netdev - Fwd: TCP receive failure

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Date:   Tue, 10 Mar 2020 12:37:51 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Netdev <netdev@...r.kernel.org>,
        Martin Pohlack <mpohlack@...zon.de>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Fwd: TCP receive failure

On Tue, Mar 10, 2020 at 12:19 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 3/10/20 2:40 AM, David Woodhouse wrote:
> > I'm chasing a problem which was reported to me as an OpenConnect packet
> > loss, with downloads stalling until curl times out and aborts.
> >
> > I can't see a transport problem though; I think I see TCP on the
> > receive side misbehaving. This is an Ubuntu 5.3.x client kernel
> > (5.3.0-40-generic #32~18.04.1-Ubuntu) which I think is 5.3.18?
> >
> > The test is just downloading a large file full of zeroes. The problem
> > starts with a bit of packet loss and a 40ms time warp:

David, would you be able to post a binary tcpdump pcap file (including
SYN/SYNACK, to get the wscale option) at a public HTTP/S URL
somewhere? A time/sequence plot may shed some light on this, but the
tools that generate those generally want a pcap file as input.

I also like Eric's idea to reproduce and take periodic ss/nstat dumps.
That might be extremely informative.

thanks,
neal