Constant SETTLE_PAST_TIMERS 

pub const SETTLE_PAST_TIMERS: Duration;

The shortest settling period that does not accuse the protocol of leaking.

RFC 3261’s Timer J and Timer K are both 64·T1 — thirty-two seconds with the default T1 — and a completed transaction is supposed to sit there for that long. Forty seconds leaves room for a run whose last call ended a little after the load did.

A soak measured over a shorter period is measuring the RFC, and the result of measuring the RFC is a failing test that somebody eventually deletes.

This is a definition of silence in the sense docs/designs/media.md gives the term: how long a hole has to be before “everything that was going to end has ended” is true, so that what is still resident afterwards is a leak rather than the specification. It is not a bound on failure and it is not a measurement — a run that settles for longer is more trustworthy, not less, which is why the constant is a floor (X-44).