User contribution in peer-to-peer communities User contribution in peer-to-peer communities Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof. ir. K. C. A. M. Luyben, voorzitter van het College voor Promoties, in het openbaar te verdedigen op maandag 6 juli 2015 om 10:00 uur door Mihai Capotă inginer diplomat, Universitatea Politehnica din București, Roemenië geboren te Buzău, Roemenië. Dit proefschrift is goedgekeurd door de: Promotor: Prof. dr. ir. D. H. J. Epema Copromotor: Dr. ir. J. A. Pouwelse Samenstelling promotiecommissie: Rector Magnificus, voorzitter Prof. dr. ir. D. H. J. Epema, Technische Universiteit Delft & Technische Universiteit Eindhoven, promotor Dr. ir. J. A. Pouwelse, Technische Universiteit Delft, copromotor Onafhankelijke leden: Prof. dr. F. M. T. Brazier Technische Universiteit Delft Prof. dr. ir. G. J. P. M. Houben Technische Universiteit Delft Prof. dr. F. E. Bustamante Northwestern University, United States of America Prof. dr. P. Michiardi EURECOM, France Dr. S. Voulgaris Vrije Universiteit Amsterdam Advanced School for Computing and Imaging This work was partly carried out in the ASCI graduate school. ASCI dissertation series number 330. Printed by: Uitgeverij BOXPress, ‘s-Hertogenbosch © 2015 Mihai Capotă ISBN 978-94-6295-269-0 doi:10.4233/uuid:a03b025b-1fd5-4cd1-9423-a5c1bd85fefe Acknowledgments A PhD represents proof of independence in research. At the same time, obtaining a PhD can only be done with help from others. I am thankful to all who have contributed to this thesis as co-authors of my publications, colleagues, and friends. Dick, you are first in the list not because you are my PhD supervisor, but be- cause you really helped me the most. You believed in me when I was behind with my publications halfway through my contract and also towards the end when, with one chapter of my dissertation missing, I switched focus away from my PhD topic toward the exciting new area of data-intensive computing. Your input helped me at all stages of research, from extracting a problem statement out of the many ideas floating around in our conversations to designing sound experiments. You always supported me, but never let me forget I have a PhD to finish. Our conversations un- related to work were always fascinating, especially the ones about the Netherlands. I could not have asked for a better supervisor. Johan, you gave me the opportunity to come to Delft and meet all the won- derful people you gathered around Tribler. I will always be immensely grateful for that. Furthermore, a large part of the work in this dissertation started during our brainstorming sessions, the remains of which will probably still be visible on the whiteboards in our offices for generations to come. Outside work, you were always a great discussion partner, whether the topic was technology, privacy, or politics. Alex, you taught me how to focus on the important things and forget about minutiae that were preventing me from making progress; in other words, you taught me how to get things done in research. Most importantly, you helped me find a path beyond my PhD and I can confidently face the fast-approaching next step in my career thanks to you. You and Ana were also real friends to me from the first day I arrived in the Netherlands. You welcomed me to your home to celebrate birthdays and other special events, we travelled to exciting places together, and we simply went out for a drink and a chat many times. Thank you both for being my friends. Naza, you were there for me when I needed you most, at the beginning of my PhD, and you taught me how to conduct research on my own. You offered plenty of help, but made it clear that I am the one responsible for my PhD. I can certainly say that the research visit to your university in Brazil you arranged together with Dick for me is the highlight of my PhD, for many more reasons that just paper we published as a result. Outside of research, I will only say one thing: you and Aida were the closest thing to a family I had in the Netherlands. Lucia, you are a great friend. We shared several fantastic years in the same office, during which we had plenty of fun, and we even wrote a paper together! I will not attempt to list all the wonderful moments we shared, but I have to thank you for introducing me to Jelle, the coolest Dutch person I have met. I hope I will be hanging out with both of you for many years to come. v vi Acknowledgments Niels, from writing papers together, to designing and enjoying the social room, to home improvement, to partying, you were always there for me. Thank you for being such a good friend. You and Janna even introduced me to the Sint Jansbrug student society, certainly the most authentic aspect of Delft student life I have experienced. Dimitra, it was very nice sharing the journey toward a PhD with you. You always lightened the atmosphere in our group with your cheerfulness. While we never got to write a paper together, our discussions were very helpful for my research. Outside the office, I fondly remember our great chats about art, especially about film. And the partying, also with Georgios! Henk, thank you for accepting my PhD application, for teaching me Dutch during lunch breaks and random conversations at the office, and for sharing your insights into research, education, TU Delft, and life in the Netherlands. I would like to thank Nicolae Țăpuș for being an excellent MSc supervisor and for giving me the opportunity to come to Delft through the collaboration between Politehnica University of Bucharest and TU Delft. Bogdan, you helped me explore new avenues for research in data-intensive com- puting. The papers we wrote together opened doors on my career path and I am very grateful for that. I also want to thank you for the many conversations we had about life, the universe, and everything. Maciek, we see eye-to-eye on so many things... I was very lucky to have met you at the beginning of my adventure in the Netherlands. Your friendship in the first year of my PhD was crucial for my sticking with the program. You are an example of honesty and fairness in research and in life. I hope we will be again geographically close soon. Tamás, your mathematics skills helped me make crucial progress in my PhD. More importantly, in research and outside research, your warm character was a bright spot in my life. Thank you for everything. Riccardo, you were always one of the most cheerful persons around and I always enjoyed hanging out with you. Incredibly, we also managed to publish a paper together and you presented it so well that we obtained a best paper award for it! This has already been very useful in my career, thank you. Dave, you helped me see that there is more to science than the narrow field of one’s PhD. Your interdisciplinary vision will always be a model for my career. Thank you for sharing your wisdom and helping me become a more open-minded person. Tim, you are the best student anyone can wish for. I am thankful for your independent hard work that got our data-intensive computing papers published and allowed me to wrap up my dissertation. Michel, you are the main author of my first paper and you gave me the oppor- tunity to present it and attend NSDI at the same time. Needless to say, that made quite an impression on me and instilled in me a desire to reach the highest level of expertise. Thank you. Nitin, thank you for being a great companion during our visit to Brazil. During both work and holidays we shared an experience that I will cherish forever. I would like to thank you as well for the helpful comments you made on my work. I will also Acknowledgments vii remember our lively conversations about US politics, which nobody else seemed to understand. Rahim, while we were colleagues for so many years, we collaborated best outside the office, on the tennis courts and at PhD parties, in Leiden and elsewhere. Adele, thank you for sharing your passion for music with me, and for putting up with my ramblings about politics. Boxun, I learned a lot about alternative Chinese culture from you, as well as about old-school video games. You made life at the office very entertaining. Thank you. Otto, you were my go-to person for everything Dutch related. I hope I returned the favor and helped you with Corina with Romanian info at least a bit. Corina, thank you for teaching me computer science at Politehnica University in Bucharest. And thank you both for the wonderful events we celebrated together in the Nether- lands. Elric, thank you for being a great nerd and talking to me about nerdy things for the past couple of years. And also for helping me with the Tribler infrastructure. Lipu, you were a great office mate. I will remember all the fun lunch breaks we had and the delicious tea you brought from China. Egbert, thank you for your help with the Tribler code. I would not have com- pleted the experiments without your help. Flávio, thank you for our fruitful collaboration which resulted in the publication of two papers. It was also nice hanging out with you in Amsterdam. Yong, you started the Graphalytics project and I am very thankful for that.