netdev - Re: [PATCH v3] net: sysctl for RA default route MTU

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Date:	Thu, 02 Apr 2015 21:08:37 +0300
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	David Miller <davem@...emloft.net>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] net: sysctl for RA default route MTU

>>  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?

Thanks,
Roman
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