I've just run on the need of a hash function with a peculiar property: f(A,B,C) = f(B,A,C); IOW, the function should return the same result if two of its parameters are swapped (all parameters are integers).

For example, I could simply do A XOR B then hash it with C, this would give the expected result, but wouldn't give a good hashing result, since it is easly exploitable: f(0x01,0x20,C) = f(0x00,0x21,C).

I need this for classifying network streams: it doesn't matter if a given packet cames from host A to host B or vice-versa, the connection is the same. Does someone have a suggestion?