Ethernet-briding a MBIM-interface

Markus Gothe nietzsche at
Fri Oct 24 00:23:20 PDT 2014

The DirectIP and CDC ECM behaves differently at least because you put the logic inside the modem firmware :-)

Having a DHCP-server inside cdc_mbim.c should not be too difficult, look at an old version of sierra_net.c

Well, MBIM is supposed to be like QMI ‘raw-ip’-mode. Maybe we could do the same mess up for that case. ;-)


On 24 Oct 2014, at 09:04 , Bjørn Mork <bjorn at> wrote:

> Markus Gothe <nietzsche at> writes:
>> Hi, Björn and thanks for your feedback.
>> 2G/3G networks are circuit switch running PPP over the backhaul. LTE
>> is circuit switch and talking IP over the backhaul.  This is why
>> Sierra’s DirectIP / sierra_net.c is a half-bridge. It is by an old
>> design. Hiding the PPP and using a DHCP-hack (older DirectIP devices
>> actually wants the DHCP-server to be implemented in sierra_net.c using
>> CnS/HIP).
> Yes, I have wondered if that wouldn't have been a better design after
> all.  Having a DHCP server inside the driver would be messy, of course.
> But having it inside the modem firmware isn't less messy, and prevents
> us from fixing it if that should become necessary.  Luckily it hasn't.
> For MBIM, I believe the best option for the host is to use the IP
> configuration request instead of trusting some random DHCP server
> implementation in the firmware.
>> The local MAC-issue will be overcome by using eatables to do MAC-DNAT
>> (been there done that before).  The use case for being able to bridge
>> MBIM is of course embedded devices. Not computers.
>> Playing around with proxy arp / parprouted might be a good starter. :-)
>> So basically you are saying that MBIM and Sierra DirecIP is
>> reassembling each other very close?
> Yup. And also any other modem presenting an ethernet like interface,
> like NCM or ECM (with or without a QMI channel)
> Some might argue that MBIM should be different because there are no
> ethernet headers, but we messed that up in the Linux driver :-)
> Bjørn
> _______________________________________________
> libmbim-devel mailing list
> libmbim-devel at

//Markus - The panama-hat hacker

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