MTU issues.

[Paul Gildea]

Hi guys, have done some more testing with this. Using ubuntu 16.04 I
installed libqmi (1.16) to test that also, on a fresh system (a laptop).
Set the MTU of the private network to 1430 and pinged with large packets
(2000) and after a couple of hundred pings this behaviour repeated itself.
When I thought that the system was fine yesterday, it turns out it just
took more pings for it to fall over. At 1500 MTU and 300 pings the same
thing occurred with the private network.
Massive amounts of input errors, modem becomes unusable until reset.

I can see pings arriving at the back end, it's when they are returning to
the modem that there appears to be an issue.
This is happening with SW and Telit with varying modems and multiple
networks.
It seems to me to be a qmi_wwan driver issue, what do you think?

Regards

--
Paul

On Thu, 23 Jan 2020 at 17:15, Paul Gildea <gildeap at tcd.ie> wrote:

> Hi,
>
> We are seeing MTU issues with the ATT network and were just wondering if
> you had heard of anything like this happening before? ATT push an MTU of
> 1430, we apply that to our linux interface and everything works fine.
> However when we try and ping with a large packet and the pings fail instead
> of fragmenting correctly, we also see input errors on the linux interface.
>
> In some scenarios the input errors never stop (getting thousands of them
> every second) after we stop pinging and the modem needs to be rebooted to
> pass any traffic again. Changed MTU values to a lot of different things and
> this always occurs.
>
> On a private network here (MTU 1404) and Vodafone (MTU 1500) doing the
> same thing with the network pushed MTU values causes no issues and
> everything works fine. I tested moving away from 1500 on vodafone to lower
> values and once the issues reoccurred, same as with ATT.
>
> Libqmi 1.24 and I have seen this behaviour with all modems tested so far:
> MC7455, EM7565, EM7511 and Telit LM960A18.
>
> Regards,
>
> --
> Paul
>
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