netdev - Re: TCP stall issue

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Date:   Fri, 26 Feb 2021 12:50:34 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Gil Pedersen <kanongil@...il.com>
Cc:     David Laight <David.Laight@...lab.com>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        "dsahern@...nel.org" <dsahern@...nel.org>,
        Netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Maciej Żenczykowski <maze@...gle.com>
Subject: Re: TCP stall issue

On Fri, Feb 26, 2021 at 11:26 AM Gil Pedersen <kanongil@...il.com> wrote:
>
>
> > On 26 Feb 2021, at 15.39, David Laight <David.Laight@...LAB.COM> wrote:
> >
> > Some thoughts...
> >
> > Does a non-android linux system behave correctly through the same NAT gateways?
> > Particularly with a similar kernel version.
> >
> > If you have a USB OTG cable and USB ethernet dongle you may be able to get
> > android to use a wired ethernet connection - excluding any WiFi issues.
> > (OTG usually works for keyboard and mouse, dunno if ethernet support is there.)
> >
> > Does you android device work on any other networks?
>
> I have done some further tests. I managed to find another Android device (kernel 4.9.113), which thankfully does _not_ send the weird D-SACKs and quickly recovers, so the problem appears to be on the original device.
>
> Additionally, I have managed to do a trace on the WLAN AP, where I can confirm that all packets seem to be transferred without unnecessary modifications or re-ordering. Ie. all segments sent from the server make it to the device and any loss will be device local. As such this points to a driver-level issue?
>
> I don't have an ethernet dongle ready. I tried to connect using cellular and was unable to replicate the issue, so this further points at a driver-level issue.
>
> Given that it now seems relevant, the device is an Android P20 Lite, running a variant of Android 9.1 with an update from this year (kernel was built jan. 05 2021).

Thanks for the details. Agreed, it does sound as if a wifi
hardware/firmare/driver issue on that particular Android device is the
most likely cause of those symptoms.

The only sequence I can think of that would cause these  symptoms
would be if the wifi hardware/firmer/driver on that device is somehow
both:

(1) duplicating each of the retransmit packets that it passes up the
network stack, and
(2) dropping the first ACK packet generated by the first of the two
copies of the retransmit

Though that sounds so unlikely that perhaps there is a different explanation...

neal