Confused, Timid and Unstable: Picking a Video Rate is Hard Te-Yuan (TY) Huang Stanford University Nov 15 th, 2012 Joint work with Nikhil Handigol, Brandon Heller, Nick McKeown, Ramesh Johari

Video is the BIG thing on the Internet Video is more than 50% of peak traffic in the US Trend: Streaming over HTTP – Content Distribution Networks (CDNs) Well-provisioned HTTP servers at the edge of the Internet Cheap (2-3 cents per GB in 2011) – Firewall friendliness 2Te-Yuan (TY) ACM IMC'12, Boston

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4 Before download started After download started Te-Yuan (TY) ACM IMC'12, Boston

What happened? Both the download and video are over HTTP – TCP shares my home link equally among all flows 5 What is the problem? Te-Yuan (TY) ACM IMC'12, Boston

Experiment Setup The Internet Bandwidth Controller CDN 1 CDN 2 CDN 3 6 Content Distribution Networks Te-Yuan (TY) ACM IMC'12, Boston

Video Rate in the Presence of a Competing Flow Video Rate 7 Available Video Rates Fair share Competing Flow Throughout Steps down all the way to the lowest quality Video Flow Throughput Te-Yuan (TY) ACM IMC'12, Boston

What If We Manually Select a Video Rate? 8 Available Video Rates Manually select 1750kb/s Video Flow Throughput Video keeps playing without any problem Video Rate Te-Yuan (TY) ACM IMC'12, Boston

What If We Manually Select a Video Rate? 9 Competing Flow Throughput Video Rate Available Video Rates Manually select 1750kb/s Competing flow throughput is brought down Video Flow Throughput Te-Yuan (TY) ACM IMC'12, Boston

Not Just One Service’s Problem This happens in all the services we measured – Hulu, Netflix, Vudu 10 Video Rate Te-Yuan (TY) ACM IMC'12, Boston

The Problem 11 Why? It picks a video rate that is much too low Video client ends up with much less throughput than its fair share Te-Yuan (TY) ACM IMC'12, Boston

Outline How does rate selection over HTTP work? The rate selection process Where does it go wrong? The complete story Conclusion 12Te-Yuan (TY) ACM IMC'12, Boston

How Does Rate Selection over HTTP Work? The Internet Client 13 Standard, commoditized HTTP Servers Rate selection logic resides at client side CDN 3 File 1: File 2: Get File 1 (1750kb/s) Serve the video with quality 1750kb/s Playout Buffer Te-Yuan (TY) ACM IMC'12, Boston

The Rate Selection Process Initial video rate Video rate for the next video segment 14 Bandwidth estimation Download & measure Pick a rate What goes wrong? Te-Yuan (TY) ACM IMC'12, Boston

The Rate Selection Process 15 Bandwidth estimation Download & measure Pick a rate Initial video rate Video rate for the next video segment Te-Yuan (TY) ACM IMC'12, Boston

TCP Throughput of the Video Flow Playout Buffer is Full 16 TCP Throughput OFF Period: 1-2 seconds Te-Yuan (TY) ACM IMC'12, Boston

Impact of OFF period on TCP TCP sender resets its congestion window – When idle more than one RTO (200ms) – Slow-start restart, RFC 2581/5681 – Linux 3.x (tcp_output.c, line 163) Throughput will be affected – Worse with a competing flow Experience packet loss during slow start 17 50% of the segments get < 1.8Mb/s (Fair Share is 2.5Mb/s) Te-Yuan (TY) ACM IMC'12, Boston

The Rate Selection Process 18 Bandwidth estimation Download & measure Pick a rate Underestimation If perceived 1.8 Mb/s, which video rate would it pick? 1400 kb/s?1750 kb/s? Even lower? Video rate for the next video segment Initial video rate Te-Yuan (TY) ACM IMC'12, Boston

Conservative Rate Selection Available BW Video Rate 19 If 1.8Mb/s BW is perceived, the client switches down to 1050kb/s video rate 19Te-Yuan (TY) ACM IMC'12, Boston

The Rate Selection Process 20 Bandwidth estimation Download & measure Pick a rate Underestimation Conservatively Is there any consequence of being conservative? Initial video rate Video rate for the next video segment Te-Yuan (TY) ACM IMC'12, Boston

Smaller Segment Size for Lower Video Rate 21 When requesting a smaller segment size, lower probability of obtaining fair share. Te-Yuan (TY) ACM IMC'12, Boston

Lower video rate leads to further bandwidth underestimation 22 Lowest Video Rate Highest Video Rate Te-Yuan (TY) ACM IMC'12, Boston

The Rate Selection Process 23 Bandwidth estimation Download & measure Pick a rate Underestimation Conservatively Request for a smaller segment Further Video rate for the next video segment Initial video rate Te-Yuan (TY) ACM IMC'12, Boston

The Complete Story Video Quality 24 Bandwidth Underestimation Conservatism Further Underestimation Te-Yuan (TY) ACM IMC'12, Boston

Conclusion The problem: Video flow gets a much lower throughput than its fair share – Problem occurred in all the services we measured The reason: Video client tries to do TCP’s job – It is afraid of causing network congestion It becomes timid – It tries to figure out its available bandwidth Without seeing the interaction between the playout buffer, segment sizes and TCP dynamics It is easily confused and becomes unstable One service provider changed its algorithm because of this work 25Te-Yuan (TY) ACM IMC'12, Boston