[Bloat] [Ecn-sane] [iccrg] Fwd: [tcpPrague] Implementation and experimentation of TCP Prague/L4S hackaton at IETF104
Bob Briscoe
ietf at bobbriscoe.net
Mon Mar 18 21:06:57 EDT 2019
David,
On 17/03/2019 18:07, David P. Reed wrote:
>
> Vint -
>
> BBR is the end-to-end control logic that adjusts the source rate to
> match the share of the bolttleneck link it should use.
>
> It depends on getting reliable current congestion information via
> packet drops and/or ECN.
>
> So the proposal by these guys (not the cable guys) is an attempt to
> improve the quality of the congestion signal inserted by the router
> with the bottleneck outbound link.
>
What do you mean 'not the cable guys'?
This thread was reasonably civil until this intervention.
> THe cable guys are trying to get a "private" field in the IP header
> for their own use.
>
There is nothing private about this codepoint, and there never has been.
Here's some data points:
* The IP header codepoint in question (ECT(1) in the ECN field) was
proposed for use as an alternative ECN behaviour in July 2105 in the
IETF AQM WG and the IETF's transport area WG (which handles all ECN
matters).
* A year later there followed a packed IETF BoF on the subject (after 2
open Bar BoFs).
* Long discussion ensued on the merits of different IP header field
combinations, on both these IETF lists, involving people active on this
list (bloat), including Dave Taht, who is acknowledged for his
contributions in the IETF draft.
* That was when it was decided that ECT(1) was most appropriate.
* The logic of the decision is written up in an appendix of
draft-ietf-ecn-l4s-id.
* David Black, one of the co-chairs of the IETF's transport area WG and
co-author of both the original ECN and Diffserv RFCs, wrote RFC8311 to
lay out the process for reclaiming and reusing the necessary codepoints.
* This work and the process of freeing up codepoints has been very
visible at every IETF ever since, with multiple drafts to fix other
aspects of the protocols working their way through the IETF process in
multiple WGs (tsvwg, tcpm, trill, etc).
* L4S has also been mentioned in IETF liaisons with the IEEE and 3GPP.
Some history:
* I had been researching the idea since 2012.
* In fact my first presentation on it was scheduled directly after Van
Jacobson's first presentation of CoDel at the IETF in July 2012. VJ
overran by nearly 20mins leaving just 3 mins for my presentation.
* Mirja Kuehlewind and I did early groundwork in 2013 and published a paper
* Then I (working for BT) brought the work into the EU RITE project
(Reducing Internet Transport Latency) with Simula, Alcatel-Lucent, etc.
* By 2015 the two main L4S proponents were Koen De Schepper from Alcatel
Lucent and myself (I had just switched from BT to Simula), along with
Olga Bondarenko (now Albisser), a PhD student at Simula who now works
for Microsoft (on something else) and is still doing her PhD part-time
with Simula
o By that time, Al-Lu and Simula had a cool prototype running.
o This was demonstrated publicly for the first time in the IETF AQM
WG over DC+core+backhaul+DSL+home networks.
https://riteproject.eu/dctth/#1511dispatchwg
* In May 2016, L4S was demonstrated at MultiMediaSystems'16 with /every/
packet from all the following simultaneous applications achieving ~1ms
queuing delay or less over a 40Mb/s Internet access link (7ms base RTT):
o cloud-rendered remote presence in a racing car within a VR headset
o the interactive cloud-rendered video already linked above
o an online gaming benchmark
o HAS video streaming
o a number of bulk file downloads
o a heavy synthetic load of web browsing
L4S has never been access-technology-specific. Indeed the cable industry
has been funding my work at the IETF to help encourage a wider L4S
ecosystem. There is nothing private to the cable industry in this:
* Al-Lu used DSL as a use-case, but L4S was relevant to all the access
technologies they supplied.
* BT was obviously interested in DSL,
* but BT's initial motivating use-case was to incrementally deploy the
low queuing delay of DCTCP over BT's data centre interconnect networks.
* In Jul 2016 the open-source Linux repo for the Coupled AQM was
published, with a fully working version to be used and abused.
