[Cake] Using firewall connmarks as tin selectors

[Toke Høiland-Jørgensen]

Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> writes:

>> On 3 Mar 2019, at 12:22, John Sager <john at sager.me.uk> wrote:
>> 
>> If you are going to do that, I would suggest using a few of the upper bits
>> of the 32-bit fwmark/connmark space available, rather than the lowest bits.
>> Then that would allow to use fwmarks other purposes, and to use the lowest
>> bits, as well in the future. As iptables allows a mask before comparison,
>> then choose a specific mask for the bits you use both for setting and testing.
>
> That’s a good point and I’m sort of hoping upstream reject the current
> submission on that basis.  I think the ‘use of fwmarks’ needs more
> thought as to how it’s done for the future - too keen to get something
> out.  Maybe it’s sufficient as is, I don’t know.

https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/commit/?id=0b5c7efdfc6e389ec6840579fe90bdb6f42b08dc

This means it'll be in 5.1; so we have until that is released (~8 weeks
or so) to set the behaviour in stone.

I do think we at least need to add masking of the mark before using it
for tin selection; the question is just which bits to use from it.

As for setting the fwmark back in conntrack, I'm not sure I agree that
this is something CAKE should be doing. Mostly because it means even
tighter coupling between CAKE and the conntrack subsystem. However, I
may be convinced by a sufficiently neat implementation, and anyway this
is definitely something that will need to wait for 5.2 for upstream.

So I think the priority is to agree on semantics for masking the fwmark
when reading, and getting that implemented in a way that is compatible
with both other uses of marks, and with anything we else we might want
to do down the road.

-Toke