Re: [PATCH net-next v3] net: tcp: accept old ack during closing

[Eric Dumazet]

On Fri, Jan 26, 2024 at 5:04 AM Menglong Dong <menglong8.dong@gmailcom> wrote:
>
> For now, the packet with an old ack is not accepted if we are in
> FIN_WAIT1 state, which can cause retransmission. Taking the following
> case as an example:
>
>     Client                               Server
>       |                                    |
>   FIN_WAIT1(Send FIN, seq=10)          FIN_WAIT1(Send FIN, seq=20, ack=10)
>       |                                    |
>       |                                Send ACK(seq=21, ack=11)
>    Recv ACK(seq=21, ack=11)
>       |
>    Recv FIN(seq=20, ack=10)
>
> In the case above, simultaneous close is happening, and the FIN and ACK
> packet that send from the server is out of order. Then, the FIN will be
> dropped by the client, as it has an old ack. Then, the server has to
> retransmit the FIN, which can cause delay if the server has set the
> SO_LINGER on the socket.
>
> Old ack is accepted in the ESTABLISHED and TIME_WAIT state, and I think
> it should be better to keep the same logic.
>
> In this commit, we accept old ack in FIN_WAIT1/FIN_WAIT2/CLOSING/LAST_ACK
> states. Maybe we should limit it to FIN_WAIT1 for now?
>
> Signed-off-by: Menglong Dong <menglong8.dong@xxxxxxxxx>
>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>