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

[Yuchung Cheng]

On Mon, Dec 14, 2020 at 9:42 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Sat, Dec 12, 2020 at 9:31 PM Alexander Duyck
> <alexander.duyck@xxxxxxxxx> wrote:
> >
> > From: Alexander Duyck <alexanderduyck@xxxxxx>
> >
> > 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@xxxxxx>
> > ---
> >
>
> SGTM, thanks !
>
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Nice work. I tested and verified it works with our packetdrill

Signed-off-by: Yuchung Cheng <ycheng@xxxxxxxxxx>

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