Show that the Internet checksum will never be 0xFFFF (that is, the final value of sum will not be 0x0000) unless every byte in the buffer is 0.

Internet specifications in fact require that a checksum of 0x0000 be transmitted as 0xFFFF; the value 0x0000 is then reserved for an omitted checksum. Note that, in ones’ complement arithmetic, 0x0000 and 0xFFFF are both representations of the number 0.