headerchecksum

Header checksum is a mechanism used by some network protocols to detect errors in the header portion of a packet. It uses a dedicated field within the header to store a numerical value that is derived from the header’s contents. The goal is to identify corruption that may occur as a packet travels through networks and routers.

In IPv4, the header checksum is a 16-bit value computed as a one's complement sum of all 16-bit words in the IP header, with the checksum field set to zero during calculation. The resulting sum is then stored in the header’s checksum field. When a receiver processes a packet, it recomputes the same sum over the header (including the received checksum). If the result is the all-ones value (0xFFFF), the header is considered intact; otherwise, the packet is rejected or marked as errored. The checksum is typically recalculated for each fragment, since fragment headers differ from the original.

IPv6 does not include a header checksum. To avoid per-router processing overhead, IPv6 relies on other mechanisms for integrity, such as link-layer checksums or transport-layer checksums. In TCP and UDP, the checksums cover both the header and the payload and involve a pseudo-header that includes source and destination addresses, protocol, and length. These transport-layer checksums are separate from the IP header checksum and provide end-to-end integrity for data.

Limitations of header checksums include protection only of header data, not payload content, and the fact that certain error patterns may go undetected. They are not security features and do not prevent all forms of data corruption.