netdev - Re: [PATCH net v2 2/3] geneve: Relax MTU constraints

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Date:	Thu, 18 Feb 2016 18:11:46 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	David Wragg <david@...ve.works>
Cc:	Jesse Gross <jesse@...nel.org>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	ovs dev <dev@...nvswitch.org>,
	David Miller <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Thomas Graf <tgraf@...g.ch>,
	Roopa Prabhu <roopa@...ulusnetworks.com>
Subject: Re: [PATCH net v2 2/3] geneve: Relax MTU constraints

On Thu, Feb 18, 2016 at 8:54 AM, David Wragg <david@...ve.works> wrote:
> Tom Herbert <tom@...bertland.com> writes:
>> Please implement like in ip_tunnel_change_mtu (or better yet call it),
>> that is the precedent for tunnels.
>
> I've made geneve_change_mtu follow ip_tunnel_change_mtu in v2.
>
> If it were to call it instead, are you suggesting just passing in
> t_hlen?  Or restructuring geneve.c to re-use the whole ip_tunnel
> infrastructure?
>
I'll leave that to you to decide if that is feasible or makes sense,
but ip_tunnel does do some other interesting things. Support for
geneve could easily be implemented using ip_tunnel_encap facility. The
default MTU on the device is set based on the MTU of the outgoing
interface and tunnel overhead-- this should mitigate the possibility
of a lot of fragmentation happening within the tunnel. Also, the
output infrastructure caches the route for the tunnel which is a nice
performance win.

> Also, I'm not sure where the 0xFFF8 comes from in
> __ip_tunnel_change_mtu.  Any ideas why 0xFFF8 rather than 0xffff?  It
> goes all the way back to the inital import of the kernel into git.
>
Yes, that's pretty ugly. Feel free to replace that with a #define or
at least put a comment about it for the benefit of future generations.

Thanks,
Tom

> David