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This paper presents an effective data collection scheme to provide group communications among appropriate members selected by each user’s geographic situation and preference (real-spatial information). When each user directly notifies central servers of user’s information via wireless network infrastructure (Wi-infra), message delivery latency and losses drastically increase due to the network congestion. Therefore, we employ representative nodes (RNs) selected in a distributed manner. The RN first collects the real-spatial information from neighboring nodes via an ad hoc network and then notifies the server via Wi-infra. From simulation experiments, our scheme can drastically reduce both message delivery latency and losses.

This article presents Axiom, a DTLS-based approach to efficiently secure multicast group communication among IoT-constrained devices. Axiom provides an adaptation of the DTLS record layer, relies on key material commonly shared among the group members, and does not require one to perform any DTLS handshake. We made a proof-of-concept implementation of Axiom based on the tinyDTLS library for the Contiki OS and used it to experimentally evaluate performance of our approach on real IoT hardware. Results show that Axiom is affordable on resource-constrained platforms and performs significantly better than related alternative approaches.

Vehicular Ad Hoc Network (VANET) is an emerging type of network which facilitates vehicles on roads to communicate for driving safety. It requires a mechanism to help authenticate messages, identify valid vehicles, and remove malevolent vehicles which do not obey the rules. Most existing solutions either do not have an effective message verification scheme, or use the public key infrastructure (PKI). In this network, vehicles are able to broadcast messages to other vehicles and a group of known vehicles can also communicate securely among themselves. So group communication is necessary for the network. However, most existing solutions either do not consider this or use pairing operation to realize this. They are either not secure or not effective. In this paper, we provide a more comprehensive set of secure schemes with Hash-based Message Authentication Code (HMAC) in VANETs to overcome their shortcomings. Of course, we still need to use Pairing operation in some place. Our scheme is composed of three schemes: (1) Communications between Vehicles and Road-Side Units (RSUs), (2) One to One Communications within a Group, (3) One to One Communications without a Group. Based on our simulation study, we show that our schemes are effective and the delay caused is much lower. The average delay caused by our first scheme is nearly thousands of times lower than prior schemes. The average delay caused by our second scheme is 0.312 ms, while the delay caused by prior scheme is 12.3 ms. Meanwhile the average delay caused by our third scheme is 0.312 ms, and the delay caused by prior scheme is about 9 s. © 2012 Elsevier B.V. All rights reserved., postprint

The core-based approach in multipoint communication enhances the solution space in terms of QoS-efficiency of solutions in inter- and intra-domain routing. In an earlier work [A. Karaman, H.S. Hassanein, Extended QoS-framework for Delay-constrained Group Communication, International Journal of Communication Systems, in press.], we showed that the constrained cost minimization solutions in core-based approach proposed to date are restrictive in their search to a subrange of solutions, and we proposed SPAN, a generic framework to process in our identified extended solution space. In this paper, we study the core selection component of SPAN and propose two novel algorithms, SPAN/COST and SPAN/ADJUST, which define the core-selection component of SPAN. SPAN/COST mainly optimizes the cost distances to be traveled between the source-core and core-receiver pairs on the multicast trees, while SPAN/ADJUST selects the cores based on the numbers of nodes they dominate and adjusting the set based on cost. Our algorithms consistently outperform their counterparts proposed to date and can be considered pioneering in their optimization range of multiple metrics and processing in the extended solution space.

Group communication is the basic infrastructure for implementing fault-tolerant replicated servers. While group communication is well understood in the context of static groups (in which the membership does not change), current specifications of dynamic group communication (in which processes can join and leave groups during the computation) have not yet reached the same level of maturity.The paper proposes new specifications -- in the primary partition model -- for dynamic reliable broadcast (simply called "reliable multicast"), dynamic atomic broadcast (simply called "atomic multicast") and group membership. In the special case of a static system, the new specifications are identical to the well known static specifications. Interestingly, not only are these new specifications "syntactically" close to the static specifications, but they are also "semantically" close to the dynamic specifications proposed in the literature. We believe that this should contribute to clarify a topic that has always been difficult to understand by outsiders. Finally, the paper shows how to solve atomic multicast, group membership and reliable broadcast. The solution of atomic multicast is close to the (static) atomic broadcast solution based on reduction to consensus. Group membership is solved using atomic multicast. Reliable multicast can be efficiently solved by relying on a thrifty generic multicast algorithm.

