Αντικείμενο αυτής της διδακτορικής διατριβής είναι η μελέτη και η υλοποίηση ενός ολοκληρωμένου κατανεμημένου συστήματος διανομής δεδομένων σε πραγματικό χρόνο. Η ταχεία ανάπτυξη του Διαδικτύου και η πολυπλοκότητα των υπηρεσιών που προσφέρονται μέσα από αυτό έχει εξαντλήσει τα περιθώρια- όρια της κλασικής αρχιτεκτονικής του εξυπηρετητή και του πελάτη , καθώς, ο συνεχώς αυξανόμενος αριθμός χρηστών που ζητούν διάφορες υπηρεσίες δημιουργούν ένα τεράστιο φορτίο στους εξυπηρετητές, το οποίο δεν είναι σε θέση πια να ικανοποιήσουν. Η αρχιτεκτονική των διομότιμων συστημάτων αποτελεί σήμερα τον πιο υποσχόμενο αντικαταστάτη της αρχιτεκτονικής του εξυπηρετητή-πελάτη για την παροχή υπηρεσιών μέσω του Διαδικτύου. Η υπόθεση αυτή δικαιολογείται, αφού αξιοποιώντας τους πόρους των ίδιων των χρηστών, που αποτελούν πλέον ενεργό κομμάτι του συστήματος, η συγκεκριμένη αρχιτεκτονική μπορεί να εξασφαλίσει κλιμάκωση των συστημάτων αυτών σε αριθμό χρηστών αλλά και σε πόρους, του οποίους και αυτό-διαχειρίζονται για την παροχή οποιασδήποτε υπηρεσίας. Η ανάπτυξη, όμως, διομότιμων συστημάτων προϋποθέτει την επίλυση ενός συνόλου προβλημάτων που προκύπτουν από την κατανεμημένη φύση τους και την πολυπλοκότητα τους. Τα τελευταία χρόνια, η επιστημονική κοινότητα έχει ασχοληθεί εκτενώς με τα συστήματα αυτά και έχει προτείνει τρόπους επίλυσης των προβλημάτων που εμφανίζουν, οι οποίες όμως επικεντρώνονται σε συγκεκριμένες πτυχές τους, με αποτέλεσμα να μην προσφέρουν ακόμα δυνατότητες επαρκούς αξιοποίησης των πλεονεκτημάτων τους. Στην παρούσα διδακτορική διατριβή μελετήθηκαν η ανάπτυξη και η υλοποίηση ενός ολοκληρωμένου κατανεμημένου συστήματος διαμοιρασμού δεδομένων σε πραγματικό χρόνο. Το σύστημα αυτό αποτελείται από τρία διακριτά υποσυστήματα: 1. Ένα διομότιμο σύστημα για το διαμοιρασμό δεδομένων σε πραγματικό χρόνο. Το υποσύστημα αυτό αποτελείται από το γράφο διασύνδεσης των κόμβων που το συγκροτούν και το χρονοπρογραμματιστή που εκτελείται σε κάθε κόμβο. 2. Ένα σύστημα υποστήριξης, το οποίο είναι υπεύθυνο για την παρακολούθηση της λειτουργίας του υποσυστήματος διαμοιρασμού και την παροχή επιπλέον εύρους ζώνης, στην περίπτωση που δεν επαρκούν οι πόροι του πρώτου. 3. Ένα διομότιμο σύστημα για την αποθήκευση και την εύρεση των αντικειμένων που είναι διαθέσιμα προς διανομή μέσω του πρώτου υποσυστήματος. Για την ανάπτυξη του πρώτου υποσυστήματος, αρχικά διερευνήθηκε η φύση της εφαρμογής και ορίστηκαν τα επιθυμητά χαρακτηριστικά. Αυτά είναι ο μικρός χρόνος στησίματος, η ανοχή του σε δυναμικά φαινόμενα, όπως είναι η δυναμική συμπεριφορά των χρηστών και του φυσικού δικτύου, η ικανότητα κλιμάκωσης ως προς τον αριθμό των κόμβων και η ικανότητα για διαμοιρασμό δεδομένων με το μεγαλύτερο δυνατό ρυθμό υπό τον περιορισμό του μέσου εύρους ζώνης των κόμβων που αποτελούν το σύστημα. Στη συνέχεια ακολούθησε η μοντελοποίηση της λειτουργίας των συστημάτων κατανεμημένου διαμοιρασμού μέσα από την οποία προέκυψε η κατάλληλη αρχιτεκτονική ενός τέτοιου συστήματος που εγγυάται τη βέλτιστη εκπλήρωση των παραπάνω χαρακτηριστικών. Η προσφορά της παρούσας διατριβής στην έρευνα του επιστημονικού πεδίου των διομότιμων συστημάτων διαμοιρασμού δεδομένων σε πραγματικό χρόνο συνοψίζεται στα παρακάτω σημεία/συμπεράσματα : • Αντίθετα με τη μέχρι τώρα πρακτική που εφαρμόζεται στα συστήματα κατανεμημένου διαμοιρασμού, είναι αναγκαία η παράλληλη ανάπτυξη του γράφου διασύνδεσης και του χρονοπρογραμματιστή έτσι ώστε να μπορεί το κάθε υποσύστημα να χρησιμοποιήσει με βέλτιστο τρόπο τα χαρακτηριστικά του άλλου. • Ο γράφος διασύνδεσης πρέπει να αντικατοπτρίζει τη θέση των κόμβων στο φυσικό υποδίκτυο και να μπορεί να αυτό-οργανώνεται στις δυναμικές αλλαγές του δικτύου ή του πληθυσμού των κόμβων. • Η λειτουργία του χρονοπρογραμματιστή γίνεται πιο αποτελεσματική όταν διαχωρίζεται σε τρεις διαφορετικούς μηχανισμούς. Στο μηχανισμό δημιουργίας κουπονιών, στο μηχανισμό προ-ενεργής αίτησης πακέτου και στο μηχανισμό απόφασης επόμενου κόμβου προς αποστολή πακέτου. Τέλος, υλοποιήθηκαν κατανεμημένοι αλγόριθμοι για τη δημιουργία και την αυτό-οργάνωση του γράφου διασύνδεσης καθώς και οι απαραίτητοι αλγόριθμοι για την υλοποίηση του χρονοπρογραμματιστή. Οι αλγόριθμοι αυτοί σχεδιάστηκαν με τέτοιο τρόπο έτσι ώστε να χρησιμοποιούν ένα ελάχιστο ποσοστό του εύρους ζώνης των κόμβων χωρίς να συμβιβάζουν την αποτελεσματικότητα και την ταχύτητα σύγκλισής τους. Το δεύτερο ζήτημα που μελετήθηκε είναι η βοηθητική χρήση εξυπηρετητών με στόχο την αδιάλειπτη διάθεση απαραίτητων δικτυακών πόρων (εύρος ζώνης) που απαιτούνται από το σύστημα για τον πλήρη και συνεχή διαμοιρασμό του αντικειμένου. Αναλυτικότερα, ο σύγχρονος διαμοιρασμός βίντεο μέσω διομότιμων συστημάτων απαιτεί τη συνεχή ύπαρξη μέσου εύρους ζώνης συμμετεχόντων κόμβων μεγαλύτερο από το ρυθμό αναπαραγωγής του αντικειμένου που διαμοιράζεται. Αντιθέτως, λόγω της δυναμικής συμπεριφοράς των χρηστών και του απρόβλεπτου μέσου όρου εύρους ζώνης που διατίθεται από τους κόμβους οδηγούμαστε συχνά στη μη ομαλή λειτουργία του συστήματος ή/και στο διαμοιρασμό ενός αντικειμένου με μικρό ρυθμό αναπαραγωγής. Η επίλυση αυτού του προβλήματος απαιτεί την εξασφάλιση του ακριβούς και σε πραγματικό χρόνο υπολογισμού των διαθέσιμων πόρων του συστήματος. Επιπλέον, προϋποθέτει το σχεδιασμό μιας αρχιτεκτονικής που είναι κλιμακούμενη, δηλαδή επιτρέπει την παρακολούθηση συστημάτων στα οποία συμμετέχει πολύ μεγάλος αριθμός χρηστών. Παράλληλα, το προτεινόμενο σύστημα παρακολούθησης και ελέγχου του εύρους ζώνης πρέπει να εισάγει στο σύστημα όσο το δυνατόν μικρότερη κατανάλωση πόρων. Ομοίως, το εύρος ζώνης που συνεισφέρουν οι εξυπηρετητές πρέπει να ελαχιστοποιείται με στόχο την ελαχιστοποίηση του κόστους λειτουργίας. Τέλος, οι συνδέσεις που δημιουργούνται μεταξύ εξυπηρετητών και κόμβων πρέπει να εισάγουν με τη σειρά τους ελάχιστο φορτίο στο δίκτυο του προτεινόμενου συστήματος. Εκμεταλλευόμενοι, λοιπόν, τις ιδιότητες του χρονοπρογραμματιστή που αναπτύχθηκε είμαστε σε θέση μετρώντας ένα μικρό μόνο υποσύνολο κόμβων να εκτιμήσουμε γρήγορα και με ακρίβεια το συνολικό διαθέσιμο