https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=3112
--- Comment #5 from Guy Harris <guy@xxxxxxxxxxxx> 2008-12-22 13:14:12 PDT ---
To quote RFC 793, which is what defines the Internet checksum:
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. If a
segment contains an odd number of header and text octets to be
checksummed, the last octet is padded on the right with zeros to
form a 16 bit word for checksum purposes. The pad is not
transmitted as part of the segment. While computing the checksum,
the checksum field itself is replaced with zeros.
So, for test 7:
test 7 data: 0xbfe29ba1 length: 1 fe
in_cksum 65281
WS CDP checksum 65281
CDP checksum 511
That's a case with an odd number of octets, so it'd be padded on the right with
zeroes, giving 0xfe00. The one's complement of that is 0x01ff, or 511.
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