IPv4 config for QMI modem

[Sven Schwermer]

Hi again!

> I’m currently using a 4.1.x kernel. I’ll try with a 4.18.x kernel now. I’ll keep you posted.

I have gotten a step further with a newer kernel. DNS works as well as simple pinging (ICMP). However, when trying to establish a TCP connection, the connection times out. I then went ahead and captured the packets both on the modem’s system side as well as on the cellular operator side. There I could see that the TCP handshake never completes because the modem complains with “Fragmentation needed”. When looking at the modem’s MTU, I could see that the IPv4 settings in the bearer called for a 68 byte MTU. That seems very small—is that common for modems like this? I also noticed that the NetworkManager doesn’t query the MTU from the ModemManager and so the MTU was incorrectly set to 1500. I corrected this via the NetworkManager connection configuration file (in the [gsm] section), but this prevented all communication from working.

The modem I’m using still has the “engineering sample” status, so maybe this can be blamed on faulty firmware, but it’s not looking too good.

Sven