The InterPlanetary File System (IPFS) is the largest decentralized content-centric storage network. While its architecture enables resilient, distributed content delivery, it can be abused to host and disseminate malicious content. Public IPFS HTTP gateways further expand this threat surface, enabling attackers to deploy phishing websites and leverage gateway reputation to evade detection. This model can keep content available even after attackers go offline and challenges traditional phishing detection systems.

Cybercriminals have long depended on domain names for phishing, spam, malware distribution, and botnet operation. To facilitate the malicious activities, they continually register new domain names for exploitation. Previous work revealed an abnormally high concentration of malicious registrations in a handful of registrars and TLDs. However, no existing study systematically analyzed the factors driving abuse, leaving a critical gap in understanding how different variables influence malicious registrations. In this paper, we carefully distill the inclinations and aversions of malicious actors during the registration of new phishing domain names. Having compiled a list of 14.5 k malicious and 15.4 k benign domains, we collect a comprehensive set of 73 features for all the domains encompassing three main latent factors: registration attributes, proactive verification, and reactive security practices. With a GLM regression analysis, we found that each dollar reduction in registration fees corresponds to a 49% increase in malicious domain registrations. The availability of free bundled services, such as web hosting, drives an 88% surge in phishing activities. Conversely, stringent registration restrictions cut down abuse by 63%, while registrars providing API access for domain registration or account creation experience a staggering 401% rise in malicious domains. The results enable intermediaries involved in domain registration to develop tailored anti-abuse practices, yet aligning them with their economic interests.

Domain Name System Security Extensions (DNSSEC) enhanced the security of conventional DNS by providing data integrity and origin authentication, but enabled zone walking as a side effect. To address this issue, the Next Secure (NSEC3) resource record provides an authenticated denial of existence mechanism based on hashes of domain names. However, an improper selection of the NSEC3 parameters may significantly degrade the performance of resolvers and authoritative name servers alike. RFC 9276 (Guidance for NSEC3 Parameter Settings) imposes additional constraints on hash computation parameters, crucial in light of emerging security threats such as CPU resource exhaustion attacks. Despite this guideline, our analysis of over 302 M registered domain names reveals that 87.8 % of 15.5 % NSEC3-enabled domains fail to adhere to RFC 9276 with a dozen using 500 additional hash iterations. Furthermore, 78.3 % of 114 K open and closed validating resolvers impose the RFC's additional constraints on hash iterations with 18.4 % returning SERVFAIL, possibly rendering non-compliant domains unreachable.

The Domain Name System (DNS) relies on response codes to confirm successful transactions or indicate anomalies. Yet, the codes are not sufficiently fine-grained to pinpoint the root causes of resolution failures. RFC~8914 (Extended DNS Errors or EDE) addresses the problem by defining a new extensible registry of error codes to be served inside the OPT resource record. In this paper, we show that four major DNS resolver vendors and three large public DNS resolvers support this standard and correctly narrow down the cause of underlying problems. Yet, they do not agree in 94% of our test cases in terms of the returned EDE codes. We reveal that Cloudflare DNS is the most precise in indicating various DNS misconfigurations via the EDE mechanism, so we use it to perform a large-scale analysis of more than 303M registered domain names. We show that 17.7M of them trigger EDE codes. Lame delegations and DNSSEC validation failures are the most common problems encountered.

\beginabstract In the context of the Factories at Major Accident Risk (FMAR), we consider a scenario consisting of multiple sensor nodes that detect dangerous conditions and raise alarms over a LoRaWAN network. Upon event detection, a large number of nodes try to transmit concurrent alarm messages and the reception of at least one message is sufficient for the central server to react. We propose a scheme in which nodes operate in a slotted-time mode after the detection of the triggering event and they can only transmit one packet in a randomly chosen slot. The choice of the transmission slot follows a specific probability distribution that maximizes the probability of successful reception of at least one packet subject to real-time latency constraints. We provide an optimization framework to find the optimal distribution for choosing a transmission slot. We validate the proposed scheme by simulations and provide numerical results to compare the performance for three different slot choice distributions: i) uniform, ii) the distribution used in the Sift protocol \cite Tay2004, and iii) the proposed optimal distribution. The results show that the proposed optimal distribution leads to much better probability of successful packet delivery than other distributions. \endabstract

DNS Security Extensions (DNSSEC) provide the most effective way to fight DNS cache poisoning attacks. Yet, very few DNS resolvers perform DNSSEC validation. Identifying such systems is non-trivial and the existing methods are not suitable for Internet-scale measurements. In this paper, we propose a novel remote technique for identifying DNSSEC-validating resolvers. The proposed method consists of two steps. In the first step, we identify open resolvers by scanning 3.1 billion end hosts and request every non-forwarder to resolve one correct and seven deliberately misconfigured domains. We then build a classifier that discriminates validators from non-validators based on query patterns and DNS response codes. We find that while most open resolvers are DNSSEC-enabled, less than 18% in IPv4 (38% in IPv6) validate received responses. In the second step, we remotely identify closed non-forwarders in networks that do not have inbound Source Address Validation (SAV) in place. Using the classifier built in step one, we identify 37.4% IPv4 (42.9% IPv6) closed DNSSEC validators and cross-validate the results using RIPE Atlas probes. Finally, we show that the discovered (non)-validators actively send requests to DNS root servers, suggesting that we deal with operational recursive resolvers rather than misconfigured machines.

DNS dynamic updates represent an inherently vulnerable mechanism deliberately granting the potential for any host to dynamically modify DNS zone files. Consequently, this feature exposes domains to various security risks such as domain hijacking, compromise of domain control validation, and man-in-the-middle attacks. Originally devised without the implementation of authentication mechanisms, non-secure DNS updates were widely adopted in DNS software, subsequently leaving domains susceptible to a novel form of attack termed zone poisoning. In order to gauge the extent of this issue, our analysis encompassed over 353 million domain names, revealing the presence of 381,965 domains that openly accepted unsolicited DNS updates. We then undertook a comprehensive three-phase campaign involving the notification of Computer Security Incident Response Teams (CSIRTs). Following extensive discussions spanning six months, we observed substantial remediation, with nearly 54% of nameservers and 98% of vulnerable domains addressing the issue. This outcome serves as evidence that engaging with CSIRTs can prove to be an effective approach for reporting security vulnerabilities. Moreover, our notifications had a lasting impact, as evidenced by the sustained low prevalence of vulnerable domains.

This article explores the capacity limits and the tradeoff between the antagonistic means of enabling reliable data delivery in a loaded LoRaWAN cell. In fact, channel attenuation and variability call for robust transmission settings but the associated load increase causes more collisions between frames. In addition to the physical-layer parameters of the LoRa modulation, this article considers the benefits and tuning of interpacket error correction codes (ECCs), which also trade transmission redundancy for reliability. We thus start by proposing a refined packet delivery ratio (PDR) model that improves the ones found in the literature in that, first, it takes into account the dependency between overcoming ambient noise and dominating colliding frames, and second, it considers the sum of interference powers when multiple colliding frames are present, even if the interference preexists. Moreover, the model extends to the case of a gateway with receiver diversity. In a second step, the model allows to set out the level of redundancy at which ECC is the most effective without hindering capacity: a coding rate of one third. When this is fixed, it allows to define the transmission parameters allocation within a cell and thus the size of the cell. We finally develop an ad-hoc Python discrete-event simulator, freely accessible, to complement the model for assessing the effect of inter-SF and near–far interference and to show the benefits of power control in this respect.

Ingress filtering, commonly referred to as Source Address Validation (SAV), is a practice aimed at discarding packets with spoofed source IP addresses at the network periphery. Outbound SAV, i.e., dropping traffic with spoofed source IP addresses as it leaves its source network, has received widespread attention in operational and research communities. It is one of the most effective ways to prevent Reflection-based Distributed Denial-of-Service (DDoS) attacks. Contrariwise, inbound SAV, i.e., dropping incoming spoofed traffic at the destination network edge, has received less attention, even though it provides protection for the deploying network. In this paper, we present the results of the Closed Resolver Project, our initiative aimed at finding networks without inbound SAV and raising awareness of the issue. We perform the first Internet-wide active measurement study to enumerate networks that enforce (or not) inbound SAV. We reach open and closed Domain Name System (DNS) resolvers in tested networks and determine whether they resolve requests with spoofed source IP addresses. Our method provides unprecedented insight into inbound SAV deployment by network operators, revealing 49% IPv4 and 26% IPv6 Autonomous Systems (AS) that suffer from a consistent or partial absence of inbound filtering. By identifying dual-stack DNS resolvers and ASes, we further show that inbound filtering is generally deployed consistently across IPv4 and IPv6. Finally, the lack of inbound SAV exposes 2.5M IPv4 and 100K IPv6 purportedly closed DNS resolvers to many types of external attacks, including NXNSAttack, zone poisoning, or zero-day vulnerabilities in DNS software.

This paper considers a LoRaWAN cell in which devices access the channel using the unslotted ALOHA protocol. We propose a model of this access method that combines the effects of collisions with channel fading, by which reception may get buried in noise. Unlike the existing models of LoRaWAN,our model takes into account the frame arrival timing: it distinguishes, on the one hand, the interference created by earlier transmissions with respect to the frame of interest, and on the other hand, the interference by the frames arriving later on. From the results of the model, we draw three observations regarding the improvement of Packet Delivery Ratio (PDR). First, it puts back under the spotlight the often overlooked fact that repeating frames is always beneficial when the desired PDR is above 60%, even though the extra packet transmissions create more collisions. Second, as soon as the node density becomes notable and collisions have a similar impact on losses as attenuation, adding a smaller spreading factor SF6 modulation into the cell list of transmission parameters allows increasing the coverage range. Third, the model shows that cell capacity sometimes grows with the distance to the gateway or with decreased node transmission power, a trend seldom observed in wireless networks.

