Thank you. I would prefer to add some texts, because I don't understand yet if (and how) the use of the entire RFC7490 definition will resolve the example case (I might need some more time to understand).
Best regards, Yasu 2024年3月29日(金) 3:35 Stewart Bryant <[email protected]>: > > An interesting point. > > The RFC 7490 definition of P-space is > > > P-space: > The P-space of a router with respect to a protected link is the > set of routers reachable from that specific router using the pre- > convergence shortest paths without any of those paths (including > equal-cost path splits) transiting that protected link. > > For example, the P-space of S with respect to link S-E is the set > of routers that S can reach without using the protected link S-E. > > > > > > > For safety the ECMP exclusion needs to apply to extended P-space as well. In > both cases unless the ECMP behaviour of all nodes on the path is examined and > the packet is specifically steered the safe way (something that cannot be > done in the general case but could be done in Ti-LFA) it is unsafe to assume > that ECMP will not cause a loop back to the PLR. > > I therefore think that either the RFC7490 definition needs to be used, or > there needs to be specific text included that provides guidance on the need > to steer the repaired packet safely though any node that the repaired packet > will be routed through that may ECMP via the failure. > > Best regards > > Stewart > > > > Begin forwarded message: > > From: Yasuhiro Ohara <[email protected]> > Subject: Question for TI-LFA > Date: 24 March 2024 at 05:54:33 GMT > To: [email protected] > Cc: Yasuhiro Ohara <[email protected]> > > 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
