[Cake] More overhead keywords

[Jonathan Morton]

Hmm. Looks like that document I found was really out of date, then. But it
got me fairly close.

For me, the overhead starts at the IP packet (so for your example, at the
1492 byte level), and excludes optional parts of the spec like FCS (since I
have a separate flag for that). So I need to take the 8 bytes for PPPoE,
the 14 for Ethernet and the 4 for PTM, total 26 - one less than my existing
figure.

Presumably the bridged version is also one byte smaller than my
calculation, 18 rather than 19. Is there also a version which transmits IP
without Ethernet framing?

You would then specify "pppoe-ptm ether-vlan via-ethernet" to set your
example connection up the friendly way, or just "overhead 16" for the terse
way (and you can already do that in the current version).

The 64/65 sync overhead is something we'll have to discover by experiment.
Luckily, it's pretty easy to tell whether we're filling up a dumb FIFO or
not.

I've gone for the technical labels for three reasons. First, it reflects
what's actually happening, which generally reduces confusion in the long
run. Second, you might underestimate the number of ADSL ISPs worldwide, as
well as the difficulty of keeping such a database up to date. Third, every
DSL modem and ISP I know of has made it reasonably easy to discover at
least the base encapsulations - autodetection of vcmux vs llc isn't
absolutely reliable, for example. They might be less forthcoming about vlan
and FCS, but one can make intelligent guesses here, based on whether it's a
converged services ISP.

Ideally, we could do with a tool (at dslreports?) which makes detecting the
actual overhead easier. This would be doable using small packets to magnify
the differences.

And if the user really can't work it out, they can always throw up their
hands and specify "conservative".

- Jonathan Morton
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