The Aloha-like access method is a major limiting factor in LoRaWAN networks, with only 18% of channel utilization at best. It also causes high packet losses when node deployment becomes more massive.In this paper, we propose a technique to enhance the reception process of the LoRaWAN gateways, so that channel utilization can reach values up to 35% in a single LoRaWAN cell.We investigate the benefits of concurrent and preemptive reception at the gateways: the capture effect allows to receive a frame even if it collides with a later frame, whereas Message in Message (MIM) reception allows the gateway to drop the current reception and switch to a new more powerful frame. An implementation in the NS-3 simulator allows us to assess the gains of this approach through extensive simulations.

In this paper, we propose a scheme for representing semantic metadata of IoT devices in compact identifiers and DNS names to enable simple discovery and search with standard DNS servers. Our scheme defines a binary identifier as a sequence of bits: a Context to use and several bits of fields corresponding to semantic properties specific to the Context. The bit string is then encoded as base32 characters and registered in DNS. Furthermore, we use the compact semantic DNS names to offer support for search and discovery. We propose to take advantage of the DNS system as the basic functionality for querying and discovery of semantic properties related to IoT devices.We have defined three specific Contexts for hierarchical semantic properties as well as logical and geographical locations. For this last part, we have developed two prototypes for managing geo-identifiers in LoRa networks, one based on Node and the Redis in-memory database, the other one based on the CoreDNS server.

Sending forged emails by taking advantage of domain spoofing is a common technique used by attackers. The lack of appropriate email anti-spoofing schemes or their misconfiguration may lead to successful phishing attacks or spam dissemination. In this paper, we evaluate the extent of the SPF and DMARC deployment in two large-scale campaigns measuring their global adoption rate with a scan of 236 million domains and high-profile domains of 139 countries. We propose a new algorithm for identifying defensively registered domains and enumerating the domains with misconfigured SPF rules by emulating the SPF check_function. We define for the first time new threat models involving subdomain spoofing and present a methodology for preventing domain spoofing, a combination of good practices for managing SPF and DMARC records and analyzing DNS logs. Our measurement results show that a large part of the domains do not correctly configure the SPF and DMARC rules, which enables attackers to successfully deliver forged emails to user inboxes. Finally, we report on remediation and its effects by presenting the results of notifications sent to CSIRTs responsible for affected domains in two separate campaigns.

Miscreants abuse thousands of domain names every day by launching large-scale attacks such as phishing or malware campaigns. While some domains are solely registered for malicious purposes, others are benign but get compromised and misused to serve malicious content. Existing methods for their detection can either predict malicious domains at the time of registration or identify indicators of an ongoing malicious activity conflating maliciously registered and compromised domains into common blacklists. Since the mitigation actions for these two types domains are different, we propose COMAR, an approach to differentiate between compromised and maliciously registered domains, complementary to previously proposed domain reputation systems. We start the paper with a thorough analysis of the domain life cycle to determine the relationship between each step and define its associated features. COMAR uses a set of 38 features costly to evade. We evaluate COMAR using phishing and malware blacklists and show that it can achieve high accuracy (97% accuracy with a 2.5% false-positive rate) without using any privileged or non-publicly available data, which makes it suitable for the use by any organization. We plan to deploy COMAR at two domain registry operators of the European country-code TLDs and set up an early notification system to facilitate the remediation of blacklisted domains.

In this paper, we investigate how to provide good transmission quality in massive deployments of LoRa networks by considering all parameters such as device heterogeneity, network topology, and deployment density. We consider the scenario with nodes progressively joining the network, i.e., new nodes joining the network are configured based on measured metrics and without modifying the configuration of nodes that already joined the network. Based on this assumption, we propose an algorithm to improve network performance by effectively allocating a spreading factor (SF) to end-devices in realistic multi-gateway deployments. The algorithm performs better than the Adaptive Data Rate (ADR) of LoRaWAN (e.g., it almost doubles the packet delivery ratio (PDR) in scenarios with 10k nodes) and enhances LoRa deployments by adapting the communication parameters of end-devices according to the network size and estimated metrics. The allocation decision is based on different metrics: link PDR, network PDR, and network distribution of SF per gateway. Nodes can easily derive the estimated metrics from gateway measurements.

In this paper, we propose Timemaps, a new scheduling scheme to improve the performance of LoRaWAN. The idea is to build a temporal map of all transmissions of IoT devices by a Gateway to schedule transmissions and avoid collisions. A device when performing a Join operation includes its traffic description in the request. Based on the traffic descriptions from all devices, the Gateway constructs a schedule for channel access that avoids collisions. The Gateway includes in the Join accept the information on the temporal position of the device transmission in the schedule, the Spreading Factor (SF) to use based on measured Signal to Noise Ratio (SNR) at the Gateway, and the channel to use. We evaluate our proposal with the NS-3 simulator in both cases of perfect clocks and clocks with a drift as well as for homogeneous and inhomogeneous node density. The simulation takes into account quasi-orthogonality and the capture effect. The results show that Timemaps benefits from remarkably higher PDR and a considerably lower collision ratio compared to LoRaWAN along with slightly increased energy consumption.

Phishing is one of the most common cyberattacks these days. Attackers constantly look for new techniques to make their campaigns more lucrative by extending the lifespan of phishing pages. To achieve this goal, they leverage different anti-analysis (i.e., evasion) techniques to conceal the malicious content from anti-phishing bots and only reveal the payload to potential victims. In this paper, we study the resilience of anti-phishing entities to three advanced anti-analysis techniques based on human verification: Google re-CAPTCHA, alert box, and session-based evasion. We have designed a framework for performing our testing experiments, deployed 105 phishing websites, and provided each of them with one of the three evasion techniques. In the experiments, we report phishing URLs to major server-side anti-phishing entities (e.g., Google Safe Browsing, NetCraft, APWG) and monitor their occurrence in the blacklists. Our results show that Google Safe Browsing was the only engine that detected all the reported URLs protected by alert boxes. However, none of the anti-phishing engines could detect phishing URLs armed with Google re-CAPTCHA, making it so far the most effective protection solution of phishing content available to malicious actors. Our experiments show that all the major serverside anti-phishing bots only detected 8 out of 105 phishing websites protected by human verification systems. As a mitigation plan, we intend to disclose our findings to the impacted anti-phishing entities before phishers exploit human verification techniques on a massive scale.

In this paper, we consider the problem of evaluating the capacity of a LoRaWAN cell. Previous analytical studies investigated LoRaWAN performance in terms of the Packet Delivery Ratio (PDR) given a number of devices around a gateway and its range. We improve the model for PDR by taking into consideration that the following two events are dependent: successful capture during a collision and successful frame decoding despite ambient noise. We consider a realistic traffic model in which all devices generate packets with the same inter-transmission times corresponding to the duty cycle limitation at the highest SF, regardless of the distance to the gateway. Based on the developed model, we optimize the Spreading Factor (SF) boundaries to even out PDR throughout the cell. We validate the analytical results with simulations, compare our model with previous work, and experimentally validate the hypothesis of Rayleigh fading for the LoRa channel. The important conclusion from our results is that a LoRa cell can handle a relatively large number of devices. We also show that there is practically no inter-SF interference (cross interference between transmissions with different SFs): interference from higher SFs comes from nodes located farther away, so they face greater attenuation and thus, they do not interfere with lower SF nodes.

Quick and accurate estimation of link quality, and more specifically packet loss probability, is the key element for efficient and effective communications in wireless multi-hop networks. We focus on IEEE 802.15.4 and we posit that losses only occur when noise and interference last long enough and are strong enough relatively to the received signal, to hinder packet reception. So, the key information for any ordered node pair is the signal to noise plus interference ratio distribution, which we obtain by combining the observed noise plus interference power at the receiver with the received signal strength. In this paper, we propose two novel schemes for the estimation of PER: Burst-NISI and Sample-NISI. Burst-NISI is based on high frequency measurements of the power level of ambient noise and interference around a given node. Sample-NISI sporadically samples the power level of ambient noise and interference when the radio operates according to a duty cycle. Using a large scale experimental platform, we show that our packet error rate estimation schemes are accurate for any packet length and diverse experimentation sites with different settings, for which the prediction is within 10 percentage points of the PER value measured a posteriori.

In this paper, we present the results of extensive experiments on a testbed in the The Things Network (TTN), a public LoRa network. We evaluate the transmission quality of LoRa links by measuring the Packet Reception Rate (PRR) as a function of the payload length. The results show that there is only a slight impact of the payload length on PRR, which means that the bit error rate does not strongly influence the probability of packet reception. Our measurements show that the LoRa channel behaves like a slow fading Rayleigh channel, which translates into probability Ps of being (or not) in a favorable condition for each frame reception: once the frame preamble is received, there is great chance that the whole frame is correctly received. Probability Ps depends on the Spreading Factor and the Signal to Noise Ratio, and often becomes a dominant factor of successful reception depending on the signal strength at a gateway.

