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

moeller0 moeller0 at gmx.de
Thu Jun 2 11:42:50 EDT 2016

Hi Jonathan,

> On Jun 2, 2016, at 16:49 , Jonathan Morton <chromatix99 at gmail.com> wrote:
>>> It would be nice if LuCI could infer information about the likely
>>> overheads from the rest of the configuration, and apply (or suggest &
>>> default) the correct keywords in sqm-scripts. That would make the
>>> feature much more widely used.
>> We can probably do this for the most common cases, but am not so sure
>> it's unambiguous when to pick what. If someone can supply a couple of
>> examples of configuration where we are fairly certain we know what to
>> pick, I can look into how that can be inferred in luci…
> The most obvious cases are where there is an ADSL or VDSL modem built into the router, as there is on my Buffalo WBMR (though I don’t currently have a working ADSL line to demonstrate a live example).  SQM on the WAN-facing interfaces which (directly or indirectly) use that modem should then infer the PPP and ATM modes from the modem’s configuration, which I know is visible to LuCI in some form.
> It’s slightly less obvious what to do with a standalone router, where there is a separate modem handling the PPP endpoint, thus the WAN interface appears in all respects to be Ethernet.  For this case, an easy way of setting the “conservative”, “bridged-ptm" or “pppoe-ptm” keywords should do the trick, perhaps with “conservative” being the sane default.  The PTM keywords are better suited to VDSL installations.  I suspect cable and cell modems may work best *without* overhead compensation.

	I do not agree that these are too helpful unless the user already has lots of knowledge, but then they are hardly an improvement on “overhead N”.

> As for the “conservative” keyword, it assumes ATM encapsulation and exactly one ATM cell of inner encapsulation overhead, the latter exceeding all known combinations of inner encapsulation used in practice (except for bizarre cases involving IP-in-IP tunnelling).  It’s intended to be a reasonable default where it is known or suspected that encapsulation is in use, but no specifics are known.

	So far I have not encountered overhead >= 48, that is true. But I have also never seen a blue whale, which I do not want to be interpreted as me doubting their existence. In reality 48 bytes still seems like a likely upper bound for per-packet overhead on an ATM link, so of all the keywords, conservative might survive, if renamed “conservative_adsl” or “conservative_atm”. “pppoe-ptm” however is a good example for an under-defined keyword… what exactly is this supposed to entail?

> Sebastian seems to have more detailed information about what encapsulation combinations are actually in use in Germany, and can doubtless advise on how these show up in LuCI on an ADSL router.

	Not at all, some details of the encapsulation are not easily extractable from the typical modem’s dsl-information tab. My attempts at measuring the per-packet-overhead culminated in what I documented under https://github.com/moeller0/ATM_overhead_detector and https://github.com/moeller0/ATM_overhead_detector/wiki I stopped believing this is “simple” a long time ago; I also failed to come up with a method to measure the actual overhead on VDSL links…

Best Regards

> - Jonathan Morton
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