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

[Eric Dumazet]

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>

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