I read UDP checksum from Wikipedia that "If the checksum calculation results in the value zero (all 16 bits 0) it should be sent as the one's complement (all 1s) as a zero-value checksum indicates no checksum has been calculated. In this case, any specific processing is not required at the receiver, because all 0s and all 1s are equal to zero in 1's complement arithmetic."
I don't understand when checksum calculation be zero in one's complement, but all 1's possible to get. My understanding Zero sum means there is no data to calculate. Please help.