An attack called the SYN flood ensures that the ack packet never returns so that the server runs out of room to process incoming connections.
有一种称为syn泛滥(SYN flood)的网络攻击，它使ack数据包无法返回，导致服务器用光内存空间，无法处理到来的连接。
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。
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