I am missing something in this discussion of multiple algorithms.

My understanding of flex-algo whether for MPLS, SRv6, SRH, or IPv6, is that you need to associated a forwarding label (e.g. MPLS label or IPv6 address) with a specific algorithm so that you can compute the next hope for the forwarding label using the proper algorithm. Then when a packet arrives, it is simply forwarded according to the forwarding table (e.g. FIB, LIB, ..)

If that is so, then I do not understand how a given prefix can be safely associated with more than one algorithm. I could imagine doing several calculations according to different algorithms. But how do you decide which one applies to the packet? As far as I know, flex-algo does not look at the QoS/CoS/ToS bits.

Yours,
Joel

PS: I will admit that it took until an operator described some "interesting" constraints before I understood why one would even do this.

On 9/29/2020 11:50 PM, Huzhibo wrote:
Hi.

Associating multiple algorithms with a given prefix is an interesting topic, 
and I think this can simplify the complexity of FlexAlgo. I wonder if the 
author would consider using cases with multiple algorithms with a given prefix.

Thanks

ZHibo

-----Original Message-----
From: Lsr [mailto:lsr-boun...@ietf.org] On Behalf Of tony...@tony.li
Sent: Tuesday, September 29, 2020 10:05 PM
To: Ron Bonica <rbonica=40juniper....@dmarc.ietf.org>
Cc: lsr@ietf.org
Subject: Re: [Lsr] New Version Notification for 
draft-bonica-lsr-ip-flexalgo-00.txt


Ron,

This is nice. It makes it clear that constraint based path computation need not 
have MPLS overhead for those that don’t want it.

One thing that you don’t talk about is how this gets used, tho that may be 
blindingly obvious: you’ll need all nodes placing their prefixes in the 
RIB/FIB, where it will need to be selected over other path computation for the 
same prefixes.  This somewhat precludes the possibility of a given prefix being 
useful in multiple flex-algos.

More text on application would be most welcome, just to ensure that we’re on 
the same page.

Tony


On Sep 29, 2020, at 6:37 AM, Ron Bonica <rbonica=40juniper....@dmarc.ietf.org> 
wrote:


Please review and comment

                                       Ron



Juniper Business Use Only

-----Original Message-----
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Sent: Tuesday, September 29, 2020 9:36 AM
To: Parag Kaneriya <pkane...@juniper.net>; Shraddha Hegde
<shrad...@juniper.net>; Ron Bonica <rbon...@juniper.net>; Rajesh M
<mraj...@juniper.net>; William Britto A J <bwill...@juniper.net>
Subject: New Version Notification for
draft-bonica-lsr-ip-flexalgo-00.txt

[External Email. Be cautious of content]


A new version of I-D, draft-bonica-lsr-ip-flexalgo-00.txt
has been successfully submitted by Ron Bonica and posted to the IETF
repository.

Name:           draft-bonica-lsr-ip-flexalgo
Revision:       00
Title:          IGP Flexible Algorithms (Flexalgo) In IP Networks
Document date:  2020-09-29
Group:          Individual Submission
Pages:          14
URL:            
https://urldefense.com/v3/__https://www.ietf.org/id/draft-bonica-
lsr-ip-flexalgo-00.txt__;!!NEt6yMaO-gk!X7PVDP-
FnUA0oCcZMw3Qde6in0C72hu_9hOZ53kPspIarR8fNDyU9Vck80Zbjoij$
Status:
https://urldefense.com/v3/__https://datatracker.ietf.org/doc/draft-bo
nica-lsr-
ip-flexalgo/__;!!NEt6yMaO-gk!X7PVDP-
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Htmlized:
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Abstract:
   An IGP Flexible Algorithm computes a constraint-based path and maps
   that path to an identifier.  As currently defined, Flexalgo can only
   map the paths that it computes to Segment Routing (SR) identifiers.
   Therefore, Flexalgo cannot be deployed in the absence of SR.

   This document extends Flexalgo, so that it can map the paths that it
   computes to IP addresses.  This allows Flexalgo to be deployed in any
   IP network, even in the absence of SR.




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