Disconnect on Data Transfer

[Reinhard Speyerer]

On Sun, Apr 14, 2019 at 04:24:34PM -0400, John Marrett wrote:
> Reinhard,
> 
> Thanks for getting back to me so quickly, much appreciated.
> 
> 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.
> >
> 
> Sierra Wireless has previously made reference to a "DM capture" tool that
> will collect logs from the modem, this tool creates swi files that need to
> be decoded by Qualcomm support. Is this what you are referring to? Are
> there open tools I could use to decode these files?
> 

Hi John,

yes "DM capture"/dmrecord is the name of the corresponding Sierra Wireless
tool. The created log files usually require commercial tools from Qualcomm
or tools from UE vendors like Sierra Wireless for decoding.

> 
> >
> > 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.
> >
> >
> I'll see if I can get access to a release with a newer kernel.
> 
> 
> > 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.
> >
> >
> AT&T also spoke about reducing MTU when we were experiencing issues; I
> wasn't able to get an reason for why they thought it would help. Why do you
> suggest lowering the MTU to 1280?
> 

In order to avoid potential problems with IP fragmentation in the core
network the MTU size should not exceed
  MTUcn - sizeof(IP-header) - sizeof(UDP-header) - sizeof(GTP-header).
A typical value returned by Qualcomm devices is 1430.

The minimum IPv6 MTU is 1280, therefore it should be a safe value to
assume for it to be supported on the mobile device.

Regards,
Reinhard