netdev - Re: [PATCH] net:ipv4: send an ack when seg.ack

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Message-ID: <CAABZP2z6=4bC7qzDdPVjFB10NFioqtspXGTgw_=62o29=VFa+w@mail.gmail.com>
Date:   Sat, 19 Mar 2022 18:34:39 +0800
From:   Zhouyi Zhou <zhouzhouyi@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>, yoshfuji@...ux-ipv6.org,
        dsahern@...nel.org, Jakub Kicinski <kuba@...nel.org>,
        pabeni@...hat.com, netdev@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Wei Xu <xuweihf@...c.edu.cn>
Subject: Re: [PATCH] net:ipv4: send an ack when seg.ack > snd.nxt

Dear Florian

Thank you for reviewing my patch ;-)

On Sat, Mar 19, 2022 at 6:04 PM Florian Westphal <fw@...len.de> wrote:
>
> zhouzhouyi@...il.com <zhouzhouyi@...il.com> wrote:
> > -     if (after(ack, tp->snd_nxt))
> > +     if (after(ack, tp->snd_nxt)) {
> > +             tcp_send_ack(sk);
> >               return -1;
> > +     }
>
> If we really need to do this we need to
>   if (!(flag & FLAG_NO_CHALLENGE_ACK))
Yes, we need to check FLAG_NO_CHALLENGE_ACK here to avoid two acks.
>         tcp_send_challenge_ack(sk);
>
> ... else this might result in two acks?
> Whats the problem thats being fixed here?
We fix the code to let it match what RFC 793 page 72 has described. I
guess this is also what the intermediate internet devices (routers,
firewalls for example) expect us to do ;-)

Thanks again
Zhouyi