Disconnect on Data Transfer

[Reinhard Speyerer]

On Sun, Apr 14, 2019 at 09:19:28AM -0400, John Marrett wrote:
> I'm experiencing an issue with disconnect during downloads on the AT&T
> network using Jasper managed IoT SIMs on a private APN. Our devices are
> quite stable for long periods of time when running with normal operational
> traffic for embedded devices. When we attempt to download a few megabytes
> of data to a device it will sometimes stop receiving data from the mobile
> network. Limiting the transfer rate reduces the chances of the issue
> occurring. Disabling the modem (mmcli -m ? --disable) usually restores the
> connection. We do not see this behaviour on other networks, including, we
> believe, devices roaming on the AT&T network from other carriers. We are
> working with AT&T to resolve the issue but so far they haven't been able to
> identify the cause of the issue.
> 
> This morning I recreated the issue running qmicli 1.22.2 and mmcli 1.10.0
> on top of a 3.14.26 kernel. Modems are Sierra Wireless MC7354 running
> SWI9X15C_05.05.58.00 r27038 carmd-fwbuild1 2015/03/04 21:30:23. Initially
> my connection was stable for about 100MB of transfer. I then cut power to
> 
> [...]
> 
> I don't see anything else that appears interesting or relevant in the log
> messages. I do have plenty of other messages including from the
> reconnection and can look for anything particularly interesting. I'm ready
> and willing to gather other info and perform other tests. I also can
> request specific captures from the AT&T team supporting us.
> 
> -JohnF

Hi John,

without Qualcomm diag logs on the MC7354 when the problem occurs it might
be very hard to find the root cause. In order to obtain them you would
probably have to define a custom udev rule to make ModemManager ignore
the diag port and also ask Sierra Wireless to provide you with their
Qualcomm logging frontend.

Alternatively you could also check whether one of the following makes the
problem go away:

1. Switch to Linux kernel 4.5 or newer for testing so that ModemManager
   uses Raw IP mode for QMI.

2. Apply commit 245d21190aec547c0de64f70c0e6de871c185a24
   ("qmi_wwan: set FLAG_SEND_ZLP to avoid network initiated disconnect")
   if not already present in the qmi_wwan version you use.

3. Reduce the MTU size to 1280 for testing.

Regards,
Reinhard