tcp: fix F-RTO may not work correctly when receiving DSACK
Currently DSACK is regarded as a dupack, which may cause
F-RTO to incorrectly enter "loss was real" when receiving
DSACK.
Packetdrill to demonstrate:
// Enable F-RTO and TLP
0 `sysctl -q net.ipv4.tcp_frto=2`
0 `sysctl -q net.ipv4.tcp_early_retrans=3`
0 `sysctl -q net.ipv4.tcp_congestion_control=cubic`
// Establish a connection
+0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0 bind(3, ..., ...) = 0
+0 listen(3, 1) = 0
// RTT 10ms, RTO 210ms
+.1 < S 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+0 > S. 0:0(0) ack 1 <...>
+.01 < . 1:1(0) ack 1 win 257
+0 accept(3, ..., ...) = 4
// Send 2 data segments
+0 write(4, ..., 2000) = 2000
+0 > P. 1:2001(2000) ack 1
// TLP
+.022 > P. 1001:2001(1000) ack 1
// Continue to send 8 data segments
+0 write(4, ..., 10000) = 10000
+0 > P. 2001:10001(8000) ack 1
// RTO
+.188 > . 1:1001(1000) ack 1
// The original data is acked and new data is sent(F-RTO step 2.b)
+0 < . 1:1(0) ack 2001 win 257
+0 > P. 10001:12001(2000) ack 1
// D-SACK caused by TLP is regarded as a dupack, this results in
// the incorrect judgment of "loss was real"(F-RTO step 3.a)
+.022 < . 1:1(0) ack 2001 win 257 <sack 1001:2001,nop,nop>
// Never-retransmitted data(3001:4001) are acked and
// expect to switch to open state(F-RTO step 3.b)
+0 < . 1:1(0) ack 4001 win 257
+0 %{ assert tcpi_ca_state == 0, tcpi_ca_state }%
Fixes: e33099f96d ("tcp: implement RFC5682 F-RTO")
Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/1650967419-2150-1-git-send-email-yangpc@wangsu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
d9157f6806