We present WiBend, a system that recognizes bending gestures as the input modalities for interacting on non-instrumented and deformable surfaces using WiFi signals. WiBend takes advantage of off-the-shelf 802.11 (Wi-Fi) devices and Channel State Information (CSI) measurements of packet transmissions when the user is placed and interacting between a Wi-Fi transmitter and a receiver. We have performed extensive user experiments in an instrumented laboratory to obtain data for training the HMM models and for evaluating the precision of WiBend. During the experiments, participants performed 12 distinct bending gestures with three surface sizes, two bending speeds and two different directions. The performance evaluation results show that WiBend can distinguish between 12 bending gestures with a precision of 84% on average.

All existing models for analyzing the performance of LoRaWAN assume a constant density of nodes within the gateway range. We claim that such a situation is highly unlikely for LoRaWAN cells whose range can attain several kilometers in real-world deployments. We thus propose to analyze the LoRa performance under a more realistic assumption: the density of nodes decreases with the inverse square of the distance to the gateway. We use the LoRaWAN capacity model by Georgiou and Raza to find the Packet Delivery Ratio (PDR) for an inhomogeneous spatial distribution of devices around a gateway and obtain the number of devices that benefit from a given level of PDR. We analyze the LoRaWAN capacity in terms of PDR for various spatial configurations and Spreading Factor allocations.

In this paper, we investigate how to provide good transmission quality in LoRa networks by considering all parameters such as device heterogeneity, network topology, deployment density, and traffic intensity. We propose three approaches to adaptation of LoRa parameters to different scenarios. Enhanced Link Adaptation (ELA) improves the classical Adaptive Data Rate (ADR) algorithm by considering network load. The ELA scheme selects the configuration according to the network parameters (e.g., network congestion) and the radio environment (e.g., link budget), and optimizes single gateway capacity by taking into account current load and capacity of each Spreading Factor (SF) allocation. The second approach exploits network densification by increasing the number of gateways. It maximizes the overall capacity of the network for different SF configurations. Finally, the joint Multi-Gateway ELA (MGjELA) strategy improves transmission quality by exploiting SF orthogonality, spatial reuse of communications, and optimized gateway capacity.

In this paper, we consider two research issues. First, we present a NS-3 module that simulates the behavior of LoRa in an accurate way. To validate the module, we compare its results with measurements on a real-world testbed and with measured values reported by other work. We also show that the module correctly represents the capture effect that lowers the packet drop rate due to collisions. Second, we want to improve the performance of LoRa devices while not impacting energy consumption, the aspect that usually is not taken into account in the literature. We use the simulator to evaluate CSMA, a simple enhancement to LoRaWAN that lowers the collision ratio. The simulation results show that CSMA considerably lowers the collision ratio while only slightly increasing energy consumption. We also observe that CSMA presents lower energy consumption than LoRa for a large number of devices. Another advantage of CSMA consists of increased throughput and larger network capacity because the ETSI restrictions on the duty cycle do not longer apply.

In this paper, we describe the design and implementation of WiSH-WalT, a framework for controllable and reproducible LoRa testbeds. The implementation of the WiSHFUL unified interface provides the means for controlling and adapting the LoRa parameters to given conditions and low energy consumption by configuring transmission power, spreading factor, bandwidth, and error coding rate. WalT provides support for running reproducible experiments based on the deployment of docker images on a distributed set of nodes. Put together, the functionalities of WiSHFUL and WalT open new possibilities for deploying reproducible LoRa testbeds. We have used WiSH-WalT to experiment with adaptive configuration of the LoRa parameters and evaluated the communication performance of LoRa motes in a setup with several gateways.

In this paper, we propose IoTChain, a combination of the OSCAR architecture [1] and the ACE authorization framework [2] to provide an E2E solution for the secure authorized access to IoT resources. IoTChain consists of two components, an authorization blockchain based on the ACE framework and the OSCAR object security model, extended with a group key scheme. The blockchain provides a flexible and trustless way to handle authorization while OSCAR uses the public ledger to set up multicast groups for authorized clients. To evaluate the feasibility of our architecture, we have implemented the authorization blockchain on top of a private Ethereum network. We report on several experiments that assess the performance of different architecture components.

This paper considers the problem of a node joining an existing 802.15.4e TSCH network. We propose DataJoin, an energy-efficient scheme based on two ideas: i) it schedules the transmission of enhanced beacon frames in a smart way by taking advantage of the information on the common TSCH schedule and ii) it piggybacks the next beacon transmission times onto some acknowledgment frames to allow a joining node to overhear this information. We compare DataJoin with the Baseline scheme that corresponds to the state-of-the-art protocol for joining TSCH networks. Simulation comparisons show lower energy consumption by the joining node under DataJoin along with similar joining delays and average network energy consumption for both schemes.

LoRa technology has emerged as an interesting solution for Low Power Wide Area (LPWA) applications. The main LPWA research directions are about large scale networks to support massive number of devices, interference issues, link optimization and adaptability. Thus heterogeneous network deployments and Spreading Factor (SF) allocation strategies need to be studied. In this paper, we investigate the performance of homogeneous networks (i.e. when all the nodes select the same LoRa configuration) and heterogeneous networks (i.e. when each node selects its LoRa configuration according to its link budget or their needs) for large scale deployments (up to 10000 nodes per gateway). For that purpose we have developed a LoRa Module, based on improved WSNet simulator, including a spectrum usage abstraction, the co-channel rejection due to the quasi-orthogonality of SFs and the gateway capture effect. Simulation results show the performance comparison in terms of reliability, network capacity and power consumption for homogeneous and heterogeneous deployments as a function of the number of nodes and the traffic intensity. The comparison shows the benefits of the heterogeneous deployment where each node selects its configuration according to its link budget.

In this paper, we propose a fast predictive resynchronization scheme that allows nodes to rejoin a 802.15.4e TSCH network with which they were previously synchronized. The idea of the scheme comes from the investigation of the internal and external root causes of the clock drift between two nodes--we have identified the causes and proposed a means for its estimation. Based on the prediction of the drift between any pair of nodes, we are able to determine the instant at which a node needs to wake up after desynchronization, thus reducing its energy consumption and the rejoin latency. We compare the proposed scheme with other state-of-the-art proposals through experiments on different hardware platforms. The experimental results show that desynchronized nodes rejoin up to 34 times faster and consume less energy by a factor of 1000 compared to the other proposals.

This paper considers the problem of joining a network of constrained IoT devices. We propose to take advantage of Singer Cyclic Difference Sets (S-CDS) for a joining scheme that results in low duty cycles and apply the proposed scheme to the joining problem in 802.15.4e TSCH networks. S-CDS distributes the active periods of nodes over time so that a joining node does not suffer from long scanning periods in contrast to other schemes. We compare S-CDS through simulation with other state-of-the-art schemes adapted to TSCH networks. The comparisons show that S-CDS achieves a better trade-off between energy consumption and joining delay than the best joining schemes while being particularly suitable for devices with strong energy constraints such as energy harvesting nodes.

The LoRa technology has emerged as an interesting solution for low power, long range IoT applications by proposing multiple ”degrees of freedom” at the physical layer. This flexibility provides either a long range at the cost of a lower data rate or higher throughput at the cost of low sensitivity, so a shorter range. In this paper, we analyze the flexibility of LoRa and propose various strategies to adapt its radio parameters (such as the spreading factor, bandwidth, and transmission power) to different deployment scenarios. We compute the energy consumption of LoRa transceivers using various radio configurations in both star and mesh topologies. Our simulation results show that in a star topology, we can achieve the optimal scaling-up/down strategy of LoRa radio parameters to obtain either a high data rate or a long range while respecting low energy consumption. In mesh networks, energy consumption is optimized by exploiting various radio configurations and the network topology (e.g., the number of hops, the network density, the cell coverage). Finally, we propose a strategy to take advantage of both star and mesh topologies.

The Internet of Things (IoT) requires a compact naming scheme, which can also bring significant advantages to service registration and discovery. We propose a novel approach, denoted as distributed naming service, which provides a new naming scheme as well as an efficient service discovery protocol for wireless sensor networks. It is based on three pillars: 1) Bloom filters, to create compact names from node descriptions; 2) message propagation strategies, to publish and discover information-not only names-within the network; and 3) distributed caches, to store names within the network. In this paper, we assume ContikiMAC at layer 2, IPv6 and Routing Protocol for Low-Power and Lossy Networks (RPL) at layer 3, and we present two particular UDP-based message propagation strategies that take advantage of the RPL protocol at layer 3. We evaluate the performance of the proposed solutions through Contiki/Cooja simulations and on a real testbed, using the open and large scale FIT IoT-LAB.

Data Pendulum [1] designates the packet backlog switch from one side of a bottleneck link to the other in presence of two antiparallel TCP connections. This paper completes the explanation of the Data Pendulum dynamics by elucidating on which side the backlog grows, while identifying the relevant parameters in the process. We also explain why and when the data backlog may switch sides without any packet loss. We propose a model for the dynamics of antiparallel TCP connections that effectively explains the throughput observed in real conditions. Moreover, the model is applicable to a more realistic case in which buffers are dimensioned in packets rather than bytes, the case that previous models could not capture.

