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

Arjen Smit smit.arjen at
Sun Oct 23 17:48:06 UTC 2022

Hi all,
I hope I am at the right place for some questions related to Modemmanager
and how host interfaces actually get their IP configuration assigned.
Apologies if I'm at the wrong place.

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.

This question actually does extend to IPv6, here also via the available
control protocol (QMI/AT/etc) the connection is initiated.  It is the modem
firmware that runs a SLAAC client process to receive a /64 prefix.  Then
the data interface on the host is able retrieve that prefix (via a
SLAAC/DHCPv6 client procedure (depending on the modem firmware
capabilities) from the modem. Also here the alternative option exists to
retrieve the IPv6 prefix (+ other items like Ipv6DNS, MTU size, or even PD
- (e.g quectel AT+QIP6CFG="PD_addr") via the control interface from the
modem and apply the result to the data interface.

What is the modem manager approach in this ? It looks like always the data
interface needs to use DHCP/SLAAC kind of client processes. As I prefer to
be independent of specific modem firmware (DHCP) server processes (which
are often outdated/have limited DHCP options supported) I am interested to
understand  the pros/cons of the above approach (and the modem manager
support of this approach).

Thanks !
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