Re: [PATCH net] tcp: Fix dup-ack detection on non-SACK bidirectional data transfer.

[Guillaume Nault <gnault@redhat.com>]

On Fri, Oct 23, 2026 at 01:04:48PM -0400, Neal Cardwell wrote:
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 99baf14afdfd..1feb370804b1 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -107,7 +107,8 @@ int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
> >  #define FLAG_TS_PROGRESS       0x40000 /* Positive timestamp delta */
> >
> >  #define FLAG_ACKED             (FLAG_DATA_ACKED|FLAG_SYN_ACKED)
> > -#define FLAG_NOT_DUP           (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
> > +#define FLAG_NOT_DUP           (FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED|\
> > +                                FLAG_SND_UNA_ADVANCED)
> >  #define FLAG_CA_ALERT          (FLAG_DATA_SACKED|FLAG_ECE|FLAG_DSACKING_ACK)
> >  #define FLAG_FORWARD_PROGRESS  (FLAG_ACKED|FLAG_DATA_SACKED)
> >
> > @@ -3959,10 +3960,10 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32
> >                 nwin <<= tp->rx_opt.snd_wscale;
> >
> >         if (tcp_may_update_window(tp, ack, ack_seq, nwin)) {
> > -               flag |= FLAG_WIN_UPDATE;
> >                 tcp_update_wl(tp, ack_seq);
> >
> >                 if (tp->snd_wnd != nwin) {
> > +                       flag |= FLAG_WIN_UPDATE;
> >                         tp->snd_wnd = nwin;
> >
> >                         /* Note, it is the only place, where
> > @@ -4354,7 +4355,6 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
> >                  */
> >                 tcp_update_wl(tp, ack_seq);
> >                 tcp_snd_una_update(tp, ack);
> > -               flag |= FLAG_WIN_UPDATE;
> >
> >                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPACKS);
> >         } else {
> > --
> > 2.47.3
> 
> This patch passes all of our internal tests, as well as the new
> tools/testing/selftests/net/packetdrill/tcp_bidir_fast_retransmit_nosack.pkt
> test that Guillaume Nault posted in this thread.
> 
Thanks a lot for the tests. I wasn't sure if I should put your Tested-by tag
to my v2. Feel free to add it :).