Rob Austein wrote:
The last time this came up for a TCP implementation I used to
maintain, our interpretation of Robustness Principle applied to this
problem dictated that we shouldn't send segments with checksum fields
set to all ones (that is, we shouldn't send ~(+0)), but that we had to
accept either ~(+0) or ~(-0) in received segments.
Strictly speaking, either zero state is completely legal,
See my other post on this, as per RFC1624 section 3, only one state
(0x0000) is legal as the complement of the checksum of non-zero data.
TCP checksummed data must contain at least one nonzero word (the one
with the TCP header length, which must be >=20).
The receiver is entirely justified in considering 0xffff as an error.
Jeo