Elad Michael Schiller

Random walk for self-stabilizing group communication in ad-hoc networks

We introduce a self-stabilizing group communication system for ad-hoc networks. The system design is based on random walks of mobile agents. Three possible settings for modeling the location of the processors in the ad-hoc network are presented; slow location change, complete random change, and neighbors with probability. The group membership algorithm is based on collecting and distributing information by a mobile agent. The new techniques support group membership and multicast, and also support resource allocation.

Virtual Mobile Nodes for Mobile Ad Hoc Networks

One of the most significant challenges introduced by mobile networks is coping with the unpredictable motion and the unreliable behavior of mobile nodes. In this paper, we define the Virtual Mobile Node Abstraction, which consists of robust virtual nodes that are both predictable and reliable. We present the Mobile Point Emulator, a new algorithm that implements the Virtual Mobile Node Abstraction. This algorithm replicates each virtual node at a constantly changing set of real nodes, modifying the set of replicas as the real nodes move in and out of the path of the virtual node. We show that the Mobile Point Emulator correctly implements a virtual mobile node, and that it is robust as long as the virtual node travels through well-populated areas of the network. The Virtual Mobile Node Abstraction significantly simplifies the design of efficient algorithms for highly dynamic mobile ad hoc networks.

Random walk for self-stabilizing group communication in ad hoc networks

We introduce a self-stabilizing group communication system for ad hoc networks. The system design is based on a mobile agent, collecting and distributing information, during a random walk. Three possible settings for modeling the location of the mobile nodes (processors) in the ad hoc network are presented: slow location change, complete random change, and neighbors with probability. The group membership algorithm is based on a mobile agent collecting and distributing information. The new techniques support group membership and multicast, and also support resource allocation.

Autonomous virtual mobile nodes

This paper presents a new abstraction for virtual infrastructure in mobile ad hoc networks. An Autonomous Virtual Mobile Node (AVMN) is a robust and reliable entity that is designed to cope with the inherent difficulties caused by processors arriving, leaving, and moving according to their own agendas, as well as with failures and energy limitations. There are many types of applications that may make use of the AVMN infrastructure: tracking, supporting mobile users, or searching for energy sources.The AVMN extends the focal point abstraction in [9] and the virtual mobile node abstraction in [10]. The new abstraction is that of a virtual general-purpose computing entity, an automaton that can make autonomous on-line decisions concerning its own movement. We describe a self-stabilizing implementation of this new abstraction that is resilient to the chaotic behavior of the physical processors and provides automatic recovery from any corrupted state of the system.

Brief announcement: virtual mobile nodes for mobile ad hoc networks

Devising algorithms for mobile networks is hard. It is important, then, to develop techniques and abstractions to ease the design of algorithms for mobile networks. We propose one such abstraction, the Virtual Mobile Node abstraction. The key difficulty in mobile networks is the completely unpredictable motion of the nodes. This complication is, of course, unavoidable: the defining feature of a mobile network is that the nodes, in fact, move. The other difficulty is the unpredictable availability of nodes that continually join the system and fail. The motion, however, also presents an opportunity: if mobile nodes moved in a useful way, algorithms could take advantage of the motion, performing even more efficiently than in static networks. This idea is illustrated by Hatzis et al. in [2], which defines the notion of a compulsory protocol, one that requires a subset of the mobile nodes to move in a specific manner. They present an efficient compulsory protocol for leader election. The routing protocols of Chatzigiannakis et al. [1] and Li et al. [3] provide further evidence that compulsory protocols are simple and efficient. It may be difficult, however, to ensure that mobile nodes move as desired, especially for highly dynamic systems where nodes may fail or be diverted from the prescribed path. Thus our objective is to retain the effectiveness of compulsory protocols without imposing requirements on the motion of the nodes. We propose executing algorithms on virtual mobile nodes, abstract nodes that move in a predetermined, predictable manner. Virtual mobile nodes (VMNs) are simulated by real nodes in the network. The motion of a VMN is determined in advance, and is known to the programs executing on the VMNs. For example, a VMN may traverse the plane in a regular pattern, or it may perform a pseudorandom walk. The motion of the VMNs may be completely uncorrelated with the motion of the real nodes: even if all the real nodes are moving in one direction, the virtual nodes may travel in the opposite direction. ∗ For a full version of this paper, see [4]. This work is supported in part by NSF grants CCR-0098305, ITR-0121277, 64961-CS, 9988304, 0311368, 9984774 and 0098305, AFSOR #F49620-00-1-0097, DARPA #F33615-01-C-1896, NTT MIT9904-12,Texas Advanced Research Program 000512-0091-2001, an IBM faculty award, the Israeli Ministry of Defense and Ministry of Trade and Industry. Copyright is held by the author/owner. PODC’04, July 25–28, 2004, St. Johns, Newfoundland, Canada. ACM 1-58113-802-4/04/0007. This abstraction simplifies the solution of many problems. VMNs can be used to route message, using compulsory protocols devised by Chatzigiannakis et al. [1]). Similarly, VMNs can be used to collect and evaluate data in a sensor network, or to implement group communication services by collecting and ordering messages. We present the Mobile Point Emulator, a new algorithm that implements robust VMNs. The main idea of the algorithm is to allow real nodes traveling near the location of a VMN to assist in emulating the VMN. In order to achieve robustness, the algorithm replicates the state of a virtual node at a number of real mobile nodes. As the execution proceeds, the algorithm continually modifies the set of replicas so that they always remain near the virtual node. We use a replicated state machine approach, augmented to support joins, leaves, and recoveries, to maintain the consistency of the replicas. As long as the path of the virtual node travels through dense areas of the network, the virtual node does not fail. If a VMN does fail, it can recover when it again reenters a dense area. The VMN framework introduces new horizons for further research. One path of investigation is to devise further applications for the VMN abstraction. While we have begun to develop some basic protocols based on VMNs that are simpler and more intuitive than traditional solutions, much analysis and evaluation remain. Another significant path of investigation is to develop improved VMN implementations. In this paper we have assumed that the path of a VMN is fixed in advance, and the set of VMNs is fixed in advance. In many cases, this is not only sufficient, but in fact advantageous, as the location of a VMN can be known a priori. However, for some applications it would be useful if the path of the VMN could be determined on-the-fly. In addition, long-term robustness of the VMN abstraction could be improved if the virtual nodes were self-stabilizing. We believe that with a few modifications, the Mobile Point algorithm can be made self-stabilizing. Finally, the correctness of the Mobile Point Emulator depends on relatively strong assumptions about the local communication among the mobile nodes. We would like to better understand the feasibility of the current model, and also to develop versions of the algorithm that rely on weaker network models.

