[PATCH net] tcp: fix F-RTO may not work correctly when receiving DSACK

[PATCH net] tcp: fix F-RTO may not work correctly when receiving DSACK

[Pengcheng Yang]

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>
---
 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 1595b76..c54accc 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3867,7 +3867,8 @@ 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))
-- 
1.8.3.1

Re: [PATCH net] tcp: fix F-RTO may not work correctly when receiving DSACK

[Neal Cardwell]

On Tue, Apr 26, 2022 at 6:04 AM Pengcheng Yang <yangpc@wangsu.com> wrote:
>
> 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>
> ---

Thanks for the fix and test! Both look good to me. The patch passes
all of our team's packetdrill tests, and this new test passes as well.

Acked-by: Neal Cardwell <ncardwell@google.com>
Tested-by: Neal Cardwell <ncardwell@google.com>

thanks,
neal

Re: [PATCH net] tcp: fix F-RTO may not work correctly when receiving DSACK

[Eric Dumazet]

On Thu, Apr 28, 2022 at 10:06 AM Neal Cardwell <ncardwell@google.com> wrote:
>

>
> Thanks for the fix and test! Both look good to me. The patch passes
> all of our team's packetdrill tests, and this new test passes as well.
>
> Acked-by: Neal Cardwell <ncardwell@google.com>
> Tested-by: Neal Cardwell <ncardwell@google.com>
>

Indeed, thank you all for a nice changelog, nice patch, and exhaustive
tests from Neal.

Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH net] tcp: fix F-RTO may not work correctly when receiving DSACK

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 26 Apr 2022 18:03:39 +0800 you wrote:
> 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`
> 
> [...]

Here is the summary with links:
  - [net] tcp: fix F-RTO may not work correctly when receiving DSACK
    https://git.kernel.org/netdev/net/c/d9157f6806d1

You are awesome, thank you!
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