Communication-space tradeoffs for unrestricted protocols

Communicating branching programs are introduced, and a general technique for demonstrating communication-space tradeoffs for pairs of communicating branching programs is developed. The technique is used to prove communication-space tradeoffs for any pair of communicating branching programs that hashes according to a universal family of hash functions. Other tradeoffs follow from this result. For example any pair of communicating Boolean branching programs that computes matrix-vector products over GF(2) requires communication-space product Ω(n ²). These are the first examples of communication-space tradeoffs on a completely general model of communicating processes