Message Forwarding in People-centric Delay Tolerant Networks Shabbir Ahmed A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy School of Computer Science and Engineering May 2010 Copyright c 2010, Shabbir Ahmed Originality Statement ’I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of mate- rial which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project’s de- sign and conception or in style, presentation and linguistic expression is acknowledged.’ Signed: Dated: Copyright Statement ’I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International. I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.’ Signed: Dated: Authenticity Statement ’I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in format- ting, they are the result of the conversion to digital format.’ Signed: Dated: Acknowledgments I would like to take this opportunity, to thank all the people, who have supported and inspired me during my doctoral research. I especially want to thank my supervisor, Dr. Salil S. Kanhere, for his constant guidance, feedback and patience during my research and study at the University of New South Wales. He has supported me in many ways during my hard times. In addition, he was always accessi- ble and willing to help his students with their research. Without his persistent support, I could not have managed to complete my thesis. A special thanks goes to Dr. Sanjay Jha and Dr. Chun Tung Chou for their cordial support during my research tenure. I would also like to thank Dr. Mahbub Hassan his valuable feedback during my annual progress reviews. Also, many thanks to Dr. Nadeem Ahmed for his encouragements to complete this thesis. All my friends at the Network Research Laboratory made it an en- joyable place to work. In particular, I would like to thank Baseem, Jerry and Xin for their friendship and help in the past four years. Many thanks to my friend Prashant for his intellectual discussions and support. I apologize that I could not mention all of my friends here individually. Thanks to all my family members, who inspired me to research and sacrifice my company for a greater cause. Thanks. Author’s Publications • Shabbir Ahmed and Salil S. Kanhere. VANETCODE: Network Coding to Enhance Cooperative Downloading in Vehicular Ad- hoc Networks. In Proceedings of International Wireless Com- munications and Mobile Computing Conference, IWCMC 2006, ACM press, Vancouver, Canada, pp. 527-532, July 2006. • Shabbir Ahmed and Salil S. Kanhere. SKVR: Scalable Knowl- edge based Routing Architecture for Public Transport Networks. In Proceedings of the 3rd ACM International Workshop on Vehic- ular Ad-hoc Networks, VANET 2006, short paper, ACM press, Los Angeles, California, USA, pp. 92-93, September 2006. • Shabbir Ahmed and Salil S. Kanhere. Cluster-based Forwarding in Delay Tolerant Public Transport Networks. In 1st IEEE Workshop on User Mobility and Vehicular Networks, IEEE LCN ON-MOVE 2007, IEEE, Dublin, Ireland, pp. 625-634, August 2007. • Shabbir Ahmed and Salil S. Kanhere. HUBCODE: Message Forwarding using Hub-based Network Coding in Delay Tolerant Networks. In The 12th ACM International Conference on Mod- eling, Analysis and Simulation of Wireless and Mobile Systems, MSWiM 2009, ACM press, Tenerife, Canary Island, pp. 288-296, October 2009. • Shabbir Ahmed and Salil S. Kanhere. A Bayesian Routing Framework for Delay Tolerant Networks. In IEEE Wireless Communications and Networking Conference 2010, IEEE, Syd- ney, Australia, pp. 1-6, April 2010. • Shabbir Ahmed and Salil S. Kanhere. Message Dissemination in Vehicular Networks. Book Chapter, Accepted for publication in Delay Tolerant Networks: Protocols and Applications, May 2010. • Shabbir Ahmed and Salil S. Kanhere. HUBCODE: Hub-based Forwarding Using Network Coding in Delay Tolerant Networks. Submitted to Wiley InterScience Journal of Wireless Communi- cations and Mobile Computing 2010. • Shabbir Ahmed and Salil S. Kanhere. Characterization of a Delay Tolerant Network. The 35th IEEE Conference on Local Computer Networks, IEEE LCN 2010. Abstract This thesis concentrates on the message forwarding issues of real- world people-centric Delay Tolerant Networks (DTN), such as Pocket Switched Networks (PSN) and Vehicular Ad-hoc Networks (VANET), which are omnipresent, and hence pertinent to examine. These people- centric DTN often