code: func (kcp *KCP) ReserveBytes(n int) bool {
  	if n >= int(kcp.mtu-IKCP_OVERHEAD) || n < 0 {
  		return false
  	}
  	kcp.reserved = n
  	kcp.mss = kcp.mtu - IKCP_OVERHEAD - uint32(n)
  	return true
  }
docs: // ReserveBytes keeps n bytes untouched from the beginning of the buffer
  // the output_callback function should be aware of this
  // return false if n >= mss