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

On Sun, 26 May 2013 00:03:04 -0700 (PDT)
David Miller [off-list ref] wrote:

From: Timo Teräs <redacted>
Date: Thu, 23 May 2013 16:15:46 +0300

quoted

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 <redacted>

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

Thanks, I did notice one problems in further testing.

The symptom is that once learned pmtu expires, it is not refreshed by
fragments. This seems to be a dst caching issue. Fundamentally it seems
to be same problem that commit 05ab86c5 (xfrm4: Invalidate all ipv4
routes on IPsec pmtu events) tried to fix by flushing everything.

This some problem now happens with ip_gre bound inner flows as the pmtu
is locally generated and not received from internet to the esp/ah
packets. I believe the flush all approach is not necessary and we can
do better.

I basically have new patch set pending testing. I'll post the new patch
set sometime in next few days.