εύρος ζώνης του συστήματος. Επιπλέον, μετρώντας κάποιες παραμέτρους του χρονοπρογραμματιστή ανταλλαγής μπλοκ εκτιμούμε δυναμικά το φορτίο που αυτός εισάγει για διαμοιρασμό ανάλογα με τις επικρατούσες συνθήκες. Ο αριθμός των κόμβων αυτών είναι αρκετά μικρός και ανεξάρτητος από τον αριθμό των συμμετεχόντων κόμβων καθιστώντας το προτεινόμενο σύστημα ικανό για εξαιρετική κλιμάκωση. Με τις μετρήσεις αυτές γίνεται εφικτός ο υπολογισμός του εύρους ζώνης που απαιτείται από τους εξυπηρετητές για την ομαλή λειτουργία του συστήματος διαμοιρασμού. Τέλος, με τη βοήθεια ενός δυναμικά προσαρμόσιμου στο δίκτυο γράφου διασύνδεσης επιτυγχάνεται η μέγιστη εκμετάλλευση του εύρους ζώνης που συνεισφέρουν οι εξυπηρετητές και ο διαμοιρασμός του σε κόμβους με τη μικρότερη δυνατή δικτυακή καθυστέρηση. Το προτεινόμενο σύστημα αξιολογήθηκε σε κάθε είδους κατάσταση όπως: αυξομειούμενο μέσο εύρος ζώνης, γρήγορες μεταβολές στο μέσο εύρος ζώνης, μέσο εύρος ζώνης μεγαλύτερο και μικρότερο από το ρυθμό αναπαραγωγής. Η αξιολόγηση απέδειξε ότι ο πλήρης διαμοιρασμός του αντικειμένου, η ελαχιστοποίηση του εύρους ζώνης που συνεισφέρουν οι εξυπηρετητές μέσω της ακριβούς εκτίμησης των διαθέσιμων πόρων και η δυνατότητα εκτίμησης μέσω ενός μικρού υποσυνόλου συμμετεχόντων κόμβων είναι εφικτά κάτω από οποιεσδήποτε συνθήκες. Ο τρίτος στόχος που επιδιώξαμε να εκπληρώσουμε είναι η δημιουργία ενός κατανεμημένου συστήματος αποθήκευσης δεδομένων. Αυτό το σύστημα βασίστηκε στους Κατανεμημένους Πίνακες Κατακερματισμού (ΚΠΚ). Σκοπός αυτού του συστήματος είναι η δημιουργία ενός κατανεμημένου αποθηκευτικού χώρου, αποτελούμενου από πόρους των συμμετεχόντων κόμβων, για την αποθήκευση και ανάκτηση δεδομένων που πρόκειται να διαμοιραστούν. Οι απαιτήσεις ενός τέτοιου συστήματος περιλαμβάνουν την γρήγορη αναζήτηση δεδομένων, τη χρησιμοποίηση του μικρότερου δυνατού ποσοστού εύρος ζώνης για τη δρομολόγηση των αναζητήσεων, τη δυνατότητα εκτέλεσης σύνθετων αναζητήσεων και τη συμμέτοχη των κόμβων στο σύστημα ανάλογα με τους διαθέσιμους πόρους τους. Οι παραπάνω απαιτήσεις είναι αδύνατον να ικανοποιηθούν από έναν μόνο γράφο διασύνδεσης, καθώς προϋποθέτουν ετερόκλητα χαρακτηριστικά από το γράφο. Προκειμένου να είναι εφικτή η γρήγορη δρομολόγηση ο γράφος πρέπει να αντανακλά τη θέση των κόμβων στο φυσικό δίκτυο συνεπώς η εισαγωγή των κόμβων στον γράφο πρέπει επίσης να βασίζεται σε αυτό το χαρακτηριστικό. Η δυνατότητα για σύνθετες αναζητήσεις και η συμμετοχή των κόμβων ανάλογα με τους διαθέσιμους πόρους τους προϋποθέτει την μη ομοιόμορφη κατανομή των δεδομένων στο γράφο καθώς και επίσης και την εισαγωγή των κόμβων σε αυτόν ανάλογα με τους πόρους τους και τα δεδομένα που επιθυμούν να αποθηκεύσουν στο δίκτυο. Στα πλαίσια αυτής της διδακτορικής διατριβής προτείνεται ένα σύστημα κατανεμημένης αποθήκευσης το οποίο αποτελείται από δύο συνδεόμενους γράφους διασύνδεσης και μπορεί να ικανοποιήσει τις απαιτήσεις που έχουν τεθεί. Αυτοί οι δύο γράφοι είναι: • Ο γράφος διασύνδεσης και δρομολόγησης ο οποίος είναι υπεύθυνος για τη δρομολόγηση των αιτήσεων αναζήτησης. Οι κόμβοι εισέρχονται σε αυτόν ανάλογα με τη θέση τους στο φυσικό δίκτυο. Η παραπάνω συνθήκη συντελεί στην ταχύτατη δρομολόγηση των αιτήσεων αναζήτησης και τη χρησιμοποίηση ελάχιστου εύρους ζώνης για την εκτέλεσή τους. Για τη δημιουργία αυτού του γράφου αναπτύχτηκαν/σχεδιάστηκαν δύο κατανεμημένοι αλγόριθμοι. Ο πρώτος είναι υπεύθυνος για την εισαγωγή ενός κόμβου στο γράφο ανάλογα με τη θέση του στο φυσικό δίκτυο. Ο δεύτερος είναι υπεύθυνος για τη βελτιστοποίηση και προσαρμοστικότητα του γράφου στις δυναμικές αλλαγές των ιδιοτήτων του φυσικού δικτύου ή του πληθυσμού των συμμετεχόντων κόμβων. • Ο γράφος αποθήκευσης δεδομένων. Αυτός ο γράφος είναι υπεύθυνος για την αποθήκευση των δεδομένων στους κόμβους του συστήματος με τέτοιο τρόπο ώστε να είναι δυνατή η σύνθετη αναζήτησή τους καθώς επίσης και η αποθήκευσή τους ανάλογα με τους διαθέσιμους πόρους κάθε κόμβου. Η αξιολόγηση του συστήματος αυτού απέδειξε ότι ο διαχωρισμός της διαδικασίας δρομολόγησης από τη διαδικασία αποθήκευσης δεδομένων με την δημιουργία δύο ξεχωριστών γράφων διασύνδεσης εξασφαλίζει την εκπλήρωση όλων των απαιτήσεων ενός τέτοιου συστήματος. / The subject of this phd thesis is the study and development of a complete distributed system for real time data distribution. The rapid growth of the Internet and the complexity of the provided services, renders the investigation for a new architectural paradigm necessary, since classic server-client architecture has reached its full potential. The main reason for the above is that the continuously increasing number of users demanding a diversity of services generates an enormous overhead on the servers, that can’t be dealt with efficiently. Today, Peer-to-Peer architecture is considered to be the most promising replacement for client-server architecture for providing such services via the Internet. This assumption can be easily justified since, taking advantage of users resources, who now become active members of the system, peer-to-peer architecture can guarantee the scalability of these systems in respect to the number of participating users as well as the amount of data that they can manage. The development, however, of peer-to-peer systems requires the clarification of a set of problems which stem from their distributed nature and their complexity. In recent years, scientific community has been focusing on these systems suggesting a number of solutions, which, however, deal with certain only aspects of them, thus are unable to provide a holistic approach that could benefit from their numerous advantages. The complete distributed system for the real time distribution of data developed in the current dissertation thesis consists of three discrete subsystems: • a peer-to-peer live streaming system. This subsystem consists of an overlay, for the interconnection of peers, and a scheduler, which runs in every peer. • a supporting system, responsible for the monitoring of live streaming system and the supply of extra