Data spaces are an emerging concept with significant potential to enable a data-centric economy by fostering seamless and secure data sharing across diverse stakeholders. These environments are designed to unlock the value of data by ensuring interoperability and collaboration, which are essential for innovation and informed decision-making. However, managing access control in data spaces poses unique challenges, as it must account for complex relationships not only among stakeholders but also among data items themselves, requiring a flexible and context-aware approach. To this end, in this paper we present the design, implementation, and evaluation of an access control solution tailored for data spaces. Our solution leverages the paradigm of Relationship-Based Access Control (ReBAC), enabling the definition and enforcement of access control policies that consider the relationships between entities within the data space, as well as data consumer organisational structures. Furthermore, we propose a distributed version of our solution to facilitate the segregation of access control management across different administrative domains. Our approach supports fine-grained, continuous access control by dynamically evaluating the context of both the protected data items and the consumers of the data space. To ensure compatibility with existing data-sharing standards, we have integrated our solution with ETSI NGSI-LD API, a standardised interface for interacting with data spaces.

Network Music Performance (NMP), where musicians perform music together over the Internet, requires ultra-low delays. Achieving a sense of presence in NMP requires both aural and visual communication, preferably coupled with immersive telepresence technologies, such as holographic communication. Existing 4G networks offer neither the ultra-low latencies needed for audio and video, nor the ample bandwidth required for video and (especially) holographic communication. In the Telepresence-Enhanced Network Music Performance (TENeMP) project we are investigating and developing solutions for immersive NMP, to be tested in the 5G testbeds of the SPIRIT project. We provide an overview of the experimental testbeds of TENeMP as well as the software tools developed by the project. We then present baseline video and audio latency measurements over 4G and 5G-NSA networks, which demonstrate NMP’s need for the capabilities of 5G-SA and beyond networks.

The proliferation of Internet of Things (IoT) devices that operate unattended providing a multitude of important and often sensitive services highlights the need for seamless interoperability and increased security. We argue that digital twins of IoT devices, with the right design, can enhance the security, reliability, auditability, and interoperability of IoT systems. The salient features of digital twins have made them key elements for the IoT and Industry 4.0. In this paper, we leverage advances in W3C's Web of Things (WoT) standards and Distributed Ledger Technologies (DLTs) to present a novel design of smart contract-based digital twins with enhanced security, transparency, interoperabilty, and reliability. We provide two different variations of that general design using two different blockchains (one public and one private, permissioned blockchain), and we present design trade-offs. Furthermore, we introduce an architecture for accessing and controlling IoT devices securely, reliably, providing full auditability, while at the same time using the proposed digital twins as an indirection mechanism (proxy). The proposed architecture leverages the blockchain to offer notable properties, namely, decentralization, immutability, auditability, non-repudiation, availability, and reliability. Moreover, it introduces mass actuation, easier management of IoT devices, enhanced security to the IoT gateways, it enables new business models, and it makes consumer devices (vendor-)agnostic.

Emerging energy management systems (EMS) involve devices and services provided by multiple stakeholders. In order to improve the interoperability of these systems, state of the art efforts propose an interoperability middleware that mediates the communication between end-user applications and EMS components. The potential lack of trust between the different stakeholders raises the need for fine-grained access control mechanisms. However, extending the middleware to support access control in a secure and usable way is a challenging problem. In this paper, we present a solution that achieves fine-grained authorization using Verifiable Credentials (VCs). Our solution leverages VC properties to enable end-users to combine authorizations issued by different entities. Additionally, our solution integrates a cloud-based VC wallet that hides the authorization process from end-user applications, thus facilitating interoperability among EMSes and the development of new, secure applications.

Digital Twins and the Internet of Things (IoT) are two of the most prominent recent concepts and technologies. The IoT supports many applications that merge the physical with the cyber world. This highlights the need for improved security. Here, we argue that digital twins can help in securing and strengthening the IoT by using them for interacting with the actual IoT devices. While most digital twins implementations are centralized, we propose to integrate Distributed Ledger Technologies (DLTs) and digital twins into the IoT to realize decentralized, secure, available, flexible, and auditable blockchain-based IoT services for IoT devices that follow the W3C Web of Things (WoT) standards. In this work, we present the design of SmartTwin, a blockchain-based digital twin framework for which we provide two different implementations using two different blockchains, we present the design trade-offs, and we discuss future research and development directions.

The proliferation of Internet of Things (IoT) devices and applications that need to cooperate unattended highlights the need for seamless interoperability and intrinsic security. We argue that Distributed Ledger Technologies (DLTs), due to their decentralized nature, transparent operations, immutability, and availability, can enhance the security, reliability, and interoperability of such IoT systems. In this paper, we advance the integration of W3C's Web of Things (WoT) standards with DLTs and smart contracts, introducing smart contracts as “Digital Twins” of (physical) devices, or whole Cyber-Physical subsystems. Namely, we introduce a DLT-based architecture for controlling devices across federated IoT systems, securely, reliably, and with full auditability. The proposed architecture provides mass actuation and service composition with notable security properties, such as full auditability, transparency, and high availability. Specifically, a single request, with multiple action parameters and conditions, can trigger the reliable and secure actuation of a large number of possibly physically dispersed actuators.

Named Data Networking (NDN) is a realization of the Information-Centric Networking (ICN) paradigm, where routing is based on content identifiers rather than on network location identifiers. The routing state in NDN can grow exponentially, not only due to the huge number of content identifiers (as opposed to network addresses) but also because it is difficult to detect "fake" routing advertisements. For example, in contrast to IP-based routing, a potentially valid routing entry in NDN can be advertised from multiple network locations, making NDN susceptible to Denial-of-Service attacks at the routing layer. In this paper, we leverage Decentralized Identifiers (DIDs) to build self-verifiable "content advertisements." With our solution, any router can verify that a content advertisement originates from an "authorized" entity, without requiring any trusted third party. We implement our solution and we evaluate it in a scenario where filtering is implemented by the edge routers. We show that our solution reduces fake routing advertisements with minimal computational overhead.

We propose a capability-based access control technique for sharing Web resources, based on Verifiable Credentials (VCs) and OAuth 2.0. VCs are a secure means for expressing claims about a subject. Although VCs are ideal for encoding capabilities, the lack of standards for exchanging and using VCs impedes their adoption and limits their interoperability. We mitigate this problem by integrating VCs into the OAuth 2.0 authorization flow. To this end, we propose a new form of OAuth 2.0 access token based on VCs. Our approach leverages JSON Web Tokens (JWT) to encode VCs and takes advantage of JWT-based mechanisms for proving VC possession. Our solution not only requires minimum changes to existing OAuth 2.0 code bases, but it also removes some of the complexity of verifying VC claims by relying on JSON Web Signatures: a simple, standardized, and well supported signature format. Additionally, we fill the gap of VC generation processes by defining a new protocol that leverages the OAuth 2.0 “client credentials” grant.

