如果在报文段中的数据未确认之前定时器超时，TCP就认为该报文段已经丢失或出现损坏，从而重传这一报文段。
If the timer expires before data in the segment is acknowledged, TCP assumes that the segment has lost or being damaged, and a retransmission is sent.
为了克服由大时延带来的重传时间间隔过大的缺点，采用了批量重传来及时重传丢失的报文段；
In order to alleviate the long retransmission intervals brought by long propagation delays, bulk retransmissions are introduced to retransmit the lost packets in time.
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