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Jake & Ingemar,<br>
<br>
<div class="moz-cite-prefix">On 16/06/2019 11:07, Ingemar Johansson
S wrote:<br>
</div>
<blockquote type="cite"
cite="mid:HE1PR07MB4425E0997EE8ADCAE2D4C564C2E80@HE1PR07MB4425.eurprd07.prod.outlook.com">
<pre class="moz-quote-pre" wrap="">Hi Jake + all
Please see inline
/Ingemar
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">-----Original Message-----
From: Holland, Jake <a class="moz-txt-link-rfc2396E" href="mailto:jholland@akamai.com"><jholland@akamai.com></a>
Sent: den 14 juni 2019 20:28
To: Ingemar Johansson S <a class="moz-txt-link-rfc2396E" href="mailto:ingemar.s.johansson@ericsson.com"><ingemar.s.johansson@ericsson.com></a>; Bob Briscoe
<a class="moz-txt-link-rfc2396E" href="mailto:ietf@bobbriscoe.net"><ietf@bobbriscoe.net></a>
Cc: <a class="moz-txt-link-abbreviated" href="mailto:tsvwg@ietf.org">tsvwg@ietf.org</a>
Subject: Re: [tsvwg] Comments on L4S drafts
Hi Ingemar,
(bcc: ecn-sane, to keep them apprised on the discussion).
Thanks for chiming in on this. A few comments inline:
On 2019-06-08, 12:46, "Ingemar Johansson S"
<a class="moz-txt-link-rfc2396E" href="mailto:ingemar.s.johansson@ericsson.com"><ingemar.s.johansson@ericsson.com></a> wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Up until now it has been quite a challenge to make ECN happen, I
believe that part of the reason has been that ECN is not judged to
give a large enough gain.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">Could you elaborate on this point?
I haven't been sure how to think about the claims in the l4s drafts that operators
will deploy it rapidly because of performance.
Based on past analyses (e.g. the classic ECN rollout case study in RFC
8170 [1]), I thought network operators had a very "safety first" outlook on these
things, and that rapid deployment for performance benefits seemed like wishful
thinking.</pre>
</blockquote>
</blockquote>
[BB] The ECN rollout case study in rfc8170 is not a useful example .
It ends hoping there will be some client roll-out (written before
Apple's decision) and doesn't even mention that network roll-out
would still be needed subsequently. So it gives no insight into what
causes network operator resistance
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<pre class="moz-quote-pre" wrap="">
But I'd be interested to know more about why that view might be mistaken.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">[IJ] I believe that it is easy to end up in a lot of speculation. I don't believe that the safety first thinking makes much sense, yes it has sometimes been used as a counter argument. Part of the problem is perhaps that ECN was introduced into 3GPP for VoLTE. And then when ECN is proposed for its original use in 3GPP (=Generic transport protocol agnostic feature) it gets hard to make it stick. With that said, ECN is supported in both LTE and NR standards (TS36.300, TS38.300). It is however rarely deployed. One could speculate around the reasons, I believe that one big reason can be that traditional ECN does not show a large enough delta improvement to make it worthwhile. I can of course be wrong , I don't possess a crystal ball 😊</pre>
</blockquote>
[BB] My experience on this comes from years inside BT. The last 15
were after ECN was standardized, and for the last few years I was in
BT's tech strategy team, regularly making business cases for various
improvements. And talking with folks from other operators, of
course. <br>
<br>
When I quantified the performance benefit of classic ECN, it was
embarrassing. You only got significant benefits in an
under-provisioned network, which most operators avoid for obvious
other reasons. Classic ECN gave next-to-no benefit with long-running
flows. The more significant benefit for short transactional flows
was primarily due to avoiding the timeout when the last packet of a
flow was dropped. I figured that could be solved e2e, and indeed in
2012 the tail loss probe was proposed to solve that problem. The
remaining benefit was mostly due to not losing SYNs and to a lesser
extent SYN/ACKs, but classic ECN couldn't be used on SYNs anyway. In
comparison the potential risks on the cost side dominated.<br>
<br>
Finally, for a large network improvement project it is nearly
impossible to squeeze the cash needed out of the relatively small
budgets assigned for regular network improvements. None of the
access equipment supported a modern AQM or ECN, so we would have had
to tender for new designs. To persuade vendors to spend that sort of
money, you need a budget line in a project that is buying kit for a
new service with a projected revenue stream (e.g. a new sports
service, a VR product, etc). That means your performance improvement
has to be necessary for that product. <br>
<br>
An alternative would have beeen to show that the performance
improvement would gain sales from competing ISPs for long enough to
pay for the costs of the improvement, but that's much harder to
argue convincingly.<br>
<br>
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<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Besides this, L4S has the nice
property that it has potential to allow for faster rate increase when
link capacity increases.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">I think section 3.4 of RFC 8257 says the rate increase would be the
same:
<a class="moz-txt-link-freetext" href="https://tools.ietf.org/html/rfc8257#section-3.4">https://tools.ietf.org/html/rfc8257#section-3.4</a>
A DCTCP sender grows its congestion window in the same way as
conventional TCP.