Reliable wireless communication even in adverse conditions is the key for building the energy efficient and dependable Internet of Things. In this paper, we explore the benefits of channel diversity for enabling efficient wireless communication: we propose MRR (Multi-channel Round-Robin), a backward-compatible evolution of beacon-enabled IEEE 802.15.4 in which energy constrained nodes take advantage of additional active periods operating on different channels in a round-robin way. Each active period starts with a beacon sent on a cyclically changing channel, which then allows an associated device to transmit data on the channel used for the beacon. MRR schedules the additional active periods at carefully selected instants to avoid direct beacon collisions. To motivate our work, we first experimentally corroborate previous findings that channel diversity is an effective way of mitigating variable or poor transmission conditions. Then, we observe that channel diversity improves the quality of transmission even better than expected - it appears that wireless sensor nodes have a radiation pattern that changes significantly from one frequency channel to another, which often results in a considerably improved gain when using the right communication channel. The evaluation of the MRR scheme through measurements on a real indoor multihop testbed shows that the proposed scheme results in significantly improved Packet Reception Ratio even without resorting to e.g. channel blacklisting. These results confirm the benefits of multichannel operation and exhibit a fully functional solution that does not add a large overhead compared to using a single channel.

The recent IEEE 802.15.4e standard has introduced two interesting modes of operation: Time Slotted Channel Hopping (TSCH) and Deterministic and Synchronous Multi-channel Extension (DSME). Both provide a mix of time and frequency division to improve the performance of the previously available synchronized MAC mode (beacon-enabled 802.15.4). In this paper, we compare the performance of DSME and TSCH with respect to the energy consumption, throughput, and delay through an analysis of their respective ways of operation. We use an energy consumption model coming from our previous experience on the design of recent energy harvesting motes for the GreenNet platform. Our results show that DSME performs slightly better in terms of the energy consumption spent in data transfers. Both protocols exhibit similar delays for a given duty cycle, nevertheless, TSCH obtains shorter delay and higher throughput for low duty cycles. For higher duty cycles, TSCH results in lower throughput - for applications that send little data, the fixed slot configuration of TSCH results in wasted bandwidth. DSME can allocate shorter slots, which is beneficial for applications that transmit short packets.

In this paper, we apply the Gilbert-Elliot model to analyze measurements on a large-scale wireless sensor testbed with nodes equipped with CC1101 and CC2420 radio chips. The model considers a packet loss process as a sequence of transitions between Good and Bad states. We analyze the Packet Reception Ratio (PRR) based on the probabilities derived from the Gilbert-Elliot model. We show that the probabilities and other parameters (run and loss lengths) can very well discriminate between the main categories of link quality such as good and weak links. Identification of link quality is crucial for routing protocols such as RPL and LOADng.

The Datagram Transport Layer Security (DTLS) protocol is the IETF standard for securing the Internet of Things. The Constrained Application Protocol, ZigBee IP, and Lightweight Machine-to-Machine (LWM2M) mandate its use for securing application traffic. There has been much debate in both the standardization and research communities on the applicability of DTLS to constrained environments. The main concerns are the communication overhead and latency of the DTLS handshake, and the memory footprint of a DTLS implementation. This paper provides a thorough performance evaluation of DTLS in different duty-cycled networks through real-world experimentation, emulation and analysis. In particular, we measure the duration of the DTLS handshake when using three duty cycling link-layer protocols: preamble-sampling, the IEEE 802.15.4 beacon-enabled mode and the IEEE 802.15.4e Time Slotted Channel Hopping mode. The reported results demonstrate surprisingly poor performance of DTLS in radio duty-cycled networks. Because a DTLS client and a server exchange more than 10 signaling packets, the DTLS handshake takes between a handful of seconds and several tens of seconds, with similar results for different duty cycling protocols. Moreover, because of their limited memory, typical constrained nodes can only maintain 3–5 simultaneous DTLS sessions, which highlights the need for using DTLS parsimoniously.

Sustainable power management techniques in energy harvesting wireless sensors currently adapt the consumption of sensors to their harvesting rate within the limits of their battery residual energy, but regardless of the traffic profile. To provide a fairer distribution of the energy according to application needs, we propose a new sustainable traffic aware duty-cycle adaptation scheme (STADA) that takes into account the traffic load in addition to previous factors. We evaluate our protocol in the specific context of multi-hop IEEE 802.15.4 beacon-enabled wireless sensor networks powered by solar energy. Simulations show that our solution outperforms traffic-unaware adaptation schemes while minimizing the variance of the quality of service provided to applications.

We consider the crucial problem of routing in wireless sensor networks. Routing protocols need to deal with topology changes while keeping the routing overhead low, especially the number of broadcasts, to save energy. In this paper, we consider the problem of building and adapting default routes for convergecast and host routes for downward traffic. We first propose a local repair scheme that allows arbitrarily long periods for rebuilding the tree/DODAG of default routes. Second, we design a scheme for preventing packet forwarding along routing loops, which may occur in the combination of broken host routes and default routes. We achieve this goal without adding an additional header to packets. The two schemes along with on-demand host route construction provides a complete routing solution that addresses many of the shortcomings of RPL. We validate the proposed schemes with Cooja emulations and an experimental evaluation on a real-world sensor network testbed.

This paper presents GreenNet, an energy efficient and fully operational protocol stack for IP-enabled wireless sensor networks based on the IEEE 802.15.4 beacon-enabled mode. The stack runs on a hardware platform with photovoltaic cell energy harvesting developed by STMicroelectronics (STM) that can operate autonomously for long periods of time. GreenNet integrates several standard mechanisms and enhances existing protocols, which results in an operational platform with the performance beyond the current state of the art. In particular, it includes the IEEE 802.15.4 beacon-enabled medium access control (MAC) integrated with lightweight IP routing for achieving very low duty cycles. It offers an advanced discovery scheme that accelerates the process of joining the network and proposes an adaptation scheme for adjusting the duty cycle of harvested nodes to the available energy for increased performance. Finally, it supports security at two levels: a basic standard secure operation at the link layer and advanced scalable data payload security. This paper describes all techniques and mechanisms for saving energy and operating at very low duty cycles. It also provides an evaluation of the performance and energy consumption of GreenNet.

In this paper, we propose stochastic fingerprints for application traffic flows conveyed in Secure Socket Layer/Transport Layer Security (SSL/TLS) sessions. The fingerprints are based on first-order homogeneous Markov chains for which we identify the parameters from observed training application traces. As the fingerprint parameters of chosen applications considerably differ, the method results in a very good accuracy of application discrimination and provides a possibility of detecting abnormal SSL/TLS sessions. Our analysis of the results reveals that obtaining application discrimination mainly comes from incorrect implementation practice, the misuse of the SSL/TLS protocol, various server configurations, and the application nature.

In this paper, we propose a new scheme that allows coupling beacon-enabled IEEE 802.15.4 with the RPL routing protocol while keeping full compliance with both standards. We provide a means for RPL to pass the routing information to Layer 2 before the 802.15.4 topology is created by encapsulating RPL DIO messages in beacon frames. The scheme takes advantage of 802.15.4 command frames to solicit RPL DIO messages. The effect of the command frames is to reset the Trickle timer that governs sending DIO messages. We provide a detailed analysis of the overhead incurred by the proposed scheme to understand topology construction costs. We have evaluated the scheme using Contiki and the instruction-level Cooja simulator and compared our results against the most common scheme used for dissemination of the upper-layer information in beacon-enabled PANs. The results show energy savings during the topology construction phase and in the steady state.

In this paper, we investigate schemes for energy-efficient multi-hop broadcasting in large-scale dense Wireless Sensor Networks. We begin with an initial simplified study of the schemes for relay selection. Our first finding is that MPR-based (Multipoint Relay) mechanisms work poorly in a dense network while the recently proposed Multicast Protocol for Low power and Lossy Networks (MPL) protocol based on Trickle performs better. However, Trickle requires to overhear packet retransmissions in the vicinity, while sensor nodes try to avoid overhearing by periodically waking up and going to sleep to save energy.

The Internet of Things requires a naming service that can also be beneficial for searching for services and applications. To avoid the traditional DNS approach in which a DNS server provides mapping of names to IP addresses in its domain, we propose a novel network service called DINAS (DIstributed NAming Service). It is based on three pillars: 1) Bloom filters for creating compact names from node descriptions, 2) distributed caches for storing names within the network, and 3) overlay routing strategies to publish and discover information - not only names - within the network. In this work, we assume ContikiMAC at Layer 2, 6LoWPAN at Layer 2.5, IPv6 and RPL (routing protocol) at Layer 3, and we present a particular UDP-based overlay routing strategy. We evaluate the proposal by Cooja simulations and compare its performance with a centralized naming service at the sink. The results show that DINAS outperforms the centralized solution.

Multi-hop harvested wireless sensor networks (HWSN) face two challenges. First, each node must control its energy consumption through duty cycle adaptation to spend less energy than nodes obtain from the environment. Second, nodes must efficiently adapt topology so that packets use the most efficient paths towards the sink. In this paper, we address the challenges in the specific context of beacon-enabled IEEE 802.15.4 networks that harvest energy from the light. We analyze and discuss the integration of duty cycle adaptation and topology reconfiguration algorithms in this kind of networks. We propose a novel scheme to identify the best route using the Expected Delay metric (ED). ED synthesizes several network path factors in a single monotonic value that may be used to choose the parent node in topology construction. We have evaluated the performance of the proposed scheme and compared it with the DEHAR routing protocol. Our simulations show the better performance of the proposed scheme.

