An Adaptable Group Communication System

The vision of ubiquitous communication is driven by increasing user mobility and the need to collaborate. Processes and workflows that were previously bound to stationary computer systems are increasingly being adapted for mobile devices. Mobile collaborative applications, such as video conferences, collaborative writing, and gaming, are already important parts of human interaction. These kinds of applications intensively use group communication. Legacy group communication systems are rarely applicable in mobile scenarios due to temporary communication outages, mobile and nomadic users, and limited device capabilities. Non-responding users are usually associated with node failures and excluded from the group in these systems. In this paper, we argue for the need of a mobility support as a primary service in mobile group communication systems. We analyze mobility issues for mobile collaboration and show that it is difficult to find a generic solution that meets the various requirements to group communication. Using the group communication protocol Moversight as an example, we show how mobility support can be assured for closed group scenarios.

Developers of modern distributed systems continuously face the impossibility result proved by the CAP theorem. In a nutshell, the theorem states that a partition-tolerant system can either guarantee consistency or availability.\n\n Most distributed programming languages implicitly make the choice between consistency or availability in their designs and implementations. Concretely, distributed programming languages can be roughly divided into two categories. A first category of languages provide abstractions to implement the consistent parts of a distributed system. A second category of languages provide abstractions to implement the available parts of a distributed system.\n\n However, real-world distributed systems often require consistency for some parts while requiring availability for others. Programmers are therefore forced to implement the abstractions missing from their chosen distributed programming language themselves or rely on external libraries.\n\n In this paper we present a novel distributed programming model. This model introduces two object-oriented abstractions: consistents and availables. The former guarantees strong consistency by sacrificing availability. The latter guarantees availability, but only provides eventual consistency. Through these constructs programmers are able to implement the entirety of their distributed system within the same language.\n\n We present a prototypical implementation of the model as a TypeScript library called CAPtain.js. To showcase the usefulness of our approach we implement a non-trivial example application. Moreover, we highlight both the functional as well as the performance characteristics of both language abstractions.

We have developed a cost-effective regional computer communication network system and examined its implementation. The system provides a LAN environment and group communication services to the customers. The main system components are: the intelligent bridge circuit, called the low-end card (LEC), the center brouter (bridging router), and the management server. The LEC is set up in the subscriber's household and provides the popular Ethernet interface (10 Base-T). The leased circuits are connected between the LEC and the center brouter. The LEC and the brouter are controlled by the management server to function as the group communication system. The brouter, which has a multiplex subscriber-port interface, is required to accommodate a lot of customers cost effectively. This paper presents the system design of a large-capacity brouter having a SDH/SONET multiplex-port interface and its operation. The brouter can accommodate up to approximately 2700 subscribers. Ethernet packets are transferred through each subscriber channel between the LEC and the brouter using the HDLC protocol. A group communication mechanism based on packet filtering via the MAC addresses of PCs is presented. The Ethernet packets are filtered by the brouter to eliminate all packets except those from/to the same group members. The group management operation among the center brouter, the LEC, and the management server is discussed.

Collaboration is used for information sharing and activity coordinating, and it exists broadly in many fields. Group communication enables efficient communication between a set of processes logically organized into groups and communicating via multicast in an asynchronous environment. One of the key technologies for collaborative applications is secure group communication. Current research on secure group communication scarcely considers the existing security mechanism in local systems. As a result, group communication systems couldn 't provide general support for collaborative applications running on a specific system. Based on the existing grid security technologies, we propose an authentication and access control framework at virtual organization (VO) level for group communication in grid environment. By introducing role-based access control (RBAC) and attribute-based approach, we define group management policies and design group control protocols. The protocols are analyzed from three aspects: compatibility, performance, and security. Finally, we implement a prototype based on GridShib.

As the Presence Service provides users with dynamic information such as status and availability of their contacts increases the likelihood for success of the user’s communications, and enhances user satisfaction. Currently the Presence Service is used importantly in VoIP and Instant Messaging Service. However, in the systems that a real-time property is required, presence service should be provided limitedly and efficiently because it is in the trade-off relation with a real-time property. In this paper, we propose more effective Presence Service in the Group Communication System based on SIP. As the Presence Service of in this paper extracts the presence information from the normal SIP messages and sends minimal changed presence information to previously defined watchers instead of the OMA defined SIMPLE framework which consists of complex presence messages such as REGISTER, SUBSCRIBE, PUBLISH and NOTIFY it can be provided more efficiently in the Group Communication System.KeywordsPresenceGroup Communication SystemSIPOMASIMPLE

The explosive growth of the Internet demands higher reliability and performance than what the current networking infrastructure can provide. This dissertation explores novel architectures and protocols that provide a methodology for grouping together multiple networking elements, such as routers, gateways, and switches, to create a more reliable and performant distributed networking system. Clustering of networking elements is a novel concept that requires the invention of distributed computing protocols that facilitate efficient and robust support of networking protocols. We introduce the Raincore protocol architecture that achieves these goals by bridging the fields of computer networks and distributed systems. In designing Raincore, we paid special attention to the unique requirements from the networking environment. First, networking clusters need to scale up the networking throughput in addition to the scaling up of computing power. Second, task switching between the different services supported by a networking element has a major negative impact on performance. Third, fast fail-over time is critical for maintaining network connections in the event of failures. We discuss in depth the design of Raincore Group Communication Manager that addresses the forgoing requirements and provides group membership management and reliable multicast transport. It is based on a novel token-ring protocol. We prove that this protocol is formally correct, namely, it satisfies the set of formal specifications that defines the Group Membership problem. The creation of Raincore has already made a substantial impact both at Caltech and the academic community as well as in the industry. The first application is SNOW, a scalable web server cluster that is part of RAIN, a collaborative project between Caltech and JPL/NASA. The second application is RainWall, a commercial solution created by Rainfinity, a Caltech spin-off company, that provides the first fault-tolerant and scalable firewall cluster. These applications exhibit the fast fail-over response, low overhead, and near-linear scalability of the Raincore protocols. In addition, we studied fault-tolerant networking architectures. In particular, we considered efficient constructions of extra-stage fault-tolerant Multistage Interconnection Networks. Multistage Interconnection Networks provide a way to construct a larger switching network using smaller switching elements. We discovered an optimal family of constructions, in the sense that it requires the least number of extra components to tolerate multiple switching element failures. We prove that this is the only family of constructions that has this optimal fault-tolerance property.