In this paper, we present a performance comparison of database replication techniques based on total order broadcast. While the performance of total order broadcast-based replication techniques has been studied in previous papers, this paper presents many new contributions. First, it compares with each other techniques that were presented and evaluated separately, usually by comparing them to a classical replication scheme like distributed locking. Second, the evaluation is done using a finer network model than previous studies. Third, the paper compares techniques that offer the same consistency criterion (one-copy serializability) in the same environment using the same settings. The paper shows that, while networking performance has little influence in a LAN setting, the cost of synchronizing replicas is quite high. Because of this, total order broadcast-based techniques are very promising as they minimize synchronization between replicas.

Smart embedded objects will become an important part of what is called the Internet of Things. Applications often require concurrent interactions with several of these objects and their resources. Existing solutions have several limitations in terms of reliability, flexibility and manageability of such groups of objects. To overcome these limitations we propose an intermediately level of intelligence to easily manipulate a group of resources across multiple smart objects, building upon the Constrained Application Protocol (CoAP). We describe the design of our solution to create and manipulate a group of CoAP resources using a single client request. Furthermore we introduce the concept of profiles for the created groups. The use of profiles allows the client to specify in more detail how the group should behave. We have implemented our solution and demonstrate that it covers the complete group life-cycle, i.e., creation, validation, flexible usage and deletion. Finally, we quantitatively analyze the performance of our solution and compare it against multicast-based CoAP group communication. The results show that our solution improves reliability and flexibility with a trade-off in increased communication overhead.

The Constrained Application Protocol (CoAP) is a new Internet protocol that is currently being standardized. CoAP allows access to the drastically increasing number of smart objects and their sensing resources from virtually anywhere. It is a light-weight protocol designed to cope with the restrictions imposed by the limited resources (CPU, memory, power,...) of many smart objects. Depending on the application, information from individual objects might not be sufficient, reliable, or useful. An application may need to aggregate and/or compare data from a group of objects in order to obtain accurate results. Although multicast may be used to transmit the same request to several objects, multicast communication with smart objects has some disadvantages. Programming individual requests is another solution but lacks flexibility and opportunities for reusability. In this paper we propose a novel CoAP-based approach for communication with a group of resources across multiple smart objects. This approach organizes the group of resources that should be accessed into a new CoAP resource, called an entity, and nicely integrates several important aspects of entity management: creation, validation, usage and manipulation. In order to demonstrate the feasibility of this approach we present an implementation and experimental validation.

Secure group management is an important issue in Wireless Sensor Networks (WSNs). The most of previous works consider the whole network as a single group managed by a central, powerful node (e.g., the base station) capable of supporting heavy communication, computation and storage cost. However, typical WSNs applications may benefit from being designed and implemented as a collection of multiple logical groups, each one is maintained by a sensor node (the group controller) with constrained resources. Furthermore, previous schemes require multicast support at the routing level to deliver rekeying messages. Unfortunately, multicast may cause a storage and communication overhead that are not a_ordable in a WSN. In order to go beyond these two limitations, we propose a new secure group management scheme with a lightweight re-keying process. The scheme allows multiple logical groups, each one is maintained and rekeyed separately by a resource-constrained sensor node without requiring multicast routing support. We prove that the scheme is secure and we evaluate its performance from several view points. Actually, we show that our scheme outperforms some previous well-known schemes such as LKH.

