Abstract of thesis entitled “Congestion Control for Transmission Control Protocol (TCP) in Wireless Networks” Submitted by Chengdi LAI for the degree of Master of Philosophy at The University of Hong Kong in August 2011 Congestion control regulates the amount of data traffic injected by end systems into communication networks, preventing persistent network over- load. In the Internet, it is typically realized by Transmission Control Protocol (TCP), which backs off transmission of end systems upon detecting packet loss, with the assistance of active queue management (AQM) algorithms, which probabilistically drop packets at the intermediate routers based on the buffer occupancy. The thesis adapts the present Internet congestion control to the wire- less environment that is gradually becoming an indispensable component of the Internet. Firstly, we propose a novel TCP variant, known as TCP for non-congestive loss (TCP-NCL), to perform effective congestion control, se- quencing control, and loss recovery over wireless networks where reordering, error-prone channels are prevalent. Conventional TCP tends to misinterpret packet reordering and wireless loss as congestive loss, and back off unnecessar- ily. We develop a smart TCP sender (STS) model that offers more reliable signals of packet loss and network congestion over reordering, error-prone channels. TCP-NCL is devised as a practical approximation of STS. Our simulation results show that TCP-NCL can accurately differentiate among the occurrences of packet reordering, congestive loss, and non-congestive loss, and can thus serve as the aforementioned unified solution. Deployment of TCP-NCL requires sender-side modification only. Secondly, we have performed a series of simulation experiments to inves- tigate the performance of AQM under wireless losses. Our results report that random early detection (RED), a classical AQM, fails to maintain a stable backlog against time-varying wireless packet error rate (WPER), resulting in poor delay performance and unfairness towards bursty traffic. We apply the internal model principle to devise a family of AQM enhancements, which can automatically adjust packet dropping rate of AQM to compensate for wireless losses, and thus stabilize the backlog. We further devise the integral controller (IC) as an embodiment of the principle. Our simulation stud- ies show that IC is robust against time-varying WPER, even in challenging scenarios like fat pipes and perturbing traffic flows. In summary, our proposed TCP and AQM enhancements can perform effective congestion control over the heterogeneous wired/wireless Internet, and help the latter to operate in the optimal region of low delay with small jitters and high throughput. (372 words) Congestion Control for Transmission Control Protocol (TCP) in Wireless Networks by Chengdi LAI B.Eng. (EComE) H.K. A Thesis Submitted in Partial Fulfilment of the Requirements for the Degree of Master of Philosophy at The University of Hong Kong August 2011 Declaration I declare that the thesis and the research work thereof represents my own work, except where due acknowledgement is made, and that it has not been previously included in a thesis, dissertation or report submitted to this Uni- versity or to any other institution for a degree, diploma or other qualifica- tions. Signed ..................................................................... Chengdi LAI i Acknowledgements The thesis marks a first milestone for my research journey, which started three years ago when my current advisors, Prof. Victor On-Kwok Li and Dr. Ka-Cheong Leung, introduced to me the prosperous realm of networking research. Over the years, their pursuit of excellence, encouragement of inno- vation, and emphasis on rigorousness have greatly influenced me. Prof. Li’s great vision and innovative insights have kept me inspired in pursuit of re- search work of profound impact. Dr. Leung has patiently led me to formulate my research methodology and taste via the numerous email exchanges and conversations. I would like to express my earnest gratitude to them. I owe my special thanks to Prof. Steven H. Low and Prof. Ricky Y.K. Kwok for serving as my thesis examiners. Their constructive comments assist me to improve the quality of this thesis. I thank Dr. K.P. Chan for chairing my thesis committee. I have been very fortunate to encounter several wonderful mentors during the past five years in the University of Hong Kong, spanning my B.Eng. and M.Phil. programs. I appreciate Prof. Kwok, Prof. Lawrence K. Yeung, and Dr. Vincent Tam for their insightful advice on career and life. I would also like to take the opportunity to thank Fr. John Coghlan for providing me three ii years’ enjoyable residence in Ricci Hall during my undergraduate study. The high