bandwidth in cases when peers’ aggregated resources are insufficient to sustain the streaming process • a peer-to-peer system for the storage and query of objects available for streaming, aided by the first subsystem described above. For the development of the first subsystem initially we investigated the nature of the application and defined the required characteristics. Those are the small setup time values, the tolerance of the system in dynamic conditions, like the dynamic behavior of the participating users and the dynamic conditions of the underlying network, the increased scalability concerning the number of supported users, and the ability to support streaming rates as high as possible having as constrain the aggregated upload bandwidth of the participating peers. The contribution of the present dissertation in the research of the scientific field of P2P real time data distribution systems is summarized below: 1. in contrast to contemporary practices regarding distributed live streaming systems the parallel development of the overlay and scheduler are necessary in order for the systems to be able to benefit from each other characteristics 2. the overlay should reflect the locations of the peers in the underlying network and be able to self-organize in response to dynamic changes of the peer population and the network conditions 3. the performance of the scheduler is enhanced when it comprises of three different mechanisms: the token generation algorithm, the mechanism of pro-active block request and the mechanism for selecting the next peer for packet transmitting. At last, distributed algorithms for the realization and self-organization of the overlay along with the necessary algorithms for the actualization of the scheduler were developed. These algorithms were designed in a way that allows for the usage of a small percentage of the nodes’ upload capacities without compromising the efficiency and the speed of their convergence A second subject that was studied was the use of supporting servers for the continuous provision of the required resources (upload bandwidth) for the complete and uninterrupted delivery of a stream. In more detail, peer-to-peer live streaming requires the constant presence of aggregated upload bandwidth greater than the rate of the stream being delivered. In contrast, the dynamic behavior of peers and the unpredictable upload bandwidth of nodes and of the conditions of the underlying network, often result in the disturbance of the streaming process and/or the delivery of a stream with low rate. Solving the above problems requires precise and real time monitoring of participating peers’ resources. Moreover, it assumes the development of an architecture which is scalable, allowing for the monitoring of systems with large peers number. Additionally, the proposed monitoring and bandwidth control system should introduce as little overhead as possible to the system, meaning that the amount of bandwidth used by the servers should be the minimum required to support peer-to-peer streaming system. Finally, connections established between servers and nodes should introduce, in their turn, the least possible overhead. Benefitting from the properties of our proposed peer-to-peer live streaming system’s scheduler we manage, by monitoring a small subset of participating peers, to measure with accuracy and in real time the aggregated upload bandwidth of the total participating peers. In addition, by measuring some parameters of the scheduler of bloc exchange we can dynamically estimate the overhead introduced for the distribution depending on the present conditions. The number of nodes is quite small and independent of the number of participant nodes allowing for the exceptional scalability of the proposed system. Because of these measurements the approximation of the bandwidth necessary for the successful performance of the distribution system becomes feasible. The evaluation process proved that the complete distribution of data, the minimization of the available servers bandwidth through the precise estimation of the available resources as well as the potential for estimation of a small subset of participating nodes are possible under any given circumstances. The third goal we tried to achieve is the development of a distributed data storage system. This system is based on DHTs. It aims to create a distributed storage space that consists of resources belonging to participating nodes, for the storage and retrieval of data about to be distributed. The prerequisites of such a system include: - fast routing process - usage of the smallest possible percentage of bandwidth for the querying process - the potential for execution of complex queries and - the participation of nodes in the system depending on their available recourses The above prerequisites can not be met by one only overlay, since they require diverse characteristics/ from the overlay. In order to achieve fast queries the overlay should reflect the location of all nodes in the physical network, therefore the introduction of nodes in the overlay should also rely on the above feature. The potential for complex queries and the participation of nodes depending on their available resources assumes a non-uniform node distribution in the overlay as well as the introduction of nodes in the system depending on their resources and the data needed to be stored in the network. In this work we propose a system for distributed storage that comprises of two interconnected overlays and can achieve all the demands set. The two overlays are described below: - LCAN is responsible for the routing process. Nodes enter this overlay in terms of their location on the physical network. The condition above leads to the fast routing of queries and the usage of the least possible bandwidth for their execution. In order to design this overlay the development of two distributed algorithms was necessary. The first one performs the introduction of nodes in the overlay according to their location in the network. The second distributed algorithm is responsible for the optimization and the adjustability of the overlay to the dynamic changes of the physical network properties or the participating nodes population. - VCAN. This is responsible for the storage of data in the nodes of the system in a way their storage according to each node’s available resources becomes feasible, while complex queries can be performed. The evaluation of the system has proved that the separation of the routing process from the data storage process with the creation of two separate overlays can result in the successful achievement of all prerequisites set by a distributed data storage system.

