Picking a Video Rate is Hard TeYuan 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 ID: 430727
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Slide1
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
JohariSlide2
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 InternetCheap (2-3 cents per GB in 2011)Firewall friendliness
2
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide3
3
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide4
4
Before
download
started
After
download
started
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide5
What happened?
Both the download and video are over HTTP
TCP shares my home link
equally among all flows5
What is the problem?
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide6
Experiment Setup
The Internet
Bandwidth Controller
CDN 1
CDN 2
CDN 3
6
Content Distribution Networks
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide7
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) Huang @ ACM IMC'12, BostonSlide8
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) Huang @ ACM IMC'12, BostonSlide9
What If
W
e
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) Huang @ ACM IMC'12, BostonSlide10
Not Just One Service’s ProblemThis happens in all the services we measured
Hulu
, Netflix,
Vudu10
Video Rate
Video Rate
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide11
The Problem11
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) Huang @ ACM IMC'12, BostonSlide12
OutlineHow does rate selection over HTTP work?
The rate selection process
Where does it go wrong?
The complete storyConclusion12
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide13
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:
1750
1750
1050
1050
1050
Get File 1 (1750kb/s)
Serve the video with quality 1750kb/s
Playout
Buffer
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide14
The Rate Selection Process
Initial
video
rateVideo rate for the next video segment
14
Bandwidthestimation
Download
& measure
Pick a rate
What goes wrong?
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide15
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) Huang @ ACM IMC'12, BostonSlide16
TCP Throughput of the Video
Fl
ow
Playout
Buffer is Full
16
TCP Throughput
OFF Period:
1-2 seconds
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide17
Impact of OFF period on TCP
TCP sender resets its congestion window
When idle more than one RTO (200ms)
Slow-start restart, RFC 2581/5681Linux 3.x (tcp_output.c, line 163)Throughput will be affectedWorse with a competing flowExperience packet loss during slow start
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50% of the segments get < 1.8Mb/s
(Fair Share is 2.5Mb/s)
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide18
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) Huang @ ACM IMC'12, BostonSlide19
Conservative Rate
S
election
Available BW
Video Rate
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If 1.8Mb/s BW is perceived,
the client switches down to
1050kb/s video rate
19
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide20
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) Huang @ ACM IMC'12, BostonSlide21
Smaller Segment Size for Lower Video Rate
21
When requesting a
smaller
segment size,
lower probability
of obtaining fair share.
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide22
Lower video rate leads to
further bandwidth underestimation
22
Lowest Video Rate
Highest Video Rate
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide23
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) Huang @ ACM IMC'12, BostonSlide24
The
Complete Story
Video Quality
24
Bandwidth Underestimation
Conservatism
Further Underestimation
Te-Yuan (TY) Huang @ ACM IMC'12, BostonSlide25
Conclusion
The problem: Video
flow gets a much lower throughput than its fair share
Problem occurred in all the services we measuredThe reason: Video client tries to do TCP’s job It is afraid of causing network congestionIt becomes timidIt 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
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Te-Yuan (TY) Huang @ ACM IMC'12, Boston