Overcast is an application-level multicasting system that can be incrementally deployed using today's Internet infrastructure. These properties stem from Overcast's implementation as an overlay network. An overlay network consists of a collection of nodes placed at strategic locations in an existing network fabric. These nodes implement a network abstraction on top of the network provided by the underlying substrate network.

In this paper, we propose a new paradigm for network file system design, serverless network file systems. While traditional network file systems rely on a central server machine, a serverless system utilizes workstations cooperating as peers to provide all file system services. Any machine in the system can store, cache, or control any block of data. Our approach uses this location independence, in combination with fast local area networks, to provide better performance and scalability than traditional file systems. Further, because any machine in the system can assume the responsibilities of a failed component, our serverless design also provides high availability via redundant data storage. To demonstrate our approach, we have implemented a prototype serverless network file system called xFS. Preliminary performance measurements suggest that our architecture achieves its goal of scalability. For instance, in a 32-node xFS system with 32 active clients, each client receives nearly as much read or write throughput as it would see if it were the only active client.

Emerging high-speed networks will allow machines to access remote data nearly as quickly as they can access local data. This trend motivates the use of cooperative caching: coordinating the file caches of many machines distributed on a LAN to form a more effective overall file cache. In this paper we examine four cooperative caching algorithms using a trace-driven simulation study. These simulations indicate that for the systems studied cooperative caching can halve the number of disk accesses, improving file system read response time by as much as 73%. Based on these simulations we conclude that cooperative caching can significantly improve file system read response time and that relatively simple cooperative caching algorithms are sufficient to realize most of the potential performance gain.

Most wide-area caching schemes are client initiated. Decisions on when and where to cache information are made without the benefit of the server's global knowledge of the situation. We believe that the server should play a role in making these caching decisions, and we propose geographical push-caching as a way of bringing the server back into the loop. The World Wide Web is an excellent example of a widearea system that will benefit from geographical pushcaching, and we present an architecture that allows a Web server to autonomously replicate HTML pages. 1 Introduction The World-Wide Web [1] operates for the most part as a cache-less distributed system. When two neighboring clients retrieve a document from the same server, the document is sent twice. This is inefficient, especially considering the ease with which Web browsers allow users to transfer large multimedia documents. To combat this problem, some Web browsers have begun to add local client caches. These prevent one client...

As the Web continues to explode in size, caching becomes increasingly important. With caching comes the problem of cache consistency. Conventional wisdom holds that strong cache consistency is too expensive for the Web, and weak consistency methods such as Time-To-Live (TTL) are most appropriate. This study compares three consistency approaches: adaptive TTL, polling-every-time and invalidation, through analysis, implementation and trace replay in a simulated environment. Our analysis shows that weak consistency methods save network bandwidth mostly at the expense of returning stale documents to users. Our experiments show that invalidation generates a comparable amount of network traffic and server workload to adaptive TTL and has similar average client response times, while polling-every-time results in more control messages, higher server workload and longer client response times. We show that, contrary to popular belief, strong cache consistency can be maintained for the Web with ...

The bandwidth demands of the World Wide Web continue to grow at a hyper-exponential rate. Given this rocketing growth, caching of web objects as a means to reduce network bandwidth consumption is likely to be a necessity in the very near future. Unfortunately, many Web caches do not satisfactorily maintain cache consistency. This paper presents a survey of contemporary cache consistency mechanisms in use on the Internet today and examines recent research in Web cache consistency. Using trace-driven simulation, we show that a weak cache consistency protocol (the one used in the Alex ftp cache) reduces network bandwidth consumption and server load more than either time-to-live fields or an invalidation protocol and can be tuned to return stale data less than 5% of the time.

Abstract In this paper, we examine several distributed caching strategies to improve the response time for accessing data over theInternet. By studying several Internet caches and workloads, we derive four basic design principles for large scale distributed

In this paper, we describe the design and implementation of an integrated architecture for cache systems that scale to hundreds or thousands of caches with thousands to millions of users. Rather than simply try to maximize hit rates, we take an end-to-end approach to improving response time by also considering hit times and miss times. We begin by studying several Internet caches and workloads, and we derive three core design principles for large scale distributed caches: (1) minimize the number of hops to locate and access data on both hits and misses, (2) share data among many users and scale to many caches, and (3) cache data close to clients. Our strategies for addressing these issues are built around a scalable, high-performance data-location service that tracks where objects are replicated. We describe how to construct such a service and how to use this service to provide direct access to remote data and push-based data replication. We evaluate our system through trace-driven simulation and find that these strategies together provide response time speedups of 1.27 to 2.43 compared to a traditional three-level cache hierarchy for a range of trace workloads and simulated environments. 1.

The rapid increase in web usage has led to dramatically increased loads on the network infrastructure and on individual web servers. To ameliorate these mounting burdens, there has been much recent interest in web caching architectures and algorithms. Web caching reduces network load, server load, and the latency of responses. However, web caching has the disadvantage that the pages returned to clients by caches may be stale, in that they may not be consistent with the version currently on the server. In this paper we describe a scalable web cache consistency architecture that provides fairly tight bounds on the staleness of pages. Our architecture borrows heavily from the literature, and can best be described as an invalidation approach made scalable by using a caching hierarchy and application-level multicast routing to convey the invalidations. We evaluate this design with calculations and simulations, and compare it to several other approaches. 1 Introduction The world-wide-web h...

Current network file system protocols rely heavily on a central server to coordinate tile activity among client workstations. This central server can become a bottleneck that limits scalabdity for environments with large numbers of clients. In central server systems such as NFS and AFS, all chent writes, cache misses, and coherence messages are handled by the server. To keep up with this workload, expensive server machines are needed, configured with high-performance CPUS, memory systems, and 1/0 channels. Since the server stores all data, tt must be physically capable of connecting to many disks. This reliance on a central server also makes current systems inappropriate for wide area network use where the network bandwidth to the server may be Ilmited. In this paper, we mvestlgate the quantitative performance effect of movmg as many of the server responsibilities as possible to client workstations to reduce the need for high-performance server machines. We have devised a cache protocol in which all data reside on clients and all data transfers proceed directly from client to client. The server is used only to coordinate these data transfers. Thm protocol]s being incorporated as part of our experi-mental file system, xFS. We present results from a trace-driven simulation study of the protocol using traces from a 237 client NFS installation. We find that the xFS protocol reduces server load by more than a factor of six compared to AFS without significantly affecting response time or file availability.