Checksum computation

The adjustment acclimated to compute the checksum is authentic in RFC 768:

Checksum is the 16-bit one's accompaniment of the one's accompaniment sum of a bogus attack of advice from the IP header, the UDP header, and the data, bedlam with aught octets at the end (if necessary) to accomplish a assorted of two octets.[7]

In added words, all 16-bit words are summed application one's accompaniment arithmetic. The sum is again one's complemented to crop the amount of the UDP checksum field.

If the checksum adding after-effects in the amount aught (all 16 $.25 0) it should be beatific as the one's accompaniment (all 1s).

The aberration amid IPv4 and IPv6 is in the abstracts acclimated to compute the checksum.