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

[Menglong Dong]

On Tue, Jan 9, 2024 at 11:12 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>
> ---
>  net/ipv4/tcp_input.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index df7b13f0e5e0..b2b19421de8b 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -6699,17 +6699,21 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
>                 return 0;
>
>         /* step 5: check the ACK field */
> -       acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
> -                                     FLAG_UPDATE_TS_RECENT |
> -                                     FLAG_NO_CHALLENGE_ACK) > 0;
> +       reason = tcp_ack(sk, skb, FLAG_SLOWPATH |
> +                                 FLAG_UPDATE_TS_RECENT |
> +                                 FLAG_NO_CHALLENGE_ACK);
>
> -       if (!acceptable) {
> +       if (reason <= 0) {
>                 if (sk->sk_state == TCP_SYN_RECV)
>                         return 1;       /* send one RST */
> -               tcp_send_challenge_ack(sk);
> -               SKB_DR_SET(reason, TCP_OLD_ACK);
> -               goto discard;
> +               /* accept old ack during closing */
> +               if (reason < 0) {
> +                       tcp_send_challenge_ack(sk);
> +                       reason = -reason;
> +                       goto discard;
> +               }
>         }
> +       SKB_DR_SET(NOT_SPECIFIED);

Oops, It should be "SKB_DR_SET(reason, NOT_SPECIFIED);" here.
Sorry that I shouldn't be too confident to compile it.

>         switch (sk->sk_state) {
>         case TCP_SYN_RECV:
>                 tp->delivered++; /* SYN-ACK delivery isn't tracked in tcp_ack */
> --
> 2.39.2
>