Sasu Tarkoma

Aalto University, Helsinki, Southern Finland Province, Finland

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Publications (129)28.13 Total impact

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    ABSTRACT: Human mobility has been empirically observed to exhibit Levy flight characteristics and behaviour with power-law distributed jump size. The fundamental mechanisms behind this behaviour has not yet been fully explained. In this paper, we analyze urban human mobility and we propose to explain the Levy walk behaviour observed in human mobility patterns by decomposing them into different classes according to the different transportation modes, such as Walk/Run, Bicycle, Train/Subway or Car/Taxi/Bus. Our analysis is based on two real-life GPS datasets containing approximately 10 and 20 million GPS samples with transportation mode information. We show that human mobility can be modelled as a mixture of different transportation modes, and that these single movement patterns can be approximated by a lognormal distribution rather than a power-law distribution. Then, we demonstrate that the mixture of the decomposed lognormal flight distributions associated with each modality is a power-law distribution, providing an explanation to the emergence of Levy Walk patterns that characterize human mobility patterns.
    08/2014;
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    ABSTRACT: The challenge of providing a resilient and scalable computational and data management solution for massive scale research environments requires continuous exploration of new technologies and techniques. In this project the aim has been to design a scalable and resilient infrastructure for CERN HEP data analysis. The infrastructure is based on OpenStack components for structuring a private Cloud with the Gluster File System. We integrate the state-of-the-art Cloud technologies with the traditional Grid middleware infrastructure. Our test results show that the adopted approach provides a scalable and resilient solution for managing resources without compromising on performance and high availability.
    Journal of Physics Conference Series 06/2014; 513(6):062047.
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  • Aaron Yi Ding, Jon Crowcroft, Sasu Tarkoma
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    ABSTRACT: The fast increase of mobile traffic from smartphone-like devices has created a huge pressure for the cellular operators to manage the network infrastructure and resources. Offloading the mobile traffic to alternative networks such as WiFi is sought as a promising direction to solve this problem cost-effectively. According to our study and experimental findings, existing research proposals are lack of concern for the complexity of network deployment and device limitations, which impedes the solution deployment. To overcome such challenge, we propose SoftOffload, a programmable framework for collaborative mobile traffic offloading. SoftOffload takes the advantage of software defined networking (SDN) paradigm in terms of openness and extensibility. We have implemented the first prototype utilising the open source Floodlight platform.
    12th ACM International Conference on Mobile Systems, Applications, and Services (ACM MobiSys), 2014, Bretton Woods, NH, USA; 06/2014
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    ABSTRACT: In recent years we have seen a fast change in the networking industry: leading by the Software Defined Networking (SDN) paradigm that separates the control plane from the data plane to enable programmability and centralized control of the network infrastructure, the SDN design not only simplifies the network management but also accelerates the innovation speed of deploying advanced network applications. Meanwhile, the landscape of the wireless and mobile industry is changing dramatically as well. Given the advance of wireless technologies such as 4G and WiFi offering a pervasive Internet access, the traffic growth from the smartphone-alike devices has placed an increasing strain on the mobile network infrastructure and infringed the profit. Since the demand is increasing together with the growth of mobile users, the incumbent legacy infrastructure is already calling for an upgrade to overcome its existing limitations in terms of network management and security. In this paper, we advocate that the way forward is to integrate SDN and fully utilize its feature to solve the problem. As the security issue has raise serious concern in the networking community recently, we focus on the security aspect and investigate how to enhance the security with SDN for the wireless mobile networks.
