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

Hi Acee, 

Thanks for your comments. 

I guess you are referring to a particular usage of MTR, when multiple topologies have different routes to the same prefix, with IP forwarding some additional mechanism is needed to match the traffic to particular topology. This can be solved with SR forwarding (using per-topology SID).

Flex-algo also allows multiple routes to the same prefix, and uses different prefix-SIDs in the SR forwarding. 

Best regards,
Jie

> -----Original Message-----
> From: Acee Lindem (acee) [mailto:acee@cisco.com]
> Sent: Friday, April 20, 2018 6:58 PM
> To: Peter Psenak (ppsenak) <ppsenak@cisco.com>; Dongjie (Jimmy)
> <jie.dong@huawei.com>; lsr@ietf.org
> Subject: Re: [Lsr] LSR Working Group Adoption Poll for Flex Algorithm Drafts
> 
> Hi Jie, Peter,
> 
> Another difference with Multiple Topology Router (MTR) is that it implies the
> abstraction of a RIB per topology so you could have multiple routes to the same
> prefix dependent on criteria other than longest prefix matching (e.g., ingress
> interface). The authors can correct me if I'm wrong, but I don't believe Flex
> Algorithm requires this abstraction.
> 
> Thanks,
> Acee
> 
> 
> On 4/20/18, 5:33 AM, "Peter Psenak (ppsenak)" <ppsenak@cisco.com> wrote:
> 
>     Dongjie,
> 
>     On 20/04/18 11:00 , Dongjie (Jimmy) wrote:
>     > Hi Peter,
>     >
>     > Please see inline:
>     >
>     >> -----Original Message-----
>     >> From: Peter Psenak [mailto:ppsenak@cisco.com]
>     >> Sent: Friday, April 20, 2018 3:31 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 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.
>     >
>     > MTR is also used to compute constrained based path:) The constraint is
> described as a sub-topology.
> 
>     you are mixing two different things - topology and path computations,
>     these are two different things.
> 
>     >
>     > With flex-algo, you need to define the algorithm first, then the
> constrained path can be computed according to the algorithm.
>     >
>     > According to your presentation in IETF101, a flex-algo specifies:
>     >
>     >    a) Set of constraints - e.g affinity exclude-any, include-any, include-all
>     >    b) Metric type - IGP metric, Delay (RFC7810), TE metric
> (RFC5305), ...
>     >    c) Algorithm type - SPF, ...
>     >
>     > As I see a) defines a constrained topology, or a sub-topology.
> 
>     again, you are mixing "set of constraints" with a "topology", these are
>     two different things.
> 
>     >
>     >>> 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.
>     >
>     > Not exactly. Multiple constrained paths can be created in the same
> sub-topology. You don't need as many topologies as the number of paths.
> 
>     if you calculate multiple constrained paths on a single MT, you need to
>     agree what the constraints are for each calculation and that is what the
>     flex-algo draft is doing.
> 
>     regards,
>     Peter
> 
>     >
>     >>>
>     >>>>> 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.
>     >
>     > OK. It seems some sync up is needed to have consistent understanding
> of what flex-algo means.
>     >
>     >>>
>     >>>>> 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.
>     >
>     > Right, in order to make them orthogonal, overlapping (if any) should be
> resolved.
>     >
>     > Best regards,
>     > Jie
>     >
>     >>
>     >> 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-alg
>     >>>>> o/
>     >>>>>
>     >>>>> 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
>     >>>>>
>     >>>
>     >>> .
>     >>>
>     >
>     > .
>     >
> 
> 