Subject: Re: UDP checksum trouble in -current
To: William Allen Simpson <>
From: Steven M. Bellovin <>
List: tech-net
Date: 01/21/2005 18:52:55
In message <>, 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
>>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,