I guess this is referring to the paced chirping for rapid growth idea presented last
time?
<a class="moz-txt-link-freetext" href="https://datatracker.ietf.org/meeting/104/materials/slides-104-iccrg">https://datatracker.ietf.org/meeting/104/materials/slides-104-iccrg</a>-
implementing-the-prague-requirements-in-tcp-for-l4s-01#page=20
I'm a little unclear on how safe this can be made, but I agree it seems useful if it
can work well.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">[IJ] Yes DCTCP use traditional additive increase. I have personally done a few experiments in this area, nothing that is good enough to show as the experiments were very limited. One possible idea can be to make the bandwidth probing in BBR(v2) more aggressive. And there may also be possibilities with Paced chirping too</pre>
</blockquote>
[BB] Note that paced chirping is not the differentiator here. It
doesn't depend on ECN, nor L4S-ECN, nor SCE-ECN for that matter. It
is delay-based, and potentially applicable to any e2e technology.<br>
<br>
The differentiator that L4S provides (and perhaps SCE if all the
problems were fixed) is the introduction of scalable congestion
control (like DCTCP), which induces a frequent amount of signalling
per RTT that remains invariant as flow rate scales. <br>
<br>
Support for a transition to scalable CC is as important as cutting
latency. Aside from being able to scale flow rate indefinitely,... <br>
<br>
...it also solves the problem of rapidly detecting when more
capacity has become available. If you normally get 2 signals per RTT
(like DCTCP), you can tell there's available capacity after 2 or 3
RTT. If you get 1 signal every few hundred RTTs (like Cubic), you
cannot tell there's available capacity for a thousand or so RTTs.
That in itself is useful.... You don't need to do the seeking with
paced chirping, which is just one attempt to get up to capacity both
with less overshoot and faster.<br>
<br>
<br>
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<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Do you think the L4S benefits will still be sufficient if this point about faster
growth doesn't hold up (and/or could be replicated regardless of L4S), or is it
critical to providing sufficient benefit in 3GPP?
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">[IJ] No, I don't believe that it is critical, it is definitely a welcome bonus if it is possible.
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">(Note: I'm not taking a position on this point, just asking about how much this
point matters to the 3GPP support, as you see it.)</pre>
</blockquote>
</blockquote>
[BB] Ingemar has described the New Radio meetings to me where he's
tried to propose ECN in the RLC layer. Like the swathes of other
proposals, he was given 2 minutes, to persuade primarily radio
people, who have already seen all the work that went into ECN for
VoLTE not being taken up.<br>
<br>
5G has promised extremely low latency. It is currently planning to
do that with 'old school' QoS - by limiting throughput into reserved
capacity. But that doesn't scale to apps that want high bandwidth
and low latency. That's when the NR working group will start
listening more carefully to ECN-based solutions.<br>
<br>
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<pre class="moz-quote-pre" wrap="">
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">I see many applications that can benefit greatly from L4S, besides
AR/VR, there is also an increased interest in the deployment of remote
control capabilities for vehicles such as cars, trucks and drones, all
of which require low latency video streaming.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">Remote control over the internet instead of a direct radio link is an interesting
use case. Do you happen to know the research about delay parameters that
make the difference between viable or not viable for RC?
</pre>
</blockquote>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">This touches on one of the reasons I've been skeptical that the benefits will drive
a rapid deployment--in most of the use cases I've come up with, it seems like
reducing delay from ~200-500ms down to ~15-30ms (as seems achievable even
for single queue with classic AQM) would give almost all the same benefits as
reducing from ~15-30ms down to 1ms.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">[IJ] The thing I like with L4S is that it reduces standing queues down to almost zero, which gives a very fast reaction time when throughput drops. In addition L4S gives frequent signals of congestion, which makes it easier for a congestion control algorithm to know when it is close to the congestion knee.</pre>
</blockquote>
[BB] I did some research on motion-to-photon latency a while ago,
with others. It was for VR/AR, but it translates to similar apps.
