Re: [tcpm] [Ecn-sane] [tsvwg] ECN CE that was ECT(0) incorrectly classified as L4S

Sebastian Moeller <> Mon, 05 August 2019 11:00 UTC

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From: Sebastian Moeller <>
Date: Mon, 05 Aug 2019 13:00:26 +0200
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Subject: Re: [tcpm] [Ecn-sane] [tsvwg] ECN CE that was ECT(0) incorrectly classified as L4S
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Hi Ruediger,

> On Aug 5, 2019, at 09:26, <> <> wrote:
> Hi Sebastian,
> the access link is the bottleneck, that's what's to be expected.

	Mostly, then again there are situations with 1Gbps Plans where it is not the actual access link, but rather the CPEs Gigabit ethernet LAN ports that are the true bottleneck, but that does not substantially change the issue, it is still the upstream shaper/policer that needs to be worked around.

> As far as I know, in the operator world shapers here by and large removed policers.

	Good to know, shapers are somewhat nicer to user traffic than hard policers, at least that is my interpretation.

> A consecutive chain of narrower links results, if the Home Gateway runs with an additional ingress or egress shaper operating below the access bandwidth, if I get you right. 

	Yes, as you state below, this only is true for the ingress direction, egress shaping works reliably and is typically not suffering from this. That said, if the egress link bandwidth is larger than a servers connection this issue can appear also for the egress direction. For example overly hot peering/transit links can cause downstream bottlenecks considerably narrower than the internet access link's upload direction, but that, while unfortunate, is not at the core of my issue.

> I understand that you aren't interested in having 300ms buffer delay and may be some jitter for a phone conversation using best effort transport.


> A main driver for changes in consumer IP access features in Germany are publications of journals and regulators comparing IP access performance of different providers.

	Good to know,

> Should one provider have an advantage over the others by deploying a solution as you (and Bob's team) work on, it likely will be generally deployed.

	I do not believe that these mechanisms are actually in play in the German market, as an example for roughly a decade the DOCSIS-ISPs offer higher bandwidth for same or less money than the incumbent telco and yet only managed to get 30% of the customers of their ~75% of possible customers, so only 75*0.3 = 22.5 % market share, with the incumbent only reaching 250/40 for the masses while the DOCSIS ISPs offer 1000/50. And unlike latency, bandwidth (or rather rate) is the number that consumers understand intuitively.
	If anything will expedite the roll-out of L4S style AQMs it is the capability to use those to implement the "special services" that the EU net neutrality regulation explicitly allows, as that is a product that can be actually sold to customers, but I might be too pessimistic here.

> As far as I can see, latency aware consumers still are a minority and gamers seem to be a big group belonging here. Interest in well performing gaming seems to be growing, I guess (for me at least it's an impression rather than a clear trend).

	Put that way, I see a way for ISPs to distinguish themselves from the rest by being gaming friendly, but unless this results in gamers paying more I fail to see the business case that management probably needs before green-lighting the funds required to implement this. This is where cablelabs approach to mandate this in the specs is brilliant. 

> I'd personally prefer an easy to deploy and operate standard solution offering Best Effort based transport being TCP friendly and at the same time congestion free for other flows at a BNG for traffic in access direction (and for similar devices in other architectures of course). 
> Fighting bufferbloat in the upstream direction the way you describe it doesn't construct a chain of links which are consecutively narrower than the bottleneck link, I think.

	Yes, fully agreed, that said, and ISPs CPE should implement an AQM to really solve the latency issues for end-users. The initial L4S paper side-stepped that requirement by making sure the uplinks were not saturated during the test, and state that that needs a real solution for proper roll-out. In theory the ISP could do the uplink shaping on its end (and to constrain users to their contracted rates, ISPs do this already) but as in the downstream case, running an AQM in front of a bottleneck as opposed to behind it makes everything much easier. Also with uplinks typically << downlinks, the typically weak CPE CPUs will still be able to AQM the uplink, nicely distributing that computation load away from the BNG/BRAS big iron ....

