In this paper, we present D-BAS, a dialog-based online argumentation system, tailored to support e-participation processes. The main idea of D-BAS is to let users exchange proposals and arguments with each other in the form of a time-shifted dialog where arguments are presented and acted upon one-at-a-time. We highlight the key research challenges that needed to be addressed in order to realize such a system, provide solutions for those challenges, report on a full scale implementation of D-BAS and summarize the findings from a real world e-participation process, where D-BAS provided the infrastructure for online argumentation.

In this demonstration we will showcase Jebediah, a social bot based on Google's Dialogflow. Jebediah is a front-end to dialog-game execution platforms that enable their seamless integration into popular social networks such as Facebook or Twitter. Users can interact with the social bot using natural language while Jebediah translates the user input to a format that can be interpreted by a dialog game execution platform and vice versa.

Enabling the reuse of arguments as entities that can be shared across multiple Internet-based discussion platforms and that can be improved upon while they are being used and reused has many benefits ranging from easier participation in an online discussion to increasing the quality of arguments. In this paper we propose a mechanism that is able to support the large-scale reuse of arguments by providing distributed version control of argument data. Building on that mechanism we have designed and implemented EDEN, a framework which enables platform providers to easily network their discussions. EDEN is designed for real-world use and provides all tools necessary to enable the reuse of arguments and their interrelation for users and providers alike.

In this position paper we propose a novel approach to online argumentation. It avoids the pitfalls of unstructured systems such as asynchronous threaded discussions and it is usable by any participant without training while still supporting the full complexity of real-world argumentation. The key idea is to let users exchange arguments with each other in the form of a time-shifted dialog where arguments are presented and acted upon one-at-a-time. We highlight the key research challenges that need to be addressed in order to realize such a system and provide first solutions for those challenges.

The development and evaluation of Vehicle-to-X (V2X) applications using mobile cellular networks by means of field tests is time consuming and expensive. Simulations can speed up the development and largely reduce evaluation costs. However, due to the complex nature of the network involved, it is quite difficult to be certain that the results obtained using simulation will actually match the observable behaviour in the real world. In this paper, we introduce a novel trace-based simulation environment for V2X applications using mobile cellular networks. It employs real world measurement traces as a basis and thus avoids many uncertainties of other simulation approaches.

Current revision control systems are commonly used in a distributed fashion but rely on centralized stores and low-latency communication. In this work we evaluate how they would fare in fully distributed high latency networks such as delay tolerant networks (DTNs). We show that current revision control systems impose significant costs under these conditions even in moderately-sized networks. By simplifying/improving the merging process, these costs can be reduced. We also show that speeding up or slowing down communication can reduce the costs significantly.

The trace of a moving object is commonly referred to as a trajectory. This paper considers the spatio-temporal information content of a discrete trajectory in relation to a movement prediction model for the object under consideration. The information content is the minimal amount of information necessary to reconstruct the trajectory, given the movement model. We show how the information content of arbitrary trajectories can be determined and use these findings to derive an approximative arithmetic coding scheme for trajectory information, reaching a level of compression that is close to the bound provided by its entropy. We then demonstrate the practical applicability of our ideas by using them to compress real-world vehicular trajectories, showing that this vastly improves upon the results provided by the best state-of-the art compression schemes for spatio-temporal data.

Traffic lights strongly impact vehicle movement and fuel consumption in cities. If drivers were aware of the situation at arrival time, they could adapt their velocity and thus reduce the number of unnecessary stops and fuel consumption. To predict the influence of the traffic light ahead on the velocity of an approaching vehicle, our vision is that drivers share their vehicle traces in a digital cloud, and in return benefit from algorithms evaluating the collected data. With Traffic Light Coordination Analysis (TLCorA), we present one such algorithm analyzing vehicle traces. When a vehicle is approaching a traffic light, TLCorA finds traces of vehicles similar to that of the vehicle at the previous traffic light, and calculates from their approach to the upcoming traffic light whether there is a representative approaching trace. For this purpose, TLCorA classifies the approaching traces with help of a clustering algorithm based on dynamic time warping. We implement TLCorA in simulations of different traffic light signalization algorithms, and study the calculated approach probabilities depending on the respective traffic light correlation level in the scenarios.