In IPFS content identifiers are constructed based on the item's data therefore the binding between an item's identifier and its data can be deterministically verified. Nevertheless, once an item is modified, its identifier also changes. Therefore when it comes to mutable content there is a need for keeping track of the “latest” IPFS identifier. This is achieved using naming protocols on top of IPFS, such as IPNS and DNSlink, that map a constant name to an IPFS identifier, allowing at the same time content owners to update these mappings. Nevertheless, IPNS relies on a cryptographic key pair that cannot be rotated, and DNSlink does not provide content authenticity protection. In this paper, we propose a naming protocol that combines DNSlink and decentralized identifiers to enable self-verifiable content items. Our protocol provides content authenticity without imposing any security requirement to DNSlink. Furthermore, our protocol prevent fake content even if attackers have access to the DNS server of the content owner or have access to the content owner secret keys. Our proof of concept implementation shows that our protocol is feasible and can be used with existing IPFS tools.

We leverage the InterPlanetary File System (IPFS) and Non-Fungible Tokens (NFTs) backed by Distributed Ledger Technologies (DLTs) to build a flexible, decentralized, and fair baseline system for trading games. Our solution creates a fully decentralized system, where new business models are enabled, as the evolvable assets of the games can be resold and priced depending on their rarity, giving also a cut to the digital artist, without the need for a trusted party. The system guarantees that assets will remain online, thus the users do not risk losing control over the artefacts or their value, even if the creator game company loses interest or goes bankrupt.

Service providers usually require detailed statistics in order to improve their services. On the other hand, privacy concerns are intensifying and sensitive data is protected by legislation, such as GDPR (General Data Protection Regulation). In this paper, we present the design, implementation, and evaluation of a marketplace that allows “data consumers” to buy information from “data providers”, which can then be used for generating meaningful statistics. Additionally, our system enables “system operators” that can select which data providers are allowed to provide data, based on filtering criteria specified by the data consumer. We leverage local differential privacy to protect the data provider's privacy against data consumers, as well as against system operators, and we build a blockchain-based solution for ensuring fair exchange, and immutable data logs. Our design targets use cases that involve hundreds or even thousands of data providers. We prove the feasibility of our approach through a proof-of concept implementation of a measurement sharing application for smart-grid systems.

Efficient congestion control is critical to the operation of MPTCP, the Multipath extension of TCP. Congestion control in such an environment primarily aims at enhancing the cumulative TCP throughput over the available paths, while preserving TCP-friendliness by fairly sharing the available bandwidth with single-path TCP flows in each path. While most existing multipath congestion control algorithms fulfill the TCP-friendliness objective in their steady state, their throughput convergence latency is high, rendering them ineffective for short-lived flows. We have proposed Normalized Multipath Congestion Control (NMCC), an MPTCP congestion control algorithm that achieves TCP-friendliness faster, by normalizing the growth of individual sub-flow throughput rather than the throughput itself. As NMCC can become unfriendly when it experiences sparse congestion events, in this paper we introduce the extended NMCC (e-NMCC) protocol that caters for TCP-friendliness upon both throughput growth and throughput reduction epochs. We analytically characterize e-NMCC in terms of TCP-friendliness and responsiveness and compare it with alternative algorithms. Finally, we assess the performance of e-NMCC through experimentation with the htsim simulator and a real Linux implementation. Our results confirm that e-NMCC accelerates throughput convergence, thus ensuring TCP-friendliness regardless of connection duration and underlying network conditions.

HTTPS enhances end-user privacy and is often preferred or enforced by over-the-top content providers, but renders inoperable all intermediate network functions operating above the transport layer, including caching, content/protocol optimization, and security filtering tools. These functions are crucial for the optimization of integrated satellite-terrestrial networks. Additionally, due to the use of end-to-end and per- session encryption keys, the advantages of a satellite's wide- area broadcasting capabilities are limited or even negated completely. This paper investigates two solutions for authorized TLS interception that involve TLS splitting. We present how these solutions can be incorporated into integrated satellite- terrestrial networks and we discuss their trade-offs in terms of deployment, performance, and privacy. Furthermore, we design a solution that leverages satellite broadcast transmission even in the presence of TLS (i.e. with the use of HTTPS) by exploiting application layer encryption in the path between the satellite terminal and the TLS server. Our findings indicate that even if no other operation than TLS splitting is performed, TLS handshake time, which involves roundtrips through possibly a Geosynchronous satellite, can be reduced by up to 94%. Moreover, by combining an application layer encryption solution with TLS splitting, broadcast transmissions can be exploited as well as proactive caching, content pushing, request aggregation, and other optimizations.

Blockchains and smart contracts are an emerging, promising technology, that has received considerable attention. We use the blockchain technology, and in particular Ethereum, to implement a large-scale event-based Internet of Things (IoT) control system. We argue that the distributed nature of the “ledger,” as well as, Ethereum's capability of parallel execution of replicated “smart contracts”, provide the sought after automation, generality, flexibility, resilience, and high availability. We design a realistic blockchain-based loT architecture, using existing technologies while by taking into consideration the characteristics and limitations of IoT devices and applications. Furthermore, we leverage blockchain's immutability and Ethereum's support for custom tokens to build a robust and efficient token-based access control mechanism. Our evaluation shows that our solution is viable and offers significant security and usability advantages.

We present and evaluate models that allow clients to access IoT resources using secure and trusted device-to-device (D2D) communication, while utilizing smart contracts to obtain the benefits of blockchain technology. These benefits include decentralized trust, immutability, transparency, and high availability. The models consider different network connection capabilities of the clients and the IoT resources, namely continuous network connectivity and D2D-only connectivity. We describe two approaches for utilizing blockchains and smart contracts in the authorization process: in the first approach, only hashes of the authorization information are recorded on the blockchain. In the second approach, a smart contract handles authorization requests. We implement the approaches using the OAuth 2.0 delegated authorization framework and evaluate the implementations on the public Ethereum testnet Rinkeby, in terms of execution cost, contract creation cost, and delay. Our evaluation quantifies the tradeoffs of blockchain cost and smart contract functionality, such as blocking and non-blocking operation, and the reduction of the transaction cost that can be achieved when multiple authorization requests are concatenated in a single transaction.

In this paper, a performance evaluation of the Quality-of-Experience (QoE) of YouTube video streaming in mobile broadband networks with active measurements is described. The measurements were collected from a field experiment campaign using the MONROE platform which provides probes in four European countries and enables the benchmarking of three mobile broadband operators. Firstly, we present a framework for the automated collection and processing of the measurements, and then, we analyze the results to identify the cache allocation policy per operator. Additionally, we examine whether the selected cache server has an effect on the delivered video quality and present the results using standardised objective methods for the estimation of the perceived quality.