Existe um interesse crescente do mercado por aplicações de multimídia em streaming via rede. Particularmente, as aplicações de live streaming que utilizam a tecnologia de redes P2P para a disseminação de conteúdo têm sido alvo de grande atenção. Aplicações como PPLive e PPStream provam que as aplicações de live streaming em redes P2P são uma realidade com relação à tecnologia atual. Os sistemas de live streaming fornecem um serviço de multicast no nível de aplicação para transmissões ao vivo na Internet. Essas aplicações de live streaming, quando executadas em redes P2P, têm potencial para serem altamente robustas, escaláveis e adaptativas devido à redundância e não dependência de recursos particulares dentre os nodos participantes. Porém, para fazer uso de todas as vantagens disponíveis, a aplicação deve contornar alguns desafios: i) manter a qualidade de playback mesmo com a inerente dinamicidade das redes P2P; ii) impedir que nodos incorretos escondam ações maliciosas atrás do anonimato que existe em P2P; iii) manter a taxa de upload dos nodos participantes da aplicação em um nível aceitável. A taxa de upload dos nodos é muito importante porque a aplicação de live streaming em P2P é uma aplicação cooperativa. Desta forma, esperase que todo novo usuário ajude a aplicação retransmitindo pacotes para outros usuários, mantendo, desta forma, a capacidade global de upload do sistema. Infelizmente, manter a cooperação em live streaming não é uma tarefa trivial, visto que cada nodo enfrenta o dilema social do interesse próprio (individualmente é melhor explorar a cooperação dos outros usuários sem reciprocidade) versus a cooperação para com o grupo. A principal contribuição deste trabalho consiste na apresentação de um modelo matemático baseado em Teoria dos Jogos Evolucionários, cujo objetivo é ajudar a compreender as aplicações de live streaming em redes P2P e os fatores que influenciam o seu correto funcionamento. Como contribuição secundária, este trabalho fornece uma análise estatística do comportamento do download e upload observado nestas aplicações. A análise estatística mostra que existe um decaimento da variância temporal de download e upload nas aplicações de live streaming, e que tal decaimento segue uma lei de potência. Os resultados evolucionários do modelo indicam que, se a queda do índice de satisfação dos usuários com a taxa de download for suave, e se a redução da satisfação devido ao custo de upload for insignificante, então existe um ambiente propício para que a cooperação entre os nodos cresça. De forma inversa, se a queda do índice de satisfação dos usuários com a taxa de download for abrupta, e a redução da satisfação devido ao custo de upload for significativa, então existe um ambiente propício para proliferação de nodos oportunistas. A realização e descrição desta pesquisa é composta de quatro etapas principais: i) a delimitação do cenário de live streaming e a definição do jogo para modelagem; ii) a definição do conjunto de estratégias e da função de utilidade; iii) a criação do modelo; iv) a análise do modelo e a apresentação dos resultados de simulação. A análise do modelo abrange três fases: i) análise estatística e comparação das características de download e upload dos dois simuladores utilizados; ii) avaliação do modelo de Teoria dos Jogos Evolucionários através de simulações; e iii) análise dos resultados evolucionários gerados pelo simulador de Teoria dos Jogos Evolucionários. / There is a growing interest in the market for networked multimedia applications. Live streaming applications that use the technology of P2P networks for distribution of live content have specially been the subject of great attention. Applications such as PPLive and PPStrem demonstrate that P2P live streaming applications are already possible with our present technology. Live streaming systems provide a multicast service in the application level for live broadcasts to users through the Internet. These systems executing in P2P networks have the potential to be highly robust, scalable and adaptive due to the characteristics of these scenarios. However, to take advantage of these potential properties, they must overcome some challenges: i) to maintain the playback quality even with the inherent dynamics of P2P networks; ii) to prevent that incorrect peers hide malicious behavior behind their anonymity; iii) to maintain the upload contribution of peers at acceptable levels. The upload contribution of peers is highly important because live streaming applications are cooperative applications. Therefore, every new user must help the application forwarding packets to other users, thereby maintaining the global upload capacity of the system. Unfortunately, the maintenance of cooperation in live streaming system is not a trivial task, since each node faces the social dilemma of self-interest (individually is always better to explore the cooperation of other users without reciprocity) versus cooperation to the group. The main contribution of this dissertation is the presentation of a mathematical model based on Evolutionary Game Theory, whose goal is to help understanding live streaming P2P applications and the factors that influence their correct operation. As a secondary