While the size and heterogeneity of wireless sensor networks confirm the need and benefit of group communication, an intelligent ap- proach that exploits the interaction pattern and network context is still missing. This paper introduces sensor middleware to dynamically select the most efficient alternative from a set of group communication mech- anisms. The proposed solution leverages on an empirical analysis of the ODMRP multicast protocol and was evaluated by a proof-of-concept pro- totype running on the SunSPOT platform. Results show that network overhead is considerably reduced when using the sensor middleware for software deployment, reconfiguration and periodic data monitoring. ispartof: pages:59-74 ispartof: Proceedings of the 2nd International ICST conference on Sensor Systems and Software vol:57 pages:59-74 ispartof: S-Cube location:Miami, Florida, USA date:13 Dec - 14 Dec 2010 status: published

Reliable distributed protocols, such as consensus and atomic broadcast, are known to scale poorly with large number of processes. Recent research has shown that algorithms providing probabilistic guarantees are a promising alternative for such environments. In this paper, we propose a specification of atomic broadcast with probabilistic liveness and safety guarantees. We present an algorithm that implements this specification in a truly asynchronous system (i.e., without assumptions about process speeds and message transmission times).

A specific class of multimedia applications is expected to be of importance for future communication networks: multi-party interactive multimedia (MIM). Based on the isolation and characterization of MIM applications, concrete network support requirements are derived. The varying degree of connectivity, the vastly different sizes in terms of participants and the reliance on a guaranteed quality of service make MIM support a difficult problem. Starting with the definition of multimedia communication abstractions, principles of solutions are sketched. For an important subclass of applications, a particularly efficient and practicable alternative implementation based on half-duplex channels is introduced. Interfaces at both the transport and network layers are also considered. >

Due to the non-deterministic behavior of some parallel and distributed programs, addressing the problem of testing in such context is a non-trivial task. A proficient testing phase must be done using tools which record information about a single execution, and which are able to force a concurrent program to exercise a given execution. This paper describes a tool for testing of programs based on the Horus system. Our approach is novel in dealing with events connected to process group handling and group communication. Moreover, we show that using the capabilities of the Horus run-time system can be really helpful in solving testing problems. Our approach has the advantages of requiring no modifications neither in the program nor in the operating system, and to avoid a centralized solution through the use of partial orders of synchronization events.

Colloque avec actes et comité de lecture. internationale.; International audience; Efficient multicast communication provides a good opportunity for scaling bandwidth intensive application. Current multicast routing involves selection of a \emph{core} also known as rendez-vous point through which most of the multicast communication is routed. The purpose of this paper is to study the performance impact of migration techniques. Several studies were performed on heuristics to choose a good core but none of them presents and implements the migration itself. Moreover, by using active networking architecture, we are able to adapt our multicast protocol to the application needs. Indeed, the application may provide the cost function used to evaluate the "quality" of the multicast tree. Experimental results reveal that active multicast routing with migration performs well and uses network resources in a very efficient way.

In this paper, we study the safety guarantees of group communication-based database replication techniques. We show that there is a model mismatch between group communication and database, and because of this, classical group communication systems cannot be used to build 2-safe database replication. We propose a new group communication primitive called \emph{end-to-end atomic broadcast} that solves the problem, i.e., can be used to implement 2-safe database replication. We also introduce a new safety criterion, called \emph{group-safety}, that has advantages both over 1-safety and 2-safety. Experimental results show the gain of efficiency of group-safety over lazy replication, which ensures only 1-safety.

Group communication in partitionable systems has been the focus of many research activities over the last decade. Fault-tolerant applications in a partitionable system model generally rely on two basic services: a group membership service and a reliable multicast service. The paper considers two of the most prominent specifications for such services that appear in the literature. Based on these specifications, the paper shows that one specification can be satisfied by a trivial implementation and the other, which basically extends the first one, requires additional system assumptions, which are discussed in the paper. The paper shows that the specifications of group communication in partitionable systems has not yet reached the level of maturity of the so called primary partition specifications.

