Several approaches have been explored in the past to tackle scalability issues faced by VoD service. IP multicast has been proposed to enhance the efficiency of one-to-many and many-to-many communication over the Internet. A series of IP Multicast-based schemes, such as Patching [1,2], Periodic Broadcast [3,4], and Stream Merging , have been developed that can drastically decrease the aggregate bandwidth requirement at the server by leveraging the native IP multicast. For instance, under Patching, clients arriving close in time form a session. The server begins to multicast the entire stream at the client playback rate upon the arrival of the first client. The following clients retrieve the stream from the multicast channel and obtain the missing initial portion, denoted as the patch, from the server over a unicast channel. It has been shown that the required server bandwidth grows as the square root of the request arrival rate . Unfortunately, IP multicast has not been widely deployed, and access to it is very limited so far, which makes the above schemes less applicable.
Other approaches to addressing the scalability issue include proxy caching and the use of content distribution networks (CDNs). Here the content is pushed from the server to proxies or CDN servers close to the clients. By strategically placing a large number of servers around the Internet, clients can choose the server that incurs the least amount of congestion. Although this method can alleviate the scalability issue, it cannot resolve the issue entirely. For example, assume a proxy is assigned to serve the clients from a region. If the number of requesting clients is very large, this proxy can be overwhelmed.
Recently, peer-to-peer networks (P2P) have been used for file sharing, application-level multicast, and more [6,7,8,9,10]. Peer nodes bring computation and storage resources into the system, hence reducing the workload placed on the server and thereby increasing the overall scalability.
Whether these techniques can be integrated to tackle the scalability issue faced by the VoD service is an intriguing technical question. In this paper, we propose P2Cast - an architecture based on a peer-to-peer approach to cooperatively stream video using the patching technique, while only relying on unicast connections among peers. In P2Cast, clients arriving close in time (within a threshold) form a session. For each session, the server, together with the P2Cast clients, form an application-level multicast tree over the unicast-only network. The entire video is streamed over the application-level multicast tree, so that it can be shared among clients. For clients that arrive later than the first client in the session and thus miss the initial part of the video, the missing part can be retrieved from the server or other clients that have already cached the initial part. The clients in P2Cast are ``active'' in the sense that they can forward the video stream to other clients, and also cache and serve the initial part of the video to other clients. Every P2Cast client actively contributes its bandwidth and storage space to the P2Cast system while taking advantage of the resources from other clients. We compare the performance of P2Cast with IP multicast-based patching and the traditional unicast-based client-server approach. Simulation experiments show that P2Cast can serve many more clients than traditional client-server unicast service, and generally out-performs the multicast-based patching if clients can cache more than of stream's initial part in our experiments.
Although P2Cast is based on patching, it is not a simple extension. The following issues need to be properly addressed before patching can be successfully applied.
We handle disruptions by (1) delaying the start of playback; and (2) applying the shifted forwarding technique to the base stream. The threshold serves as a knob that can adjust the balance between the scalability and the clients' viewing quality in P2Cast.
In summary, the major contributions of this paper lie in the following three aspects:
The remainder of the paper is organized as follows. In Section 2 we describe P2Cast. In Section 3 we present the Best Fit algorithm along with two variations. Section 4 is dedicated to performance evaluation. Recovery from client early departures and underlying network failures is investigated in Section 5. Section 6 introduces related works, and conclusions and future work are included in Section 7.