Billions of smart, but constrained objects wirelessly connected to the global network require novel paradigms in network design. New protocol standards, tailored to constrained devices, have been designed taking into account requirements such as asynchronous application traffic, need for caching, and group communication. The existing connection-oriented security architecture is not able to keep up-first, in terms of the supported features, but also in terms of the scale and resulting latency on small constrained devices. In this paper, we propose an architecture that leverages the security concepts both from content-centric and traditional connection-oriented approaches. We rely on secure channels established by means of (D)TLS for key exchange, but we get rid of the notion of the “state” among communicating entities. We provide a mechanism to protect from replay attacks by coupling our scheme with the CoAP application protocol. Our object-based security architecture (OSCAR) intrinsically supports caching and multicast, and does not affect the radio duty-cycling operation of constrained objects. We evaluate OSCAR in two cases: 802.15.4 Low Power and Lossy Networks (LLN) and Machine-to-Machine (M2M) communication for two different hardware platforms and MAC layers on a real testbed and using the Cooja emulator. We show significant energy savings at constrained servers and reasonable delays. We also discuss the applicability of OSCAR to Smart City deployments.

IEEE 802.15.4 defines a popular MAC standard for wireless sensor and actuator networks. With the default parameters, under medium to high load, 802.15.4 generates excessive collisions and packet losses. Low duty cycles even exacerbate the problem, because more nodes become active after long periods of sleep and contend for channel access. In this paper, we have applied the models that led to the Idle Sense access method for 802.11 to the 802.15.4 slotted CSMA/CA, taking into account the central role of the coordinator and also the bursty nature of the traffic. Surprisingly, the approach perfectly applies to 802.15.4 even if the principles of the two access methods fundamentally differ. Based on the model, we propose ABE, an adaptation method that adjusts the contention window to optimal values so that the network obtains high throughput along with low duty cycles leading to low energy consumption. The method converges to near-optimal back off values even under bursty traffic and for any number of contending nodes.

In wireless sensor networks, the PHY layers use robust modulations and spread spectrum techniques as in IEEE 802.15.4, which often allows to capture a frame successfully even if it collides with another transmission. In this work, we propose eMiM (Enhanced MiM), a new capture scheme that takes into account both SINR and the collision timing in the switching decision. We have carried out extensive simulations to evaluate the performance gains compared to the MiM scheme. The simulation results show that the proposed capture scheme results in a substantial improvement of the frame reception ratio, as well as fairness.

In IEEE 802.15.4 networks, nodes access the medium using a CSMA/CA method to avoid collisions. As the networks use robust physical layers, they benefit from the capture effect: a receiver can correctly receive a frame even if it collides with another transmission. The capture effect and the access scheme result in significant unfairness between nodes depending on their positions. In this paper, we propose new adaptive medium access control algorithms to improve fairness and performance. The algorithms adapt the contention parameters and transmission power depending on the observed busy channel and transmission failure rates to achieve a target frame delivery rate. We have carried out extensive simulations to evaluate the performance gains of the algorithms compared to legacy IEEE 802.15.4 with static transmission power and default contention parameters. Simulations show important improvement in terms of the frame delivery rate, energy consumption, and fairness.

We report results of measurements on SensLAB, an indoor wireless sensor network testbed with a large number of nodes equipped with a CC1101 radio chip. We analyze RSSI and LQI to find the best way to discriminate good links from weak ones and in particular, to derive a metric that estimates the Packet Reception Ratio (PRR) the best. To obtain an estimator of PRR, we have fitted a Fermi-Dirac function to the scatter diagram of the average and standard variation of LQI. The function enables us to find PRR for a given level of LQI. We evaluate the estimator by computing PRR over a varying size window of transmissions and comparing with the estimator.

In this paper, we study current routing issues in Low Power and Lossy Networks (LLNs) and propose two enhancements to the RPL routing protocol. First, we focus on the establishment and maintenance of the Destination Oriented Directed Acyclic Graph (DODAG) for upward traffic and propose a low overhead mechanism that guarantees the absence of loops and quickly reacts to topology changes using a link reversal algorithm. Second, we propose either proactive or on demand establishment of downward routes from the DODAG root to leaf nodes. Simulation results show that our enhancements reduce the convergence time, generate less routing messages, and improve the performance in a network subject to link failures.

RPL, the routing protocol proposed by IETF for IPv6/6LoWPAN Low Power and Lossy Networks has significant complexity. Another protocol called LOADng, a lightweight variant of AODV, emerges as an alternative solution. In this paper, we compare the performance of the two protocols in a Home Automation scenario with heterogenous traffic patterns including a mix of multipoint-to-point and point-to-multipoint routes in realistic dense non-uniform network topologies. We use Contiki OS and Cooja simulator to evaluate the behavior of the ContikiRPL implementation and a basic non-optimized implementation of LOADng. Unlike previous studies, our results show that RPL provides shorter delays, less control overhead, and requires less memory than LOADng. Nevertheless, enhancing LOADng with more efficient flooding and a better route storage algorithm may improve its performance.

In multi-hop IEEE 802.15.4 networks with nodes operating in the beacon-enabled mode, the time and energy spent in the cluster-tree construction may be long and highly variable especially if there are no a priori restrictions on duty-cycle durations or on the channels used in the network. In this paper, we propose a Multi-Channel Beacon Train (MCBT) protocol in which coordinator nodes send trains of beacons on all channels at random instants during the inactive part of a superframe, so that nodes entering the network can quickly acquire the network parameters or routing metrics and join the network. The scheme drastically shortens the delay for topology construction and lowers the consumed energy.

In this paper, we consider the problem of detecting Skype traffic and classifying Skype service flows such as voice calls, skypeOut, video conferencing, chat, file upload and download. We propose a classification method for Skype encrypted traffic based on the Statistical Protocol IDentification (SPID) that analyzes statistical values of some traffic attributes. We have evaluated our method on a representative dataset to show excellent performance in terms of Precision and Recall.

We consider extended LANs composed of two different parts: a static one with hosts and routers, and a dynamic one with mobile hosts. We propose a scheme for interconnecting the two parts that builds on OLSR to establish compact routing tables, by aggregating addresses into prefixes whenever possible. Prefix advertisements imply possible conflicts of prefixes covering the same address ranges, thus we propose an extension to OLSR for solving this issue. We report on performance improvement measured on an experimental testbed.

We propose MCCT (Multi-Channel Cluster Tree), a cluster-tree construction protocol for nodes in IEEE 802.15.4 beacon-enabled mode. By multiplexing transmissions across orthogonal channels, we reduce collisions between control and data frames, which leads to better packet delivery rate and fairness. We propose a method for constructing a cluster-tree suitable for minimizing beacon collisions. The protocol builds on a neighbor discovery procedure that uses a dedicated control channel while still sticking to the superframe structure of IEEE 802.15.4. We also specify a channel assignment and superframe scheduling method that takes into account channel diversity. We evaluate the proposed protocol through simulation and compare with other proposals: standard 802.15.4 and a representative of distributed solutions to the superframe scheduling problem — MeshMAC. The simulation results show that MCCT significantly improves packet delivery ratio, delay, and fairness. It also results in very good packet delivery ratio for increased network density.

The paper presents a simple probabilistic analysis of the energy consumption in preamble sampling MAC protocols. We validate the analytic results with simulations. We compare the classical MAC protocols (B-MAC and X-MAC) with LA-MAC, a method proposed in a companion paper. Our analysis highlights the energy savings achievable with LA-MAC with respect to B-MAC and X-MAC. It also shows that LA-MAC provides the best performance in the considered case of high density networks under traffic congestion.

The paper presents LA-MAC, a low-latency asynchronous access method for efficient forwarding in wireless sensor networks. It is suitable for current and future sensor networks that increasingly provide support for multiple applications, handle heterogeneous traffic, and become organized according to some complex structure (tree, DAG, partial mesh). It takes advantage of the network structure so that a parent of some nodes becomes a coordinator that schedules transmissions in a localized region. Allowing burst transmissions improves the network capacity so that the network can handle load fluctuations. At the same time, the method reduces energy consumption by decreasing the overhead of node coordination per frame. The paper reports on the results of extensive simulations that compare LA-MAC with B-MAC and X-MAC, two representative methods based on preamble sampling. They show excellent performance of LA-MAC with respect to latency, delivery ratio, and consumed energy.

The exponential increase in high rate traffic driven by a new generation of wireless devices is expected to overload cellular network capacity in the near future. Femtocells have recently been proposed as an efficient and cost-effective approach to enhance cellular network capacity and coverage. However, dense and unplanned deployment of additional Base Stations and their uncoordinated operation may increase the system power consumption. Thus, efficient schemes are essential for managing femtocells activity and improving the system performance. In this paper, we investigate the effect of femtocell deployment on the cellular network energy efficiency. The goal is twofold: first, we aim to analyse how classic femtocell access schemes affect the system energy consumption; second, we propose a novel cell selection scheme for Open Access femtocells that allows the effective deployment of femtocells in the cellular network reducing power consumption and limiting the effect of interference.

In this paper, we propose an accurate sampling scheme for defeating SYN flooding attacks as well as TCP portscan activity. The scheme examines TCP segments to find at least one of multiple ACK segments coming from the server. The method is simple and scalable, because it achieves good detection performance with false positive rate close to zero even for very low sampling rates. Our trace-based simulations show that the effectiveness of the proposed scheme only relies on the sampling rate regardless on the sampling method.

