Congestion Window Control Algorithms

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Congestion Window Control Algorithms

• David Wei
• For Pizza Talk _at_ Netlab

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Topics

• History
• Component of Congestion Window Control Algorithm
• FAST (sc2002 version)
• Synchronization Model
• Layered Architecture based on Time Scale
• Open Issues

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History of TCP Congestion Control

• John Nagles
• Lixia Zhangs, Karns Algorithm
• DEC-bit, Tahoe, CARD
• Tri-S, DUAL
• Reno
• Vegas
• New Reno
• HSTCP, Scalable TCP, FAST, Westwood

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???

• Raj Jains Three Questions that a congestion
  control algorithm should answer
• How frequent to make decision
• Decision function
• How to Increase/Decrease Window

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How Frequent?

• Time Scale of a Congestion control algorithm

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Layered-Architecture based on Time Scale
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Decision Function
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Decision Function under Binary Signal
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Congestion Signal

• Delay
• Loss
• ECN

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Congestion Signal

• Transition Point
• Non-Transition Point

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Increase/Decrease Function

• Under Synchronization Model (Chui and Jain)

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Congestion Control Algorithms
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FAST (sc2002) Algorithm

• Do the following every other RTT
• per acknowledgement
• Calculate
• If w(t)gtw w(t)w(t)-1
• If w(t)ltw w(t)w(t)1
• Or Randomize the Increase/Decrease

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Synchronization Model

• Assume
• All the sources make the decision slow enough
  that the queues converge whenever the sources
  make its decision. (queue dynamics has a faster
  time scale than source dynamics)

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Results

• Single link identical sources converges
• Single link heterogeneous sources bounded by
  converging sequences
• General case Contraction Mapping

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Simulation with Matlab

• baseRTT 550ms,600ms,650ms,700ms,750ms
• Alpha500
• C5000/12 pkt/ms

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Simulation with Matlab

• baseRTT 550ms,600ms,650ms,700ms,750ms
• Alpha500
• C5000/12 pkt/ms

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Verification of Synchronization Model

• sources 3
• baseRTT200ms
• BW 100Mbps
• Green Sync. Model
• Magenta Fluid Model
• Red avg queue size
• Blue avg var queue size

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Verification of Synchronization Model

• of sources may not be enough

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Layered-Architecture

• Whats an architecture?
• Functions that TCP Congestion control should be
  considered
• Rate smoothening
• Available bandwidth estimation
• Response to Loss/Retransmission

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Layered-Architecture based on Time Scale
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Open Issues

• Synchronization Model vs Fluid Model
• Properties of different congestion control
  algorithms under the synchronization model
• New optimization model based on time scale
  architecture
• Alpha adaptation based on large time scale
  signals (loss, long averaged RTT)
• Conformance Testing Method based on the time
  scale architecture

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Questions?

• Thanks?