MASTER'S THESIS Analysis of UDP-based Reliable Transport using Network Emulation Andreas Vernersson 2015 Master of Science in Engineering Technology Computer Science and Engineering Luleå University of Technology Department of Computer Science, Electrical and Space Engineering Abstract The TCP protocol is the foundation of the Internet of yesterday and today. In most cases it simply works and is both robust and versatile. However, in recent years there has been a renewed interest in building new reliable transport protocols based on UDP to handle certain problems and situations better, such as head-of-line blocking and IP address changes. The first part of the thesis starts with a study of a few existing reliable UDP-based transport protocols, SCTP which can also be used natively on IP, QUIC and uTP, to see what they can offer and how they work, in terms of features and underlying mechanisms. The second part consists of performance and congestion tests of QUIC and uTP imple- mentations. The emulation framework Mininet was used to perform these tests using controllable network properties. While easy to get started with, a number of issues were found in Mininet that had to be resolved to improve the accuracy of emulation. The tests of QUIC have shown performance improvements since a similar test in 2013 by Connectify, while new tests have identified specific areas that might require further analysis such as QUIC’s fairness to TCP and performance impact of delay jitter. The tests of two different uTP implementations have shown that they are very similar, but also a few differences such as slow-start growth and back-off handling. A previously identified problem with increasing queuing-delay could be replicated with both uTP implementations but new experiments show that a small amount of packet loss seems to prevent this problem. This work has shown how Mininet can be used to build test systems to analyze UDP- based transport protocol implementations. iii Contents Chapter 1 – Introduction 1 1.1 Background..................................2 1.2 Problem description and goals........................2 1.3 Demarcations.................................3 1.4 Thesis outline.................................3 Chapter 2 – Theory 5 2.1 A short history of TCP............................7 2.2 TCP mechanisms...............................8 2.3 Why UDP................................... 10 2.4 Overview of UDP-based protocols...................... 11 2.5 Delays, BDP and fairness.......................... 12 2.6 SCTP..................................... 14 2.7 QUIC..................................... 16 2.7.1 QUIC Implementation........................ 18 2.7.2 Related work on QUIC........................ 18 2.8 uTP...................................... 21 2.8.1 uTP Implementations........................ 23 2.8.2 Related work on uTP......................... 24 2.9 Mininet.................................... 25 2.9.1 Python example............................ 26 2.10 Brief overview of used tools......................... 27 Chapter 3 – Methodology 29 3.1 Alternative test methods........................... 30 3.2 Selection of tests............................... 31 Chapter 4 – Mininet 33 4.1 Rationale................................... 33 4.2 Test system setup............................... 34 4.3 Test system verification............................ 35 4.3.1 Packet delay and jitter........................ 35 4.3.2 Packet loss............................... 36 4.3.3 Queue length............................. 37 4.3.4 Bandwidth and congestion...................... 38 4.4 Issues and solutions.............................. 38 Chapter 5 – Google QUIC 47 5.1 Test setup................................... 47 5.2 Connectify performance comparison test.................. 48 5.2.1 Connectify performance comparison results............. 48 5.3 New performance tests............................ 50 5.3.1 New performance tests results.................... 52 5.4 Congestion tests................................ 56 5.4.1 Congestion tests results....................... 56 5.5 Conclusions for QUIC............................ 59 Chapter 6 – uTorrent Transport Protocol (uTP) 61 6.1 Test setup................................... 61 6.2 Performance tests............................... 62 6.2.1 Performance tests results....................... 62 6.3 Congestion tests................................ 66 6.3.1 Congestion tests results....................... 67 6.4 Growing queuing delay............................ 68 6.4.1 Growing queuing delay results.................... 69 6.5 Conclusions for uTP............................. 74 Chapter 7 – Conclusions 75 7.1 Discussion................................... 75 7.2 Conclusions.................................. 76 7.3 Further work................................. 77 References – 79 Appendix A– Test system details I Appendix B– List of abbreviations III vi Chapter 1 Introduction The Transmission Control Protocol (TCP) is the foundation of the Internet of yesterday and today. In most cases it simply just works and is both robust and versatile. However, in recent years there has been a renewed interest in building new reliable transport protocols based on the unreliable User Datagram Protocol (UDP). In this thesis, we will first study three reliable UDP-based protocols to learn what prob- lems and situations they are trying to handle better and how they are different from TCP. Stream Control Transport Protocol (SCTP) is a generic transport protocol that designed to run directly over IP[1], but can also be used over UDP when the operating systems that lacks a kernel implementation of SCTP[2]. Thereby, a user space implementation of SCTP can be used. Google’s Quick UDP Internet Connections (QUIC) is used for transporting web requests and responses[3]. uTorrent Transport Protocol (uTP) which is used by BitTorrent for peer to peer file sharing[4]. Looking at SCTP, QUIC and uTP, several features that are not widely possible with TCP emerge. For example QUIC connections can handle IP address changes[3], which are not uncommon when smart phones switch between Wi-Fi and mobile networks, and uTP that can transfer files using low bandwidth priority[4]. 1 2 Introduction The second part of this thesis examines QUIC and uTP in more detail to test actual protocol implementations using network emulation. These tests are performed to evaluate the implementations’ performance characteristics in different network situations, and their congestion handling to see how they react to concurrent TCP connections. The tests were performed using Mininet[5], a software based network emulator, which makes it possible to test implementations using different network properties in a con- trolled environment. 1.1 Background The background of this thesis was to learn more about new UDP-based reliable transport protocols and the limitations of TCP. Not for implementing or improving a specific appli- cation, but to be able to consider alternative protocols when developing new applications in the future. Depending on the situation and the set of requirements, there might be a more suitable protocol than TCP. The tests are performed to evaluate the implementations, both to gain better experience and understanding of them but also to create a test system that can be useful to test other implementations in the future. The performance characteristics in different net- work situations and comparison with TCP are important decision factors when selecting an implementation. To check their congestion handling is important to avoid causing unintended effects for other users or the networks. QUIC was selected because it’s a new and relatively complex protocol that has already been deployed by Google servers and in Chrome browser[3][6], and because it has not seen much public analysis yet. uTP was selected because of its unique low priority con- gestion control and it’s wide usage[7] and that multiple implementations exists and can be compared[8][9]. 1.2 Problem description and goals The goal of this thesis is to examine the design of the protocols SCTP over UDP, QUIC and uTP and to test the implementations of QUIC and uTP to evaluate their network performance and congestion handling. 1.3. Demarcations 3 In particular, the thesis tries to answer the following questions: • What limitations of TCP can reliable UDP-based protocols handle better? • Which features do reliable UDP protocols offer compared to TCP and how do they work? • How can Mininet be used to test reliable UDP implementations? • How does the QUIC implementation perform compared to TCP? • What differences are there between the popular uTP implementations and how do they perform compared to TCP? 1.3 Demarcations To limit the scope of the thesis and keep time constraints, the following was not included. • Detailed review of the protocol design or implementations. • Complete evaluations of the implementations, for example their compatibility, scal- ability or security. • The effects when emulating corruption and reordering to network packets. 1.4 Thesis outline Chapter 2 begins with a short refresher on reliable transport protocols, a few relevant TCP mechanisms, why UDP is necessary and a few words about delays and fairness. Thereafter follows the study of three existing reliable UDP transport protocols to see what problems they are trying to solve and how they work, including related work. The chapter ends with an introduction to Mininet, the network emulator used to test and measure different aspects of the implementations. Chapter 3 describes