code: func (c *cubicState) updateSlowStart(packetsAcked int) int {
  	// Don't let the congestion window cross into the congestion
  	// avoidance range.
  	newcwnd := c.s.sndCwnd + packetsAcked
  	enterCA := false
  	if newcwnd >= c.s.sndSsthresh {
  		newcwnd = c.s.sndSsthresh
  		c.s.sndCAAckCount = 0
  		enterCA = true
  	}
  
  	packetsAcked -= newcwnd - c.s.sndCwnd
  	c.s.sndCwnd = newcwnd
  	if enterCA {
  		c.enterCongestionAvoidance()
  	}
  	return packetsAcked
  }
docs: // updateSlowStart will update the congestion window as per the slow-start
  // algorithm used by NewReno. If after adjusting the congestion window we cross
  // the ssThresh then it will return the number of packets that must be consumed
  // in congestion avoidance mode.