Severe packet drop on Border Router with certain CoAP packet sizes

[Nopik]

Hello,

I asked about this on openthread-users (https://groups.google.com/forum/#!topic/openthread-users/F8_NmMRDOFE) and got redirected here. Generally, I observed a severe packet drop on certain CoAP packet sizes, when using Border Router. I.e. when I was sending packet with about 90 bytes of payload, it was working fine (~99% packet delivery). But with about 100 bytes of payload, not even half of the packets were delivered successfully. If I increased the payload size by another 10 bytes, it started to be reliable again. In my particular case the window was between 94 and 102 bytes (might be dependent on stuff like url length) - any payload of that size was dropped a lot. This was repeated every 127 bytes, so, ~220 bytes payload had the same window of few bytes which big drops were observed.

The scenario was: regular nRF52840 ot-ftd-cli (0.11 Thread SDK) was sending CoAP over radio to NCP (ot-ftd-ncp), which was in turn handled on the linux side of the border router. Node-node communication had not triggerred this problem.

For more details please refer to the openthread-users.

Any suggestions / insights?

[Robert Lubos]

Hi,

I've been trying to reproduce the issue with current OpenThread master, but it either does not occur anymore or I wasn't able to trigger it.

My setup was RPi running current OpenThread border router on top of Raspbian, NCP connected through USB CDC-ACM and CLI application, both on nRF52840. On my RPi I've created CoAP server with our python CoAP implementation (https://github.com/NordicSemiconductor/piccata), CLI app was slightly modified to enable to send series of CoAP requests. I've sent mutliple series with different payload sizes, mostly near the size that caused packet fragmentation. Whatever payload size I've used, I had always got nearly 100% packet delivery.

In your description I was missing a single detail - have you been using Border Router shipped with our recent SDK (0.11.0) or OpenThread's Border Router? What was the CoAP server that you have used on RPi?

Anyway, we are going to release updated SDK with up-to-date libraries in the near future, so hopefully you could verify if the issue persists on your setup.

Best Regards,
Robert