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Identifying Performance Bottlenecks in CDNs through TCP-Level Monitoring. Peng Sun Minlan Yu, Michael J. Freedman, Jennifer Rexford Princeton University August 19, 2011. Performance Bottlenecks. CDN Servers. Internet. Clients. Client Insufficient receive buffer. Internet

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identifying performance bottlenecks in cdns through tcp level monitoring
Identifying Performance Bottlenecks in CDNs through TCP-Level Monitoring

Peng Sun

Minlan Yu, Michael J. Freedman, Jennifer Rexford

Princeton University

August 19, 2011

performance bottlenecks
Performance Bottlenecks

CDN Servers




Insufficient receive buffer


Network congestion


Write too slowly

Server OS

Insufficient send buffer or Small initial congestion window

reaction to each bottleneck
Reaction to Each Bottleneck

CDN Servers



Client is bottleneck:

Notify client to change

Internet is bottleneck:

Circumvent the congested part of network

APP is bottleneck:

Debug application

Server OS is bottleneck:

Tune buffer size, or upgrade server

previous techniques not enough
Previous Techniques Not Enough

Application logs:

No details of network activities

Packet sniffing:

Expensive to capture

Transport-layer stats:

Directly reveal perf. bottlenecks

Active probing:

Extra load on network

how tcp stats reveal bottlenecks
How TCP Stats Reveal Bottlenecks

Insufficient data in send buffer

CDN Server Applications

Receive window too small

Packet loss

Send buffer full or Initial congestion window too small


Network Stack


Network Path

CDN Servers



measurement framework
Measurement Framework
  • Collect TCP statistics
    • Web100 kernel patch
    • Extract useful TCP stats for analyzing perf.
  • Analysis tool
    • Bottleneck classifier for individual connections
    • Cross-connection correlation at AS level
      • Map conn. to AS based on RouteView
      • Correlate bottlenecks to drive CDN decisions
how bottleneck classifier works
How Bottleneck Classifier Works

Small initial Cwin

Slow start limits perf.

Network Stack is bottleneck

BytesInSndBuf = Rwin

Rwin limits sending

Client is bottleneck

Cwin drops greatly

and Packet loss

Network path is bottleneck

coralcdn experiment
CoralCDN Experiment
  • CoralCDN serves 1 million clients per day
  • Experiment Environment
    • Deployment: A Clemson PlanetLab node
    • Polling interval: 50 ms
    • Traces to Show: Feb 19th – 25th 2011
    • Total # of Conn.: 209K
    • After removingCache-Miss Conn.: 137K (Total 2008 ASes)
  • Log Space overhead
    • < 200MB per Coral server per day
what are major bottleneck for individual clients
What are Major Bottleneck for Individual Clients?
  • We calculate the fraction of time that the connection is under each bottleneck in lifetime


Slow CPU or scarce disk resources of the PlanetLab node

Our suggestion:

Filter them out of decision making


Congestion window rises too slowly for short conn.

(>80% of the connections last <1 second)

Our suggestion:

Use larger initial congestion window


Spotty network (discussed in next slide)


Receive buffer too small (Most of them are <30KB)

Our suggestion:

Use more powerful PlanetLab machines

as level correlation
AS-Level Correlation
  • CDNs make decision at the AS level
    • e.g., change server selection for
  • Explore at the AS level:
    • Filter out non-network bottlenecks
    • Whether network problems exist
    • Whether the problem is consistent
filtering out non network bottlenecks
Filtering Out Non-Network Bottlenecks
  • CDNs change server selection if clients have low throughput
  • Non-network factors can limit throughput
  • 236 out of 505 low-throughput ASes limited by non-network bottlenecks
  • Filtering is helpful:
    • Don’t worry about things CDNs cannot control
    • Produce more accurate estimates of perf.
network problem at as level
Network Problem at AS Level
  • CDN make decision at AS level
  • Whether conn. in the same AS have common network problem
  • For 7.1% of the ASes, half of conn. have >10% packet loss rate
  • Network problems are significant at the AS level
consistent packet loss of as
Consistent Packet Loss of AS
  • CDNs care about predictive value of measurement
  • Analyze the variance of average packet loss rates
    • Each epoch (1 min) has nonzero average loss rate
    • Loss rate is consistent across epochs (standard deviation < mean)
conclusion future work
Conclusion & Future Work
  • Use TCP-level stats to detect performance bottlenecks
  • Identify major bottlenecks for a production CDN
  • Discuss how to improve CDN’s operation with our tool
  • Future Works
    • Automatic and real-time analysis combined into CDN operation
    • Detect the problematic AS on the path
    • Combine TCP-level stats with application logs to debug online services