The KARYON project: Predictable and safe coordination in cooperative vehicular systems

KARYON, a kernel-based architecture for safety-critical control, is a European project that proposes a new perspective to improve performance of smart vehicle coordination. The key objective of KARYON is to provide system solutions for predictable and safe coordination of smart vehicles that autonomously cooperate and interact in an open and inherently uncertain environment. One of the main challenges is to ensure high performance levels of vehicular functionality in the presence of uncertainties and failures. This paper describes some of the steps being taken in KARYON to address this challenge, from the definition of a suitable architectural pattern to the development of proof-of-concept prototypes intended to show the applicability of the KARYON solutions. The project proposes a safety architecture that exploits the concept of architectural hybridization to define systems in which a small local safety kernel can be built for guaranteeing functional safety along a set of safety rules. KARYON is also developing a fault model and fault semantics for distributed, continuous-valued sensor systems, which allows abstracting specific sensor faults and facilitates the definition of safety rules in terms of quality of perception. Solutions for improved communication predictability are proposed, ranging from network inaccessibility control at lower communication levels to protocols for assessment of cooperation state at the process level. KARYON contributions include improved simulation and fault-injection tools for evaluating safety assurance according to the ISO 26262 safety standard. The results will be assessed using selected use cases in the automotive and avionic domains.

Hovering data clouds: a decentralized and self-organizing information system

With ever-increasing numbers of cars, traffic congestion on the roads is a very serious economic and environmental problem for our modern society. Existing technologies for traffic monitoring and management require stationary infrastructure. These approaches lack flexibility with respect to system deployment and unpredictable events (e.g., accidents). Moreover, the delivery of traffic reports from radio stations is imprecise and often outdated. In the project AutoNomos we aim at developing a decentralized system for traffic monitoring and managing, based on vehicular ad-hoc networks (VANETs). Our objective is to design a system for traffic forecasting that can deliver faster and more appropriate reactions to unpredictable events. In our design, cars collect traffic information, extract the relevant data, and generate traffic reports. A key concept are so-called Hovering Data Clouds (HDCs), which are based on the insight that many crucial structures in traffic (e.g., traffic jams) lead an existence that is independent of the individual cars they are composed of. The result is an elegant, robust and self-organizing distributed information system. In this paper we demonstrate first experimental results.

Self-stabilizing end-to-end communication in (bounded capacity, omitting, duplicating and non-FIFO) dynamic networks

End-to-end communication over the network layer (or data link in overlay networks) is one of the most important communication tasks in every communication network, including legacy communication networks as well as mobile ad hoc networks, peer-to-peer networks and mash networks. We study end-to-end algorithms that exchange packets to deliver (high level) messages in FIFO order without omissions or duplications. We present a self-stabilizing end-to-end algorithm that can be applied to networks of bounded capacity that omit, duplicate and reorder packets. The algorithm is network topology independent, and hence suitable for always changing dynamic networks with any churn rate.

Communication adaptive self-stabilizing group membership service

This paper presents the first (randomized) algorithm for implementing self-stabilizing group communication services in an asynchronous system. Our algorithm converges rapidly to legal behavior and is communication adaptive, namely, the communication volume is high when the system recovers from the occurrence of faults and is low once a legal state is reached. Communication adaptability is achieved by a new technique that combines transient fault detectors.

Autonomous TDMA Alignment for VANETs

The problem of local clock synchronization is studied in the context of media access control (MAC) protocols, such as time division multiple access (TDMA), for dynamic and wireless ad hoc networks. In the context of TDMA, local pulse synchronization mechanisms let neighboring nodes align the timing of their packet transmissions, and by that avoid transmission interferences between consecutive timeslots. Existing implementations for Vehicular Ad-Hoc Networks (VANETs) assume the availability of common (external) sources of time, such as base-stations or geographical positioning systems (GPS). This work is the first to consider autonomic design criteria, which are imperative when no common time sources are available, or preferred not to be used, due to their cost and signal loss. We present self-*pulse synchronization strategies. Their implementing algorithms consider the effects of communication delays and transmission interferences. We demonstrate the algorithms via extensive simulations in different settings including node mobility. We also validate these simulations in the MicaZ platform, whose native clocks are driven by inexpensive crystal oscillators. The results imply that the studied algorithms can facilitate autonomous TDMA protocols for VANETs.