Re: [Lsr] draft-decraene-lsr-isis-flooding-speed & IETF 111

["Les Ginsberg (ginsberg)" <[e-mail]>]

Guillaume –

Thanx for the thoughtful response.
Responses inline.

From: [e-mail] <[e-mail]>
Sent: Thursday, July 29, 2021 3:20 AM
To: Les Ginsberg (ginsberg) <[e-mail]>; [e-mail]; [e-mail]
Cc: [e-mail]
Subject: Re: [Lsr] draft-decraene-lsr-isis-flooding-speed & IETF 111

Hello Les,

Jumping in since I have some insight as well.
Le 29/07/2021 à 08:51, Les Ginsberg (ginsberg) a écrit :
Bruno –

Resuming this thread…

I assure you that we have the same goals.
We are not yet in agreement on the best way to achieve those goals.
Your slides show indeed we have the same goal, and we agree on one way to deal with the matter (congestion control).


Looking forward to the WG discussion on Friday.

To get some discussion going in advance – if you have time to do so (which I know is challenging especially during IETF week) – I call your attention to slides 15-18 in the presentation we have prepared:

https://datatracker.ietf.org/meeting/111/materials/slides-111-lsr-21-isis-flood-scale-00

I do not intend to present these slides during my portion of the presentation time – but I included them as potential points of discussion during the discussion portion of the meeting (though the WG chairs will decide how best to direct that portion of the time).
I call your attention specifically to Slide 16, which discusses the functional elements in the input path typically seen on router platforms.
Each of these elements has controls associated with it – from queue sizes to punt rates, etc. that play a significant role in delivery of incoming IS-IS PDUs to the protocol running in the control plan.

Your slides – https://datatracker.ietf.org/meeting/111/materials/slides-111-lsr-22-flow-congestion-control-00
 focus only on the direct input queue to IS-IS in the control plane. I do not see how the state of the other staging elements on the path from PDU ingress to IS-IS implementation reading the PDUs in the control plane is known and/or used in determining the flow control state signaled to the transmitting neighbor. If, for example, PDUs were being dropped on ingress and never made it to IS-IS in the control plane, how would your algorithm be aware of this and react to this?

In my experience, the state of these lower level staging elements plays a significant role.

I can imagine that some form of signaling from the dataplane to the control plane about the state of these lower level elements could be possible – and that signaling could be used as input to a receiver based control algorithm. However, given the differences as to how individual platforms implement these lower level elements,  I see it as challenging to get each platform to provide the necessary signaling in real time and to normalize it in a way so that IS-IS flow control logic in the control plane can use the data in a platform independent way. I believe this represents a significant bar to successful deployment of receiver-based flow control.

That's one point we want to clarify, the flow control algorithm does not focus on the "IO path" between the line card and the control plane. There is no magic there, it does not directly deals with congestion on the IO path. It happens it has some nice properties even in case of drops before reaching the control plane, but it's arguably not sufficient. That's why we also propose a congestion control algorithm. While it is not necessary to establish a standard since it's only local, it helps having a baseline if one does not want to spend time re-developing its own algorithm.

Our slides also show the result of our congestion control algorithm, which is the part that deals with IO path losses. Very much like your algorithm, ours sees this IO path as a black box.

[LES:] This is an aspect on which I need further clarification.

From the POV of the control plane, in the absence of enhanced signaling (which I believe is problematic to implement – and you seem to agree) you simply have no knowledge as to whether incoming PDUs have been dropped or are simply delayed.

On Slide 6 you say: “Sender will never send more than RWin unacknowledged LSPs”. On Slide 7 you describe how to choose RWIN. But all of these methods are fixed in size – not adaptive. And since the input queue of packets to IS-IS is not “per interface”, the number you choose for RWIN seems to have nothing at all to do with current state/neighbor.

You then go on to discuss RTT (which seems to be a configured or pre-determined value??) and LPP (LSPs acked/PSNP) – which is only meaningful for the LSPs that have actually made their way to IS-IS – no way to account for those that have been dropped or delayed.

So it is difficult for me to understand how you are actually accounting for the current state of the I/O path in real time on a per neighbor basis.

This is one major reason why we prefer a Tx based flow control mechanism. Tx based flow control simply focuses on the relative rates of LSP transmission and reception of Acknowledgments. Whether slower acks are due to PDU drops at ingress, slow punt path operation, lack of CPU time for IS-IS to process the incoming packets, etc. matters not. Whatever the reason, if acks are not keeping pace with transmission we know we have to slow down.

As you can see in the slides & draft, we also have a congestion control algorithm and we show the results. This congestion control algorithm only works with the ACKs (like yours), and gives results in the case of "IO congestion" (like yours).

[LES:] As best I understand it, ACKs in your case are from the receiver’s POV – which makes it dependent on what LSPs the receiver has actually seen.

