This unsigned variable typically contains a checksum over the entire packet, i.e all the fields described above. However, the actual size and contents of this field is determined again by both the Protocol Id and Address Type fields in the packet. This allows the use of no checksums, or checksums that are appropriate for the type of lower layers, e.g a forward error correcting checksum when low bandwidth HF links are utilised.
For most protocol/address type pairs, it is expected that the checksum will be calculated according to the ISO 3309 (HDLC) specification [ISO 3309 ].