Theoretical Modeling And Protocol Improvement For P2P Streaming Media System

With the increase of user access bandwidth and the number of Internet users, as well as the improvement of PC capacity, P2P streaming media system becomes one of the most popular Internet applications, due to its huge system capacity, stable quality of service (QoS), outstanding scalability and low deployment cost. P2P streaming media system can support millions of users watching high definition video at the same time and then greatly speed up the transmission of information over Internet. Though gaining rapid development, P2P streaming media system still has some pending issues, for instance, the QoS characteristic has not been revealed, the assignment of bandwidth resource between peers is not reasonable and the signal overhead is too high. Towards these issues, we study P2P streaming media system in terms of theoretical modeling, network measurement and protocol design. The main achievement is below:(1) Towards issues in P2P VoD (Video-on-Demand) system that the relationship between QoS and important parameters is not revealed and the playback interruption characteristic is not disclosured, we establish a QoS evaluation model towards playback interruption. In this model, we first use the Brownian motion with drift to model the peer downloading process and video playback process, then establish the mathematical relationship among playback continuity, video playback rate, peer download rate and prefetched data amount (PDA) and finally get the mathematical expression of the playback interruption probability, the first interruption time and the number of interruptions during a viewing process. From above models, it is observed that the dynamics of peer download speed and video playback rate is the main cause of QoS decline; increasing PDA or download speed can improve playback continuity but the former delays the playback interruption. During a viewing process, the interruption occurs at the beginning with great probability; many interruptions may happen with small probability. To verify the playback interruption characteristic, we do a network measurement towards PPLive VoD system with millions of users for continuous five days and the measurement results are consistent with the conclusion derived from the model, which not only reveals the unique characteristic of P2P VoD system, but also lays the foundation for new bandwidth assignment algorithms.(2) Towards the issue that different users in P2P VoD system have different watching experience because the bandwidth obtained by one peer is not consistent with the bandwidth required by that peer (namely the bandwidth obtained by one peer can not dynamically and in real time change with the bandwidth requirement of that peer), we establish a peer bandwidth demand model, which shows the mathematical relationship among expected bandwidth, PDA, playback rate and playback continuity. Based on this model, a distributed upload bandwidth assigning algorithm is proposed. This algorithm achieves real-time and dynamic bandwidth assignment among heterogeneous peers; specifically, this algorithm makes peers with too much prefetched data or lower playback rate release some bandwidth, which then is assigned to those peers with too little prefetched data or higher playback rate. In this way, all peers in P2P VoD system can obtain the same playback continuity. Simulation results show that compared with traditional greedy strategy this algorithm gains shorter startup delay and less server bandwidth.(3) Towards the polarization of peer QoS due to no prefetching limitation in P2P VoD system, we propose a moderate prefetching strategy based on video slicing mechanism. In this strategy, peers with more prefetched data will stop downloading content and the released bandwidth is assigned to those peers with less prefetched data. In this way, low download capacity peers can gain the same playback continuity as the high download capacity peers. Towards this strategy, we propose a performance evaluation model from which it is observed that under this strategy peers can obtain moderate prefetched content and high playback continuity. Finally, this strategy is applied to PPLive VoD system and the measurement results show that the server bandwidth saving rate of PPLive VoD system is as high as95%but that of UUSee VoD system is just70%. Also, the average number of interruptions during a viewing process in PPLive VoD system is as small as0.8, which means that the user viewing experience is wonderful. In a word, this strategy not only reduces server bandwidth but also improves the system QoS.