Re: [Lsr] WG Adoption Poll for “Using IS-IS Multi-Topology (MT) for Segment Routing based Virtual Transport Network” - draft-xie-lsr-isis-sr-vtn-mt-03

[Tony Przygienda <[e-mail]>]

Having had a long history with RFC5120 (and with the concept before IETF
was even afoot really) and their deployment in its time I cannot resist
finally chiming in a bit ;-)

Well, first, it seems there is agreement that the drafts state fairly
straightforward things but as informational probably nothing wrong with
having it captured and I'd agree with that.

Second, as to realities of MT in such architecture some musings based on
experience;

a) MT was best used when faced with the problem of "alien" topologies, e.g.
introduction of v6 into a network when a lot of boxes/links couldn't do v6
yet (there was a good set of discussions in its time around MT strictly per
node or per link being best approach and for many reasons link based
architecture was chosen) or need for drastically different forwarding
decision deviating from simple SPF (RPF problems).
b) using underlay to "slice" is an expensive proposition of course. The
costliest resources in large networks is really ASIC space in the core and
if the MTs are properly separated, that will take ASIC state. in simplest
form e.g. topology being mirrored into per DSCP bits lookup or something
like this. Nothing wrong with that if one has the $ (or CNY ;-) to foot the
bill for that and slicing underlay gives of course the best quality
assurance compared to overlay approaches AFAIS.
c) MT (well, at least in ISIS ;-) has been built to scale ;-), originally 8
bits were suggested and then I asked others "what's the biggest you can
imagine" to which answer was 1024 MT which needed of course a
multiplication by 4 to make sure stuff will still work in 25 years where we
are today ;-)
d) SPF is to an extent only limitation of MT. the tree itself can be
computed very efficiently today (and there are techniques like incremental
SPF which can drastically lessen the cost of a computation and one can go
to the extent of throwing the SPF computations to a beefy side-box even and
most extreme, ASIC [unbelievale or not, this has been done for trees up to
a certain space ;-)]), the limiting factor is more prefix attachement and
prefix download on massive changes (but there is no reason any LFA would
not work with MT BTW, even to the point where an MT can protect another MT
or MT use all links when protecting disregarding MT itself, I'm not aware
of implementations but the theoretical thinking/whiteboarding has been done
here long ago albeit one has to know a fair bit of math [metric sets
largely]) to not blow up on a mine ;-)
e) main limiting factor of MT deployment was IME operational discontinuity
of topologies that happens much more often than one thinks it does. That's
why MT negotiates on every link the topologies so continuity is clearly
provisioned and can be tracked. Tools to visualize, manage all that are
much harder to operate than one would think, I assume because humans
generally have poor intuition concerning topological spaces under different
metrics. Just look how confused we are when we try to strike something in a
glass of water ;-)
b) yes, TE can be advertised in topologies but here is bunch of things that
fall out of it
i) oversubscription is a fact of life in such an architecture and
discussions about this will start quickly
ii) except in case of massive under-subscription (but even then) the
correct scheduling of MTs will become an issue and possibly pre-emption in
HW when the timing becomes tight enough. DetNet is an interesting romping
ground of ideas for that right now
iii) Even with under-subscription interesting problems crop up like parties
that don't need much resources under normal circumstances but in case of
emergencies are willing to pay any amount to allocate resources their need
and pre-empt any other traffic.

--- tony






On Sat, Mar 27, 2021 at 4:47 AM Gyan Mishra <[e-mail]> wrote:

>
> Hi Acee
>
> As an operator, I  support adoption of the draft and would like to provide
> answers to your questions.
>
> I would like to start by stating that as this is an informational
> document,  as nothing new is proposed other then the recommendation to use
> MT for VTN provisioning as a component of the 5G network slicing solutions,
> the benefit is that this concept can be used immediately as other NS
> features are still being developed.
>
> The solution for network slicing resource isolation is multi faceted
> involves Enhanced VPN+ provisioning, resource SIDs for provisioning
> underlay resources, SR-TE Per VPN or flow path steering to meet NS SLO
> requirements, as well as a method to isolate IGP resources for a VTN.
>
> MT is a component of the entire NS solution so there is really nothing to
> market as it’s not the only component used to provision the VTN.
>
> As their is a need for providing a viable means of provisioning IGP
> underlay resources VTN network slice and the ability to provide forwarding
> plane isolation via topological RIB without consuming tremendous control
> plane resources per instance as with MI.
>
> This draft does fill the gap of a means of forwarding plane isolation on
> shared infrastructure even though it does have scalability considerations.
>
> As other ideas of IGP forwarding plane isolation come about we are open to
> other solutions as well.
>
> As their are scalability concerns in section 5 that should be expanded,
> when MT should be used to support VTN and when should not. Agreed.
>
> I would deploy in a limited fashion taking into account the scalability
> concerns.
>
> Enhanced VPN  VPN+ scalability issue are described in detail in this draft
> below.  Lots of variables related to how many slices based on services
> which will eventually scale up but I think MT may suffice well in the
> beginning stages.
>
> https://tools.ietf.org/html/draft-dong-teas-enhanced-vpn-vtn-scalability-01
>
>
> There are many drafts and solutions in the works across many different WGs
> that are working on development of solution as to how network slicing and
> SLO can be realized  by operators for 5G services.
>
> Of these drafts below there are a number of Enhanced VPN Framework VPN+
>  related drafts that are critical to the provisioning of various components
> of network slicing.
>
> https://datatracker.ietf.org/doc/html/draft-ietf-teas-enhanced-vpn-07
>
>
> https://datatracker.ietf.org/doc/draft-dong-teas-enhanced-vpn-vtn-scalability/
>
> https://tools.ietf.org/html/draft-drake-bess-enhanced-vpn-06
>
> https://tools.ietf.org/html/draft-ietf-spring-resource-aware-segments-02
>
> Kind Regards
>
> Gyan
>
> On Thu, Mar 25, 2021 at 2:21 PM Acee Lindem (acee) <acee=
> [e-mail]> wrote:
>
>> Speaking as WG chair:
>>
>>
>>
>> There has been considerable support for this document. However, there has
>> also been objections to the document. The objections are either that there
>> is nothing to standardize given that all pieces exist and that the MT isn’t
>> a viable option for VTNs since it isn’t scalable.
>>
>>
>>
>> Since most of the draft’s support is from “friends and family”, I’d like
>> to know of the WG members who supported it, would you really want to market
>> it as a VTN solution? Those of you who operate networks, would you actually
>> consider deploying it?
>>
>>
>>
>> In any case, section 5 needs to be expanded on the scalability and where
>> using MTs to support VTNs would make sense and where it wouldn’t.
>>
>>
>>
>> Thanks,
>> Acee
>>
>>
>>
>>
>>
>> *From: *Lsr <[e-mail]> on behalf of "Acee Lindem (acee)"
>> <acee=[e-mail]>
>>
>>
>> *Date: *Tuesday, March 2, 2021 at 6:28 PM
>> *To: *"[e-mail]" <[e-mail]>
>> *Subject: *[Lsr] WG Adoption Poll for “Using IS-IS Multi-Topology (MT)
>> for Segment Routing based Virtual Transport Network” -
>> draft-xie-lsr-isis-sr-vtn-mt-03
>>
>>
>>
>> This information draft describes how MT could be used for VTN
>> segmentation. The authors have asked for WG adoption.
>>
>>
>>
>> This begins a three week LSR Working Group Adoption Poll for “Using IS-IS
>> Multi-Topology (MT) for Segment Routing based Virtual Transport Network” -
>> draft-xie-lsr-isis-sr-vtn-mt-03. I’m giving it three weeks due to the IETF
>> next week. Please register your support or objection on this list prior to
>> the end of the adoption poll on 3/24/2020.
>>
>>
>>
>> Thanks,
>>
>> Acee
>>
>>
>>
>>
>> _______________________________________________
>> Lsr mailing list
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>> https://www.ietf.org/mailman/listinfo/lsr
>>
> --
>
> <http://www.verizon.com/>
>
> *Gyan Mishra*
>
> *Network Solutions A**rchitect *
>
> *Email [e-mail] <[e-mail]>*
>
>
>
> *M 301 502-1347*
>
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