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。
TCP may generate an immediate acknowledgment (a duplicate ack) when an out - of-order segment is received.
当收到一个出问题的分片，Tcp立即产生一个应答。
TCP may generate an immediate acknowledgment (a duplicate ack) when an out - of-order segment is received.
当收到一个出问题的分片，Tcp立即产生一个应答。
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