A Tale of Three CDNs: An Active Measurement Study of Hulu and Its CDNs Vijay Kumar Adhikari * , Yang Guo † , Fang Hao † , Volker Hilt † and Zhi-Li Zhang * * University of Minnesota, † Bell-Labs/Alcatel-Lucent Abstract— We study the Hulu online video service via active measurements. It is known that Hulu utilizes multiple CDNs to serve users’ video requests. The focus of this study is on how Hulu selects CDNs and how each CDN allocates resources (i.e., servers) to serve user requests. Based on our analysis of measurement data, we find that Hulu frequently changes preferred CDNs for users. However, once a CDN is selected, Hulu clients try to stay with the same CDN during the entire length of the movie even when performance of that CDN degrades. While the preferred CDN selection is not fixed, we observe that Hulu attempts to divide video requests among CDNs to attain a fixed target ratio. In terms of CDNs, we find that different CDNs employ different amounts of resources (servers) to serve Hulu content. I. I NTRODUCTION Large-scale video content delivery is becoming the single most important contributor to the Internet traffic. For instance, in [1] Labovitz et al. reports that Google is one of the largest source of Internet traffic and a significant amount of its traffic is due to the YouTube video delivery. Within North America, Netflix uses about 30% of the peak-time downstream bandwidth for fixed-access networks, according to a recent report by Sandvine [2]. Moreover, according to comScore[3], more than 82 percent of the U.S. Internet audience viewed online video in February 2011; and Hulu (with more than 27, 257, 000 unique viewers per month) is one of the largest online video service provider. Just as these online video services continue to grow, so does the demand for higher quality video viewing experience. Many of these services make money either by serving advertisement or through monthly subscription fees. Therefore, poor user viewing experience will likely impact their revenue. In order to serve video content at a sufficiently good quality, some providers, for example, Google, build their in-house content distribution systems. However, because of the formiddable infrastructure costs and the vast expertise needed to operate such a content distribution infrastructure, a large majority rely on one or more third-party content distribution network (CDN) providers such as Akamai and Limelight to help distribute content to users. How a content provider select CDNs and how many resources (e.g., video servers) each CDN utilizes to serve content for the content provider is one of the most important factors affecting user experience. Likewise, how an online video service reacts and adapts to diverse and changing This study was done while Vijay Kumar Adhikari was a summer intern at Bell Labs, Alcatel-Lucent. It was supported in part by the NSF grants 08-31734, CNS-0905037, CNS-1017092 and CNS-1017647. network conditions also plays a crucial role in providing good user viewing experiences. In this paper, we study the video delivery system employed by Hulu – one of most popular online video services serving primarily TV shows – via active measurements. It is known that Hulu utilizes multiple CDNs to serve users’ video re- quests. The focus of this study is on i) how Hulu selects CDNs and ii) how each CDN allocate resources (i.e., servers) to serve user requests. For this purpose, we have conducted extensive data collection, careful analysis and inference and systematical experimentation to verify and validate our inferences and findings. We find that Hulu appears to distribute user requests among the CDNs in accordance with certain predetermined ratio. The selection of the “preferred” CDN for a given user does not seem to be taking into account the current condition of the network performance (between the user and the selected CDN). Further, Hulu frequently changes preferred CDNs for each user. However, once a CDN is selected, Hulu clients try to stay with the same CDN during the entire length of the movie even when performance of that CDN degrades. The CND is only changed when the chosen CDN cannot serve the lowest quality level of the video. In terms of CDNs, we find that the CDNs employ varying number of servers at different locations to serve Hulu content. The CDN resources (servers) allocated to Hulu are also used for other online video services that employ the same video streaming protocol suite; they are not used for other online video services that employ different video streaming technologies. The rest of this paper is organized as follows. A brief overview of Hulu is presented in Section II. In Section III, we describe in detail how Hulu uses multiple CDNs and how it picks a CDN to serve a given request and in Section IV, we explore how the selected CDN decides which server resource to use to serve that request. We describe related work in Section V and conclude the paper in Section VI. II. OVERVIEW In this section we provide an overview of Hulu’s video delivery system. We explain what technologies and protocols Hulu uses for video delivery and what bit-rates they support. The most popular service that Hulu offers is the free service for desktop users. In addition, Hulu also offers a subscription based service called HuluPlus which supports additional plat- forms such as set top boxes and mobile devices, and also supports HD video quality. Video advertisement is another Global Internet Symposium 2012 978-1-4673-1017-8/12/$31.00 ©2012 IEEE 7