Hi Jan, Thanks for you your answer. Reading more I have found more evidence that the “digital network twin” should be single/unified for a network of any size: - I did mention below OpenConfig that had merged configuration and operational data models many years ago, - RFC8342 is asking for the same☺, - RFC8969 is going a step further and asking to have the cross-layer convergence to the unified YANG model. It looks obvious – all relationships are better to have in the single consistent YANG data storage. It is slowly happening in the IETF (examples above).
Yet RFC9182 has many disclaimers like: “the L3NM is not defined as an augment to the L3SM, because a specific structure is required to meet network-oriented L3 needs” “initial design was interpreted as if the deployment of the L3NM depends on the L3SM, while this is not the case”. I see a couple of reasons for it: - to minimize the disruption for many available implementations, - to trade off unification and interoperability for functionality and flexibility. After L3NM was disconnected from L3SM, many mapping between these models should be done in a proprietary way by coders. Imagine that one product could map an event “A” in the L3NM to be related to the configuration XpathA in the L3SM, But the different product would map an event “A” to be related to the configuration XpathB in the L3SM. Then they would act inconsistently. The other example could be when provisioning something in L3SM would be mapped to different Xpath in L3NM by different products. It is again a problem if L3NMs from different vendors should support the same VPN. If humans (with really the best knowledge and intelligence in respective IETF WGs) were not capable of automatically mapping L3SM to L3NM, Then no hope that the algorithm developed by coders could. It would for sure break the “closed loop control” cross-vendor. In reality, it would create challenges even for a single-vendor solution because coders developing it may be not strong enough. I suspect that this problem has to be revisited after some additional years of not satisfactory automation progress. Not many would agree to single-vendor where it is not a critical roadblock. I agree that for the single vendor environment just L2NM and completely disconnected L2SM are already big progress (teaching people how to develop big systems, a sort of educational value). It is much better than no recommendations at all. When the design started top-down from L3SM – it was right. But when automatic mapping between L2NM and L3SM was lost – it was a mistake. It did break top-down design that is very needed here. Eduard From: Jan Lindblad [mailto:[email protected]] Sent: Wednesday, December 7, 2022 8:35 PM To: Vasilenko Eduard <[email protected]> Cc: [email protected]; [email protected]; Paolo Volpato <[email protected]>; Xipengxiao <[email protected]> Subject: Re: [netmod] How many "digital twins" every single network should have? Who would map between "twins"? Hi Eduard, Hi Automation Gurus, YANG modules may be treated like a “digital twin” of the network with different resolution/accuracy (depending on Module details). It looks like RFC 8969 is discussing that different YANG models (for different layers or functions) of the same network should be the clarification of the same “digital twin”. Below are some excerpts from RFC 8969 that make me believe in the common Data Model after all YANG modules clarification for the same network. But comparing RFC 8299 (L3SM) with RFC 9182 (L3NM) I conclude that “Data Models” are different (could not be automatically mapped). Yet they should describe/represent the same network. That is right. There are multiple attempts at modeling the use cases at each level of the management stack. This is not unlike how standards develop in other areas. Initially, and sometimes even after a long time, there are often competing standards. Sometimes even from within the same SDO. It is evident in this situation that a big job for the vendor is needed to *map* Data Model of L3SM to the Data Model of L3NM. I think you should be careful with the word "vendor", here as we're talking about an entire vendor eco-system. It is not typical that a router product would contain this mapping, but you are right that an NMS or OSS product from some vendor might. The mapping from network use cases to network device configuration is happening widely today, and a fair portion of all that is using YANG in some way. It is not just a cost/time, additionally, it is a big source of interoperability issues. Engineers from different vendors would never map it in the same way. I could pose similar examples for the other RFCs (like L2SM and L2NM, and many more). Of course. Just like two router vendors would not implement a given IETF routing YANG model the same way, NMS/OSS vendors and any service providers that choose to do this on their own, will have the same freedom of implementation at their level. This freedom does not remove the value of standardized service YANG models in any way. Why is IETF not following RFC 8969? It looks pretty evident. Why “Data Models” for the same network are not automatically mapped?!? How could they be automatically mapped? Such mappings necessarily depend on use case, network circumstances and operator traditions/preferences, so I can't see any one-size-fits-all mapping here. Sure, you can make one mapping and declare it the one and only. But others may not agree and prefer to go with their own mapping. It was logical to define initially top-level approximation for the network (the service model is probably the loosest one), Then extend Data Model (augment in RFC 7950 terminology) to the network model and so on (continue to clarify more details). As it is rightfully stated in RFC 8969: only a top-down approach permits resolving the challenge of “closed loop control”. I would add “in the multivendor environment”. If I understand right (not sure): it was the primary idea of OpenConfig to have the common Data Model for Configuration and Assurance at every layer (the unified “Digital twin” for the network). The value of hundreds of already developed YANG modules looks questionable because vendor mapping by different vendors between functional and layered YANG modules could produce m*n^2 permutations. It may not permit interoperability in the multi-vendor environment. We certainly experience the concrete value of the many thousands of device level YANG modules out there when implementing NMS/OSS type of functionality. Anyone in that business should come prepared to navigate combinatorial explosions, but I can't say I have seen any traces of the specific m*n^2 permutations you speak of above, relating to combinations of device level YANGs and service level YANGs. I could imagine some reasons why it may not be possible in some cases but the general rule should be to always use “augment” of the parent YANG model. I'm afraid I can't decipher this statement. Feel free to elaborate. Best Regards, /jan
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