Relay Path Selection for Peer-to-peer Voice-over- IP Systems A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy Quang Duc Bui MSc., B.Eng. Electrical and Computer Engineering College of Science, Engineering and Health RMIT University August 2010 © Copyright by Quang D. Bui 2010 All Rights Reserved ii To my wife, Duong, and my family. iii ABSTRACT Selecting one or multiple peers to relay voice calls is a critical component of large- scale peer-to-peer (P2P) voice-over-IP (VoIP) systems, such as Skype. The challenge is to determine good relay paths for a given voice call in a practical manner. We study different VoIP relay path selection schemes which are usually instantiated during the call session initiation and refreshed periodically. This not only allows end hosts behind Network Address Translations (NATs) or firewalls to establish voice calls but also allows communication in periods of poor performance to be switched to alternate relay paths quickly. We propose an enhanced version of existing algorithms for selecting VoIP relay paths and demonstrate that it improves voice quality. Then, we provide a series of simulations and analytical discussions on the correspondences of relay path performance to overlay network scenarios. We show that there are more opportunities for P2P VoIP systems to obtain good relay paths when selecting relay nodes located at highly connected Transit domains. For better relay path performance, we recommend to select relay nodes whose distances are less than four hops from the source. We also show that, in general, increasing relay node density can reduce relay path length but generate more hop overlaps to the default path due to the breadth first search manner in existing algorithms. Finally, we fully address the problem of selecting VoIP relay path by taking a new approach, including the development of overall formulation, a new network model to effectively identify the optimal solution space, and a new heuristic algorithm for selecting VoIP relay paths. iv ACKNOWLEDGEMENTS I acknowledge with great pleasure the contributions of my supervisor, Professor Andrew Jennings, who accepted me to study with his team a few years ago. Without him this work would not have been possible. At the beginning of my study, I was not really imagining what kind of experience I was running in. Professor Jennings has provided guidance, knowledge, advice, and direction whenever it was needed. He has always been very patient and encouraging me whenever I met obstacles. I appreciated very much Jennings’s very pragmatic approach to problem solving. I would also like to thank him for improving my writing skills, providing supports to me to participate in conferences. Besides that, a large part of the ideas presented in this thesis belong to Jennings. The simulations in my thesis work were based on the simulation platform that I was studied during the time working for the Traffic Engineering for Quality of Service in the Internet, at Large Scale (TEQUILA) project as a part of my Master program in University of Surrey, UK. Under this project, I have received guidance from Professor George Pavlou and Doctor Panos Trimintzios. I have learnt from them how to do research in the field of Internet Traffic Engineering, as well as to develop programs from the TEQULA’s simulation platform. I would thank to Doctor Himanshu Agrawal in the School of Electrical and Computer Engineering, RMIT University, AU who was studying with me under the supervision of Professor Jennings. He has always been willing to spend his valuable time to discuss the problems with me with patience, and shared his experiences and expertise without any reservation. I would also thank to the research people working in the iPLANE project in the Department of Computer Science, Washington University, USA. During the time I was trying to make a simulation of the real Internet configuration, they have been very helpful to instruct me to use their inter-domain path latency database. All of my questions to them have been responded usefully and without delay. v Finally, I would thank all of my family for their supports and encouragements during my study. I would not have done this without your supports. vi TABLE OF CONTENTS ABSTRACT.........................................................................................................................iv ACKNOWLEDGEMENTS..................................................................................................v LIST OF PAPERS ................................................................................................................x LIST OF TABLES...............................................................................................................xi LIST OF FIGURES ............................................................................................................xii CHAPTER 1 INTRODUCTION ..........................................................................................1 1.1 Problem Statement and Research Objectives .......................................................1 1.2 Limitations of the Dissertation..............................................................................3 1.3 Contributions.........................................................................................................4 1.4 Dissertation Organisation......................................................................................5 CHAPTER 2 BACKGROUND ............................................................................................7 2.1 Unstructured P2P Networks..................................................................................9 2.1.1 Non-hierarchical P2P Networks .....................................................................10 2.1.1.1 k-Walker Random Walk .........................................................................10 2.1.1.2 Directed BFS and Routing Indices Based Search...................................11 2.1.1.3 Local Indices Based Search ....................................................................12 2.1.2 Hierarchical Unstructured P2P Networks.......................................................13 2.1.3 Summary.........................................................................................................13 2.2 Structured P2P Networks....................................................................................14 2.2.1 Non-hierarchical Structured P2P Networks....................................................15 2.2.1.1 Chord.......................................................................................................15 2.2.1.2 Pastry.......................................................................................................16 2.2.2 Hierarchical Structured P2P Networks ...........................................................17 2.2.3 Summary.........................................................................................................18 2.3 Locality-aware Peer-to-Peer Algorithms ............................................................19 2.3.1 Network Proximity in Distributed Hash Tables..............................................19 2.3.1.1 Geographic Layout..................................................................................20 vii 2.3.1.2 Proximity Routing...................................................................................21 2.3.1.3 Proximity Neighbour Selection...............................................................21 2.3.2 eQuus: a Locality-aware P2P System.............................................................23 2.3.3 Summary.........................................................................................................25 2.4 Peer-to-Peer VoIP Systems.................................................................................26 2.4.1 VoIP Performance Parameters........................................................................27 2.4.2 Skype...............................................................................................................29 2.4.2.1 Skype System Overview.........................................................................29 2.4.2.2 Skype Functions......................................................................................31 2.4.2.3 Skype Related Research..........................................................................34 2.4.3 P2P SIP Telephony .........................................................................................34 2.4.4 Summary.........................................................................................................37 2.5 Relay Path Selection ...........................................................................................37 2.5.1 Resilient Overlay Network .............................................................................38 2.5.2 Detour .............................................................................................................40 2.5.3 PDF .................................................................................................................42 2.5.4 AS-Aware Peer-relay Protocol .......................................................................44 2.5.5 Earliest Divergence Rule ................................................................................46 2.5.6 Summary.........................................................................................................48 CHAPTER 3 AS-LEVEL TOPOLOGY AND SIMULATION DESIGN .........................49