IPv4 config for QMI modem

[Dan Williams]

On Mon, 2018-11-12 at 14:02 +0100, Sven Schwermer wrote:
> 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 

Yeah, 68 byte MTU is the minimum IPv4 Ethernet MTU per RFC791.  That's
quite odd and likely way too small.  Most of the MTUs I've seen
reported from modems are in the 1400-1500 range.

MM/NM usually try to follow what the firmware tells them to do. I don't
know how the modem firmware gets that MTU, but that sounds like a
firmware or network-side bug to me.  Perhaps the thing that configures
the interface (NM or whatever) could enforce a minimum MTU, but it
would be interesting to know if very low MTUs are used in practice too.

Dan

> 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
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