    Computer Networks 06/2014; 66:94–101. · 1.23 Impact Factor
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    ABSTRACT: Multimedia streaming to mobile devices is challenging for two reasons. First, the way content is delivered to a client must ensure that the user does not experience a long initial playback delay or a distorted playback in the middle of a streaming session. Second, multimedia streaming applications are among the most energy hungry applications in smartphones. The energy consumption mostly depends on the delivery techniques and on the power management techniques of wireless access technologies (Wi-Fi, 3G, and 4G). In order to provide insights on what kind of streaming techniques exist, how they work on different mobile platforms, their efforts in providing smooth quality of experience, and their impact on energy consumption of mobile phones, we did a large set of active measurements with several smartphones having both Wi-Fi and cellular network access. Our analysis reveals five different techniques to deliver the content to the video players. The selection of a technique depends on the mobile platform, device, player, quality, and service. The results from our traffic and power measurements allow us to conclude that none of the identified techniques is optimal because they take none of the following facts into account: access technology used, user behavior, and user preferences concerning data waste. We point out the technique with the best playback buffer configuration, which provides the most attractive trade-offs in particular situations.
    Pervasive and Mobile Computing 05/2014; · 1.63 Impact Factor
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    ABSTRACT: With bounded receive buffers, the aggregate bandwidth of multipath transmission degrades significantly in the presence of path heterogeneity. The performance could even be worse than that of single-path TCP, undermining the advantage gained by using multipath transmit. Furthermore, multipath transmission also suffers from delay and jitter even with large receive buffers. In order to tolerate the path heterogeneity when the receive buffer is bounded, we propose a new multipath TCP protocol, namely SC-MPTCP, by integrating linear systematic coding into MPTCP. In SC-MPTCP, we make use of coded packets as redundancy to counter against expensive retransmissions. The redundancy is provisioned into both proactive and reactive data. Specifically, to send a generation of packets, SC-MPTCP transmits proactive redundancy first and then delivers the original packets, instead of encoding all sent-out packets as all the existing coding solutions have done. The proactive redundancy is continuously updated according to the estimated aggregate retransmission ratio. In order to avoid the proactive redundancy being underestimated, the pre-blocking warning mechanism is utilized to retrieve the reactive redundancy from the sender. We use an NS-3 network simulator to evaluate the performance of SC-MPTCP with and without the coupled congestion control option. The results show that with bounded receive buffers, MPTCP achieves less than 20%20% of the optimal goodput with diverse packet losses, whereas SC-MPTCP approaches the optimal performance with significantly smaller receive buffers. With the help of systematic coding, SC-MPTCP reduces the average buffer delay of MPTCP by at least 80%80% in different test scenarios. We also demonstrate that the use of systematic coding could significantly reduce the arithmetic complexity compared with the use of non-systematic coding.
    Computer Networks 05/2014; · 1.23 Impact Factor
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    ABSTRACT: At the age of Web 2.0, Web content becomes live, and users would like to automatically receive content of interest. Popular RSS subscription approach cannot offer fine-grained filtering approach. In this paper, we propose a personalized subscription approach over the live Web content. The document is represented by pairs of terms and weights. Meanwhile, each user defines a top-k continuous query. Based on an aggregation function to measure the relevance between a document and a query, the user continuously receives the top-k most relevant documents inside a sliding window. The challenge of the above subscription approach is the high processing cost, especially when the number of queries is very large. Our basic idea is to share evaluation results among queries. Based on the defined covering relationship of queries, we identify the relations of aggregation scores of such queries and develop a graph indexing structure (GIS) to maintain the queries. Next, based on the GIS, we propose a document evaluation algorithm to share query results among queries. After that, we re-use evaluation history documents, and design a document indexing structure (DIS) to maintain the history documents. Finally, we adopt a cost model-based approach to unify the approaches of using GIS and DIS. The experimental results show that our solution outperforms the previous works using the classic inverted list structure.
