Congestion Avoidance and Control

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Congestion Avoidance and Control

• Van Jacobson and Michael Karels
• Presented by Sui-Yu Wang

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Introduction

• Congestion problem has become more severe as the
  computer network grows
• New algorithm forcing the packet conservation
  can be used to achieve network stability
• i) round-trip-time variance estimation
• ii) exponential retransmit timer backoff
• iii) slow-start
• iv) more aggressive receiver ack policy
• v) dynamic window sizing on congestion

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Three ways for packet conservation to fail

• The connection doesnt get to equilibrium
• A sender injects a new packet before an old
  packet has exited
• The equilibrium cant be reached because of
  resource limits along the path

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Getting to equilibrium slow-start

• Self-clocking
• Gradually increase the data in transit
• Source of the picture fig1 in the paper

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Getting to equilibrium slow-start

• Add a congestion window to the per-connection
  state.
• When starting or restarting after a loss, set
  congestion window to on packet
• On each ack for new data, increase congestion
  window by one packet
• When sending, send the minimum of the receivers
  advertised window and congestion window

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• Source of the picture fig2 in the paper

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Conservation at equilibrium round-trip timing

• TCP
• Estimating mean round trip time
• Next packet sent
• Exponential backoff

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• Source of the picture fig4 in the paper

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• Source of the picture fig3 in the paper

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Adapting to the path congestion avoidance

• Reasons that cause time out
• Packets damaged in transit
• Packets lost due to insufficient buffer
• Congestion avoidance
• The network must be able to signal the transport
  endpoints that congestion is occurring
• The endpoints must have a policy that decreases
  utilization

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Adapting to the path congestion avoidance

• Signal of a congested network drop of packets
• Measuring network load
• Smooth network
• Congested network
• Sender policy

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Adapting to the path congestion avoidance

• On any timeout, set congestion window to half the
  current window size
• On each ack for new data, increase the congestion
  window by 1/cwnd
• When wending, send the minimum of the receivers
  advertised window and cwnd