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 ................ [email protected] [email protected] Dirtside Systems ......... Web: <http://www.dirtside.com/>
