NCP vs RCP - Opportunities and Tradeoffs

[Rob Crouthamel]

I'd like to better understand the practical pros/cons of NCP vs. RCP Co-processor designs in the context of an OpenThread Border Router (OTBR).  Please chime in if you have anything to add or correct.

Background:

• Basic definitions of NCP and RCP are here: https://openthread.io/platforms/co-processor
• In an NCP configuration, the OpenThread stack is running on the Radio Controller (e.g., nRF52840 or '833)
• An expanded explanation of RCP is here: https://openthread.io/platforms#radio-co-processor-rcp
• The RCP configuration requires a very simple radio control layer on the Radio Controller (e.g., nRF52840, '833, or '811)
• In order to use an RCP, therefore, the Border Router must be running a Thread stack -- this is where the OpenThread Posix core comes in.  See https://github.com/openthread/openthread/tree/master/src/posix

Benefits of NCP:

• Host processor can sleep to save power, leaving the Radio Controller to keep the network going. Not really applicable for a Border Router.
• More responsive than RCP for local Thread network traffic (packets don't need to be forwarded to the Linux host for processing just to be sent back out on the Thread interface)
• Supported by the OTBR Docker Image (easier to get started with development)

Benefits of RCP:

• Can operate on a lower cost Thread Radio IC (e.g., Nordic nRF52811)
• The Border Router host (Linux) can access the full Thread CLI functionality, not just commands implemented by wpantund.  It can do this without disturbing wpantund.  This is helpful for troubleshooting, even remotely.
• Updates to the OpenThread stack can be applied to the Border Router without re-flashing the NCP firmware (assumes RCP implementation is complete/stable).

Are there other reasons you've selected RCP or NCP for your project?  Please share.

Thanks,
Rob

[Jonathan Hui]

Another potential benefit of running OpenThread on host (i.e. RCP) is easier debugging (e.g. host debugging tools, more resources, etc.).

Another potential benefit of NCP model is isolation from host instabilities.

Note that we could also have a docker image that supports RCP as well.

--

Jonathan Hui

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