Presence Of Churn Research Articles

Wide-Scale Internet Disconnection: Impact and Recovery on Social-Based P2P Overlays

In the recent years, there have been situations where wide-scale network fraction of Internet users have been disconnected from the rest of the network, due to natural hazards or national censorships (e.g., a government cuts connectivity to the outside world as a mechanism for suppression of uprisings). Peer-to-peer systems are known to be resilient in the presence of churn and uncorrelated failures. However, their behavior in extreme scenarios, where massive correlated failures occur, is not well-studied. In this paper, we consider nodes within a relatively small geographical region of the network (2.5 percent or fewer Internet users), and study whether users can communicate with their ( $n$ -hop away) social neighbors in a peer-to-peer fashion after network partitions. The contribution of our work is twofold: 1) we study the effect of such partitions on topology and routing of structured and social-based unstructured P2P overlays, including a newly proposed Social-aware overlay; 2) for the overlay with better routability observed from the first step, we propose and evaluate a social-based bootstrapping method for nodes to join the overlay, under the constraints imposed by firewalls and NATs. In our analysis, we consider both real and synthetic datasets of online social networks. The results of our evaluation show that structured P2P overlay routability is severely hampered by wide-scale partition events. In addition, the proposed social-based unstructured overlay network provides improved routability while maintaining a smaller number of links. Furthermore, in the evaluation of bootstrapping, by varying several input parameters, from 90 to 97 percent of nodes in Social-aware overlay could bootstrap and reach their direct-neighbors.

A comparative study of spanning tree and gossip protocols for aggregation

SummaryDistributed aggregation queries like average and sum can be implemented in different paradigms like gossip and hierarchical approaches. In the literature, these two paradigms are routinely associated with stereotypes such as ‘trees are fragile and complicated’ and ‘gossip is slow and expensive’. However, a closer look reveals that these statements are not backed up by systematic studies. A fair and informative comparison is clearly needed. However, this is a hard task because the performance of protocols from the two paradigms depends on different subtleties of the environment and the implementation of the protocols. We tackle this problem by carefully designing the comparison study. We use state‐of‐the‐art algorithms and propose the problem of monitoring the network size in the presence of churn as the ideal problem for comparing very different paradigms for global aggregation. Our simulation study helps us identify the most important factors that differentiate between gossip and spanning tree aggregation: the time needed to compute a truly global output, the properties of the underlying topology, and sensitivity to dynamism. We demonstrate the effect of these factors in different practical topologies and scenarios. Our results help us to choose the right protocol in the light of the topology and dynamism patterns. Copyright © 2015 John Wiley & Sons, Ltd.

Implementing a Regular Register in an Eventually Synchronous Distributed System Prone to Continuous Churn

Due to their capability to hide the complexity generated by the messages exchanged between processes, shared objects are one of the main abstractions provided to developers of distributed applications. Implementations of such objects, in modern distributed systems, have to take into account the fact that almost all services, implemented on top of distributed infrastructures, are no longer fully managed due to either their size or their maintenance cost. Therefore, these infrastructures exhibit several autonomic behaviors in order to, for example, tolerate failures and continuous arrival and departure of nodes (churn phenomenon). Among all the shared objects, the register object is a fundamental one. Several protocols have been proposed to build fault resilient registers on top of message-passing system, but, unfortunately, failures are not the only challenge in modern distributed systems and new issues arise in the presence of churn. This paper addresses the construction of a multiwriter/multireader regular register in an eventually synchronous distributed system affected by the continuous arrival/departure of participants. In particular, a general protocol implementing a regular register is proposed and feasibility conditions associated with the arrival and departure of the processes are given. The protocol is proved correct under the assumption that a constraint on the churn is satisfied.

Analyzing the Resilience-Complexity Tradeoff of Network Coding in Dynamic P2P Networks

Most current-generation P2P content distribution protocols use fine-granularity blocks to distribute content to all the peers in a decentralized fashion. Such protocols often suffer from a significant degree of imbalance in block distributions, especially when the users are highly dynamic. As certain blocks become rare or even unavailable, content availability and download efficiency are adversely affected. Randomized network coding may improve block diversity and availability in P2P networks, as coded blocks are equally innovative and useful to peers. However, the computational complexity of network coding mandates that, in reality, network coding needs to be performed within segments, each containing a subset of blocks. In this paper, we quantitatively evaluate how network coding may improve content availability, block diversity, and download performance in the presence of churn, as the number of blocks in each segment for coding varies. Based on stochastic models and a differential equation approach, we explore the fundamental tradeoff between the resilience gain of network coding to peer dynamics and its inherent coding complexity. We conclude that a small number of blocks in each segment is sufficient to realize the major benefits of network coding, with acceptable coding cost.

Performance evaluation of a Kademlia-based communication-oriented P2P system under churn

The phenomenon of churn has a significant effect on the performance of Peer-to-Peer (P2P) networks, especially in mobile environments that are characterized by intermittent connections and unguaranteed network bandwidths. A number of proposals have been put forward to deal with this problem; however, we have so far not seen any thorough analysis to guide the optimal design choices and parameter configurations for structured P2P networks. In this article, we present a performance evaluation of a structured communication-oriented P2P system in the presence of churn. The evaluation is conducted using both simulation models and a real-life prototype implementation. In both evaluation environments, we utilize Kademlia with some modifications as the underlying distributed hash table (DHT) algorithm, and Peer-to-Peer Protocol (P2PP) as the signaling protocol. The results from the simulation models created using Nethawk EAST (a telecommunication simulator software) suggest that, in most situations, a lookup parallelism degree of 3 and resource replication degree of 3 are enough for guaranteeing a high resource lookup success ratio. We also notice that, with the parallel lookup mechanism, a good success ratio is achieved even without the KeepAlive traffic that is used for detecting the aliveness of nodes. A prototype system that works in mobile environment is implemented to evaluate the feasibility of mobile nodes acting as full-fledged peers. The measurements made using the prototype show that, from the viewpoints of CPU load and network traffic load, it is feasible for the mobile nodes to take part in the overlay. Through energy consumption measurements, we draw the conclusion that in general the UMTS access mode consumes slightly more power than the WLAN access mode. Protocol packets with sizes of 200bytes or less are observed to be the most energy efficient in the UMTS access mode.

Resilience of structured P2P systems under churn: The reachable component method

Users in a peer-to-peer (P2P) system join and leave the network in a continuous manner. Understanding the resilience properties of P2P systems under high rate of node churn becomes important. In this work, we first find that a lifetime-based dynamic churn model for a P2P network that has reached stationarity is reducible to a uniform node failure model. This is a simple yet powerful result that bridges the gap between the complex dynamic churn models and the more tractable uniform failure model. We further develop the reachable component method and derive the routing performance of a wide-range of structured P2P systems under varying rates of churn. We find that the de Bruijn graph based routing systems offer excellent resilience under extremely high rate of node turnovers, followed by a group of routing systems that include CAN, Kademlia, Chord and randomized-Chord. We show that our theoretical predictions agree well with large-scale simulation results. We finish by suggesting methods to further improve the routing performance of dynamic P2P systems in the presence of churn and failures.