contribution, this work provides a statistical analysis of download and upload behaviors of peers in live streaming P2P systems. The statistical analysis indicates that there is a decay in the download and upload variances, and that this decay follows a power law. The evolutionary results of the model indicate that, if the satisfaction of users with the download rate is smooth, and the reduction of satisfaction due to the upload cost is negligible, then there is a favorable environment for the growth of cooperation. Conversely, if the satisfaction of users with the download rate is abrupt, and the reduction of satisfaction due to the upload cost is significant, then there is a favorable environment to the proliferation of opportunistic nodes. The realization and description of this research is composed of four main steps: i) the definition of the live streaming scenario and the definition of the game to model this scenario; ii) the definition of the strategy set and of the utility function; iii) the suggestion of a model; iv) the analysis of the proposed model and the presentation of obtained results. The model analysis comprehends three phases: i) the statistical analysis and the comparison of the characteristics of download and upload of the two simulators used in this work; ii) the evaluation of the Evolutionary Game Theory model through simulation; and iii) the analysis of the results generated by the Evolutionary Game Theory simulator.

Existe um interesse crescente do mercado por aplicações de multimídia em streaming via rede. Particularmente, as aplicações de live streaming que utilizam a tecnologia de redes P2P para a disseminação de conteúdo têm sido alvo de grande atenção. Aplicações como PPLive e PPStream provam que as aplicações de live streaming em redes P2P são uma realidade com relação à tecnologia atual. Os sistemas de live streaming fornecem um serviço de multicast no nível de aplicação para transmissões ao vivo na Internet. Essas aplicações de live streaming, quando executadas em redes P2P, têm potencial para serem altamente robustas, escaláveis e adaptativas devido à redundância e não dependência de recursos particulares dentre os nodos participantes. Porém, para fazer uso de todas as vantagens disponíveis, a aplicação deve contornar alguns desafios: i) manter a qualidade de playback mesmo com a inerente dinamicidade das redes P2P; ii) impedir que nodos incorretos escondam ações maliciosas atrás do anonimato que existe em P2P; iii) manter a taxa de upload dos nodos participantes da aplicação em um nível aceitável. A taxa de upload dos nodos é muito importante porque a aplicação de live streaming em P2P é uma aplicação cooperativa. Desta forma, esperase que todo novo usuário ajude a aplicação retransmitindo pacotes para outros usuários, mantendo, desta forma, a capacidade global de upload do sistema. Infelizmente, manter a cooperação em live streaming não é uma tarefa trivial, visto que cada nodo enfrenta o dilema social do interesse próprio (individualmente é melhor explorar a cooperação dos outros usuários sem reciprocidade) versus a cooperação para com o grupo. A principal contribuição deste trabalho consiste na apresentação de um modelo matemático baseado em Teoria dos Jogos Evolucionários, cujo objetivo é ajudar a compreender as aplicações de live streaming em redes P2P e os fatores que influenciam o seu correto funcionamento. Como contribuição secundária, este trabalho fornece uma análise estatística do comportamento do download e upload observado nestas aplicações. A análise estatística mostra que existe um decaimento da variância temporal de download e upload nas aplicações de live streaming, e que tal decaimento segue uma lei de potência. Os resultados evolucionários do modelo indicam que, se a queda do índice de satisfação dos usuários com a taxa de download for suave, e se a redução da satisfação devido ao custo de upload for insignificante, então existe um ambiente propício para que a cooperação entre os nodos cresça. De forma inversa, se a queda do índice de satisfação dos usuários com a taxa de download for abrupta, e a redução da satisfação devido ao custo de upload for significativa, então existe um ambiente propício para proliferação de nodos oportunistas. A realização e descrição desta pesquisa é composta de quatro etapas principais: i) a delimitação do cenário de live streaming e a definição do jogo para modelagem; ii) a definição do conjunto de estratégias e da função de utilidade; iii) a criação do modelo; iv) a análise do modelo e a apresentação dos resultados de simulação. A análise do modelo abrange três fases: i) análise estatística e comparação das características de download e upload dos dois simuladores utilizados; ii) avaliação do modelo de Teoria dos Jogos Evolucionários através de simulações; e iii) análise dos resultados evolucionários gerados pelo simulador de Teoria dos Jogos Evolucionários. / There is a growing interest in the market for networked multimedia applications. Live streaming applications that use the technology of P2P networks for distribution of live content have specially been the subject of great attention. Applications such as PPLive and PPStrem demonstrate that P2P live streaming applications are already possible with our present technology. Live streaming systems provide a multicast service in the application level for live broadcasts to users through the Internet. These systems executing in P2P networks have the potential to be highly robust, scalable and adaptive due to the characteristics of these scenarios. However, to take advantage of these potential properties, they must overcome some challenges: i) to maintain the playback quality even with the inherent dynamics of P2P networks; ii) to prevent that incorrect peers hide malicious behavior behind their anonymity; iii) to maintain the upload contribution of peers at acceptable levels. The upload contribution of peers is highly important because live streaming applications