ModemManager and muxed network interfaces

Hey all,

I'm working on setting up muxed network interfaces with ModemManager so that e.g. you can connect to multiple 
APNs at the same time, each with its own subnet on a different virtual network interface link created from 
the "master" wwan modem interface. These links can be done either with mbim (using VLANs) or qmi (using the 
built-in muxing capabilities of qmi_wwan or with rmnet).

Now, in order to integrate this with NetworkManager, the main difference that NM will see upon connecting a 
modem through MM will be that the data port reported in the Bearer info that Simple.Connect() returns is no 
longer the master wwan interface, but a new "qmimux0" interface (name varies depending on the kernel backed 

I already have this logic setup in a custom branch, but I'm lacking 2 things right now:
  * When setting up the muxing support, we need to configure the MTU of the master wwan interface to a 
specific value that the modem tells us to use (the maximum data aggregation size).
  * And also, before NetworkManager can bring up the "qmimux0" link interface, the "master" wwan modem 
interface needs to be already up or the link ifup will fail.

I can definitely add those two things to the ModemManager connection logic, but wanted to ask, is there any 
way NetworkManager could setup both the master network interface AND then the muxed qmimux0 interface? E.g. 
if MM exposed all those things in DBus, could NM setup both those interfaces? Or is that too much change in 
the NM logic because it truly expects to setup one interface at a time?

I think the way to go is to add it in MM, so that theoretically NM does not need to do absolutely anything, 
but wanted to ask first anyway.
Any comments or suggestions?


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