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

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Message-Id: <20160106.155950.1007160228570301281.davem@davemloft.net>
Date:	Wed, 06 Jan 2016 15:59:50 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	david@...ve.works
Cc:	netdev@...r.kernel.org, dev@...nvswitch.org
Subject: Re: [PATCH net 0/2] vxlan: Set a large MTU on ovs-created vxlan
 devices

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.
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