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## Re: [Fwd: [Fwd: I-D ACTION:draft-ietf-rtgwg-microloop-analysis-00.txt]]

 Subject: Re: [Fwd: [Fwd: I-D ACTION:draft-ietf-rtgwg-microloop-analysis-00.txt]] Alia Atlas Thu, 28 Jul 2005 11:06:55 -0700
 ```On 7/27/05, Russ White <[email protected]> wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > > > > a) shortest paths > > b) downstream paths: the next-hop's distance to D is less than S's > > distance to D. Shortest paths are a subset of these. > > c) loop-free paths: the next-hop's shortest path(s) to D do not go > > through S. Downstream paths are a subset of these. > > d) looping paths: the set of next-hops with at least one shortest > > path that does go through S. > > Agreed. All of C and all of D fall into Dopt(S,D) > Dopt(N,D), based on > your definition of B. You can't tell which is which by relying on the > metrics. See my example in my other email.... > > > Perhaps I should have said that the right side is the cost of the > > shortest possible "looping" path that would go via S. > > Hmmm.... I think what you're saying here is that as long as a neighbor's > cost is less than my cost, it must be loop free, since any additional > cost, even 1 more, could be a loop. In fact, if a neighbor has an equal > cost, it could be a loop, in theory, in the case of equal cost multipaths. > > > The question is whether the shortest path from N to D goes via S. > > The shortest distance that such a path can be is D_opt(N,S) + D_opt(S, > > D). If D_opt(N, D) is less than this, then the path from N to D > > cannot go through S. > > Okay, so what you're trying to actually figure out is whether the path > from N goes through me, S. So, what you're actually trying to split is > the second case, not the third, from what it looks like. Going back to > my three cases: No, I'm trying to split the third case. The case of downstream neighbors where D_opt(N,D) < D_opt(S,D) isn't the issue here. Those are loop-free by virtue of a each hop along a shortest path being monotonically decreasing. > 1. This is the lowest cost path. This is loop free. > 2. My neighbor's cost is less than my cost. This is loop free. > 3. My neighbor's cost is more than my cost. This may, or may not, be > loop free, but I can't tell from the metrics. > > What you're actually trying to determine seems to be whether or not N > will, or is, using you as its next hop, in case 2. To do this, you have > to add your cost to N, and see if that's lower or higher than your cost > to D. If it is, then N must be using the alternate path to D. If it's > not, then N must be using you as it's alternate path. I'm trying to determine this for case 3. The question is how will N decide to forward a packet to D. Is it via S or not? N will use the shortest path - so if N->S + S->D is shorter than any other known path from N->D, N will use that. > Is that right? If so, you're not messing with the third case, you're > breaking the second case down into it's two options. > > >>Huh? Again, there are three options for a path, based on the metric: > >> > >>- -- The path is your best path. This is known to be loop free. > >>- -- Your neighbor's cost is less than your total cost. This is known to > >>be loop free, but not the "best." > >>- -- Your neighbor's cost is more than your total cost. You don't know if > >>this path is loop free or not, and there's no way to tell from the metrics. > > > > > > And, again, the third one case CAN and IS broken into 2 groups: > > loop-free & looping. Why do you believe that this can't be told from > > the metrics??? > > See my other example. You can't know the difference between the two. > > > Why do you think a router S can't tell if a neighbor N's path is > > loop-free to a particular destination? > > > > Are you making an assumption that S has only run a single SPF from its > > own perspective? Why do you think this is unknowable?!?! > > Because of my other example, and my long experience with EIGRP, a > protocol that uses this specific property of paths to determine a loop > free feasible successor. :-) I could probably draw you a large number of > easy to complicated networks where a specific router, under the > conditions where Dopt(S,D) > Dopt(N,D), may or may not be a loop, and > the metrics look the same in all cases. Now, breaking the second case > down into it's two possibilities is useful in the link state case (it's > not in EIGRP's case, by the way, for various reasons), because you can't > tell the difference between the two without the second step of > determining if N must be using S as it's next hop or not. I have not thought about this in the context of EIGRP (for obvious reasons :-) but I am confused by why you are unwilling to consider the knowable information for N->S. Again, I am not concerned with the downstream neighbors case, which is, as I understand it, what EIGRP uses. Also, the question is not whether N is using S as its next-hop, but whether S is in the path from N to D. > I do think the draft's wording is a little confusing--I've read it three > or four times, and couldn't make out what it was trying to say in this area. Did you have a chance to loop at the framework & LFA drafts? Clearly, from this conversation, the PLSN draft needs some clarifications & rewording. :-) Alia _______________________________________________ Rtgwg mailing list [email protected] https://www1.ietf.org/mailman/listinfo/rtgwg ```
 Current Thread [Fwd: [Fwd: I-D ACTION:draft-ietf-rtgwg-microloop-analysis-00.txt]], Russ White Message not available Message not available Message not available Message not available Re: [Fwd: [Fwd: I-D ACTION:draft-ietf-rtgwg-microloop-analysis-00.txt]], Russ White Re: [Fwd: [Fwd: I-D ACTION:draft-ietf-rtgwg-microloop-analysis-00.txt]], Alia Atlas Re: [Fwd: [Fwd: I-D ACTION:draft-ietf-rtgwg-microloop-analysis-00.txt]], Russ White Re: [Fwd: [Fwd: I-D ACTION:draft-ietf-rtgwg-microloop-analysis-00.txt]], Alia Atlas <= Message not available Message not available Message not available