James S. Pascoe

Middleware enhancements for metropolitan area wireless Internet access

Abstract The effectiveness of mission critical wireless applications such as those deployed in mobile commerce depends to a large extent on network service quality. Based on the observation that wireless networks, particularly in metropolitan areas, are subject to pockets of lowered quality and disconnectivity, we propose mechanisms to detect and adapt to these ‘trouble spots’. In metropolitan area wireless networks, we postulate that the majority of network problem areas are caused by location and environmental factors. End applications that are susceptible to brief disconnectivity when entering these trouble spots can exploit pre-emptive adaptation at the protocol level to overcome potentially serious problems at the application level. Based on this premise, we propose a mechanism that gathers semantic data pertaining to specific geographic areas which cause communication problems. Prior knowledge of such locations can be used by higher-level communication protocols to pre-emptively adapt, thereby avoiding undesirable effects at the application layer. To facilitate the categorization of trouble spots, we propose a series of wireless service evaluation metrics. We report on our experiences with using these metrics and present initial results from an experimental evaluation of their effectiveness.

The Agreement Problem Protocol Verification Environment

The Agreement Problem Protocol Verification Environment (APPROVE) for the automated formal verification of solutions to agreement problems is presented. Agreement problems are characterized by the need for a group of processes to agree on a proposed value and are exemplified by group membership, consensus and leader election schemes. Generally it is accepted by practitioners in both academia and industry that the development of reliable and robust solutions to agreement problems is essential to the usability of group communication infrastructures. Thus, it is important that the correctness of new agreement algorithms be verified formally. In the past, the application of manual proof methods has been met with varying degrees of success, suggesting that a less error prone automated tool approach is required. Furthermore, an observation made during a review of such proofs is that a significant amount of effort is invested into repeatedly modeling re-usable themes. The APPROVE project addresses these issues by introducing a usable Spin based framework that exploits the potential for model re-use wherever possible.

Collaborative Group Membership

In this paper we present a novel approach to fault-tolerant group membership for use predominantly in collaborative computing environments. As an exemplar, we use the Collaborative Computing Transport Layer which offers reliable atomic multicast capabilities for use in collaborative environments such as the Collaborative Computing Frameworks (CCF). Specific design goals of the approach are the elimination of processing overhead due to heartbeats, support for partial failures and extensibility. These goals are satisfied in an approach, termed Collaborative Group Membership (CGM), which uses a quiescent weak failure detector and two election based algorithms to form consensus on the membership of a failing group. Failure detection operates through a reliable multicast primitive and as such eliminates the need for explicit keep-alive packets; thus in a failure free environment, CGM imposes no overhead.

Mobile Wide Area Wireless Fault-Tolerance

This paper presents work-in-progress that is developing a novel fault-tolerant mechanism for use in mobile wide area wireless networks. As a developmental platform, we are using the Ricochet service which offers ubiquitous metropolitan scale wireless network coverage in several major US cities. We postulate that the majority of network failures in infrastructures such as Ricochet are caused by environmental factors. From this, we propose a GPS based mechanism that intelligently gathers semantic data pertaining to specific geographic areas (or trouble spots) which cause communication problems. To facilitate the categorisation of trouble spots, we propose a list of suitable metrics to analyse the status of a wireless connection. Finally, we experimentally evaluate their effectiveness.

An election based approach to fault-tolerant group membership in collaborative environments

In this paper we present a novel approach to fault-tolerant group membership for use predominantly in collaborative computing environments. As an exemplar, we use the Collaborative Computing Transport Layer which offers reliable atomic multicast capabilities for use in collaborative environments such as the Collaborative Computing Frameworks (CCF). Specific design goals of the approach are the elimination of processing overhead due to heartbeats, support for partial failures and extensibility These goals are satisfied in an approach which uses an IP multicast failure detector and two election based algorithms. By basing failure detection on IP multicast, the need for explicit keep-alive packets is removed, thus in the absence of failures the approach imposes no overhead.

On Group Communication Systems: Insight, a Primer, and a Snapshot

This paper contributes a concise introduction to the field of group communication systems and is structured as three integrated parts. In the first instance, this paper aims to share the practical insight gained from the implementation of several group communication projects back into the community. This is discussed in a form that can be used to guide and steer subsequent projects. Secondly, the paper aims to benefit newcomers to the subject by offering an introduction to some of the more pertinent areas of the field through a snapshot of its current state. The subjects of failure detectors, group membership (including virtual synchrony variants) and security are discussed. Although this paper presents a general view on these subjects, it alludes to the exemplars of the Collaborative Computing Frameworks (CCF) and IceT where necessary.

Working Towards Strong Wireless Group Communications: The Janus Architecture

Strong wired group communication systems have been employed in numerous applications, often characterised by some degree of replication. With the development of wireless technologies, a new series of group applications is emerging and due to the effectively asymmetric characteristics of these networks, existing group communication systems can not be employed directly. Differences between wireless and wired networks in terms of connectivity, latency, bandwidth and other characteristics necessitate alternative algorithms and mechanisms, especially in group communication protocols. In particular, protocols must adapt to highly variable network characteristics in the wireless domain, whilst providing standard group communication semantics and delivering traditional qualities of service. In addition, arbitration between wired and wireless communication entities is required to reconcile the order-of-magnitude (or more) difference in network parameters between the two.The Janus project is developing a strong group communications framework for use in wireless environments. It is envisaged that the findings from our research, as well as the software that results, will be of value in a number of emerging domains such as collaborative computing and multiway application sharing that are moving from fully wired to hybrid wireless and land-line networks.

Collaborative Computing and E-learning

In this paper we present the Collaborative Computing Frameworks (CCF) as an integration platform for e-learning. The capabilities of the CCF facilitate mixed modes of delivery and the possibility to integrate already existing e-learning platforms. The CCF features allow to form different groups during the learning process. This coupled with peer-to-peer communication facilities, promotes the efficient implementation of collaborative technologies as an important component of the Dialogue phase of the learning process. The new Collaborative Computing Transport Layer (CCTL) will allow wireless devices to be experimented with for the purposes of e-learning. It is envisaged that this direction will dramatically widen the possibilities for content delivery.

Introducing Fault-Tolerant Group Membership into the Collaborative Computing Transport Layer

In this paper we introduce the novel election based fault tolerance mechanisms recently incorporated into the Collaborative Computing Transport Layer (CCTL). CCTL offers the atomic reliable multicast facilities used in the Collaborative Computing Framework (CCF). Our approach utilizes a reliable IP multicast primitive to implement two electorial algorithms that not only form consensus, but efficiently deliver a compact matrix based view of the network. This matrix can subsequently be analyzed to identify specific network failures (e.g. partitioning). The underlying premise of the approach being that by basing fault tolerance on a reliable multicast primitive, we eliminate the need for specific keep-alive packets such as heartbeats.

The ncast Primitive for Peer-to-Peer Networks

Many emerging applications of peer-to-peer networking will benefit from new, more appropriate communication primitives at the transport level. This paper presents the design and preliminary implementation of two variants of a novel primitive termed ncast. Unlike conventional multiway communications, the ncast primitives are not based on identity; their semantics specify the number of message deliveries in a given collaborative peer group. Both variants provide a means for fulfilling message delivery quotas in a peer-to-peer network, but they differ in the strength of the semantics they provide. A description of this operation and its semantics is followed by examples and implementation outlines. The paper concludes with a brief discussion of issues relating to the utility of such primitives in collaborative peer group networks.