    World Wide Web 01/2014; · 1.20 Impact Factor
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    ABSTRACT: Wireless data transmission consumes a significant part of the overall energy consumption of smartphones, due to the popularity of Internet applications. In this paper, we investigate the energy consumption characteristics of data transmission over Wi-Fi, focusing on the effect of Internet flow characteristics and network environment. We present deterministic models that describe the energy consumption of Wi-Fi data transmission with traffic burstiness, network performance metrics like throughput and retransmission rate, and parameters of the power saving mechanisms in use. Our models are practical because their inputs are easily available on mobile platforms without modifying low-level software or hardware components. We demonstrate the practice of model-based energy profiling on Maemo, Symbian, and Android phones, and evaluate the accuracy with physical power measurement of applications including file transfer, web browsing, video streaming, and instant messaging. Our experimental results show that our models are of adequate accuracy for energy profiling and are easy to apply.
    IEEE Transactions on Mobile Computing 01/2014; 13(8):1760-1773. · 2.40 Impact Factor
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    ABSTRACT: There is little information from independent sources in the public domain about mobile malware infection rates. The only previous independent estimate (0.0009%) [5], was based on indirect measurements obtained from domain name resolution traces. In this paper, we present the first independent study of malware infection rates and associated risk factors using data collected directly from over 55,000 Android devices. We find that the malware infection rates in Android devices estimated using two malware datasets (0.28% and 0.26%), though small, are significantly higher than the previous independent estimate. Using our datasets, we investigate how indicators extracted inexpensively from the devices correlate with malware infection. Based on the hypothesis that some application stores have a greater density of malicious applications and that advertising within applications and cross-promotional deals may act as infection vectors, we investigate whether the set of applications used on a device can serve as an indicator for infection of that device. Our analysis indicates that this alone is not an accurate indicator for pinpointing infection. However, it is a very inexpensive but surprisingly useful way for significantly narrowing down the pool of devices on which expensive monitoring and analysis mechanisms must be deployed. Using our two malware datasets we show that this indicator performs 4.8 and 4.6 times (respectively) better at identifying infected devices than the baseline of random checks. Such indicators can be used, for example, in the search for new or previously undetected malware. It is therefore a technique that can complement standard malware scanning by anti-malware tools. Our analysis also demonstrates a marginally significant difference in battery use between infected and clean devices.
    12/2013;
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    ABSTRACT: Multipath TCP (MPTCP) is a major extension of TCP that aims to offer higher aggregate bandwidth and robustness by pooling multiple paths within one transport connection. One of the current weaknesses of the protocol is the Delayed ACK scheme inherited from TCP. The Delayed ACK is an option of TCP that allows the receiver to delay sending an ACK for every other packet within a window given by the Delayed ACK timer. At the sender, the RTO should be no less than the Minimum RTO to avoid spurious timeouts. This strategy can lead to significant performance degradation in the presence of timeouts, especially in high speed networks, where RTT is usually one- or two-order of magnitude smaller than the Minimum RTO. When a subflow occurs a timeout, the receiver has to buffer data from all the subflows until the missing packet is received. The data may overrun the receive buffer to cause flow control at the sender, which seriously impacts the overall performance. In order to avoid MPTCP performance degradation, we propose a new Delayed ACK aiming to remove the Minimum RTO constraint at the sender while to reserve the Delayed ACK function at the receiver. Our solution requires only minor modification to the legacy Delayed ACK scheme and it introduces negligible computational overhead and no extra traffic overhead. We use a NS-3 network simulator to evaluate the performance of our new Delayed ACK in three typical network environments. The results indicate that MPTCP using new Delayed ACK scheme requires much smaller aggregate buffer than MPTCP using the legacy Delayed ACK scheme, especially in high speed networks where the buffer requirement reduces one- or two-order of magnitude.
    GLOBECOM 2013 - 2013 IEEE Global Communications Conference; 12/2013
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    ABSTRACT: This paper proposes a novel energy-efficient multimedia delivery system called EStreamer. First, we study the relationship between buffer size at the client, burst-shaped TCP-based multimedia traffic, and energy consumption of wireless network interfaces in smartphones. Based on the study, we design and implement EStreamer for constant bit rate and rate-adaptive streaming. EStreamer can improve battery lifetime by 3x, 1.5x and 2x while streaming over Wi-Fi, 3G and 4G respectively.
