lkml.org 
[lkml]   [2020]   [Jan]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received
On Fri, Jan 31, 2020 at 1:12 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
>
>
> On 1/31/20 7:10 AM, Neal Cardwell wrote:
> > On Fri, Jan 31, 2020 at 7:25 AM <sjpark@amazon.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
> >
> > AFAICT this sequence is not quite what would happen, and that it would
> > be different starting in line 8, and would unfold as follows:
> >
> > 08 close()
> > 09 LAST_ACK
> > 10 <--FIN/ACK---
> > 11 TIME_WAIT
> > 12 ---ACK-->
> > 13 CLOSED CLOSED
> >
> >
> >> 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,
> >
> > AFAICT that is where the bug starts.
> >
> > AFAICT, from first principles, when process A receives the FIN/ACK it
> > should move to TIME_WAIT even if it has not received a preceding ACK.
> > That's because ACKs are cumulative. So receiving a later cumulative
> > ACK conveys all the information in the previous ACKs.
> >
> > Also, consider the de facto standard state transition diagram from
> > "TCP/IP Illustrated, Volume 2: The Implementation", by Wright and
> > Stevens, e.g.:
> >
> > https://courses.cs.washington.edu/courses/cse461/19sp/lectures/TCPIP_State_Transition_Diagram.pdf
> >
> > This first-principles analysis agrees with the Wright/Stevens diagram,
> > which says that a connection in FIN_WAIT_1 that receives a FIN/ACK
> > should move to TIME_WAIT.
> >
> > This seems like a faster and more robust solution than installing
> > special timers.
> >
> > Thoughts?
>
>
> This is orthogonal I think.
>
> No matter how hard we fix the other side, we should improve the active side.
>
> Since we send a RST, sending the SYN a few ms after the RST seems way better
> than waiting 1 second as if we received no packet at all.
>
> Receiving this ACK tells us something about networking health, no need
> to be very cautious about the next attempt.

Yes, all good points. Thanks!

> Of course, if you have a fix for the passive side, that would be nice to review !

I looked into fixing this, but my quick reading of the Linux
tcp_rcv_state_process() code is that it should behave correctly and
that a connection in FIN_WAIT_1 that receives a FIN/ACK should move to
TIME_WAIT.

SeongJae, do you happen to have a tcpdump trace of the problematic
sequence where the "process A" ends up in FIN_WAIT_2 when it should be
in TIME_WAIT?

If I have time I will try to construct a packetdrill case to verify
the behavior in this case.

thanks,
neal

>
>
>

\
 
 \ /
  Last update: 2020-01-31 23:13    [W:0.071 / U:0.204 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site