Re: [Lsr] Working Group Last Call for "Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints" - draft-ietf-lsr-flex-algo-bw-con-07

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

Draft authors -

Regarding Section 5, which defines the rules for deriving Bandwidth metric,
Rule #1 states:

"If the Generic Metric sub-TLV with Bandwidth metric type is advertised for the link as described in Section 4, it MUST be used during the Flex-Algorithm calculation."

I think this constraint does not fit all possible use cases.

(Note: For brevity, I am inventing the acronym GMBM to represent "Generic Metric sub-TLV with Bandwidth metric type".)

Consider two nodes A, B that are connected by two parallel links - call them AB-1 and AB-2.

Suppose a customer deploys Flex-Algo 128, which specifies use of Bandwidth Metric, but also uses affinity to exclude link AB-2. Customer configures advertisement of GMBM for link AB-1 - all is working as expected.

Now customer deploys Flex-Algo 129, also specifying use of bandwidth metric, but wants both links (AB-1, AB-2) to be used and wants automatic bandwidth metric
calculation to be used so metric automatically adapts to the number of links which are up between A-B. Since there is advertisement of GMBM for link AB-1, 
automatic bandwidth calculation cannot include Link AB-1 - which means the customer does not have a way to get what is desired without:

o Removing the advertisement of GMBM for AB-1
o Adding automatic bandwidth to the FAD for Flex-Algo 128

This might be quite surprising to a customer - especially if Flex-Algo 128 has been deployed for some time and has been working well.

There are probably multiple ways this limitation could be overcome.

One way would be to define an additional bit in the bandwidth constraint sub-TLVs - call it the I-bit - meaning ignore GMBM even when advertised.
This would specify use of the automated calculation based on the bandwidth of all links even in the presence of an explicit GMBM on one or more members of the Group.

Related to this, I think some clarification as regards the existing G-bit is required i.e., I assume that automatic calculation only includes the bandwidth for links which do not have a GMBM advertisement - but the current text is not clear on that point.

    Les

> -----Original Message-----
> From: Lsr <[e-mail]> On Behalf Of Acee Lindem
> Sent: Monday, February 19, 2024 2:26 PM
> To: lsr <[e-mail]>
> Cc: [e-mail]
> Subject: [Lsr] Working Group Last Call for "Flexible Algorithms: Bandwidth,
> Delay, Metrics and Constraints" - draft-ietf-lsr-flex-algo-bw-con-07
> 
> 
> This starts the Working Group Last call for draft-ietf-lsr-flex-algo-bw-con-07.
> At least some of the flex algorithm enhancements described in the document
> have been implemented.
> 
>  Please send your support or objection to this before March 5th, 2024.
> 
> Thanks,
> Acee
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