    ACM Transactions on Multimedia Computing Communications and Applications 11/2013; · 0.94 Impact Factor
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    ABSTRACT: We present novel accelerometer-based techniques for accurate and fine-grained detection of transportation modes on smartphones. The primary contributions of our work are an improved algorithm for estimating the gravity component of accelerometer measurements, a novel set of accelerometer features that are able to capture key characteristics of vehicular movement patterns, and a hierarchical decomposition of the detection task. We evaluate our approach using over 150 hours of transportation data, which has been collected from 4 different countries and 16 individuals. Results of the evaluation demonstrate that our approach is able to improve transportation mode detection by over 20% compared to current accelerometer-based systems, while at the same time improving generalization and robustness of the detection. The main performance improvements are obtained for motorised transportation modalities, which currently represent the main challenge for smartphone-based transportation mode detection.
    Proceedings of the 11th ACM Conference on Embedded Networked Sensor Systems; 11/2013
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    ABSTRACT: We introduce CoSense, a collaborative sensing platform for mobile devices that opportunistically distributes sensing tasks between familiar devices in close proximity. We use empirical energy measurements together with data collected from everyday transportation behaviour to demonstrate that our solution can significantly reduce power consumption while maintaining the best possible sensing accuracy.
    Proceedings of the 11th ACM Conference on Embedded Networked Sensor Systems; 11/2013
  • 11/2013;
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    ABSTRACT: When a TCP connection experiences a timeout, the sender must wait at least RTOmin (Minimum Retransmission Timeout) before doing the retransmission, during which the channel may be completely idle, undermining the throughput and channel efficiency. In this paper, we investigate the origin of RTOmin and find that it is needed to mitigate against spurious timeouts when the Delayed ACK (DA) scheme for TCP is implemented. Motivated by this observation, we propose a deployable and TCP-compatible new Delayed ACK (NDA) to replace the legacy DA. Our solution differs with previous work is that instead of using complex algorithms or fine-grained timer to tune RTOmin, we modify the DA scheme with minor changes to allow the sender to remove the RTOmin constraint while reserving the delayed ACK function at the receiver. In order to eliminate the aggressiveness of RTO (Retransmission Timeout) after removing RTOmin, we use coding techniques to encode the timeout retransmitted packets to make the potential spurious retransmissions useful. The simulation results demonstrate that in lossy wireless networks, NDA is efficient, because it achieves much higher TCP goodput and channel efficiency compared to DA. The gain we obtain by the use of NDA comes from two-order effects. One effect comes from removing the RTOmin constraint, because small RTO timer makes TCP react quickly to timeouts, resulting in small transmission idle. The other effect comes from eliminating consecutive RTO by allowing the receiver to acknowledge each timeout retransmission, which further reduces the RTO idle.
    Proceedings of the 16th ACM international conference on Modeling, analysis & simulation of wireless and mobile systems; 11/2013
  • Weixiong Rao, Lei Chen, Sasu Tarkoma
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    ABSTRACT: In recent years, the content-based publish/subscribe [12], [22] has become a popular paradigm to decouple information producers and consumers with the help of brokers. Unfortunately, when users register their personal interests to the brokers, the privacy pertaining to filters defined by honest subscribers could be easily exposed by untrusted brokers, and this situation is further aggravated by the collusion attack between untrusted brokers and compromised subscribers. To protect the filter privacy, we introduce an anonymizer engine to separate the roles of brokers into two parts, and adapt the k-anonymity and `-diversity models to the contentbased pub/sub. When the anonymization model is applied to protect the filter privacy, there is an inherent tradeoff between the anonymization level and the publication redundancy. By leveraging partial-order-based generalization of filters to track filters satisfying k-anonymity and ℓ-diversity, we design algorithms to minimize the publication redundancy. Our experiments show the proposed scheme, when compared with studied counterparts, has smaller forwarding cost while achieving comparable attack resilience.
