diff options
Diffstat (limited to 'src/include')
| -rw-r--r-- | src/include/ipxe/tcpip.h | 38 |
1 files changed, 36 insertions, 2 deletions
diff --git a/src/include/ipxe/tcpip.h b/src/include/ipxe/tcpip.h index c3528c9c9..be4ac838b 100644 --- a/src/include/ipxe/tcpip.h +++ b/src/include/ipxe/tcpip.h @@ -19,11 +19,38 @@ struct io_buffer; struct net_device; struct ip_statistics; +/** Positive zero checksum value */ +#define TCPIP_POSITIVE_ZERO_CSUM 0x0000 + +/** Negative zero checksum value */ +#define TCPIP_NEGATIVE_ZERO_CSUM 0xffff + /** Empty checksum value * - * This is the TCP/IP checksum over a zero-length block of data. + * All of our TCP/IP checksum algorithms will return only the positive + * representation of zero (0x0000) for a zero checksum over non-zero + * input data. This property arises since the end-around carry used + * to mimic one's complement addition using unsigned arithmetic + * prevents the running total from ever returning to 0x0000. The + * running total will therefore use only the negative representation + * of zero (0xffff). Since the return value is the one's complement + * negation of the running total (calculated by simply bit-inverting + * the running total), the return value will therefore use only the + * positive representation of zero (0x0000). + * + * It is a very common misconception (found in many places such as + * RFC1624) that this is a property guaranteed by the underlying + * mathematics. It is not; the choice of which zero representation is + * used is merely an artifact of the software implementation of the + * checksum algorithm. + * + * For consistency, we choose to use the positive representation of + * zero (0x0000) for the checksum of a zero-length block of data. + * This ensures that all of our TCP/IP checksum algorithms will return + * only the positive representation of zero (0x0000) for a zero + * checksum (regardless of the input data). */ -#define TCPIP_EMPTY_CSUM 0xffff +#define TCPIP_EMPTY_CSUM TCPIP_POSITIVE_ZERO_CSUM /** TCP/IP address flags */ enum tcpip_st_flags { @@ -88,6 +115,13 @@ struct tcpip_protocol { int ( * rx ) ( struct io_buffer *iobuf, struct net_device *netdev, struct sockaddr_tcpip *st_src, struct sockaddr_tcpip *st_dest, uint16_t pshdr_csum ); + /** Preferred zero checksum value + * + * The checksum is a one's complement value: zero may be + * represented by either positive zero (0x0000) or negative + * zero (0xffff). + */ + uint16_t zero_csum; /** * Transport-layer protocol number * |