Re: [PATCH net v2 1/2] tcp: refresh TS.Recent for accepted old ACKs

[Eric Dumazet]

On Fri, Sep 25, 2026 at 12:45 AM Jeff Jo <jeffjo@xxxxxxxxxx> wrote:
>
> 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. ACK validation already rejects old ACKs in SYN_RECV before this
> path, so no additional state check is needed.
>
> 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@xxxxxxxxxx>
> ---
>  net/ipv4/tcp_input.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 92bc60716f33..99baf14afdfd 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4465,6 +4465,12 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
>         return 1;
>
>  old_ack:
> +       /* An old ACK can carry new data. Update TS.Recent before SACK
> +        * processing can trigger a retransmission.
> +        */
> +       if (flag & FLAG_UPDATE_TS_RECENT)
> +               tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
> +

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>