Why data_transfer_size and max_spdm_msg_size must be equal when chunk capabilities are disabled?

[ssunder-xilinx]

May I know why the responder generating the SPDM_ERROR_CODE_INVALID_REQUEST error when the chunk capabilities are disabled and data_transfer_size and max_spdm_msg_size are not equal?
The DMTF specification says that the data_transfer_size should be less than max_spdm_msg_size. Also, there is already a check for this but why do we need an additional check?
This is the code snippet in question,

external/product/libspdm/library/spdm_responder_lib/libspdm_rsp_capabilities.c

if (spdm_request->header.spdm_version >= SPDM_MESSAGE_VERSION_12) {
        if ((spdm_request->data_transfer_size < SPDM_MIN_DATA_TRANSFER_SIZE_VERSION_12) ||
            (spdm_request->data_transfer_size > spdm_request->max_spdm_msg_size)) {
            return libspdm_generate_error_response(spdm_context,
                                                   SPDM_ERROR_CODE_INVALID_REQUEST, 0,
                                                   response_size, response);
        }
        if (((spdm_request->flags & SPDM_GET_CAPABILITIES_REQUEST_FLAGS_CHUNK_CAP) == 0) &&
            (spdm_request->data_transfer_size != spdm_request->max_spdm_msg_size)) {
            return libspdm_generate_error_response(spdm_context,
                                                   SPDM_ERROR_CODE_INVALID_REQUEST, 0,
                                                   response_size, response);
        }
    }

Thanks,
Shyam

[steven-bellock]

This was clarified in 1.3 and will eventually be clarified in 1.2.2 when that gets published.

On the flip-side, if the Requester does not support chunking then what does it mean to have DataTransferSize != MaxSPDMmsgSize? DataTransferSize is the size of the transmission buffer. MaxSPDMmsgSize is the size of the backing buffer into which chunks are re-assembled. If chunking is not supported then the two buffers, and their sizes, are the same.

[ssunder-xilinx]

Thanks @steven-bellock