무선 네트워크에서의 TCP 성능 향상 기법

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2015. 8. 박세웅.TCP (Transmission Control Protocol), one of the most essential protocol for the Internet, has carried the most of the Internet traffic since its birth. With the deployment of various types of wireless networks and proliferation of smart devices, a rapid increase in mobile data traffic volume has been observed and TCP has still carried the majority of mobile traffic, thus leading to huge attention again on TCP performance in wireless networks. In this dissertation, we tackle three different problems that aim to improve TCP performance in wireless networks. Firstly, we dealt with the downstream bufferbloat problem in wireless access networks such as LTE and Wi-Fi. We clarify the downstream bufferbloat problem in resource competitive environments such as Wi-Fi, and design a receiver-side countermeasure for easy deployment that does not require any modification at the sender or intermediate routers. Exploiting TCP and AQM dynamics, our scheme competes for shared resource in a fair manner with conventional TCP flow control methods and prevents bufferbloat. We implement our scheme in commercial smart devices and verify its performance through real experiments in LTE and Wi-Fi networks. Secondly, we consider the upstream bufferbloat problem in LTE networks. We clarify that the upstream bufferbloat problem can significantly degrade multitasking users QoE in LTE networks and design a packet scheduler that aims to separate delay-sensitive packets from non delay-sensitive packets without computational overhead. We implement the proposed packet scheduler in commercial smart devices and evaluate the performance of our proposed scheme through real experiments in LTE networks. Lastly, we investigate the TCP fairness problem in low-power and lossy networks (LLNs). We confirm severe throughput unfairness among nodes with different hop counts and propose dynamic TX period adjustment scheme to enhance TCP fairness in LLNs. Through experiments on the testbed, we evaluate how much the proposed scheme enhances fairness index.1 Introduction 1 1.1 Motivation 1 1.2 Background and Related Work 3 1.3 Outline 7 2 Receiver-side TCP Countermeasure to Bufferbloat in Wireless Access Networks 8 2.1 Introduction 8 2.2 Dynamics of TCP and AQM 11 2.3 Receiver-side TCP Adaptive Queue Control 14 2.3.1 Receiver-side Window Control 15 2.3.2 Delay Measurement and Queue Length Estimation 17 2.3.3 Configuration of RTAC 19 2.4 Experimental Setup and Configuration 20 2.4.1 Receiver Measurement Errors and Configuration 21 2.5 Experimental Results 27 2.5.1 Bufferbloat in Wireless Access Networks 27 2.5.2 Prevention of Bufferbloat 31 2.5.3 Fairness of TCP Flows with Various Receiver Types 32 2.5.4 The Impact of TCP Variants 39 2.5.5 The Impact of Upload Bufferbloat 46 2.5.6 Coexistence with the Unlimited Sender 48 2.6 Summary 48 3 Dual Queue Approach for Improving User QoE in LTE Networks 51 3.1 Introduction 51 3.2 User QoE Degradation in Multitasking Scenarios 54 3.2.1 Unnecessary Large Upload Queueing delay 54 3.2.2 Negative Impact on Performance in Multitasking Scenarios 55 3.3 SOR based Packet Classification with Multiple Transmit Queue 58 3.3.1 Dual Transmit Queue 59 3.3.2 SOR based Packet Classification Algorithm 61 3.4 Experiment Results 63 3.4.1 Packet Classification Metric: Sendbuffer Occupancy Ratio (SOR) 64 3.4.2 Improving RTT performance of Interactive Applications 68 3.4.3 Improving Download Performance 69 3.4.4 Fairness among Competing Upload Flows 71 3.5 Summary 74 4 Uplink Congestion Control in Low-power and Lossy Networks 75 4.1 Introduction 75 4.2 System Model 78 4.3 Proposed Scheme 79 4.3.1 Tx Period 79 4.3.2 Dynamic TX Period Adjustment 80 4.4 Experimental Results 82 4.4.1 Experimental Setup 82 4.4.2 Throughput analysis vs. Measurement 84 4.4.3 TCP Performance in Low-power Lossy Networks 87 4.4.4 Fairness improvement of DTPA 89 4.5 Summary 92 5 Conclusion 93 5.1 Research Contributions 93 5.2 Future Research Directions 95Docto