JOURNAL OF NETWORKS, VOL. 9, NO. 7, JULY 2014 1721 Optimal Rate Allocation for ISP-friendly P2P VoD Systems Yun He, Zhongyang Xiong, and Xiaolin Tan Department of Computer Science, Chongqing University, Chongqing 400044, China Email: {heyun, zyxiong, allen_txl}@cqu.edu.cn Abstract—The rapid growth of Peer-to-Peer (P2P) Video-on- down P2P applications [7], in turn leading P2P content Demand (VoD) applications has posed significant pressure providers into an arms race to evade restrictions. on both content providers and Internet Service Providers To alleviate the aforementioned tussle, several recent (ISPs). In this paper, we focus on rate allocation algorithms studies [8-13] have proposed locality-aware neighbor which not only improve the streaming performance of P2P selection to make P2P systems more ISP-friendly. The VoD systems but also cut down on inter-ISP traffic. We first propose two centralized rate allocation algorithms which basic idea is to direct peers to select adjacent neighbors in serve as the benchmarks: one is Server-First Optimal (SFO) the same ISP domain when the overlay connections are algorithm that takes minimum server load as the first order established. Although locality-aware neighbor selection objective, and the other is Locality-First Optimal (LFO) has been shown to be effective in reducing inter-ISP algorithm that takes minimum inter-ISP traffic as the first traffic, it may prevent peers from utilizing idle upload order objective. By introducing network flow theory, SFO bandwidth resource at the remote peers. Such resource is and LFO can be solved as a series of maximum or min-cost usually critical to the streaming performance of P2P VoD flow problems. Then we propose a distributed rate systems, as it helps the peers maintain smooth playback allocation algorithm which utilizes a novel hybrid neighbor and combat user churn. selection strategy and models the design goals as a multi- objective optimization problem. Through simulation studies, For most network environments, the peers’ upload we show that our algorithms substantially outperform bandwidth is the most important resource due mainly to traditional and state-of-the-art approaches. the widespread deployment of asymmetric access links, such as ADSL and cable modem connections [14, 15]. In Index Terms—P2P, Video-on-Demand, Rate Allocation, ISP- P2P VoD systems, it is difficult for younger peers to friendly reciprocate older peers who have started watching the same video earlier because of the lack of playback synchronization among users [16]. A recent measurement I. INTRODUCTION study [17] has shown that more than half of the video Following the flourish of P2P live streaming, large- chunks may still need to be supplied from the media scale P2P VoD streaming has become one of the most server. If the locality-aware neighbor selection popular Internet applications. There are a number of P2P mechanism is employed, the case may be further VoD systems that have been successfully deployed over exacerbated as the server burden will increase. the Internet, such as PPLive VoD [1], Joost [2] and Therefore, an ideal traffic control mechanism is needed Funshion [3]. The major advantage of the P2P for P2P VoD systems to achieve the tradeoffs between architecture is that by utilizing the distributed resource of the inter-ISP traffic and streaming performance. In this participating peers, the system achieves better robustness paper, we propose several rate allocation algorithms that and scalability than the traditional client-server fulfill the following design goals and achieve the architecture. As recently reported in [4, 5], more than tradeoffs among them if there is any conflict. hundreds of thousands of users can be simultaneously Low server load: From the viewpoint of content participating in these P2P systems. providers, the system with low server burden has Although P2P solutions can provide highly efficient great scalability and cost benefits. content distribution services, they have posed Low inter-ISP traffic: From the viewpoint of ISPs, unprecedented pressure on ISPs by generating a P2P applications should be carefully designed to tremendous amount of inter-ISP traffic. According to an constrain traffic within ISPs’ boundaries. Internet study report by IPOQUE [6], P2P applications High peer prefetching: To improve user perceived account for approximately 73% of total Internet traffic service quality, upload bandwidth of peers should worldwide. There exists a serious topology mismatch be fully utilized to allow VoD users to download problem between the P2P overlay network and the faster and cache video content for future use. physical underlay network, which significantly increases Note that optimizing the three goals simultaneously is ISPs’ operational costs. It is not surprising that a number a difficult task. For example, reducing the inter-ISP of ISPs have attempted to detect, throttle, suppress or shut traffic of the system may make the download rates of some peers drop below their required playback rates. This © 2014 ACADEMY PUBLISHER doi:10.4304/jnw.9.7.1721-1731 