We present and experimentally evaluate procedures for efficient IoT data collection while achieving target requirements in terms of data accuracy and privacy protection. The procedures adjust the time period between consecutive measurements following an additive increase and multiplicative decrease (AIMD) scheme based on a target data accuracy and add noise to measurements using differential privacy techniques. The experimental evaluation involves real temperature and humidity measurements obtained from two testbeds through the FIESTA-IoT platform. Our results show that the AIMD adaptation of the measurement period is robust to different types of measurements from different testbeds, without having any tuning parameters, and the addition of noise to the sensor measurements using differential privacy has a negligible effect on the aggregate statistics.

We analyze the design space for implementing the Constrained Application Protocol (CoAP) within Content-Centric Networks (CCN), identifying several CoAP specific scenarios and seeing how they map to CCNs. We present an evaluation, recommendations for implementations and extensions, and directions for future work. Our key result is that while several protocol features and flows map naturally, some communication patterns are more difficult to capture without modifications or additions to the CCN model. In addition to many conceptual and also non-performance related advantages, our experimental evaluation demonstrates that it is feasible and useful to implement CoAP over CCN, with the performance similar to that of CoAP over UDP over IP, and in several cases outperforming it significantly.

Name resolution is at the heart of Information-Centric Networking (ICN), where names are used to both identify information and/or services, and to guide routing and forwarding inside the network. The ICN focus on information, rather than hosts, raises significant concerns regarding the scalability of the required Name Resolution System (NRS), especially when considering global scale, inter-domain deployments. In the route-by-name approach to NRS construction, name resolution and the corresponding state follow the routing infrastructure of the underlying inter-domain network. The scalability of the resulting NRS is therefore strongly related to the topological and routing characteristics of the network. However, past work has largely neglected this aspect. In this paper, we present a detailed investigation and comparison of the scalability properties of two route-by-name inter-domain NRS designs, namely, DONA and CURLING. Based on both real, full-scale inter-domain topology traces and synthetic, scaled-down topologies, our work quantifies a series of important scalability-related performance aspects, including the distribution of name-resolution state across the Internet topology and the associated processing and signaling overheads. We show that by avoiding DONA's exchange of state across peering links, CURLING results in deployment costs proportional to the total number of downstream customers of each Autonomous System. This translates to a 62-fold global state size reduction, at the expense of a 2.78-fold increase in lookup processing load, making CURLING a feasible approach to ICN name resolution.

We study a linear multihop network that is formed by wireless devices that can directly communicate pairwise whenever two devices are within range of each other. This Device-to-Device communication model is expected to play a significant role in future 5G wireless networks due to its advantages (e.g., cellular offloading, increased throughput and low cost/energy communication). In such networks, devices are typically selfish and compete for channel access aiming at maximizing their own throughput while at the same time avoiding packet collisions. In this setup, we study how an efficient coexistence of these devices may be achieved, using a game-theoretic approach. First, we model the contention for the channel as a game and study the structural properties of the resulting Nash Equilibria (NE). Then, we design a distributed, round-based scheme that is guaranteed to converge to a NE. We compare quantitatively and qualitatively this scheme with previous work. We show that this scheme converges faster to a NE, in a number of rounds that is proportional to the logarithm of the number of nodes of the network. Moreover, the convergence is monotonic, meaning that the percentage of nodes that finalize their strategy is increasing in each round.

Mobile application development for smartphones is a trend in the telecommunications industry. However, their deployment is not seamless since many applications are not “mobile network-friendly.” A key problem that frequently arises is an excessive number of signaling messages, known as signaling storms. This leads to very high overhead and a decrease in operator income. We focus on this problem and propose an approach that is based on an information-centric networking deployment at the access network. We compute the number of signaling messages and derive the conditions under which our approach leads to fewer messages than the approach that is used in current networks. We also argue about the network and application layer modifications that are needed for the adoption of our method.

Nowadays, the Internet connects more objects than people. These devices generate vast amounts of information. Furthermore, identification technologies - such as Radio Frequency Identification (RFID) - enable the association of information with identifiable objects, not necessarily connected to the Internet. All this information is organized into vertical silos. These silos usually belong to different administrative domains and use their own specific communication protocols. In this paper, we present our vision for a global, all-encompassing Internet of Things (IoT) realized through an integrating architecture relying on information and its identifiers/names. We envision the IoT as the architecture that will interconnect all these silos and will make the information generated by or associated with objects globally accessible. Moreover, we argue that the Information-Centric Networking (ICN) paradigm is the ideal candidate architecture for the realization of that IoT vision. In line with this premise, we propose a research agenda for the realization of a full-fledged ICN-based IoT architecture.

We focus on the problem of efficiently integrating wireless users in future Information-Centric Networks (ICN), where communication is based on publish-subscribe primitives. The current host-centric Internet paradigm is abandoned in favor of information-oriented, rendezvous-based communication, where multicast data delivery is the norm. However, Wi-Fi, the predominant means of local wireless connectivity today, but also 3G and 4G technologies, are known to suffer from poor multicast performance. Data destined to a broadcast or multicast address are typically transmitted at lower rates to increase reliability for clients with poor signal conditions, causing unfavorable delays for high-rate users. One approach to this problem is to designate a subset of the clients as relays who re-broadcast packets for other clients at a higher rate. Given that different types of content have different performance requirements, we exploit content-awareness, inherent in our environment, to optimize for different criteria on a per-content basis. For this purpose, we provide a multi-objective optimization formulation for the problem of relay selection and rate assignment, which can capture the tradeoff among reliability, performance and energy cost.

This short paper describes the objectives and initial results of project I-CAN: Information-Centric Future Mobile and Wireless Access Networks. I-CAN seeks to radically advance the integration of cellular and wireless access technologies by developing and evaluating architectures and procedures for future access networks based on Information-Centric Networking (ICN).

We present a distributed proactive caching approach that exploits user mobility information to decide where to proactively cache data to support seamless mobility, while efficiently utilizing cache storage using a congestion pricing scheme. The proposed approach is applicable to the case where objects have different sizes and to a two-level cache hierarchy, for both of which the proactive caching problem is hard. Our evaluation results show how various system parameters influence the delay gains of the proposed approach, which achieves robust and good performance relative to an oracle and an optimal scheme for a flat cache structure.

ABSTRACT Revealing one's interests in communication has been recognized as a growing problem in the Internet. We postulate that it is desirable for future information retrieval systems to provide privacy in both what information is requested and what information is received, without raising obstacles to the deployment of accounting and access control mechanisms. This paper outlines a solution that fulfills this requirement in the context of broker‐based systems, that is, systems in which brokers facilitate the communication between a consumer and a provider (of information). Broker‐assisted communication is a common paradigm used in many settings, including contemporary information‐centric networking approaches. We present the design and the evaluation of a solution that conceals consumers' interests, without hiding consumer identity or location. The developed solution is applied over a system of hierarchically organized brokers; similar systems are used in many information lookup services. Because in these systems, information is distributed in various locations, traditional private information retrieval (PIR) protocols exhibit significant communication overhead. Our solution achieves up to 97% less communication overhead compared with a PIR protocol, without additional computational overhead. Copyright © 2013 John Wiley & Sons, Ltd.

