netdev - Re: question: update_pmtu doesn't update dst mtu

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Message-ID: <20140403121410.GC13354@order.stressinduktion.org>
Date:	Thu, 3 Apr 2014 14:14:10 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	"Ilya V. Matveychikov" <i.matveychikov@...uritycode.ru>
Cc:	netdev@...r.kernel.org
Subject: Re: question: update_pmtu doesn't update dst mtu

On Thu, Apr 03, 2014 at 04:07:21PM +0400, Ilya V. Matveychikov wrote:
> On 03.04.2014 15:58, Hannes Frederic Sowa wrote:
> > On Thu, Apr 03, 2014 at 03:37:33PM +0400, Ilya V. Matveychikov wrote:
> >> Looking through the code gives me that rt_pmtu is always 0 for the skb->dst
> >> entry and ipv4_mtu that called via the dst->ops->mtu() uses dev->mtu :(
> > 
> > At this point you have to drop skb_dst and have to relookup the route. During
> > that a new dst will be created which gets the mtu value from the next hop
> > exception, which got created by update_pmtu.
> > 
> > Normally routes are checked with dst_check if they are still valid
> > and a relookup should happen. In your example just do the relookup
> > unconditionally.
> > 
> 
> Does it mean that the next packet must have an updated route without any
> problems? I meant that if the first packet xmitting leads to updating the route
> PMTU via the exception creating (or updating) so the next packets must have an
> updated route? Am I right?

In case the next packet causes a lookup in the routing table, yes. Or
if it does cache the routing lookup in some structure and checks the
route with dst_check and conditionally does a relookup (which kernel
implementation should already do) then you should also get the updated
mtu value.

Bye,

  Hannes

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