Re: [LARTC] Patch to allow for the ATM "cell tax"

[Andy Furniss <andy.furniss@dsl.pipex.com>]

Adam James wrote:

> As Markus mentioned in another post on this thread, Jesper Dangaard
> Brouer (http://www.adsl-optimizer.dk) has already written an iproute2
> and Linux kernel patch that implements the above. ATM cell alignment is
> done in tc_core.c, and the per packet overhead is passed to the relevant
> kernel modules.
> 
> I have made some very minor changes to the patches, so that they apply
> cleanly to iproute2-20051007 and kernel 2.6.15. You will find them
> attached to this mail.

Just remember to take 14 from your overhead if your modem is connected 
via eth rather than ppp etc. This means you need to put a negative 
overhead (can you?) if using pppoa/vcmux


> Setting the MTU to 1478 is probably a good idea to prevent excessive
> cell usage. Reasoning behind this being: 1488 is the largest multiple of
> 48 under 1500, less 2 bytes for the PPP header and 8 for the AAL5
> footer.
> 
> Perhaps you can confirm this isn't way off the mark Andy? :)

1478 is optimal if your overhead is 10 - I use it.

For the pppoes I don't think you gain much as you loose tcp data 
efficiency by using smaller packets.

Andy.
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