netdev - Re: RE: [PATCH 9/9] batman-adv: Use packing of 2 for all headers before an ethernet header

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Message-ID: <7016771.yWLQhgLl2P@sven-laptop.home.narfation.org>
Date:	Wed, 21 Nov 2012 14:51:02 +0100
From:	Sven Eckelmann <sven@...fation.org>
To:	David Laight <David.Laight@...lab.com>
Cc:	Antonio Quartulli <ordex@...istici.org>, davem@...emloft.net,
	netdev@...r.kernel.org, b.a.t.m.a.n@...ts.open-mesh.org,
	Marek Lindner <lindner_marek@...oo.de>
Subject: Re: RE: [PATCH 9/9] batman-adv: Use packing of 2 for all headers before an ethernet header

On Wednesday 21 November 2012 13:36:54 David Laight wrote:
> > All packet headers in front of an ethernet header have to be completely
> > divisible by 2 but not by 4 to make the payload after the ethernet header
> > again 4 bytes boundary aligned.
> 
> I'm not sure that statement is correct - whether the patches are
> correct rather depends on the actual packet format(s) you are
> generating/modifying.
> 
> If you are adding data to an ethernet packet you'll need
> to add a multiple of 4 bytes in order maintain the alignment
> of the IP header (which needs to be 4 byte aligned and is
> usually 14 bytes from the start of the receive data).
> 
> This means that the data you add probably has to be 4n+2
> aligned - since I guess you are adding between the source
> MAC address and ethertype?
> If you are inserting data after the ethertype (ie after some
> protocol identifier), then your header can be 4 byte aligned
> provided it ends with the replacement ethertype (0800 or 0806
> for IP protocols).

No, it looks like this

ethernet header | batman-adv stuff | ethernet header | ip header

Kind regards,
	Sven
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