虽然 SIP 是请求-响应协议，但每个请求不一定只有一个响应与之对应。
While SIP is a request-response protocol, there is not necessarily only one response to every one request.
Active：目前登录到系统的已命名用户，但不一定是处于提交一个请求或者等待一个请求完成的过程中(比如说一个用户正在阅读一个报告)。
Active: Named users currently logged onto the system but not necessarily in the process of submitting a request or waiting for one to finish (say like a user reading a report).
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