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) <[email protected]>: > > 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 <[email protected]> namens Yasuhiro Ohara <[email protected]> > Verzonden: zondag 24 maart 2024 6:54 > Aan: [email protected] <[email protected]> > CC: Yasuhiro Ohara <[email protected]> > Onderwerp: Question for TI-LFA > > [Some people who received this message don't often get email from > [email protected]. Learn why this is important at > https://aka.ms/LearnAboutSenderIdentification ] > > CAUTION: This is an external email. Please be very careful when clicking > links or opening attachments. See the URL nok.it/ext for additional > information. > > > > 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 > > _______________________________________________ > rtgwg mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/rtgwg _______________________________________________ rtgwg mailing list [email protected] https://www.ietf.org/mailman/listinfo/rtgwg