Denial of Service (DoS) attacks aiming to exhaust the resources of a server by overwhelming it with bogus requests have become a serious threat. Especially protocols that rely on public key cryptography and perform expensive authentication handshakes may be an easy target. A well-known countermeasure against DoS attacks are client puzzles. The victimized server demands from the clients to commit computing resources before it processes their requests. To get service, a client must solve a cryptographic puzzle and submit the right solution. Existing client puzzle schemes have some drawbacks. They are either parallelizable, coarse-grained or can be used only interactively. In case of interactive client puzzles where the server poses the challenge an attacker might mount a counterattack on the clients by injecting fake packets containing bogus puzzle parameters. In this paper we introduce a novel scheme for client puzzles which relies on the computation of square roots modulo a prime. Modular square root puzzles are non-parallelizable, i.e., the solution cannot be obtained faster than scheduled by distributing the puzzle to multiple machines or CPU cores, and they can be employed both interactively and non-interactively. Our puzzles provide polynomial granularity and compact solution and verification functions. Benchmark results demonstrate the feasibility of our approach to mitigate DoS attacks on hosts in 1 or even 10 GBit networks. In addition, we show how to raise the efficiency of our puzzle scheme by introducing a bandwidth-based cost factor for the client.

The trace of a moving object is commonly referred to as a trajectory. This paper considers the spatio-temporal information content of a discrete trajectory in relation to a movement prediction model for the object under consideration. The information content is the minimal amount of information necessary to reconstruct the trajectory, given the movement model. We show how the information content of arbitrary trajectories can be determined and use these findings to derive an approximative arithmetic coding scheme for trajectory information, reaching a level of compression that is close to the bound provided by its entropy. We then demonstrate the practical applicability of our ideas by using them to compress real-world vehicular trajectories, showing that this vastly improves upon the results provided by the best existing schemes. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Fuel-efficient driving is difficult in unknown or complex environments. To aid the driver with this task, we present a novel method of tactical route optimization by calculating a short-term fuel-reduced velocity profile. This profile is based on knowledge of location-dependent velocity profiles that are collected by the vehicles over time and shared with other vehicles. To determine a fuel-efficient velocity profile, we first split the planned route into segments. We cluster the historical velocity profiles within each segment using a Dynamic Time Warping algorithm, obtaining classes of velocity profiles and their probabilities. We construct a transition graph between velocity profile classes from adjacent segments and calculate the most probable path through the next segments ahead. This path represents the most likely future velocity profile under the assumption that the driver behaves like previous drivers on the same segment. Given the constraints defined by this profile, we calculate the fuel-reduced velocity profile with help of a shortest path algorithm in a vehicle-specific fuel-consumption graph. First results in an urban environment indicate possible fuel savings of about 8.3% compared to the most probable profile.

Many applications in vehicular communications require the collection of vehicular position traces. So far this has been done by recording and transmitting unencoded or merely linearly filtered position samples. Depending on the sample frequency and resolution, the resulting data load may be very large, consuming significant storage and transmission resources. In this paper, we propose a method based on two-dimensional cubic spline interpolation that is able to reduce the amount of the measurement data significantly. Our approach allows for a configurable accuracy threshold and performs in O(n3). We evaluate our approach with real vehicular GPS movement traces and show that it is able to reduce the volume of the measurement set by up to 80% for an accuracy threshold of 20 centimeters.

Access point sharing communities are an attractive solution for low-cost global mobile Internet access. However, existing communities all require a central authority in form of a certificate authority or an authentication server. In this paper, we show how such a community network can be created without a central entity. We introduce a mechanism called remote station approval; a host can use it to offer Internet access without requiring a central instance and without the risk of legal responsibility for the guests' traffic. We discuss how our system preserves the anonymity of its users and present a prototype implementation and some first performance figures.

Offline trading allows users to trade rights associated with digital content or any other digital asset without immediate access to a central authority. To conduct an offline trade all involved parties sign a contract describing the transaction. Later this contract may then be submitted to a central authority (e.g., a digital content repository or a bank) to conduct the actual change of ownership as specified in the contract. In this paper we show that there is a crucial problem when using offline trading: whoever signs the contract first is providing the other parties with an option to unilaterally control the contract. The other parties may sign the contract later and submit it to complete the transaction. Or they may simply discard it. If the value of the involved goods change over time, this option may have a significant value. We introduce an approach to limit the value of options granted through offline contract signing and discuss its impact, design alternatives as well as related aspects.

Searching for witnesses in case of road accidents is a challenging task for the police and involved persons. In this paper, we propose a mechanism that helps to find witnesses. Our solution preserves the potential witnesses' anonymity and gives them a free hand to decide whether to step forward as a witness or not. We analyze the performance of our application using a realistic model of a German city.