Now at: https://github.com/L4STeam/sch_dualpi2_upstream
* Of course, DCTCP was already open-sourced in Linux and FreeBSD, as
well as available in Windows
* In Jul 2016, the main IETF BoF on L4S was held:
o Ingemar Johansson from Ericsson was one of the proponents, focused
on using L4S in LTE
o along with Kevin Smith from Vodafone and
o Praveen Balasubramanian from Microsoft (who maintains the Windows
TCP stack, including DCTCP).
o Ingemar has since written an open-source L4S variant of the SCReAM
congestion controller for real-time media:
https://github.com/EricssonResearch/scream/
o Mirja Kuehlewind of ETHZ (and now Ericsson) implemented the
necessary feedback in TCP https://github.com/mirjak/linux-accecn
* In summer 2017 CableLabs started work on Low Latency DOCSIS, and hired
me later in the year to help develop and specify it, along with support
for L4S
o In Jan 2019 the Low Latency DOCSIS spec was published and is now
being implemented.
o You can find the primary companies involved at the end of the White
Paper.
https://cablela.bs/low-latency-docsis-technology-overview-february-2019
o Operators:
Liberty Global
Charter
Rogers
Comcast
Shaw
Cox Communications
o Equipment vendors
ARRIS
Huawei
Broadcom
Intel
Casa
Nokia
Cisco
Videotron
* Nicolas Kuhn of CNES has been assessing the use of L4S for satellite.
* Magnus Westerlund of Ericsson with a team of others has written the
necessary ECN feedback into QUIC
* L4S hardware is also being implemented for hi-speed switches at the
moment
(the developer wants to announce it themselves, so I have been
asked not to identify them).
There's a lot more stuff been going on, but I've tried to pick out
highlights.
All this is good healthy development of much lower latency for Internet
technology.
I find it extremely disappointing that some people on this list are
taking such a negative attitude to the major development in their own
field that they seem not to have noticed since it first hit the
limelight in 2015.
L4S has been open-sourced since 2016 so that everyone can develop it and
make it better...
If I was in this position, having overlooked something important for
multiple years, I would certainly not condemn it while I was trying to
understand what it was and how it worked. Can I suggest everyone takes a
step back, and suspends judgement until they have understood the
potential, the goals and the depth of what they have missed. People who
know me, know that I am very careful with Internet architecture, and
particularly with balancing public policy against commercial issues.
Please presume respect unless proven otherwise.
Best Regards
Bob
PS. Oh and BBR would be welcome to use the ECT(1) codepoint to get into
the L4S queue. But only if it can keep latency down below around 1ms.
Currently those ~15-25ms delay spikes would not pass muster. Indeed, its
delay is not consistently low enough between the spikes either.
> -----Original Message-----
> From: "Vint Cerf" <vint at google.com>
> Sent: Saturday, March 16, 2019 5:57pm
> To: "Holland, Jake" <jholland at akamai.com>
> Cc: "Mikael Abrahamsson" <swmike at swm.pp.se>, "David P. Reed"
> <dpreed at deepplum.com>, "ecn-sane at lists.bufferbloat.net"
> <ecn-sane at lists.bufferbloat.net>, "bloat" <bloat at lists.bufferbloat.net>
> Subject: Re: [Ecn-sane] [Bloat] [iccrg] Fwd: [tcpPrague]
> Implementation and experimentation of TCP Prague/L4S hackaton at IETF104
>
> where does BBR fit into all this?
> v
>
> On Sat, Mar 16, 2019 at 5:39 PM Holland, Jake <jholland at akamai.com
> <mailto:jholland at akamai.com>> wrote:
>
> On 2019-03-15, 11:37, "Mikael Abrahamsson" <swmike at swm.pp.se
> <mailto:swmike at swm.pp.se>> wrote:
> L4S has a much better possibility of actually getting
> deployment into the
> wider Internet packet-moving equipment than anything being
> talked about
> here. Same with PIE as opposed to FQ_CODEL. I know it's might
> not be as
> good, but it fits better into actual silicon and it's being
> proposed by
> people who actually have better channels into the people
> setting hard
> requirements.
