Equationbased Congestion Control for Unicast Applications

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Equation-based Congestion Control for Unicast
Applications

• S. Floyd, M. Handley, J. Padhye and J.Widmer
• Presented by
• Hang Yu, 10/29/2004
• Dept. of ECE, UIUC
• CS598 RHK Fall 2004

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Outline

• Motivations and Problem Statement
• Overview of TFRC
• Loss event estimation
• Average Loss Interval Method
• History Discounting
• Inter-packet spacing
• Rate-based slow-start
• Experimental Results

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Motivations

• Window-based congestion control
• Halve the window in response to congestion
• Reliable data transfer applications
• Reliability
• Rate-based congestion control
• Smooth response to congestion
• Real-time media streaming applications
• Less Jitter and low delay
• Equation-based rate is determined by an equation

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TCP-friendliness

• The majority of the flows on Internet should
  constrain its rate to archive global stability.
• How ?
• Behave like TCP !
• TCP-friendly flow
• A flow that uses less or equal bandwidth in
  stable state than a conformant standard TCP flow
  on the same link.

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TCP Response function

• Throughput T(R,p,tRTO,s)
• Roundtrip time R
• Loss rate p
• Timeout tRTO
• Segment size s

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Problem Statement and Approaches

• Design a new congestion control mechanism that
  satisfies
• Smoother response to congestion than TCP
• Global stability
• Approaches
• Smoother rate-based control to replace
  window-based control
• Increase the sending rate slowly when more
  bandwidth is available
• Do not halve the sending rate to a single packet
  loss.
• Stable use TCP response function to determine
  rate
• Halve the sending rate in severe congestion

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Proposed solution TFRC

• TFRC TCP Friendly Rate Control
• TFRC is actually a mechanism to control rate, NOT
  a transport protocol itself.
• Use of TFRC Datagram Congestion Control Protocol
  (DCCP)
• Basic functionalities
• Sender
• R, tRTO , s
• Receiver
• p

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Loss Event Rate Estimation

• Loss event there exist loss(es) during one RTT
• Requirements of the estimation
• Smooth in a stable loss event rate
• Respond strongly to loss events in successive
  RTTs
• Increase only in response to a new loss event
• Decrease only in response to a new longer loss
  interval
• Possible Methods
• Dynamic History Window (DHW)
• Exponentially Weighted Moving Average (EWMA)
• Average Loss Interval (ALI)

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Average Loss Interval Method

• Loss interval
• number of packets between two loss events
• Average with unequal weights
• Include current loss interval if large enough
• Heuristic when there is no history (during start
  stage)

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An Example of ALI
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History Discounting

• ALI is too slow to respond to decrease in the
  loss rate as it need a large loss interval to
  respond, eg. 100-gt50 and 100-gt200
• If most recent loss interval is twice the
  averaged loss interval, the weight for the old
  loss intervals are discounted.
• More general adaptive weight is not explored yet.
  

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Packet spacing and Slow-Start

• Adjust inter-packet spacing
• Effect of exponential weight on most recent RTT
• Large short term oscillations
• Small not sensitive to RTT changes, easy to
  overshoot bandwidth
• Solution small weight plus modifications to T
• tinter-packets T-1 (R0)0.5 M-1
• Rate-based slow-start
• TCP response function does not model slow-start
• Double rate bounded by fed-back received rate
• Loss occurs slow start ends and set initial rate
  half of the current rate

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Persistent congestion and quiescent senders

• Response to persistent congestion
• 4-8 RTTs to reduce to ½ rate in congestion
• 0.14 packet/RTT after congestion
• Response to quiescent senders
• Application does not have bulk data all the time
• Solutions
• Always bounded by twice the received rate
• Halve the permitted rate every 2 RTT when no
  feedback
• Need to reduce rate less aggressively and recover
  by slow start to old sending rate ( in
  exploration).

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Experiments

• Measures Equivalent Ratio
• Mean and CoV
• Simulations
• Long-duration background traffic
• ON/OFF background traffic
• Real Internet Experiments
• Two transcontinental links
• Effects on queue dynamics

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Different traffic types
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Real Internet Experiment
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Queue Dynamics
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Conclusions from Experiments

• TFRC co-exist well with TCP over a wide range of
  network conditions.
• CoV of TFRC is lower.
• When is TFRC less well-behaved ?
• Overloaded links
• With Some TCP variants
• Phase effect

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More topics and references

• Open issues
• Multicast TFRC
• How to limit feedback ?
• How to determine RTT ?
• Effects of ECN
• Duplex TFRC
• Variable Packet Size
• Transient performance
• Comparison with GAIMD
• ICIR TFRC www.icir.org/tfrc/