linux-kernel - Re: [PATCH v2] net: sysctl for RA default route MTU

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Date:	Wed, 25 Mar 2015 19:14:15 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Roman Gushchin <klamm@...dex-team.ru>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net: sysctl for RA default route MTU

Hi,

On Wed, Mar 25, 2015, at 17:33, Roman Gushchin wrote:
> 25.03.2015, 18:52, "Hannes Frederic Sowa" <hannes@...essinduktion.org>:
> > On Wed, Mar 25, 2015, at 12:07, Roman Gushchin wrote:
> >>  --- a/net/ipv6/route.c
> >>  +++ b/net/ipv6/route.c
> >>  @@ -1714,6 +1714,14 @@ int ip6_route_add(struct fib6_config *cfg)
> >>
> >>           rt->rt6i_flags = cfg->fc_flags;
> >>
> >>  +       if ((cfg->fc_flags & (RTF_ADDRCONF | RTF_DEFAULT | RTF_GATEWAY))
> >>  ==
> >>  +           (RTF_ADDRCONF | RTF_DEFAULT | RTF_GATEWAY)) {
> >>  +               u32 mtu = idev->cnf.ra_default_route_mtu;
> >>  +
> >>  +               if (mtu && mtu >= IPV6_MIN_MTU && mtu <= idev->cnf.mtu6)
> >>  +                       dst_metric_set(&rt->dst, RTAX_MTU, mtu);
> >>  +       }
> >>  +
> >
> > Could you move this RA specific snippet over to ndisc.c?
> 
> Ok, no problem.

Thanks!

> >
> > Hmm
> >
> > How do you use this option?
> 
> We want to set and keep normal (~1500) MTU on default route for external
> connections
> without an additional userspace effort, while link MTU is 9000 to support
> jumbo frames
> on other routes.
> 
> > You use jumbo frames on the on-link network and announce all routes via
> > route options where you also want to communicate to with jumbo frames?
> 
> Yes, exactly.
> 
> >
> > I wonder if an offlink_mtu parameter would be more suitable?
> 
> If I understand you correctly, the difference is which MTU will have
> routes, announced via RIO.
> Am I right?
> If so, it will not help in our case, because only default route should
> have "small" MTU,
> and there is no way to announce per-route MTUs for RIO routes.
> I thought about two separate knobs (ra_default_route_mtu and
> ra_rt_info_route_mtu, for example),
> but it seemed to me too excessive.

Hmm. I revert my opinion on offlink_mtu parameter.

So the approach would be to just basically leave your patch as-is and if
another segment can be talked to with jumbo frames one could just let
the RA speaker add another route announcement which should get a more
specific route into the tables with the jumbo MTU from the RA packet.
Only default routes will get the overwritten MTU value from the new
knob. Am I correct? So your approach seems to be the most flexible
option.

Thanks and looking forward to the new patch,
Hannes
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