Header checksum

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Header Checksum refers to the basic protection used in the header of IP packets against data corruption. This checksum is calculated only for the header bytes and is 16 bit long and is a part of the IP packet header.

This checksum is calculated by word aligning the IP header bytes and forming a ones complement of that. At every node of the router, the checksum is verified. The result should be all 1's if there is no corruption. At each hop, the checksum is recalculated and the packet will be discarded upon checksum mismatch.The router may add incremental IP header checksum updating when the only change to the IP header is the time to live.^[1]