I'm not clear on why coroutines or an explicit state machine are necessary to implement network protocols. We have been doing it for decades by writing straigh

I'm not clear on why coroutines or an explicit state machine are necessary to implement network protocols. We have been doing it for decades by writing straightforward imperative C code and for years by writing C++ template classes that are templated on the downstream type (CRTP). Both of these approaches result in near-zero overhead (computers are good at fetching and decoding consecutive instructions) or even negative apparent overhead (if the compiler sees caller and callee, inlining and dead code elimination can potentially eliminate lower-level protocol handling irrelevant to the higher-level protocol). I would guess that transforming coroutine-based code into state machines hampers performance over the long term by scaring the programmer away from wanting to touch it. However, I would also guess that the direct cost of a coroutine context switch (save and restore all callee-save registers and stack pointer then indirect-jump somewhere unpredictable) outweighs the direct cost of a state machine transition (update some variable). I do not want to pass judgment on the approach beyond saying that its attendant overhead is too high for some workloads.