Abstract: The present invention is directed to a method of updating a checksum associated with a packet of information to be transferred between correspondents. The method comprises the steps of: changing the value of a field; computing a complement; computing a difference; computing a one's complement difference from the first difference; computing a first intermediate checksum value equal to the sum of the complement of the checksum HC and the one's complement difference; computing a one's complement intermediate checksum from the intermediate checksum; computing a complement of the one's complement intermediate checksum; and replacing the checksum in the header with the updated checksum.