RE: Should an acknowledged message that replies a lot of data be broadcasted into the mesh?

user91456 — Thu, 12 May 2022 14:32:35 GMT

Hi Hung Bui,

Thank you very much for the link and the summary!

RE: Should an acknowledged message that replies a lot of data be broadcasted into the mesh?

Hung Bui — Thu, 12 May 2022 13:40:50 GMT

Hi BlueMike,

We had a related discussion here that might be useful for you: https://devzone.nordicsemi.com/f/nordic-q-a/87396/nrf-mesh-sending-hundreds-of-bytes-packets-to-the-mesh-nodes-causes-nrf_mesh_evt_sar_failed

The take out from the discussion is that:

- When sending SAR message to a unicast address, the segmented messages are ACKed and retransmitted if needed on lower transport layer. When sending SAR message to a group address they are not ACKed and no retransmission happens. This is requires multiple re-tranmission on the upper transport layer to compensate but still it's not the best way of delivering reliable message.

- When sending SAR message from a phone, at least in our implementation on the Android app, there is no retransmission on upper transport layer. So you may need to do that in the application.

- So what we would suggest is to try to keep the message under 11 bytes, to avoid SAR. If the data is above 11 bytes, you can think of doing the segmentation on the application. Try to divide the data into chunks and can send and verify all nodes have acknowledged the chunks, if not you can resend the data. Then continue with the next chunk.