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

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Message-ID: <CAEh+42hHR==wFWveA8CVnetU0DnVpXWWunM8RKwNZprnPbU+0A@mail.gmail.com>
Date:	Wed, 6 Jan 2016 14:53:02 -0800
From:	Jesse Gross <jesse@...nel.org>
To:	David Miller <davem@...emloft.net>
Cc:	david@...ve.works,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	dev@...nvswitch.org
Subject: Re: [PATCH net 0/2] vxlan: Set a large MTU on ovs-created vxlan devices

On Wed, Jan 6, 2016 at 12:59 PM, David Miller <davem@...emloft.net> wrote:
> From: David Wragg <david@...ve.works>
> Date: Wed,  6 Jan 2016 13:33:04 +0000
>
>> Prior to 4.3, openvswitch vxlan vports could transmit vxlan packets of
>> any size, constrained only by the ability to transmit the resulting
>> UDP packets.  4.3 introduced vxlan netdevs corresponding to vxlan
>> vports.  These netdevs have an MTU, which limits the size of a packet
>> that can be successfully vxlan-encapsulated.  The default value for
>> this MTU is 1500, which is awkwardly small, and leads to a conspicuous
>> change in behaviour for userspace.
>>
>> These two patches set the MTU on openvswitch-crated vxlan devices to
>> be 65465 (the maximum IP packet size minus the vxlan-on-IPv6
>> overhead), effectively restoring the behaviour prior to 4.3.  In order
>> to accomplish this, the first patch removes the MTU constraint of 1500
>> for vxlan netdevs without an underlying device.
>
> Is this really the right thing to do?  Won't we get a lot of fragmentation
> by using such a large MTU, especially since you're making it the default
> for OVS setups?
>
> Things like path MTU discovery hinge strongly upon accurate MTU settings.
> Otherwise they won't function properly.

At a minimum, I don't think this should be VXLAN specific. But I agree
that I'm not sure this is the right thing to do.
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