Rarest-First With Probabilistic-Mode-Suppression (RFwPMS)

Recent studies suggested that the BitTorrent’s rarest-first (RF) protocol, owing to its work-conserving nature, can become unstable in the presence of non-persistent users. Consequently, for any provably stable protocol, many peers, at some point, have to be forced to hold off their file-download activity. In this work, we propose a tunable piece-selection policy that minimizes this (undesirable) requisite by combining the (work-conserving but not stabilizing) RF protocol with only an appropriate share of the (stabilizing but not work-conserving) mode-suppression (MS) protocol. We refer to this policy as “Rarest-First with Probabilistic Mode-Suppression” or simply RFwPMS. We study RFwPMS using a stochastic abstraction of the BitTorrent network that is general enough to capture a multi-swarm setting of non-persistent users—each swarm having its own altruistic preferences that may or may not overlap with those of other swarms. Using Lyapunov drift analysis, we show that for all kinds of inter-swarm behaviors and all arrival-rate configurations, RFwPMS is stable. Then, using the Kingman’s moment bound technique, we further show that the steady-state expected sojourn time of RFwPMS is independent of the arrival-rate in the single-swarm case (under a mild additional assumption). Finally, our simulation-based performance evaluation confirms our theoretical findings, and shows that the steady-state expected sojourn time is linear in the file-size (compared to our loose estimate of a polynomial with degree 6). Overall, an improved performance is observed in comparison to previously proposed stabilizing schemes like MS.