We can improve the performance of wireless mesh networks by using multiple interfaces tuned to non-overlapping channels. A Channel and Interface Assignment (CIA) decides when to switch interfaces and which channel to use. Surprisingly, the impact of CIAs on connectivity has received little attention so far. In this paper, we present a comparison and performance evaluation of the existing CIA strategies addressing the connectivity issues: network topology, density of connections, and neighbor discovery. The results presented in this paper provide guidelines for network designers in planning multi-channel multi-interface network deployments.

We consider the problem of running RPL on top of the IEEE 802.15.4 MAC layer-the two layers operate over two different structures, a directed acyclic graph in the case of RPL and a cluster-tree for IEEE 802.15.4. We propose to adapt the cluster-tree of IEEE 802.15.4 so that it can efficiently work coupled with RPL. Nodes in our modified cluster-tree can associate with several parent nodes by taking advantage of an adequate organization of superframes at the MAC layer. Building on this modified MAC layer, we define an opportunistic forwarding scheme that extends RPL with the possibility of forwarding packets over multiple paths. Instead of always using a preferred parent, a node opportunistically forwards packets through other parents as long as their routes towards the sink are better. We take advantage of the opportunistic forwarding to support higher-priority delay-sensitive alarms that need to arrive in sink before a given deadline along with low-intensity monitoring data considered as best-effort. We compare our opportunistic version of RPL to its basic version through detailed simulations in terms of packet delivery ratio, incurred delay, and overhead.

The paper presents DA-MAC, a density aware access method for efficient forwarding in multi-hop dense wireless sensor networks. Its principle is to offer a configurable channel sensing phase during which nodes request transmission opportunity in a way that avoids collisions. The receiver can thus schedule transmissions so that nodes may return to sleep and only wake up at their scheduled transmission instants. Allowing burst transmissions improves network capacity and the network can handle load fluctuations. The paper presents simulation results on the performance of DA-MAC compared with two other adaptive access methods: B-MAC with Contention Window and SCP-MAC. They show excellent performance of DA-MAC with respect to latency, packet delivery ratio, and power consumption.

Within IEEE 802.11 Wireless Local Area Networks (WLANs), client stations can move freely, but because of the short range of their Access Points (APs), they usually need to reassociate with different APs to continue to communicate. When changing APs, a client station starts a process known as a handoff that can take up to 2 seconds, which is too long for real-time applications such as Voice over IP (VoIP). Various solutions have been proposed to change or improve the client behaviour when doing a handoff. Previously, we proposed the idea of Virtual Access Points (VAP) implemented on APs in which a client station changes APs without disrupting its current communication. Based on this new concept, we have developed a solution called Multichannel Virtual Access Points (mVAP) to take advantage of APs operating on multiple channels. We have implemented mVAP using PACMAP, a tool for packet manipulation, and evaluated its performance. Our results show that mVAP is a new efficient technique for seamless handoffs without performance degradation.

The femtocell deployment in 3GPP/LTE sets new challenges to interference mitigation techniques and Radio Resource Management (RRM). Traditional schemes are mainly designed for classical cellular networks while the ad hoc nature of femtocells notably limits the complexity of possible algorithms. Thus, efficient RRM schemes are essential for limiting the interference impact on end-user performance. The goal of this paper is to achieve effective spectral reuse between macrocells and femtocells while guaranteeing the QoS of users served by both macro and femto base stations. We propose a novel resource management scheme that limits the overall interference per chunk generated outside the coverage range of a femtocell while reducing the transmission power in each Resource Block (RB). Our simulation results show that the proposed RRM scheme enhances the energy efficiency of femtocells and improves both macrocell and femtocell throughput.

We propose a new energy efficient network structure for maintaining synchronization in access methods based on Synchronous Preamble Sampling. Our scheme limits the number of synchronization messages and increases network capacity through the use of multiple non-interfering virtual channels. It consists in constructing independent clusters based on the Weakly Connected Dominating Set (WCDS) so that they can use different virtual channels and only need to maintain internal synchronization, while still offering global connectivity. We define a distributed and self-stabilizing algorithm for constructing and maintaining the clusters. Our simulation results show that the proposed scheme has comparable energy consumption to Scheduled Channel Polling, but results in better network capacity. Moreover, it achieves better energy savings and network capacity than recently proposed Crankshaft access method.

One way of improving performance of wireless mesh networks is to use multiple non-overlapping channels. At the same time, the mesh network must continuously self-adapt to varying radio conditions and topology changes. Thus, we propose a neighbor discovery protocol that fits the requirements of multichannel networks while dealing at the same time with the deafness problem. The proposed scheme also takes advantage of neighbor discovery to continually monitor channel activity. We provide a theoretical analysis of the average discovery time. Our simulations show that the solution integrated within Molecular MAC, a multichannel MAC, results in efficient discovery and channel assignment.

Multichannel solutions are increasingly used to cope with the problem of low capacity in sensor networks resulting from high contention during wake-up periods of nodes. We can consider wake-up schedules as virtual channels, because nodes using different schedules cannot communicate with each other. In this paper, we show that multiple virtual channel solutions come at the cost of an increased energy consumption in broadcasts. We analyze the problem of broadcasting over multiple virtual channels under different classes of MAC methods and propose Clustered Virtual Channels (CVC), a new network structure that limits the number of frames needed for maintaining multiple virtual channels synchronized. Our simulation results show that CVC reduces the cost of maintaining synchronization and increases capacity while making all types of broadcasts possible.

In this paper, we study the problem of designing propagation protocols for network-wide localization based on two-way ranging. At the beginning, a network contains a few localized anchor nodes and a large number unlocalized nodes. Unlocalized nodes in the communication range of anchor nodes perform two-way ranging, estimate their positions, and become anchor nodes. The process repeats until all nodes know their positions. We consider three protocols for this propagation process, analyze their convergence speed, and evaluate the communication costs related to the energy consumption. We show that the proposed Optimized Beacon protocol requires much less messages than two other considered protocols while achieving almost the same convergence delay as the Beacon protocol.

Wireless LANs suffer from performance problems caused by insufficient medium access opportunity given to the access point. Consequently, the downlink buffer fills up, which often leads to packet losses. We propose to address this problem by using a size-based scheduling approach, which is known to favor short flows and the start up of new ones-a very appealing property from the user's perspective as interactive applications and new flows are serviced quickly. Still, size-based scheduling policies have a well-known Achilles heel: large flows can block each other for long periods of time and low rate multimedia transfers may end up with a low priority when their accumulated transferred volume becomes large. To solve the above deficiencies, we propose a new packet scheduling scheme called Least Attained Recent Service (LARS) that applies a temporal decay to the volume of data associated with each flow. In this way, its priority depends more on what has happened recently. With this strategy, LARS can bound the impact of a new arriving flow on ongoing flows, thus limiting lock out durations. It can also efficiently protect low rate multimedia transfers irrespectively of the load conditions.

We present a novel geographical routing scheme for spontaneous wireless mesh networks. Greedy geographical routing has many advantages, but suffers from packet losses occurring at the border of voids. In this paper, we propose a flexible greedy routing scheme that can be adapted to any variant of geographical routing and works for any connectivity graph, not necessarily Unit Disk Graphs. The idea is to reactively detect voids, backtrack packets, and propagate information on blocked sectors to reduce packet loss. We also propose an extrapolating algorithm to reduce the latency of void discovery and to limit route stretch. Performance evaluation via simulation shows that our modified greedy routing avoids most of packet losses.

A multichannel MAC improves throughput in wireless mesh networks by multiplexing transmissions over orthogonal channels. In this paper, we propose an efficient way for constructing the wireless mesh structure associated with Molecular MAC, a multichannel MAC layer designed for efficient packet forwarding. Molecular MAC outperforms other classical approaches, but requires a specific structure for efficient operation. First, we propose a centralized protocol that provides an upper bound for constructing such a molecular structure through a MILP (Mixed Integer Linear Programming) formulation that maximizes network capacity. Then, we present two distributed self-stabilizing heuristic protocols derived from the protocols for constructing respectively a Maximum Independent Set and a Spanning Tree. We compare the performance of the proposed protocols in terms of network capacity and route length.

A multi-channel MAC is a promising approach for improving network throughput by multiplexing transmissions over orthogonal channels. Molecular MAC has recently adopted this approach by proposing to modify the standard IEEE 802.11 DCF. It requires role and channel assignment to nodes: some of them use a static channel while others dynamically switch to neighbor channels on-demand. To assign roles and channels, we extend the notion of the Weakly Connected Dominating Set, a structure already used in clustering. We adapt it by introducing new constraints to define what we call a reversible WCDS (r-WCDS), which is particularly suitable for Molecular MAC. We propose a divide-and-conquer scheme that partitions the network into clusters with one leader per cluster solving a MILP formulation to assign roles in its cluster. By appropriately defining the roles at the border of clusters, we maintain global connectivity in the r-WCDS. Our simulations show that the performance of our strategy is close to a centralized algorithm.

We propose a novel view on efficient packet forwarding in wireless mesh networks based on a molecular analogy in which mesh routers are either nuclei or electrons in an atom that corresponds to a 802.11 cell. In this view, a mesh network appears as a collection of spatially distributed 802.11 cells operating on different channels. We define Molecular MAC that uses dynamic channel switching at neighbor mesh routers to efficiently forward packets over multiple hops. To avoid deafness, nuclei notify electrons about pending packets before electrons pull them from nuclei for reception or further forwarding. We evaluate the proposed scheme through simulation and compare with other proposals. Our results show that Molecular MAC obtains much better performance in terms of throughput, packet delivery rate, end-to-end delay, and fairness.

