code: func (r *renoState) updateSlowStart(packetsAcked int) int {
  	// Don't let the congestion window cross into the congestion
  	// avoidance range.
  	newcwnd := r.s.sndCwnd + packetsAcked
  	if newcwnd >= r.s.sndSsthresh {
  		newcwnd = r.s.sndSsthresh
  		r.s.sndCAAckCount = 0
  	}
  
  	packetsAcked -= newcwnd - r.s.sndCwnd
  	r.s.sndCwnd = newcwnd
  	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 SSthreshold then it will return the number of packets that
  // must be consumed in congestion avoidance mode.