Sirio Scipioni

Coupling-Based Internal Clock Synchronization for Large-Scale Dynamic Distributed Systems

This paper studies the problem of realizing a common software clock among a large set of nodes without an external time reference (i.e., internal clock synchronization), any centralized control, and where nodes can join and leave the distributed system at their will. The paper proposes an internal clock synchronization algorithm which combines the gossip-based paradigm with a nature-inspired approach, coming from the coupled oscillators phenomenon, to cope with scale and churn. The algorithm works on the top of an overlay network and uses a uniform peer sampling service to fulfill each nodes local view. Therefore, differently from clock synchronization protocols for small scale and static distributed systems, here, each node synchronizes regularly with only the neighbors in its local view and not with the whole system. An evaluation of the convergence speed and the synchronization error of the coupled-based internal clock synchronization algorithm has been carried out, showing how convergence time and the synchronization error depends on the coupling factor and the local view size. Moreover, the variation of the synchronization error with respect to churn and the impact of a sudden variation of the number of nodes have been analyzed to show the stability of the algorithm. In all these contexts, the algorithm shows nice performance and very good self-organizing properties. Finally, we showed how the assumption on the existence of a uniform peer-sampling service is instrumental for the good behavior of the algorithm and how, in system models where network delays are unbounded, a mean-based convergence function reaches a lower synchronization error than median-based convergence functions exploiting the number of averaged clock values.

An adaptive coupling-based algorithm for internal clock synchronization of large scale dynamic systems

This paper proposes an internal clock synchronization algorithm which combines the gossip-based paradigm with a nature-inspired approach coming from the coupled oscillators phenomenon. The proposed solution allows a very large number of clocks to self-synchronize without any central control, despite node departure and arrival. This addresses the needs of an emergent class of large-scale peer-to-peer applications that have to operate without any assumptions on the underlying infrastructure. Empirical evaluation shows extremely good convergence and stability under different network settings.

Practical Uniform Peer Sampling under Churn

Providing independent uniform samples from a system population poses considerable problems in highly dynamic settings, like P2P systems, where the number of participants and their unpredictable behavior (e.g., churn, crashes etc.) may introduce relevant bias. Current implementations of the Peer Sampling Service are designed to provide uniform samples only in static settings and do not consider that biased samples can directly affect the correctness of algorithms relying on a uniformity property or be exploited by a malicious adversary to increase the effectiveness of its attacks to the system. In this paper we provide a practical solution to the biasing problem by deploying a fully distributed Peer Sampling Correction Module on top of a given, possibly biased, peer sampling service. Samples provided by the peer sampling service will be locally processed by this module, using computationally efficient hashing functions, before getting to the application. The effectiveness of our approach is evaluated through an extensive simulation-based study.

Churn resilience of peer-to-peer group membership: a performance analysis

Partitioning is one of the main problems in p2p group membership. This problem rises when failures and dynamics of peer participation, or churn, occur in the overlay topology created by a group membership protocol connecting the group of peers. Solutions based on Gossip-based Group Membership (GGM) cope well with the failures while suffer from network dynamics. This paper shows a performance evaluation of SCAMP, one of the most interesting GGM protocol. The analysis points out that the probability of partitioning of the overlay topology created by SCAMP increases with the churn rate. We also compare SCAMP with DET – another membership protocol that deterministically avoids partitions of the overlay. The comparison points out an interesting trade-off between (i) reliability, in terms of guaranteeing overlay connectivity at any churn rate, and (ii) scalability in terms of creating scalable overlay topologies where latencies experienced by a peer during join and leave operations do not increase linearly with the number of peers in the group.

A Peer-to-Peer Filter-Based Algorithm for Internal Clock Synchronization in Presence of Corrupted Processes

This paper proposes an internal clock synchronization algorithm for very large number of processes that is able to (i) self-synchronize their local clocks without any central control and (ii) resist to attacks of an adversary whose aim is to put out-of-synchronization as many correct processes as possible. To cope with scale the algorithm utilizes the gossip-based paradigm where each process has a limited view of the system, while to resist to attacks the algorithm employs a filtering mechanism based on the notion of ¿-trimmed mean to filter out out-of-range clock values. The algorithm shows nice convergence in presence of networks errors and in absence of the adversary. When the adversary takes control of some of the processes in the system, we define two goals for the adversary, actually two predicates, to measure the strength of the attack. The first one captures the percentage of time in which at least one correct is out of synchronization and the second one when all correct processes are out of synchronization. The paper presents an extensive simulation study showing under which conditions (in terms of number of corrupted processes and size of local views) these two goals can be achieved by the adversary. Interestingly, these results can be exploited by applications that can tolerate either a certain time in which some correct process is non-synchronized or a certain percentage of correct processes that is non-synchronized.

Event-Based Data Dissemination on Inter-Administrative Domains: Is it Viable?

Middleware for timely and reliable data dissemination is a fundamental building block of the event driven architecture (EDA), an ideal platform for developing air traffic control, defense systems, etc. Many of these middlewares are compliant to the data distribution service (DDS) specification and they have been traditionally designed to be deployed on managed environments where they show predictable behaviors. However, the enterprise setting can be unmanaged and characterized by geographic inter-domain scale and heterogeneous resources. In this paper we present a study aimed at assessing the strengths and weaknesses of a commercial DDS implementation deployed on an unmanaged setting. Our experiments campaign outlines that, if the application manages a small number of homogeneous resources, this middleware perform timely and reliably. In a more general setting with fragmentation and heterogeneous resources, reliability and timeliness rapidly degenerate pointing out a need of research in self-configuring scalable event dissemination with QoS guarantee on unmanaged settings.

Communication Channel Management for Maintenance of Strong Overlay Connectivity

A fundamental problem for both structured and unstructured peer-to-peer networks is how to maintain connected the topology of a network in the presence of processes that, possibly concurrently, join and leave the network. In this paper we firstly define a model of the computation well-suited to analyze connectivity maintenance among processes carrying out a distributed computation considering unbounded concurrency and infinite participation. Secondly upon this model we provide a specification of the connectivity maintenance problem. We finally present a protocol that guarantees connectivity maintenance by arranging processes of the computation on a tree. The protocol handles both joins and leaves concurrently and actively (i.e., some piece of code is executed by a leaving/joining process interacting with its neighbors in the topology).

A theoretical evaluation of peer-to-peer internal clock synchronization

Synchronized clocks are usually considered as a prerequisite for many distributed applications. Existing solutions mainly deal with this problem in static environments with well defined characteristics and limits. The needs of an emergent class of large-scale peer-to-peer applications that have to operate without any assumptions on the surrounding environment have recently revitalized this research area with the proposals of new solutions characterized by self-organization capabilities and strong adaptability to dynamic settings. This paper reports about the properties of a clock synchronization algorithm for large scale applications. The algorithm implements an internal clock synchronization mechanism which combines the gossip-based paradigm with a nature-inspired approach coming from the coupled oscillators phenomenon. Using a theoretical approach, the paper focuses on the convergence properties of the algorithm, characterizing its synchronization speed (decay factor) the final synchronization point and error.

Multidisciplinary models and guidelines for eprocurement projects: the ereadiness phase

This paper presents a multidisciplinary methodology for developing public eProcurement (eProc.) projects in countries whose economy are either emerging, developing or in transition. The focus is on the first part of the methodology, namely eReadiness. Our aim is to bring out a number of issues, which are currently challenging many eGovemment projects through an integrated approach by encompassing sociological, economic, organizational, legal and technological factors.