Before revision:  If peers depart after obtaining a complete collection, they can tend to leave before helping other peers receive the rare piece  .
After revision:  If peers depart after obtaining a complete collection, they can tend to leave before helping other peers receive the rare piece . Assuming that peers arrive with no pieces, there is a single seed, random peer contacts are made, random useful pieces are downloaded, and peers depart upon receiving the complete file, the system is stable if the seeding rate (in pieces per time unit) is greater than the arrival rate, and is unstable if the seeding rate is less than the arrival rate. The result persists for any piece selection policy that selects from among useful pieces, such as rarest first, and it persists with the use of network coding .