In the TX based algorithm, we don’t care/know whether the receiver has seen anything – we just know whether we have got timely ACKs or not. And since it is possible that drops/delays could occur on the Tx side as well as the Rx side, this approach seems much more robust.

Flow and congestion control are not mutually exclusive; in fact it is almost certain it will be necessary to have both at some point. The main benefit of limiting the number of unacked packets inflight is to avoid loosing packets in case of CPU contention. As this should be a common situation (in part for reasons in your slide 15), flow control as we propose seems very relevant.

For example, in your Slowing Down scenario, if the slowing down occurs at the Control Plane, a congestion control algorithm will lose packets. If on top of your algorithm, you limit the number of unacked LSPs (flow control), these losses cannot occur anymore as the sender will stop sending before overflowing the socket buffer. It's an (almost) free win.

[LES:] In both approaches it is the control plane that is adapting. And Tx based approach does react to the number of unacked packets.

In addition, slide 17 talks about signalling in real time; I am unsure of your point. As the socket size is static (or at least long lived), there is no need to change the advertised value in real time. Maybe the previous explanations helped in clarifying the proposed changes. I don't really understand the point of slide 18 neither. I would be interested in more details.

[LES:] The point of Slide 17 is that if we want the algorithm to react to transient changes in the receiver’s capability (e.g., due to bursts unrelated to IS-IS LSP activity), to be effective we have to do so quickly. And since this coincides with the high input of IS-IS PDUs, the likelihood of delays in processing the PDUs used to send the signal is higher than normal. Look at our Slide #8 Row #2 for an example of the consequences of not adapting quickly.

If, as you say, there is no need to signal in real time, this tells me that you are simply advertising conservative values not based on the actual real time performance, in which case the issues highlighted on our Slide #15 are relevant. You seem to be acknowledging you have no intent to adapt to transient changes – you are simply going to limit things to something you have determined via offline evaluation should be safe. But such values have to account for the “worst case” in terms of # of neighbors, # of LSPs in the network, …all the things listed on our Slide #15 – so either they are overly conservative, or the customer somehow has to determine what value to configure based on the network. So you aren’t actually proposing anything adaptive at all it seems. ???

Slide 18 is highlighting the differences between operation of a TCP session and operation of IS-IS Update process. One of the arguments used for the Rx based approach has been “this is the way TCP has done it for years”. We are just highlighting why that isn’t a very good analogy. To be fair, I note you have acknowledged some (but not all) of this in your presentation as well e.g., on your Slide 25 you acknowledge that “packet reordering” isn’t applicable to IS-IS.

Thanks for your remarks,

[LES:] Thank you for your response. I hope we can continue this dialogue.

   Les

Guillaume

Please comment on these points as you have time.

Thanx.

   Les

From: [e-mail]<mailto:[e-mail]> <[e-mail]><mailto:[e-mail]>
Sent: Monday, July 12, 2021 1:48 AM
To: Les Ginsberg (ginsberg) <[e-mail]><mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Cc: [e-mail]<mailto:[e-mail]>
Subject: RE: draft-decraene-lsr-isis-flooding-speed & IETF 111

Les,

Faster flooding may be achieved without protocol extension. But if we are at changing flooding, it would be reasonable to try to make it good (rather than just faster than today).
In particular some goals are:
- faster flooding when the receiver has free cycles
- slower flooding when the receiver is busy/congested (either by flooding, or any CPU computation including not coming from IS-IS)
- avoiding/minimizing the parameters that the network operator is been asked to tune
- avoiding/minimizing the loss of LSPs
- robust to a wide variety of conditions (good ones and bad ones)

You seem to agree on changing the flooding behaviour on both the sender and the receiver so that they can better cooperate. That’s protocol extension to me (and IMHO much bigger than the sending of info in one TLV)

Bruno

From: Les Ginsberg (ginsberg) [mailto:[e-mail]]
Sent: Friday, July 9, 2021 7:49 PM
To: DECRAENE Bruno INNOV/NET <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Cc: [e-mail]<mailto:[e-mail]>
Subject: RE: draft-decraene-lsr-isis-flooding-speed & IETF 111

Bruno –

Neither of us has presented anything new of substance in the last few IETFs.
There were two presentations recently - one by Arista and one by Huawei – each of which simply demonstrated that it is possible to flood faster - and that in order to do so it is helpful to send acks faster - both points on which there is no disagreement.

To have a productive discussion we both need to present new data - which is why having the discussion as part of the meeting at which the presentations occur makes sense to me.

We removed the example(sic) algorithm from our draft because it was only an example, is not normative, and we did not want the discussion of our approach to be bogged down in a debate on the specifics of the example algorithm.
Based on your response, seems like we were right to remove the algorithm. 😊

Regarding WG adoption, one of the premises of our draft is that faster flooding can be achieved w/o protocol extensions and so there is no need for a draft at all. I am sure we do not yet agree on this - but I do hope that makes clear why adopting either draft at this time is premature.

