[dpdk-users] checksums?

[William Herrin]

Hi folks,

I've recently done a deep dive in to IP checksums and I've run in to
something I don't understand. Any insight would be helpful.

rte_ipv4_cksum() is implemented as:

return (cksum == 0xffff) ? cksum : (uint16_t)~cksum;

Which means: if the sum is zero, return -0 (0xffff) never +0 (0x0000).
Welcome to the wonderful world of 1's complement arithmetic.

RFC 1624, on the other hand, says:

"In one's complement, there are two representations of zero: the all zero
and the all one bit values, often referred to as +0 and -0. One's
complement addition of non-zero inputs can produce -0 as a result, but
never +0.  Since there is guaranteed to be at least one non-zero field in
the IP header, and the checksum field in the protocol header is the
complement of the sum, the checksum field can never contain ~(+0), which is
-0 (0xFFFF).  It can, however, contain ~(-0), which is +0 (0x0000)."

Which I understand to mean that +0 (0x0000) is a legal value in an IPv4
checksum field, but -0 (0xffff) is not.

Is this a bug? Is there a more authoritative source for which zero is
correct in an IPv4 header? Please help me find the error in my
understanding.

Thanks,
Bill Herrin


-- 
William Herrin ................ herrin at dirtside.com  bill at herrin.us
Dirtside Systems ......... Web: <http://www.dirtside.com/>