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:

quoted

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