exhibit discernible network properties, which can be leveraged to develop efficient message forwarding schemes. In this thesis, we analyse the mobility traces of a large-scale public transport network. This type of network is an illustrative example of people- centric DTN. Our extensive analysis of its spatio-temporal communi- cation graph identifies a power-law behaviour and periodicity in the mobility patterns. It also reveals the existence of hubs (a group of highly connected nodes) in these networks. Based on our observa- tions of these networks, we propose three classes of functional mes- sage forwarding schemes for people-centric DTN. In particular, we propose a network coding based forwarding strategy, called the Hub- Code, which seeks to exploit the hubs as message relays in an efficient manner. We formulate a mathematical model for estimating the mes- sage delivery delay and present a closed-form expression for the same. The large-scale simulation results show that under pragmatic assump- tions, which account for short contact durations between nodes, our schemes outperform comparable strategies by more than 20%. Next, by utilizing the clustering property of people-centric networks, we propose a cluster-based forwarding scheme and a generic cluster for- mation method based on encounter frequencies between nodes, which uses the dynamic programming paradigm to reduce the complexity by an order, compared to traditional graph-theoretic approach. Our cluster-based forwarding approach outperforms the Spray and Wait scheme by about 12% in terms of delivery ratio. Finally, when the history of mobility patterns about the underlying network is avail- able, we propose a statistical forwarding scheme which leverages this information in predicting the next best forwarder. In particular, we have proposed a Bayesian classifier based DTN message forwarding framework, that adopts a systematic approach for computing the for- warding metrics by utilizing the network parameters (e.g. spatial and temporal information at the time of message forwarding), which cap- ture the periodic behaviour of DTN nodes. Simulation results show that even a naive instantiation of our Bayesian forwarding framework outperforms comparable gradient-based schemes by 25%, in terms of delivery ratio. It is worth mentioning that our proposed forwarding schemes, which are based on the key characteristics of a real-world public transport network, are generic enough to be applicable for a wide range of people-centric DTN. This is due to the fact that the other real-world people centric DTN (e.g. pocket switched network) also share similar properties (e.g. existence of hubs) with the aforementioned network. Contents Nomenclature xviii 1 Introduction 1 1.1 DelayTolerantNetworks . 1 1.2 People-centricDTN. .. .. .. 5 1.3 Challenges in Message Forwarding . 7 1.3.1 Store-Carry-and-Forward Paradigm . 10 1.4 RelatedWorkandtheResearchGap . 11 1.4.1 Problems of Multi-copy Schemes . 14 1.5 OurContribution ........................... 18 1.6 ThesisOrganization. 23 2 Literature Review 24 2.1 Characterization of People-centric DTN . 24 2.1.1 Human Mobility Based Traces . 25 2.1.2 WiFi Access Point Based traces . 26 2.1.3 VehicleBasedTraces . 29 2.2 MessageForwardinginDTN. 31 2.2.1 Knowledge-less Forwarding . 32 2.2.1.1 DirectContact . 32 2.2.1.2 Two-hopRelay . 33 2.2.1.3 Tree-basedFlooding . 33 2.2.1.4 Epidemic ...................... 34 2.2.1.5 NetworkCoding . 35 2.2.2 Knowledge-based Forwarding . 37 ix CONTENTS 2.2.2.1 GradientForwarding . 39 2.2.2.2 Cluster-based Forwarding . 42 2.3 Summary ............................... 43 3 Characterization of People-centric DTN 45 3.1 MobilityTraceDetails . 47 3.1.1 Post-processing of the Mobility Trace . 47 3.1.2 Physical Properties of the Network . 48 3.2 Power-lawproperties . 52 3.2.1 Inter-contactDurations. 53 3.2.2 ContactDurations . 55 3.2.3 NodeDegreeDistribution . 56 3.3 Clustering ............................... 58 3.3.1 Clustering Coefficient . 58 3.4 Periodicity............................... 62 3.5 Summary ............................... 64 4 Forwarding using Hub-based Network Coding 66 4.1 HubCode ............................... 68 4.1.1 HubCodeV1.......................... 69 4.1.1.1 Source........................ 69 4.1.1.2 Hubs......................... 69 4.1.1.3 Destination .