ACK_HDR_LEN
memcpy(req->data, &state->buf[ACK_HDR_LEN], req->data_len);
(!write && (req->data_len > BUF_LEN - ACK_HDR_LEN))) {
rlen = ACK_HDR_LEN;
memcpy(req->rbuf, &state->buf[ACK_HDR_LEN], req->rlen);
req->rlen > (BUF_LEN - ACK_HDR_LEN - CHECKSUM_LEN)) {
rlen = ACK_HDR_LEN + req->rlen + CHECKSUM_LEN;