This survey presents the evolution of live media streaming and the technological developments behind today's IP-based low-latency live streaming systems. Live streaming primarily involves capturing, encoding, packaging and delivering real-time events such as live sports, live news, personal broadcasts and surveillance videos. Live streaming also involves concurrent streaming of linear TV programming off the satellite, cable, over-the-air or IPTV broadcast, where the programming is not necessarily a real-time event. The survey starts with a discussion on the latency and latency continuum in streaming applications. Then, it lays out the existing live streaming workflows and protocols, followed by an in-depth analysis of the latency sources in these workflows and protocols. The survey continues with the technology enablers, low-latency extensions for the popular HTTP adaptive streaming methods and enhancements for robust low-latency playback. An entire section is dedicated to the detailed summary and findings of Twitch's grand challenge on low-latency live streaming. The survey concludes with a discussion of ongoing research problems in this space.

翻译：本综述阐述了直播媒体流的演进历程以及当代基于IP的低延迟直播系统背后的技术发展。直播主要涉及实时事件的采集、编码、打包和分发，例如体育赛事直播、新闻直播、个人直播以及监控视频。直播还涉及将线性电视节目从卫星、有线、无线或IPTV广播进行并发流化传输，这些节目未必是实时事件。本综述首先讨论了流媒体应用中的延迟及延迟连续体概念。随后，梳理了现有的直播工作流与协议，并深入分析了这些工作流与协议中的延迟来源。接下来，综述介绍了技术推动因素、面向主流HTTP自适应流媒体方法的低延迟扩展，以及增强鲁棒低延迟回放的技术。专门用一整节详细总结并阐述了Twitch低延迟直播大挑战的发现。综述最后讨论了该领域当前待解决的研究问题。