[Cerowrt-devel] Update to "Setting up SQM for CeroWrt 3.10" web page. Comments needed.

Sebastian Moeller moeller0 at gmx.de
Sat Dec 28 05:01:29 EST 2013

Hi Rich,

great! A few comments:

Basic Settings:
[Is 95% the right fudge factor?] I think that ideally, if we get can precisely measure the useable link rate even 99% of that should work out well, to keep the queue in our device. I assume that due to the difficulties in measuring and accounting for the link properties as link layer and overhead people typically rely on setting the shaped rate a bit lower than required to stochastically/empirically account for the link properties. I predict that if we get a correct description of the link properties to the shaper we should be fine with 95% shaping. Note though, it is not trivial on an adel link to get the actually useable bit rate from the modem so 95% of what can be deduced from the modem or the ISP's invoice might be a decent proxy…

[Do we have a recommendation for an easy way to tell if it's working? Perhaps a link to a new Quick Test for Bufferbloat page. ] The linked page looks like a decent probe for buffer bloat.

> Basic Settings - the details...
> CeroWrt is designed to manage the queues of packets waiting to be sent across the slowest (bottleneck) link, which is usually your connection to the Internet.

	I think we can only actually control the first link to the ISP, which often happens to be the bottleneck. At a typical DSLAM (xDSL head end station) the cumulative sold bandwidth to the customers is larger than the back bone connection (which is called over-subscription and is almost guaranteed to be the case in every DSLAM) which typically is not a problem, as typically people do not use their internet that much. My point being we can not really control congestion in the DSLAM's uplink (as we have no idea what the reserved rate per customer is in the worst case, if there is any).

> CeroWrt can automatically adapt to network conditions to improve the delay/latency of data without any settings.

	Does this describe the default fq_codels on each interface (except fib?)?

> However, it can do a better job if it knows more about the actual link speeds available. You can adjust this setting by entering link speeds that are a few percent below the actual speeds. 
> Note: it can be difficult to get an accurate measurement of the link speeds. The speed advertised by your provider is a starting point, but your experience often won't meet their published specs. You can also use a speed test program or web site like http://speedtest.net to estimate actual operating speeds.

	While this approach is commonly recommended on the internet, I do not believe that it is that useful. Between a user and the speediest site there are a number of potential congestion points that can affect (reduce) the throughput, like bad peering. Now that said the sppedtets will report something <= the actual link speed and hence be conservative (interactivity stays great at 90% of link rate as well as 80% so underestimating the bandwidth within reason does not affect the latency gains from traffic shaping it just sacrifices a bit more bandwidth; and given the difficulty to actually measure the actually attainable bandwidth might have been effectively a decent recommendation even though the theory of it seems flawed)

> Be sure to make your measurement when network is quiet, and others in your home aren’t generating traffic.

	This is great advise.

I would love to comment further, but after reloading http://www.bufferbloat.net/projects/cerowrt/wiki/Setting_up_AQM_for_CeroWrt_310 just returns a blank page and I can not get back to the page as of yesterday evening… I will have a look later to see whether the page resurfaces…


On Dec 27, 2013, at 23:09 , Rich Brown <richb.hanover at gmail.com> wrote:

>> You are a very good writer and I am on a tablet.
> Thanks!
>> Ill take a pass at the wiki tomorrow.
>> The shaper does up and down was my first thought...
> Everyone else… Don’t let Dave hog all the fun! Read the tech note and give feedback!
> Rich
>> On Dec 27, 2013 10:48 AM, "Rich Brown" <richb.hanover at gmail.com> wrote:
>> I updated the page to reflect the 3.10.24-8 build, and its new GUI pages.
>> http://www.bufferbloat.net/projects/cerowrt/wiki/Setting_up_AQM_for_CeroWrt_310
>> There are still lots of open questions. Comments, please.
>> Rich
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