Re: [Cerowrt-devel] Correctly calculating overheads on unknown connections

Linux Advanced Routing and Traffic Control

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Sebastian Moeller wrote:

Maybe you mean overhead calculated by a script?

Well in cerowrt’s SQM-scripts we expose the stab options so users can
take link layer and overhead into account. If you naively determine
the overhead, either with the help of the scrips I posted earlier or
by looking it up on a table (if the encapsulation options are known)
you will end up not handling the kernel’s auto-added overhead well.
Currently SQM scripts does not expose PPP devices only ge00
(ethernet) so -14 seems currently the best recommendation in
combination with “please test”.

Oh, OK - I know nothing about wrt.

What I am curious after your message
is what happens if the kernel terminates a pppoe connection but is
connected to a “modem” via ethernet, what does the kernel do. And
thanks to your pointers I know have an idea of how to test that ;)

Well I can't say I know - testing is always best.

I think we are "seeing" skbs just as they enter an interface - so what form they take depends on the particular interface they have just been made for.

It's possible to have multiple pppoes/vlans on an eth and use the eth normally at the same time. What you see I suppose depends on where you are "attached". I guess shaping a pppoe on the eth rather than on the actual ppp is doable with a bit of filtering - in which case you may need to allow for the +14 macs/ethertype and that 8 ppp are already in the payload - a totally untested theory :-)

On ppp skb->len = ip len

On eth skb->len = ip len + 14

On vlan skb->len = ip len + 18

So this is the information I actually wanted to find and then somehow
thought qdisc_pkt_len_init() was the place. Do you by chance have any
pointer where this assignment is handled?

No, sorry I don't know the code.

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