Access to and Encoding of VRML State Information

In this paper we propose a concept for transparent access of VRML state information. Our approach enhances VRML-browsers to provide additional functionality instead of placing the burden for state access on content developers. The enhanced functionality is realized as an extension to the External Authoring Interface (EAI). Any application which relies on a VRML-browser as 3D presentation engine can use the new EAI functionality to get and set the state of arbitrary VRML content. In order to support diverse applications, the proposed methods not only allow to retrieve the full state of a complete world, but also the state of single objects and state changes. Since the results of state access should be independent of browser implementations, we also specify an encoding for state information. Data in this form is either produced or consumed during state access. For the encoding of state information we use an easy-to-parse and efficient binary encoding

Consistency in Continuous Distributed Interactive Media

In this paper we investigate how consistency can be ensured for continuous distributed interactive media, i.e. distributed media which change their state in reaction to user initiated operations as well as because of the passing of time. Existing approaches to reach consistency in discrete distributed 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 trade off relationship exists between the responsiveness of the medium and the appearance of short term inconsistencies. Currently this trade off is 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 and it enables the distribution of continuous interactive media which 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

Report Dagstuhl Seminar 10402 - Working Group on Fundamental Limits and Opportunities

This working group investigated first steps towards finding a theoretical foundation for inter-vehicle communication. The main outcome is a sketch of a roadmap for future work in this direction

A Survey on TCP-Friendly Congestion Control (extended version)

New trends in communication, in particular the deployment of multicast and real-time audio/video streaming applications, are likely to increase the percentage of non-TCP traffic in the Internet. These applications rarely perform congestion control in a TCP-friendly manner, i.e., they do not share the available bandwidth fairly with applications built on TCP, such as web browsers, FTP- or email-clients. The Internet community strongly fears that the current evolution could lead to a congestion collapse and starvation of TCP traffic. For this reason, TCP-friendly protocols are being developed that behave fairly with respect to co-existent TCP flows. In this article, we present a survey of current approaches to TCP-friendliness and discuss their characteristics. Both unicast and multicast congestion control protocols are examined, and an evaluation of the different approaches is presented

Probabilistic Congestion Control for Non-Adaptable Flows

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 adaption of the quality is not possible (e.g., since it is already at the lowest possible quality). 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 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 simulations with the network simulator ns-2

Application-Aware Distribution Trees for Application-Level Multicast

In this paper, we present an algorithm to construct application-aware distribution trees for application-level multicast. Unlike existing approaches, the distribution trees do not solely depend on network characteristics but also on the application semantics of the transmitted packets. In the presented algorithm, the application may specify an individual priority for each packet-receiver pair. The distribution tree is then constructed such that the higher the priority, the more direct the path from the sender to the packet\'s destination. This comes at the cost of an increase in link stress -- the more direct a path, the less likely it is that it can be integrated efficiently into an overlay multicast distribution tree. Our algorithm takes this tradeoff into account and constructs efficient application-aware distribution trees. We demonstrate the performance and characteristics of the algorithm through extensive simulation

A Reactive Location Service for Mobile Ad Hoc Networks

We present and analyze a reactive location service RLS for mobile ad hoc networks. RLS provides a mobile node in a wireless ad-hoc network with the means to inquire the current geographical position of another node on-demand and can be used as a building block for location-based routing. We provide a comparison of RLS to an ideal omniscient location service as well as to the complex Grid Location Service (GLS). In addition, we compare the performance of greedy location-based routing in combination with RLS to the performance of a non-location-based ad hoc routing approach, namely Dynamic Source Routing (DSR). DSR was chosen for the comparison since RLS can be considered an adaptation of DSR\'s route discovery mechanisms to the location-based domain. We also introduce and study possible optimizations for RLS, in particular caching, random re-broadcast jitter, and re-broadcast suppression. The quantitative results of our NS-2 simulation study show a very good perform! ance of RLS combined with greedy routing, outperforming GLS and DSR for scenarios with high mobility and high node density

Position-Based Multicast Routing for Mobile Ad-Hoc Networks

In this paper we present Position-Based Multicast (PBM), a multicast routing algorithm for mobile ad-hoc networks which does neither require the maintenance of a distribution structure (e.g., a tree or a mesh) nor resorts to flooding of data packets. Instead a forwarding node uses information about the positions of the destinations and its own neighbors to determine the next hops that the packet should be forwarded to and is thus very well suited for highly dynamic networks. PBM is a generalization of existing position-based unicast routing protocols such as face-2 or GPSR. The key contributions of PBM are rules for the splitting of multicast packets and a repair strategy for situations where there exists no direct neighbor that makes progress toward one or more destinations. The characteristics of PBM are evaluated in detail by means of simulation