Hi Ketan,
please see inline (##PP3):
On 13/04/2022 06:00, Ketan Talaulikar wrote:
Hi Peter,
Please check inline below with KT2. I am trimming everything other than
the one point of continuing debate.
> >
> > 2) The relationship between the algo usage for IP FlexAlgo
and other
> > data planes (e.g. FlexAlgo with SR) is not very clear.
There arise
> > complications when the algo usage for IP FlexAlgo overlap
with other
> > (say SR) data planes since the FAD is shared but the node
> participation
> > is not shared. While Sec 9 suggests that we can work
through these
> > complications, I question the need for such complexity.
The FlexAlgo
> > space is large enough to allow it to be shared between
various data
> > planes without overlap. My suggestion would be to neither
carve out
> > parallel algo spaces within IGPs for various types of
FlexAlgo data
> > planes nor allow the same algo to be used by both IP and
SR data
> planes.
> > So that we have a single topology computation in the IGP
for a given
> > algo based on its FAD and data plane participation and
then when it
> > comes to prefix calculation, the results could involve
> programming of
> > entries in respective forwarding planes based on the
signaling of
> the
> > respective prefix reachabilities. The coverage of these
aspects in a
> > dedicated section upfront will help.
>
> ##PP
> I strongly disagree.
>
> FAD is data-pane/app independent. Participation is data-plane/app
> dependent. Base flex-algo specification is very clear about
that. That
> has advantages and we do not want to modify that part.
>
>
> KT> No issue with this part.
>
>
> Topology calculation for algo/data-plane needs to take both
FAD and
> participation into account. You need independent calculation
for each
> data-plane/app in the same algo.
>
>
> KT> So, an implementation now needs to potentially support
performing
> multiple topology computations for each algo. This is a
complication for
> which I do not see the justification. Why not just pick different
> algorithms for different data planes for those (rare?)
deployments where
> someone wants multiple data planes?
##PP2
flex-algo architecture supports multiple apps/data-planes per algo,
with
unique participation per app/data-plane. That requires per-algo/per
app/data-plane calculation. What is complicated on it?
KT2> This specific and precise statement that you have provided is not
covered in either draft-ietf-lsr-flex-algo or this document. For
starters, this needs to be clarified and covered so that it gets the
attention of any reader during the review. This has implications for
implementations.
##PP3
sure we can add it explicitly there, but if you read the base flex-algo
draft carefully, it is quite clear. I will add that exact statement in
the next re-spin of the base spec.
If your implementation does not want to support it, fine, but the
architecture allows it and there is/are implementation(s) that already
support it. This is not defined in this draft, it's defined in base
flex-algo spec.
KT2> I am not sure if it is really an option for implementation once it
is in the specification. And this is not about "my" implementation :-).
So it is not that because some implementations can do (or does) it that
it should be in the specification. The determination on whether it
should be in a specification needs to be based on the tradeoff between
requiring multiple computations per algo with the potential benefit or
use case that is enabled by it.
##PP3
again, this is how things have been defined from day one, and for a good
reason. Requiring per app flex-algo even though I want to use the same
metric and constraints for both app would be inefficient.
>
>
> The fact that the same FAD is shareable between all apps has it
> advantages and use cases - e.g. if the participation for algo
X is the
> same in SR and IP data-planes, one can use SR to protect IP
in that
> algo.
>
>
> KT> Would this protection use case not violate the base FlexAlgo
rule
> that the protection has to remain within the specific topology.
If there
> is an SR data plane, then why would one want an IP data plane as
well?
##PP2
if the participation in two app/data-planes is the same for the algo,
the resulting topology is the same. If your implementation is smart, it
can only run a single computation for that case. There is no violation
here whatsoever.
KT2> If the resulting topology is the same between SR data plane and IP
data plane, what is the need to enable the IP data plane? Why not just
steer the IP traffic over the FlexAlgo data plane? And when it is not
the same topology, then we cannot really do the protection for IP
FlexAlgo using SR FlexAlgo. So what is really the use case or benefit
for enabling this?
##PP3
I just gave you an example where this might be useful. You may not like
it, but it will have no impact on the defined architecture.
> IP forwarding can be steered over the SR-based FlexAlgo topology
along
> with the protection provided by it. Am I missing something?
##PP2
topology for both primary and backup computation must be the same.
KT2> I see the primary use case for IP FlexAlgo (or another data plane)
to be that the data plane is used by itself. In the (rare?) case where
multiple data planes are required to coexist, it is simpler both from
implementation and deployment POV to use different algos. It would be
good to have operator inputs here. The only cost that I see for this is
that the same FAD may get advertised twice only in the case where it is
identical for multiple data planes. So I am still not seeing the benefit
of enabling multiple (i.e. per data plane) computations for a single
algo rather than just keeping it a single computation per algo where a
single data plane is associated with a specific algo.
##PP3
I really do not see the problem. As you stated above repeating the same
FAD for multiple algos would be inefficient. The beauty of FAD is that
it is app independent and can be used by many of them.
If you like to repeat it, fine it will still work. But we do not want to
mandate that in the spec.
thanks,
Peter
Thanks,
Ketan
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