anchao commented on a change in pull request #5598:
URL: https://github.com/apache/incubator-nuttx/pull/5598#discussion_r813578014
##########
File path: net/tcp/tcp_send_buffered.c
##########
@@ -574,6 +547,37 @@ static uint16_t psock_send_eventhandler(FAR struct
net_driver_s *dev,
ninfo("ACK: wrb=%p seqno=%" PRIu32 " pktlen=%u sent=%u\n",
wrb, TCP_WBSEQNO(wrb), TCP_WBPKTLEN(wrb), TCP_WBSENT(wrb));
}
+
+#ifdef CONFIG_NET_TCP_FAST_RETRANSMIT
+ /* Fast Retransmit (RFC 5681): an acknowledgment is considered a
+ * "duplicate" when (a) the receiver of the ACK has outstanding data,
+ * (b) the incoming acknowledgment carries no data, (c) the SYN and
+ * FIN bits are both off, (d) the acknowledgment number is equal to
+ * the greatest acknowledgment received on the given connection
+ * and (e) the advertised window in the incoming acknowledgment equals
+ * the advertised window in the last incoming acknowledgment.
+ */
+
+ if (conn->tx_unacked < conn->sent &&
+ (flags & TCP_NEWDATA) == 0 &&
+ (tcp->flags & (TCP_SYN | TCP_FIN)) == 0 &&
+ ackno == conn->snd_prev_ack &&
+ conn->snd_wnd == conn->snd_prev_wnd)
+ {
Review comment:
There is another problem here. If the segment loss occurs before
multiple segments are sent, it will trigger more invalid retransmissions:
e.g:
```
1 -> good
2 -> lost
3 -> good
4 -> good
5 -> good
6 -> good
7 -> good
8 -> good
9 -> good
10 -> good
snd_dup_acks 0 <- 1 (ACK 1)
snd_dup_acks 1 <- 2 (DUP ACK 1)
snd_dup_acks 2 <- 3 (DUP ACK 1)
snd_dup_acks 3 <- 4 (DUP ACK 1)
2 -> rexmit(snd_dup_acks = 0)
snd_dup_acks 1 <- 5 (DUP ACK 1)
snd_dup_acks 2 <- 6 (DUP ACK 1)
snd_dup_acks 3 <- 7 (DUP ACK 1)
2 -> rexmit(snd_dup_acks = 0)
snd_dup_acks 1 <- 8 (DUP ACK 1)
snd_dup_acks 2 <- 9 (DUP ACK 1)
snd_dup_acks 3 <- 10 (DUP ACK 1)
2 -> rexmit(snd_dup_acks = 0)
```
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