http://jaseywang.me/2012/12/20/%E7%9B%91%E6%8E%A7%E7%BD%91%E7%BB%9C%E8%AE%BE%E5%A4%87%E6%B5%81%E9%87%8F/
http://docstore.mik.ua/orelly/networking_2ndEd/snmp/appa_01.htm
