Contract Type Sequencing for Reallocative Negotiation

The capability to reallocate items|e.g. tasks, securities, bandwidth slices, Mega Watt hours of electricity, and collectibles|is a key feature in automated negotiation. Especially when agents have preferences over combinations of items, this is highly nontrivial. Marginal cost based reallocation leads to an anytime algorithm where every agent's utility increases monotonically over time. Di erent contract types head toward di erent locally optimal task allocations, and contracts from a recently introduced comprehensive contract type, OCSM-contracts, head toward the global optimum. Reaching it can take impractically long, so it is important to trade o solution quality against negotiation time. To construct negotiation protocols that lead to the best achievable allocations in a bounded amount of time, we compared sequences of four contract types: original, cluster, swap, and multiagent contracts. The experiments show that it is pro table to use multiple contract types in the sequence: signi cantly better solutions are reached, and faster, than if only one contract type is used. However, the best sequences only include original and cluster contracts. Swap and multiagent contracts lead to bad local optima quickly. Interestingly, the number of contracts using any given contract type does not always decrease over time: contracts play the role of enabling further contracts.