From: Pengcheng Yang yangpc@wangsu.com
[ Upstream commit d9157f6806d1499e173770df1f1b234763de5c79 ]
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: e33099f96d99 ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_input.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index c25a95c74128..2e267b2e33e5 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -3814,7 +3814,8 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag) tcp_process_tlp_ack(sk, ack, flag);
if (tcp_ack_is_dubious(sk, flag)) { - if (!(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP))) { + if (!(flag & (FLAG_SND_UNA_ADVANCED | + FLAG_NOT_DUP | FLAG_DSACKING_ACK))) { num_dupack = 1; /* Consider if pure acks were aggregated in tcp_add_backlog() */ if (!(flag & FLAG_DATA))