Abstract class defines a reference to a member of the multicast group. 抽象类定义了对多播组成员的引用。
Then the server, by sending a single packet of data to that multicast group, would be sending to all the clients as the packets are replicated somewhere along the way. 然后服务器将单个数据包发送到多播组里，当信息包沿着路线被重复某地操作时，将发送到所有的客户机上。
