congestion avoidance and control van jacobson and michael karels presented by sui-yu wang
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Congestion Avoidance and Control
Van Jacobson and Michael Karels
Presented by Sui-Yu Wang
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
Three ways for packet conservation to fail
• The connection doesn’t get to equilibrium
• A sender injects a new packet before an old packet has exited
• The equilibrium can’t be reached because of resource limits along the path
Getting to equilibrium: slow-start
• Self-clocking• Gradually increase the data in transit
Source of the picture: fig1 in the paper
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 receiver’s advertised window and congestion window
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
MRR )1( MRR )1(
R
Source of the picture: fig4 in the paper
Source of the picture: fig3 in the paper
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
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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1 iLNLi
uWW ii 1
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 receiver’s advertised window and cwnd
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