Probe and Adapt: Rate Adaptation for HTTP Video Streaming At Scale

Today, the technology for video streaming over the Internet is converging towards a paradigm named HTTP-based adaptive streaming (HAS). HAS comes with two unique flavors. First, by riding on top of HTTP/TCP, it leverages the network-friendly TCP to achieve firewall/NATS traversal and bandwidth sharing. Second, by pre-encoding and storing the video in a number of discrete bitrate levels, it introduces video bitrate adaptivity in a scalable way that the video encoding is excluded from the closed-loop adaptation. A conventional wisdom is that the TCP throughput observed by a HAS client indicates the available network bandwidth, thus can be used as a reliable reference for the video bitrate selection. We argue that this no longer holds true when HAS becomes a substantial fraction of the Internet traffic. We show that when multiple HAS clients compete at a network bottleneck, the presence of competing clients and the discrete nature of the video bitrates would together create confusion for a client to correctly perceive its fair-share bandwidth. Through analysis and real experiments, we demonstrate that this fundamental limitation would lead to, for example, video rate oscillation that negatively impacts the video watching experiences. We therefore argue that it is necessary to implement at the application layer a "probe-and-adapt" mechanism for HAS video rate adaptation, which is akin but orthogonal to the transport-layer network rate adaptation achieved by TCP. We present PANDA -- a client-side rate adaptation algorithm for HAS -- as an embodiment of this idea. Our testbed results show that compared to conventional algorithms, PANDA is able to reduce the instability of video rate by 60%, at a given risk of buffer underrun.