Receiving MTU errors when trying to conduct OTA DFU

RE: Receiving MTU errors when trying to conduct OTA DFU

Vidar Berg — Fri, 27 Aug 2021 18:44:27 GMT

[quote user="tjhasan"]But how do I tell the GATT server that the first 20 bytes is NOT the end of the data I want to write?[/quote]

You don't have to, the packets will be re-assembled by DFU target as they are received. And the bootloader knows how many bytes/packets to expect because of the DFU protocol (DFU Protocol: BLE DFU Profile).

RE: Receiving MTU errors when trying to conduct OTA DFU

tjhasan — Fri, 27 Aug 2021 17:43:26 GMT

I see. I don't really have access to the source code for the nRF Connect Mobile app so I can't comment on why I didn't receive the error on my phone. I did indeed use the same packet length.

I guess my follow-up question would be how would I send write commands 20 bytes at a time? For example, if I had a 40 byte packet I wanted to send, I would have to send it as 2 separate 20 byte write commands. But how do I tell the GATT server that the first 20 bytes is NOT the end of the data I want to write? How does the GATT server know how much data to expect from a given write command?

RE: Receiving MTU errors when trying to conduct OTA DFU

Vidar Berg — Fri, 27 Aug 2021 13:11:18 GMT

Hello,

Legacy DFU did not have support for long ATT MTUs, so the payload was effectively limited to 20 bytes per write (23 byte MTU - 3 Byte header). In other words, the PC client app will have to split the data into 20 bytes packets before sending.

I'm not sure why you didn't see error when you sent the DFU packet from your phone though. Did you use the same packet length? If so, I guess the phone must have split the packet across multiple on-air BLE packets (i.e. performed what's known as a long write - GATTC Characteristic or Descriptor Value Long Write). But that is an inefficient way to transfer data and is something you should avoid if you care about throughput performance. It's faster to send GATT write commands 20 bytes at a time.

Best regards,

Vidar