Bandwidth Allocation in BitTorrent-like VoD Systems under Flashcrowds

The efficiency of BitTorrent in content distribution has inspired a number of peer-to-peer (P2P) protocols for on-demand video (VoD) streaming systems (henceforth BitTorrent-like VoD systems). However, the fundamental quality-of-service (QoS) requirements of VoD (i.e. providing peers with a smooth playback continuity and a short startup delay) make the design of these systems more challenging than normal file-sharing systems. In particular, the bandwidth allocation strategy is an important aspect in the design of BitTorrent-like VoD systems, which becomes even more crucial in a scenario where a large number of peers joins in a short period of time, a phenomenon known as flashcrowd. In fact, the new joining peers all demand for content while having few or no pieces of content to offer in return yet. An unwise allocation of the limited bandwidth actually available during this phase may cause peers to experience poor QoS. In this work, we analyze the effects of a flashcrowd on the scalability of a BitTorrent-like VoD system and propose a number of mechanisms to make the bandwidth allocation in this phase more effective. In particular, we derive an upper bound for the number of peers that can be admitted in the system over time and we find that there is a trade-off between having the seeders minimize the upload of pieces already injected recently and high peer QoS. Based on the insights gained from our analysis, we devise some flashcrowd-handling algorithms for the allocation of peer bandwidth to improve peer QoS during flashcrowd. We validate the effectiveness of our proposals by means of extensive simulations.