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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net v2 2/3] geneve: Relax MTU constraints
Date: Thu, 18 Feb 2016 15:07:15 -0500 (EST)
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From: David Wragg <david@weave.works>
Date: Thu, 18 Feb 2016 16:54:14 +0000

> Tom Herbert <tom@herbertland.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?
> 
> 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.

Some 8 byte multiple requirement, perhaps to do with fragmentation.