07.04.2015, 18:58, "Hannes Frederic Sowa" <han...@stressinduktion.org>:
> On Do, 2015-04-02 at 21:08 +0300, Roman Gushchin wrote:
>>>> The next question I have is about the behavior of the new setting
>>>> in the presence of an RA MTU option. It seems like the sysctl
>>>> doesn't override that RA MTU option, but rather just clamps it.
>>>>
>>>> And then if it's in range, this controls only whether the default
>>>> route has it's MTU adjusted.
>>>>
>>>> That doesn't make any sense to me if we then go and do the
>>>> rt6_mtu_change() call unconditionally. The route metric update
>>>> and the rt6_mtu_change() go hand in hand.
>>> Agreed but that gets interesting:
>>>
>>> I guess during testing the cnf.mtu6 value was equal to the newly
>>> announced mtu value, so the rt6_mtu_change call does not happen. We
>>> update cnf.mtu6 so a second RA packet would actually bring the system
>>> into the desired state but we have a moment where the default route
>>> carries a too big MTU. That's not good.
>> Agreed.
>>> Easiest solution is to reorder those calls but that also leaves us with
>>> a time frame where we carry the incorrect MTU on the default route.
>>> Otherwise we must conditionally filter out the default routes.
>>> Roman, any ideas?
>> I think, such approach will work on practise, but looks not very beatiful.
>>
>> May be, a better idea is to serarate per-route and per-device MTU,
>> so an updating of per-device MTU will not affect per-route MTU.
>> Actual MTU can always been calculated as min(route_mtu, device_mtu),
>> but we wouldn't need to update mtu on each route on receiving RA MTU
>> option,
>> for instance.
>>
>> Do you see any problems with such approach?
> If I understood you correct this actually seems to be quite an intrusive
> change? :/ Can you show me some code how to do this?
Too intrusive, really)
> I would also dislike adding a filtering capability to the route mtu
> updates. Currently I don't have a god idea, sorry.
Hmm, I thought a bit more about this issue... And It seems to me now, that
there is no issue at all.
If RA MTU is larger than ra_default_route_mtu, rt6_mtu_change() will not update
it,
because dst_mtu(&rt->dst) != idev->cnf.mtu6 :
if (rt->dst.dev == arg->dev &&
!dst_metric_locked(&rt->dst, RTAX_MTU) &&
(dst_mtu(&rt->dst) >= arg->mtu ||
(dst_mtu(&rt->dst) < arg->mtu &&
dst_mtu(&rt->dst) == idev->cnf.mtu6))) {
dst_metric_set(&rt->dst, RTAX_MTU, arg->mtu);
}
So, it's ok.
Otherwise, if RA MTU is lower than ra_default_route_mtu, rt6_mtu_change() will
lower default route mtu, and it's ok too. There is a short period of time, when
a newly created default route has too large MTU, but it's not scary. And it's
exactly as it works now if new RA advertise MTU smaller than previous.
Do I miss something?
Thanks!
Regards,
Roman
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