We present a novel protocol for restructuring a tree- based overlay network in response to the workload of the application running over it. Through low-cost restructuring operations, our protocol incrementally adapts the tree so as to bring nodes that tend to communicate with one another closer together in the tree. It achieves this while respecting degree bounds on nodes so that, e.g., no node degenerates into a "hub" for the overlay. Moreover, it limits restructuring to those parts of the tree over which communication takes place, avoiding restructuring other parts of the tree unnecessarily. We show via experiments on PlanetLab that our protocol can significantly reduce communication latencies in workloads dominated by clusters of communicating nodes.

We present Quiver, a system that coordinates service proxies placed at the "edge" of the Internet to serve distributed clients accessing a service involving mutable objects. Quiver enables these proxies to perform consistent accesses to shared objects by migrating the objects to proxies performing operations on those objects. These migrations dramatically improve performance when operations involving an object exhibit geographic locality, since migrating this object into the vicinity of proxies hosting these operations will benefit all such operations. Other workloads benefit from Quiver, dispersing the computation load across the proxies and saving the costs of sending operation parameters over the wide area when these are large. Quiver also supports optimizations for single-object reads that do not involve migrating the object. We detail the protocols for implementing object operations and for accommodating the addition, involuntary disconnection, and voluntary departure of proxies. We also evaluate Quiver through experiments on PlanetLab. Finally, we discuss the use of Quiver to build an e-commerce application and a distributed network traffic modeling service.

We present an algorithm by which nodes arranged in a tree, with each node initially knowing only its parent and children, can construct a fault-tolerant communication structure (an expander graph) among themselves in a distributed and scalable way. The tree overlayed with this logical expander is a useful structure for distributed applications that require the intrinsic "treeness" from the topology but cannot afford any obstruction in communication due to failures. At the core of our construction is a novel distributed mechanism that samples nodes uniformly at random from the tree. In the event of node joins, node departures or node failures, the expander maintains its own fault tolerance and permits the reformation of the tree. We present simulation results to quantify the convergence of our algorithm to a fault tolerant network having both good vertex connectivity and expansion properties.

A correctly working domain name system (DNS) is essential for the Internet. Due to its significance and because of deficiencies in its current design, the DNS is vulnerable to a wide range of attacks. This paper presents the design and implementation of a secure distributed name service on the level of a DNS zone. Our service is able to provide fault tolerance and security even in the presence of a fraction of corrupted name servers, avoiding any single point of failure. It further solves the problem of storing zone secrets online without leaking them to a corrupted server, while still supporting secure dynamic updates. Our service uses state-machine replication and threshold cryptography. We present results from experiments performed using a prototype implementation on the Internet in realistic setups. The results show that our design achieves the required assurances while servicing the most frequent requests in reasonable time.

A capture protection server protects a cryptographic key on a device that may be captured by authenticating the user of the device (e.g., by password) before permitting the key to be used. Delegation from one capture protection server to another enables the new server to perform this capture protection function for the device. Delegation, however, opens the system to new vulnerabilities, including difficulties in limiting online password-guessing attacks and in disabling a device that has been stolen by an attacker who knows the password. Here we propose a lightweight protocol for coordinating capture protection servers that eliminates these vulnerabilities. We also report on the implementation of our protocol in a JCA-compliant cryptographic service provider, and ramifications of the JCA interfaces for our approach.

Data Grids are currently proposed solutions to large scale data management problems including efficient file transfer and replication. Large amounts of data and the world-wide distribution of data stores contribute to the complexity of the data management challenge. Recent architecture proposals and prototypes deal with replication of read-only files but do not address the replica synchronisation problem. We propose a new Grid service, called Grid Consistency Service (GCS), that sits on top of existing Data Grid services and allows for replica update synchronisation and consistency maintenance. We give models for different levels of consistency provided to the Grid user and discuss how they can be included into a replica consistency service for a Data Grid. 1

Data replication is a key issue in a data grid and can be managed in different ways and at different levels of granularity: for example, at the file level or the object level. In the high-energy physics community, data grids are being developed to support the distributed analysis of experimental data. We have produced a prototype data replication tool, the Grid Data Management Pilot (GDMP) that is in production use in one physics experiment, with middleware provided by the Globus toolkit used for authentication, data movement and other purposes. We present a new, enhanced GDMP architecture and prototype implementation that uses Globus data-grid tools for efficient file replication. We also explain how this architecture can address object replication issues in an object-oriented database management system. File transfer over wide-area networks requires specific performance tuning in order to gain optimal data transfer rates. We present performance results obtained with GridFTP, an enhanced version of FTP, and discuss tuning parameters.