Search indices for distributed information are an important building block for many mobile, decentralized applications. However, the network load caused by nodes registering their information in the index can become quite significant especially in mobile networks like vehicular ad-hoc networks (VANETs), where nodes need to register periodically due to their mobility. Our goal in this paper is to encode this registration information as efficiently as possible. For this purpose, we first analytically study hash keys and Bloom filters as alternative encoding types of nodes' registration data. It turns out that in many situations, Bloom filters are more bandwidth efficient than hash keys, and that they are even more efficient when nodes cooperate. Many such forms of cooperation are conceivable; we define one specific scheme targeting metropolitan VANETs. Our accompanying simulation study shows that this first algorithm already allows for large bandwidth savings in content registration, compared to the alternative of registering hash keys.

The distribution of dynamic information from many sources to many destinations is a key challenge for VANET applications such as cooperative traffic information management or decentralized parking guidance systems. In order for these systems to remain scalable it has been proposed to aggregate the information within the network as it travels from the sources to the destinations. However, so far it has remained unclear by what amount the aggregation scheme needs to reduce the original data in order to be considered scalable. In this paper we prove formally that any suitable aggregation scheme must reduce the bandwidth at which information about an area at distance d is provided to the cars asymptotically faster than 1/d2. Furthermore, we constructively show that this bound is tight: for any arbitrary ε>0, there exists a scalable aggregation scheme that reduces information asymptotically like 1/d(2+ε).

Repeatability has most often been neglected in experiments with wireless multihop networks. In this paper, we propose to consider repeatability in a coarse-grained fashion on a topological level. For this, a metric for comparing the similarity of topologies in static and mobile setups is presented and used to examine the level of repeatability achievable in such experiments. This metric is able to classify experiments according to the presence or absence of interference and to changes in the behavior of mobile nodes. It can be used to identify experimental runs where application layer performance is influenced by topological effects.

In this paper, we present a methodology and a toolkit to conduct tightly controlled real-world experiments in wireless multihop networks. The main features of our EXC toolkit are a plug-in architecture for easy integration of new functionality, simple scheduling of experiment runs, direct monitoring of experiments, easy management of node mobility, and high portability. EXC does not require specific hardware and can be used for investigating applications and algorithms in real-world environments. We explain the basic features of the toolkit and examine these by means of an experiment with a special focus on repeatability.

In mobile ad-hoc networks, the multicast paradigm is of central importance. It can help to save scarce medium bandwidth if packets are to be delivered to multiple destinations. We consider the problem of congestion control for multicast traffic in wireless multihop networks. We propose to apply a congestion control concept which is tailored to the very special properties of the wireless multihop medium: implicit hop-by-hop congestion control. The idea, so far only having been considered for unicast traffic, is here generalized to multicast. We implement it in the Backpressure Multicast Congestion Control (BMCC) protocol, with a focus on how to realize it in combination with geographic multicast routing in the Scalable Position-Based Multicast (SPBM) protocol. Our evaluation points out a number of highly desirable properties of the proposed scheme. In particular, it achieves and maintains high throughput and high packet delivery ratios at low packet latencies, even in the presence of significant network load.

A central problem in today's Internet is unsolicited bulk email: spam. The SMTP protocol lacks a mechanism for verifying the source of a message, and respective protocol extensions are still far from becoming standard. Content-based automatic spam filters are thus often used, and simple origin-based filtering techniques like black- and whitelists are also very common. In this paper we first analyze the retry behavior of Internet mail sources upon delivery errors. We then propose a novel spam filtering approach, founded on the results of this analysis. It is based on the observed reaction of a mail source host to temporary errors. Furthermore, evaluation results from the production use of our filter on real mail servers are given, underlining its very good performance in practice.

In the context of vehicular ad-hoc networks (VANETs), a number of highly promising convenience applications have been proposed. These include collecting and distributing information on the traffic situation, distributed monitoring of road and weather conditions, and finding available parking places in a distributed, cooperative manner. Unfortunately, all of these applications face major problems when a VANET is used as a means to distribute the required information. In particular a large number of vehicles needs to be equipped with dedicated VANET technology before these applications can provide a useful service. Even if customers were willing to purchase a system which is not immediately useful, it would still take quite some time until the required density of equipped cars is reached. In contrast, affordable always-on mobile Internet access is already mainstream. Such Internet connectivity could be used to build the proposed applications in a different fashion: by using peer-to-peer communication, essentially creating a peer-to-peer network of cars sharing traffic information. This allows to overcome the limitations of VANETs, while it preserves their key benefits of decentralization and robustness. In this paper, we describe the technical challenges that arise from such an approach, point out relevant research directions, and outline possible starting points for solutions.

