Re: Question for TI-LFA

[Yasuhiro Ohara <[e-mail]>]

Thank you.

The example makes sense if it were the extended P-space, P'().

We'd better bear in mind that the use of P'() increases computational
complexity.
As an implementation developer, computation of an SPT (the routes to
every destination in the intra-net)
for all neighbors also requires some extra caution (such as upper
bounds for memory).

But yes, the I-D would be fine if the error in the example is fixed.
Please add some text if the extended P-space should be used
for the correct results in the example.

Best regards,
Yasu

2024年3月29日(金) 1:03 Dirk Goethals (Nokia) <[e-mail]>:
>
> I think the P space calculation in Section 6 is the extended P-space
> as defined in RFC7490, i.e. path to R1 is no longer ECMP once N2
> is selected as next hop.
> See also  ecmp path to node C In figure 1 of that RFC7490.
> Dirk
> ________________________________
> Van: rtgwg <[e-mail]> namens Yasuhiro Ohara <[e-mail]>
> Verzonden: zondag 24 maart 2024 6:54
> Aan: [e-mail] <[e-mail]>
> CC: Yasuhiro Ohara <[e-mail]>
> Onderwerp: Question for TI-LFA
>
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>
> Hi,
>
> I have a question for the draft-ietf-rtgwg-segment-routing-ti-lfa-13.
> I wonder if it needs a fix.
>
> In the I-D, the Section 3. "Terminology" defines
> the P-space as the following.
>
> > The P-space P(R,X) of a router R with regard to a resource X (e.g. a
> > link S-F, a node F, or a SRLG) is the set of routers reachable from R
> > using the pre-convergence shortest paths without any of those paths
> > (including equal-cost path splits) transiting through X.
>
> The Figure 1 (Section 6) in the same I-D,
> the resulting P(S, N1) includes R1,
> but one of the S's ECMPs to R1 includes N1.
> S's ECMPs to R1: [(S-N1-R1), (S-N2-R1)].
> How can we include R1 in the P(S,N1),
> given the P-space definition?
>
> My current guess is that P-space definition needs additional
> explanation on the ECMP part.
> My guess for the correct definition is:
>         A router (say 'U') can be included in the P(R,X)
>         as long as the R can exclude all the nexthops
>         possibly transiting through X.
>
> I think we are implicitly assuming that S can eliminate sending
> through N1 to R1 by itself, and so the R1 can be include in P(S,N1)
> in Section 6.
>
> As a search for other problematic example,
> we can manipulate(generate artificially)
> the topology such that S's ECMPs to R1 consist of:
> S-X-A-R1
> S-B-R1
> S-C-X-R1
> S-D-E-R1
> S-D-X-R1
>
> In this case, R1 can be included only if S can eliminate the
> X, C, D from the nexthops to R1.
> S-X-A-R1 (NG, easily avoidable)
> S-B-R1 (OK)
> S-C-X-R1 (NG, avoidable after path calculation)
> S-D-E-R1 (NG, hard to avoid unless we compute ECMP from D to R1)
> S-D-X-R1 (NG, hard to avoid unless we compute ECMP from D to R1)
>
> The current definition seems to worry about inclusion of D nexthop case,
> and contradicts with the raised example which includes B nexthop case.
>
> By the way, I think Q-space definition is correct as is
> in the current version.
>
> Best regards,
> Yasu
>
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