>
> I suggest you consider joining them instead of opposing them.
>
>
> Hi Mikael,
>
> I agree it makes sense that fq_anything has issues when you're talking
> about the OLT/CMTS/BNG/etc., and I believe it when you tell me PIE
> makes better sense there.
>
> But fq_x makes great sense and provides real value for the uplink in a
> home, small office, coffee shop, etc. (if you run the final rate limit
> on the home side of the access link.) I'm thinking maybe there's a
> disconnect here driven by the different use cases for where AQMs
> can go.
>
> The thing is, each of these is the most likely congestion point at
> different times, and it's worthwhile for each of them to be able to
> AQM (and mark packets) under congestion.
>
> One of the several things that bothers me with L4S is that I've seen
> precious little concern over interfering with the ability for another
> different AQM in-path to mark packets, and because it changes the
> semantics of CE, you can't have both working at the same time unless
> they both do L4S.
>
> SCE needs a lot of details filled in, but it's so much cleaner that it
> seems to me there's reasonably obvious answers to all (or almost
> all) of
> those detail questions, and because the semantics are so much cleaner,
> it's much easier to tell it's non-harmful.
>
> <aside regarding="non-harmful">
> The point you raised in another thread about reordering is mostly
> well-taken, and a good counterpoint to the claim "non-harmful relative
> to L4S".
>
> To me it seems sad and dumb that switches ended up trying to make
> ordering guarantees at cost of switching performance, because if it's
> useful to put ordering in the switch, then it must be equally
> useful to
> put it in the receiver's NIC or OS.
>
> So why isn't it in all the receivers' NIC or OS (where it would render
> the switch's ordering efforts moot) instead of in all the switches?
>
> I'm guessing the answer is a competition trap for the switch vendors,
> plus "with ordering goes faster than without, when you benchmark the
> switch with typical load and current (non-RACK) receivers".
>
> If that's the case, it seems like the drive for a competitive
> advantage
> caused deployment of a packet ordering workaround in the wrong network
> location(s), out of a pure misalignment of incentives.
>
> RACK rates to fix that in the end, but a lot of damage is already
> done,
> and the L4S approach gives switches a flag that can double as
> proof that
> RACK is there on the receiver, so they can stop trying to order those
> packets.
>
> So point granted, I understand and agree there's a cost to abandoning
> that advantage.
> </aside>
>
> But as you also said so well in another thread, this is
> important. ("The
> last unicorn", IIRC.) How much does it matter if there's a
> feature that
> has value today, but only until RACK is widely deployed? If you were
> convinced RACK would roll out everywhere within 3 years and SCE would
> produce better results than L4S over the following 15 years, would
> that
> change your mind?
>
> It would for me, and that's why I'd like to see SCE explored before
> making a call. I think at its core, it provides the same thing
> L4S does
> (a high-fidelity explicit congestion signal for the sender), but with
> much cleaner semantics that can be incrementally added to congestion
> controls that people are already using.
>
> Granted, it still remains to be seen whether SCE in practice can match
> the results of L4S, and L4S was here first. But it seems to me
> L4S comes
> with some problems that have not yet been examined, and that are
> nicely
> dodged by a SCE-based approach.
>
> If L4S really is as good as they seem to think, I could imagine
> getting
> behind it, but I don't think that's proven yet. I'm not certain, but
> all the comparative analyses I remember seeing have been from more or
> less the same team, and I'm not convinced they don't have some
> misaligned incentives of their own.
>
> I understand a lot of work has gone into L4S, but this move to jump it
> from interesting experiment to de-facto standard without a more
> critical
> review that digs deeper into some of the potential deployment problems
> has me concerned.
>
> If it really does turn out to be good enough to be permanent, I'm not
> opposed to it, but I'm just not convinced that it's non-harmful,
> and my
> default position is that the cleaner solution is going to be better in
> the long run, if they can do the same job.
>
> It's not that I want it to be a fight, but I do want to end up
> with the
> best solution we can get. We only have the one internet.
>
> Just my 2c.
>
> -Jake
>
>
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________________________________________________________________
Bob Briscoe http://bobbriscoe.net/
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