Sanjay G. Rao

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.

Opportunities and Challenges of Peer-to-Peer Internet Video Broadcast

There have been tremendous efforts and many technical innovations in supporting real-time video streaming in the past two decades, but cost-effective large-scale video broadcast has remained an elusive goal. Internet protocol (IP) multicast represented an earlier attempt to tackle this problem but failed largely due to concerns regarding scalability, deployment, and support for higher level functionality. Recently, peer-to-peer based broadcast has emerged as a promising technique, which has been shown to be cost effective and easy to deploy. This new paradigm brings a number of unique advantages such as scalability, resilience, and effectiveness in coping with dynamics and heterogeneity. While peer-to-peer applications such as file download and voice-over-IP have gained tremendous popularity, video broadcast is still in its early stages, and its full potential remains to be seen. This paper reviews the state-of-the-art of peer-to-peer Internet video broadcast technologies. We describe the basic taxonomy of peer-to-peer broadcast and summarize the major issues associated with the design of broadcast overlays. We closely examine two approaches - tree-based and data-driven - and discuss their fundamental tradeoff and potential for large-scale deployment. Lastly, we outline the key challenges and open problems and highlight possible avenues for future directions.

Cloudward bound: planning for beneficial migration of enterprise applications to the cloud

In this paper, we tackle challenges in migrating enterprise services into hybrid cloud-based deployments, where enterprise operations are partly hosted on-premise and partly in the cloud. Such hybrid architectures enable enterprises to benefit from cloud-based architectures, while honoring application performance requirements, and privacy restrictions on what services may be migrated to the cloud. We make several contributions. First, we highlight the complexity inherent in enterprise applications today in terms of their multi-tiered nature, large number of application components, and interdependencies. Second, we have developed a model to explore the benefits of a hybrid migration approach. Our model takes into account enterprise-specific constraints, cost savings, and increased transaction delays and wide-area communication costs that may result from the migration. Evaluations based on real enterprise applications and Azure-based cloud deployments show the benefits of a hybrid migration approach, and the importance of planning which components to migrate. Third, we shed insight on security policies associated with enterprise applications in data centers. We articulate the importance of ensuring assurable reconfiguration of security policies as enterprise applications are migrated to the cloud. We present algorithms to achieve this goal, and demonstrate their efficacy on realistic migration scenarios.

Dissecting Video Server Selection Strategies in the YouTube CDN

In this paper, we conduct a detailed study of the YouTube CDN with a view to understanding the mechanisms and policies used to determine which data centers users download video from. Our analysis is conducted using week-long datasets simultaneously collected from the edge of five networks - two university campuses and three ISP networks - located in three different countries. We employ state-of-the-art delay-based geolocation techniques to find the geographical location of YouTube servers. A unique aspect of our work is that we perform our analysis on groups of related YouTube flows. This enables us to infer key aspects of the system design that would be difficult to glean by considering individual flows in isolation. Our results reveal that while the RTT between users and data centers plays a role in the video server selection process, a variety of other factors may influence this selection including load-balancing, diurnal effects, variations across DNS servers within a network, limited availability of rarely accessed video, and the need to alleviate hot-spots that may arise due to popular video content.

Mercury: a scalable publish-subscribe system for internet games

Todays network games suffer from scalability and performance limitations caused by centralized client-server architectures and/or broadcast communication. In this paper, we argue that the communication between components of a game can be modeled as a publish-subscribe system. We present the design of MERCURY, a completely distributed publish-subscribe system, which supports a content-based publish-subscribe model of communication and performs distributed matching using a novel content-based routing protocol. We also present preliminary simulation results identifying key design decisions affecting the scalability and network efficiency of the system.

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.

Enabling contribution awareness in an overlay broadcasting system

We consider the design of bandwidth-demanding broadcasting applications using overlays in environments characterized by hosts with limited and asymmetric bandwidth, and significant heterogeneity in outgoing bandwidth. Such environments are critical to consider to extend the applicability of overlay multicast to mainstream Internet environments where insufficient bandwidth exists to support all hosts, but have not received adequate attention from the research community. We leverage the multi-tree framework and design heuristics to enable it to consider host contribution and operate in bandwidth-scarce environments. Our extensions seek to simultaneously achieve good utilization of system resources, performance to hosts commensurate to their contributions, and consistent performance. We have implemented the system and conducted an Internet evaluation on Planet-Lab using real traces from previous operational deployments of an overlay broadcasting system. Our results indicate for these traces, our heuristics can improve the performance of high contributors by 10-240% and facilitate equitable bandwidth distribution among hosts with similar contributions.

Configuration management at massive scale: system design and experience

The development and maintenance of network device configurations is one of the central challenges faced by large network providers. Current network management systems fail to meet this challenge primarily because of their inability to adapt to rapidly evolving customer and provider-network needs, and because of mismatches between the conceptual models of the tools and the services they must support. In this paper, we present the Presto configuration management system that attempts to address these failings in a comprehensive and flexible way. Developed for and used during the last 5 years within a large ISP network, Presto constructs device-native configurations based on the composition of configlets representing different services or service options. Configlets are compiled by extracting and manipulating data from external systems as directed by the Presto configuration scripting and template language. We outline the configuration management needs of large-scale network providers, introduce the PRESTO system and configuration language, and reflect upon our experiences developing PRESTO configured VPN and VoIP services. In doing so, we describe how PRESTO promotes healthy configuration management practices.

The impact of heterogeneous bandwidth constraints on DHT-Based multicast protocols

In this paper, we consider support for bandwidth-demanding applications such as video broadcasting using DHTs. Our investigations focus on the impact of heterogeneity in the outgoing bandwidth capabilities of nodes on Scribe, a representative and relatively mature DHT-based multicast protocol. We expose important issues that arise due to the mismatch between the ID space that underlies the DHT and the outgoing bandwidth constraints on nodes.