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The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero. If there is no corruption, the result of summing the entire IP header, including checksum, and then taking its one's complement should be zero.
The checksum algorithms most used in practice, such as Fletcher's checksum, Adler-32, and cyclic redundancy checks (CRCs), address these weaknesses by considering not only the value of each word but also its position in the sequence. This feature generally increases the cost of computing the checksum.
A checksum of a message is a modular arithmetic sum of message code words of a fixed word length (e.g., byte values). The sum may be negated by means of a ones'-complement operation prior to transmission to detect unintentional all-zero messages. Checksum schemes include parity bits, check digits, and longitudinal redundancy checks.
When TCP runs over IPv4, the method used to compute the checksum is defined as follows: [16] The checksum field is the 16-bit ones' complement of the ones' complement sum of all 16-bit words in the header and text. The checksum computation needs to ensure the 16-bit alignment of the data being summed.
So, the simple checksum is computed by adding together all the 8-bit bytes of the message, dividing by 255 and keeping only the remainder. (In practice, the modulo operation is performed during the summation to control the size of the result.) The checksum value is transmitted with the message, increasing its length to 137 bytes, or 1096 bits.
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fieldbus networks [50] 0x1DCF 0xF3B8 0xE771 0x8EE7 odd Fletcher-16 Used in Adler-32 A & B Checksums Often confused to be a CRC, but actually a checksum; see Fletcher's checksum: CRC-17-CAN CAN FD [51] 0x1685B 0x1B42D 0x1685B 0x1B42D even CRC-21-CAN CAN FD [51] 0x102899 0x132281 0x064503 0x18144C even CRC-24 FlexRay [39] 0x5D6DCB 0xD3B6BA ...