Hi Stewart, Please see inline
Orange Restricted From: Stewart Bryant <[email protected]> Sent: Monday, October 16, 2023 2:08 PM To: [email protected]; rtgwg-chairs <[email protected]>; [email protected] Cc: Stewart Bryant <[email protected]> Subject: draft-ietf-rtgwg-segment-routing-ti-lfa : A simple pathological network fragment During the operations directorate early review of draft-ietf-rtgwg-segment-routing-ti-lfa Gyan Mishra points to a simple pathological network fragment that I think deserves wider discussion. https://datatracker.ietf.org/doc/review-ietf-rtgwg-segment-routing-ti-lfa-11-opsdir-early-mishra-2023-08-25/ I am not aware of any response to the RTGWG by the draft authors concerning the review comment and I cannot see obvious new text addressing this concern. The fragment is as follows CE1 -R1- R2-/-R3-CE2 | | R4 - R5 -R6 In the pre converged network R4 is ECMP CE2 via R5 (cost 4) and via R1 (cost also 4). We can easily build a TI-LFA repair path from R2 under link failure to CE2 (so long as we remember that R4 is an ECMP path to CE2), but the problem occurs during convergence. If R1 converges before R4, R4 may ECMP packets addressed to CE2 back to R1 in a micro loop. Meanwhile since no packets for R3 are reaching R2 the Ti-LFA repair is not doing anything useful. The Ti-LFA text leads the reader to conclude that it is a loop-free solution, but gives no guidance on how to determine when this assumption breaks down. There is an informational reference to draft-bashandy-rtgwg-segment-routing-uloop, but this short individual draft does little in the way of helping the reader determine when loop avoidance strategy needs to be deployed and the loop-free approach it describes does not seem to be fully developed. I am worried that proceeding with the Ti-LFA draft without noting that there is a real risk that simple network fragments can micoloop, and providing a fully formed mitigation strategy is a disservice to the operator community given the industry interest in Ti-LDA and the insidious nature of unexpected micro loop network transients, I am wondering what the view of the working group is on how to proceed. One approach would be for the Ti-LFA draft to incorporate detailed guidance on how to determine the risk of a micro loop in a specific operator network, and to provide specific mitigation advice. Another approach would be to reference a developed loop avoidance strategy and recommending its preemptive deployment. Another approach would be to make draft-bashandy-rtgwg-segment-routing-uloop a normative reference and tie the fate of the two drafts. Another approach would be to elaborate on the risks and their manifestations but declare it a currently unsolved problem. I am sure there are other options that the WG may formulate. What is the opinion of the working group on how we should proceed with draft-ietf-rtgwg-segment-routing-ti-lfa when considering the possible formation of micro loops? FRR takes place between the failure (detection) and the IGP reconvergence. Those are two consecutive steps that the WG has so far addressed with different solutions and documents. That's not new and that's not specific to TI-LFA. E.g., that's applicable to RLFA. Would the below text, taken verbatim from RFC 7490 (RLFA), work for you? Or would you say that the text is not good enough? "When the network reconverges, micro-loops [RFC5715<https://datatracker.ietf.org/doc/html/rfc5715>] can form due to transient inconsistencies in the forwarding tables of different routers. If it is determined that micro-loops are a significant issue in the deployment, then a suitable loop-free convergence method, such as one of those described in [RFC5715<https://datatracker.ietf.org/doc/html/rfc5715>], [RFC6976<https://datatracker.ietf.org/doc/html/rfc6976>], or [ULOOP-DELAY<https://datatracker.ietf.org/doc/html/rfc7490#ref-ULOOP-DELAY>], should be implemented." https://datatracker.ietf.org/doc/html/rfc7490#section-10 Of course, we could update the list of informative references. E.g., by adding another informative reference to draft-bashandy-rtgwg-segment-routing-uloop and by removing informative references to [RFC6976] and [ULOOP-DELAY] which are probably outdated. --Bruno - Stewart ____________________________________________________________________________________________________________ Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc pas etre diffuses, exploites ou copies sans autorisation. 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