【作者】 李立；

【导师】 徐恪；

【作者基本信息】 清华大学 ， 计算机科学与技术， 2018， 博士

【摘要】 随着高速铁路的迅速普及和发展,人民群众对提升高速移动场景中网络体验的需求日益强烈。然而,常用的网络协议,比如TCP和UDP,在高速移动场景下的性能和行为还没有得到全面深入的研究。迫切需要通过测量来评估这些协议的性能,发现存在的问题,并提出解决方案。为此,在峰值速度为310km/h的中国高铁列车场景下,在3G/4G移动蜂窝网络中对TCP本身,和基于UDP的音/视频通话应用——Skype进行了长周期、长距离的大规模测量研究,在测量的基础上提出了采用多运营商多路径传输来提升网络协议性能的解决方案。本文完成的主要工作包括:1)适应高速移动场景的测量和分析方法。开发了能自动采集地形、车速、网络类型、切换和断网等诸多因素信息的测量工具,设计了能定量分析各因素独立影响的分析方法。针对Skype协议不公开,无法分析关键操作的难题,提出了通过引入参考流探测网络条件来评价协议效率的测量和分析方法。2)对高速移动场景下的TCP协议进行了全面深入的测量研究。测量历时3年,采集了500GB数据。发现高速移动下,RTT和丢包率急剧上升,吞吐量显著下降。TCP在重传、拥塞控制、连接建立和关闭等关键操作呈现严重的异常行为。发现大流比小流性能下降更为严重。3)对高速移动场景下的Skype应用进行了全面深入的测量研究。测量历时8个月,采集50GB数据。发现在高速移动场景下各种客观性能指标(包括RTT、发送速率和呼叫建立时间等),和主观指标(QoE)都显著降低,Skype算法在应用层路由、速率控制、状态更新、自动挂断等关键操作中都呈现出异常行为。4)提出在高速移动场景下通过多运营商多路径传输来提升性能的建议。对多路径传输协议—MPTCP进行了广泛深入的测量,历时22个月,收集超过2.7TB的数据。发现MPTCP虽然能提升鲁棒性,但效率不高。为提高效率,设计了面向高速移动场景的MPTCP HSR协议,实验结果表明MPTCP HSR性能比MPTCP LIA有显著提升。更多还原

【Abstract】 With the rapid popularization and development of high speed railway,the demand for acceptable QoE of network service in the high-speed mobility scenarios has been unprecedentedly strong.However,the performance and behavior of commonly used network protocols,such as TCP and UDP,have not been fully and profoundly studied in such scenarios.We urgently need to measure these network protocols,find existing problems,and propose solutions.For this purpose,this paper has conducted a large-scale measurement study on TCP,and Skype,a voice/video call application based on UDP,in 3G/4G networks on high speed rails with a peak speed of 310km/h in China.Finally,multipath transmission with multiple carriers is proposed to improve performance.The main contributions are as follows:1)Innovative measurement and analysis methods.We have developed a measurement tool that can automatically collect the information of many influencing factors,such as terrain,speed,network type,handoff,network disconnection etc.We have designed an analytical method that can analyze the independent influences of each factor quantitatively.In addition,when measuring Skype,to overcome the challenge that Skype is proprietary software,and there is very limited public information about its encoding and transmission algorithms,we bring in two reference flows to detect network condition and evaluate the efficiency of Skype.2)A comprehensive and in-depth measurement on TCP.The measurement lasted three years and we collected 500 GB of data.It was found that RTT and packet loss rate increased sharply,and throughput decreased significantly in high speed mobility.Many key operations of TCP,such as retransmission,congestion control connection,and establishment/closure show serious abnormal behavior.It was found that big flows suffer much more significant performance degradation than small flows.3)A comprehensive and in-depth measurement on Skype.The measurement lasted eight months and we collected 50 GB of data.Various objective performance metrics(including RTT,sending rate and call setting time)and a subjective performance metric(QoE)were found to be significantly reduced.Many key operations of Skype show serious abnormal behavior,including routing in application layer,rate control,state updates and call termination.4)It is proposed to improve the performance through multi-path transmission with multiple cellular carriers.A mature multi-path transport protocol,MPTCP,was measured on high speed rails.During a 22-month period,we collected more than2.7 TB of data.It is found that although MPTCP can improve robustness,it is not efficient.In order to improve the efficiency,we designed a new protocol,MPTCP HSR.Experiment results show that MPTCP HSR performs much better than MPTCP LIA.更多还原