header checksum (HCS)

In the IP header, the Header Checksum( HCS) is a 16-bit long data field that contains the checksum from the IP header data and any option field used; but without the payload.

This data from the HCS field is used tocheck the IP header for errors. When IP datagrams are fragmented, the checksum of the header changes, as does when the TTL time is decremented. The checksum must then be recalculated.

IP data packets with errors are simply discarded and requested again by the sending data station.