这不是一个只看数据包“头”的技术。 通过分析数据包体（Payload) 可以对Proxi进行处理。I.e., through deep-content analysis nothing can be skipped
Yahoo is just one example. One can specify a list of web sites on which need to be blocked.
Another advantage is to identify worms very fast since every packet is analyzed.
It seems US market is really diferent from China one. In USA this is getting hot, especially within universities. However the requirement is different since it is illegal to block the Internet access. What we can do is to restrict the traffic speed as soon as certain P2P applications are identified.