The search for free parking places is a promising application for vehicular ad hoc networks (VANETs). In order to guide drivers to a free parking place at their destination, it is necessary to estimate the occupancy state of the parking lots within the destination area at time of arrival. In this paper, we present a model to predict parking lot occupancy based on information exchanged among vehicles. In particular, our model takes the age of received parking lot information and the time needed to arrive at a certain parking lot into account and estimates the future parking situation at time of arrival. It is based on queueing theory and uses a continuous-time homogeneous Markov model. We have evaluated the model in a simulation study based on a detailed model of the city of Brunswick, Germany.

Abstract Congestion control is a key problem in mobile ad hoc networks. The standard congestion control mechanism of the Transmission Control Protocol (TCP) is not able to handle the special properties of a shared wireless multi‐hop channel well. In particular, the frequent changes of the network topology and the shared nature of the wireless channel pose significant challenges. Many approaches have been proposed to overcome these difficulties. In this paper, we give an overview over existing proposals, explain their key ideas and show their interrelations. Copyright © 2007 John Wiley & Sons, Ltd.

No abstract available.

Discrete-event network simulation is a major tool for the research and development of mobile ad-hoc networks (MANETs). These simulations are used for debugging, teaching, understanding, and performance-evaluating MANET protocols. For the first three tasks, visualization of the processes occurring in the simulated network is crucial for verification and credibility of the generated results. Working with the popular network simulator ns-2, we have not yet found a visualization toolkit capable of reading native ns-2 trace files and providing means to change the evaluated parameters without changing the visualization software. Thus, we developed Huginn, a software providing an intuitive way to visualize simulation properties and to determine how they should be displayed without the need of programming. In addition, Huginn has a 3D interface allowing an improved exploitation of the (human) user's perceptual system. It helps to handle the significant cognitive load associated with the mental reconstruction of simulated network processes. Besides presenting the software interface and architecture, we describe algorithmic solutions that might be of a more general interest for similar problems.

Recording systems and media servers for networked audio and video streams have become an important part of today's Internet. In contrast to this, only a few recording and playback solutions currently exist for the data streams of interactive media applications (e.g., shared whiteboards and distributed virtual environments). So far these solutions are application-specific: individual algorithms and implementations are required for each application that is to be recorded. In this paper, we are proposing generic algorithms for the recording and playback of interactive media streams. These algorithms are based on a common model for the class of interactive media. They enable full random access to recordings by initializing the replaying applications with the required state information (e.g., the current slide in a recorded presentation). We have implemented these algorithms in the Interactive Media on Demand (IMoD) system. In order to interpret the semantics of an interactive media stream, the system requires that the Real-Time Application-Level Protocol for Distributed Interactive Media (RTP/I) protocol is used for the framing of the transmitted data. Any application using RTP/I can be recorded directly using the system without any modification. Interactive media streams not using RTP/I can be recorded using the generic recording algorithms. However, they require an adaptation of the system so that it is able to extract a minimal set of information from the application-level protocol of these streams. In addition to the generic recording algorithms, we present the architecture and major design considerations of the system and discuss the experiences gained from recording different interactive media applications.

In this paper, we investigate how consistency can be established for replicated applications changing their state in reaction to user-initiated operations as well as the passing of time. Typical examples of these applications are networked computer games and distributed virtual environments. We give a formal definition of the terms consistency and correctness for this application class. Based on these definitions, it is shown that an important tradeoff relationship exists between the responsiveness of the application and the appearance of short-term inconsistencies. We propose to exploit the knowledge of this tradeoff by voluntarily decreasing the responsiveness of the application in order to eliminate short-term inconsistencies. This concept is called local-lag. Furthermore, a timewarp scheme is presented that complements local-lag by guaranteeing consistency and correctness for replicated continuous applications. The computational complexity of the timewarp algorithm is determined in theory and practice by examining a simple networked computer game. The timewarp scheme is then compared to the well-known dead-reckoning approach. It is shown that the choice between both schemes is application-dependent.

