Variability in TCP Roundtrip Times Extent, Causes, and Impact

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Variability in TCP Round-trip TimesExtent,
Causes, and Impact
The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL
Faculty Jasleen Kaur Don Smith Diane Pozefsky
(IBM) Vidyadhar Kulkarni (OR) Michele
Weigle Kevin Jeffay
Students Jay Aikat Sushant Rewaskar Vivek
Sawant Li Cheng (OR)
http//www.cs.unc.edu/jasleen/research/RTT
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TCP Round-trip Times (RTTs)

• Popular belief
• RTTs do not vary significantly within TCP
  connections
• Mean RTT can be used to approximate per-segment
  RTTs
• TCP analytical models
• Altman00, Kumar98, Lakshman97, Mathis97,
  Padhye98, Zhang02
• RTT of initial segments is typical for a TCP
  connection
• Jiang02

Do per-segment RTTs vary significantly within a
TCP connection?
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Data Source
UNC Campus
ibiblio web servers
Regional health-care center
Departments Research labs
Rest of the world
Residence halls
ISP router
Wireless users
VPNs Modem
Monitor
Rich data source!
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Trace Statistics
Large data set!
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Variability Within Connections

• Median RTT
• 30 of connections see a median RTT more than
  twice the min RTT
• 90 RTT
• 22 of connections see a 90 RTT more than 5
  times the min RTT
• 90 RTT increases with min RTT

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Causes of Variability

• Correlation with hop-count
• Correlation with location
• Prefix-aggregation
• Behavior with time
• Timescales
• Congestion
• Path changes

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Per-Segment RTTs Mean or Distributions?

• Is mean RTT a good approximation for per-segment
  RTTs?
• TCP analytical models
• TCP evaluation (simulations)

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Impact of variability

• Models for per-connection RTTs
• Accurate simulation environments
• Accurate analysis
• Impact of RTT variability on past work
• TCP analytical models
• Delay-based congestion control
• TCP evaluation (simulations)
• TCP-based analysis

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The SYN/(SYNACK) RTT

• RTT yielded by the SYN and SYNACK pair
• Differs by more than 10 from min RTT for 14 of
  connections
• Differs by more than 10 from median RTT for 50
  of connections