Show that the Internet checksum will never be $0 x FFFF$ (that is, the final value of sum will not be $0x 0000$) unless every byte in the buffer is $0$. (Internet specifications in fact require that a checksum of $0x 0000$ be transmitted as $0x FFFF$; the value $0x 0000$ is then reserved for an omitted checksum. Note that, in ones complement arithmetic, $0x 0000$ and $0x FFFF$ are both representations of the number $0$).
