Hi Levente, Thanks a lot for the detailed review and comments. I will try to address them inline.
Best Regards, -Pushpasis From: Levente Csikor <[email protected]<mailto:[email protected]>> Date: Tuesday, October 27, 2015 at 9:26 PM To: "[email protected]<mailto:[email protected]>" <[email protected]<mailto:[email protected]>>, Pushpasis Sarkar <[email protected]<mailto:[email protected]>>, "[email protected]<mailto:[email protected]>" <[email protected]<mailto:[email protected]>> Subject: Re: FW: New Version Notification for draft-ietf-rtgwg-rlfa-node-protection-04.txt Dear Pushpasis, I've read the new version of the draft, and I find some ambiguous statements/terms and mistakes. Sorry that I did not read it before the 4th revision. 1)Terms: To be consistent, in Section 2. first-hop should be named/termed as next-hop, since in each latter case it is termed as next-hop (or nexthop). And later the term primary node also rises the question whether it means next-hop, or something else. [Pushpasis] They all mean the same I.e. Primary next-hop(s) except for the cases which talks about the backup/alternate next-hops. Alternate/backup next hops maybe immediate neighbor (in case of standard RFC5286 LFA) or remote nodes (in case of Remote LFA). I will modify the text to be consistent in next version. 2)Ambiguous things: On page 4, the Topology 2 introduced in Fig.2., is a bit ambiguous, since we are talking about (remote) loop-free alternates, but some cells in Table 2's Remote-LFA Back Path column contain loops, namely S=>N=>E=>R3->E->D1,S=>N=>E=>R3->E. In my opinion, this case can be understood if I take a look on the topology over and over again (to find and indentify remote LFAs), but according to RFC 7490, shouldn't we rely on simple LFA (node N in the topology) in such cases when they are available? [Pushpasis] Yes RFC 7490 says that LFA backup paths should be more preferrable than RLFA bcakup paths. However this table talks about possible RLFA paths via the PQ-nodes to depict cases where while the backup paths to certain PQ-nodes computed provides node-protection it does not for others. We spent a lot of time to come up with example to depict the same, but could not come up with a better example to depict that. Probably you can help us with a better topology if you have in mind :) Probably a better and unambiguous example topology would be better, or it should be stated that in such case node N could be a pure link-protecting LFA. 3)Mistake: Section 2.2.1 on page 6 states the following about link protecting extended P-space: "A node Y is in link-protecting extended P-space w.r.t to the link (S-E) being protected, if and only if, there exists at least one direct neighbor of S, Ni, other than primary nexthop E, that satisfies the following condition. D_opt(Ni,Y) < D_opt(Ni,S) + D_opt(S,Y)" The document also states that this inequality is already defined in RFC7490. However, this inequality seems to be wrong, or it is not properly prepared. To me, in essence, this inequality states (assuming that Y is the neighbor of Ni) is that my neighbor's (Ni) neighbor (Y) is closer to my neighbor (Ni) than me (S), which is almost every time true, but what if my neighbor's (Ni) neighbor (Y) is my (S) neighbor as well? Consider the topology below, where 'x' denotes only the failure on link S-E, while all the links are of unit cost except the link Ni-Y, where the cost is 2: Y 2/ \ Ni--S--x--E | / B D \ / \ / \ / A In this case, Y does not fulfill the inequality stated above, however, it's in extended P-space, moreover, it's in P-space as well, and as far as I remember, ext.P-space always consists of the (smaller) P-space. I think the main problem here is that there is no cross-reference to the failed link itself (as it is in node-protecting P-space inequality). Therefore, this could be resolved by the following inequality: D_opt(Ni,Y) < D_opt(Ni,S) + D_opt(S,E) + D_opt(E, Y) Or, the statement should emphasize in the beginning that node Y is not in the P-space of S w.r.t. the failed link S-E. Pls tell me if I'm wrong. Btw, the inequality for node-protecting extended P-space is valid. [Pushpasis] I think you might have misunderstood it.. Ni is not a single neighbor of S.. The term Ni in the above inequality stands for all neighbors of S other than E (primary next hop).. Here is the text once more.. "A node Y is in link-protecting extended P-space w.r.t to the link (S-E) being protected, if and only if, there exists at least one direct neighbor of S, Ni, other than primary nexthop E, that satisfies the following condition. D_opt(Ni,Y) < D_opt(Ni,S) + D_opt(S,Y)" So one possible value for Ni can be Y itself.. So now if you substitute Y for Ni in the above inequality it satisfies the inequality.. D_opt(Y, Y) < D_opt(Y,S) + D_opt(S,Y) 0 1 1 However to make it more clear let me rename the Nodes Ni and Y in the above diagram as N1 and N2. So the diagram now looks like.. N2 2/ \ N1--S--x--E | / B D \ / \ / \ / A Ni = {N1, N2} Now with N2 as the candidate, substituting Ni = N1 D_opt(N1, N2) < D_opt(N1,S) + D_opt(S,N2) is not satisfied 2 1 1 However substituting Ni = N2 D_opt(N2, N2) < D_opt(N2,S) + D_opt(S,N2) is indeed satisfied 0 1 1 So N2 is in link-protecting Ext-P-Space of S, wrt S-E link. Essentially for Y to be in link-protected Extended P-Space(S, S-E) S (or one of the neighbors of S other than E) should be able to reach Y without taversing the S-E