We propose an efficient way of constructing the wireless mesh structure associated with Molecular MAC, a multichannel access method designed for efficient packet forwarding. We base our role assignment on a restricted Weakly Connected Dominating Set structure. After presenting a formal definition of the role assignment problem, we prove its NP-completeness. Then, we propose a centralized 2-approximation algorithm that maximizes the sum of radio link capacities in the molecular structure. Finally, we extend this protocol so that it can operate in a distributed way still providing the same guarantee. This distributed protocol is self-stabilizing thus robust to topology changes. Our simulation results show that the 2-approximation distributed protocol provides an improvement in throughput with respect to other protocols.

We provide an analytical framework for preamble sampling techniques for MAC protocols in wireless sensor networks, from which we derive closed-form formulas for lifetime and reliability calculations. In addition to take into account transmitter behavior that controls the form and the content of the transmitted preamble, our model also considers receiver behavior that controls the duration of preamble reception in case of successful and failed reception. Along with both transmitter and receiver behavior, our model considers a non-perfect channel and thus takes into account the impacts of transmission errors and retransmissions on lifetime and reliability of preamble sampling protocols. Numerical results show that no protocol is universally optimal; that is, each protocol has its own optimal operation point that depends on the given channel and load conditions.

Many researchers have addressed the problem of QoS differentiation in 802.11 wireless networks, however no method proposed so far benefits from all desirable properties: high aggregate throughput even for a large number of contending stations, fair allocation to all stations in the same class, fast adaptation to changing conditions, and support for absolute priorities. If we consider the IEEE 802.11e standard, its EDCA (enhanced distributed channel access) access method suffers from an increased collision rate when the number of stations increases. In this paper, we propose a novel access method that supports both relative proportional throughput allocation and absolute priorities in 802.11 wireless networks. The method is efficient, scalable, and fair. It builds on the idea of the Idle Sense method that provides the optimal throughput and fairness for 802.11 WLANs [1]: each station adjusts its contention window based on the observed average number of idle slots. We achieve absolute priority differentiation by setting the target value for the number of idle slots to a small value, so that the absolute priority class gains all the available throughput. The method also supports relative proportional throughput allocation in which several classes share the available throughput according to desired ratios. Our simulations show that the proposed method achieves its objectives of relative and absolute differentiation both with respect to the aggregated throughput and the speed of convergence. Unlike 802.11e EDCA, it presents very good scalability - the throughput remains almost constant in function of the number of contending stations.

We address the problem of improving the throughput in multi-hop ad hoc networks. Many approaches can be considered to address this issue. We focus on those that imply the use of multiple radio interfaces with separate radio channels. In this context, we need to address two important issues, namely channel assignment and routing. We propose a throughput-oriented scheme called IC2A2R. It allows nodes to use the best channel available in their neighborhood based on a channel selection algorithm. It also extends the OLSR protocol with a new routing metric to take into account both channel diversity and channel alternation. Simulation tests of the enhanced OLSR protocol show that it significantly improves the throughput and outperforms existing solutions such as WCETT.

We propose a solution which aims at improving the throughput in multi-hop ad hoc networks. This solution equips nodes with multiple radio interfaces and dedicates a separate radio channel to each. It firstly assigns channels to transmission links so as nodes receive and emit data simultaneously. Then it enhances the routing protocol so that it takes into account both the channel diversity and the channel alternation while choosing the routes. Finally, it adapts the node transmission power in order to favor the channel re-use in each group of nodes that shared the same channel. We detail its principle while it operates with the AODV routing protocol. Analysis of simulation results prove that such an enhanced AODV protocol performs better than existing metrics such as WCETT and can provide a 370% throughput gain.

We propose Binary Waypoint Routing, a novel geographical routing protocol for wireless mesh networks. Its idea is to learn and maintain source routes to a small number of nodes called binary waypoints that are placed in subspaces constructed as a result of binary space partitioning. A source node sends a packet to a waypoint for a given destination and intermediate nodes try to adapt the packet route by aiming at waypoints that are closer to the destination. Our simulation results show that the proposed scheme achieves high packet delivery rate with a traffic pattern similar to the Optimal Shortest Path Routing.

In this paper, we review the most important performance characteristics of the 802.11 DCF wireless networks, point out some false common knowledge, and report on recent improvements.

We propose a packet scheduling discipline called virtual flow queueing (VFQ) to improve the end-to-end performance of TCP connections over IEEE 802.11 WLANs. VFQ is based on an inter-layer approach: the IP layer schedules packets over the wireless link according to the information coming from both the transport and MAC layers. The scheduler computes virtual transmission times that take into account the channel time at the MAC layer needed to transfer both TCP and ACK segments. The scheduling discipline achieves fair allocation of wireless channel capacity to TCP connections in both directions (upload and download). VFQ also supports different weights like in the classical WFQ scheduling so that the access point can for instance give preference to download connections. In this paper, we describe the VFQ implementation in the Linux kernel and present the results of measurements that show very good performance behavior of VFQ compared to the FIFO scheduling discipline currently deployed in 802.11 access points.

In atypical deployment of IEEE 802.11 wireless LANs in the infrastructure mode, an access point acts as abridge between the wireless and the wired parts of the network. Under the current IEEE 802.11 Distributed Coordination Function (DCF) access method, which provides equal channel access probability to all devices in a cell, the access point cannot relay all the frames that it receives on the downlink. This causes significant unfairness between upload and download connections, long delays, and frame losses. This unfairness problem comes from the not-so-complex interaction of transport-layer protocols with the MAC-layer access method. The main problem is that the access point requires more transmission attempt probability than wireless stations for correct operation at the transport layer. In this paper, we propose to solve the unfairness problem in a simple elegant way at the MAC layer. We define the operation of an Asymmetric Access Point that benefits from a sufficient transmission capacity with respect to wireless stations so that the overall performance improves. The proposed method of operation is intrinsically adaptive so that when the access point does not need the increased capacity, it is used by wireless stations. We validate the proposed access method by simulation to compare it with other solutions based on IEEE 802.11e. Unlike many papers in this domain, which only validate MAC-layer modifications through simulation or analytical modeling, we provide measurement data gathered on an experimental prototype that uses wireless cards implementing the proposed method.

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We analyze greedy geographical routing in spontaneous wireless mesh networks to show several interesting properties. First, we can approximate the dependence of packet loss probability on the mean node rank with a Fermi-Dirac function. When the mesh network grows, it becomes opaque to packets regardless of the average node rank. We also show that packet loss probability in mesh networks with greedy geographical routing does not exhibit the behavior of percolating systems. Finally, we propose an analytical model of greedy geographical routing and use it to derive packet loss probability.

In this paper, we present an evaluation of chosen wireless LAN access methods involving stations with different bit error rates: n-1 stations in ideal transmission conditions (BER = 0) and 1 station with a given bit error rate (BER ne 0). The simulation results show that the IEEE 802.11 DCF and its modifications (Slow Decrease, AOB) are very sensitive to transmission errors, whereas Idle Sense provides good channel access fairness: the value of the contention window is almost the same regardless of transmission errors, so that the throughput difference between stations subject to different bit error rates corresponds only to the proportion of lost frames.

In this paper, we consider the problem of blocked stations that appears in some spatial configurations of multi-hop wireless networks based on the 802.11 DCF (distributed coordination function). The problem leads to starvation of at least one station caused by the presence of neighbor stations within its carrier sensing range that do not hear each other. We propose Forced Transmissions, a simple and efficient solution to this problem. It consists of detecting that a station is blocked by others stations and forcing a transmission. This results in a collision that increases the contention windows of blocking stations and leaves some channel time to the blocked station for transmitting. The blocked station forces transmission only with some probability adjusted in function of the time spent waiting for the channel. Our simulations show that the proposed method increases the minimal throughput of the blocked stations at the expense of a slight degradation of the total throughput, which is unavoidable in any DCF based solution.

In this paper, we introduce and evaluate novel adaptive schemes for neighbor discovery in Bluetooth-enabled ad-hoc networks. In an ad-hoc peer-to-peer setting, neighbor search is a continuous, hence battery draining process. In order to save energy when the device is unlikely to encounter a neighbor, we adaptively choose parameter settings depending on a mobility context to decrease the expected power consumption of Bluetooth-enabled devices. For this purpose, we first determine the mean discovery time and power consumption values for In different Bluetooth parameter settings through a comprehensive exploration of the parameter space by means of simulation validated by experiments on real devices. The fastest average discovery time obtained is 0.2 s, while at an average discovery time of I s the power consumption is just 1.5 times that of the idle mode on our devices. We then introduce two adaptive algorithms for dynamically adjusting the Bluetooth parameters based on past perceived activity in the ad-hoc network. Both adaptive schemes for selecting the discovery mode are based only on locally-available information. We evaluate these algorithms in a node mobility simulation. Our adaptive algorithms reduce energy consumption by 50% and have up to 8% better performance over a static power-con serving scheme

In this paper, we address the issue of evaluating performance of wireless LANs in multicell scenarios. We try to understand the complex behavior of the DCF (distributed coordination function) access method defined in the IEEE 802.11 standard [1] and its modifications proposed for improving performance: slow decrease [2], asymptotically optimal backoff [3], and idle sense [4]. We analyze the influence of overlapping cells and large multicell environments on their performance. Our results show that the IEEE 802.11 DCF and its two modifications (slow decrease and AOB) exhibit important unfairness between stations close to the access point and those near the border of a neighbor cell. Idle Sense performs much better: it provides much better fairness than the IEEE 802.11 DCF and its modifications. It also obtains the highest throughput when stations adapt their bit rate to channel conditions.