1722 JOURNAL OF NETWORKS, VOL. 9, NO. 7, JULY 2014 may increase the server load as these peers have to selection strategy is proposed in [11], where each peer download video data from the media server directly. Thus, chooses the majority of its neighbors from the same ISP we make a general assumption that reducing server load domain, and only a few from other ISP domains. Li et al. and inter-ISP traffic are more important than increasing [12] propose a novel neighbor selection algorithm called peer prefetching when the required playback rate is UTAPS, in which each peer selects neighbors based on satisfied, as peers gain little utility from having a round trip time (RTT) and hop count information by download speed much higher than the necessary to utilizing the knowledge of underlying topology. In preserve stream continuity [18]. Precisely, the TopBT [13], each peer utilizes lightweight pings or contributions of this paper can be summarized as follows: traceroute probes to its connected neighbors periodically 1) To minimize server load, minimize inter-ISP to obtain corresponding link hops or AS hops. The above traffic and maximize peer prefetching in a mechanisms have been shown to be effective in reducing sequential manner, we propose two centralized inter-ISP traffic and beneficial for the ISPs. However, the rate allocation algorithms: one is Server-First pure locality-aware neighbor selection prevents peers Optimal (SFO) algorithm which takes minimum from utilizing idle upload bandwidth resources at the server load as the first order objective, and the remote peers, and hence may degrade the streaming other is Locality-First Optimal (LFO) algorithm performance of P2P VoD systems. which takes minimum inter-ISP traffic as the first A simple random neighbor selection scheme is likely order objective. By introducing network flow to stress the older peers and waste network bandwidth in theory [19], we show that SFO and LFO can be the P2P VoD service. To alleviate the server load, some solved as a series of maximum or min-cost flow researchers have suggested that the design of neighbor problems. The two centralized algorithms serve selection mechanism should consider the load balance of as benchmarks for our later comparison study. peers in the system. The iPASS [20] adopts a neighbor 2) We then propose a distributed rate allocation selection strategy based on the buffering progresses of algorithm called Improved Weighted Sum with peers to obtain high peer bandwidth utilization with low Hybrid Neighbor selection (IWSHN), which maintenance cost. Moraes et al. [21] proposes a lifetime- utilizes a novel hybrid neighbor selection strategy based neighbor selection mechanism, where each peer and models the tradeoffs among the server load, chooses other peers with close lifetimes as its neighbors inter-ISP traffic and peer prefetching as a multi- to increase the probability of finding chunks of interest. objective optimization problem. The objective Wen et al. [22] propose an efficient request peer selection function of the optimization problem is the mechanism called CPF (Closest Playback-point First), in weighted sum of the above three design goals. which each peer sends the download request to the 3) We conduct simulation studies on our centralized neighbor that has the smallest playback-point difference and distributed algorithms, and compare them with itself. A similar request peer selection mechanism with traditional and state-of-the-art approaches. called LPP (Least Loaded Peer) is proposed in [23]. In The simulation results confirm the effectiveness this method, each peer sends the download request to the of our proposed algorithms. neighbor with the shortest queen size, among those that The rest of this paper is organized as follows. In have the needed chunk. However, the above four studies Section 2, we survey the related works and present a do not take inter-ISP traffic into consideration. preliminary description of network flow theory. Then, we Among others, the studies by Yu et al. [24] and Wang present the centralized rate allocation algorithms in et al. [25] are closely related to ours. In [24], the authors Section 3 and the distributed algorithm in Section 4. In propose a centralized rate allocation framework for P2P Section 5, illustrative simulation results are presented. VoD systems to achieve server stress minimization when Finally, the paper is summarized in Section 6. the playback continuities of users are guaranteed. Their framework is also based on the network flow theory and II. RELATED WORK AND PRELIMINARIES the maximum network flow problem. However, the centralized algorithm that they propose in the study aims A. Related Work only at minimizing server load, whereas our SFO and In recent years, ISP-friendly P2P applications have LFO algorithms are proposed to achieve the three design attracted lots of research activities. Aggarwal et al. [8] goals mentioned in Section 1. propose a cooperative solution where ISPs help P2P In [25], the authors propose a distributed rate systems by offering a service called “oracle”.