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Vantage: Optimizing Video Upload for Time-Shi Ed Viewing of Social Live Vantage: Optimizing video upload for time-shied viewing of social live streams Devdeep Ray Jack Kosaian Carnegie Mellon University Carnegie Mellon University [email protected] [email protected] K. V. Rashmi Srinivasan Seshan Carnegie Mellon University Carnegie Mellon University [email protected] [email protected] ABSTRACT ACM Reference Format: Social live video streaming (SLVS) applications are becoming in- Devdeep Ray, Jack Kosaian, K. V. Rashmi, and Srinivasan Seshan. 2019. creasingly popular with the rise of platforms such as Facebook- Vantage: Optimizing video upload for time-shifted viewing of social live streams. In SIGCOMM ’19: 2019 Conference of the ACM Special Interest Group Live, YouTube-Live, Twitch and Periscope. A key characteristic that on Data Communication, August 19–23, 2019, Beijing, China. ACM, New York, differentiates this new class of applications from traditional live NY, USA, 14 pages. https://doi.org/10.1145/3341302.3342064 streaming is that these live streams are watched by viewers at dif- ferent delays; while some viewers watch a live stream in real-time, 1 INTRODUCTION others view the content in a time-shifted manner at different delays. Mobile live video traffic has grown significantly over the last In the presence of variability in the upload bandwidth, which is decade [14]. This growth has been propelled by improvements typical in mobile environments, existing solutions silo viewers into in mobile camera technology, computing power, and wireless tech- either receiving low latency video at a lower quality or a higher nology, which enables the capture, encoding, and transmission quality video with a significant delay penalty, without accounting of high-quality video in real-time from mobile devices. Applica- for the presence of diverse time-shifted viewers. tions for video-conferencing and live broadcasting have become In this paper, we present Vantage, a live-streaming upload so- ubiquitous on mobile devices today. Social live video streaming lution that improves the overall quality of experience for diverse (SLVS) applications like Facebook Live [1], YouTube Live [11], and time-shifted viewers by using selective quality-enhancing retrans- Periscope [7], are a new and increasingly popular class of applica- missions in addition to real-time frames, optimizing the encoding tions that bring the power of live streaming to individuals. schedules to balance the allocation of the available bandwidth be- A unique feature that differentiates SLVS platforms from tradi- tween the two. Our evaluation using real-world mobile network tional live video applications is the ability to view real-time, time- traces shows that Vantage can provide high quality simultaneously shifted, and archival versions of a single stream. SLVS applications for both low-latency and delayed viewing. enable viewers to interact with broadcasters via comments and For delayed viewing, Vantage achieves an average improvement reactions in real-time [40, 41], and also archive the video to enable of 19.9% over real-time optimized video streaming techniques across viewing after the live streaming session has ended. Furthermore, all the network traces and test videos, with observed gains of up to platforms like Hangouts-on-air [2] and Facebook Live [4] allow mul- 42.9%. These benefits come at the cost of an average drop in real- tiple users to broadcast simultaneously on a single live-stream. For time quality of 3.3%, with a maximum drop of 7.1%. This represents viewers using interactive features such as real-time comments, as a significant performance improvement over current techniques well as broadcasters taking part in collaborative broadcasting [2, 4], used for SLVS applications, which primarily optimize the video it is critical to deliver the video stream at a low-latency, whereas a upload for real-time viewing. higher streaming latency is acceptable for the other viewers. CCS CONCEPTS In contrast, traditional live video streaming applications target a single viewing delay. In video conferencing applications, par- • Information systems → Multimedia streaming; ticipants require low-latency for interactivity, while viewers of a broadcast event can typically tolerate tens of seconds of delay. Ex- KEYWORDS isting approaches for handling network bandwidth variations are Video delivery, adaptive bitrate algorithms, live streaming, VOD tailored for one particular viewing delay. In low-latency applications Permission to make digital or hard copies of all or part of this work for personal or like videoconferencing, video bitrate is chosen to closely follow the classroom use is granted without fee provided that copies are not made or distributed available bandwidth in order to ensure that frames are received for profit or commercial advantage and that copies bear this notice and the full citation before their real-time playback deadline, at the expense of lower on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, quality during periods of low bandwidth. On the other hand, when to post on servers or to redistribute to lists, requires prior specific permission and/or a higher delays are acceptable, applications use buffers to absorb fee. Request permissions from [email protected]. network variations and the video bitrate is chosen to match the av- SIGCOMM ’19 August 19–23, 2019 Beijing, China © 2019 Association for Computing Machinery. erage bandwidth. This results in higher video quality and smoother ACM ISBN 978-1-4503-5956-6/19/08...