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Envoy, contains all the "hard" tech (the data plane) while creating a "service mesh", basically comes down to creating a management layer on top of it.I'd argue this is backwards. Envoy has a fairly tightly defined boundary with relatively strong guarantees of consistency given by hardware -- each instance is running on a single machine, or in a single pod, with a focus on that machine or pod.
The control plane is dealing with the nightmare of good ol' fashioned distributed consistency, with a dollop of "update the kernel's routing tables quickly but not too quickly" to go with it. It's "simple" insofar as you don't need to be good at lower-level memory efficiency and knowing shortcuts that particular CPUs give you. But that's detail complexity. The control plane faces dynamic complexity.