Question for TI-LFA

[Yasuhiro Ohara <[e-mail]>]

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