link.. The above inequality is satisfied by substituting Ni as Y. 4) Section 2.2.3 "The Remote-LFA [RFC7490] draft already defines this. The Q-space for a link S-E being protected is the set of routers that can reach primary node E,..." In this case, the term primary node is again equivocal, since if it means next-hop, then the definition is wrong. RFC7490 defines Q-spaces for the routers/nodes, however, if we talk about to find a remote LFA for a given source-destination pair w.r.t. a failed element, then (ext.) P-space of the source, and the Q-space of the destination should be evaluated/calculated (w.r.t. the failed element). Later, in section 2.2.5.: "A node Y is in candidate node-protecting PQ space w.r.t to the node(E) being protected, if and only if, Y is present in both node-protecting extended P-space and the Q-space for the link being protected." To me, the term "..Q-space for the link being protected" is again not properly stated. [Pushpasis] Q-Space by definition [RFC 7490] is always wrt to primary next-hop link S-E being protected. There is no separate link-protecting or node-protecting Q-Space. Node-protecting P, Ext-P and PQ-space is wrt to both the primary next-hop link S-E and primary next-hop node E being protected. I will modify the text accordingly. 5) My final problem probably remains my problem :), but from section 2.2.5, there are a lot of reference to the term PQ-space, or PQ-node, for instance, in section 2.3.1: "As mentioned in Section 2.2.2, to consider a PQ-node as candidate node-protecting PQ-node, there must be at least one direct neighbor Ni of S, such that all shortest paths from Ni to the PQ-node does not traverse primary nexthop node E." or "To determine if a given candidate node-protecting PQ-node provides node-protecting alternate for a given destination, the primary nexthop node should not be on any of the shortest paths from the PQ-node to the given destination." and it occurs in many sentences. Basically what a bit confusing here is that according to RFC7490, if a set of routers is termed PQ-nodes, or even PQ-space, then they already fulfill the inequalities for (ext.) P-space and Q-space. So, in the above-mentioned sentences, it's a bit confusing why are we checking Q-space inequality if it's already a PQ-node. Probably better placements of the word "candidate" could resolve this issue, or we should rely on "node in P-space" or "node in Q-space" instead of PQ-node candidate. On the other hand, if we talk about a PQ-node candidate, then it means (at least to me) that the node fulfills at least one of the inequalities, i.e., it is already in the Q-space or the (ext.) P-space, and therefore we say it's a candidate if it fulfills the remaining inequality as well. [Pushpasis] The term 'PQ-node' here means RFC 7490 computed link-protecting PQ-node. However the inequality being applied here is for a link-protecting PQ-node that is also considered as a candidate for being a node-protecting PQ-node. Once again I will try modifying the text to remove any such confusion. 6) Typo on page 10 "As seen in the above example above" -> As seen in the example above [Pushpasis] Will correct it. Thanks, and please don't consider my observations offending, I'm just try to understand the whole concept of remote LFAs and try to help and improve the draft itself. [Pushpasis] Not all. Once again thank you very much for the detailed review. Best regards, Levente On 10/14/2015 11:51 AM, Pushpasis Sarkar wrote: Hi Mike, I have addressed all the comments I have received so far. Here is the updated version of the draft. Thanks -Pushpasis On 10/14/15, 3:19 PM, "[email protected]"<mailto:[email protected]> <[email protected]><mailto:[email protected]> wrote: A new version of I-D, draft-ietf-rtgwg-rlfa-node-protection-04.txt has been successfully submitted by Pushpasis Sarkar and posted to the IETF repository. Name: draft-ietf-rtgwg-rlfa-node-protection Revision: 04 Title: Remote-LFA Node Protection and Manageability Document date: 2015-10-14 Group: rtgwg Pages: 16 URL: https://www.ietf.org/internet-drafts/draft-ietf-rtgwg-rlfa-node-protection-04.txt Status: https://datatracker.ietf.org/doc/draft-ietf-rtgwg-rlfa-node-protection/ Htmlized: https://tools.ietf.org/html/draft-ietf-rtgwg-rlfa-node-protection-04 Diff: https://www.ietf.org/rfcdiff?url2=draft-ietf-rtgwg-rlfa-node-protection-04 Abstract: The loop-free alternates computed following the current Remote-LFA [RFC7490] specification guarantees only link-protection. The resulting Remote-LFA nexthops (also called PQ-nodes), may not guarantee node-protection for all destinations being protected by it. This document describes procedures for determining if a given PQ-node provides node-protection for a specific destination or not. The document also shows how the same procedure can be utilised for collection of complete characteristics for alternate paths. Knowledge about the characteristics of all alternate path is precursory to apply operator defined policy for eliminating paths not fitting constraints. Please note that it may take a couple of minutes from the time of submission until the htmlized version and diff are available at tools.ietf.org. The IETF Secretariat _______________________________________________ rtgwg mailing list [email protected]<mailto:[email protected]>https://www.ietf.org/mailman/listinfo/rtgwg
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