[Bloat] [Ecn-sane] 2019-12-31 docsis strict priority dual queue patent granted

Sebastian Moeller moeller0 at gmx.de
Fri Jan 24 07:08:20 EST 2020

Hi Dave,

> On Jan 24, 2020, at 09:59, Dave Taht <dave.taht at gmail.com> wrote:
> To be deliberately contrarian - (I do try to only pay attention to
> this a few days a month) - after also re-reading
> https://www.cablelabs.com/technologies/low-latency-docsis and the
> associated white papers (yes, 24 hours on a plane can do this to you)
> 1) I've never been able to figure out where the 99 percentile latency
> figure so often cited came from. on the upstream which typically runs
> well below 20Mbit, a single IW10 burst at 10Mbit is 1.3ms, so I've
> generally figured it was either a long term figure, or calculated from
> a much higher (100mbit? 1gbit?) downstream rate against some load
> that's never been documented. (that I know of, please note that I
> don't
> read much of the traffic about this stuff)

	Thinn air and/or tests with DCTCP, probably (or paced fixed-rate flows)?

> 2) There is a lot of valuable looking stuff in the lower level aspects
> of the docsis LL standard.

	I agree, they propose a nice mid-life MAC do-over, but I am not sure whether ISPs/Manufacturers will follow as IIRC some of that stuff is either not mandatory to implement and/or to activate.

> I'd noted it when I first read it, but
> achieving .9ms baseline a/g latency finally does make it competitive
> with fiber with whatever the heck "pgm" is. So far as I knew, the
> overlapping grant request and estimator functions

	I initially thought that is going to be tricky, but realistically if there is traffic on a link (immediate) history probably is a decent predictor of (immediate) future traffic need, so a bit of temporal prediction can go a long way there to lessen the impact of the grant request cycle on average latency (one could even think about not only tracking past traffic speed but also acceleration). 

> documented in the
> patent are already present in most cablemodems already, and not really
> tied to the ll spec... but that data would be interesting to get out
> of the modem itself, somehow. The histogram is made available via a
> MIB to the operator. It would be nice if those MIBs were also visible
> to the user somehow.
> 3)
> In the docsis-ll white paper and spec it lays out cmts requirements
> also. With the cmtses currently exhibiting 500+ms of latency at
> 100Mbit loaded,  from a mere "solving bufferbloat" perspective -
> getting just pie there to work would be *marvelous* - it would be
> superior to any of the fiber deployments I know of. dualpi, even if
> not configured for l4s ecn support,

	Well, especially if not configured for l4s ECN, as dualpi is misdesigned in giving the L4S queue dominance over the non-L4S queue... Ironic giving all the prose about dualpi not being a priority based AQM (and driven by the IMHO faulty assumption that rate and delay are fully orthogonal). And that failure to do the one job it was designed for has been documented (or shall I say buried) in the L4S measurement papers since early on, and yet no-one bothered to actually go fix this. 
	But to your point, sure, if used as a single queue AQM it will give us a PIE variant with a reference delay of 15ms (which according to theory should be good up to a RTT of ~300ms) at the head-end, a significant improvement on the status quo, albeit only for docsis users..

> would be a godsend. The ECO for
> cablemodems at least, went out over a year ago.
> some aqm tech becoming common on these head ends would also spur
> deployment of aqm (or fq + aqm) tech on fiber also. But I've seen no
> info as to what's going into cmtses today. Haven't seen any
> announcements...
> I still have no idea what is going to happen on 5G.
> My initial experiments with the intel ax200 wifi card have been dismal.
> On Fri, Jan 24, 2020 at 12:24 AM Dave Taht <dave.taht at gmail.com> wrote:
>> Jeeze, you guys are up early. I read this stuff on the plane home from
>> australia, and am still a bit under the weather.
>> On Fri, Jan 24, 2020 at 12:01 AM Sebastian Moeller <moeller0 at gmx.de> wrote:
>>> Hi Jonathan,
>>>> On Jan 24, 2020, at 08:44, Jonathan Morton <chromatix99 at gmail.com> wrote:
>>>>> On 24 Jan, 2020, at 7:37 am, Dave Taht <dave.taht at gmail.com> wrote:
>>>>> "Otherwise, this exemplary embodiment enables system configuration to
>>>>> discard the low-priority packet tail, and transmit the high-priority
>>>>> packet instead, without waiting."
>>>> So this really *is* a "fast lane" enabling technology.  Just as we suspected.
>> Well, there are weasel words elsewhere in the patent, and the dualq
>> code for linux merely cleared a lane for L4S traffic and hardcoded the
>> ect(1) as an identifier. It would be good to have more data on
>> rtt-fairness, and on CE reordering of rfc3168 ecn packets.
>> I spent time dreaming up also all the ways "queue protection" could be
>> used against the user. Given the rigor of the l4s spec required, and
>> how one misbehaving application can screw it all up,  I could see
>> queue protection of unknown sources that can be squelched on demand
>> being a desirable "feature". This can be used to stop "unauthorized"
>> mac addresses from participating in this design as one example.
>> I like the idea of queue protection - there is a lot of malicious
>> traffic worth throttling - but without a reporting scheme to the user,
>> nor a means for the user to set it up, and the mechanism under the
>> sole control of the ISP - not so much.
>> My other in-flight entertainment was cory doctorow's latest piece,
>> which was so good I submitted it to slashdot. (
>> https://arstechnica.com/gaming/2020/01/unauthorized-bread-a-near-future-tale-of-refugees-and-sinister-iot-appliances/
>> )
>>>        They seem to be setting their customers up for a head-on collision with the European Union's net neutrality rules, according to which "special services/fast lanes" are permissible under the condition thay they are realized with completely dedicated addition bandwidth. Just looking at their patent diagram there is one common input path to the classifier. So either that fast lane is not going to be a paid for fast lane, or the ISPs rolling this out will be in hot water with the respective national regulators (at least in the EU). The one chance would be to give the end-user control over the classification engine, or if the strict priority path is only used for ISP originated VoIP traffic (I seem to recall there are weasel words in the EU rules that would allow that and ISPs are doing something like that already, and I agree that it is nice to be able to field an emergency call independent of access link load).
>> Well, one country at a time. NN is currently quite dead in the USA,
>> and only a change in regime might change that, and it's unclear if any
>> of the candiates understand the issues. Certainly with twin subsidies
>> being aimed at 5G and broadband deployment in pending legislation, I
>> have no idea what will happen here next. I view 5G with fear, watching
>> frontier file for bankruptcy, also... I really wish all the fiber
>> being run for 5G was being run into the home instead.
>>> Best Regards
>>>        Sebastian
>>>> - Jonathan Morton
>>>> _______________________________________________
>>>> Ecn-sane mailing list
>>>> Ecn-sane at lists.bufferbloat.net
>>>> https://lists.bufferbloat.net/listinfo/ecn-sane
>> --
>> Make Music, Not War
>> Dave Täht
>> CTO, TekLibre, LLC
>> http://www.teklibre.com
>> Tel: 1-831-435-0729
> -- 
> Make Music, Not War
> Dave Täht
> CTO, TekLibre, LLC
> http://www.teklibre.com
> Tel: 1-831-435-0729

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