code: func (r *receiver) getSendParams() (rcvNxt seqnum.Value, rcvWnd seqnum.Size) {
  	// Calculate the window size based on the current buffer size.
  	n := r.ep.receiveBufferAvailable()
  	acc := r.rcvNxt.Add(seqnum.Size(n))
  	if r.rcvAcc.LessThan(acc) {
  		r.rcvAcc = acc
  	}
  
  	return r.rcvNxt, r.rcvNxt.Size(r.rcvAcc) >> r.rcvWndScale
  }
docs: // getSendParams returns the parameters needed by the sender when building
  // segments to send.