Re: [PATCH v2.1 1/2] tcp: Reduce SYN resend delay if a suspicous ACK is received

[Eric Dumazet]

On 2/1/20 6:53 AM, sj38.park@xxxxxxxxx wrote:
> From: SeongJae Park <sjpark@xxxxxxxxx>
> 
> When closing a connection, the two acks that required to change closing
> socket's status to FIN_WAIT_2 and then TIME_WAIT could be processed in
> reverse order.  This is possible in RSS disabled environments such as a
> connection inside a host.
> 
> For example, expected state transitions and required packets for the
> disconnection will be similar to below flow.
> 
> 	 00 (Process A)				(Process B)
> 	 01 ESTABLISHED				ESTABLISHED
> 	 02 close()
> 	 03 FIN_WAIT_1
> 	 04 		---FIN-->
> 	 05 					CLOSE_WAIT
> 	 06 		<--ACK---
> 	 07 FIN_WAIT_2
> 	 08 		<--FIN/ACK---
> 	 09 TIME_WAIT
> 	 10 		---ACK-->
> 	 11 					LAST_ACK
> 	 12 CLOSED				CLOSED
> 
> In some cases such as LINGER option applied socket, the FIN and FIN/ACK
> will be substituted to RST and RST/ACK, but there is no difference in
> the main logic.
> 
> The acks in lines 6 and 8 are the acks.  If the line 8 packet is
> processed before the line 6 packet, it will be just ignored as it is not
> a expected packet, and the later process of the line 6 packet will
> change the status of Process A to FIN_WAIT_2, but as it has already
> handled line 8 packet, it will not go to TIME_WAIT and thus will not
> send the line 10 packet to Process B.  Thus, Process B will left in
> CLOSE_WAIT status, as below.
> 
> 	 00 (Process A)				(Process B)
> 	 01 ESTABLISHED				ESTABLISHED
> 	 02 close()
> 	 03 FIN_WAIT_1
> 	 04 		---FIN-->
> 	 05 					CLOSE_WAIT
> 	 06 				(<--ACK---)
> 	 07	  			(<--FIN/ACK---)
> 	 08 				(fired in right order)
> 	 09 		<--FIN/ACK---
> 	 10 		<--ACK---
> 	 11 		(processed in reverse order)
> 	 12 FIN_WAIT_2
> 
> Later, if the Process B sends SYN to Process A for reconnection using
> the same port, Process A will responds with an ACK for the last flow,
> which has no increased sequence number.  Thus, Process A will send RST,
> wait for TIMEOUT_INIT (one second in default), and then try
> reconnection.  If reconnections are frequent, the one second latency
> spikes can be a big problem.  Below is a tcpdump results of the problem:
> 
>     14.436259 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [S], seq 2560603644
>     14.436266 IP 127.0.0.1.4242 > 127.0.0.1.45150: Flags [.], ack 5, win 512
>     14.436271 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [R], seq 2541101298
>     /* ONE SECOND DELAY */
>     15.464613 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [S], seq 2560603644
> 
> This commit mitigates the problem by reducing the delay for the next SYN
> if the suspicous ACK is received while in SYN_SENT state.
> 
> Following commit will add a selftest, which can be also helpful for
> understanding of this issue.
> 
> Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> ---
>  net/ipv4/tcp_input.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 2a976f57f7e7..baa4fee117f9 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5893,8 +5893,14 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
>  		 *        the segment and return)"
>  		 */
>  		if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
> -		    after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt))
> +		    after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
> +			/* Previous FIN/ACK or RST/ACK might be ignored. */
> +			if (icsk->icsk_retransmits == 0)
> +				inet_csk_reset_xmit_timer(sk,
> +						ICSK_TIME_RETRANS,
> +						TCP_TIMEOUT_MIN, TCP_RTO_MAX);
>  			goto reset_and_undo;
> +		}
>  
>  		if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
>  		    !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
> 

Please add my

Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>

Please resend the whole patch series as requested by netdev maintainers.


vi +134 Documentation/networking/netdev-FAQ.rst

Q: I made changes to only a few patches in a patch series should I resend only those changed?
---------------------------------------------------------------------------------------------
A: No, please resend the entire patch series and make sure you do number your
patches such that it is clear this is the latest and greatest set of patches
that can be applied.