Content pollution is one of the most common attacks against peer-to-peer file-sharing systems. As such, systems are usually open to users, and the deployed security mechanisms merely examine the sanity of the downloaded files—content pollution attacks can be easily launched. InfoRanking is a mechanism that tries to mitigate this security risk by ranking content items. In this paper, we show through analysis, fluid modeling, and simulation that when InfoRanking is used, attackers can deceive users only when they share corrupted copies of legitimate file versions. Nevertheless, as corrupted files can be immediately detected after being downloaded, this attack is only effective when users enter the system at very low rate and leave relatively fast.

The Internet of Things is an emerging paradigm that allows the association of information with objects. The information about an object is stored in databases, distributed around the globe, maintained by various stakeholders that participate in the object's supply chain. A Discovery Service (DS) is responsible for collecting all these database URIs and feeding them to clients that query about an object. DSs are usually centralized and are designed to effectively aggregate as many information sources as possible, rather than trying to respond optimally to unforeseen user queries. In this paper we propose a novel, Information-Centric Networking (ICN) inspired, architecture that eliminates the need for a separate DS, enabling at the same time multi-ownership and flexible management of information that is associated with an object. In our ICN approach, information about an object is organized in scopes. Each scope has its own access control rules allowing easy control of information dissemination. Moreover scopes can be hierarchically organized, creating complex access structures that can reflect business relationships. In our architecture companies provide information about an object to the appropriate scopes, but they never lose control of this information. To access information associated with an object,a user queries the scope that corresponds to the desired context, which will forward the query to a service that will respond with the appropriate data. The query will not reveal any extra information, not authorized to be retrieved.

ABSTRACT The vision towards the Network of the Future cannot be separated from the fact that today's networks, and networking services are subject to sophisticated and very effective attacks. When these attacks first appeared, spoofing and distributed denial‐of‐service attacks were treated as apocalypse for networking. Now, they are considered moderate damage, whereas more sophisticated and inconspicuous attacks, such as botnets activities, might have greater and far reaching impact. As the Internet is expanding to mobile phones and ‘smart dust’ and as its social coverage is liberalized towards the realization of ubiquitous computing (with communication), the concerns on security and privacy have become deeper and the problems more challenging than ever. Re‐designing the Internet as the Network of the Future is self‐motivating for researchers, and security and privacy cannot be provided again as separate, external, add‐on, solutions. In this paper, we discuss the security and privacy challenges of the Network of the Future and try to delimit the solutions space on the basis of emerging techniques. We also review methods that help the quantification of security and privacy in an effort to provide a more systematic and quantitative treatment of the area in the future. Copyright © 2011 John Wiley & Sons, Ltd.

The low cost and ease of installation of Wi-Fi equipment operating in unlicensed spectrum have made dense Wi-Fi deployments a reality in most modern urban areas. With the lack of many non-overlapping frequencies to operate on, interference among neighbor Wi-Fi cells can cause significant performance degradation. Here we study the problem of topology discovery in such dense deployments, which is necessary in order to combat interference. To this end, we apply a client-driven scheme, where client devices sense the spectrum and report overlapping cells. However, reporting entities cannot always be assumed trustworthy. We therefore study cases where reporters attack the discovery process by submitting fake information and propose simple countermeasures to tackle some attacker strategies. We show analytically and via simulations that, in realistic urban scenarios, our mechanisms are effective, even in the presence of a large number of attackers.

Information-Centric Networking (ICN) is constantly gaining momentum within the Future Internet research community. In the PURSUIT research project we are developing a clean-slate Pub/Sub Internetworking (PSI or Ψ) approach with integrated seamless mobility support. The novel ICN mechanisms supported in Ψ, along with smartly placed in-network caches, enable the architecture to handle both mobile and fixed devices in a uniform way. This paper presents a blueprint for optimizing mobility support in Ψ without modifications to the architecture or add-on solutions. We demonstrate a micro-mobility scenario that describes the functionality of Ψ's core components in supporting mobility and then sketch our plans for future work and a proper assessment of these designs.

The efficient management of the radio spectrum is a key functionality in every type of wireless network. Wireless nodes generally have heterogeneous QoS targets, which sometimes cannot be satisfied for all of them due to the high interference levels that frequently arise, even in sparse topologies. In this work, we propose a distributed negotiation-based power control algorithm that aims at maximizing the number of nodes achieving their QoS targets. Our algorithm combines the influential Foschini-Miljanic power control algorithm with a bargaining-inspired phase (among the unsatisfied nodes only). In particular, all nodes are endowed with an initial budget; unsatisfied nodes randomly pick others to negotiate with for the level of their transmission powers; if a negotiation leads to an agreement, a node gives some (predefined) reward to the other and the latter reduces its power to the agreed level; the process is then repeated, using the updated budgets. Simulations show that, under various negotiation scenarios, our scheme is more efficient than previously proposed approaches that impose on the "weakest" nodes (those that are further from their targets) to turn off their power completely. More importantly, our scheme leads to a statistical rotation of the set of nodes that achieve their target, independently of the initial budget allocation, and hence is more fair.

We present our work on a sensor-based smart system automatically trained to recognize the activities of individuals in their home. In this paper we present and analyze a method for recognizing the indoor everyday activities of a monitored individual. This method is based on the data mining technique of association rules and Allen's temporal relations. Our experimental results show that for many (but not all) activities, this method produces a recognition accuracy of nearly 100%, in contrast to other methods based on data mining classifiers. The proposed method is accurate, very flexible and adaptable to a dynamic environment such as the "Smart Home" and we believe that it deserves further attention.

In densely populated cities, Wi-Fi networks-private or otherwise-are ubiquitous. We focus on the provision of citywide broadband communication capability to mobile users through private Wi-Fi networks that are in range but belong to others. We form a club that relies on indirect reciprocity: Members participate in the club and provide free Wi-Fi access to other members in order to enjoy the same benefit when they are away from their own Wi-Fi network. Our club scheme does not require registration with an authority and does not rely on centrally issued club identities: Members create their own identities (public-private key pairs) and receive signed digital receipts when they provide Wi-Fi service to other members. These receipts form a distributed receipt graph, parts of which are used as input to an indirect reciprocity algorithm that classifies club members according to their contribution. We show that our algorithm can sustain cooperation within the club and is robust to attacks by free-riders. We implement and evaluate our proposed club algorithms on commodity Wi-Fi routers and dual-mode cellular/Wi-Fi phones. Because we anticipate that Wi-Fi telephony will be a popular club application, we present and evaluate a secure and decentralized architecture for citywide voice (and multimedia) communications that is compatible with our club both from an architectural as well as an incentives perspective.

