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

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Date:	Tue, 16 Feb 2016 12:33:42 +0000
From:	David Wragg <david@...ve.works>
To:	Jesse Gross <jesse@...nel.org>, Tom Herbert <tom@...bertland.com>
Cc:	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

Jesse Gross <jesse@...nel.org> writes:
> On Wed, Feb 10, 2016 at 3:21 PM, Tom Herbert <tom@...bertland.com> wrote:
>> 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.
>
> Sure, we might as well prevent the extreme edge cases that can never
> be valid. In the case of Geneve though, this would be the minimum
> header size, not the maximum, since it's possible that the user
> actually knows how big the options are that they want to use.
>
> But as I said, the practical impact is low because IP_MAX_MTU is so
> much larger than the MTU of real devices. There will always be many
> values that the MTU can be set to that result in dropped packets. This
> is true of all tunnel types.

I agree with Jesse, and if there was a debate about lifting the MTU
limit on all tunnel types to IP_MAX_MTU, I'd be for that.

But for the other tunnel types, I followed the precedent of the code
that was already there, and geneve might as well be consistent.

Patch sent to the lists.

David