Video streaming over P2P networks: Challenges and opportunities

Abstract

A robust real-time video communication service over the Internet in a distributed manner is an important challenge, as it influences not only the current Internet structure but also the future Internet evolution. In this context, Peer-to-Peer (P2P) networks are playing an imperative position for providing efficient video transmission over the Internet. Recently, several P2P video transmission systems have been proposed for live video streaming services or video-on-demand services over the Internet. In this paper, we describe and discuss existing video streaming systems over P2P. Efficient (delay tolerant and intolerant) data sharing mechanisms in P2P and current video coding trends are elaborated in detail. Moreover, video streaming solutions (live and on-demand) over P2P from the perspective of tree-based and mesh-based systems are explained. Finally, the conclusion is drawn with key challenges and open issues related to video streaming over P2P.

Introduction

Future media Internet will need to be able to distribute high quality video contents in an efficient, supple and personalized way through dynamic and heterogeneous network environments. Multimedia content distribution over Internet has already become a major application due to users’ growing demand of multimedia content and extraordinary growth of network technologies. A broad assortment of such applications can be found in these days, e.g. as video streaming, video conferencing, surveillance, media broadcasting, e-learning and storage, etc. In customary, video streaming techniques typically employ the client-server model and the usage of Content Distribution Networks (CDN) along with IP multicast has been the most desirable solutions to support media streaming over Internet. However, it has been shown that there are significant limitations on the number of simultaneous users for bandwidth intensive video streaming in such client/server architectures, due to a bandwidth bottleneck at the server side from which all users request the content. In contrast, Peer-to-Peer (P2P) video streaming protocols, motivated by the great success of file sharing applications, have attracted a lot of interests from academia and industry. With respect to conventional approaches, a major advantage in using P2P is that each peer involved in a content delivery contributes with its own resources to the streaming session. However, in order to efficiently provide high quality of service in P2P networks, several mechanisms in the P2P protocols should be modified such that they can support unique multimedia characteristics as well as the video coding/transmission technology needs to be able to cope with varying bandwidth capacities inherent to P2P systems and end-user characteristics such as decoding and display capabilities usually tend to be non-homogeneous and dynamic.

Conventionally, P2P systems can be classified into two main categories namely tree-based and mesh-based (or data-driven)¹ based on the type of distribution graphs they implement for delivery of data. Tree-based overlays employ a tree distribution graph, entrenched at the source of the content. The gigantic mainstream of the existing research on P2P focuses on tree-based approaches, where peers are organized into fixed structures (i.e., trees) for delivering data. Nodes in the structure typically have well-defined relationships, e.g., “parent-child” relationships in trees. Parents are responsible for forwarding data packets they receive to their children. Consequently, the structure plays an essential role in the average user experience and thus has to be fully optimized. If peers do not change too frequently, such a system requires little overhead; in fact, packets can be forwarded from node to node without the need for extra messages. However, in high churn environments, the tree structures is a very challenging since nodes can join and leave the group at any time with a high rates and nodes must buffer data for at last the time required to repair the tree, in order for smooth delivery of data. This maintenance is especially crucial for time-constrained (or delay-sensitive) media content. In contrast to tree-based approaches, mesh-based overlays are not confined to a static topology but are based on a mesh distribution graph, where each node contacts a subset of peers to exchange data chunks. In order for efficient chunk exchange, the information about which chunks are owned by other peers needs to be known to each peer so that it can explicitly pulls the chunks it needs. This type of scheme involves overhead, due in part to the exchange of buffer maps between nodes. As each node relies on multiple peers to retrieve content, mesh-based overlay systems provide good resilience to node failures. In addition to these overlays, researchers are working on hybrid solution that utilized both above mentioned overlays.

Video streaming systems [1], [2] based on P2P can be broadly classified and deployed in two main categories: live video streaming and on-demand video streaming. These P2P streaming systems attain immense triumph due to high scalability, low cost and robustness to churn. In live video streaming systems over P2P, the source peer broadcasts the live video content in the form of chunks and the peers can display the video content once they receive the chunks of video where they want to play. Thus, the chunks must arrive in a predefined sequential order, so that they can be played at the end node/peer with minimal delay latency. In live video streaming deployments, a minimal end-to-end delay is more enviable for high Quality of Experience (QoE) at receiver. On the other hand, Video-on-Demand (VoD) is a user driven interactive multimedia application in which user has an option to perform VCR-like operations (i.e., forward, backward, skip, etc.). The video contents [3] are pre-encoded and stored in the form of chunked at source peer so liveliness is irrelevant in VoD systems.

In the rest of this paper, we provide in depth survey on the existing techniques of video streaming over P2P. Section 2 provides an overview and basics of P2P systems. The most recent trends in video coding are explained in Section 3. Advanced techniques of VoD and live video streaming over P2P are explained in Section 4. Section 5 concludes the paper with several open research issues.