netdev - Re: [PATCH net-next] veth: extend features to support tunneling

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Date:	Sun, 17 Nov 2013 09:20:47 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Or Gerlitz <or.gerlitz@...il.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	John Fastabend <john.r.fastabend@...el.com>
Subject: Re: [PATCH net-next] veth: extend features to support tunneling

On Sat, 2013-11-16 at 23:31 -0800, Alexei Starovoitov wrote:

> In case of VMs sending gso packets over tap and tunnel in the host,
> ip_forward is not in the picture.
> 

I was specifically answering to #2 which should use ip forwarding, of
course. Note that my patch was a POC : We have many other places where
the typical MTU check is simply disabled as soon as skb is GSO.

> when host mtu doesn't account for overhead of tunnel, the neat trick
> we can do is to decrease gso_size while adding tunnel header.

That would be very broken to change gso_size, this breaks DF flag
semantic. You need to send an ICMP, and the sender will take appropriate
action.

GRO + GSO request that forwarded segments are the same than incoming
ones. It's not like a proxy that can chose to aggregate as it wants.

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