netdev - Re: [PATCH v2] tcp: don't ignore ECN CWR on pure ACK

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Date:   Wed, 24 Jun 2020 12:20:30 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Denis Kirjanov <kda@...ux-powerpc.org>,
        Netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        "Scheffenegger, Richard" <Richard.Scheffenegger@...app.com>,
        Bob Briscoe <ietf@...briscoe.net>
Subject: Re: [PATCH v2] tcp: don't ignore ECN CWR on pure ACK

On Wed, Jun 24, 2020 at 11:47 AM Eric Dumazet <edumazet@...gle.com> wrote:
> Do we really want to trigger an ACK if we received a packet with no payload ?
>
> I would think that the following is also needed :
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 12fda8f27b08bdf5c9f3bad422734f6b1965cef9..023dc90569c89d7d17d72f73641598a03a03b0a9
> 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -261,7 +261,8 @@ static void tcp_ecn_accept_cwr(struct sock *sk,
> const struct sk_buff *skb)
>                  * cwnd may be very low (even just 1 packet), so we should ACK
>                  * immediately.
>                  */
> -               inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
> +               if (TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq)
> +                       inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_NOW;
>         }
>  }

Good point, Eric. Denis, would you mind respinning with Eric's addition?

thanks,
neal