Best Regards

> Regards,
> Ruediger
> -----Ursprüngliche Nachricht-----
> Von: Sebastian Moeller <> 
> Gesendet: Freitag, 2. August 2019 15:15
> An: Geib, Rüdiger <>
> Cc: Jonathan Morton <>;; ECN-Sane <>;
> Betreff: Re: [Ecn-sane] [tsvwg] ECN CE that was ECT(0) incorrectly classified as L4S
> Hi Ruediger,
>> On Aug 2, 2019, at 10:29, <> <> wrote:
>> Hi Jonathan,
>> could you provide a real world example of links which are consecutively narrower than sender access links?
> 	Just an example from a network you might be comfortable with, in DTAGs internet access network there typically are traffic limiting elements at the BNGs (or at the BRAS for the legacy network), I am not 100% sure whether these are implemented as policers or shapers, but they tended to come with >= 300ms buffering. Since recently, the BNG/BRAS traffic shaper's use the message field of the PPPoE Auth ACK to transfer information about the TCP/IPv4 Goodput endusers can expect on their link as a consequence of the BNG/BRAS"s traffic limiter. In DOCSIS and GPON networks the traffic shaper seems mandated by the standards, in DSL networks it seems optional (but there even without a shaper the limited bandwidth of the access link would be a natural traffic choke point).
> Fritzbox home router's now use this information to automatically set egress (and I believe also) ingress traffic shaping on the CPE to reduce the bufferbloat users experience. I have no insight in what Telekom's own Speedport routers do, but I would not be surprised if they would do the same (at least for egress). 
> 	As Jonathan and Dave mentioned, quite a number of end-users, especially the latency sensitive ones, employ their own ingress and egress traffic shapers at their home routers as the 300ms buffers of the BNG's are just not acceptable for any real-timish uses (VoIP, on-line twitch gaming, even for interactive sessions like ssh 300ms delay are undesirable). E.g. personally, I use an OpenWrt router with an FQ AQM for both ingress and egress (based on Jonathan's excellent cake qdisc) that allows a family of 5 to happily share a 50/10 connection between video streaming and interactive use with very little interference between the users, the same link with out the FQ-AQM active makes interactive applications feel like submerged in molasses once the link gets saturated...
> 	As far as I can tell there is a number of different solutions that offer home-router based bi-directional traffic shaping to solve bufferbloat" from home (well, not fully solve it, but remedy its consequences), including commercial options like evenroute's iqrouter, and open-source options like OpenWrt (with sqm-scripts as shaper packet). 
> 	It is exactly this use case and the fact that latency-sensitive users often opt for this solution, that causes me to ask the L4S crowd to actually measure the effect of L4S on RFC3168-FQ-AQMs in the exact configuration it is actually used today, to remedy the same issue L4S wants to tackle.
> Best Regards
> 	Sebastian
>> I could figure out a small campus network which has a bottleneck at the Internet access and a second one connecting the terminal equipment. But in a small campus network, the individual terminal could very well have a higher LAN access bandwidth, than the campus - Internet connection (and then there's only one bottleneck again).
>> There may be a tradeoff between simplicity and general applicability. Awareness of that tradeoff is important. To me, simplicity is the design aim. 
>> Regards,
>> Ruediger 
>> -----Ursprüngliche Nachricht-----
>> Von: tsvwg <> Im Auftrag von Jonathan Morton
>> Gesendet: Dienstag, 9. Juli 2019 17:41
>> An: Bob Briscoe <>
>> Cc: tcpm IETF list <>;; tsvwg IETF list <>
>> Betreff: Re: [tsvwg] [Ecn-sane] ECN CE that was ECT(0) incorrectly classified as L4S
>>> On 13 Jun, 2019, at 7:48 pm, Bob Briscoe <> wrote:
>>>     1.  It is quite unusual to experience queuing at more than one
>>>         bottleneck on the same path (the available capacities have to
>>>         be identical).
>> Following up on David Black's comments, I'd just like to note that the above is not the true criterion for multiple sequential queuing.
>> Many existing TCP senders are unpaced (aside from ack-clocking), including FreeBSD, resulting in potentially large line-rate bursts at the origin - especially during slow-start.  Even in congestion avoidance, each ack will trigger a closely spaced packet pair (or sometimes a triplet).  It is then easy to imagine, or to build a testbed containing, an arbitrarily long sequence of consecutively narrower links; upon entering each, the burst of packets will briefly collect in a queue and then be paced out at the new rate.
>> TCP pacing does largely eliminate these bursts when implemented correctly.  However, Linux' pacing and IW is specifically (and apparently deliberately) set up to issue a 10-packet line-rate burst on startup.  This effect has shown up in SCE tests to the point where we had to patch this behaviour out of the sending kernel to prevent an instant exit from slow-start.
>> - Jonathan Morton
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