Quoting:<br>
<pre class="newpage"> MTP Latency: AR/VR developers generally agree that MTP latency
becomes imperceptible below about 20 ms [<a href="https://tools.ietf.org/html/draft-han-iccrg-arvr-transport-problem-01#ref-Carmack13" title=""Latency Mitigation Strategies"">Carmack13</a>]. However,
some research has concluded that MTP latency must be less than
17ms for sensitive users [<a href="https://tools.ietf.org/html/draft-han-iccrg-arvr-transport-problem-01#ref-MTP-Latency-NASA" title=""HEAD TRACKING LATENCY IN VIRTUAL ENVIRONMENTS: PSYCHOPHYSICS AND A MODEL"">MTP-Latency-NASA</a>]. Experience has shown
that standards bodies tend to set demanding quality levels, while
motivated humans often happily adapt to lower quality although
they struggle with more demanding tasks. Therefore, we must be
clear that this 20 ms requirement is designed to enable immersive
interaction for the same wide range of tasks that people are used
to undertaking locally.
...
For a summary of numerous references
concerning the limit of human perception of delay see the thesis of
Raaen [<a href="https://tools.ietf.org/html/draft-han-iccrg-arvr-transport-problem-01#ref-Raaen16" title=""Response time in games : requirements and improvements"">Raaen16</a>].</pre>
<br>
Let's say 20ms is too pedantic and you've got 50ms round trip MTP
budget (John Carmack says that 50ms feels responsive, but the slight
lag is still subtly unnatural, and merely defers the onset of
VR-sickness). <br>
<br>
We projected [<a moz-do-not-send="true"
href="https://tools.ietf.org/html/draft-han-iccrg-arvr-transport-problem-01#appendix-A.1.2">latency
budget</a>] that, with some expected advances, it should possible
to get the total of all delays except propagation and queuing down
to about 13ms.<br>
<br>
If one subtracts the delays you just stated for queuing, you get the
following left for propagation:<br>
<br>
<table width="100%" cellspacing="2" cellpadding="2" border="1">
<tbody>
<tr>
<td valign="top"><br>
</td>
<td valign="top">target for 'responsive'</td>
<td valign="top">non-network<br>
</td>
<td valign="top">queuing<br>
</td>
<td valign="top">left for 2-way propagation<br>
</td>
<td valign="top">reach in fibre<br>
</td>
</tr>
<tr>
<td valign="top">2nd gen. AQM</td>
<td valign="top">50ms<br>
</td>
<td valign="top">- 13ms<br>
</td>
<td valign="top">- 30ms<br>
</td>
<td valign="top">= 7ms<br>
</td>
<td valign="top">700km (440miles)</td>
</tr>
<tr>
<td valign="top">3rd gen. AQM</td>
<td valign="top">50ms<br>
</td>
<td valign="top">- 13ms<br>
</td>
<td valign="top">- 1ms<br>
</td>
<td valign="top">= 36ms<br>
</td>
<td valign="top">3600km (2250miles)</td>
</tr>
</tbody>
</table>
<br>
5 times greater reach means responsive interaction between Los
Angeles and Atlanta, rather than just Los Angeles and Phoenix.<br>
<br>
For communicating with a data centre, 5 times greater reach means
equivalent coverage from 25 times fewer sites (coverage area is the
square of reach). Concentration of sites is surely a very important
cost factor for a CDN.<br>
<br>
<br>
Note that, for real-time comms you need to watch the 99 or 99.9
percentile, not just median. See these percentiles on a log-scale at
slide 24 here:<br>
<a class="moz-txt-link-freetext" href="https://www.files.netdevconf.org/f/4ebdcdd6f94547ad8b77/?dl=1">https://www.files.netdevconf.org/f/4ebdcdd6f94547ad8b77/?dl=1</a><br>
This was under rather extreme load (600 web sessions per second -
see slide for details).<br>
<br>
<br>
Whatever, @Jake, I think you will agree that SCE's aim is to cut
queuing to similarly low levels. So arguing that we don't need such
low delay also argues against SCE. <br>
<br>
<br>
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<pre class="moz-quote-pre" wrap="">
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Of course, there's a difference in that last 14-29ms, but for instance for gaming
reaction time it's well under the thresholds that make a difference for humans
(the low end of which is at 45ms, according to [2]), so it seems like the value in
that market would be captured by classic ECN, and therefore since classic ECN
deployment hasn't caught on yet, I had to conclude that the performance gains
to enable that market aren't sufficient to drive wide adoption.
