mtpcp not usable with ipv6 if built as module

mtpcp not usable with ipv6 if built as module

[Björn JACKE]

Hi,

I know that this is a known issue and https://www.mptcp.dev/setup.html mentions
this but I would like to encourage the developers to fix the problem that mptcp
for ipv6 isn't available if ipv6 is built as a module.

The problem that mptcp doesn't work with ipv6 if built as a module is also not
very obvious when you are working on such a system. It simply does not work,
but no dmesg message or so is mentioning the lack of mptcp support at runtime.

The system where I hit this was as raspi system, where ipv6 is built as a
module due to size reasons, which is reasonable for small memory systems. See
https://github.com/raspberrypi/linux/issues/6518 and
https://github.com/raspberrypi/linux/pull/5487

Maybe this is a bit of a motivation to fix the module build of mptcp with ipv6,
raspberry users (and some more probably) would very much apreciate it.

Cheers
Björn

Re: mtpcp not usable with ipv6 if built as module

[Matthieu Baerts]

Hi Björn,

On 05/03/2025 14:13, Björn JACKE wrote:
> Hi,
> 
> I know that this is a known issue and https://www.mptcp.dev/setup.html mentions
> this but I would like to encourage the developers to fix the problem that mptcp
> for ipv6 isn't available if ipv6 is built as a module.
> 
> The problem that mptcp doesn't work with ipv6 if built as a module is also not
> very obvious when you are working on such a system. It simply does not work,
> but no dmesg message or so is mentioning the lack of mptcp support at runtime.
> 
> The system where I hit this was as raspi system, where ipv6 is built as a
> module due to size reasons, which is reasonable for small memory systems. See
> https://github.com/raspberrypi/linux/issues/6518 and
> https://github.com/raspberrypi/linux/pull/5487
> 
> Maybe this is a bit of a motivation to fix the module build of mptcp with ipv6,
> raspberry users (and some more probably) would very much apreciate it.

In most deployments, it doesn't make sense today to have IPV6 as a
module, because very likely, the IPV6 module will be loaded in RAM if
there is a network connection. Or simply when having the loopback
interface with the ::1 IPv6 address assigned to it.

Also, the effort looks consequent to properly support MPTCP as a module.

I just added a comment on the ticket you mentioned. To me, MPTCP_IPV6
should be enabled on all RPI >= 2, like it is on other major
distributions for the RPIs.

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.