netdev - Re: L2TPv3 offset

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Date:   Tue, 19 Feb 2019 18:54:35 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     James Chapman <jchapman@...alix.com>
Cc:     t.martitz@....de, davem@...emloft.net,
        netdev <netdev@...r.kernel.org>
Subject: Re: L2TPv3 offset

On Tue, Feb 19, 2019 at 04:36:55PM +0000, James Chapman wrote:
> On 19/02/2019 13:09, t.martitz@....de wrote:
> >
> > Because everytime a LCCE decapsulates such traffic it'll suffer from
> > unaligned access to the inner IP header (likewise for the outer IP
> > header when encapsulating). It's a fundamental assumption that the IP
> > header is word-aligned in Linux so I'm surprised this isn't solved
> > already. And now the only way "fix" without patching the kernel is
> > being removed.
> >
IIUC, you'd be perfectly fine running without offset if the stack was
properly handling the alignment problems created by the encapsulated
ethernet header.

Unaligned headers is a fundamental problem when tunneling ethernet
frames. I'd expect other encapsulation modules (vxlan, geneve,
gretap...) to have the same issues (but I'd be happy to be proven
wrong).

The topic has been discussed a few years ago:
https://lore.kernel.org/netdev/20160922.015242.735026657310158125.davem@davemloft.net/
I might very well have missed some discussions, but I don't remember
any patch aimed at tackling this problem so far.