Re: [LARTC] HTB MPU

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

Jason Boxman wrote:
> On Friday 28 May 2004 14:54, Andy Furniss wrote:
> <snip>
> 
>>Reading your other post I see your small traffic is ~100b - this would
>>use three cells, so as a temporary kludge you could set your mpu to 159
>>and see how it goes.
>>
>>AFAIK the author of the HTB patch is looking into modifying it to do the
>>sums properly for DSL. There isn't one answer though - Eds' formula is
>>fine doing the cells bit, but before this you need to add a ppp overhead
>>to the IP packet length and this varies with pppoa+vc mux/pppoe/bridged
>>pppoe and probably other varieties of dsl implementations.
> 
> 
> But there's no tried and true method of determining that information?
> 
> You mention at least three methods of mangling PPP with Ethernet/ATM.  And the 
> overhead of each kind of setup also would vary depending on the specifics of 
> that setup?  (i.e., knowing you have bridged PPPoE doesn't instantly qualify 
> you as having an overhread of 123i.)
> 
> Sounds particularly complicated.
> 
> But the overhead would be a fixed cost, no?  If that is the case you can play 
> whack-a-mole with that until you find a 'good' number.  But, as I see it, 
> without a realtime ATM cost scheduler, even if I figure out my true 
> 'overhead' it won't make much difference.
> 
> Thoughts, anyone?

You can find it by experementation - if you get a cell count from your 
modem it's easy.

If you are on BT in the UK using pppoa/vc mux it's 10 (you can't even 
look that up - the RFC says 9 or 10). ping -s 10 uses 1 cell -s 11 2. 10 
data + 20 IP + 8 ICMP = 38, ATM cell data size = 48 so ppp overhead is 10.

Like ED I haven't really looked at the code - but will eventually If it 
doesn't get done by anyone else first :-)

Andy.



_______________________________________________
LARTC mailing list / LARTC@mailman.ds9a.nl
http://mailman.ds9a.nl/mailman/listinfo/lartc HOWTO: http://lartc.org/