Calculates a 16-bit 1's complement checksum over buffer (of length length) and message, returning the bit complement of the sum. length should be even and the buffer must be aligned on a 16-bit boundary. length may be zero.
u_short inCkSum(Msg *message, u_short *buffer, int length)
Returns the 1's complement sum of the count 16-bit words pointed to by hdr, which must be aligned on a 16-bit boundary.
u_short ocsum(u_short *hdr, int count)