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net/tcp: implement the fast retransmit
RFC2001: TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms ... 3. Fast Retransmit Modifications to the congestion avoidance algorithm were proposed in 1990 [3]. Before describing the change, realize that TCP may generate an immediate acknowledgment (a duplicate ACK) when an out- of-order segment is received (Section 4.2.2.21 of [1], with a note that one reason for doing so was for the experimental fast- retransmit algorithm). This duplicate ACK should not be delayed. 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. Since TCP does not know whether a duplicate ACK is caused by a lost segment or just a reordering of segments, it waits for a small number of duplicate ACKs to be received. It is assumed that if there is just a reordering of the segments, there will be only one or two duplicate ACKs before the reordered segment is processed, which will then generate a new ACK. If three or more duplicate ACKs are received in a row, it is a strong indication that a segment has been lost. TCP then performs a retransmission of what appears to be the missing segment, without waiting for a retransmission timer to expire. Change-Id: Ie2cbcecab507c3d831f74390a6a85e0c5c8e0652 Signed-off-by: chao.an <anchao@xiaomi.com>
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