Re: [Lsr] draft-lin-lsr-flex-algo-metric-00

Hi Peter,

Sorry to reply late. Please see inline.

Thanks,

Mengxiao Chen

-----Original Message-----
From: Peter Psenak [mailto:ppsenak@cisco.com]
Sent: Friday, March 25, 2022 4:16 PM
To: chenmengxiao (RD) <chen.mengxiao@h3c.com>; lsr@ietf.org
Subject: Re: [Lsr] draft-lin-lsr-flex-algo-metric-00

Hi Mengxiao,

please see inline:

On 25/03/2022 07:35, Chenmengxiao wrote:

> Hi Peter,

>

> Thank you for the comments.

>

> Time for the presentation is very limited. Glad to receive your comments

> here.

>

> I agree that flex-algo is much easier to deploy and manage than MT,

> which make it a popular tool now.

>

> But I think using algorithm-specific link metric will not cause the loss

> of this property.

>

> For example, flex-algo 128 and flex-algo 129 need to use different

> metrics of the same metric-type for the same link.

>

> Solution A: Use algorithm-based metric for algorithm 128 and algorithm 129

>

> Solution B: Use metric-type-based metric for metric-type 128 and

> metric-type 129 (according to Shraddha's suggestion,

> draft-ietf-lsr-flex-algo-bw-con)

>

> Both in the two solutions, extra link metric tlvs need to be advertised,

> and they are stored locally, no matter algorithm-based or metric-type-based.

yes, but for B, any other algorithm may share the metric used for algo

128 or 129. That is not possible with A.

You may argue that in your case you only have two algos, so the result

ts similar. Yes, but from the architecture perspective there is a

difference.

[MXC] Yes, from the perspective of protocol, metric-type 128 and 129 in solution B are possible to be shared by other algorithm. But if we focus on the case here, metric-type 128 and 129 are used to configure their unique metrics for the same public type (bandwidth or TE metric), so the chance of sharing may be very low in practice.

From another perspective, if we use user-defined metric-type, we need to advertise it for every link. But if we use algorithm-specific link metric, the algorithm may only advertise the algorithm-specific metrics of a few links whose metric value are different from the common metric.

TLV-22 of Solution A:

   metric   <- IGP metric (used by all algorithms, if no algorithm-specific advertised)

    generic metric  <- other type (used by all algorithms, if no algorithm-specific advertised)

    ASLA

        link attribute    <- TE metric/delay (used by all algorithms, if no algorithm-specific advertised)

        algorithm-specific metric  <- metric for algorithm 128

        algorithm-specific metric  <- metric for algorithm 128

TLV-22 of Solution B:

   metric   <- IGP metric (used by all algorithms)

    generic metric  <- other type (used by all algorithms)

    generic metric  <- type 128 (for algorithm 128)

    generic metric  <- type 129 (for algorithm 129)

    ASLA

        link attribute    <- TE metric/delay (used by all algorithms)

Not to mention that for A we would need to extend the FAD.

[MXC] That’s true. I think a new bit in the Flexible Algorithm Definition Flags can be used for such extension. We’ve missed it in the current version of draft. Thank you for pointing out.

>

> During path computation, the difference is that, solution A needs to

> check if there is algorithm-based metric for the link. If yes,

> algorithm-based metric is preferred. If no, common metric is used.

>

> IMHO, even if the algorithm-specific link metric is used, flex-algo is

> still light weight, compared with MT.

not really, because it advertise algo specific metric that can not be

used by any other algo, which is similar to topology specific metric in MT.

I would encourage you to consider option B as a solution for your problem.

[MXC] Thanks for your suggestion. I don’t doubt that solution B can solve our problem at current stage. In solution B, user-defined metric-type is used as a substitution for algorithm-specific metric of a standard metric-type.

But if there are strong requirements to advertise different metrics  for different algorithms, I would prefer solution A (algorithm-specific link metric). Assume that, an algorithm uses metric-type X for path computation, and it also uses metric-type Y as constraints. And it has its own metrics for both metric-type X and Y. The user-defined metric-type may not be adequate in such case.

Besides, I still think that flex-algo with algorithm-specific link metric will not become MT. The control plane and data plane are so different.

thanks,

Peter

>

> Thanks,

>

> Mengxiao Chen

>

> -----Original Message-----

> From: Lsr [mailto:lsr-bounces@ietf.org] On Behalf Of Peter Psenak

> Sent: Thursday, March 24, 2022 11:09 PM

> To: lsr@ietf.org<mailto:lsr@ietf.org>

> Subject: [Lsr] draft-lin-lsr-flex-algo-metric-00

>

> Hi,

>

> as I was unable to make my comments during the presentation, let me put

>

> them here:

>

> When we started with the flex-algo work, there has been a lengthy

>

> discussion on why not to use the multi-topology (MT) instead. We always

>

> wanted flex-algo to be light weight and as a result easy to deploy and

>

> manage. I believe that was one of the reasons why flex-algo has been

>

> successfully deployed. We do not want to loose that very property of the

>

> flex-algo architecture.

>

> What you are proposing is what MT was invented for and we do not want

>

> flex-algo to become a new version of MT.

>

> You have two ways to solve your problem:

>

> a) use MT

>

> b) if you want to solve your problem with flex-algo, please use what

>

> already exists in flex-algo. Please look at

>

> draft-ietf-lsr-flex-algo-bw-con as others have pointed out.

>

> thanks,

>

> Peter

>

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