One’s Compliment Checksum

• IP, like many protocols, uses a checksum to detect loss of packet integrity

• • Packets can have accidentally flipped bits, be truncated, or maliciously changed

• The type of checksum used by IP and a number of other Internet protocols is called one’s compliment

• • If you’ve studied one’s compliment math before, you’ll realize soon that the one’s compliment used by IP is different
  • IP protocols use 16-bit chunks for the calculations

• The IP one’s compliment checksum process

• • Add together the array of 16-bit values “carrying over the one” when the sum spills into the 17th bit
  • Once the addition is complete, invert the result