Re: [Lsr] LSR Working Group Adoption Poll for Flex Algorithm Drafts

Hi Dongjie,

please see inline:

On 20/04/18 05:04 , Dongjie (Jimmy) wrote:
> Hi Peter,
>
> Thanks for the prompt response. Please see inline:
>
>> -----Original Message-----
>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>> Sent: Thursday, April 19, 2018 4:28 PM
>> To: Dongjie (Jimmy) <jie.dong@huawei.com>; Acee Lindem (acee)
>> <acee@cisco.com>; lsr@ietf.org
>> Subject: Re: [Lsr] LSR Working Group Adoption Poll for Flex Algorithm Drafts
>>
>> Hi Dongjie,
>>
>> please see inline:
>>
>> On 19/04/18 09:10 , Dongjie (Jimmy) wrote:
>>> Hi,
>>>
>>> Here are some comments on the Flex Algo drafts.
>>>
>>> SR algorithm as defined in draft-ietf-isis-segment-routing-extensions
>>> is about the algorithm used for path calculation, such as SPF, strict SPF, etc.
>>>
>>> In the Flex Algo drafts, the definition of algorithm is extended to
>>> include topological constraints and the metric type used in
>>> calculation, which makes its functionality analogous to multi-topology routing
>> (MTR).
>>
>> not really. MTR is defined on a per link basis and each MTR participation needs
>> to be advertised on a per link basis. There is no such concept in flex-algo draft.
>
> Both mechanisms have the capability to define a specific sub-topology in the network, that's why I say they are analogous in functionality. Flex-algo uses link affinity to describe the constraints of the corresponding topology, which is also a link attribute and needs to be configured on a per-link basis.
>
> The difference is in topology advertisement. In MTR a consistent topology is constructed by each node advertising its own adjacent links in the topology. While in flex-algo, the whole topology is advertised as part of the algorithm definition by each node, and priority based selection is used to reach a consistent view by all nodes.
>
>> Flex-algo works on top of existing IGP topologies.
> 	
> Do you mean flex-algo can work on top of the default IGP topology, and can also work on top of multiple IGP topologies created with MTR?

yes

> In the latter case, it seems you would create sub-topologies on top of a sub-topology (MTR) of the default topology,

no. We don't create any topologies with flex-algo. We compute 
constrained based paths.

> which sounds quite complicated. Maybe another way is to use MTR to create the sub-topology needed, and define the metric type and computation algorithm using a particular flex-algo?

what we propose is simple - compute multiple constrained based paths on 
top of a given topology.

What you propose is indeed complicated - create as many topologies as 
many constrained based paths you need. That solution does not scale.

>
>>> Section 4.1 of the Flex Algo drafts says "Flex-Algorithm definition is
>>> topology independent", while in some places Flex Algo is described as
>>> a "light weight alternative" to MTR.
>>
>> there is no mention of MTR in the document.
>
> I was referring to another relevant draft: draft-wijnands-mpls-mldp-multi-topology-00. Sorry for the confusion caused. It seems that draft considered MTR and flex-algo as comparable candidates for creating sub-topology.

then please talk to the authors of that draft.

>
>>> It would be necessary if the relationship between Flex-Algo and MTR
>>> can be further clarified. Whether the two mechanisms are complementary
>>> to each other, or Flex-Algo will be used to replace MTR?
>>
>> they are orthogonal.
>
> If as you said they are orthogonal, it would be better to avoid overlapping functionalities in topology definition and creation.

orthogonal does not mean overlapping.

thanks,
Peter

>
> Best regards,
> Jie
>
>
>> thanks,
>> Peter
>>
>>>
>>> And if it is claimed that Flex-Algo is light weight than MTR, it would
>>> be helpful to give a thorough comparison of the two mechanisms somewhere.
>>>
>>> Best regards,
>>>
>>> Jie
>>>
>>> *From:*Lsr [mailto:lsr-bounces@ietf.org] *On Behalf Of *Acee Lindem
>>> (acee)
>>> *Sent:* Tuesday, April 17, 2018 10:44 PM
>>> *To:* lsr@ietf.org
>>> *Subject:* [Lsr] LSR Working Group Adoption Poll for Flex Algorithm
>>> Drafts
>>>
>>> This begins a two-week adoption poll for the following Flex Algorithm
>>> drafts:
>>>
>>> https://datatracker.ietf.org/doc/draft-hegdeppsenak-isis-sr-flex-algo/
>>>
>>> https://datatracker.ietf.org/doc/draft-ppsenak-ospf-sr-flex-algo/
>>>
>>> The adoption poll will end at 12:00 AM EST on May 2^nd , 2018. Please
>>> indicate your support of opposition of the drafts.
>>>
>>> Additionally, the authors are amenable to combining the drafts into a
>>> single draft. If you have an opinion, please state that as well.
>>>
>>> Thanks,
>>>
>>> Acee
>>>
>>>
>>>
>>> _______________________________________________
>>> Lsr mailing list
>>> Lsr@ietf.org
>>> https://www.ietf.org/mailman/listinfo/lsr
>>>
>
> .
>