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P2P(Peer-to-Peer) Streaming Systems

Peer-to-Peer(P2P) computer network has been an active research area in the past decade. Different forms of file distribution systems have been developed based on P2P. A pure P2P file-sharing network does not have a centralized server; instead peers are responsible for uploading and downloading the data among themselves. This is the major difference compared with the traditional file-sharing system, where communication is usually directed to and from a server. As a result, given a stable P2P network, files can be shared effectively and network scalability can increase with lower cost.

With the power of P2P network, users become interested in sharing large volume of data (especially multimedia contents) across the Internet. Therefore, video on demand and live media streaming systems are created. In order to achieve the highest quality of service, the most important principle is cooperation among different peers. This article will focus on P2P streaming systems and discuss several incentive mechanisms that reward peers for cooperation and punish free riders.

P2P Streaming Systems

In P2P multimedia streaming, the video/audio stream is split into several smaller streams. Each stream is stamped with a numerical sequence number so that it can be placed in the correct sequence for playback. In order to guarantee each stream is delivered, a forward error correction code will be used. Different peers join the streaming session and exchange availability information. A peer retrieves data by requesting data from other peers, while supplying available data to other peers. These operations are monitored by an application-level streaming system.

Tree-based Multicast System

A traditional design of P2P streaming system is the tree-based multicast system that is based on a single tree (Figure 1a). In this system, a peer is either an interior node or a leaf node. If a peer is an interior node, it will carry all the loads to forward the data to other nodes. On the other hand, if a peer is a leaf node, no forwarding is required. There are two problems with this design. First, the system is not fair. When the number of fanouts of the tree increases, the number of leaf nodes increases much faster than the number of interior nodes. Since all the loads are within the interior nodes, the system becomes very unbalanced. Second, peers who act as interior nodes may not be able to handle high-bandwidth application (e.g. high quality video) due to network capacity limitation.

Split Stream Multicast System

In order to overcome the unbalanced loads on peers, an improved system was proposed in, using a split stream approach (Figure 1b). Split stream system works by splitting the stream into multiple stripes which uses separate multicast trees to distribute each stripe to the peers. The goal is to ensure that vast majority of peers are interior nodes in only one tree, and they will be leaf nodes in all other trees. As a result, the system distributes forwarding workloads among all peers and solves the first problem in the traditional streaming system. This method becomes useful when a large number of cooperative peers are interested in a streaming session to share the content.

With this approach, peers choose to join a subset of the stripes to control their inbound bandwidths. On the other hand, peers opt to limit the number of children nodes they adopt to control their outbound bandwidths. Thus, the split stream system accommodates peers with different bandwidths and solves the second problem in the traditional streaming system. Other advantages of striping across multiple trees include improved robustness to node failure since a node failure only causes the loss of a single stripe on average, and the system also guarantees quick recovery from node failure.

The major drawback of this design is no guarantee of finding an optimal forest, even if the network has sufficient capacity.

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