When this technology was originally introduced to the IETF in 2015 the key selling point, and the key point of innovation, was that in order to prevent the formation of micro-loops it was necessary for the repair path to exactly follow the post convergence path.
All the simulations in Appendix B were, as I recall made on that assumption. As work progressed, segment routing was usefully introduced to avoid the need for directed LDP sessions or preconfigured tunnels. It now seems that the author have quietly backtracked on the need for the congruence between the repair path and the post convergence path. If there is no need for this path congruence then the draft can succinctly be described as “repair according to RFC7490, but take advantage of segment routing to build stateless repair paths”. That would be a very simple and short draft to write. If partial path congruence is deployed then there needs to be some detailed discussion about the consequences and mitigations of this, because any micro loops that form will have an impact on both the traffic micro looping and any traffic that is innocently using those paths. I am forming the view that the draft needs to be returned to the working group for further consideration with a number of possible approaches. One approach is to split the draft into a draft that explains how to use SR to support RFC7490 operation, and another draft that explains the application of path selection that follows the path convergence path, including text on operation in a network that only partially includes these constraints. That way the two aspects of the problem are independently approached and the costs and constraints of (partial) path congruence are bought clearly to the fore. Another approach is to make it much clearer that congruence between the post convergence path and the repair path is an optional extension to a segment routing repair based on the methods described in RFC7490. There then needs to be further text as indicated above making it clear to the user what the consequences of partial congruence is so that they can make an informed choice of the consequence of accepting this limitation. The draft is with the IESG and it is their decision, but having reflected at length on this, I have the view that moving from an absolute requirement to follow the post convergence path to making this an optional requirement is a game changer compared to the original proposition, and as such it needs considered joint work by the authors and the other experts in RTGWG in the context the RTGWG. Best Regards Stewart _______________________________________________ rtgwg mailing list -- rtgwg@ietf.org To unsubscribe send an email to rtgwg-le...@ietf.org