are cooperative applications. Therefore, every new user must help the application forwarding packets to other users, thereby maintaining the global upload capacity of the system. Unfortunately, the maintenance of cooperation in live streaming system is not a trivial task, since each node faces the social dilemma of self-interest (individually is always better to explore the cooperation of other users without reciprocity) versus cooperation to the group. The main contribution of this dissertation is the presentation of a mathematical model based on Evolutionary Game Theory, whose goal is to help understanding live streaming P2P applications and the factors that influence their correct operation. As a secondary contribution, this work provides a statistical analysis of download and upload behaviors of peers in live streaming P2P systems. The statistical analysis indicates that there is a decay in the download and upload variances, and that this decay follows a power law. The evolutionary results of the model indicate that, if the satisfaction of users with the download rate is smooth, and the reduction of satisfaction due to the upload cost is negligible, then there is a favorable environment for the growth of cooperation. Conversely, if the satisfaction of users with the download rate is abrupt, and the reduction of satisfaction due to the upload cost is significant, then there is a favorable environment to the proliferation of opportunistic nodes. The realization and description of this research is composed of four main steps: i) the definition of the live streaming scenario and the definition of the game to model this scenario; ii) the definition of the strategy set and of the utility function; iii) the suggestion of a model; iv) the analysis of the proposed model and the presentation of obtained results. The model analysis comprehends three phases: i) the statistical analysis and the comparison of the characteristics of download and upload of the two simulators used in this work; ii) the evaluation of the Evolutionary Game Theory model through simulation; and iii) the analysis of the results generated by the Evolutionary Game Theory simulator.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-05-30T20:25:10Z No. of bitstreams: 1 franciscohenriquecerdeiraferreira.pdf: 1810078 bytes, checksum: b5d9796bdd636be5e4f0f3212220ad0a (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-06-01T11:36:20Z (GMT) No. of bitstreams: 1 franciscohenriquecerdeiraferreira.pdf: 1810078 bytes, checksum: b5d9796bdd636be5e4f0f3212220ad0a (MD5) / Made available in DSpace on 2017-06-01T11:36:20Z (GMT). No. of bitstreams: 1 franciscohenriquecerdeiraferreira.pdf: 1810078 bytes, checksum: b5d9796bdd636be5e4f0f3212220ad0a (MD5) Previous issue date: 2013-03-11 / Nos últimos anos, as aplicações P2P de transmissão de vídeo ao vivo despertaram um grande interesse na comunidade cientí ca. Essas aplicações geram um grande volume de dados que afetam diretamente o desempenho da rede. Apesar de existir um grande número de trabalhos dedicados a entender as aplicações P2P de transmissão de vídeo ao vivo, a maioria deles se baseia em uma visão estática desses sistemas. Estes trabalhos não se preocupam em entender a dinâmica do sistema, ou seja, como eles evoluem ao longo do tempo. Através de experimentos realizados na plataforma PlanetLab, este trabalho apresenta uma caracterização da aplicação SopCast, um dos mais importantes sistemas P2P de transmissão de vídeo ao vivo. O estudo é realizado levando-se em consideração a dinamicidade da topologia construída. Também é investigada a formação de comunidades na rede sobreposta e a correlação dessas comunidades com os Sistemas Autônomos da Internet (AS). Os resultados mostram que a formação de comunidades é bem de nida pela troca de tráfego e uma pequena porção de participantes desses grupos é responsável por sustentar toda a comunidade. Além disso, não existe indícios que os participantes se agrupam de acordo com seus Sistemas Autônomos. De fato, a probabilidade de uma comunidade ser formada com mais da metade de membros pertencentes a um mesmo AS é inferior a 10%. Finalmente, as caracterizações apresentadas fornecem informações importantes para o desenvolvimento de novas aplicações P2P de transmissão de vídeo ao vivo. Mais que isso, será possível algoritimos de formação de parcerias e grupos de tal forma que seja possível reduzir os custos de transmissão entre redes diferentes. / P2P live streaming systems have attracted a lot of attention from the research community in the last years. Such systems generate a large amount of data which impacts the network performance. Despite large number of works devoted to understand P2P live streaming applications, most of them rely on characterizing the static view of these systems. These works do not worry about either understanding the systems dynamics or analyzing how it evolves over time. Through experiments in PlanetLab platform, we present a characterization of the SopCast application, one of the most important P2P live streaming system. We focus on characterizing the dynamics of P2P overlay topology. We also investigated the community formation process in SopCast overlay and the correlation of these communities with the Autonomous Systems of the Internet (AS). Our results show that the formation of communities is well de ned by the exchange of tra c and a small portion of peers of these groups is responsible for supporting the entire community. Furthermore, there is a evidence that peeers are not grouped according to their Autonomous Systems. Indeed, the probability of a community be formed with more 50% of members belonging to the same AS is less than 10%. Finally, the characterizations we conduct provide important information to new P2P live streaming protocols and membership algorithms design. Moreover, the presented characterization may be useful to developers create algorithms that reduces the transmission cost on the P2P network.

