[Cake] cake/tc - removal of atm/ptm/ethernet specific overhead keywords

Kevin Darbyshire-Bryant kevin at darbyshire-bryant.me.uk
Thu Jun 2 05:37:22 EDT 2016

Greetings All,

In a bid to create yet another day of cake controversy, here's my latest 
pull request https://github.com/dtaht/tc-adv/pull/12

It removes all the atm/ptm/ethernet specific overhead keywords.

I find myself largely in agreement with Sebastian where he said:

I would prefer very much if we could just rip the keywords out and just 
leave “overhead N” in there. As is the keywords are inconsistent in that 
some act as modifiers that “add” to already specified values while 
others replace older values whole-sale. Also these keywords do not 
really simplify the challenges in choosing the correct overhead at all, 
so my vote would be to go from today’s:

[ atm | noatm* ] [ overhead N | conservative | raw* ]

[ atm | noatm* ] [ overhead N | raw* ]

So, instead of documenting the keywords, remove even more (and add an 
info/error message to tc to warn users still using the old ones). If 
that is not acceptable then I would propose to simply create named 
keywords for the individual components that make up the overhead and 
apply these in an additive fashion, like:

ppp pppoe ethernet llc snap atmpad aal5sar

which would translate into:

2 + 6 + 14 + 3 + 5 + 2 + 8 = 40

So, I propose to just take the components and their names from 
and ad vlan, mac, and fcs to the mix (and maybe the effective additional 
overhead required to shape ethernet, IFG and friends).

But honestly I believe we would be better off by directing users to 
decent information how to deduce the applicable overhead in each 
individual case. Say the link above and maybe a link to 
https://github.com/moeller0/ATM_overhead_detector might be of more help 
than trying to simply a complex situation?
While at it, I also would remove the “conservative” keyword since with 
that we make a promise to our users we might not be able to keep as we 
have no guarantee of a maximum overhead possible.

I agree that pointing people to decent info and/or having a small table 
in the cake man page with the same info as the now dropped overhead 
commands would be far better.  My concession is to keep the conservative 
keyword.  For people who are unwilling to go into this (and I expect 
those same people would not be interested in Sebastian's ppp pppoe 
ethernet llc....' keyword set) it is *likely* that the addition of a 
whole ATM user cell is going to cover the overhead.

I'd be sort of interested to know if anyone is actually using those 
keywords: ipoa-vcmux, ipoa-llcsnap, bridged-vcmux, bridged-llcsnap, 
ppoa-vcmux, pppoa-llc, pppoe-vcmux, pppoe-llcsnap, pppoe-ptm, 
bridged-ptm, via-ethernet, ether-phy, ether-all, ether-fcs, ether-vlan.

How many actually knew they even existed?

Yours controversially,


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