So I'm curious to know more about the use cases that get over that hump from
an operator's point of view, and what you've seen that leads you to believe the
additional gains of L4S from will make the difference on those use cases where
classic ECN wasn't adequate.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">[IJ] I guess for this part, there need to be more input from operators</pre>
</blockquote>
[BB] When Kjetil [2] says 45ms is good enough for today's games, I'd
trust that. But you can't burn all that with queuing - if I had
aimed for 45ms not 50 ms above, I'd have been left with 2ms for
propagation. <br>
<br>
When I showed Kjetil the demo of L4S using finger-gestures to pan
and zoom cloud-rendered video, he agreed that humans are much more
sensitive to the lag that the eye sees between their real hand
controlling a movement and seeing the thing move under their hand.
It depends how much freedom we want to give game developers to
explore new user interfaces and delivery platforms (e.g. a Wii
interacting with cloud-rendering).<br>
<br>
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<pre class="moz-quote-pre" wrap="">
</pre>
<blockquote type="cite">
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">My bottomline is that I believe L4S provides with a clear benefit that
is large enough to be more widely accepted in 3GPP. SCE is as I see it
more like something that is just a minor enhancement to ECN and is therefore
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">much
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">harder to sell in to 3GPP.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">Thanks, this is good to know.
To me one benefit of SCE over L4S is that it seems safer to avoid relying on an
ambiguous signal (namely a CE that we don't know which kind of AQM set it) in a
control system, while still providing high- fidelity info about the network device
congestion, where available.
I agree that it's not completely clear exactly how the congestion controllers can
capitalize on that info, but to me it still seems worth considering.
So although I'll support L4S if it really covers all the safety issues and performs
better, I'd be more comfortable with the signaling if there's a way to make SCE
do the same job, especially if the endpoint implementation is simpler to get
robustly deployed.</pre>
</blockquote>
</blockquote>
[BB] How much is this a case of, "There aren't any problems with the
SCE endpoint because we haven't thought about the problems yet"?<br>
<br>
As well as the straightforward engineering showstoppers that I have
highlighted (which I'll repeat 1-by-1 in later emails), there's also
algorithmic stuff that hasn't even been identified yet in SCE, let
alone addressed theoretically, let alone implemented. <br>
<br>
For instance, a shift to fine-grained signals also shifts the
smoothing from the network to the sender. That means the sender has
to smooth the SCE
signal and not the CE. So you have to deal with the cases where the
two controllers interact and one overtakes the other. I don't
believe stability is understood in such a system (you can be
pessimistic when slowing down, but you also have to ensure stability
when speeding up). <br>
<br>
It's not as if SCE can just ride on the back of the CC research we
and others have already done - it also introduces its own new
research problems.
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<pre class="moz-quote-pre" wrap="">
So really, I'm hoping for a bakeoff to decide this, because one of my concerns is
that L4S still doesn't have an implementation that does all the things the drafts
say are needed for safety on the internet, even though the initial proof of
concept demoing the performance gains was presented 7 years ago. </pre>
</blockquote>
</blockquote>
[BB] It was Jul 2015 (nearly 4 years ago, not 7). <br>
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<pre class="moz-quote-pre" wrap="">It's good
that it's getting closer, but the long implementation cycle (which still doesn't
have all the features required by the drafts) is a concern for me from the
"running code" point of view.</pre>
</blockquote>
</blockquote>
[BB] The SCE endpoint will need all the features required by the
drafts as well. Unless it is going to solely require FQ.<br>
<br>
I should also add that we (the L4S proponents) never envisaged that
we would have to do all the endpoint stuff. We were all from
network-focused companies. Altho we all had background in congestion
control for video, we weren't allowed/expected to do such work on
company time.<br>
<br>
What we didn't realize was that researchers aren't getting funding
to do such work these days (those that haven't been collected by
Google). So we eventually had to grasp the nettle and find ways to
do the endpoint stuff ourselves.<br>
<br>
For instance, on a personal note, CableLabs is only funding a
fraction of my working week, and will only pay for time on tasks in
my contract, which are nearly all about the network aspects. I am
self-funding nearly all work I do on end-system stuff.<br>
<br>
<br>
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<pre class="moz-quote-pre" wrap="">
On this point of view, it's possible that a parallel track might get further faster,
especially if it doesn't need the same special cases to be safe, which is part of
why I've been tentatively supportive.</pre>
</blockquote>
</blockquote>
[BB] Let me first see if I can get the SCE proponents to address the
show-stoppers that I have highlighted. By remaining silent, they
seem to have convinced everyone that these show-stoppers don't
exist.<br>
<br>
If necessary, it sounds like it would help to address the only
outstanding concern with L4S (classic ECN fall-back), irrespective
of whether we think the problem actually exists or will ever exist.<br>
<br>
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<pre class="moz-quote-pre" wrap="">
And although I can see how the queue classification is a major issue that could
make the difference, especially with the very promising dualq proposal, it also
seems true that in addition to CPEs, there are promising avenues for carrier-
scale FQ systems (e.g [3], [4]) that could solve that. It makes me think that even
if SCE only gets low-latency with FQ and otherwise causes no harm, it's not clear
it'll be a slower path to ubiquitous deployment (and by the way, this approach
also would handle the opt-in access control problem).</pre>
</blockquote>
</blockquote>
[BB] You (@Jake) are right to point out that different people have
different ideas of what they think might happen in the future.
