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

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Message-ID: <CALx6S36kvXJUZM8_vEmF8L3bxnRiBh98Rs0wAd-HGBJ9Yyde_Q@mail.gmail.com>
Date:	Wed, 10 Feb 2016 15:21:01 +0100
From:	Tom Herbert <tom@...bertland.com>
To:	Jesse Gross <jesse@...nel.org>
Cc:	David Wragg <david@...ve.works>,
	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 Wed, Feb 10, 2016 at 12:59 PM, Jesse Gross <jesse@...nel.org> wrote:
> On Wed, Feb 10, 2016 at 12:41 PM, David Wragg <david@...ve.works> wrote:
>> Tom Herbert <tom@...bertland.com> writes:
>>> The correct thing to do is determine the maximum amount of
>>> encapsulation overhead that can ever be set in a packet and use for
>>> setting the MTU. For instance, when RCO is enable in GUE, the size of
>>> the option is included in tunnel->encap_hlen even though it will not
>>> be used in all packets (via ip_tunnel_change_mtu). If there is no way
>>> to determine a maximum overhead a priori from configuration, then
>>> maximum overhead could be assumed to be maximum possible encapsulation
>>> header size which for Geneve is 132 bytes IIRC.
>>
>> Ok, I'll come up with a patch to address this.
>
> I don't think that this really applies in this situation. The concerns
> here relate to what the MTU is actually set to but this patch affects
> the range of MTUs allowed to be set by the user. I don't see a reason
> to disallow the user from setting a precise value if they know what it
> should be.
>
Right, but if the user sets a bad value and packets are silently
dropped on the floor then that seems like a bad result that could have
easily been prevented.

> In any case, I don't think it is likely to have much impact. By
> default with tunnels the output device is not fixed and therefore the
> base MTU that is used is IP_MAX_MTU. Subtracting some tunnel overhead
> amount from this is still likely quite a bit higher than any physical
> MTU.
>
> If you really want, I would subtract the base Geneve header size from
> IP_MAX_MTU to get the true max but it's probably not a big deal in any
> case.