In recent years, we have witnessed a trend towards open wireless access, mainly driven by the low cost of IEEE 802.11-capable equipment and its operation in unlicensed spectrum. This trend has many faces; municipalities set up free Wi-Fi hotspots for Internet connectivity in public spaces, while Wi-Fi access is provided as an extra service to customers in other venues, such as restaurants or hotels. Also, in many metropolitan areas worldwide, community-initiated wireless mesh networks have emerged. Their members use inexpensive technologies to build multi-hop wireless networks and communicate autonomously. In this work, we document the structure and evolution of one of the largest community wireless mesh networks worldwide, the Athens Wireless Metropolitan Network (AWMN). We focus on how the network has grown in size, complexity and service offerings in the last few years. To be able to dynamically discover the structure of such a network, we have developed a suitable topology discovery methodology.

Abstract Service discovery can be greatly enhanced in terms of efficiency, both regarding service discoverability and energy consumption, by piggybacking service information into routing messages. Thus, service discovery does not generate additional messages and a node requesting a service, in addition to discovering that service, it is simultaneously informed of the route to the service provider. We extended the Zone Routing Protocol in order to encapsulate service information in its routing messages. Our extended protocol, E‐ZRP, may be seen as a representative of routing layer protocols providing service discovery functionality. Simulations demonstrate the superiority of this routing layer‐based service discovery scheme over that of a similar, but application layer based service discovery scheme. In order to have a thorough evaluation of our approach we introduced a new metric, called Service Availability Duration (SAD), which characterizes the ‘quality’ of discovered services and experimentally examines the implications of network density and node mobility on the availability of services discovered with E‐ZRP, as a typical representative of routing layer based service discovery protocols. Copyright © 2008 John Wiley & Sons, Ltd.

IEEE 802.11k is an extension of the IEEE 802.11 specification for radio resource measurements. In an IEEE 802.11k-enabled wireless LAN, an access point or other network element may request from a client or another access point to monitor and report the load of a channel. We call the latter a channel monitoring station. In this paper we propose a mechanism for a channel monitoring station to efficiently derive accurate values of channel load.We especially focus on optimizing the duration of channel monitoring and thus minimize the impact on applications. Note that such mechanisms are critical for the success of new sharing regimes such as Cognitive Radio and Open Spectrum Access.

Driven by their low cost and ease of deployment, as well as their operation in unlicensed spectrum bands, IEEE 802.11-based Wireless Local Area Networks (WLANs), also termed Wi-Fi, have been established as the de facto access technology for local area wireless connectivity. Especially in densely-populated urban areas, WLAN presence is ubiquitous. Residential WLAN owners, municipalities and venue owners, among others, set up wireless hotspots for private or public use. However, one can identify two important problems that have to be efficiently tackled. On the one hand, while Wi-Fi is present practically everywhere in modern metropolitan areas, access to roaming users is typically restricted. At the same time, the broadband Internet connections WLAN Access Points (APs) are attached to may have excess capacity, thus leaving resources underutilized. On the other hand, unplanned Wi-Fi deployment leads to significant interference problems among neighbor WLANs. In this work, we exploit our prior work on WLAN sharing communities to jointly tackle the above problems. The solution we propose is based on offering users QoS benefits as an incentive to perform spectrum sensing and supply interference reports to the WLAN APs they are connected to. We present the design of our mechanisms, discuss some of their properties and investigate their expected performance overhead, especially on delay-sensitive applications like VoIP.

The use of licensed spectrum for wireless communication is driven by the need to control interference between different operators. However, with this mode of regulation, spectrum utilization is far from efficient and the growth of wireless networks is hindered by the shortage of free frequency bands and the vast investments for the acquisition of a license. In view of this situation, we present an alternative evolution path for the unobstructed growth of wireless networks and the efficient use of spectrum. The proposed architecture is based on the use of unlicensed spectrum and the open access of users to all public networks without prior contracts with operators. We highlight and discuss the inherent technical challenges that must be tackled before the proposed solution can be realized. Special attention is paid to the inherent need for alternative interference mitigation strategies.

In the past few years there has been increasing research interest in service discovery protocols for mobile ad hoc networks (MANET). The most promising approaches so far address energy efficiency by merging the service discovery process with the routing process. Actually the service information is piggy-backed into routing messages, so that a node is simultaneously informed of available services and of routes towards the corresponding service providers. This cross layering leads to improved adaptation to the network conditions and at the same time to significant energy savings. Those savings are infeasible if the two processes are implemented separately, because then each one would have to use its own messages and create additional (if not) redundant network overhead. In this paper we propose such a hybrid adaptive protocol for energy consumption-avert users. This protocol, named AVERT, is based on the independent zone routing framework (IZR) and has the ability to allow each node to adapt its zone range (similarly to the way it is done in IZR). We also add a mechanism to adapt the sending rate of proactive messages on each node based only on local traffic monitoring. Through simulations we show that using this mechanism, the energy efficiency achieved is substantially higher compared to similar hybrid service and route discovery protocols.

Optimized service provisioning is a challenging problem in dynamic environments such as mobile ad hoc networks (MANETs). Most of the existing approaches assume an environment, where service provision is free (and dictated) and servers do not have an incentive to maximize their benefit. In this paper we consider the nodes in MANETs to be independent, rational agents trying to maximize their profits through service provision. We model this problem as a generalized assignment problem (GAP). We adopt a pay-as-you-go model and introduce into the proposed profit maximization algorithm expected payoffs based on estimates of server-to-client connectivity. We define connectivity as the lifetime of the network connection between a client and a server. We experimentally study cases with non-cooperative and cooperative servers and investigate the gain of the estimate based maximization algorithm versus a classic maximization algorithm, which does not take into account the networkpsilas dynamics that affect server-to-client connectivity. The results show that our approach achieves up to three-fold improved server profits compared to the classical one and is especially suited for MANETs with high-mobility.

The introduction of mobile devices and wireless communications in the context of the Grid computing paradigm has recently drawn the attention of the research community. We point out fundamental issues and problems emerging from this introduction and further describe our ongoing work for the investigation of key design decisions and optimizations.

Recent advances in mobile communications and computing and strong interest of the scientific community in the Grid have led to research into the Mobile Grid. We discuss various approaches proposed in the literature and try to point out the fundamental issues and problems emerging from the introduction of mobile devices and wireless communications in the context of the Grid computing paradigm. We further propose an architecture for the realization of a Mobile Grid and investigate key design decisions and optimizations.

Recent advances in mobile communications and computing and strong interest of the scientific community in the Grid have led to research into the Mobile Grid. We discuss various approaches proposed in the literature and try to point out the fundamental issues and problems emerging from the introduction of mobile devices and wireless communications in the context of the Grid computing paradigm. We further describe an architecture for the realization of a Mobile Grid and investigate key design decisions and optimizations.

Nowadays wireless networks are becoming increasingly popular in urban areas, university campuses and corporate environments. This, along with the widespread deployment of mobile devices with advanced computational capabilities, creates more opportunities for developing collaborative/social applications. The development of such applications often raises a number of challenges for the developer mostly due to synchronization issues, increased network failures of the wireless environment and limited resource availability. In this paper we present a novel framework that tackles the above problems and releases the developer from the burden of dealing with cross-domain problems. We focus on the development of a certain type of applications, inside the context of ubiquitous gaming, in a small scale peer-to-peer manner.

Abstract—Wireless Community Networks (WCNs) are wide-area wireless networks whose nodes are owned and managed by volunteers. We focus on the provision of Internet access to mobile users through WCN-controlled wireless LAN access points (APs). We rely on reciprocity: a person participates in the WCN and provides ‘free ’ Internet access to mobile users in order to enjoy the same benefit when mobile. Our reciprocity scheme is compatible with the distinctive structure of WCNs: it does not require registration with authorities, relying only on uncertified free identities (public-private key pairs). Users sign digital receipts when they consume service. The receipts form a receipt graph, which is used as input to a reciprocity algorithm that identifies contributing users using network flow techniques. Simulations show that this algorithm can sustain reciprocal cooperation. We have implemented our algorithm to run on common APs. I.

We have designed and implemented P2PWNC, a fully distributed, open to all, autonomous WLAN roaming scheme that can be used in a community to provide various multimedia communication services. Here we discuss various security related issues and the support provided by P2PWNC for several aspects of communication and roaming privacy. We also report on aspects of its performance, focusing on the performance penalty of security-related operations. The core P2PWNC scheme assumes that community members are selfish and do not trust each other and uses a secure incentive technique to encourage their contribution. It protects the real-world identities of community providers and clients by relying only on disposable opaque identifiers and does not rely on any authority to resolve disputes or control membership. It could easily complement cellular networks for low-mobility users in metropolitan areas where some wireless community networks provide wide coverage

Only recently, for the first time, private individuals are in a position to provide telecommunication services. This was brought on by the emergence of low-cost Wireless Local Area Network (WLAN) technologies. The density of WLANs in many urban areas is high and increasing. The access bandwidth that WLANs can offer to individual users is greater than what cellular offers-even if the backhaul is a simple DSL connection. In addition, certain new mobile phones support WLAN technology and protocols. Thus, the stage is set for an alternative public cellular network, one that relies on numerous WLAN access points owned and managed by private individuals.We view home WLANs as an underexploited resource. We believe that a controlled WLAN sharing scheme is interesting, viable, and urgently needed. Such a scheme would allow outsiders who are passing by home WLANs to access services on the network in a systematic way. Why would individuals share their WLANs when potential direct and indirect costs are involved? The basic idea is that they would do it in exchange of expected similar services in the future on other WLANs.We have defined, designed, and evaluated a system for sharing WLANs with others, called Peer-to-Peer Wireless Network Confederation (P2PWNC). The system is relying on the principle of indirect reciprocity. We show through simulations that our reciprocity algorithms encourage cooperation in a P2PWNC system (where we assume that all peers are independent and selfish and that software and hardware modules can be tampered with). In addition, a P2PWNC system: (1) is open to all and allows participants to use Uncertified Public Keys as their identities, (2) is fully distributed, relying on a decentralized subsystem for storing transaction history, (3) uses standard hardware and software we developed for some of the main relevant platforms (Linux-based WLAN access points and Windows Mobile-based cell phones.More information can be found at http://mm.aueb.gr/research/P2PWNC/.This is joint work with (my now graduated Ph.D. student) Elias Efstathiou, (my current Ph.D. student) Pantelis Frangoudis, and the whole P2PWNC team.

Service discovery in mobile ad hoc networks is an essential process in order for these networks to be self-configurable with zero or minimal administration overhead. In this paper we argue that service discovery can be greatly enhanced in terms of efficiency (regarding service discoverability and energy consumption), by piggybacking service information into routing layer messages. Thus, service discovery does not generate additional messages and a node requesting a service, in addition to discovering that service, it is simultaneously informed of the route to the service provider. We extended the zone routing protocol (ZRP) in order to encapsulate service information in its routing messages. Extensive simulations demonstrate the superiority of this routing layer-based service discovery scheme over that of a similar, but application layer based service discovery scheme.

We present a self-managed scheme that could fuel the deployment of free public wireless networks in cities; we call it peer-to-peer wireless network confederation (P2PWNC). Unlike existing approaches, P2PWNC does not rely on central planning but on an ad hoc community of broadband Internet subscribers (the peers) with Wi-Fi access points (APs). These APs provide wireless access to peers that are away from home but within the range of another P2PWNC AP. In the P2PWNC scheme, wireless service is provided to those peers who consistently provide service to passerby peers, based on an algorithm that detects non-simultaneous multi-way peer-to-peer exchanges. This indirect reciprocity algorithm runs in isolation on every peer AP, resists Sybil attacks, and promotes cooperation without relying on trusted authorities, certified identities, or tamperproof modules. We discuss P2PWNC's design and show preliminary results that support its feasibility.

Although TCP and UDP application performance over wireless links may be substantially improved via link layer error recovery, different schemes are appropriate for each application class. We present a multi-service link layer architecture that simultaneously enhances the performance of diverse applications by supporting multiple error recovery mechanisms in parallel. We simulated concurrent file transfers and WWW browsing over TCP and continuous media distribution over UDP using our architecture. The results show that each application achieves similar improvements as when it operates alone over its preferred scheme, despite the parallel execution of diverse applications.

We present the peer-to-peer wireless network confederation (P2PWNC), a P2P system designed to enable the sharing of WLAN bandwidth among residential hotspots. The benefits of joining the confederation outweigh the costs, and its token-based incentive mechanism prevents free-riding.

We address the incentive issues that arise in a peer-to-peer WLAN consortium (P. Antoniadis et al., 2003). We explore the use of flexible rules on reciprocity to guide domain policies and develop a suitable economic model that demonstrates the basic characteristics of our system.

The rapid advances in wireless communications technology and mobile computing have enabled personal mobile devices that we use in everyday life to become information and service providers by complementing or replacing fixed-location hosts connected to the wireline network. Such mobile resources is highly important for other moving users, creating significant opportunities for many interesting and novel applications. The MobiShare architecture provides the infrastructure for ubiquitous mobile access and mechanisms for publishing, discovering and accessing heterogeneous mobile resources in a large area, taking into account the context of both sources and requestors. Any wireless communication technology could be used between a device and the system. Furthermore, the use of XML-related languages and protocols for describing and exchanging metadata gives the system a uniform and easily adaptable interface, allowing a variety of devices to use it. The overall approach is data-centric and service-oriented, implying that all devices are treated as producers or requestors of data wrapped as information services.

An analysis of a plane cover multiple access (PCMA) cellular system in the presence of non-uniform traffic is presented. PCMA seeks to maximize the number of parallel transmissions among cells by defining virtual cells in which users transmit using different reuse factors. It is demonstrated how PCMA is novel in its ability to more efficiently service users in a system with non-uniform traffic load among cells and to adapt in real-time to changes in traffic distribution. After formally stating the optimal coverage problem, a greedy algorithm is presented and used to improve substantially over PCMA with simple, uniform resource allocation.

