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For questions about component-level electronic circuits, tools and equipment.
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You could give PJON a try https://github.com/gioblu/PJON
This seems more than interesting. Will check it out in detail, since I haven't heard about it until now. Thanks for the suggestion!
MQTT?
MQTT isn't peer-to-peer, it requires a centralized broker that all devices connect to.
If OP were looking at network protocols, CoAP would be the right option, but it looks like they don't even want to bother with IP.
CAN BUS is relatively easy to implement and ticks your boxes
Thank you for your suggestion. Is it really that easy to implement? So far the people I talked with irl told me otherwise, but I will look into it and judge myself whether I am up to the task.
Exactly, hard/easy depends on your background. It's been almost 20 years since I worked with it, I'm sure there are ready made libraries or chips implementing 99% of the protocol these days.
Sounds like you want ethernet.
Hmmm... It does tick a lot of the boxes, but the ethernet protocol is way too complex with all of its layers and not reasonably implementable on a low-power microcontroller. Also it requires separate hubs for connecting multiple devices together unlike i2c, which is daisy chainable.
A lot of microcontrollers, ESP32 or STM32F4 to give two examples, have the PHY built in.
But if that's too much for your taste, then CAN bus might be what you're looking for.