Oliver P. Waldhorst

A special-purpose peer-to-peer file sharing system for mobile ad hoc networks

Establishing peer-to-peer (P2P) file sharing for mobile ad hoc networks (MANET) requires the construction of a search algorithm for transmitting queries and search results as well as the development of a transfer protocol for downloading files matching a query. In this paper, we present a special-purpose system for searching and file transfer tailored to both the characteristics of MANET and the requirements of peer-to-peer file sharing. Our approach is based on an application layer overlay network. As innovative feature, overlay routes are set up on demand by the search algorithm, closely matching network topology and transparently aggregating redundant transfer paths on a per-file basis. The transfer protocol guarantees low transmission overhead and a high fraction of successful downloads by utilizing overlay routes. In a detailed ns-2 simulation study, we show that both the search algorithm and the transfer protocol outperform off-the-shelf approaches based on a P2P file sharing system for the wireline Internet, TCP and a MANET routing protocol.

A distributed search service for peer-to-peer file sharing in mobile applications

In this paper, we present the concept of passive distributed indexing, a general-purpose distributed search service for mobile file sharing applications, which is based on peer-to-peer technology. The service enables resource-effective searching for files distributed across mobile devices based on simple queries. Building blocks of PDI constitute local broadcast transmission of query- and response messages, together with caching of query results at every device participating in PDI. Based on these building blocks, the need for flooding the entire network with query messages can be eliminated for most application. In extensive simulation studies, we demonstrate the performance of PDI. Because the requirements of a typical mobile file sharing application are not known-or even do not exist at all-we study the performance of PDI for different system environments and application requirements. We show that due to the flexible design PDI can be employed for several kinds of applications.

Performance analysis of time-enhanced UML diagrams based on stochastic processes

In this paper, we propose extensions to UML state diagrams and activity diagrams in order to allow the association of events with exponentially distributed and deterministic delays. We present an efficient algorithm for the state space generation out of these UML diagrams that allows a quantitative analysis by means of an underlying stochastic process. We identify a particular stochastic process, the generalized semi-Markov process (GSMP), as the appropriate vehicle on which quantitative analysis is performed. As innovative feature the algorithm removes vanishing states, i.e. states with no timed events active, and considers branching probabilities within activity diagrams. Furthermore, we introduce a performance evaluation framework that allows a system designer to predict performance measures at several steps in the design process. The applicability of our approach for practical performance and dependability projects is demonstrated by an UML specification of the General Packet Radio Service, a packet switched extension in GSM wireless networks.

The Underlay Abstraction in the Spontaneous Virtual Networks (SpoVNet) Architecture

Overlay-based services are a popular approach for providing functions like multicast, quality of service or security in the Internet without requiring infrastructure support. This paper presents the Underlay Abstraction Layer in the Spontaneous Virtual Networks (SpoVNet) architecture that enables easy and flexible creation of such services. Also building on an overlay approach, the Underlay Abstraction provides generic functionality to cope with mobility, multi-homing, and heterogeneity. It manages node mobility by separating node identifiers from network locators and it provides persistent connections by transparently switching locators. Multi-homing is supported by choosing the most appropriate pair of network locators for each connection. In order to cope with network and protocol heterogeneity, it uses dedicated overlay nodes, e.g., for relaying between IPv4 and IPv6 hosts. Since the functionality provided by the Underlay Abstraction can be used by several overlay-based services in parallel, redundant functionality is removed from services and applications.

Modeling epidemic information dissemination on mobile devices with finite buffers

Epidemic algorithms have recently been proposed as an effective solution for disseminating information in large-scale peer-to-peer (P2P) systems and in mobile ad hoc networks (MANET). In this paper, we present a modeling approach for steady-state analysis of epidemic dissemination of information in MANET. As major contribution, the introduced approach explicitly represents the spread of multiple data items, finite buffer capacity at mobile devices and a least recently used buffer replacement scheme. Using the introduced modeling approach, we analyze seven degrees of separation (7DS) as one well-known approach for implementing P2P data sharing in a MANET using epidemic dissemination of information. A validation of results derived from the analytical model against simulation shows excellent agreement. Quantitative performance curves derived from the analytical model yield several insights for optimizing the system design of 7DS.

