In message <41F18074.1050908@greendragon.com>, William Allen Simpson writes:
>john heasley wrote:
>
>>A value of zero is a clear
>>indication not to check it for UDP.  It seems like little pain to be
>>"correct".
>>
>>I do not know what could be done for IP or TCP checksums.  But, if I'm not
>>mistaken, zero is not a valid value for either and might be a good indicator
>>that no checksumming was performed.
>>  
>>
>Both IP and TCP checksums MUST be performed.
>
>Zero is not a valid value for UDP, and indicates no checksum.  The field
>still must be set to something.  Don't leave uninitialized fields.
>

You can use a zero checksum for IPv4 UDP, but not for IPv6, I believe.

		--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb