Confused, Timid and Unstable: Picking a Video Rate is Hard Te-Yuan (TY) Huang Stanford University...

Confused, Timid and Unstable:Picking a Video Rate is Hard

Te-Yuan (TY) HuangStanford University

Nov 15th, 2012

Joint work with Nikhil Handigol, Brandon Heller, Nick McKeown, Ramesh Johari

Te-Yuan (TY) Huang @ ACM IMC'12, Boston 2

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


Before download started

Afterdownload started


What happened?

• Both the download and video are over HTTP– TCP shares my home link equally among all flows

What is the problem?


Experiment Setup

The Internet

Bandwidth Controller

CDN 1

CDN 2

CDN 3

Content Distribution Networks


Video Ratein the Presence of a Competing Flow

Video Rate

AvailableVideo Rates

Fair share

Competing FlowThroughout

Steps down all the way to the lowest quality

Video FlowThroughput


What If We Manually Select a Video Rate?

AvailableVideo Rates Manually select

1750kb/s

Video FlowThroughput Video keeps playing

without any problem

Video Rate


What If We Manually Select a Video Rate?Competing FlowThroughput

Video Rate

AvailableVideo Rates Manually select

1750kb/s

Competing flow throughput is brought downVideo Flow

Throughput


Not Just One Service’s Problem

• This happens in all the services we measured– Hulu, Netflix, Vudu

Video Rate

Video Rate


The Problem

Why?

It picks a video rate that is much too low

Video client ends up with much less throughput than its fair share


Outline

• How does rate selection over HTTP work?• The rate selection process• Where does it go wrong?• The complete story• Conclusion


How Does Rate Selection over HTTP Work?

The Internet

Client

Standard, commoditized HTTP Servers

Rate selection logic resides at client side

CDN 3

File 1:

File 2:

1750 1750

1050 1050 1050Get File 1 (1750kb/s)

Serve the video with quality 1750kb/s

PlayoutBuffer


The Rate Selection Process

Initialvideo rate

Video rate for the next video segment

BandwidthestimationDownload

& measure Pick a rate

What goes wrong?



BandwidthestimationDownload

& measure Pick a rate

Initialvideo rate



TCP Throughput of the Video Flow

Playout Buffer is Full

TCP Throughput

OFF Period: 1-2 seconds


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

50% of the segments get < 1.8Mb/s(Fair Share is 2.5Mb/s)



Bandwidthestimation

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?


Initialvideo rate


Conservative Rate Selection

Available BW

Video Rate

19

If 1.8Mb/s BW is perceived,the client switches down to1050kb/s video rate



Bandwidthestimation

Download& measure

Pick a rateUnderestimation

Conservatively

Is there any consequence of being conservative?

Initialvideo rate



Smaller Segment Size for Lower Video RateWhen requesting a smaller segment size, lower probability of obtaining fair share.


Lower video rate leads to further bandwidth underestimation

Lowest Video Rate

Highest Video Rate



Bandwidthestimation

Download& measure

Pick a rateUnderestimation

Conservatively

Request for a smaller segment

Further


Initialvideo rate


The Complete Story

Video QualityBandwidth Underestimation

Conservatism

Further Underestimation


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

Confused, Timid and Unstable: Picking a Video Rate is Hard Te-Yuan (TY) Huang Stanford University...

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