Users are storing ever-increasing amounts of information digitally, driven by many factors including government regulations and the public’s desire to digitally record their personal histories. Unfortunately, many of the security mechanisms that modern systems rely upon, such as encryption, are poorly suited for storing data for indefinitely long periods of time—it is very difficult to manage keys and update cryptosystems to provide secrecy through encryption over periods of decades. Worse, an adversary who can compromise an archive need only wait for cryptanalysis techniques to catch up to the encryption algorithm used at the time of the compromise in order to obtain “secure ” data. To address these concerns, we have developed POT-SHARDS, an archival storage system that provides longterm security for data with very long lifetimes without using encryption. Secrecy is achieved by using provably secure secret splitting and spreading the resulting shares across separately-managed archives. Providing availability and data recovery in such a system can be difficult; thus, we use a new technique, approximate pointers, in conjunction with secure distributed RAID techniques to provide availability and reliability across independent archives. To validate our design, we developed a prototype POTSHARDS implementation, which has demonstrated “normal ” storage and retrieval of user data using indexes, the recovery of user data using only the pieces a user has stored across the archives and the reconstruction of an entire failed archive. 1

Social networks offering unprecedented content sharing are rapidly developing over the Internet. Unfortunately, it is often difficult to both locate and manage content in these networks, particularly when they are implemented on current peer-to-peer technologies. In this paper, we describe Wayfinder, a peer-to-peer file system that targets the needs of medium-sized content sharing communities. Wayfinder seeks to advance the state-of-the-art by providing three synergistic abstractions: a global namespace that is uniformly accessible across connected and disconnected operation, content-based queries that can be persistently embedded into the global namespace, and automatic availability management. Interestingly, Wayfinder achieves much of its functionality through the use of a peer-to-peer indexed data storage system called PlanetP: essentially, Wayfinder constructs the global namespace, locates specific files, and performs content searches by posing appropriate queries to PlanetP. We describe this query-based design and present preliminary performance measurements of a prototype implementation.

In this paper, we present Mobile Tapestry, an extension to the Tapestry overlay network protocol, that enables scalable, fault-tolerant, and timely delivery of network messages, including multimedia streams, to and from rapidly moving nodes. Mobile Tapestry efficiently supports individual mobile nodes and, by using an approach we call hierarchical mobility, it also supports large groups of mobile nodes simultaneously moving together. Mobile Tapestry leverages the Tapestry's locality mechanisms to reduce the latency and bandwidth of mobility update traffic, while eliminating the routing inefficiencies and availability problems of IP-based mobility protocols, such as Mobile IP. Our

WheelFS is a wide-area distributed storage system intended to help multi-site applications share data and gain fault tolerance. WheelFS takes the form of a distributed file system with a familiar POSIX interface. Its design allows applications to adjust the tradeoff between prompt visibility of updates from other sites and the ability for sites to operate independently despite failures and long delays. WheelFS allows these adjustments via semantic cues, which provide application control over consistency, failure handling, and file and replica placement. WheelFS is implemented as a user-level file system and is deployed on PlanetLab and Emulab. Three applications (a distributed Web cache, an email service and large file distribution) demonstrate that WheelFS’s file system interface simplifies construction of distributed applications by allowing reuse of existing software. These applications would perform poorly with the strict semantics implied by a traditional file system interface, but by providing cues to WheelFS they are able to achieve good performance. Measurements show that applications built on WheelFS deliver comparable performance to services such as CoralCDN and BitTorrent that use specialized wide-area storage systems. 1

Reputation mechanisms help peers in a peer-to-peer (P2P) system avoid unreliable or malicious peers. In application-level networks, however, short peer life-times mean reputations are often generated from a small number of past transactions. These reputation values are less “reliable, ” and more vulnerable to bad-mouthing or collusion attacks. We address this issue by introducing proactive reputations, a first-hand history of transactions initiated to augment incomplete or short-term reputation values. We present several mechanisms for generating proactive reputations, along with a statistical similarity metric to measure their effectiveness. 1.

Wide-area distributed applications often reinvent the wheel for their storage needs, each incorporating its own special-purpose storage manager to cope with distribution, intermittent failures, limited bandwidth, and high latencies. This paper argues that a distributed file system could provide a reusable solution to these problems by coupling a standard interface with a design suited to widearea distribution. For concreteness, this paper presents such a file system, called WheelFS, which allows applications to control consistency through the use of semantic cues, and minimizes communication costs by adhering to the slogan read globally, write locally. WheelFS could simplify distributed experiments, CDNs, and Grid applications.

In this paper, we study the viability of a peer-to-peer backup system on nowadays internet connections. In particular, we show that peer lifetime estimation can be used to reduce the maintenance cost of peer-to-peer backup. Previous studies [5] have shown that lifetimes in a peer-to-peer system follow a Pareto distribution. Consequently, peers can be sorted on their expected lifetimes, depending only on the length of their history in the system. By carefully selecting the peers on which backup data is stored, repairing cost can be highly reduced for long-term backup users, while it is still acceptable for new users. The efficiency of this technique is evaluated through simulations of a state-of-the-art peer-topeer backup system. Therefore, our long term objective is to provide a new alternative backup system, free, efficient, reliable, yet still easyto-use, to help people protect data on their personal computers. Our system would be based on the exchange of free disk space between its members. To lower the cost and to bring a high resilience to faults, it has to be decentralized and to work in a peer-to-peer (P2P) way. In the literature, there are already some P2P backup systems. But most of them are still in prototype phase and has not yet been tested in acceptable conditions. The other ones are too restrictive and do not offer the conditions we want. Before building a complete system, we want to simulate such systems in order to know what their viability is, based on common properties of already designed systems. 1.

by Yanbin Zhao

Abstract—In this paper, we investigate the roles of replication vs. repair to achieve durability in large-scale distributed storage systems. Specifically, we address the fundamental questions: How does the lifetime of an object depend on the degree of replication and rate of repair, and how is lifetime maximized when there is a constraint on resources? In addition, in real systems, when a node becomes unavailable, there is uncertainty whether this is temporary or permanent; we analyze the use of timeouts as a mechanism to make this determination. Finally, we explore the importance of memory in repair mechanisms, and show that under certain cost conditions, memoryless systems, which are inherently less complex, perform just as well. I.

Many wide-area storage systems replicate data for durability. A common way of maintaining the replicas is to detect node failures and respond by creating additional copies of objects that were stored on failed nodes and hence suffered a loss of redundancy. Reactive techniques can minimize total bytes sent since they only create replicas as needed; however, they can create spikes in network use after a failure. These spikes may overwhelm application traffic and can make it difficult to provision bandwidth.