在序列生命周期的任一时间，R m源都可以发送一个包含ackrequested头部区域的消息，这一消息请求RM目的地端发送SequenceAcknowledgement消息来响应。

At any time during the life of a Sequence, the RM Source can send a message containing an AckRequested header block which requests that the RM Destination send a SequenceAcknowledgement.

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