MAC protocols based on preamble sampling techniques like WiseMAC and BMAC offer significant energy savings for multihop wireless sensor networks. Although preamble sampling based techniques efficiently reduce idle listening in low-rate data networks, they introduce extra overhead due to possible reception of some irrelevant frames. In this paper, we identify the problem of receiving irrelevant frames and propose to replace the continuous preamble by a series of small frames that we call micro-frames. The resulting Micro-Frame Preamble (MFP) scheme makes it possible for a node to switch its radio off to avoid receiving irrelevant frames and improve energy savings. In the paper, we evaluate the theoretical energy savings of MFP compared with traditional preamble sampling techniques. We also discuss some experiences with our implementation of MFP on the Freescale MC13192 SARD.

Previous studies have shown that preamble MAC protocols have higher energy efficiency compared to traditional low power MAC protocols based on common sleep/wakeup schedules. One efficient implementation of preamble MAC protocols is frame preamble MAC in which the MAC preamble is replaced by a series of frames. In this paper, we consider four frame preamble MAC protocols, namely: persistent MFP (micro frame preamble), non persistent MFP, persistent DFP (data frame preamble), and non persistent DFP. We study their energy cost and communication reliability by assuming a simple binary symmetric channel error model

We consider wireless LANs such as IEEE 802.11 operating in the unlicensed radio spectrum. While their nominal bit rates have increased considerably, the MAC layer remains practically unchanged despite much research effort spent on improving its performance. We observe that most proposals for tuning the access method focus on a single aspect and disregard others. Our objective is to define an access method optimized for throughput and fairness, able to dynamically adapt to physical channel conditions, to operate near optimum for a wide range of error rates, and to provide equal time shares when hosts use different bit rates.We propose a novel access method derived from 802.11 DCF [2] (Distributed Coordination Function) in which all hosts use similar values of the contention window CW to benefit from good short-term access fairness. We call our method Idle Sense, because each host observes the mean number of idle slots between transmission attempts to dynamically control its contention window. Unlike other proposals, Idle Sense enables each host to estimate its frame error rate, which can be used for switching to the right bit rate. We present simulations showing how the method leads to high throughput, low collision overhead, and low delay. The method also features fast reactivity and time-fair channel allocation.

We analyze in this paper the fairness of the 802.11 DCF (distributed coordination function) access method. Short-term fairness is an important property of a MAC layer for obtaining short delays. We show that contrary to the common wisdom, a 802.11 cell with two hosts does not exhibit short-term unfairness. We propose a new fairness index: the number of inter-transmissions that other hosts may perform between two transmissions of a given host. By means of this index, we analyze the fairness of 802.11 for the case of two hosts and derive the probability distribution of the number of inter-transmissions. Our results show that even on the short term time scale the 802.11 DCF access method provides fairness to competing hosts. When compared with slotted ALOHA, a multiple access randomized protocol with good fairness properties, 802.11 presents even better fairness. To validate the model, we compare the analytical results with experimental histograms obtained via simulations and measurements.

We consider the problem of interconnecting hosts in spontaneous edge networks composed of various types of wired or wireless physical and link layer technologies. We argue that this kind of networks requires a more sophisticated approach than standard IP forwarding: communication paths should be managed on a per flow basis, multiple paths need to be maintained to cope with link failures or changing topologies, and the interconnection architecture should provide a means for acquiring the information on destination reachability. To experiment with our approach, we have designed and implemented Lilith, a prototype of an interconnection node for spontaneous edge networks. We handle network dynamics by establishing MPLS (multi protocol label switching) label switched paths (LSP) on demand with a reactive ad hoc routing protocol. We present some measurements that show good performance with respect to the standard IP forwarding and important performance gains when multiple paths are used.

We propose to provide QoS differentiation at the IP level based on the DiffServ model over the standard distributed coordination function (DCF) method of 802.11. By constraining the output rate of each host in a cell via DiffServ scheduling and traffic shaping mechanisms, we can keep the 802.11 network in the state of nonsaturation so that the time-sensitive expedited forwarding (EF) class benefits from stable short delays. We analyze the performance of 802.11b to find how we should constrain the rate of DiffServ sources and present extensive measurements that show how the QoS differentiation is achieved.

The performance of the IEEE 802.11b wireless local area networks is analyzed. We have observed that when some mobile hosts use a lower bit rate than the others, the performance of all hosts is considerably degraded. Such a situation is a common case in wireless local area networks in which a host far away from an access point is subject to important signal fading and interference. To cope with this problem, the host changes its modulation type, which degrades its bit rate to some lower value. Typically, 802.11b products degrade the bit rate from 11 Mb/s to 5.5, 2, or 1 Mb/s when repeated unsuccessful frame transmissions are detected. In such a case, a host transmitting for example at 1 Mb/s reduces the throughput of all other hosts transmitting at 11 Mb/s to a low value below 1 Mb/s. The basic CSMA/CA channel access method is at the root of this anomaly: it guarantees an equal long term channel access probability to all hosts. When one host captures the channel for a long time because its bit rate is low, it penalizes other hosts that use the higher rate. We analyze the anomaly theoretically by deriving simple expressions for the useful throughput, validate them by means of simulation, and compare with several performance measurements.

International audience

We present an architecture for managing multiple media streams included in temporal HTML documents. The architecture provides generic services for negotiating media streams on servers, controlling their delivery, adapting their characteristics to varying network and host conditions, and synchronizing multiple remote streams. Integration of the streaming support within a synchronization architecture leads to the concept of QoP (quality of presentation) that characterizes global quality of document playback from remote servers.

We have define a network programming model called Mobile Assistant Programming (MAP) for development and execution of communication applications in large scale networks of heterogeneous computers. MAP assistants are high-level interpreted programs that can move between nodes, create clones and report results. Their execution is asynchronous and persistent to take into account client disconnections and node failures. This paper presents the implementation of the MAP model using the World-Wide Web framework and the Scheme programming language. Our measures and the first experience with MAP applications show that significant performance improvement can be achieved by moving computation closer to data: both the elapsed time and network traffic are reduced.

Locating and accessing information in a large distributed system is a difficult problem of growing importance. This paper reports on our experience building and using a prototype system for transparent, user-guided associative access to the contents of a large, distributed set of WAIS servers. Our system is based on content routing, an architecture that makes use of content labels for locating and accessing information in large distributed systems. Our content router for WAIS servers is implemented as a Semantic File System that constructs content labels from WAIS source and catalog files. The content router guides locating documents by suggesting terms that frequently appear with a given query term in document headlines. Sufficiently narrowed queries are routed to WAIS servers and processed in parallel. We have successfully used our content router to locate documents on a large number of WAIS servers. Along with demonstrating the feasibility of distributed finding in a large network of information servers, the prototype shows interactive performance limited primarily by the processing time at WAIS servers.>

The author presents an analysis of three object replication strategies based on reliable multicast protocols. A multiclass queuing model is used for obtaining performance characteristics of each strategy. The strategies are compared for a wide set of parameters showing the best strategy suitable for a given situation: the parallel update strategy is interesting for low load conditions and for the low read/write ratio.>

Epsilon is a testbed for monitoring distributed applications involving heterogeneous computers, including microcomputers, interconnected by a local area network. Such a hardware configuration is usual but raises difficulties for the programmer. First, the interprocess communication mechanisms provided by the operating systems are rather cumbersome to use. Second, they are different from one system to another. Third, the programmer of distributed applications should not worry about system and/or network aspects that are not relevant for the application level. The authors present the solution chosen in Epsilon. A set of high-level communication primitives has been designed and implemented to provide the programmer with an interface independent of the operating system and of the underlying interprocess communications facilities. A program participating in a distributed application can be executed on any host without any change in the source code except for host names.>

This paper deals with queueing systems which reveal timedependent behavior. The goal of the paper is to present a set of results which can be applied to the analysis of computer-communication systems. Some new results on the time-dependent analysis of theGI/GI/1queue and of general queueing networks are given. All these results are based on a transient diffusion model of a single server queue. A parametric-decomposition method of the time-dependent analysis of queueing networks is presented. The method permits the analysis, of the networks where the arrival, service processes and also the routing matrix are time-dependent. It makes possible the analysis of various aspects of computer-communication systems, especially the adaptive routings, the strategies of flow and congestion control, the influence of short overload peaks and other time-varying phenomena in such systems. To illustrate these applications, a simple example of the analysis of the centralized adaptive routing in packet-switching networks is presented.

There are many situations where information about the transient behaviour of computer systems is wanted. In this paper a diffusion approximation to the transient behaviour of some queueing systems is proposed. The solution of the forward diffusion equation on the real positive line with the elementary return barrier having almost general holding time distribution provides the approximation for the GI/G/1 queue. The solution of the diffusion equation in a finite region approximates the transient behaviour for the two-server cyclic system and gives approximations for the first overflow time in the GI/G/1/N system and for the maximum number of customers in the GI/G/1 system. Also the approximation for the busy period of the GI/G/1 system and of the two-server cyclic system are presented.