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Variability in TCP Round-trip Times Extent, Causes, and Impact

The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL. Variability in TCP Round-trip Times Extent, Causes, and Impact. Faculty : Jasleen Kaur Don Smith Diane Pozefsky (IBM) Vidyadhar Kulkarni (OR) Michele Weigle Kevin Jeffay. Students : Jay Aikat Sushant Rewaskar Vivek Sawant

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Variability in TCP Round-trip Times Extent, Causes, and Impact

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  1. The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL Variability in TCP Round-trip TimesExtent, Causes, and Impact • 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

  2. 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?

  3. 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!

  4. Trace Statistics Large data set!

  5. 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

  6. Causes of Variability • Correlation with hop-count • Correlation with location • Prefix-aggregation • Behavior with time • Timescales • Congestion • Path changes

  7. Per-Segment RTTs: Mean or Distributions? • Is mean RTT a good approximation for per-segment RTTs? • TCP analytical models • TCP evaluation (simulations)

  8. 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

  9. The SYN/(SYN+ACK) RTT • RTT yielded by the SYN and SYN+ACK 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

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