Ethernet-briding a MBIM-interface

Bjørn Mork bjorn at
Fri Oct 24 00:04:23 PDT 2014

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 :-)


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