netdev - Re: [ovs-dev] [PATCH net 0/2] vxlan: Set a large MTU on ovs-created vxlan devices

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Date:	Thu, 7 Jan 2016 18:21:37 +0100
From:	Thomas Graf <tgraf@...g.ch>
To:	Jesse Gross <jesse@...nel.org>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	David Wragg <david@...ve.works>,
	David Miller <davem@...emloft.net>, dev@...nvswitch.org,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [ovs-dev] [PATCH net 0/2] vxlan: Set a large MTU on ovs-created
 vxlan devices

On 01/07/16 at 08:35am, Jesse Gross wrote:
> On Thu, Jan 7, 2016 at 3:49 AM, Thomas Graf <tgraf@...g.ch> wrote:
> > A simple start could be to add a new return code for > MTU drops in
> > the dev_queue_xmit() path and check for NET_XMIT_DROP_MTU in
> > ovs_vport_send() and emit proper ICMPs.
> 
> That could be interesting. The problem in the past was making sure
> that ICMPs that are generated fit in the virtual network appropriately
> - right addresses, etc. This requires either spoofing addresses or
> some additional knowledge about the topology that we don't currently
> have in the kernel.

Are you worried about emitting an ICMP with a source which is not
a local host address?

Can't we just use icmp_send() in the context of the inner header and
feed it to the flow table to send it back? It should be the same as
for ip_forward().

skb->dev or skb->dst should lead us to the real MTU which can be
included in the ICMP frag needed. It's a bit tricky because we would
have to know whether it was encapsulated or not and adjust
accordingly.
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