El objetivo de este estudio realizado es para que el público tenga más en cuenta lo que un movimiento urbano o una cultura urbana puede llegar a crecer y evolucionar con el paso de tiempo. El freestyle es un movimiento que empezó por los ochenta en Brooklyn, Estados Unidos, desde ese entonces ha sido catalogado por varios calificativos negativos por el mismo hecho que iniciado en la calle, por ejemplo, la violencia, la delincuencia, etc. Con el paso del tiempo, el freestyle ha evolucionado de manera increíble. antes del 2010, solo había un evento por año, hoy en día hay más de dos eventos por mes. Esto se ha debido por varios factores, tanto como los mismos freestylers, la comunidad de freestyle, el live streaming, etc. Por ello, la pregunta de investigación es: ¿Cómo el live stream ha beneficiado el crecimiento de The Urban Roosters en los últimos 3 años? En este contexto, la investigación ha ayudado explorar esos beneficios que se ha ido otorgando con el pasar de los años y los motivos también. La pregunta de investigación se responde a través de los estudios que se ha realizado en transcurso de todo este periodo. se ha indagado por fuentes confiables que han aportado con datos importantes, de esa manera se pueda entender todo el tema a más profundidad. Si bien es cierto, ha habido varios factores importantes para el tema, se han tomado en cuento los que han sido más relevantes e importantes en transcurso de los años. / The main reason of this investigation is to make the public more aware of what an urban movement or urban culture can grow and evolve over the pass of time. Freestyle is a movement that began in the eighties in Brooklyn, United States, since then it has been classified by several negative labels for the same fact that it started on the street, for example, violence, crime, etc. Over time, freestyle has grown and evolved in an incredible way. Before 2010, there was only one event per year, nowadays there are more than two events per month. This has been due to several factors, as well as the freestylers themselves, the freestyle community, live streaming, etc. So, the research question is: How has live streaming benefited The Urban Roosters' growth in the past 3 years? In this context, the research that I´ve been looking for has helped explore those benefits that have been granted over the years and the reasons as well. The research question is answered through the studies that have been carried out throughout this period. It has been investigated by reliable sources that have provided important data, so that the whole topic can be understood in more depth. Although it is true, there have been several important factors for the subject, those that have been most relevant and important over the years have been taken into account. / Trabajo de investigación

In this thesis, we explore the use of a centrally-coordinated peer-to-peer overlay as a possible solution to the live streaming problem. Our contribution lies in showing that such approach is indeed feasible given that a number of key challenges are met. The motivation behind exploring an alternative design is that, although a number of approaches have been investigated in the past, e.g. mesh-pull and tree-push, hybrids and best-of-both-worlds mesh-push, no consensus has been reached on the best solution for the problem of peer-to-peer live streaming, despite current deployments and reported successes. In the proposed system, we model sender/receiver peer assignments as an optimization problem. Optimized peer selection based on multiple utility factors, such as bandwidth availability, delays and connectivity compatibility, make it possible to achieve large source bandwidth savings and provide high quality of user experience. Clear benefits of our approach are observed when Network Address Translation constraints are present on the network. We have addressed key scalability issues of our platform by parallelizing the heuristic which is the core of our optimization engine and by implementing the resulting algorithm on commodity Graphic Processing Units (GPUs). The outcome is a Linear Sum Assignment Problem (LSAP) solver for time-constrained systems which produces near-optimal results and can be used for any instance of LSAP, i.e. not only in our system.   As part of this work, we also present our experience in working with Network Address Translators (NATs) traversal in peer-to-peer systems. Our contribution in this context is threefold. First, we provide a semi-formal model of state of the art NAT behaviors. Second, we use our model to show which NAT combinations can be theoretically traversed and which not. Last, for each of the combinations, we state which traversal technique should be used. Our findings are confirmed by experimental results on a real network. Finally, we address the problem of reproducibility in testing, debugging and evaluation of our peer-to-peer application. We achieve this by providing a software framework which can be transparently integrated with any already-existing software and which is able to handle concurrency, system time and network events in a reproducible manner. / QC 20110426

In the previous two decades, live streaming has become a popular trend in the music industry. From TV and radio broadcast to live streaming on social media, the relationship between audience and performers has been transforming over time. In 2019, due to the sudden spread of Covid-19, the popularity of watching live streaming events reached an unprecedently peak. According to some market studies, this trend will continue even when the pandemic will be over. This research aims to explore how live streaming has changed the interaction between audiences and performers as a consequence of this great event.  In order to understand the current music live-streaming culture, an online survey, interviews and field studies were conducted to obtain insights about potential problems and needs in live streaming. Through the project, a new design feature, the “Cheering” function, is suggested as an additional function to the existing live-streaming platforms, to enhance and enrich the interaction between audience and performers.

A mainstream course has several components that deaf and hard-of-hearing (DHH) students must reconcile. In class, components can include the instructor, projection display, whiteboard, interpreting, and real-time captioning. Outside the classroom, components can include materials from the instructor, notes generated by a note-taker, and a lecture transcript generated via real-time captioning. Web conferencing software can be harnessed to create inclusive experiences for DHH students. Such software can place all components of a class session on a single screen to create a composite screen solution that can be viewed by students in real-time and recorded for later, self-paced review. A composite screen solution may increase performance and comprehension of DHH students in mainstream courses, along with their hearing counterparts. This mixed-methods study focused on the implementation of web conferencing software in a mainstream, college course to explore utilization of a composite screen solution by students inside the classroom. Quantitative data were collected and analyzed to determine impact on student performance. Qualitative data were also collected and analyzed to investigate participant perceptions about the intervention. There was no significant impact on student performance found based on student self-selected usage of the composite screen solution throughout the term. Hearing students utilized the composite screen solution significantly more than DHH students to compensate for obstructed views due to the design of a classroom and to make follow-along demonstrations easier. Alternatively, DHH students had unobstructed views with the projection screen and ASL interpreter in their field of vision. Overall, the live stream of the composite screen solution was not widely utilized by the intended target audience, DHH students, as anticipated. Instead, the results illustrate the importance of design and how a solution has the potential to help an unintended audience with unanticipated issues. The generalizability of the results, as they pertain to DHH students, are limited given the number of DHH participants.