The deferred update technique is a widely used approach for building replicated database systems. Its fame stems from the fact that read-only transactions can execute locally to any single database replica, providing good performance for workloads where transactions are mostly of this type. In this paper, we analyze the deferred update technique and show a number of characteristics and limitations common to any replication protocol based on it. Previous works on this replication method usually start from a protocol and then argue separately that it is based on the deferred update technique and satisfies serializability. Differently, ours starts from the abstract definition of a serializable database and gradually changes it into an abstract deferred update protocol. In doing that, we can formally characterize the deferred update technique and rigorously prove its properties. Moreover, our specification can be extended to create new protocols or used to prove existing ones correct.

This paper presents Atum, a group communication middleware for a large, dynamic, and hostile environment. At the heart of Atum lies the novel concept of volatile groups: small, dynamic groups of nodes, each executing a state machine replication protocol, organized in a flexible overlay. Using volatile groups, Atum scatters faulty nodes evenly among groups, and then masks each individual fault inside its group. To broadcast messages among volatile groups, Atum runs a gossip protocol across the overlay. We report on our synchronous and asynchronous (eventually synchronous) implementations of Atum, as well as on three representative applications that we build on top of it: A publish/subscribe platform, a file sharing service, and a data streaming system. We show that (a) Atum can grow at an exponential rate beyond 1000 nodes and disseminate messages in polylogarithmic time (conveying good scalability); (b) it smoothly copes with 18% of nodes churning every minute; and (c) it is impervious to arbitrary faults, suffering no performance decay despite 5.8% Byzantine nodes in a system of 850 nodes.

Designing and correctly implementing Group Communication Systems (GCSs) is notoriously difficult. Assuming that processes fail only by crashing provides a powerful means to simplify the theoretical development of these systems. When making this assumption, however, one should not forget that clean crash failures provide only a coarse approximation of the effects that errors can have in distributed systems. Ignoring such a discrepancy can lead to complex GCS-based applications that pay a large price in terms of performance overhead yet fail to deliver the promised level of dependability. This paper provides a thorough study of error effects in real systems by demonstrating a \emph{error-injection-driven design methodology}, where error injection is integrated in the core steps of the design process of a robust fault-tolerant system. The methodology is demonstrated for the \emph{Fortika} toolkit, a Java-based GCS. Error injection enables us to uncover subtle reliability bottlenecks both in the design of Fortika and in the implementation of Java. Based on the obtained insights, we enhance Fortika's design to reduce the identified bottlenecks. Finally, a comparison of the results obtained for Fortika with the results obtained for the OCAML-based Ensemble system in a previous work, allows us to investigate the reliability implications that the choice of the development platform (Java versus OCAML) can have.

Total order broadcast and multicast (also called atomic broadcast or atomic multicast) is an important problem in distributed systems, especially with respect to fault-tolerance. In short, the primitive ensures that messages sent to a set of processes are delivered by all these processes in the same total order. The problem has inspired an abundant literature, with a plethora of proposed algorithms. This paper proposes a classification of total order broadcast and multicast algorithms based on their ordering mechanisms, and addresses a number of other important issues. The paper surveys about sixty algorithms, thus providing by far the most extensive study of the problem so far. The paper discusses algorithms for both the synchronous and the asynchronous system models, and studies the respective properties and behavior of the different algorithms., リサーチレポート（北陸先端科学技術大学院大学情報科学研究科）

Atomic broadcast in particular, and group communication in general, have mainly been specified and implemented in a system model where processes do not recover after a crash. The model is called crash-stop. The drawback of this model is its inability to express algorithms that tolerate the crash of a majority of processes. This has led to consider group communication in the so-called crash-recovery model, in which processes have access to stable storage, to log their state periodically. This allows them to recover a previous state after a crash. However, the existing specifications of atomic broadcast in the crash-recovery model are not satisfactory, and the paper explains why. The paper also proposes a new specification of atomic broadcast in the crash-recovery model that addresses these issues. Specifically, our new specification allows to distinguish between a uniform and a non-uniform version of atomic broadcast. The non-uniform version logs less information, and is thus more efficient. The uniform and non-uniform atomic broadcast have been implemented, and compared with a published atomic broadcast algorithm. Performance results are presented.