it rounds and reaches a fixed point before collapsing to 0)
Original issue reported on code.google.com by [email protected]
on 31 Jan 2013 at 1:39
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