Yang-hua Chu

A case for end system multicast (keynote address)

The conventional wisdom has been that IP is the natural protocol layer for implementing multicast related functionality. However, ten years after its initial proposal, IP Multicast is still plagued with concerns pertaining to scalability, network management, deployment and support for higher layer functionality such as error, flow and congestion control. In this paper, we explore an alternative architecture for small and sparse groups, where end systems implement all multicast related functionality including membership management and packet replication. We call such a scheme End System Multicast. This shifting of multicast support from routers to end systems has the potential to address most problems associated with IP Multicast. However, the key concern is the performance penalty associated with such a model. In particular, End System Multicast introduces duplicate packets on physical links and incurs larger end-to-end delay than IP Multicast. In this paper, we study this question in the context of the Narada protocol. In Narada, end systems self-organize into an overlay structure using a fully distributed protocol. In addition, Narada attempts to optimize the efficiency of the overlay based on end-to-end measurements. We present details of Narada and evaluate it using both simulation and Internet experiments. Preliminary results are encouraging. In most simulations and Internet experiments, the delay and bandwidth penalty are low. We believe the potential benefits of repartitioning multicast functionality between end systems and routers significantly outweigh the performance penalty incurred.

Enabling conferencing applications on the internet using an overlay muilticast architecture

In response to the serious scalability and deployment concerns with IP Multicast, we and other researchers have advocated an alternate architecture for supporting group communication applications over the Internet where all multicast functionality is pushed to the edge. We refer to such an architecture as End System Multicast. While End System Multicast has several potential advantages, a key concern is the performance penalty associated with such a design. While preliminary simulation results conducted in static environments are promising, they have yet to consider the challenging performance requirements of real world applications in a dynamic and heterogeneous Internet environment.In this paper, we explore how Internet environments and application requirements can influence End System Multicast design. We explore these issues in the context of audio and video conferencing: an important class of applications with stringent performance requirements. We conduct an extensive evaluation study of schemes for constructing overlay networks on a wide-area test-bed of about twenty hosts distributed around the Internet. Our results demonstrate that it is important to adapt to both latency and bandwidth while constructing overlays optimized for conferencing applications. Further, when relatively simple techniques are incorporated into current self-organizing protocols to enable dynamic adaptation to latency and bandwidth, the performance benefits are significant. Our results indicate that End System Multicast is a promising architecture for enabling performance-demanding conferencing applications in a dynamic and heterogeneous Internet environment.

Measurement-based optimization techniques for bandwidth-demanding peer-to-peer systems

Measurement-based optimization is one important strategy to improve the performance of bandwidth-demanding peer-to-peer systems. However, to date, we have little quantitative knowledge of how well basic lightweight measurement-based techniques such as RTT probing, 10KB TCP probing, and bottleneck bandwidth probing may work in practice in the peer-to-peer environment. By conducting trace-based analyses, we find that the basic techniques can help achieve 40 to 50% optimal performance. To deepen our understanding, we analyze some of the intrinsic properties of these techniques. Our analyses reveal the inherent difficulty of the peer selection problem due to the extreme heterogeneity in the peer-to-peer environment, and that the basic techniques are limited because their primary strength lies in eliminating the low-performance peers rather than reliably identifying the best-performing one. However, our analyses also reveal two key insights that can potentially be exploited by applications. First, for adaptive applications that can continuously change communication peers, the basic techniques are highly effective in guiding the adaption process. In our experiments, typically an 80% optimal peer can be found by trying less than 5 candidates. Secondly, we find that the basic techniques are highly complementary and can potentially be combined to better identify a high-performance peer, thus even applications that cannot adapt may benefit. Using media file sharing and overlay multicast streaming as case studies, we have systematically experimented with several simple combined peer selection techniques. Our results show that for the nonadaptive media file sharing application, a simple combined technique can boost performance to 60% optimal. In contrast, for the continuously adaptive overlay multicast application, we find that a basic technique with even low-fidelity network information is sufficient to ensure good performance. We believe our findings will help guide the future designs of high-performance peer-to-peer systems.

A case for taxation in peer-to-peer streaming broadcast

Most existing research on peer-to-peer (p2p) has been on file sharing applications. In this paper, we focus on p2p streaming applications. In particular, we argue that the Bit-for-Bit model, widely adopted in p2p file sharing, is not applicable in p2p streaming. In p2p streaming, the bottleneck resource is the upstream bandwidth capacity. Our empirical experience with p2p streaming indicates that a large percent of peers on the Internet have limited upstream bandwidth capacity, and the Bit-for-Bit model severely limits the amount of bandwidth these resource-poor peers can receive. To address this issue, we propose a taxation model.In the taxation model, resource-rich peers contribute more bandwidth to the system, and subsidize for the resource-poor peers. This redistribution of wealth improves social welfare. Such a model is applicable in the streaming context because the publisher of the video stream has the means to enforce taxation on peers and the will to maximize their collective social welfare. We design a simple linear taxation scheme and incorporate it in a distributed streaming protocol. Our simulation results indicate that taxation can significantly improve social welfare without incurring a significant overhead to the system.

Considering altruism in peer-to-peer internet streaming broadcast

In peer-to-peer overlay or video broadcast, peers contribute a portion of the bandwidth to the overlay in return or the service. In the presence of network heterogeneity, it is not well understood how much bandwidth peers should contribute and receive in return. Existing protocols implicitly assume peers are either completely altruistic (which leads to airness concerns) or completely selfish (which leads to sub-optimal performance). In this paper, we argue that altruism should be explicitly considered. We propose a policy framework in which a wide range of altruism can be modeled and parameterized. The key findings are (i) the level of altruism has significant implication on the overall performance of the receivers; even a small degree of altruism goes a long way in improving their performance, and (ii) a wide range of altruism policy can be implemented efficiently in a distributed fashion. We validate these claims using simulation, with traces from real Internet broadcast events.