Thread Communication Maximum Packet Size

[Harikesh]

Hi, I'm currently playing around with the nRF5340 dev boards, in order to see if I can implement thread communication. I'm trying to test the limits of the thread protocol, and I had some confusion with the maximum packet size. I see the IEEE 802.15.4 specification says that the maximum frame should be 127 bytes, and that after that it will split the packets into fragments. When testing this on WireShark I noticed that the biggest packet I'm able to send before it fragments, is 141 bytes. There's an extra 14 bytes from the ethernet frame on top of the 802.15.4 frame, which means the maximum 802.15.4 frame size I can get is 125 bytes. Is there a limit set on the 802.15.4 frame in the nRF boards to 125 bytes, or am I missing something?

[Sigurd Hellesvik]

Hi,

In the 802.15.4 drivers, the max length is still 127 at least: https://github.com/nrfconnect/sdk-nrfxlib/blob/main/nrf_802154/serialization/include/host/nrf_802154_const.h#L161

he biggest packet I'm able to send before it fragments, is 141 bytes

Can you explain how you send the packet?

Regards,
Sigurd Hellesvik

[Harikesh]

I see, that's interesting. So the way I've tried sending is with both a Ping and a UDP send. I have two devices flashed with the OpenThread CLI application which are connected to my host computer (an nRF5340dk and an nRF52840 dongle). I capture all the packets with a sniffer using WireShark. I send the following ping command:

This gives the same packet information as the image I attached earlier. If I change the ping size from 79 to 80, it sends fragments the packet.

I also use this UDP command:

This also generates a packet send of 141 bytes (data of 125 Bytes), a larger udp send fragments the packet.

Is there a better way to send data, and see the maximum data packet size?

[Sigurd Hellesvik]

I think your approach seem reasonable.
Could of course be that ping or udp send limit this somehow, but I do not know yet.

Thought I should reproduce what you see, but I had some issues with installing the sniffer.

I will continue to try tomorrow, and return with more info tomorrow or early next week.

[Harikesh]

I see thank you! Please do keep me updated on what you receive on your end as well!

[Sigurd Hellesvik]

Got Wireshark set up now:

Using nRF Connect SDK v2.4.0.

nRF52840DK + nRF52833DK, both with openthread CLI sample.

I can do pings up to 96 before things break:

See my first message of 95 before fragmenting and then 96 with fragmenting

Looks like the Length max out at 125 for me.Since I send this over the air, I do not get the extra 14 bytes as you do, meaning I see the same behavior.

I will continue to look into this.

[Harikesh]

Ah I see, thank you so much! I'm glad to see the error is not an irregularity from my end. Something to note is that I also tried this exact method to send with a different brand of thread devices (Cascoda) and I experienced a similar ping/udp send limit at 123 bytes. Since the limits are different I'm not sure if the ping/udp is limiting the maximum packet but it might be a board specific limit?

[Sigurd Hellesvik]

I think this might be the same as explained in https://github.com/zephyrproject-rtos/zephyr/issues/4934.

Do you agree?

[Harikesh]

I see, I agree, I think that issue explains it. Looks like 127 is the maximum size, but the packet limit is set at 125 bytes because an automatic FCS of 2 bytes is generated in the hardware. Am I understanding that correctly?

[Sigurd Hellesvik]

That is how I understand it as well