wireshark 如何知道一个报文是快速重传报文

Question

快速重传的触发条件是连续出了三个相同ack的冗余报文。也就是说要有4个相同ack的报文。

只有两个冗余，三个相同ack报文，为什么能触发快速重传呢？

是有其他触发快速重传的条件，还是说wireshark识别错了，其实这条报文不是快速重传报文，只是来得比较慢。

Answer 1

TCP Fast Retransmission

Set when all of the following are true:

• This is not a keepalive packet.
• In the forward direction, the segment size is greater than zero or the SYN or FIN is set.
• The next expected sequence number is greater than the current sequence number.
• We have more than two duplicate ACKs in the reverse direction.
• The current sequence number equals the next expected acknowledgement number.
• We saw the last acknowledgement less than 20ms ago.