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

[SeongJae Park <sj38.park@gmail.com>]

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