Storskaliga öppna nätbaserade kurser, MOOCar, med självstyrd studietakt skapar en lärandesituation där kursdeltagaren står i centrum och aktivt kan påverka sin egen studieprocess. Ett problem som kursdeltagare upplever är att de får begränsat stöd och hjälp av lärare eller andra kursdeltagare. Möjligheten till social interaktion är avgörande för främjandet av lärandeprocessen i nätbaserade utbildningar visar tidigare forskning. Studien ämnar därför att utforska om direktuppspelning (livestreaming) kan användas som ett pedagogiskt verktyg för att främja den sociala interaktionen mellan kursdeltagare samt mellan kursdeltagare och lärare i MOOCar med självstyrd studietakt. Utifrån en systematisk litteraturstudie utformades ett moment med direktuppspelning via Twitch som testades i praktiken i en MOOC som gavs av Kungliga Tekniska högskolan (KTH). Vi utvärderade sedan momentet genom en enkät till kursdeltagarna och intervju med läraren. En intervju genomfördes också med ytterligare en lärare på KTH som tidigare genomfört ett antal kursmoment där direktuppspelning använts. MOOCar karakteriseras av att de skall vara tillgängliga för alla, därav valdes användarvänlig och kostnadsfri plattform och programvara. På grund av att en direktuppspelning inom MOOCar ska kunna stödja ett stort deltagarantal gavs kursdeltagarna enbart möjlighet att kommunicera via chatt medan läraren kunde kommunicera via såväl chatt som ljud och video. Studien visade att såväl kursdeltagare som lärarna upplevde direktuppspelning som ett givande komplement till de kommunikationsmöjligheter som redan ges i MOOCar med självstyrd studietakt men att läraren bär ett stort ansvar i att möjliggöra de sociala interaktionerna som direktuppspelning ger möjlighet till. Direktuppspelning visades även ge en personlig kontakt som kan hjälpa minimera känslan av att vara isolerad. Baserat på studien utformades en manual för att underlätta implementeringen för lärare i framtida kurser. / Self-paced massive open online courses, MOOCs, provide individual learners with the opportunity to take control of their learning. However, MOOCs require learners to be able to fulfil the course with limited help from teachers and peers - something that is highlighted as an essential factor to enhance the learning experience in distance education. Therefore this study aims to explore the use of live streaming as a pedagogic tool in self-paced MOOCs to enhance communication between learners and between learners and the teacher. Based on a systematic literature study, a live streaming event was designed and conducted via Twitch in a self-paced MOOC given by KTH Royal Institute of Technology. To evaluate the event surveys were handed out to the learners and the teacher was interviewd. An interview was also conducted with another teacher at KTH who had experience with planning and executing live streaming events. Since MOOCs are characterized as being accessible to everyone a user-friendly and free platform plus software were selected. Due to the fact that a live streaming event in MOOCs must be able to support a large number of learners, we chose to allow learners to communicate only via chat while the teacher was able to communicate via chat, audio and video. The study showed that both the learners and the teacher perceived this moment as a rewarding complement to the opportunities for communication already provided in self-paced MOOCs but that the teacher has a major responsibility in enabling the interactions that live streaming allows. The teacher also felt that live streaming provided a personal contact and could therefore be a means to minimize the sense of isolation that is often associated with self-paced MOOCs. The study further designed a manual to facilitate implementation in future courses.

The media industry has during the last decade migrated services from dedicated medianetworks to more shared resources and lately also the public internet and public data centers. Inorder to cater for such transition, several protocols have been designed to meet the demand forhigh-quality media transport over lossy infrastructure, protocols such as SRT and RIST. Thepurpose of Reliable Internet Stream Transport (RIST) and Secure Reliable Transport (SRT) is tohave all vendors of broadcasting equipment support an interoperable way of communication. Thelack of interoperability locks consumers into one particular vendor’s family of products - most oftenthis equipment only supports a proprietary technology. Interoperability creates a more competitivemarket space which benefits consumers and gives vendors an incentive to be more innovative intheir solutions. The purpose of this thesis is to assess the performance of these protocols by comparing theirperformance to a proprietary solution (named ÖÖÖ in this thesis and seen as an establishedsolution in the industry). The challenge is to test these protocols in a lab environment, but have theresults represent real-world use. For this, a large subset of samples is needed along with samplesmeasured over a long period. This sampling was made possible by writing a script which automatesthe sampling process. The results indicate that the versions of RIST and SRT tested in this thesis to some extentcompare well to the selected established protocol (ÖÖÖ). In many scenarios, SRT even did muchbetter, mainly when a line with a single feed was tested. For instance, when the network suffered a2% drop rate and utilized retransmission SRT performed the best and was the only protocol whichhad some samples where no packets were dropped during one hour of measurements. Whenrunning all three protocols at the same time, SRT also did the best in a network with up to 12% droprate. The results in this thesis should give a broadcaster an idea of which of these protocols willfulfill their requirements in a broadcast application. / I mediabranschen finns det en efterfrågan på utrustning som har inslag av interoperabilitet.Anledningen till detta är att någon som köper produkter från en viss återförsäljare inte vill låsas in idenna återförsäljares ”ekosystem” i flera år framöver. Då en studio sällan uppgraderar hela sinproduktionskedja på samma gång ger interoperabilitet möjligheten att köpa utrustning från andraåterförsäljare när man ska uppgradera något i produktionslinan. Detta leder till en merkonkurrenskraftig marknad samt ger incentiv till nya innovativa lösningar. Detta examensarbete går ut på att utvärdera lösningar som tagits fram för att främjainteroperabilitet och jämföra dem med en existerande proprietärlösning. Reliable Internet StreamTransport (RIST) och Secure Reliable Transport (SRT) är två protokoll som tagits fram för just dettasyfte. Utmaningen med att utvärdera dessa protokoll är att i en labbmiljö få resultat som reflekteraranvändandet av protokollen i verkligheten. Detta har gjorts med hjälp av ett program som tagitsfram i detta examensarbete. Med detta program har testandet kunnat automatiseras. Resultaten i detta examensarbete visar potential hos båda RIST och SRT. SRT är i vissascenarion till och med bättre än den proprietära lösningen. Protokollen visar något buggigtbeteende i vissa instanser, såsom att i vissa fal sluta fungera och inte kunna återgå till normalfunktion utan manuell interaktion. Allt som allt är dock protokollen i de flesta fallen testade i dettaexamensarbete ett godtyckligt alternativ till den jämförda proprietära lösningen.