FAST TCP

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FAST TCP

• Anwis Das
• Ajay Gulati

Slides adapted from IETF presentation
slides Link http//netlab.caltech.edu/FAST/index.
html
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Background

• Last time we saw reliable communication requires
  a transport level protocol
• But users are selfish, leading to congestion
• Van Jacobson solved the problem by adapting TCP
• Slow Start
• Congestion Avoidance
• Accurate Estimation of RTT, etc
• Is the solution still valid considering todays
  technology??

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Hypothesis

• TCP Reno is a good solution for low speed
  networks but not a viable solution for high-speed
  networks
• Too conservative, not stable, and requires
  extremely small equilibrium loss probability
• There exists an equation that governs the flow of
  all variants of TCP
• FAST provides a solution to this equation and
  shows that it is stable and reaches equilibrium
  quickly

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Philosophy of Reno

• Packet level
• Designed and implemented first
• Flow level
• Understood afterwards
• Flow level dynamics determines
• Equilibrium performance, fairness
• Stability

• Design flow level equilibrium stability
• Implement flow level goals at packet level

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Problems with TCP Reno

• Equilibrium problem
• Packet level AI too slow, MI too drastic
• Flow level required loss probability too small
• Dynamic problem
• Packet level must oscillate on binary signal
• Flow level unstable at large window

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TCP Flow Dynamics
Reno TCP
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TCP Variant Performance

• Reno AIMD (1, 0.5)

ACK W ? W 1/W Loss W ? W 0.5W

• HSTCP AIMD (a(w), b(w))

ACK W ? W a(w)/W Loss W ? W b(w)W

• STCP MIMD (1/100, 1/8)

ACK W ? W 0.01 Loss W ? W 0.125W
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Flow level Reno, HSTCP, STCP, FAST

• Generic flow level dynamics!

window adjustment
control gain
flow level goal

• Different gain k and utility Ui
• They determine equilibrium and stability
• Different congestion measure pi
• Loss probability (Reno, HSTCP, STCP)
• Queueing delay (Vegas, FAST)

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Problem binary signal
oscillation
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Solution multibit signal
FAST
stabilized
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FAST TCP Dynamics

• Generic equation

window adjustment
control gain
flow level goal

• FAST TCP equation

• Can map generic equation to FAST TCP equation and
  understand how FAST TCP stabilizes faster than
  other TCP variants

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Window control algorithm

• Full utilization
• regardless of bandwidth-delay product
• Globally stable
• exponential convergence
• Fairness
• weighted proportional fairness
• parameter a

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Window control algorithm

• Theorem (Jin, Wei, Low 03)
• In absence of delay
• Mapping from w(t) to w(t1) is contraction
• Global exponential convergence
• Full utilization after finite time
• Utility function ai log xi (proportional
  fairness)

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Dynamic sharing 3 flows
FAST
Linux
Steady throughput
HSTCP
STCP
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Dynamic sharing

• Dynamic sharing on Dummynet
• capacity 800Mbps
• delay120ms
• 3 flows
• iperf throughput
• Linux 2.4.x (HSTCP UCL)

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queue
FAST
Linux
loss
throughput

• Dynamic sharing on Dummynet
• capacity 800Mbps
• delay120ms
• 14 flows
• iperf throughput
• Linux 2.4.x (HSTCP UCL)

STCP
HSTCP
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queue
FAST
Linux
loss
throughput
STCP
HSTCP
HSTCP
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Aggregate throughput

• Dummynet cap 800Mbps delay 50-200ms flows
  1-14 29 expts

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Fairness

• Dummynet cap 800Mbps delay 50-200ms flows
  1-14 29 expts

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Stability

• Dummynet cap 800Mbps delay 50-200ms flows
  1-14 29 expts

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Open issues

• 1 BaseRTT estimation
• route changes, dynamic sharing
• does not upset stability
• 2 Small network buffer
• at least like TCP
• adapt a on slow timescale, but how?
• 3 TCP-friendliness
• friendly at least at small window
• tunable, but how to tune?
• 4 Reverse path congestion
• should react? rare for large transfer?
• 5 Did not compare to Vegas
• Future plans

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What people on the street are saying about FAST
TCP

• PC MagazineThe fastest hardware in the world
  will not do you much good if your software can't
  take advantage of it.
• Business Week Connections that leave broadband
  in the dust
• Steven Low This is OK for driving slowly in a
  parking lot. But on the open road you need to be
  able to look further ahead That's what we are
  doing with Fast TCP.
• Future Zone Scientists have developed a new
  data transfer protocol for the Internet fast
  enough to download a full-length DVD movie in
  less than five seconds

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And Of Course

• Anwis and Ajay (Rice CS) FAST TCP Rocks!!! We
  cant wait to use it!

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