In the prevailing model of business-to-business commerce on the Internet, transactions flow through large, centralized exchanges. These exchanges play two valuable roles: aggregation and facilitation. As aggregators, they bring a group of dispersed trading partners together into a virtual marketplace. As facilitators, they provide software tools and protocols that enable the traders to do business electronically, exchanging information, processing offers and bids, coming to terms on deals, and following through on them.