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 &# 39 ; 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 &# 39 ; s complement difference ; computing a one &# 39 ; s complement intermediate checksum from the intermediate checksum ; computing a complement of the one &# 39 ; s complement intermediate checksum ; and replacing the checksum in the header with the updated checksum .