   Les


From: [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Sent: Friday, July 9, 2021 9:15 AM
To: Les Ginsberg (ginsberg) <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Cc: [e-mail]<mailto:[e-mail]>
Subject: RE: draft-decraene-lsr-isis-flooding-speed & IETF 111

Les,

> From: Les Ginsberg (ginsberg) [mailto:[e-mail]]
[…]
> I also think it would be prudent to delay WG adoption

For how long exactly would it be “prudent to delay WG adoption”? (in addition to the past two years)
Until what condition?

It’s been two years now since draft-decraene-lsr-isis-flooding-speed brought this subject to the WG (and even more in private discussions).
Two years during which we have presented our work to the WG, discussed your comments/objections, been asked to provide more data and consequently worked harder to implement it and obtain evaluation results.
What’s precisely the bar before a call for WG adoption be initiated?
We have data proving the benefit, so after those two years, what are your clear and precise _technical_ objections to the mechanism proposed in draft-decraene-lsr-isis-flooding-speed?

Coming back to draft-decraene-lsr-isis-flooding-speed,
we have a specification and the flow control part is stable.
We have an implementation and many evaluations demonstrating that flow control alone is very effective in typical conditions.
we have an additional congestion control part which is still been refined but this part is a local behavior which don’t necessarily needs to be standardized and which is mostly useful when the receivers of the LSP is not CPU-bound which does not seem to be the case from what we have seen. (in most of the cases, receivers are CPU bound. In fact, we needed to artificially create I/O congestion in order to evaluate the congestion control part) .


Regarding your draft, in the latest version of your draft, published yesterday, you have removed the specification of your proposed congestion control algorithm… Based on this, I don’t see how technical discussion and comparison of the specification can be achieved.
You have an implementation. This is good to know and we are ready to evaluate it under the same conditions than our implementation, so that we can compare the data. Could you please send us an image? We may be able to have data for the interim.

--Bruno

From: Les Ginsberg (ginsberg) [mailto:[e-mail]]
Sent: Friday, July 9, 2021 5:00 PM
To: DECRAENE Bruno INNOV/NET <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: RE: draft-decraene-lsr-isis-flooding-speed & IETF 111

As is well known, there are two drafts in this problem space:

https://datatracker.ietf.org/doc/draft-decraene-lsr-isis-flooding-speed/
and
https://datatracker.ietf.org/doc/draft-ginsberg-lsr-isis-flooding-scale/


Regarding the latter, we also have a working implementation and we also have requested a presentation slot for IETF 111 LSR WG meeting.

I agree with Bruno that the time available in the WG meeting will likely be inadequate to present full updates for both drafts. In addition, I think it is important that the WG have
an opportunity to discuss publicly in an interactive way, the merits of each proposal. The likelihood that time will be available in the scheduled WG meeting for that discussion as well seems low.

I therefore join w Bruno in suggesting that an interim meeting dedicated to the flooding speed topic be organized.
Given the short time available before IETF 111, I would suggest that we look at scheduling an interim meeting after IETF 111 - but I leave it to the WG chairs to decide when to schedule this.

I also think it would be prudent to delay WG adoption calls for either draft until after such an interim meeting is held. In that way the WG can make a more informed decision.

   Les

From: Lsr <[e-mail]<mailto:[e-mail]>> On Behalf Of [e-mail]<mailto:[e-mail]>
Sent: Friday, July 9, 2021 2:01 AM
To: [e-mail]<mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>
Subject: [Lsr] draft-decraene-lsr-isis-flooding-speed & IETF 111

Hi chairs, WG,

Over the last two years, we have presented and the WG discussed draft-decraene-lsr-isis-flooding-speed at IETF 105 and “107”
IETF 105: https://datatracker.ietf.org/meeting/105/proceedings#lsr    Note: that the presentation is in first slot/video but a large part of the discussion is in the second one.
IETF 107/interim: https://datatracker.ietf.org/meeting/interim-2020-lsr-02/materials/agenda-interim-2020-lsr-02-lsr-01-07.html

The goal is to improve flooding performance and robustness to make it both faster when the receiver have free cycles, and slower when the receiver is congested.

In addition to technical discussions, a feedback was that implementation and tests/evaluation would be good in order to evaluate the proposal.

We are reporting that we have an implementation of [1] based on the open source Free Range Routing implementation.
We are now ready to report the evaluation to the WG. We have a lot of data so ideally would need around an hour in order to cover the whole picture.

We have requested a slot for IETF 111 LSR meeting. If the IETF 111 slot is short, we’d like to request for an interim meeting. In order to keep the context, the sooner/closer to IETF 111 seems the better.

Since we have an implementation, we have requested for a code point, in order to avoid squatting on one. This is currently under review by the designed experts.

Finally, given the two-years work, the specification, the implementation and extensive evaluation, we’d like to ask for WG adoption.

Thanks,
Regards,
--Bruno


[1] https://datatracker.ietf.org/doc/html/draft-decraene-lsr-isis-flooding-speed


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