Example KML overlay network link file

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
<Document>
  <NetworkLink>
    <Link>
      <href>overlay.kml</href>
      <refreshMode>onExpire</refreshMode>
    </Link>
  </NetworkLink>
</Document>
</kml>