Note that the acknowledgement isn't just for one message, it acknowledges all the messages successfully received by the RMD.
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。
2
Secondly, the RMD doesn't have to acknowledge the messages it has received.
其次，RMD并非必须对它已接收到的全部消息作出确认。
3
A sequence can be thought of as the "reliability contract" under which the RMS and RMD agree to reliably transfer messages from the sender to the receiver.