| Most of current Peer-to-Peer (P2P) Video-on-Demand (VoD) platforms are totally dependent on index servers while constructing overlay network. In those platforms, if peers execute VCR operations frequently, index servers suffer from overwhelmed load and unbearably long response time to peers'requests. Besides, it is difficult to extend and maintain"tree"structure which is key to those platforms when environment changes dynamically. Especially, pivotal peers'invalidation will lead a subversive influence on overlay network.Therefore, GridCast platform, one of P2P VoD platforms in which overlay network is incompletely dependent on index servers is developed. GridCast maintains and rebuilds the topology periodically according to existing logic relations. In GridCast, when a peer sends a request for index service, watches programs or performs VCR operations, it does not access the index server. Meanwhile, according to time adjacency, peers establish"unstructured"overlay adaptively, and form a reasonable relationship. Thus, the total performance of GridCast is improved. The basic thought of constructing such overlay is: the index server manages information of all peers in this platform, adopts adjacent/different time-slip peer search algorithm to calculate time adjacency, and searches information of adjacent/different time-slip peers according to the result. Then it creates the neighborhood relationship between these peers with shake-hand protocol, and initializes the overlay.Although overlay is constructed, due to peers'fluctuation with the change of playback time, or some other reasons, original overlay is breached, the overlay requires maintainance. To describe exact information of peers who fluctuate with the change of playback time, it is necessary to update neighbor peers'information and obtain new adjacent/different time-slip neighbors. Firstly, it can be achieved by recording information of adjacent/different time-slip peers, neighbors and partners for all online peers through peer management of GridCast. Then peers communicate with each other using communication and information exchange algorithms and adopt adjacent/different time-slip peer selection algorithm to choose more peers. In this way, the overlay is maintained periodically. As the peers perform VCR operations, original overlay is breached too. So topology rebuilding mechanism is used for these peers to communicate with former neighbors. And new adjacent/different time-slip peers are selected according to the principle mentioned above. The shake-hand protocol is adopted for establishment of neighborhood and the rebuild is completed for the overlay among peers.GridCast platform is based on Windows series and developed using Visual.NET platform. And the overlay management function of GridCast is implemented using C++. The performance of the system is evaluated by analyzing the log data which is obtained during GridCast practical run. The results show that the number of neighbours in a same channel is close to the ideal neighbour number. And the partnership success rate maintains at 0.9. Load on the index server is basically the same for both normal VCR operations and non-VCR operations. Peers are easier to acquire data from the adjacent/different time slip node than from the index server, which is shortened by 35ms. And the CPU, memory and network bandwidth occupancy rate on the media server is lowered by 50%, 30% and 45%, compared with the Client/Server model. These show that the platform's performance has an obvious improvement. |
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