However, I think it is a bit of a stretch to imagine that ubiquitous
deployment of FQ might happen...<br>
<br>
FQ assumes L4 headers are accessible, which assumes the Internet is
an unencrypted L3 network. In 4G and 5G the eNodeB or gNodeB where
ECN would need to be marked is a L2 node. A node deeper into the
network has already compressed, tunnelled and encapsulated the IP
headers. So how would FQ here access L4 port numbers? it can't do
the cake trick - creating an artificial bottleneck where the IP
header is accessible, because this concerns radio capacity, which
varies hugely and continually.<br>
<br>
Not to mention... all my other unanswered points about where SCE
doesn't work at all, e.g. <br>
<ul>
<li>all tunnels will have to propagate the ECT(1) codepoint, when
the spec saying this isn't even out of WGLC yet, <br>
</li>
<li>and the optional TCP option for AccECN will be needed to feed
back ECT(1), when no major OS is going to implement the TCP
option, because they don't want to handle all the pain of
middlebox mangling, <br>
</li>
<li>and... <my other 3 points that I'll get to in later
emails></li>
</ul>
<br>
If the last unicorn goes to a solution that will rarely work, and
becomes renowned as ineffective and unconvincing, we will have
wasted the last unicorn.<br>
<br>
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<pre class="moz-quote-pre" wrap="">
Of course, this will presumably collapse to one answer at some point, but I'll
argue that it's worthwhile to give a good look to the alternate proposal...
Anyway, thanks for the comments, I think it's good to see more discussion on
this.</pre>
</blockquote>
</blockquote>
[BB] Having alternative(s) is v important, even if strawmen. Proper
discussion is good too - I've been close enough to this that I can
identify problems v quickly, but the wider community needs
discussion time to get steeped in it all.<br>
<br>
Thank you v much for all the time you're putting into this. <br>
Cheers<br>
<br>
<br>
Bob<br>
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<pre class="moz-quote-pre" wrap="">
Best regards,
Jake
[1] Appendix A.1 RFC 8170 <a class="moz-txt-link-freetext" href="https://tools.ietf.org/html/rfc8170#appendix-A.1">https://tools.ietf.org/html/rfc8170#appendix-A.1</a>
[2] <a class="moz-txt-link-freetext" href="https://protect2.fireeye.com/url?k=997ee527-c5f43093-997ea5bc">https://protect2.fireeye.com/url?k=997ee527-c5f43093-997ea5bc</a>-
866a015dd3d5-
1d25c70963170b1e&q=1&u=https%3A%2F%2Fojs.bibsys.no%2Findex.php%2FNI
K%2Farticle%2Fview%2F9 says 45ms [3] <a class="moz-txt-link-freetext" href="http://ppv.elte.hu/">http://ppv.elte.hu/</a> [4]
<a class="moz-txt-link-freetext" href="https://ieeexplore.ieee.org/document/8419697" moz-do-not-send="true">https://ieeexplore.ieee.org/document/8419697</a>
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<pre class="moz-signature" cols="72">--
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Bob Briscoe <a class="moz-txt-link-freetext" href="http://bobbriscoe.net/">http://bobbriscoe.net/</a></pre>
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