netdev - Re: [net-next PATCH v3] tcp: Add logic to check for SYN w/ data in tcp_simple

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Date:   Mon, 14 Dec 2020 10:52:26 -0800
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        kernel-team <kernel-team@...com>
Subject: Re: [net-next PATCH v3] tcp: Add logic to check for SYN w/ data in tcp_simple_retransmit

On Mon, Dec 14, 2020 at 9:42 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Sat, Dec 12, 2020 at 9:31 PM Alexander Duyck
> <alexander.duyck@...il.com> wrote:
> >
> > From: Alexander Duyck <alexanderduyck@...com>
> >
> > There are cases where a fastopen SYN may trigger either a ICMP_TOOBIG
> > message in the case of IPv6 or a fragmentation request in the case of
> > IPv4. This results in the socket stalling for a second or more as it does
> > not respond to the message by retransmitting the SYN frame.
> >
> > Normally a SYN frame should not be able to trigger a ICMP_TOOBIG or
> > ICMP_FRAG_NEEDED however in the case of fastopen we can have a frame that
> > makes use of the entire MSS. In the case of fastopen it does, and an
> > additional complication is that the retransmit queue doesn't contain the
> > original frames. As a result when tcp_simple_retransmit is called and
> > walks the list of frames in the queue it may not mark the frames as lost
> > because both the SYN and the data packet each individually are smaller than
> > the MSS size after the adjustment. This results in the socket being stalled
> > until the retransmit timer kicks in and forces the SYN frame out again
> > without the data attached.
> >
> > In order to resolve this we can reduce the MSS the packets are compared
> > to in tcp_simple_retransmit to -1 for cases where we are still in the
> > TCP_SYN_SENT state for a fastopen socket. Doing this we will mark all of
> > the packets related to the fastopen SYN as lost.
> >
> > Signed-off-by: Alexander Duyck <alexanderduyck@...com>
> > ---
> >
>
> SGTM, thanks !
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Nice work. I tested and verified it works with our packetdrill

Signed-off-by: Yuchung Cheng <ycheng@...gle.com>

>
> > v2: Changed logic to invalidate all retransmit queue frames if fastopen SYN
> > v3: Updated commit message to reflect actual solution in 3rd paragraph
> >