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

Peter Naulls peter at
Tue Oct 25 13:40:00 UTC 2022

On 10/25/22 09:25, Aleksander Morgado wrote:
> Hey Peter!

Hey Alek!

> 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.

Ah thanks, that clarifies some unrelated matters for me.

>> 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.

When was this introduced? I guess it's not in any release yet. What I do
know for sure is that there isn't any/good handling in older setups, we
added explicit monitoring to check the assigned IP and then reconfigure
the interface.

The further problem is that there isn't a good way to test this other
than just waiting - sometimes after 12 hours or so it might get triggered
or not at all, or it has to be moved location - i.e, a different cell tower.
If there was a way to trigger this situation upstream, that'd be great.

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