Ack: Indicates that the acknowledgment number field is significant.
ACK：表明确认号字段是有意义的。
Send an immediate ACK to the customer confirming the request and supplying a confirmation number.
向客户发送即时的ACK，确认请求并提供校验码。
The purpose of this duplicate ACK is to let the other end know that a segment was received out of order, and to tell it what sequence number is expected.
这个相同应答的意图是让对端知道一个分片被收到的时候出现问题，并且告诉它希望得到的序列号。
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