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

SeongJae Park
2020-02-01T14:36:19Z

On Sat, 1 Feb 2020 08:51:48 -0500 Neal Cardwell <ncardwell@google.com> wrote:

> On Sat, Feb 1, 2020 at 2:19 AM <sj38.park@gmail.com> wrote:
> >
> > From: SeongJae Park <sjpark@amazon.de>
> >
> > 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@amazon.de>
> > ---
> > 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..980bd04b9d95 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_ATO_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,
> > --
>
> Scheduling a timer for TCP_ATO_MIN, typically 40ms, sounds like it
> might be a bit on the slow side. How about TCP_TIMEOUT_MIN, which is
> typically 2ms on a HZ=1000 kernel?
>
> I think this would be closer to what Eric mentioned: "sending the SYN
> a few ms after the RST seems way better than waiting 1 second as if we
> received no packet at all."

Agreed, it seems much better! Because this is just a small change in a tiny
patchset containing only two patches, I will send the updated version of only
this patch in reply to this mail, as soon as I finish tests.

Thanks,
SeongJae Park
>
> neal