1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +---------------+---------------+-------+----------------------+ | Kind | Length |Subtype| (reserved) |F|m|M|a|A| +---------------+---------------+-------+----------------------+ | Data ACK (4 or 8 octets, depending on flags) | +--------------------------------------------------------------+ | Data Sequence Number (4 or 8 octets, depending on flags) | +--------------------------------------------------------------+ | Subflow Sequence Number (4 octets) | +-------------------------------+------------------------------+ | Data-Level Length (2 octets) | Checksum (2 octets) | +-------------------------------+------------------------------+