CDN的全称是Content Delivery Network，即内容分发网络
http://baike.baidu.com/link?url=Wd-IGGgslfJemdpuT3Y0BUi88RPQYBCbztAVlo2RWSj66BbbeXGiEWEIKPWxwt3MeIgAXUK-tdg1u2lIkj5feq
