其次，RMD并非必须对它已接收到的全部消息作出确认。
Secondly, the RMD doesn't have to acknowledge the messages it has received.
窗口大小决定了已发送且当前处于未完成状态(等待接收确认消息)的消息的数量。
The window size determines the number of messages that have been sent but are currently outstanding, waiting for an acknowledgement to be received.
注意，这种确认并不是与消息一一对应的，而是代表对所有由RMD成功接收的消息的确认。
Note that the acknowledgement isn't just for one message, it acknowledges all the messages successfully received by the RMD.
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