[Cerowrt-devel] VDSL

[Aaron Wood]

Rich, Sebastian, (and others),

First, a hello.  I've been lurking on the bufferbloat mailing list for a
bit, and just joined here as well to better follow what's going on, and see
if there's any way I can help.

Next, I have an ADSL+ link in Paris (Free.fr), and am willing to run a
number of tests using the various LLA options and overhead estimations.
 But 3 hours on a dead-quiet link could be hard to deal with.  I'm happy to
run an hour's worth of netperf tests to a nearby server, slowly working
through the parameter space, and then comparing the results.

My ad-hoc comparisons last week with various modes showed that too high of
settings for the overhead (coupled with the already reduced bw limit in the
shaper) killed the bulk upload/download performance (which I care about on
a meager 18Mbps/1Mbps link).  But I found that setting the bw shaper limit
to the reported line speed (from the modem), and then adjusting the
overhead parameters got me the same bulk performances, with the same
latencies (or what appeared to be the same, I need to do more data
crunching on the results, and run again in a quieter setting).

It would also help if my target server was closer than it is.  The only
server I know of is in Germany, 55ms away (unloaded).

OTOH, I can say that any changes here over the defaults are still gilding
the lily (to an end user).  But Free.fr's router/modem already uses codel,
so it wasn't that bad to begin with (vs. Numericable on a docsis 3 Netgear
router/modem).

===

I can also say that I found the current verbiage on that particular page a
bit "clear as mud".  Even knowing what my network is (to some degree, since
the Free.fr modem can tell me), it was difficult to follow, and I quickly
found myself at about 3/4 my previous speeds, with no visible improvement
in latency (although that could have been a measurement tool issue, as I
was doing 60-second runs using the rrul netperf loads).

The main question I have is:

- Should we both limit the bandwidth well below the reported line rate (or
measured IP rate) AND use the link layer adaption settings? (85-90% of
bandwidth)
 or
- rely on the LLA settings to do the overhead, and shape to just a tiny bit
under the reported line rate? (95-99% of bandwidth)

Thanks,

- Aaron Wood


On Sat, Jan 11, 2014 at 8:06 PM, Rich Brown <richb.hanover at gmail.com> wrote:

> Hi Sebastian,
>
> >>>     Well, that looks like a decent recommendation for the wiki. The
> SQM configuration page still needs to expose all three values, atm,
> ethernet, and  none so that people can actually change things...
> >>
> >> So two questions really:
> >>
> >> 1) (From my previous note) What’s the difference between the current
> “Ethernet” (for VDSL) and “None” link layer adaptations?
> >
> >       Currently, "none" completely disables the link layer adjustments,
> "ethernet" enables them, but will only use the overhead (unless you specify
> an tcMPU, but that is truly exotic).
> >
> >>
> >> 2) When we distinguish the Ethernet/VDSL case, I would really like to
> use a different name from “Ethernet” because it seems confusingly similar
> to  having a real Ethernet path/link (e.g., direct connection to internet
> backbone without any ADSL, cable modem, etc.)
> >
> >       On the one hand I agree, but the two options are called "ATM"
> (well for tc "adsl" is a valid alias for ATM) and "ethernet" if you pass
> them to tc (what we do), and I would really hate it to hide this under
> fancy names. I see no chance of renaming those options in tc, so we are
> sort of stuck with them and adding another layer of indirection seems too
> opaque to me. This is why I put some explanation behind the option names in
> the list box…
>
> Now I see how it works. (I didn’t understand that “None” really meant
> NONE.) The following choices in the Link Layer Adaptation would have eased
> my confusion:
>
> - ATM (almost every type of ADSL or DSL)
> - Ethernet with overhead
> - None (default)
>
> Then the text can say:
>
> —
> You must set the Link Layer Adaptation options so that CeroWrt can perform
> its best with video and audio chat, gaming, and other protocols that rely
> on short packets. The general rule for selecting the Link Layer Adaption is:
>
> * If you use any kind of DSL/ADSL connection to the Internet (that is, if
> you get your internet service through the telephone line), you should
> choose the “ATM (almost every type of ADSL or DSL)" item. Set the
> Per-packet Overhead to 44.
>
> * If you know you have a VDSL connection, you should choose “Ethernet with
> overhead" and set the Per-packet Overhead to 8.
>
> * If you use Cable modem, Fiber, or another kind of connection to the
> Internet, you should choose “None (default)”. All other parameters will be
> ignored.
>
> If you cannot tell what kind of link you have, first try using "None",
> then run the [[Quick Test for Bufferbloat]]. If the results are good,
> you’re done. Next,  try the ADSL/ATM choice, then the VDSL choice to see
> which performs best. Read the **Details** (below) to learn more about
> tuning the parameters for your link.
> —
>
> Would that be better? Thanks.
>
> Rich
> _______________________________________________
> Cerowrt-devel mailing list
> Cerowrt-devel at lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/cerowrt-devel
>
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