level of professionalism of the support staff in the Department of Electrical and Electronic Engineering, the University of Hong Kong, has greatly facilitated my research work. In particular, I would like to thank Ms. Danita Lee, Ms. Julie Hung, and Ms. Lily Lo for their assistance on ad- ministrative issues, and Mr. C.L. Chan and Mr. Andy Fok for their technical support. I am obligated to my colleagues, Dr. Albert Y.S. Lam, Dr. Guanghua Yang, Dr. Yanhui Geng, Mr. Ze Zhao, Mr. Yile Yang, Mr. Jin Xu, Mr. Xiaoguan Fang, Dr. Qiong Sun, Dr. Jun Hong, Dr. Kuang Xu, and Dr. Jialing Xu, for their support and assistance. Last, but not least, I am deeply indebted to my parents and my girlfriend, Miss Shuzhen Shen, for their love, and for always being there for me through thick and thin. iii Contents Declaration i Acknowledgements ii Contents iv List of Figures vi List of Tables viii 1 Introduction 1 1.1 ChapterOverview......................... 1 1.2 InternetCongestionControl . 2 1.2.1 TCP............................ 3 1.2.2 AQM............................ 8 1.3 WirelessChannels. .. .. 10 1.4 Thesis Statement and Outline of Thesis . 12 1.5 ConcludingRemarks . 14 2 Related Work 16 2.1 ChapterOverview. .. .. 16 2.2 Wireless TCP Enhancements . 16 2.3 DesignandAnalysisofAQM . 19 2.4 ConcludingRemarks . 21 3 A Serialized-Timer Approach for Enhancing Wireless TCP 23 3.1 ChapterOverview. .. .. 23 3.2 STSModel ............................ 25 3.2.1 RDTimer ......................... 31 3.2.2 CDTimer ......................... 32 3.3 TCP-NCL............................. 43 3.3.1 NRUProcess ....................... 43 iv 3.3.2 RDandCDTimers. .. 46 3.3.3 RTOTimer ........................ 50 3.3.4 KernelImplementation . 51 3.4 PerformanceEvaluation . 54 3.4.1 Non-congestiveloss . 56 3.4.2 PacketReordering . 61 3.4.3 CongestiveLoss. 63 3.5 ConcludingRemarks . 68 4 Enhancing AQM to Combat Wireless Losses 69 4.1 ChapterOverview. .. .. 69 4.2 AQMunderWirelessLosses . 72 4.3 SystemModel........................... 77 4.4 Robust AQM based on Internal Model Principle . 82 4.4.1 IC ............................. 88 4.5 PerformanceEvaluation . 94 4.5.1 One Wireless Bottleneck Link . 94 4.5.2 Two Wireless Bottleneck Links . 103 4.6 ConcludingRemarks . .105 5 Conclusions and Future Research 107 5.1 ChapterOverview. .107 5.2 Contributions ...........................107 5.3 FutureResearch. .. .. .111 5.3.1 A Theoretical Framework for Wireless TCP . 111 5.3.2 A Unified AQM Solution for Wireless Networks . 113 5.4 ConcludingRemarks . .115 Bibliography 116 Acronyms 125 v List of Figures 1.1 Fastretransmitandfastrecovery. 5 3.1 AflowchartforSTSmodel. 26 ∗ 3.2 Evaluation of τCDi . ........................ 41 3.3 SoftwarearchitectureforTCP.. 52 3.4 The network topologies used for performance comparison ... 53 3.5 Goodput performance against packet error rate over infrastructure- basedwirelessnetwork. 55 3.6 Goodput performance against delay of the wireless link over infrastructure-based wireless network. 57 3.7 Goodput performance against bandwidth of the wireless link over infrastructure-based wireless network. 58 3.8 Goodput performance over multihop ad-hoc wireless network . 62 3.9 Jain’s fairness index (J) over wired network with a dumbbell topology............................... 64 3.10 GBR over wired network with a dumbbell topology. 65 4.1 Awirelessbottlenecklink. 72 4.2 Queue length dynamics under varying wireless losses in the dumbbell topology with TCP NewReno/RED. 74 4.3 Queue length dynamics under varying wireless losses in the dumbbell topology with TCP-NCL/RED. 75 4.4 Control block diagram of the linearized congestion control sys- tem. ................................ 78 4.5 Two wireless bottleneck links. 92 4.6 Comparison of queue length dynamics with same configura- tionsasinFigure4.2.. 97 4.7 Comparison of queue length dynamics under different wireless losses. ............................... 99 4.8 Comparison of queue length dynamics with different bottle- necklinkparameters. .100 vi 4.9 Comparison of queue length dynamics with the presence of othertraffic.............................102 4.10 Comparison of queue length dynamics in the topology of two wireless bottleneck links. 104 vii List of Tables 3.1 Generalnotations. .. .. 29 3.2 Notationsforevents. 30 3.3 Network configurations for the infrastructure-based wireless network .............................. 59 4.1 Network configurations for the single wireless bottleneck net- work................................. 96 viii Chapter 1 Introduction 1.1 Chapter Overview The thesis adapts the present Internet congestion control to the wireless en- vironment that is gradually becoming an indispensable component of the Internet. In this chapter, we bring the background materials on Internet congestion control and the