From: Florian Westphal <fw@strlen.de>
To: Stefano Brivio <sbrivio@redhat.com>
Cc: Florian Westphal <fw@strlen.de>,
netdev@vger.kernel.org, aconole@redhat.com
Subject: Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu discovery on encap sockets
Date: Mon, 13 Jul 2020 10:04:13 +0200 [thread overview]
Message-ID: <20200713080413.GL32005@breakpoint.cc> (raw)
In-Reply-To: <20200713003813.01f2d5d3@elisabeth>
Stefano Brivio <sbrivio@redhat.com> wrote:
> Hi,
>
> On Sun, 12 Jul 2020 22:07:03 +0200
> Florian Westphal <fw@strlen.de> wrote:
>
> > vxlan and geneve take the to-be-transmitted skb, prepend the
> > encapsulation header and send the result.
> >
> > Neither vxlan nor geneve can do anything about a lowered path mtu
> > except notifying the peer/upper dst entry.
>
> It could, and I think it should, update its MTU, though. I didn't
> include this in the original implementation of PMTU discovery for UDP
> tunnels as it worked just fine for locally generated and routed
> traffic, but here we go.
I don't think its a good idea to muck with network config in response
to untrusted entity.
> As PMTU discovery happens, we have a route exception on the lower
> layer for the given path, and we know that VXLAN will use that path,
> so we also know there's no point in having a higher MTU on the VXLAN
> device, it's really the maximum packet size we can use.
No, in the setup that prompted this series the route exception is wrong.
The current "fix" is a shell script that flushes the exception as soon
as its added to keep the tunnel working...
> > Some setups, however, will use vxlan as a bridge port (or openvs vport).
>
> And, on top of that, I think what we're missing on the bridge is to
> update the MTU when a port lowers its MTU. The MTU is changed only as
> interfaces are added, which feels like a bug. We could use the lower
> layer notifier to fix this.
I will defer to someone who knows bridges better but I think that
in bridge case we 100% depend on a human to set everything.
bridge might be forwarding frames of non-ip protocol and I worry that
this is a self-induced DoS when we start to alter configuration behind
sysadmins back.
> I tried to represent the issue you're hitting with a new test case in
> the pmtu.sh selftest, also included in the diff. Would that work for
> Open vSwitch?
No idea, I don't understand how it can work at all, we can't 'chop
up'/mangle l2 frame in arbitrary fashion to somehow make them pass to
the output port. We also can't influence MTU config of the links peer.
> If OVS queries the MTU of VXLAN devices, I guess that should be enough.
What should it be doing...?
next prev parent reply other threads:[~2020-07-13 8:04 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-12 20:07 [PATCH net-next 0/3] vxlan, geneve: allow to turn off PMTU updates on encap socket Florian Westphal
2020-07-12 20:07 ` [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu discovery on encap sockets Florian Westphal
2020-07-12 22:38 ` Stefano Brivio
2020-07-13 8:04 ` Florian Westphal [this message]
2020-07-13 10:04 ` Stefano Brivio
2020-07-13 10:51 ` Numan Siddique
2020-07-14 20:38 ` Aaron Conole
2020-07-15 11:58 ` Stefano Brivio
2020-07-13 13:25 ` David Ahern
2020-07-13 14:02 ` Florian Westphal
2020-07-13 14:41 ` David Ahern
2020-07-13 14:59 ` Florian Westphal
2020-07-13 15:57 ` Stefano Brivio
2020-07-13 16:22 ` Florian Westphal
2020-07-14 12:33 ` Stefano Brivio
2020-07-14 12:33 ` Stefano Brivio
2020-07-15 12:42 ` Florian Westphal
2020-07-15 13:35 ` Stefano Brivio
2020-07-15 14:33 ` Florian Westphal
2020-07-17 12:27 ` Stefano Brivio
2020-07-17 15:04 ` David Ahern
2020-07-17 18:43 ` Florian Westphal
2020-07-18 6:56 ` Stefano Brivio
2020-07-18 17:02 ` David Ahern
2020-07-18 17:58 ` Stefano Brivio
2020-07-18 18:04 ` Stefano Brivio
2020-07-19 18:43 ` David Ahern
2020-07-19 21:49 ` Stefano Brivio
2020-07-20 3:19 ` David Ahern
2020-07-26 17:01 ` Stefano Brivio
2020-07-12 20:07 ` [PATCH net-next 2/3] vxlan: allow to disable path mtu learning on encap socket Florian Westphal
2020-07-16 19:33 ` Jakub Kicinski
2020-07-17 10:13 ` Florian Westphal
2020-07-12 20:07 ` [PATCH net-next 3/3] geneve: allow disabling of pmtu detection on encap sk Florian Westphal
2020-07-12 22:39 ` [PATCH net-next 0/3] vxlan, geneve: allow to turn off PMTU updates on encap socket Stefano Brivio
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