>> Is it proper to substitute a resulting value of 0x0000 with 0xffff
>> [in the IP header checksum] as is done in UDP?
> No.  This question could easily be answered by checking the RFC for
> IP (it is fairly easy to read).  See
> ftp://ftp.rfc-editor.org/in-notes/rfc791.txt .

Citation?  I just had a look, and I don't find anything that would
forbid using ffff when the computed checksum is 0000.  But I don't have
time to reread the whole thing now, so I depended on searching for
"checksum".  What did I miss?

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