Hi Benoit, I have seen your presentations on Service Assurance for Intent-Based Networking Architecture and read your drafts with interest (draft-claise-opsawg-service-assurance-yang-05 and draft-claise-opsawg-service-assurance-architecture-03). Interesting stuff on which I do have a couple of comments.
The basis for the drafts is in essence a proposal for Model-Based Reasoning, in which you capture dependencies between objects and make inferences by traversing the corresponding graph. MBR based on dependency graphs allows to reason about the impact and propagation of the status or health of one object on the status or health of dependent objects "downstream" from it. Likewise, traversing the same graph in the opposite direction (from the "downstream" or dependent objects) allows to identify potential root causes for symptoms observed by those objects, although this seems to be not so much your focus. While MBR as a concept makes sense and has a long tradition in network management, there are also a number of considerable issues with it, and I was wondering about your perspective and mitigation strategies for these. For one, their effectiveness depends on the model being "complete". In most cases, there are myriads of interdependencies which are difficult to capture comprehensively. The model is still useful for many applications as a starting point, but rarely captures the full reality. As long as users are clear about that, this is not an issue. However, the one thing where I have a bit of concern in your model is that you use it to draw conclusions about the health of the dependent objects (for example, your end-to-end service). It seems that a derived health score will be no substitute for monitoring the actual health, and should not lull users into a false sense of security that as long as they monitor components of a system or service, that they don't need to be concerned with monitoring the system or service as a whole.. In reality I believe the value (although there still is a value) is more limited than that. I believe that this should be clearly acknowledged and discussed in the drafts. A second set of issues concerns the intensity of maintaining the graph and of continuously updating the dependencies. In a realistic system you will have many objects with even more interdependencies. Maintaining derived health state can become computationally very expensive, which suggests a number of mitigation strategies: for one, don't continuously maintain this but compute this only "on demand". Second, perhaps don't maintain this on the server at all, at least to the extent that you expect the server to be a networking device. It seems much more feasible to perform these type of Model-Based Reasoning computations in an Operations Support System or application outside the network, not within the network. However, it is not clear that YANG models and Netconf/Restconf would be applied there. It seems to me the drafts should add clarification on where those models would be expected to be deployed and how/would keep them updated. As an OSS tool, your proposal makes sense, but trying to process this on networking devices strikes me as very heavy, in particular given the limitations as per the earlier point. So, IMHO I think you may want to consider adding an according section that discusses these aspects in the draft, specifically the architecture draft. Cheers --- Alex
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