netdev - Re: [PATCH RFC] net/ipv4: Use next hop exceptions also for input routes

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Message-Id: <20130526.000304.483513823383996098.davem@davemloft.net>
Date:	Sun, 26 May 2013 00:03:04 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	timo.teras@....fi
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH RFC] net/ipv4: Use next hop exceptions also for input
 routes

From: Timo Teräs <timo.teras@....fi>
Date: Thu, 23 May 2013 16:15:46 +0300

> Commit d2d68ba9 (ipv4: Cache input routes in fib_info nexthops)
> assmued that "locally destined, and routed packets, never trigger
> PMTU events or redirects that will be processed by us".
> 
> However, it seems that tunnel devices do trigger PMTU events in certain
> cases. At least ip_gre, ip6_gre, sit, and ipip do use the inner flow's
> skb_dst(skb)->ops->update_pmtu to propage mtu information from the
> outer flows. These can cause the inner flow mtu to be decreased. If
> next hop exceptions are not consulted for pmtu, IP fragmentation will
> not be done properly for these routes.
> 
> It also seems that we really need to have the PMTU information always
> for netfilter TCPMSS' clamp-to-pmtu feature to work properly.
> 
> So for the time being, cache separate copies of input routes for
> each next hop exception.
> 
> Signed-off-by: Timo Teräs <timo.teras@....fi>

Thanks for working on this Timo, I am actively reviewing this change
and thinking about alternatives.
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