The RTT distribution of TCP flows on the Internet and its impact on TCP based flow control

Authors: 
Srinivas Shakkottai, R. Srikant, Nevil Brownlee, Andre Broido and kc claffy
Published: 
CAIDA technical report
Year: 
2004

This paper looks at RTT and how they affect TCP flows. Because of limited data they look at how to derive RTTs from analysis of tcpdump data (unidirectional).

Methods of getting estimates of RTT are:

  • SYNSYN-ACKACK – the time stamp between SYN and ACK in triple handshake.

  • Flight method – look at packets with near identical inter-packet times. Calculate time between start of flights (attempt to ignore rate-limited flows).

  • Rate chage – RTT= sqrt{MTU/(d^2x/dy^2)} – estimate latter by change of rate.

Three methods produce broadly comparable results.

Data:

  • OC48 Seattle – San Jose 2002 (1 hour) non anon

  • OC48 Seattle – San Jose 2003 (1 hour) non anon

  • Abeline (Kansas – Ohio) (1 hour)

Proposition 1: During TCP congestion avoidance, in interval [t_0,t_1] average rate for interval (with no drops) reached at time hat{t} = (t_0+t_1)/2 (middle of interval). The proposition is misstated in paper (sign error) but correct in equation (3).

Methodology: To calculate rate they take the time taken for ten packets and the number of bits in those packets. RTT is calculated as a distribution over all IP pairs so distribution is for entire trace not disaggregated by IP pairs. Flights are identified as being related to bandwidth delay product and do not exist for all flows.

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