[PATCH net] tcp: fix tcp_disordered_ack() vs usec TS resolution

[PATCH net] tcp: fix tcp_disordered_ack() vs usec TS resolution

[Eric Dumazet]

After commit 939463016b7a ("tcp: change data receiver flowlabel after one dup")
we noticed an increase of TCPACKSkippedPAWS events.

Neal Cardwell tracked the issue to tcp_disordered_ack() assumption
about remote peer TS clock.

RFC 1323 & 7323 are suggesting the following:
  "timestamp clock frequency in the range 1 ms to 1 sec per tick
   between 1ms and 1sec."

This has to be adjusted for 1 MHz clock frequency.

This hints at reorders of SACK packets on send side,
this might deserve a future patch.
(skb->ooo_okay is always set for pure ACK packets)

Fixes: 614e8316aa4c ("tcp: add support for usec resolution in TCP TS values")
Co-developed-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Morley <morleyd@google.com>
---
 net/ipv4/tcp_input.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 90de838a274519110ce0ac84552c4caedb826eb7..701cb87043f28079286044208128c2d687908991 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4368,6 +4368,23 @@ EXPORT_SYMBOL(tcp_do_parse_auth_options);
  * up to bandwidth of 18Gigabit/sec. 8) ]
  */
 
+/* Estimates max number of increments of remote peer TSval in
+ * a replay window (based on our current RTO estimation).
+ */
+static u32 tcp_tsval_replay(const struct sock *sk)
+{
+	/* If we use usec TS resolution,
+	 * then expect the remote peer to use the same resolution.
+	 */
+	if (tcp_sk(sk)->tcp_usec_ts)
+		return inet_csk(sk)->icsk_rto * (USEC_PER_SEC / HZ);
+
+	/* RFC 7323 recommends a TSval clock between 1ms and 1sec.
+	 * We know that some OS (including old linux) can use 1200 Hz.
+	 */
+	return inet_csk(sk)->icsk_rto * 1200 / HZ;
+}
+
 static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
@@ -4375,7 +4392,7 @@ static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
 	u32 seq = TCP_SKB_CB(skb)->seq;
 	u32 ack = TCP_SKB_CB(skb)->ack_seq;
 
-	return (/* 1. Pure ACK with correct sequence number. */
+	return	/* 1. Pure ACK with correct sequence number. */
 		(th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&
 
 		/* 2. ... and duplicate ACK. */
@@ -4385,7 +4402,8 @@ static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
 		!tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&
 
 		/* 4. ... and sits in replay window. */
-		(s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <= (inet_csk(sk)->icsk_rto * 1024) / HZ);
+		(s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <=
+		tcp_tsval_replay(sk);
 }
 
 static inline bool tcp_paws_discard(const struct sock *sk,
-- 
2.43.0.472.g3155946c3a-goog

Re: [PATCH net] tcp: fix tcp_disordered_ack() vs usec TS resolution

[patchwork-bot+netdevbpf]

Hello:

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

On Thu,  7 Dec 2023 18:13:42 +0000 you wrote:
> After commit 939463016b7a ("tcp: change data receiver flowlabel after one dup")
> we noticed an increase of TCPACKSkippedPAWS events.
> 
> Neal Cardwell tracked the issue to tcp_disordered_ack() assumption
> about remote peer TS clock.
> 
> RFC 1323 & 7323 are suggesting the following:
>   "timestamp clock frequency in the range 1 ms to 1 sec per tick
>    between 1ms and 1sec."
> 
> [...]

Here is the summary with links:
  - [net] tcp: fix tcp_disordered_ack() vs usec TS resolution
    https://git.kernel.org/netdev/net/c/9c25aae0132b

You are awesome, thank you!
-- 
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