code: func (c *cubicSender) BandwidthEstimate() Bandwidth {
  	srtt := c.rttStats.SmoothedRTT()
  	if srtt == 0 {
  		// If we haven't measured an rtt, the bandwidth estimate is unknown.
  		return 0
  	}
  	return BandwidthFromDelta(c.GetCongestionWindow(), srtt)
  }
docs: // BandwidthEstimate returns the current bandwidth estimate