[PATCH net 1/2] tcp: refresh TS.Recent for accepted old ACKs

[Jeff Jo <jeffjo@openai.com>]

A TCP packet can carry new data while acknowledging traffic in the
opposite direction. With overlapping traffic in both directions, a
delayed packet's acknowledgment can be older than one Linux has already
accepted, even when that packet fills a gap in the received data.

Linux accepts the data, but tcp_ack() takes the old_ack path and skips
updating TS.Recent, the timestamp saved for outgoing acknowledgments.
The reply therefore echoes an older timestamp. If the sender uses this
echo to measure round-trip time after a long idle period, its estimate
includes the idle time and can reduce its sending rate.

Update TS.Recent in old_ack using tcp_replace_ts_recent(), before SACK
processing can trigger a transmission. This reuses the existing timestamp
and sequence checks, including PAWS protection against old duplicate
packets. Leave SYN_RECV unchanged because its caller rejects old ACKs;
established and closing connections accept them.

Echoing the timestamp of the packet that fills the receive gap follows
RFC 7323 section 4.3. In a socket reproduction with 300 seconds idle,
controlled reordering and retransmission to exercise timestamp-based RTT
sampling, the sender's smoothed round-trip time was 37.5 seconds without
the fix and 15.5 ms with it.

Fixes: 12fb3dd9dc3c ("tcp: call tcp_replace_ts_recent() from tcp_ack()")
Assisted-by: LLM sparse
Signed-off-by: Jeff Jo <jeffjo@openai.com>
---
 net/ipv4/tcp_input.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 92bc60716f33..f3f4c905ce00 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4465,6 +4465,13 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	return 1;
 
 old_ack:
+	/* Closing connections also accept old ACKs. SYN_RECV rejects them in
+	 * the caller, so leave its timestamp unchanged. Update before SACK
+	 * processing can trigger a retransmission.
+	 */
+	if (sk->sk_state != TCP_SYN_RECV && (flag & FLAG_UPDATE_TS_RECENT))
+		flag |= tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
+
 	/* If data was SACKed, tag it and see if we should send more data.
 	 * If data was DSACKed, see if we can undo a cwnd reduction.
 	 */
-- 
2.55.0