A fiield in a datagram
, or IP packet header
. This is an error correction field that ensures the integrity of header values. The checksum is formed by calculating a 16-bit one's complement checksum, including the original checksum. This must be recalculated at each router owing to the TTL
. See RFC1141
If no errors have occurred, the resulting checksum will be all ones. The checksum is not calculated for the encapsulated data.