In this paper we present a TCP-friendly congestion control scheme for non-adaptable flows. The main characteristic of these flows is that their data rate is determined by an application and cannot be adapted to the current congestion situation of the network. Typical examples of non-adaptable flows are those produced by networked computer games or live audio and video transmissions where adaptation of the quality is not possible (e.g., since it is already at the lowest possible quality level). We propose to perform congestion control for non-adaptable flows by suspending them at appropriate times so that the aggregation of multiple non-adaptable flows behaves in a TCP-friendly manner. The decision whether or not a flow is to be suspended is based on random experiments. In order to allocate probabilities for these experiments, the data rate of the non-adaptable flow is compared to the rate that a TCP flow would achieve under the same conditions. We present a detailed discussion of the proposed scheme and evaluate it through extensive simulation with the network simulator ns-2.

No abstract available.

In this paper we present a generic consistency control service for distributed interactive media, i.e. media which allow a distributed group of users to interact with the medium itself. Consistency control is vital to these media since they typically require that a local copy of the medium's state be maintained by each user's application. Our service helps the applications to keep the local state copies consistent. The main characteristics of this service are as follows: a signicant number of inconsistencies are prevented by using a mechanism called local lag. Inconsistencies that cannot be prevented are repaired by an improved timewarp algorithm that can be executed locally without burdening the network or the applications of other users. Exceptional situations and consistency during late-join situations are supported by a consistent state request mechanism. Moreover, the service also supports the application in detecting intention conicts between the actions of distinct users. The major part of this functionality is based on a media model and the application level protocol for distributed interactive media (RTP/I) and can thus be reused by arbitrary RTP/I-based applications. In order to demonstrate the feasibility of our approach and to evaluate its performance we have integrated the generic consistency service into a shared whiteboard system.

Distributed interactive media are media that involve communication over a computer network as well as user interactions with the medium itself. Examples of this kind of media are shared whiteboard presentations and networked computer games. One key problem of this media class is that a large amount of common functionality is currently redesigned and redeveloped for each single medium. In order to solve this problem we present a media model and an application level protocol called RTP/I. Derived from the experience gained with audio and video transmission using RTP, RTP/I is defined as a new protocol framework which reuses many aspects of RTP while it is thoroughly adapted to meet the demands of distributed interactive media. By identifying and supporting the common aspects of distributed interactive media RTP/I allows the reuse of key functionality in form of generic services. Furthermore RTP/I makes it possible for applications of different vendors to interact with each other in a standardized way.

In this paper we investigate how consistency can be ensured for replicated continuous interactive media, i.e., replicated media which change their state in reaction to user initiated operations as well as because of the passing of time. Typical examples for this media class are networked computer games and distributed VR applications. Existing approaches to reach consistency for replicated discrete interactive media are briefly outlined and it is shown that these fail in the continuous domain. In order to allow a thorough discussion of the problem, a formal definition of the term consistency in the continuous domain is given. Based on this definition we show that an important tradeoff relationship exists between the responsiveness of the medium and the appearance of short-term inconsistencies. Until now this tradeoff was not taken into consideration for consistency in the continuous domain, thereby severely limiting the consistency related fidelity for a large number of applications. We show that for those applications the fidelity can be significantly raised by voluntarily decreasing the responsiveness of the medium. This concept is called local lag. It enables the distribution of continuous interactive media that are more vulnerable to short-term inconsistencies than, e.g., battlefield simulations. We prove that the concept of local lag is valid by describing how local lag was successfully used to ensure consistency in a 3D telecooperation application.

We propose a novel late join algorithm for distributed applications with a fully replicated architecture (e.g. shared whiteboards). The term 'late join algorithm' is used to denote a mechanism that allows a late-coming participant rejoin an ongoing session. Generally, this requires that participants in the session provide the latecomer with the current state of the shared application. We identify the key issues of late join algorithms and propose a set of requirements which a 'good' late join approach should satisfy. Based on these requirements, we evaluate existing late join algorithms and explain why we opted instead to develop a new, advanced late join algorithm for our own shared whiteboard. This late join approach is general enough to be used for arbitrary distributed applications.

In this paper we present a generic late-join service for distributed interactive media, i.e, networked media which involve user interactions. Examples for distributed interactive media are shared whiteboards, networked computer games and distributed virtual environments. The generic late-join service allows a latecomer to join an ongoing session. This requires that the shared state of the medium is transmitted from the old participants of the session to the latecomer in an efficient and scalable way. In order to be generic and useful for a broad range of distributed interactive media, we have implemented the late-join service based on the Real Time Application Level Protocol for Distributed Interactive Media (RTP/I). All applications which employ this protocol can also use the generic late-join service. Furthermore the late-join service can be adapted to the specific needs of a given application by specifying policies for the late-join process. Applications which do use a different application level protocol than RTP/I may still use the concepts presented in this work. However, they will not be able to profit from our RTP/I based implementation.