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

Conservation at equilibrium round-trip timing TCP –Estimating mean round trip time –Next packet sent Exponential backoff

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

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