Decentralized Bootstrapping of P2P Systems: A Practical View

So far, bootstrapping constitutes the only centralized task in otherwise decentralized peer-to-peer (P2P) systems. As a contribution to the development of generally applicable decentralized bootstrap mechanisms, in this paper we analyze two decentralized approaches from a practical point of view. We consider local host caches and random address probing for bootstrapping into the BitTorrent DHT as an example for a widely deployed P2P system. Based on the results of an extensive measurement study we show that local host caches allow rejoining the P2P system quickly after short times of disconnection, but are impracticable for infrequent or first-time users. Furthermore, random address probing is feasible using a direct Internet connection with high bandwidth, but is subject to practical issues raised by typical NAT routers and the distribution of ports used by BitTorrent clients. We propose two mechanisms for increasing the performance of random address probing: (1) probing multiple ports per host and (2) hash-based filter-resistant port selection, making distributed bootstrapping feasible even from a practical point of view.

Spontaneous Virtual Networks: On the road towards the Internet's Next Generation

Abstract Novel Internet applications demand global availability of complex services that can adapt dynamically to application requirements. At the same time, pervasive Internet usage and heterogeneous access technologies impose new challenges for service deployment. We present Spontaneous Virtual Networks (SpoVNet), a methodology that enables easy development of new services with transparent support for mobility, multi-homing, and heterogeneous environments. This article presents the overlay-based architecture of SpoVNet that supports the spontaneous deployment of new services as well as a seamless transition towards future networks. SpoVNet´s architecture offers support for the underlay aware adaptation of overlays by the use of cross-layer information. In the context of two exemplary services like a group communication service and an event service as well as two demanding applications – a realtime online game and a video streaming application – we illustrate how SpoVNet is of value in establishing services and applications for the Next Generation Internet.

Exploiting epidemic data dissemination for consistent lookup operations in mobile applications

This paper presents a general-purpose distributed lookup service, denoted Passive Distributed Indexing (PDI). PDI stores entries in form of (key, value) pairs in index caches located at mobile devices. Index caches are filled by epidemic dissemination of popular index entries. By exploiting node mobility, PDI can resolve most queries locally without sending messages outside the radio coverage of the inquiring node. For keeping index caches coherent, configurable value timeouts implementing implicit invalidation and lazy invalidation caches implementing explicit invalidation are introduced. Inconsistency in index caches due to weak connectivity or node failure is handled by value timeouts. Lazy invalidation caches reduce the fraction of stale index entries due to modified data at the origin node. Similar to index caches, invalidation caches are filled by epidemic distributions of invalidation messages. We evaluate the performance of PDI for a mobile P2P file sharing a mobile instant messaging application. Simulation results show that with the suitable integration of both invalidation mechanisms, up to 80% of the lookup operations return correct results and more than 90% of results delivered by PDI index caches are up-to-date.

Consistency mechanisms for a distributed lookup service supporting mobile applications

This paper presents a general-purpose distributed lookup service, denoted Passive Distributed Indexing (PDI). PDI stores entries in form of (key, value) pairs in index caches located in each mobile device. Index caches are filled by epidemic dissemination of popular index entries. By exploiting node mobility, PDI can resolve most queries locally without sending messages outside the radio coverage of the inquiring node. Thus, PDI reduces network traffic for the resolution of keys to values. For keeping index caches coherent, configurable value timeouts implementing implicit invalidation and lazy invalidation caches implementing explicit invalidation are introduced. Inconsistency in index caches due to weak connectivity or node failure is handled by value timeouts. Lazy invalidation caches reduce the fraction of stale index entries due to modified data at the origin node. Similar to index caches, invalidation caches are filled by epidemic distributions of invalidation messages. Simulation results show that with the suitable integration of both invalidation mechanisms, more than 95% of results delivered by PDI index caches are up-to-date for the considered scenario.

Energy-aware resource sharing with mobile devices

Sharing ad hoc resources by offering remote services through an appropriate infrastructure enables new applications for mobile devices. However, the willingness of device owners to contribute resources to such applications remains low as long as they cannot control the amount of energy spent in sharing. In this paper, we explore the potential, limitations, and tradeoffs of ad hoc resource sharing in a mobile context from an energy perspective. We show by a simulation that the battery lifetime of participating devices can be extended up to 27% beyond a random service selection through the use of appropriate service selection policies that take into account the energy cost of a requested service. Motivated by these results, we present a framework for energy-aware resource sharing that consists of a generic energy estimator for forecasting the energy consumption of a remote service call. The estimator can be customized easily for a specific platform by solving a linear equation system with coefficients derived from benchmark measurements. We present an exemplary case study of this for the Nokia N810 platform and show by extensive experiments that the average estimation error is below 5% for 88 % of the service calls. Furthermore, measurements conducted with a prototype implementation of the resource sharing framework show that battery lifetime can in fact be extended by energy-aware service selection policies.