RFC: AT-controlled USB-Ethernet cellular modems: extend comgt-ncm or new netifd protocol?

[Michael Wyraz]

Hello,

I'm porting a device with an ML352 LTE modem that exposes an RNDIS 
network interface and a separate AT control port.

On this device, the IP address reported by AT+CGPADDR is also assigned 
to the host via RNDIS DHCP. After a cellular reconnection, that address 
can change while the host retains its old DHCP address. The network 
interface does not provide netifd with a usable link transition in this 
case. My current workaround polls the modem over AT, compares the 
addresses, and triggers a fresh DHCP discovery when they differ.

I have observed the same stale-DHCP behaviour with a SIMCom A7672E in 
RNDIS mode, so this does not appear to be limited to the ML352.

The current ML352 supervisor also configures the APN and roaming policy 
and reads registration and radio technology for status LEDs. I'm 
considering moving the modem and network lifecycle into a netifd 
protocol. Since comgt-ncm already supports AT command profiles and 
dynamic DHCP interfaces, would you prefer extending or refactoring it 
for AT-controlled RNDIS modems, or adding a separate protocol?

The PDP/DHCP address comparison would only be enabled for modems that 
assign the PDP address to the host; NAT-based RNDIS modems need 
different handling.

Thanks,
Michael