    IEEE Transactions on Knowledge and Data Engineering 11/2013; 25(11):2644-2657. · 1.89 Impact Factor
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    ABSTRACT: Spaceify is a novel edge architecture and an ecosystem for smart spaces --- a technology that extends the mobile user view of today's common space services (e.g., WiFi) to a richer portfolio of space-centric, localized services and space-interactive applications.
    Proceedings of the 19th annual international conference on Mobile computing & networking; 09/2013
  • Weixiong Rao, Lei Chen, Pan Hui, Sasu Tarkoma
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    ABSTRACT: Nowadays Web search engines are experiencing significant performance challenges caused by a huge amount of Web pages and increasingly larger number of Web users. The key issue for addressing these challenges is to design a compact structure which can index Web documents with low space and meanwhile process keyword search very fast. Unfortunately, the current solutions typically separate the space optimization from the search improvement. As a result, such solutions either save space yet with search inefficiency, or allow fast keyword search but with huge space requirement. In this paper, to address the challenges, we propose a novel structure bitlist with both low space requirement and supporting fast keyword search. Specifically, based on a simple and yet very efficient encoding scheme, bitlist uses a single number to encode a set of integer document IDs for low space, and adopts fast bitwise operations for very efficient boolean-based keyword search. Our extensive experimental results on real and synthetic data sets verify that bitlist outperforms the recent proposed solution, inverted list compression [23, 22] by spending 36.71% less space and 61.91% faster processing time, and achieves comparable running time as [8] but with significantly lower space.
    Proceedings of the VLDB Endowment. 08/2013; 6(13):1522-1533.
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    ABSTRACT: Searching for mobile data offloading solutions has been topical in recent years. In this paper, we present a collaborative WiFi-based mobile data offloading architecture - Metropolitan Advanced Delivery Network (MADNet), targeting at improving the energy efficiency for smartphones. According to our measurements,WiFi-based mobile data offloading for moving smartphones is challenging due to the limitation ofWiFi antennas deployed on existing smartphones and the short contact duration with WiFi APs. Moreover, our study shows that the number of open-accessible WiFi APs is very limited for smartphones in metropolitan areas, which significantly affects the offloading opportunities for previous schemes that use only open APs. To address these problems, MADNet intelligently aggregates the collaborative power of cellular operators, WiFi service providers and end-users. We design an energy-aware algorithm for energy-constrained devices to assist the offloading decision. Our design enables smartphones to select the most energy efficient WiFi AP for offloading. The experimental evaluation of our prototype on smartphone (Nokia N900) demonstrates that we are able to achieve more than 80% energy saving. Our measurement results also show that MADNet can tolerate minor errors in localization, mobility prediction, and offloading capacity estimation.
    Sensor, Mesh and Ad Hoc Communications and Networks (SECON), 2013 10th Annual IEEE Communications Society Conference on; 01/2013

Publication Stats

520 Citations
28.13 Total Impact Points

Institutions

  • 2010–2014
    • Aalto University
      • • Department of Computer Science and Engineering
      • • Helsinki Institute for Information Technology HIIT
      Helsinki, Southern Finland Province, Finland
  • 2002–2014
    • University of Helsinki
      • • Department of Computer Science
      • • Helsinki Institute for Information Technology HIIT
      Helsinki, Southern Finland Province, Finland
  • 2003–2011
    • Helsinki Institute for Information Technology HIIT
      Helsinki, Southern Finland Province, Finland
  • 2008
    • Nokia Research Center
      Palo Alto, California, United States
    • Birla Institute of Technology and Science Pilani
      Pilāni, Rajasthan, India
  • 2007
    • Nokia
      Esbo, Southern Finland Province, Finland
    • Nokia Siemens Networks
      Esbo, Southern Finland Province, Finland