What are the DCA Slot and Perimeter rules?  The long-standing slot and perimeter regulatory structure is essential for maintaining the operational and economic balance between the small, space-constrained and over-crowded Reagan National (DCA) and the much-larger Dulles International (IAD), which is equipped to handle bigger planes and longer flights as the region’s growth airport. The Slot rule manages congestion by controlling the frequency of takeoffs and landings.