$15.00 playback at the cost of higher latency. https://doi.org/10.1145/3341302.3342064 SIGCOMM ’19 August 19–23, 2019 Beijing, China Devdeep Ray, Jack Kosaian, K. V. Rashmi, Srinivasan Seshan Due to lack of better alternatives, current SLVS platforms make where the upload path connection is terminated. We refer to this the same tradeoffs between latency and quality as traditional live ingestion point as the upload endpoint. The video is then re-encoded streaming, despite the diversity of viewing delays. Operating at a at various bitrates and handed off to a content delivery network, single point on the latency-quality tradeoff spectrum is inadequate which delivers the video to viewers using a variety of techniques for providing high quality-of-experience (QoE) for all the viewers that have been widely studied in the past [23, 24, 30]. This paper of SLVS streams. The problem is further exasperated by the fact focuses on improvements for the upload path of SLVS applications. that SLVS streams are commonly initiated from mobile devices, which have particularly unpredictable network behavior [44]. 2.2 Time-shifted viewing in SLVS In this paper, we present Vantage, a live video upload solution SLVS differs from traditional live-streaming in that it enables explicitly designed to address the time-shifted viewing characteris- viewing of the same video stream at different delays. Traditional tic of SLVS in the face of bandwidth variations. Vantage exploits live streaming applications are tailored either for interactive, low- the variability of the upload path to it’s advantage: periods with latency streaming (e.g., Skype and Hangouts) or for high quality high bandwidth can be used to correct for a loss in quality due to viewing at larger delays (e.g., ESPN and CNN). On the other hand, previous periods of network impairment. Vantage optimizes the SLVS platforms enable both real-time and delayed viewing of the video upload process across different time-shifted viewing delays same stream. We term this characteristic “time-shifted viewing.” by using quality-enhancing retransmissions in conjunction with a Time-shifted viewing takes a number of forms within an SLVS low-latency video stream. Vantage formulates bitrate selection and stream. Some viewers interact with broadcasters via comments transport scheduling as a joint optimization problem that maxi- and reactions, and thus require real-time latencies [40, 41]. SLVS mizes the video quality across the diverse viewing delays. platforms also archive video streams to allow for viewing after Several challenges need to be addressed to make the use of the live stream has ended, and also allow viewers to seek back to quality-enhancing retransmissions practical and effective: (1) allo- older segments during the live stream and watch the video with a cating bandwidth and scheduling transmissions for the real-time time-shifted delay. Moreover, collaborative broadcasting platforms and retransmitted frames such that the QoE is optimized for all like Hangouts-on-air [2] and Facebook Live [4] allow collaborative users, (2) handling the computational overheads and latencies asso- streaming where the co-broadcasters have stronger low latency ciated with complex optimization decisions and video compression, requirements compared to the viewers. and (3) dealing with the network unpredictability and its impact on In summary, SLVS streams have audiences with a wide variety of scheduling decisions. Vantage incorporates several system design viewing delays, and thus have varying degrees of latency tolerance. choices like approximations, pipelining, and fallback mechanisms to This presents a new and important dimension for improving the handle the challenges related to optimization and unpredictability. quality of experience of SLVS platforms. We have implemented Vantage and evaluated it on a wide vari- ety of mobile network traces [31] and videos [10]. Our evaluation 2.3 Variability in the upload path shows that Vantage achieves high quality for low-latency and time- Many SLVS broadcasts are initiated from mobile devices over cel- shifted viewing simultaneously. Specifically, for delayed viewing, lular networks like LTE, which experience frequent bandwidth . Vantage achieves an average improvement of 19 9% over real-time fluctuations [44]. This forces the broadcaster’s device to either optimized streaming techniques across all the network traces and adapt the bitrate of encoded video (i.e., alter quality), or use large . test videos, with observed gains of up to 42 9%. The quality achieved sender side buffers (i.e., alter the delay of transmission).
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