Host data interface IP configuration via DHCP vs via control channel.

Aleksander Morgado aleksandermj at
Tue Oct 25 13:25:55 UTC 2022

Hey Peter!

> >> Assuming a mobile connection via any of the available control protocols
> (QMI/AT/etc) has been initiated.
> >> The mobile modem firmware has established a bearer connection and via the
> (3GPP PCO -Protocol Control Options amongst others) an IPv4 address has been
> received from the mobile network.  Now for the actual data interface on the host
> a DHCPv4 client could be run to retrieve the IP address from the modem (this
> assumes the Modem runs a DHCP server). But likewise via control protocol
> (QMI/AT/etc) one could retrieve the IP address from the modem and configure the
> data interface with the retrieved result.
> >>
> >
> > Yes. Not all modems have a DHCP server though, beware.
> The DHCP is the upstream gateway, surely?

No, the DHCP server is running in the modem, to ease the configuration
process on the host side.  Qualcomm modems have it for sure, there are
several other manufacturers that don't.
There is no DHCP protocol used in between the modem and the network,
IP assignment is done via 3GPP protocols.

> Anyway, it's certainly true that DHCP
> is not always provided.  But also, with a static IP assignment as it is done
> in OpenWrt, there's no mechanism if the IP changes - this can happen due to
> expiry of the IP, changing stations (i.e, a moving devicde) or other upstream
> network churn.  In such cases, you continue to be connected to the network, but
> lose IP connectivity unless you have another mechanism to watch for such changes.

If I'm not mistaken, a network-initiated disconnection is reported in
such cases (which openwrt git master branch supports), and the modem
just needs to re-establish the bearer with a reconnection. If this is
not always the case, we should definitely trigger a disconnection of
the modem ourselves upon detecting the IP reconfiguration. I'll ask
around to see what modems usually do in this regard.


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