CRC_LEN
#define MAX_RX_HDLC (1 + RX_HDLC_PAYLOAD + CRC_LEN)
#define PKT_MAXBUF_SIZE (MAX_MTU+ETH_HLEN+CRC_LEN)
#define PKT_MINBUF_SIZE (MIN_MTU+ETH_HLEN+CRC_LEN)
uint8_t crc[CRC_LEN];
if ((ext_o_len + CRC_LEN) < str_max_len)
(str_o_len <= (str_max_len - ext_o_len - CRC_LEN)))
len = min_t(int, CRC_LEN, str_max_len - str_o_len);