We develop a new media access control strategy, called plane cover multiple access (PCMA), that provides a means of allocating wireless bandwidth in a packet-based cellular system. PCMA seeks to maximize the number of parallel transmissions among cells by defining virtual cells in which users transmit using a given reuse factor. By keeping the reuse factors low, system throughput can be maximized. We show that the throughput of a simple system designed using PCMA is up to 82% more efficient than capture division packet access (CDPA), the best known alternative for the cellular mobile environment.

Abstract We identify the importance of adaptation for the ubiquitous access to Internet multimedia content. The variety of media types combined with the diversity of Internet connection characteristics raises momentous challenges to the achievement of this goal. With adaptation, the characteristics of various media can be adjusted to better match those of the network path and the end device. As a result, adaptive streams enjoy superior robustness and provide substantially better presentation quality than traditional, non‐adaptive ones, especially in variable, capacity‐limited environments. We examine several important factors that influence the design and optimization of the adaptation architecture. First, the location of the adaptation mechanism on the end‐to‐end path must be chosen. Second, the adaptation policy, i.e., the agility in adapting to variations in network path performance, must be specified. Third, the adaptation mechanism needs to be aware of the value of the perceptual quality that the different representations of each medium and the necessary resources to sustain it. Finally, we emphasize the role of supporting mechanisms in enhancing the adaptation process, namely, prioritization, admission control and hand‐off notifications. Based on the aforementioned criteria, we classify existing research approaches and present some of them as case studies. We also describe current commercial solutions and, finally, we discuss future trends in application adaptation in conjunction with recent developments towards wireless access to the Internet. Copyright © 2001 John Wiley & Sons, Ltd.

We present a comparison of the quality of service performance of several multi-access protocols in packet switched wireless cellular networks that support real-time connections. Our analysis shows that plane cover multiple access (PCMA) provides for the greatest use of system capacity while maintaining a given level of service quality measured in either cellular overload probability or mean overload period. For instance, given a target level of quality of service of 0.001 for maximum probability of cellular overload, over twice as many calls can be accepted in the downlink under PCMA than under capture division packet access (CDPA). Furthermore, almost 2.7 times as many calls can be accepted than under one-seventh reuse. If the target quality of service metric is mean overload period then the improvements are even more extreme with improvements of up to 2.24 times the call capacity of CDPA and up to 3.27 times the capacity of one-seventh reuse. In all cases the improvements in levels of admissible calls are even greater than the straight improvement in system throughput of PCMA over the other protocols. These results also held under a simulation study which served to validate the analytical model used.

We present a comprehensive set of measurements of a 2.4 GHz DSSS wireless LAN and analyze its behavior. We examine issues such as host and interface heterogeneity, bidirectional (TCP) traffic and error modeling, that have not been previously analyzed. We uncover multiple problems with TCP and UDP performance in this system. We investigate the causes of these problems (radio hardware, device drivers, network protocols) and discuss the effectiveness of proposed improvements.

We introduce a new scheduling policy which provides guaranteed service for a session based on a flexible service specification called the service curve. This policy, referred to as the service curve based earliest deadline first policy (SCED), is a generalized policy to which well-known policies such as virtual clock and the earliest deadline first (EDF) can be mapped as special cases, by appropriate specification of the service curves. Rather than characterizing service by a single number, such as minimum bandwidth or maximum delay, service curves provide a wide spectrum of service characterization by specifying the service using a function. The flexibility in service specification allows a user, or the network, to specify a service that best matches the quality-of-service required by the user, preventing an over-allocation of network resources to the user. For a single server, we show that the SCED policy is optimal in the sense of supporting the largest possible schedulability region, given a set of delay-bound requirements and traffic burstiness specifications. For the case of a network of servers, we show that the SCED policy has a greater capability to support end-to-end delay-bound requirements than other known scheduling policies. The key to this capability is the ability of SCED to allocate and guarantee service curves with arbitrary shapes.

While support for IP multicasting continues to spread enabling new applications, an increasing number of hosts connects to the worldwide Internet via low bandwidth point-to-point links, such as wireline or wireless telephone lines. We discuss existing proposals for local and wide area IP multicasting and their implications for point-to-point links, identify problems with their integration in this environment, and propose alternative special purpose mechanisms to solve these problems. The main problems are overhead due to IGMP leave latency and unnecessary continuous probing of potentially power constrained hosts. Our solution is an alternative to IGMP mechanisms based on join/leave messages for tracking group membership over PtP networks. After presenting the implementation requirements of the proposed and existing mechanisms, we compare them with respect to the performance, interoperability, robustness and implementation complexity, demonstrating that our join/leave protocol is uniformly superior.

We present protocol extensions supporting real-time traffic performance guarantees for Fiber Distributed Data Interface (FDDI) implementations lacking a synchronous mode of operation. We provide a brief description of the FDDI protocol, with an emphasis on the synchronous mode and network parameter tuning to meet real-time application demands, and then discuss hardware limitations of some first-generation FDDI adapters which make the synchronous node ineffective and unable to support the required performance guarantees. By modifying the operating system kernel we can regulate the flow of information to the FDDI adapters and implement guarantees in higher-level software. Specifically, we added a two-tiered regulation facility which, at the lowest level, would place an upper bound on asynchronous transmission rate from the device driver buffers to the adapter. The next level up added the notion of standard and guaranteed performance system queues which user-level traffic could be directed into. Finally, a distributed user-level software mechanism was added to monitor and appropriately regulate the throughput from each adapter on the LAN in response to varying bandwidth requirements.

We propose a new scheduling policy, called SCED (service curve-based earliest deadline first), which provides guarantees to virtual circuits in packet-switched networks. This scheduling policy is developed for a general framework for service provisioning based on service curves proposed by Cruz (1992). Instead of explicitly guaranteeing a specific quality of service measure, such as maximum delay, SCED guarantees the service curve for a connection. Quality of service guarantees for the connection can then be expressed as simple functions of the service curve guarantee and the traffic burstiness constraint of the connection. A simple and convenient condition under which SCED can simultaneously guarantee a set of service curves is proved. The service curve specification gives greater flexibility to a server in allocating its resources to meet diverse delay and throughput requirements. We demonstrate by an example that SCED provides a larger schedulability region than scheduling policies such as virtual-clock and PGPS with rate proportional assignment.

