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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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