而且这种机制是在网络层实现的，不必对MAC层协议进行修改。
NRED is located at the network layer, and has no need of any modification in MAC layer.
修改了网络协议，这样客户端跑615版的时候不会再看到服务器运行上的老版本。
Change in network protocol, therefore clients running 0.615 won't be able to see servers running older versions.
会话发起协议(SIP)是IP网络应用层的控制协议，用于建立、修改和终止两个或多个参与者之间的会话。
The session Initiation protocol (SIP) is an application-layer control protocol for creating, modifying and terminating sessions among two or more parties.
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