We present a parameterizable methodology for profiling Internet traffic flows at a variety of granularities. Our methodology differs from many previous studies that have concentrated on end-point definitions of flows in terms of state derived from observing the explicit opening and closing of TCP connections. Instead, our model defines flows based on traffic satisfying various temporal and spatial locality conditions, as observed at internal points of the network. This approach to flow characterization helps address some central problems in networking based on the Internet model. Among them are route caching, resource reservation at multiple service levels, usage based accounting, and the integration of IP traffic over an ATM fabric. We first define the parameter space and then concentrate on metrics characterizing both individual flows as well as the aggregate flow profile. We consider various granularities of the definition of a flow, such as by destination network, host-pair, or host and port quadruple. We include some measurements based on case studies we undertook, which yield significant insights into some aspects of Internet traffic, including demonstrating (i) the brevity of a significant fraction of IP flows at a variety of traffic aggregation granularities, (ii) that the number of host-pair IP flows is not significantly larger than the number of destination network flows, and (iii) that schemes for caching traffic information could significantly benefit from using application information.>

Understanding the characteristic I/O behavior of scientific applications is an integral part of the research and development efforts for the improvement of high performance I/O systems. This study focuses on application level I/O behavior with respect to both static and dynamic characteristics. We observed the San Diego Supercomputer Center's Cray C90 workload and isolated the most I/O intensive applications. The combination of a low-level description of physical resource usage and the high-level functional composition of applications and scientific disciplines for this set reveals the major sources of I/O demand in the workload. We selected two applications from the I/O intensive set and performed a detailed analysis of their dynamic I/O behavior. These applications exhibited a high degree of regularity in their I/O activity over time and their characteristic I/O behaviors can be precisely described by one and two, respectively, recurring sequences of data accesses and computation periods.>

The results of a measurement study of the T1 NSFNET backbone are presented. The measurement environment and the approach to data collection are discussed. Measurements results are then presented for: long-term growth in traffic volume, including attribution to domains and protocols; trends in average packet size on the network, over both long- and medium-term intervals; most popular sources, destinations, and site pairs; traffic locality; international distribution of traffic; mean utilization statistics of the overall backbone as well as of specific links of interest: and delay statistics.>

Past research on high performance computers for scientific applications has concentrated on CPU speed and exploitation of parallelism, but has, until very recently, neglected 1/0 considerations.This paper presents a study of the production workload at the San Diego Supercomputer Centerfrom an 110 requirements and characteristics perspective.Results of our analyses support our hypothesis that a significant proportion of 110 intensive, long running, frequently executed scientific applications have predictable 110 requirements.Pemnission to copy Wbcut fee all or pm of this material is granted, provided that th copes am nci made or distributed for &red commercial advantage, the ACM copyright mice and the title of the Wblication and its date appear, and notice is given that ccpying is by penmsmon of the Association for Cbm@ng Mec+mte~.To copy othemvise, or to repubhsh, reqlires a fee andlar specWc psrtnission.

The relative performance of different data collection methods in the assessment of various traffic parameters is significant when the amount of data generated by a complete trace of a traffic interval is computationally overwhelming, and even capturing summary statistics for all traffic is impractical. This paper presents a study of the performance of various methods of sampling in answering questions related to wide area network traffic characterization. Using a packet trace from a network environment that aggregates traffic from a large number of sources, we simulate various sampling approaches, including time-driven and event-driven methods, with both random and deterministic selection patterns, at a variety of granularities. Using several metrics which indicate the similarity between two distributions, we then compare the sampled traces to the parent population. Our results revealed that the time-triggered techniques did not perform as well as the packet-triggered ones. Furthermore, the performance differences within each class (packet-based or time-based techniques) are small.

The authors apply their queuing-theoretic delay analysis methodology to several variations of the FCFS 0.487 conflict resolution algorithm. In this approach, the main component of the packet delay is viewed as a queuing problem in which each window selected by the channel access algorithm is a customer requiring conflict resolution as its service. The authors' methodology is extended to handle all the features of the full FCFS 0.487 algorithm, including variable-size windows, arrival-time addressing (and hence true FCFS scheduling), and biased interval splitting. Some of these extensions involve approximations, but they make it possible to obtain the Laplace transform and moments of the packet delay. The authors also present an exact analysis for the three-cell algorithm, which is the 0.487 algorithm with a few modifications to satisfy the separability condition that reduces its capacity to 0.48. Comparisons made with other analyses (which provide bounds on the mean delay) and with extensive simulations show that the present results for both the mean and the variance of the packet delay are extremely accurate.>

The authors present heuristics for multicast tree construction for communication that depends on: bounded end-to-end delay along the paths from source to each destination and minimum cost of the multicast tree, where edge cost and edge delay can be independent metrics. The problem of computing such a constrained multicast tree is NP-complete. It is shown that the heuristics demonstrate good average case behavior in terms of cost, as determined by simulations on a large number of graphs.>

A thorough investigation of the performance of the rotating slot generator (RSG) scheme, based on simulation, is presented. RSG is a medium access control protocol appropriate for high-capacity long-distance metropolitan area networks (MANs). It uses the looped bus architecture of the distributed queue dual bus (DQDB) in which the slot generators for both busses are colocated inside the same station. However in RSG, all the stations, one after the other in a cyclic order, undertake the task of generating and destroying the slots on both busses. In this way the location of the station relative to the slot generator changes dynamically, and its effect on the performance is drastically reduced. The authors investigate the fairness and performance of RSG under symmetric and asymmetric loading, underload and overload conditions, and under the presence of a single or multiple priority classes of traffic. They also compare its performance with different variations of DQDB.>

The authors investigate multicast routing for high-bandwidth delay-sensitive applications in a point-to-point network as an optimization problem. They associate an edge cost and an edge delay with each edge in the network. The problem is to construct a tree spanning the destination nodes, such that it has the least cost, and so that the delay on the path from the source to each destination is bounded. Since the problem is computationally intractable, the authors present an efficient approximation algorithm. Experimental results through simulations show that the performance of the heuristic is near optimal.>

Expressions are found for the throughput and delay performance of a Tree Conflict Resolution Algorithm that is used in a Local Area Network with carrier sensing (and possibly also collision detection). We assume that Massey's constant size window algorithm is used to control access to the channel, and that the resulting conflicts (if any) are resolved using a Capetanakis-like preorder traversal tree algorithm with d-ary splitting. We develop and solve functional equations for various performance metrics of the system and apply the “Moving Server” technique to calculate the main component of the delay. Our results compare very favorably with those for CSMA protocols, which are commonly used in Local Area Networks that support sensing.

Multiple-access protocols control access to a broadcast communication channel. Tree conflict resolution algorithms are the heart of some distributed multiple-access protocols with nice properties like stability, high capacity, and low delay under light load. We consider a random access protocol based on a tree conflict resolution algorithm similar to one first proposed by Capetanakis, but in which constant size windows on the arrival time axis are used to admit packets into the algorithm instead of the more common "free" or "blocked" access methods. We obtain exact recursive relationships for the (steady-state) distribution of packet delay, and thus, the exact throughput-delay curve for any finite (in general, nonhomogeneous) configuration under a Bernoulli-per-window arrival model. Then, we consider the address assignment problem. We show that by choosing an appropriate addressing scheme, we can improve the performance of the algorithm with respect to both mean delay and maximum throughput.