Re: [Lsr] New Version Notification for draft-bonica-lsr-ip-flexalgo-00.txt

[Peter Psenak <[e-mail]>]

Hi Joel,

On 30/09/2020 06:04, Joel M. Halpern wrote:
> I am missing something in this discussion of multiple algorithms.

not really.

> 
> 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, ..)

right.

For SR MPLS, the flex-algo forwarding is associated with the algo 
specific label that is derived from the algo specific SID. That unique 
algo specific label allows the traffic for the same prefix to be 
forwarded using many algorithms by simply using the right MPLS label.

For SRv6, the SRv6 Locator is associated with a single Algorithm.

The proposal here is to associate the prefix with the unique flex-algo 
only and use that association on all routers. This is similar to SRv6 
locators.

> 
> 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.

absolutely not. Doing classification of the data traffic at each hop 
does not work. It has been attempted in the past, but without much success.

thanks,
Peter


> 
> 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:[e-mail]] On Behalf Of [e-mail]
>> Sent: Tuesday, September 29, 2020 10:05 PM
>> To: Ron Bonica <rbonica=[e-mail]>
>> Cc: [e-mail]
>> 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=[e-mail]> wrote:
>>>
>>>
>>> Please review and comment
>>>
>>>                                         Ron
>>>
>>>
>>>
>>> Juniper Business Use Only
>>>
>>>> -----Original Message-----
>>>> From: [e-mail] <[e-mail]>
>>>> Sent: Tuesday, September 29, 2020 9:36 AM
>>>> To: Parag Kaneriya <[e-mail]>; Shraddha Hegde
>>>> <[e-mail]>; Ron Bonica <[e-mail]>; Rajesh M
>>>> <[e-mail]>; William Britto A J <[e-mail]>
>>>> 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-
>>>> FnUA0oCcZMw3Qde6in0C72hu_9hOZ53kPspIarR8fNDyU9Vck8x7e5ZqI$
>>>> Htmlized:
>>>> https://urldefense.com/v3/__https://datatracker.ietf.org/doc/html/dra
>>>> ft-
>>>> bonica-lsr-ip-flexalgo__;!!NEt6yMaO-gk!X7PVDP-
>>>> FnUA0oCcZMw3Qde6in0C72hu_9hOZ53kPspIarR8fNDyU9Vck82w_6CyU$
>>>> Htmlized:       https://urldefense.com/v3/__https://tools.ietf.org/html/draft-
>>>> bonica-lsr-ip-flexalgo-00__;!!NEt6yMaO-gk!X7PVDP-
>>>> FnUA0oCcZMw3Qde6in0C72hu_9hOZ53kPspIarR8fNDyU9Vck81_QrJ_p$
>>>>
>>>>
>>>> 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.
>>>>
>>>>
>>>>
>>>>
>>>> Please note that it may take a couple of minutes from the time of
>>>> submission until the htmlized version and diff are available at tools.ietf.org.
>>>>
>>>> The IETF Secretariat
>>>>
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