CoAP Block-wise Transfer over OpenThread (tested via OT-CLI)

[gc0rreiab]

I’m trying to transmit a payload of up to 2,450 bytes using CoAP over OpenThread. Before jumping into firmware development, I want to evaluate the feasibility of transmitting large payloads using the OT-CLI sample.

As a starting point, let me clarify that my CoAP server is up and running. To obtain the synthesized IPv6 address from the OTBR, I run:

$ ot dns resolve4 <CoAP server IPv4>

1) Standard transmission (non-block-wise transmission)

When I transmit short payloads using:

$ ot coap put <synthesized IPv6 address> <uri> con {"test":123} 

I can see the data arriving at the CoAP server, and I also receive the corresponding ACK on my OpenThread device running the OT-CLI sample.

To evaluate the transmission of a payload of up to 2,450 bytes, I used the OT-CLI sample with the following command:

$ ot coap put <synthesized IPv6 address> <uri> con <payload> 

where <payload> is a 2,450-byte JSON payload. However, I noticed that the command input appears to be limited to approximately 415 bytes.

I then inspected the OT-CLI sample and found that there is a Kconfig parameter in the prj.conf file that limits the maximum length of shell commands. I increased this limit and rebuilt the sample:

CONFIG_SHELL_ARGC_MAX=26
CONFIG_SHELL_CMD_BUFF_SIZE=2600 

After this change, the entire command appears to be accepted, but I now get the following error:

Error 6: Parse 

What does this error mean? Is it actually possible to use the OT-CLI command with payloads of this size?

2) Block-wise transmission 

To further evaluate the feasibility of transmitting large payloads over CoAP with OpenThread, I looked into the block-wise transfer implementation, since this is the recommended approach for large payloads to avoid IPv6 fragmentation due to the small MTU size of IEEE 802.15.4. Since I need to send a payload of up to 2,450 bytes, using a block size of 64 bytes would require transmitting approximately 40 blocks.

To test this, I used the following command:

$ ot coap put <synthesized IPv6 address> <uri> block-64 40

From the documentation, this command will transmit 40 data blocks of 64 bytes each to the CoAP server. However, no payload ever arrives at my CoAP server. From the Graylog logs, it appears that the server is trying to parse the value 40 as the payload, even though in this command 40 is intended to represent the number of blocks to send, with the actual payload consisting of random bytes. Is the block-wise implementation in OT-CLI working as expected?

The documentation also states:

“The block- type requires OPENTHREAD_CONFIG_COAP_BLOCKWISE_TRANSFER_ENABLE to be set.”

Do I need to enable this configuration? If so, can I do it in the prj.conf file like this?

CONFIG_OPENTHREAD_CONFIG_COAP_BLOCKWISE_TRANSFER_ENABLE=y

[Achim Kraus]

Not as answer to you question, more a general note on blockwise (with a decade of experience ;-) ):

Using PUT blockwise comes with the challenge, that in case of failures it's not clear how to recover and usually requires a "full retry".

Exchanging the roles (usually easy with CoAP though all messages share the same basic structure, regardless or request or response) enables in common cases to use a GET blockwise, which is much easier to recover from failures, as long as the resources to read is "stable enough".