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Over the last few years, Cloud storage systems and so-called NoSQL datastores have found widespread adoption. In contrast to traditional databases, these storage systems typically sacrifice consistency in favor of latency and availability as mandated by the CAP theorem, so that they only guarantee eventual consistency. Existing approaches to benchmark these storage systems typically omit the consistency dimension or did not investigate eventuality of consistency guarantees. In this work we present a novel approach to benchmark staleness in distributed datastores and use the approach to evaluate Amazon's Simple Storage Service (S3). We report on our unexpected findings.

The bicycling recently has gained a lot of popularity in many applications, such as transportation, recreation, and exercise. Bikers may ride in different speeds due to their body strengthes and bike performance such that group members using smart phones form two-tier vehicular long-thin networks along the common cycling path, which consist of a 3G/3.5G high-tier interface and an IEEE 802.11-based low-tier interface. In this paper, we design a Google Android and Maps-based solution for cycling group communications. On the handset side, an Android program is developed for group member localization, communication, and transmission. In addition, the augmented reality interface is integrated to show the biker information on real-time camera views of smart phones. On the web side, a biker community site is built for the biker interaction, grouping, and dedicated album. To provide efficient group communications and reduce the transmission delay, ad-hoc communications are adopted to locally exchange data among bikers. Also, a dynamic grouping mechanism has been designed to maintain the stable member connectivity in ad-hoc environments. Furthermore, the gateway selection in a group has taken the similar mobility pattern of members into account for minimizing 3G/3.5G communications.

In large-scale replicated distributed storage systems consisting of hundreds to thousands of nodes, node failures are not rare and can cause data blocks to lose their replicas and become faulty. A simple but effective approach to prevent data loss from the node failures, i.e., ensuring reliability, is to shorten the identification time of the node failures and faulty blocks, which is determined by both timeouts and check intervals for node states. However, to maintain low repair network traffic, the identification time is actually relatively long and even dominates repair processes of critical blocks. In this paper, we propose a novel scheme, named RICK, to explore the potential in the identification time, and thus improve data reliability of replicated distributed storage systems while maintaining a low repair cost. First, by introducing an additional replica state, critical blocks (with two or more lost replicas) have individual short timeouts while sick blocks (with only one lost replica) preserve the long timeouts. Second, by replacing the static check intervals for node states with adaptive ones, the check intervals and the identification time of critical blocks are further shortened, which improves data reliability. Meanwhile, due to the low ratio of critical blocks in all faulty blocks, the repair network traffic remains low. The results from our simulation and prototype implementation show that RICK improves data reliability of replicated distributed storage systems by a factor of up to 14 in terms of mean time to data loss. Meanwhile, the extra repair network traffic caused by RICK is less than 1.5 percent of the total network traffic for data repairs.

A multisignature scheme for implementing safe delivery rule in group communication systems (MSGC) was recently proposed by Rahul and Hansdah. In this paper we show that the MSGC scheme is insecure against forgery attack and signature integrity attack. We propose an improved scheme that resists the weaknesses of MSGC scheme.

In Group Communication Systems (GCS), causal message ordering is an essential tool to ensure interaction among group members in a consistent way. In several group-based applications, the exchanged information is often diffused to all members. Using a multicast protocol to ensure this broadcast should make the definition of data structures not optimal for this kind of applications. In this paper, we propose a simple and optimal causal broadcast protocol which copes with the dynamically changing groups in mobile environments. The protocol depends on two simple, yet powerful ideas. The first depends on the use of the immediate dependency relationship in the construction of control information, resulting in O(1) message overhead. When the second original idea depends on considering the join and leave requests as data messages. This ensures a consistent perception of the communication done in the group and makes no need to a coordination phase in the installation of a new view.

This paper presents the design of OPEN EDEN, an implementation of the Fault Tolerant CORBA specification based on the use of a group communication system, called EDEN. The design of OPEN EDEN has been driven by the desire to use only portable techniques (mainly portable interceptors) to implement the interaction between the EDEN group communication system and CORBA. The EDEN system itself is built by configuring, assembling and deploying components connected to a generic agreement component that implements a consensus algorithm. The resulted platform is particularly portable, since it can be easily migrated to different architectures, and transparent for applications programmers, since requests to a replicated service are transparently intercepted and redirected to the EDEN system. We discuss the main difficulties we encountered and we draw some conclusions about the adequacy of our choices.

We investigate performance characteristics of secure group communication systems (GCSs) in mobile ad hoc networks that employ intrusion detection techniques for dealing with insider attacks tightly coupled with rekeying techniques for dealing with outsider attacks. The objective is to identify optimal settings including the best intrusion detection interval and the best batch rekey interval under which the system lifetime (mean time to security failure) is maximized while satisfying performance requirements. We develop a mathematical model based on stochastic Petri net (SPN) to analyze tradeoffs between security and performance properties, when given a set of parameter values characterizing operational and environmental conditions of a GCS instrumented with intrusion detection tightly coupled with batch rekeying. We compare our design with a baseline system using intrusion detection integrated with individual rekeying to demonstrate the effectiveness.