code: func (d *Downloader) requestTTL() time.Duration {
  	var (
  		rtt  = time.Duration(atomic.LoadUint64(&d.rttEstimate))
  		conf = float64(atomic.LoadUint64(&d.rttConfidence)) / 1000000.0
  	)
  	ttl := time.Duration(ttlScaling) * time.Duration(float64(rtt)/conf)
  	if ttl > ttlLimit {
  		ttl = ttlLimit
  	}
  	return ttl
  }
docs: // requestTTL returns the current timeout allowance for a single download request
  // to finish under.